A review of experimental approaches, trends and opportunities in plasma-based gas conversion research

Abstract

Plasma-based gas conversion has emerged as a sustainable and promising approach for chemical production, attracting increasing attention in recent years. Significant progress has been achieved in areas such as nitrogen fixation, CO ₂ conversion, methane activation, and others, driven by the contributions of researchers from diverse disciplines. Given that most research in this field is experimental, the methodologies employed play a pivotal role and demand careful consideration. However, due to the interdisciplinary nature of the field and variations in research objectives, available resources, and laboratory standards, experimental set-ups and approaches often differ significantly. Moreover, critical details regarding operational techniques and key methodologies are sometimes overlooked. This paper provides a comprehensive review of the methodologies and experimental approaches used in the study of plasma-based gas conversion for chemical production. It first examines experimental systems, including plasma reactor design, plasma-catalyst integration, and set-up configuration. Subsequently, operational schemes, conditions, and analytical procedures are discussed, with examples showcasing state-of-the-art advancements. Finally, discussion on emerging research trends and potential opportunities are presented, aiming to inspire further advancements and broaden the scope of this growing field.

Graphical abstract

Keywords

plasma catalysis / plasma reactor / CO ₂ utilization / nitrogen fixation / methane conversion

1 Introduction

The use of plasma-based gas conversion for chemical production is not a new concept, with its roots stretching back over 100 years. Early research dates to 1857 when Siemens introduced silent discharge for ozone generation, laying the foundation for later applications. The Birkeland-Eyde process, which synthesized nitric oxide via thermal arc plasma, was commercialized in the early 1900s, marking it the first industrial plasma process. Even plasma-catalysis—a topic receiving substantial attention today—can be traced back to 1921 when Ray and Anderegg combined plasma discharge with silver catalysts to oxidize carbon monoxide [1]. However, plasma technology has not been widely applied in the chemical industry. Over the past century, the industry has been largely dominated by thermochemical processes powered by inexpensive fossil fuels. In recent decades, with increasing environmental concerns and a global push for energy transition, there has been a shift toward renewable electricity and the electrification of the chemical production sector. Plasma technology, particularly non-thermal plasma, has emerged as a promising green alternative. It offers significant advantages, including mild operating conditions (e.g., atmospheric pressure and low temperature), instantaneous control, and flexibility in scales. These characteristics make plasma highly suitable for coupling with intermittent renewable energy sources, aligning with the “Power-to-X” concept to produce chemicals and store energy.

Driven by the growing interest from both academia and industry, plasma-based gas conversion research has seen a dramatic surge in the past 15 years. Researchers worldwide have made significant advancements, as reflected by the substantial growth in scientific publications in Fig.1(a). Major research efforts focus on the activation of small molecules such as N₂, CO₂, and CH₄. The activation of these molecules typically requires high energy input from elevated temperatures in conventional processes. This has been considered as a key opportunity for plasma chemistry and plasma-catalysis. To date, several review papers have been published, covering topics such as nitrogen fixation, CO₂ conversion, and CH₄ activation with detailed background information, fundamental principles, the state of the art, and recent progress in the field. Tab.1 summarizes notable reviews as our recommendation for readers seeking additional insights. Tab.2 listed the examples of the most studied reactions, presenting their current technological readiness level (TRL) and the typical range of reported energy consumption. While lower energy consumption has been observed in some cases, especially with recent advancements, the values listed represent the commonly reported value, providing a general overview of the state of the art.

While computational studies have contributed to the field, the majority of research focuses on experimental work, where methodologies play a critical role. Plasma-based gas conversion is inherently multidisciplinary, requiring expertise in plasma physics, plasma chemistry, chemical engineering, and electrical engineering. Researchers from different backgrounds may interpret the subject differently and employ diverse methodologies. Consequently, experimental set-ups, techniques, and performance metrics vary significantly. Furthermore, important details about research methodologies and practical techniques are often overlooked, particularly given the growing interest and rapid expansion of this field. To address these gaps, this paper provides a comprehensive review of the experimental methodologies employed in plasma-based gas conversion research with a focus on small molecule activation for chemical production. It aims to offer valuable insights to readers from diverse disciplines. Examples from the literature, primarily focused on CO₂ conversion due to its extensive research base, are used to illustrate key points. The structure of this paper is outlined in Fig.1(b). This introduction offers an overview of the field, including its historical development, emerging trends, and key reactions of interest. Subsequent sections will present a detailed review of experimental systems, methodologies, and approaches. The paper concludes with a personal perspective on future trends and opportunities in this rapidly evolving field.

2 Experimental systems

2.1 Experimental set-up

Over the years, numerous experimental set-ups have been developed for plasma-based gas conversion research. These systems are tailored to meet specific research objectives and experimental designs, incorporating a variety of equipment configurations. Despite this variation, most laboratory-scale experiments share common components, which can be broadly categorized into three major parts, as illustrated in Fig.2.

2.1.1 Gas handling and condition control

Gas reactants are typically fed to the reactor continuously at a constant flow rate and desired concentration. Mass flow controllers are widely used to regulate flow rates, and these devices require calibration prior to experiments and periodic recalibration for accuracy. Gas pressure within the system, especially in the plasma reactor, is critical as it directly affects plasma properties and chemical performance. Most plasma experiments are conducted at or near atmospheric pressure, leveraging a key advantage of plasma chemistry over conventional methods through reduced costs and simplified process complexity. In some cases where plasma is operated at higher pressure, additional components like pressure control valves, back-pressure regulators, and pressure relief valves for safety need to be installed in the system. Conversely, in microwave plasma reactors, sub-atmospheric pressures are often employed, necessitating vacuum systems with pumps, control valves, pressure gauges, and sometimes a dedicated chamber. It needs to be noted that stable pressure, flow rate, and a compatible temperature profile are essential to ensure accurate measurements by downstream analytical equipment such as FTIR or GC. Additional devices such as diaphragm pumps can be utilized as needed.

Temperature control is another crucial factor in experimental system design. Unlike conventional thermal catalysis, the role of temperature in plasma or plasma-catalysis is more complex. Temperature variations can influence both plasma characteristics and catalyst activities, while the plasma itself generates heat, raising gas and catalyst temperatures. To address these effects, systems with various temperature control strategies have been established. Biset-Peiró et al. [23] categorized plasma reactors based on heat management strategies into three types: isothermal, adiabatic, and pseudo-adiabatic. Fig.3 compares experimental systems reported in the literature for these categories. In isothermal systems, comparisons with thermal catalysis at equivalent temperatures are often made to isolate plasma effects. However, it should be noted that the set temperature from external heating, or the measured average temperature does not reflect the practical temperature profile due to the plasma heating which may cause localized temperature increases.

Pseudo-adiabatic systems are widely used, particularly in plasma reactors without catalysts. External heating or cooling should be considered alongside reactor design to achieve optimal outcomes. An example is the water-electrode dielectric barrier discharge (DBD) reactor, which integrates cooling and plasma generation, although external water/coolant circulation is still required [24,25]. The impact of cooling has also been studied in microwave plasma reactors. For instance, Fuente’s study on a surface wave microwave plasma reactor demonstrated a significant impact of cooling rates on CO₂ conversion, underscoring the importance of cooling system design for process optimization [26]. In warm plasmas like microwave [27] and gliding arc (GA) [28], external cooling is often essential to prevent overheating and maintain stable plasma operation.

Apart from the reactor, thermal management can also be applied to inlet and outlet gas streams, influencing experimental results. For example, Paulussen et al. [29] showed that preheating inlet gases to achieve temperatures between 30 and 170 °C led to a modest increase in CO₂ conversion from 26% to 28.5%. In addition, for reactions such as CO₂ hydrogenation, DRM, where water or liquid products may form, heating the pipelines and connectors is necessary to prevent condensation.

2.1.2 Plasma system

The fundamentals of non-thermal plasma generation including discharge mechanism, criteria and sources follow the general principles which have been described in many papers and textbooks [30,31]. However, establishing a plasma system requires practical considerations and cross-disciplinary knowledge. The two major components in a plasma system are the reactors and the power supply unit (PSU). To date, various reactor designs have been studied, and the design of reactors itself remains a significant research topic in this field. The design of plasma reactors is reviewed in Section 2.2.

The development of an efficient power supply for plasma systems is also a topic worth exploring, especially in the case of large-scale applications in which the overall energy consumption and cost of the entire process should be emphasized. This area intersects with electrical engineering, particularly with research on transformer systems and power electronics, including materials, design, and topologies, as mentioned in several studies [32−34]. As for laboratory-scale experimental studies, the power supplies used vary widely, ranging from simple homemade devices with readily available components to high-precision commercial products. Output power can range from a few watts to kilowatts, with voltage signals in various forms, including constant (direct current (DC) power supply), sinusoidal (alternating current (AC) power supply), or pulsed, depending on the discharge type and reactor design. Most studies involving AC power supply use a single-phase output with adjustable power and a frequency range in kHz. While 50 Hz or three-phase systems which are closer to the industrial standard, have only been studied in a few cases [35−37]. The PSU plays a crucial role in experimental studies, particularly in enabling precise control and variation of key parameters during operation. This typically includes the modulation of output voltage, power, and frequency, all of which can significantly affect the performance of the plasma reactor. Commercially available PSUs often come equipped with built-in features for adjusting these parameters, as well as advanced functions such as feedback control and pulsed operation modes. In contrast, customized or home-made PSUs may require circuit modifications to achieve similar functionalities, which can be more complex.

Pulsed power supplies can be employed for plasma generation in various reactors, including DBD, corona, and plasma jets. In recent years, there has been growing interest in direct-driven nanosecond plasma discharge. The use of nanosecond pulsed power excitation provides a higher instantaneous power density and reduces the electric field, enhancing energy efficiency for reactions and preventing gas and reactor overheating that typically occurs with continuous operation [38]. It is important to mention that generating high-voltage (HV) nanosecond pulses and managing source-plasma interactions are critical to the system’s overall energy efficiency [39]. However, these factors are often not considered in reported research, particularly in plasma-based CO₂ conversion, as they may fall outside the primary scope of plasma chemistry, plasma-catalysts or reactor development. Nonetheless, key parameters have been investigated in a few studies. For example, Montesano et al. studied the effect of different pulse patterns on the reduction of CO₂, and their results highlighted the importance of inter-pulse duration [40]. They also observed a delay of CO₂ dissociation after breakdown, and proposed a possible way to improve energy efficiency by shortening the interpulse period [41].

An essential point to consider is that the plasma discharge reactor is typically viewed as an unconventional load connected to the power supply, which influences the circuit’s characteristics. For instance, a GA reactor behaves as a dynamic load with nonlinear and fast-changing V-I characteristics [42], while a DBD reactor is often considered a capacitive component with a distorted current profile that cannot function with a steady-state DC power source. Additionally, other components may be connected to the circuit for protection, monitoring, or plasma modulation. These components include but are not limited to, ballast resistors to limit current, resistive shunts to monitor current, and capacitors in series with DBD reactors to generate Lissajous figures.

In addition to conventional electrical power supplies, a new type of system based on triboelectric nanogenerators (TENGs) has recently been applied in the field of plasma-based gas conversion. Triboelectric plasma systems are relatively simple and can directly utilize various types of mechanical energy to generate plasma using TENGs characterized by HVs. Several studies have reported plasma generation using TENG-based systems [43,44], and explored their potential applications, such as nitrogen fixation [45,46], and CO₂ conversion [47,48]. However, the current TENG systems operate at very low power levels and may only be suitable for microplasma applications.

2.1.3 Monitoring and diagnostic systems

Equipment and devices used for monitoring and diagnostic purposes in plasma-based gas conversion experiments generally operate within two types of systems: operando and offline/ex-situ. Devices for monitoring experimental conditions, such as pressure and temperature, are usually part of the gas handling and control system. These include flow meters, pressure gauges, and thermocouples. Electrical diagnostics for plasma discharges are typically integrated into the plasma system and include equipment such as HV probes, Rogowski coils, oscilloscopes, and measuring capacitors or resistors. Proper selection of these devices is crucial, depending on the type of plasma discharge and measurement objectives. For instance, an oscilloscope with a high sampling rate is essential for capturing fine details of fast discharge phenomenon. Separately, an online gas analyzer is a key component of the monitoring and diagnostic system, often installed downstream of the reactor. Common gas analysis equipment includes GC, FTIR, and mass spectrometry (MS) for identifying and quantifying reaction products. Proper calibration of these instruments, particularly using internal standards based on specific experimental conditions, is crucial for obtaining accurate results. Furthermore, practical consideration needs to be taken during system integration. For instance, GA reactors typically operate at flow rates (several to tens of L·min^–1) that may be too high for direct GC sampling or could cause pressure drop in infrared (IR) gas cell [49]. In such cases, an online measurement with a gas bypass line or additional components like flow controllers may be required. Conversely, in some cases with DBD reactors operating at very low flow rates, a significant measurement delay may occur due to the extended travel time for the gas to reach the sampling point.

Optical diagnostic methods are commonly employed to study plasma properties, with systems such as optical emission spectroscopy (OES) and laser absorption spectroscopy in use. The complexity of these systems varies from simple spectrometers to integrated laser set-ups. It is often critical to use reactors, gas cells, and lenses tailored for specific measurement purposes. Examples of advanced in situ diagnostics for plasma-catalytic reactors are discussed in Section 5.

Offline/ex-situ diagnostic tools focus primarily on studying material properties, particularly catalysts and electrodes used in plasma-based experiments. Equipment commonly used such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller analysis are similar to those used in conventional catalysis research. Although these devices are not directly integrated into the experimental set-up, it is essential to consider them during experimental planning. For example, the preparation and transfer of catalyst samples from the plasma reactor can be critical to maintaining material integrity for accurate analysis.

2.2 The design of plasma reactors

The plasma reactors used in this field are predominantly custom-designed, with materials and configurations tailored to meet specific experimental goals and laboratory conditions. Typically, the reactor dimensions reported in the literature range from a few to several tens of centimeters, although larger reactors, reaching up to meters in scale, have been documented in limited cases. These larger designs are less common, as most studies focus on laboratory-scale reactors with low TRL. The fundamental requirements for reactor development are ensuring stable and safe plasma operation. However, advanced reactor design also aims to enhance performance, particularly by improving energy efficiency and conversion. This calls for research on optimizing plasma properties and distribution, minimizing gas bypass around the plasma zone, and facilitating beneficial interactions between plasma and catalysts. Achieving these improvements often requires detailed study of plasma operation, electrode configuration, gas flow patterns, as well as heat and mass transfer within the reactor. The fundamentals of various reactor types for plasma-based gas conversion have been reviewed in several previous studies [30,50,51], to which we refer for background information. This section provides an overview focused specifically on the design and development of these reactors.

2.2.1 Non-catalytic reactors

Although reactor designs vary widely, they typically adhere to a few fundamental configurations. The most commonly studied types include DBD, GA, corona, microwave, and radiofrequency (RF) plasma reactors.

2.2.1.1 DBD reactors

DBD is one of the most studied plasma discharge types which has a history that could date back to 1857 when the first experimental study by Siemens was reported [52]. The configuration of a DBD reactor consists of two electrodes, a gas gap and at least one dielectric barrier. With the HV supply connected to the electrodes, a strong electric field is created to generate plasma in the gap, while the existence of the dielectric barrier prevents the formation of sparks, hence keeping the plasma with non-thermal equilibrium characteristics. There have been many different designs of DBD reactors which can be found in the review of Brandenburg [53]. In the case of gas conversion, DBD reactors are often manufactured as planar or coaxial configuration, as shown in Fig.4(a) and Fig.4(b). There are also designs of plasma jet reactors which are based on the DBD concept as shown in Fig.4(c). The coaxial configuration provides better performance due to more uniform gas velocity distribution across the discharge zone. The key reactor parameters include gap size and discharge length, which determine the discharge volume and, consequently, the residence time of the reactants. The thickness and material properties of the dielectric barrier (e.g., dielectric constant), as well as the choice of electrode material, can influence reactor performance. This has been reported in several papers including the one from Wu et al. [54] who observed a better performance using an aluminum rod as the inner electrode than a copper or stainless-steel rod in their study of CO₂ conversion with a coaxial DBD reactor. The author believes that aluminum is preferred due to its moderate thermal conductivity and electrical conductivity, while copper works better as an external electrode due to its high electrical conductivity.

Several attempts have been made to investigate different designs of a DBD reactor to enhance the conversion of CO₂. For example, Belov et al. [55] studied the influence of the conductive carbonaceous coating on the electrical properties and CO₂ conversion; Wang et al. [56] studied the design with segmented outer electrode; and Niu et al. [57] reported their reactor design with multi-electrode system. Although it is possible to improve the conversion or energy efficiency through the design of the DBD reactor, such improvement is not significant. Based on the reported literature so far, DBD reactor alone shows a low energy efficiency in general, making it less suitable for industrial conversion of relatively stable molecules such as CO₂ and N₂ [50].

2.2.1.2 GA reactors

The GA discharge is a promising type of “warm” plasma which has attracted much attention in the field of gas conversion during the past 30 years. In fact, this type of discharge phenomenon has been observed and studied for an even longer period, such as the typical example of the “Jacob’s ladder” [58]. A key advantage of GA reactors is their potential to achieve high energy efficiency in chemical conversion. This efficiency primarily arises from the electron temperature (e.g., 0.4–3.4 eV as reported in literature [59]), which is optimal for the vibrational excitation of molecules such as CO₂ and N₂, while the high arc temperature also facilitates the thermal activation of inert molecules. Additionally, GA reactors are characterized by robust designs and the ability to operate at atmospheric pressure. The fundamental physics and chemical properties of GA reactors, along with their applications, have been described by Fridman et al. [60]. Additionally, Liu et al. [59] provide a specific review of CO₂ conversion using GA technology. For further details, we direct readers to these publications.

In principle, the classic GA reactor design is based on a pair of divergent electrodes as shown in Fig.5(a). The arc ignites at the narrowest gap and then propagates to a wider gap due to the buoyant force and the gas flow. The arc elongates during the process until it reaches its maximum and becomes extinct, then a new cycle starts with ignition at the narrowest gap again. This classic GA reactor design is often referred to as the “2D” GA (2D: two-dimensional), primarily due to its nearly 2D arc propagation. Numerous studies have investigated various design factors of 2D GA reactors, including configurations, materials, and operational parameters, to enhance performance in chemical conversion. For example, Sun et al. [61] investigated the influence of different electrode materials and graphite outperformed iron, stainless steel, and brass regarding energy efficiency. The author linked these results with the motion characteristics of GA when different material was used. In the study of Li et al. [62], the effect of the flow rate, diameter of the injector nozzle and its distance to the electrodes as well as the structure of the quartz cover have been investigated, providing insight into the optimization of the reactor design parameters for better performance in CO₂ dissociation. Another study from the same group reported an enhancement effect achieved by applying an external magnetic field [63]. This enhancement was attributed to the expanded plasma region, particularly at low gas flow rates. The Lorentz force can influence the arc, thereby increasing processing capacity, as also noted by Chen et al. [64]. However, it is important to consider the direction of the external electric field and operational parameters, especially the flow rate. Ivanov et al. [28] investigated the use of a magnetic field in different orientations to accelerate or stabilize the GA for CO₂ conversion. They found no improvement when attempting to accelerate the arc; however, good performance was observed at low flow rates when stabilizing the arc, after which conversion dropped drastically with increasing flow rates.

It is important to note several drawbacks that need to be addressed in classical 2D GA reactors. A major issue is that only a limited fraction of the gas is exposed to plasma, as some gas bypasses the arc within the reactor. Additionally, conventional GA reactors face other challenges, including the need for high gas velocity, significant convective heat loss, and electrode degradation over time [67]. These issues are critical focus areas for further research in reactor design.

Apart from the 2D GA, more sophisticated three-dimensional (3D) or rotating GA reactor designs have been developed by many groups. Two pioneer examples can be found are designs of plasmatron by Bromberg et al. [68,69] from Massachusetts Institute of Technology and Fridman et al. [70,71] from Drexel University. In principle, the rotating motion of arc in 3D GA reactors can be created in different ways, including using magnetic field to drive the arc such as in the study by Liu et al. [72], or mechanical rotation similar to the plasma-shade reactor reported in literature [73]. However, the most common way is to implement the gas flow, often with gas inlets in the tangential direction, to create a forward or reversed vortex. Several designs of forward vortex GA reactors have been reported [74,65], and an example is shown in Fig.5(b). Two electrodes are located coaxially, and the gas can be introduced to the reactor via tangential inlets at the bottom of the reactor. The outer electrode is normally cylindrical while the inner electrode can be a corn-shaped rod, or a spiral wire as shown in the figure to guide the arc propagation along the electrode under the effect of the gas flow. In the case of reverse vortex GA, an example of reactor design is shown in Fig.5(c). When the arc ignites between the electrodes, it moves to the center of the reactor and is stabilized under the effect of the reverse vortex flow. The motion of the arc is associated with strong convective cooling and thermal insulation from the reactor wall, hence providing better thermal efficiency [59]. The study of Ramakers et al. [75] revealed the arc dynamics and its correlation with CO₂ conversion in a reverse vortex GA reactor.

Research efforts have been made to explore the design of 3D GA reactors for improved CO₂ conversion. A study by Trenchev et al. [67] highlighted their design of a dual vortex plasmatron reactor as shown in Fig.5(d). This design with a novel electrode configuration can elongate the arc in two directions to increase the residence time, hence improving the conversion of CO₂. The results showed that 41% energy efficiency with 9% conversion can be achieved by this reactor. At the same time, the gas convection and the rotation of the arc can also prevent electrode degradation by cooling the cathode spot. Vertongen et al. [66] evaluated several new electrode configurations in a GA plasmatron and observed the influence of different designs, but the improvement in CO₂ conversion was limited. This indicates a performance limit of this reactor design, and the author recommended modification on the post-plasma region for possible further improvement. This recommendation is in line with the research of Dinh et al. [76] who investigated the effect of attaching a nozzle downstream of the GA in the reactor. In their study on DRM, 75% higher CO₂ conversion and 47% higher energy efficiency were achieved with the nozzle which enhanced heat transfer from the arc to the reactant and reduced the heat loss to the wall. There is also another study by Kwon et al. [77] who modified the downstream of the GA but with a quenching device. In their case, the selectively of H₂ was increased by preventing its consumption via suppression of the reverse water gas shift reaction. Besides, new ideas and innovative designs in this research direction could be highly beneficial for further exploration and the reactor design as well as creating new opportunities. Nagasou et al. [78] reported an interesting design which combines a solar receiver and a GA reactor. This solar-plasma reactor is aimed to decompose CO₂ by using both concentrated solar energy and plasma driven by renewable energy. Their results demonstrated the interaction between plasma and solar radiation under different configurations.

2.2.1.3 RF and microwave plasma reactors

In the case of RF plasma, RF power is applied to plasma reactors via either inductive or capacitive coupling as in Fig.6(a) and Fig.6(b). An impedance matching unit/matching box as one important component, is connected in the circuit with the external RF generator and the reactor for stable plasma operation and reduced power loss. While a typical microwave plasma system consists of several more components as indicated in Fig.6(c). HV power supply is used to drive the magnetron to produce microwave power, and the microwave-to-plasma applicator transfers microwave energy to plasma. Other components in the system such as the isolator (e.g., circulator) and matching circuit maintain the necessary practical function including preventing the damage from reflected power and matching the impedance of the magnetron with that of the load [79]. Recently reported studies on CO₂ conversion by microwave plasma have been reviewed in Refs. [80] and [81]. In reactor design and optimization research, efforts have focused on enhancing quenching, which is crucial for improving CO₂ conversion. Rapidly reducing gas temperature in the post-plasma region helps prevent CO from reacting with oxygen species, such as O or O₂, which would otherwise reform CO₂ [82,83]. Hecimovic et al. [84] demonstrated this approach by using a downstream nozzle, testing various nozzle diameters and distances at pressures from 100 to 900 mbar. The results demonstrated an enhancement at a pressure close to atmospheric with the conversion and energy efficiency comparable to the values achieved at 200 mbar. Another study by Mercer et al. [85] developed a converging-diverging nozzle attached to a microwave plasma reactor. Such nozzle design resulted in a 21% and 71% increase in energy efficiency and CO₂ conversion at 700 and 300 mbar, respectively.

2.2.1.4 Other types of reactors

There are also studies on CO₂ conversion with plasma reactors based on other types of discharges such as corona, spark or atmospheric glow. Corona discharge reactors have one or more electrodes with sharp edges or points that could create a highly divergent electric field. Due to this feature, the plasma generated is not uniform and often limited in volume, which is not preferred considering the limited fraction of gas treated which leads to a low conversion. Therefore, despite relevant research prior to 2010, only a limited number of cases have been reported in recent years [87]. The spark discharge reactor operates with a pair of electrodes between which streamers develop into spark channels that periodically extinguish and reignite. Various electrode configurations, such as plate-to-plate, point-to-plate, or sphere-to-sphere, can be used at different distances. While limited research has focused on CO₂ splitting using spark discharge reactors [71], there has been more emphasis on DRM [88−90], with studies reporting high conversion efficiencies and low energy consumption (e.g., CO₂ conversion > 84% with energy consumption of 0.218 MJ·mol^–1 [75]).

Glow discharge reactors also present a promising option, and their reactor design can be similar to corona or spark, often using point-plate electrode configurations. Traditionally, glow discharge experiments were conducted at pressures below atmospheric, and some recent studies continue to operate at low pressure [91,92]. However, there is growing interest in atmospheric-pressure glow discharge (APGD) due to the operational advantages at atmospheric conditions. A critical consideration for APGD is preventing the transition from glow to arc discharge at high pressures, which can be managed through circuit and power supply design. An example can be found in the study of Renninger et al. who reported APGD driven by appropriate power supply with low complexity, high efficiency above 70%, and a maximum overall system efficiency of 22% has been achieved [93]. Also, in this study, they used a magnetic field to expand the discharge volume which could lead to a higher fraction of gas treated. A different approach was made by Wanten et al. [94] and Trenchev et al. [95] by confining the reactor volume to maximize the fraction of gas treated by plasma, and improved reactor performance was achieved in this way. The operation parameters could further enhance the performance of APGD reactors for CO₂ conversion. This has been demonstrated by Meng et al. [96] who conducted a relevant study and concluded that key parameters including gap distance, power and flow rate play synergistic roles in improving the conversion and energy efficiency.

2.2.2 Plasma-catalyst integration

The core of plasma-catalytic reactor design lies in optimizing plasma-catalyst integration to achieve maximum synergy. Two primary configurations exist: in-plasma catalysis (IPC) and post-plasma catalysis (PPC). In IPC, also known as a one-stage configuration, catalysts are placed directly within the plasma zone, allowing intense interactions between plasma-generated species (both short- and long-lived) and the catalyst. However, the presence of catalysts in the plasma zone also impacts plasma properties. Conversely, in PPC (or two-stage configuration), the catalysts are located downstream of the plasma zone, where only long-lived species interact with the catalyst surface. Building on these two configurations, multi-stage designs have been explored, allowing for different catalysts or plasma treatments at each stage for optimized performance [97].

Applying the IPC configuration is not always straightforward in plasma reactors; for example, in “warm” plasmas with high temperatures, a two-stage configuration is often a more practical choice. Examples can be seen in studies on GA [98], microwave [99] and glow discharges [100].

Zhang et al. [101] investigated CO₂ splitting using a GA reactor packed with photocatalyst TiO₂ located downstream of the discharge gap. By varying the distance of the catalyst bed, both IPC and PPC configurations were tested. The presence of catalysts led to enhancements of 138% in CO₂ conversion and 133% in energy efficiency for the IPC configuration, while the improvement was negligible in the PPC configuration. Interestingly, the presence of the catalyst container downstream reduced reactor performance at low flow rates but significantly improved performance at higher flow rates. This effect is attributed to backflow introduced by the container, which increases the fraction of gas treated. This example also illustrates the potential influence of PPC beyond mere catalytic effects.

To date, most studies have employed IPC configurations due to the critical role of short-lived species in plasma-induced reactions, particularly for surface reactions. For instance, vibrationally excited species at atmospheric pressure have lifetimes on the order of nanoseconds, which is too short for effective utilization in PPC [3]. Different reactor set-ups, such as packed bed, structured bed, and fluidized bed, have been explored for IPC, and will be discussed in this section.

2.2.2.1 Packed bed

Directly packing catalysts (in the form of pellets or spheres) into a DBD reactor is the most used approach so far due to the simplicity and convenience of experimental tests. Very often comparisons between a DBD reactor with or without catalysts packing were made in many studies to reveal the effect of the catalysts or packing materials. The purpose of packing catalysts is to create more surface and introduce catalytic surface reaction in DBD reaction. At the same time, there are other important factors that need to be considered which make the reactor packed with catalysts very different from an empty reactor. When packing material is introduced into the reactor, the gas channel volume decreases, forcing gas to flow only through the voids of the packaging. This results in shorter residence time at the same flow rate and could cause pressure drop, especially at high flow rates. Additionally, the effective gas gap is reduced by packing, which lowers the voltage range required for plasma generation and generally reduces the power delivered compared to an empty reactor under similar conditions. More importantly, the discharge mode is modified, resulting in a combination of discharges within the void spaces and surface discharges on the packing material [53]. The existence of packing (mainly dielectric material) enhances the electric field, especially near the contact points between pellets, leading to plasma generation and increased electron energy which is normally referred to as packed-bed effect [102]. The physical properties of the packing material, including size, shape, surface area, and dielectric constant, play a crucial role and can impact chemical conversion even without considering the catalytic surface reactions introduced. These factors should be carefully considered when selecting catalysts, designing reactors, or comparing results obtained from different experimental set-ups or reported literature. Nevertheless, these influences have already been the focus of several studies, with investigations through both experimental work [103−105] and modeling [106]. Readers are encouraged to refer to these sources for more comprehensive insights.

In addition to fully packed DBD reactors, where catalysts fill the entire discharge zone and significantly alter the discharge mode, partially packed reactors have also been explored in the literature. Unlike full packing, which changes the discharge characteristics considerably, partial packing can preserve strong filamentary discharges while enabling effective plasma-catalyst interactions. This effect was demonstrated by Tu et al. [107] in their study of DRM using Ni/γ-Al₂O₃ catalysts, where enhanced physical and chemical interactions were observed. Gadkari et al. [108] further examined the influence of partial packing in a DBD reactor through a 2D numerical fluid model. They found that, compared to an empty DBD reactor, partial packing enhanced the electric field at the top surface of the packing and at contact points between the packing and dielectric layer, resulting in increased power density and electron energy. Apart from partial packing, mixed or sectioned packing configurations have also shown an impact on reactor performance. Wang et al. [109] investigated CO₂ hydrogenation using a packed DBD reactor with Co catalysts. They tested four different packing arrangements: fully and partially packed catalysts, a mixture of catalysts with Al₂O₃, and separate sections for catalysts and Al₂O₃. The latter configuration achieved a significant increase in C₂ hydrocarbon selectivity, highlighting the importance of packing arrangement, particularly when multiple materials are involved.

2.2.2.2 Structured bed

Instead of using packed pellets or spheres, structured materials can also be utilized in plasma reactors. A common example is the honeycomb discharge, which can feature various configurations, as illustrated in Fig.7(a–c). In this type of reactor, the fixed gas channels within the catalyst bed allow for high throughput, enabling the processing of gas at large flow rates with minimal pressure drop, an advantage that has made honeycomb structures popular in commercial systems such as the case of car exhaust catalysts. However, key challenges include achieving stable, uniform plasma generation within the reactor and ensuring optimal interaction between plasma species and the catalytic surface. Depending on the reactor design, charge particle loss can occur due to interactions with the channel walls, often requiring higher voltages; in some cases, sparking between electrodes through the channels can lead to damage to the catalyst bed [110]. This necessitates careful design of the catalyst structure and the electrode configuration to balance these challenges. There are different designs of honeycomb discharge reactors reported during the last 20 years, including wires-to-wires [111], mesh-to-mesh [112,113], sliding discharge-based designs [114] and many others.

Similarly, foam-type materials can also be utilized in plasma reactors. For instance, Taghvaei et al. [116] demonstrated CO₂ decomposition using a DBD reactor packed with polyurethane foam, as illustrated in Fig.7(d). This foam-filled arrangement enabled uniform distribution of packing materials, with the foam serving as a support for different metal oxides to compare their effects on conversion and energy efficiency. BaTiO₃-coated foam achieved a 124% increase in conversion compared to foam-only reactors, and a 203% improvement over empty reactors. However, research on plasma-catalytic conversion of gas with structured catalyst beds remains limited. Current studies focus more on catalyst development than on reactor design. At laboratory scale, where large flow rates are not typically required, structured bed reactors receive less attention. However, structured beds are more frequently applied in plasma reactors for volatile organic compound (VOC) removal, where low concentrations of reactants necessitate high flow rates. Nevertheless, structured beds remain a promising area for exploration, especially considering the development of plasma-catalytic reactors for large-scale applications.

2.2.2.3 Fluidized bed

The combination of plasma technology with fluidized bed reactors represents a promising approach that leverages the benefits of both plasma chemistry and fluidization, particularly in enhancing gas-solid processing through improved mass and heat transfer. Numerous studies have focused on plasma-fluidized bed reactors primarily in the realm of material processing, including catalyst treatment [117,118], polymer processing [119], and surface modification of nanoparticles [120] and powders [121]. While there has been substantial research on thermal plasma-based fluidized bed reactors for material applications, as reviewed by Du et al. [122], comparatively fewer studies have explored in the case of non-thermal plasma and their potential for gas conversion, despite the significant advantages this configuration could offer in enhancing the interaction between plasma and catalysts. A notable contribution by Chen et al. [9] has highlighted the use of fluidized bed plasma reactors for the conversion of CH₄ and CO₂ in their review paper along with the fundamental principles and recent developments in the field.

In a fluidized bed reactor, the motion of catalyst particles is a crucial factor, especially under plasma conditions. The fluidization behavior of particles within the reactor is primarily governed by the superficial gas velocity, which is determined by the reactor geometry and gas flow rate. At low gas flow rates, the pressure drop across the bed is much smaller than the gravitational force acting on the particles, resulting in a fixed bed where particles remain stationary. When the gas flow rate exceeds the minimum fluidization velocity, particles begin to move randomly, achieving a state known as incipient fluidization. As the gas flow rate is further increased, the fluidization becomes homogeneous. With even higher flow rates, the system transitions into inhomogeneous fluidization patterns, such as bubbling. When the gas flow rate reaches the terminal settling velocity of the particles, the gravitational force and the upward hydrodynamic force are balanced. If the gas flow rate continues to increase beyond this point, particles will be carried out of the reactor due to pneumatic transport. Generally, the terminal settling velocity (U_t) can be estimated using the following equation [123]:

(1)U_{\mathrm{t}}={\left(\frac{4d_{\mathrm{s}}\left({\rho }_{\mathrm{s}}-{\rho }_{\mathrm{g}}\right)g}{3{\rho }_{\mathrm{g}}{C}_{\mathrm{D}}}\right)}^{\frac{1}{2}},

where ρ_s and ρ_g are the density of the particle and gas media, d_s is the particle size, C_D is the drag coefficient and g is the gravitational acceleration. This equation highlights the importance of particle properties in achieving effective fluidization, and it can be used to guide the selection of both operating parameters and particle characteristics to ensure proper fluidization. However, it is important to note that fluidized bed plasma reactors offer less flexibility than fixed-bed reactors, as they require careful control of gas flow to maintain fluidization. Consequently, reactor design and selection of catalysts need to be made to couple optimal plasma conditions with fluidization.

In addition, it should be noted that Eq. (1) only considers conventional fluidized bed conditions whereas the existence of plasma could introduce more complexity to the particles’ motion via possible effects such as the enhanced Coulomb force between charged particles or due to external electric field. It is not a surprise that precipitation could occur on the reactor wall or electrode since the electrostatic precipitator was developed based on such a principle. Increasing the particle size can be a way to solve this issue [110]. On the other hand, it is also possible that the existence of plasma facilitates the fluidization of the particles inside the reactor. A study on acetaldehyde removal using a fluidized bed DBD reactor was conducted by Jia et al. [124]. In their study, 6.5 g catalysts in the form of pellets with a mass of 20 mg per pellet were inserted into a co-axial DBD reactor which has a 2 cm radius. The flow rate tested was 100 mL·min^–1 which is not enough to cause fluidization. While plasma with a low frequency (1–50 Hz) was applied, the pellets were charged and moved alternatively between electrodes, leading to an electrically enhanced fluidization which is very different from the conventional gas-dominated fluidization. The authors also noted that this electrically enhanced fluidization effect was absent at higher frequencies, as the rapid alternation of the electric field does not allow sufficient time for the particles to move and achieve fluidization.

In addition, catalysts in plasma fluidized bed reactors can significantly impact discharge behavior. Pou et al. investigated a plasma fluidized bed reactor with multiple electrodes for CO₂ decomposition [125]. In this study, copper powder mixed with aluminum oxide was used, serving not only as a catalyst for the reaction but also to improve the uniformity of discharge distribution within the fluidized bed. This setup resulted in more randomized discharge points, reducing the formation of primary discharge channels and “hot spots”. The authors suggested that the increased randomness of discharges, achieved by adjusting voltage, frequency, and anode positioning in fluidized bed plasma reactors, could enhance CO₂ conversion efficiency.

So far, fluidized/spouted bed plasma reactor designs based on DBD and GA have been reported, two reported examples of the reactor design are shown in Fig.8(a) and Fig.8(b). In the case of DBD, a comparison between fluidized bed and fixed bed has been made by Wang et al. [126] in their study on DRM. Using Ni/γ-Al₂O₃ catalytic particles, the conversion of methane with and without plasma under different temperatures was compared along with the synergy effect which is indicated by the synergetic factor I_SE (Eq. (16) in Section 3.3). Although higher conversion of CO₂ and CH₄ were observed in the case of a packed bed, a higher synergetic factor was achieved with a fluidized bed reactor in the temperature range from 673 to 798 K. Higher I_SE of CO₂ was also achieved with an increased discharge power. In another study conducted by Bouchoul et al. [127], the comparison was made for DBD reactor with γ-Al₂O₃. When a fluidized bed was used, an increase in the conversion was obtained from 8.5% to 12.1% and from 3.4% to 6.2% for CH₄ and CO₂, respectively, along with the increase of surface area of alumina from 260 to 312 m²·g^–1. The opposite trend was observed in the case of packed bed reactors. This discrepancy is attributed to the enhanced surface reaction in fluidized bed plasma reactors. In packed bed configurations, plasma is generated near the contact point of catalysts due to the packed-bed effect, limiting the plasma-catalysts interaction. In contrast, fluidized bed reactors allow catalysts to be dispersed throughout the plasma zone, making more catalyst surface sites available for reactions and thus achieving higher conversions. This distinction underscores the unique advantages of fluidized bed designs for plasma-catalytic processes, particularly in enhancing gas conversion. Chen et al. [128] reported their design of a FB DBD reactor and conducted a detailed study on DRM with La-Ni/Al₂O₃ as catalysts. The reactor uses a coaxial structure with a hollow internal electrode to feed gas in, creating a jet to sustain fluidization for the power in a reversed direction. The authors demonstrated that the reaction was promoted by combining DBD with a fluidized bed due to the extended surface area of powder catalysts, interaction with reactive species generated by plasma, enhanced heat and mass transport as long as long residence time. Without plasma, such promotion is limited, indicating the crucial role of plasma in breaking the C–H bond which is considered as the rate-limiting step. In addition, the power and mean discharge current in the case of a fluidized bed DBD reactor is 1/3 of that in a packed bed reactor at fixed sustain voltage and frequency. The doubled pressure also caused a reduction in E/N.

Regarding the case with GA, the reactor design in combination with the spouted bed offers the advantage of inserting catalyst particles inside the discharge zone which is difficult to achieve with a packed bed of IPC configuration due to the discharge behavior (arc path, high temperature etc.), and it led to more intensive plasma-catalysts interaction than a PPC configuration. A typical spouted bed consists of a central region where gas entrains solid particles, causing them to move upwards, and an outer region where solids move in a slower, counter-current flow. Particles rise in the central spout and descend in the outer region, and then move toward the center at lower parts of the reactor to be accelerated again, hence the circulation of particles can be achieved [122]. For such functions, cone shape or other designs rather than flat bottoms are used to avoid dead zones. This could match the divergent electrode geometry of the classical GA reactor. The high gas velocity required for spouted bed operation aligns well with the characteristics of a GA reactor. Several studies have already reported the conversion of methane with catalysts [129−131], and the conversion of CO₂ with biochar as a co-reactant [132], in spouted-bed GA reactors. In the study of Schmidt-Szalowski et al. [131], the effect of a spouted bed on the electrical characteristics of GA discharge has been observed. A much smoother voltage and current waveforms with decreased periods between active parts of discharge were observed when the spouted bed was applied. The authors stated that the recombination of ions and electrons on the particle surface could be a reason, and the influence of charged particles on the breakdown voltage should be considered as well. Notably, the existence of catalyst particles may not always be beneficial. This has been evidenced in the research of Martin-Del-Campo et al. [133] who used a rotating GA reactor in combination with a spouted bed using Ni-supported catalysts for DRM. The results from their experiments showed a decrease in the conversion of CO₂ and CH₄ due to the catalysts interfering with the arc formation, including reduced volume accessible and altering the arc itself, which limits the production of active species. However, they also observed an increase in selectivity of H₂ and CO, and a reduction of carbon deposition on the electrode. Moreover, they compared two support materials in the reactor and achieved more stable plasma operation with Al₂O₃ over SiO₂ due to a higher melting point. This leads to a discussion about the effect of exposure to high-temperature arc on the catalysts in spouted bed GA reactor, which is a general situation that should be considered when designing such plasma-catalysts system.

2.2.2.4 Other types of arrangement

Coating provides an effective approach to introducing catalysts onto the walls of plasma reactors. With a thickness of only a few micrometers—significantly thinner than the discharge gap, typically in the millimeter or centimeter range—these coatings do not substantially alter the reactor geometry, residence time, or the distribution of the electric field. However, this may be different when a microgap or microplasma is considered. Coating offers practical advantages, such as reducing pressure drop and avoiding the packed-bed effect. The material properties (e.g., dielectric constant, conductivity) and surface characteristics (e.g., surface area, pore size) of the coating are critical factors. Xia et al. [135] used a frosted quartz tube coated with CeO₂ as reactor wall of a DBD reactor for the decomposition of CO₂, as shown in Fig.8(c). The conversion and energy efficiency obtained was 23.3% and 8%, which indicated an enhancement compared to an uncoated reactor with 16.3% conversion and 6% energy efficiency. The author stated that this enhancement is not mainly caused by the strong CO₂ adsorption ability of CeO₂, but more intensive microdischarges which are reflected by the current waveform measured. In another study, Ding et al. [134] demonstrated that applying a ZrO₂ coating to the reactor wall improved CO₂ conversion and energy efficiency. This enhancement was primarily attributed to the high dielectric constant of ZrO₂, which increased the reactor’s effective capacitance, promoted more frequent micro-discharges, and elevated the mean electron energy.

Another method to introduce catalysts is through catalytic electrodes, achieved by coating, depositing catalysts on electrodes, or using catalytic materials directly as electrodes. Subrahmanyam et al. [136−138] demonstrated this in early studies on VOC decomposition using a DBD reactor with a sintered metal fiber electrode impregnated with Mn and Co oxides. Although limited research has explored catalytic coatings or electrodes specifically for CO₂ conversion, these approaches represent promising avenues for advancing reactor design.

3 Experimental approaches

3.1 Operation schemes

The operation of experimental systems in plasma-catalysis research is typically straightforward, with most studies employing plasma reactors at atmospheric pressure, and reactants are continuously flowing through the reactor, where they are exposed to plasma to initiate chemical reactions. Tests with variations of feed gas flow rate and electrical parameters such as voltage and power are often performed. This general setup is effective for testing reactors and catalyst performance under standard conditions. However, the operation can also be more flexible, with different parameters to form operation schemes that can be tailored to specific research objectives. There are different options can be selected, as shown in Fig.9. In this section, three key factors are discussed.

3.1.1 Pressure condition

Operating plasma reactors at atmospheric pressure has significant advantages, such as reducing overall process complexity and costs by eliminating the need for pressure modulation equipment. As a result, atmospheric-pressure operation is the primary focus in most experimental studies. However, pressure is a critical factor that influences plasma properties and, consequently, reactor performance. For this reason, studies on modulated pressure conditions have also been explored, particularly for certain types of plasma sources like microwave plasma, where low-pressure conditions are frequently investigated. Belov et al. [139] studied CO₂ dissociation in microwave plasma across a wide range of pressures, from 200 mbar to 1 bar, and observed a general decline in both conversion and energy efficiency as pressure increased. As the pressure increases toward the atmospheric pressure, the rotational and vibration temperature tends to reach equilibrium, leading to increased temperatures and reduced efficiency in CO₂ dissociation due to depletion of vibrational excited CO₂ via V-T relaxation. Additionally, the recombination reaction which oxidizes the produced CO to CO₂ becomes more important at higher pressure. Besides, increased pressure can result in a transition of microwave discharge from the diffusive regime to the contracted regime, leading to changes in plasma properties, mass and heat transfer which ultimately impacts reactor performance [81,140].

There are a few examples of pressure modulation in the case of DBD, GA and corona discharge reactors. Rad et al. [141] conducted a study on CO₂ splitting in a DBD reactor under increased pressure, showing a threefold improvement in CO₂ conversion and energy efficiency from 1 to 2 bar. Considering the general laboratory-scale reactor with a discharge gap in mm to cm range, higher pressure operation above atmospheric requires higher breakdown voltage for plasma generation according to Paschen’s law, and it is also associated with instability of discharges. Hence in their study, Argon dilution was used to stabilize the plasma at moderate voltages of up to 13.5 kV. In another example, Yong et al. [142] examined CO₂ dissociation using nanosecond pulsed discharges at pressures from 5 to 12 bar. The author indicated that the motivation of such tests is to understand how pressure-dependent reactions are coupled with the non-equilibrium electron energy transfer and plasma chemistry, which is relevant to large-scale carbon capture, utilization, and storage applications. Despite the demonstrated benefits of low- or higher-pressure operation for enhancing CO₂ conversion and energy efficiency, atmospheric pressure operation remains the main research focus for practical applications. Nonetheless, pressure modulation is a valuable approach for exploring both fundamental reaction mechanisms and practical reaction conditions.

3.1.2 Reactant feed

The majority of plasma-based gas conversion studies utilize flow reactors, where a continuous feed gas stream with fixed flow rates and compositions is used. This setup allows for a straightforward assessment of reactor and catalyst performance under consistent flow conditions. However, modulating the reactant feed during plasma operation has been considered a technique for specific purposes, such as investigating surface reactions in plasma-catalyst systems. One example of feed modulation is in the study by van Raak et al. [143], who explored ammonia synthesis using a DBD reactor packed with Ru/CeO₂ and Ru/Ti-CeO₂ catalysts. A titration technique named “unimolecular plasma operation scheme” was used to understand the surface reaction mechanism. This scheme involved alternating the feed gas to generate H₂ plasma and N₂ plasma in different sequences within the catalytic reactor, as shown in Fig.10(a). A significant increase in ammonia production was observed with the Ru/Ti-CeO₂ catalyst, which yielded 2.8 times more ammonia when using N₂ plasma compared to the maximum yield of Ru/CeO₂ under H₂ plasma. In a study by Barboun et al. [144], ammonia synthesis was investigated using Ni/Al₂O₃ catalysts under sequential N₂ and H₂ plasma or thermal treatment, as shown in Fig.10(b). This experimental approach enabled a clear distinction between plasma-phase and plasma-catalytic reactions. The results revealed that surface nitrogen species generated by plasma actively participate in subsequent hydrogenation reactions, which are driven either thermally or through H₂ plasma exposure.

When liquid reactants, such as water, are involved in plasma processes, they can be introduced in batch mode while the gas phase continuously flows through the plasma zone. This semi-batch reactor configuration is more commonly observed in water treatment experiments rather than gas conversion studies. Another frequently used approach is feeding H₂O into the reactor via controlled evaporation, typically achieved using an evaporator or mixer. The vaporized water is mixed with a carrier gas, often another reactant gas or dilution gas such as Ar. Examples of this method can be found in the literature [145]. Additionally, a noteworthy case is sorption-based gas conversion using plasma, which operates in batch mode. For instance, plasma-assisted CO₂ capture and conversion was demonstrated in our previous study on carbon capture and conversion using a plasma-sorbent system [146]. In this study, the sorbent was first saturated with a CO₂-containing stream, allowing CO₂ adsorption over a set period. Subsequently, the CO₂-saturated sorbent was flushed and exposed to plasma, enabling simultaneous desorption and conversion. A detailed discussion of this concept will be provided in Section 4.2.

3.1.3 Power supply mode

Depending on the power supply configuration, plasma reactors can operate in either continuous or discontinuous modes. In most studies, a continuous power supply is employed, with AC or DC power set to generate plasma discharges at fixed levels of power, voltage or frequency. Discontinuous operation is primarily represented by pulsed power discharges, in which power is delivered as voltage pulses in the microsecond or nanosecond range, as discussed in Section 2.1(b). The duty cycle, defined as the fraction of the period during which the power is on, indicates the degree of discontinuity in the plasma discharge and is an important parameter that influences the performance of the plasma reactor.

Apart from pulsed power discharges, other discontinuous modes have been explored in several research. For example, Ozkan et al. [147] investigated the AC burst mode operation of a DBD reactor for the conversion of CO₂. This method involves switching the power supply on and off periodically. Compared to continuous AC operation under the same total power input, burst mode offers higher instantaneous power and electric fields, resulting in more energetic electrons and hence improved the conversion of CO₂. A similar mode was applied in the study of Rohit et al. [148] who studied reverse water gas shift reaction, as shown in Fig.11(a). In their study, millisecond periodic switching reduced the temperature and offered a better microdischarge distribution, which has been considered an additional advantage.

In the case of GA reactor, a study by Li et al. [149] has used periodic switching of AC power supply, namely AC pulsed mode, to investigate the spatial profile of classical 2D GA reactor, as shown in Fig.11(b). This method couples the periodic operation with the natural periodic behaviors of the GA discharges. By adjusting the duty cycle, the plasma volume can be controlled, and the spatial profile of the reactor’s performance can be deduced. These studies highlight that discontinuous operation can serve as an effective research technique for investigating the reactor design and operation. Moreover, beyond simply adjusting operational parameters, selecting an appropriate power supply mode is essential for optimizing reactor performance.

3.2 Diagnostic approaches

Various diagnostic methods can be applied to the study of plasma-based gas conversion, and a generic presentation of targeted parameters/phenomena is demonstrated in Fig.12. As mentioned in 2.1(c), both operando and offline/ex-situ approaches can be applied. Offline/ex-situ diagnostics primarily involve the characterization of catalysts, typically by measuring changes in physical and chemical properties before and after plasma exposure. This helps verify the effect of plasma on the catalysts. For operando diagnostics, two commonly used methods are optical and electrical diagnostics.

3.2.1 Electrical diagnostics

3.2.1.1 Equipment

For laboratory-scale reactors with mm- to cm-sized discharge gaps operating at atmospheric pressure, the voltage typically falls within the kV range. This necessitates the use of HV probes to ensure safe and accurate measurements. A HV probe is a device based on a voltage dividing circuit, which scales down the target voltage to a measurable range suitable for measurement devices, such as an oscilloscope. When selecting an appropriate HV probe, three key parameters must be considered: the measurement range (voltage ratio), bandwidth, and input impedance. A high bandwidth allows for the accurate measurement of fast-changing voltage signals, while a high input impedance minimizes loading effects, thereby improving measurement accuracy.

For discharge current measurement, two common methods are widely employed. The simplest method involves installing a shunt resistor, also known as a current viewing resistor, in series with the reactor and measuring the voltage across this resistor. The discharge current is then calculated using Ohm’s law. This technique can be applied to both AC and DC discharges. To minimize energy losses and ensure the voltage drop remains within the measurable range, the resistor’s value should be low. The second method involves using a current probe, such as a Rogowski coil, which converts the AC current signal into a voltage signal by detecting the magnetic field generated by the current changes. When selecting a current probe, its sensitivity (e.g., the A-to-V ratio), bandwidth, and measurement range must be carefully considered.

To record and analyze the measured signals, an oscilloscope is required, and its specifications—sampling rate, bandwidth, and resolution (ADC bits)—should be taken into account. A higher sampling rate allows for more data points to be captured per unit of time, resulting in higher precision. However, this comes at the cost of generating larger data sets, which may require additional storage space or longer processing times. Resolution, which determines the smallest step size in the signal that the oscilloscope can display, is calculated using Eq. (2). Typically, an 8-bit oscilloscope is sufficient for most discharge experiments. However, for transient discharges where current can vary significantly, such as GA discharges, an oscilloscope with 10-bit or higher resolution is preferred to ensure better accuracy.

(2)\begin{array}{c}Stepsize={\displaystyle \frac{V_{max}-V_{min}}{2^m}}.\end{array}

It is worth mentioning that when considering the bandwidth of the devices, including both the probe and the oscilloscope, the overall bandwidth of the system must be taken into account. This overall bandwidth is lower than that of any individual device and can be calculated using the following equation:

(3)\begin{array}{rl}& Systembandwidth=\\ & \frac{1}{\sqrt{{\displaystyle \frac{1}{{probebandwidth}^2}}+{\displaystyle \frac{1}{{scopebandwith}^2}}}}.\end{array}

To accurately measure the amplitude and capture necessary details (third and fifth harmonics) of the signal, the bandwidth of the oscilloscope and the probe should be at least 3–5 times the frequency of the measured signal. Additionally, for plasma discharges, particularly nanosecond pulse discharges, which exhibit highly transient characteristics, the rise time (T_r) and its corresponding frequency (f_r) or equivalent bandwidth, must be considered instead of solely relying on the signal frequency. f_r can be calculated from f_r = C/T_r. Here, C is a constant that depends on the type of system response, with values of 0.35 for Gaussian-type responses and 0.4–0.45 for maximally flat responses. Most oscilloscopes with a bandwidth of 1 GHz or below typically exhibit a Gaussian-type response.

3.2.1.2 Discharge characteristics and analysis

The discharge characteristics are commonly revealed through the analysis of electrical properties, as indicated by the measured voltage and current waveforms. These waveforms vary depending on the discharge types, with the observed differences reflecting underlying physical phenomena, as explained in literature [30,150]. Fig.13 provides a general overview of the discharge regimes, represented through their voltage-current (V-I) characteristics.

DBD is one of the most extensively studied discharge types, and its electrical characteristics have been thoroughly investigated and systematically summarized by many researchers [152,153]. The ideal voltage and current waveforms of a DBD reactor operated under an AC sinusoidal voltage are shown in Fig.14(a). Plasma is periodically ignited and terminated (the period indicated from circled i to t), which is reflected by the many current peaks superimposed on the AC components of the current waveforms. The magnitude and number of these peaks can serve as a qualitative indication of the intensity of the plasma discharges. However, it is important to clarify that each current peak does not directly correspond to individual microdischarges in the DBD reactor.

Fig.14(b) illustrates a classic equivalent circuit for a DBD reactor. The reactor is represented by two capacitors, C_g and C_d, connected in series, corresponding to the gas gap and the dielectric barrier, respectively. During the discharge period, the plasma generated acts as a conductive medium, allowing charge transfer across the gap. This behavior is modeled by a variable resistor R(t), connected in parallel with C_g. In practical scenarios, discharges often do not cover the entire electrode area due to factors such as uneven gaps or non-uniform surfaces of the barrier or electrodes. For these partially discharged DBD reactors, Peeters et al. [154] proposed an equivalent circuit. In this model, the capacitances C_g and C_d are further divided into discharging (β) and non-discharging (α) fractions. Additionally, most experimental set-ups include parasitic capacitances, which can influence the analysis of electrical characteristics but are often omitted from the classic equivalent circuit. For packed-bed DBD reactors, which are widely used in plasma-catalysis studies, the reactor’s capacitance becomes more complex. In the study by Rad et al. [141], the equivalent circuit of a packed-bed DBD reactor replaced the gap capacitance C_g with the packed-bed capacitance C_pb. C_pb represents the overall capacitance of the packed zone, which includes contributions from the gas phase, the packing material, and surface discharges. However, the individual contributions of these components to the electrical characteristics cannot be distinguished. The equivalent circuit approach for diagnosing DBD reactors has been extensively studied and documented in the literature, where further details and examples can be explored [155].

The Q-V Lissajous plot is a widely used tool for analyzing the electrical properties of DBD reactors. In this method, a measurement capacitor with a known capacitance (C_m) is connected in series with the reactor, and the voltage across the capacitor (V_m) is measured. To ensure accurate results, the capacitance of the measurement capacitor should be significantly larger than that of the DBD reactor, typically around 1000 times larger as a general rule of thumb [53]. If C_m is too small, it could lead to a high V_m value that exceeds the voltage measurement range of the oscilloscope. Key parameters, such as the capacitance of the reactor (C_cell), the effective capacitance of the dielectric barrier, and the self-sustaining voltage (U_min), can be extracted from the Q-V Lissajous diagram. Fig.14(c) illustrates an ideal Q-V Lissajous figure with these parameters clearly indicated.

While the approach is well-established, certain challenges can arise in non-ideal scenarios. For instance, the shape of the diagram may deviate from the ideal parallelogram to an elliptical shape due to charge residues during the plasma-off phase or may become noisy due to measurement inaccuracy. These issues are discussed in detail in [153], which provides further insights. One notable variant of the typical Q-V parallelogram is the almond-shaped diagram, often observed in packed-bed reactors and surface discharge reactors. This shape arises from the gradual expansion of the plasma volume across the packing material or dielectric surface during the discharge phase, resulting in an increased effective capacitance. The curved slope of the Q/V relationship produces the characteristic almond shape [157]. Other cases that exhibit deformation of the Lissajous figure include pulsed-driven DBDs and reactors with asymmetric configurations. In these cases, the V-Q plot deviates significantly from the standard parallelogram, making the interpretation less straightforward, and the typical analysis methods for standard parallelograms are often not applicable. However, specific methods have been developed for the analysis of pulsed-driven DBDs [158,159]. Fig.14(d) provides an example from a study [156], where the reactor capacitance (C_cell) and the capacitance of the dielectric barrier (C_d) can be determined. This is achieved by analyzing the slopes formed by the maximal points (Q_max-V_max) obtained under different applied voltages, with and without plasma ignition respectively.

In the case of typical GA discharges, the discharge characteristics span a broad range of time scales, from nanoseconds to seconds. Taking an AC GA discharge in a classical 2D reactor as an example, the voltage and current waveforms are shown in Fig.15 [160]. The individual discharges are characterized by pulses in nanosecond scale, while the collective behavior of these discharges, which corresponds to the dynamics of the GA development, spans milliseconds or even seconds per cycle. This collective behavior is typically represented as an “envelope” in the voltage waveform. The envelope time corresponds to the full propagation of the GA—from its ignition at the narrowest gap of the reactor to its cut-off at the widest gap. A high sampling rate is required to capture the fine details of individual voltage and current pulses, while measurements over a larger time scale are also essential to register the characteristics of the entire GA cycle. This dual requirement imposes significant demands on equipment, such as oscilloscopes, and necessitates the processing of large volumes of data for analysis.

In practical experiments, the variation of the GA cycle from ignition to cut-off exhibits a degree of randomness, leading to non-identical electrical characteristics, such as variations in envelope length. These variations become especially prominent in systems with high gas velocities or large reactor sizes. As a result, analysis must often be performed over multiple cycles, averaging the data to draw reliable conclusions. Furthermore, due to the transitional nature of the GA as a “warm plasma”, a wide range of current variations is typically observed within a single GA cycle. This can include a “glow-like” mode with currents in the milliampere range and a “spark-like” mode where currents can reach tens of amps or even hundreds of amperes. These two modes correspond to different plasma properties, which significantly influence the chemical performance of the plasma reactor. This large range of current variation poses significant challenges for measurement, requiring high-resolution equipment, such as an oscilloscope with a high ADC bit depth, to capture both ends of the spectrum accurately in a single measurement. If such equipment is unavailable, separate measurements at different current ranges need to be conducted. These measurements must then be combined, and multiple measurements are required to obtain an average value, ensuring reliability and accuracy in the analysis.

3.2.1.3 Power calculation

In principle, the reactor power can be calculated by multiplying the measured voltage by the discharge current, and the energy consumed can be determined by integrating the calculated power over the plasma operation period. While this approach is straightforward in most cases, special attention is required in practice to avoid inaccuracies. For instance, there can be a measurement delay between the voltage and current signals, which can lead to a miscalculation of the discharge power. This issue is particularly significant in cases with highly transient discharge characteristics, such as nanosecond pulsed discharges, where precise synchronization is critical. The delay may be caused by various factors, including the influence of measurement probes (e.g., compensation circuits), the physical distance between voltage and current probes, and different lengths of signal cables. Accurate power calculation demands that such delays be accounted for. For example, in a study of nanosecond discharges by Montesano et al. [40], this delay was quantified by superimposing the first derivative of the voltage and the current before breakdown, during which the current primarily exhibited a capacitive nature.

Another scenario requiring special attention during power calculation is the GA discharge. The challenges here stem from measurement difficulties associated with the nature of the discharges, including wide variations in current (ranging from milliamperes to tens of amperes) and their periodic behavior due to arc propagation. Achieving accurate power analysis in such systems often necessitates multiple measurements taken at carefully selected time intervals, processing large amounts of data, and the use of high-performance equipment as discussed in the previous section.

For experiments involving DBD, a widely adopted method for power calculation is the Lissajous method, originally proposed by Manley [161]. Based on the product of instantaneous voltage and current, the derivation of this method is expressed in Eq. (4) [153]. The area enclosed by the Q-V Lissajous plot, calculated as \ointV(t)dQ(t), corresponds to the average energy per operational cycle. By incorporating the applied frequency f, the power can then be determined.

(4)\begin{array}{rl}P=& \frac{1}{T}{\int }_0^{T}V\left(t\right)\cdot i\left(t\right)\mathrm{d}t=\frac{1}{T}{\int }_0^{T}V\left(t\right)\cdot C_{\mathrm{m}}\mathrm{d}{V}_{\mathrm{m}}\left(t\right)=\\ & f\oint V\left(t\right)\mathrm{d}Q\left(t\right).\end{array}

In the case of discontinuous operation, such as burst mode or pulsed operation as mentioned in Section 3.1.3, the voltage and current signals measured are discontinuous. The calculation of power typically begins by determining the “instantaneous power” during the “plasma-on” period. This is then averaged over the entire operation period, including the “plasma-off” phase, by accounting for the duty cycle. In such scenarios, the instantaneous power provides insights directly related to plasma properties, whereas the average power reflects the overall performance profile of the reactor. By comparing both calculated power values with those of continuously operated plasma reactors at the same total power, valuable insights can be gained, such as the effects of high instantaneous power, or potential phenomena like plasma quenching and re-ignition.

Apart from ensuring the accuracy of measurements and calculations, it is crucial to use the correct terminology for power to enhance clarity, accuracy, and comparability. Unfortunately, confusion and incorrect conclusions are evident in some literature due to a lack of proper clarification. There are several power definitions: reactor power, PSU output power, plug power, and process power. The power calculated from the voltage across the reactor and the discharge current only considers the reactor itself and is therefore referred to as “reactor power”, which is often the focus of plasma studies. On the other hand, the PSU output power, sometimes directly displayed on commercial power supplies, represents the total power of all components connected to the PSU. This includes reactor power, line consumption, and the power drawn by other electrical components such as ballast resistors. The “plug power”, which reflects the total power consumption of the entire plasma system, includes the reactor power, the standalone power of the PSU, and the power consumption of all other components in the circuit. Plug power can often be directly measured using a power meter. However, while the plug power provides an overview of the entire plasma system, it is not ideal for comparing data across the literature since electrical systems differ significantly among experimental set-ups used by various researchers. Therefore, both plug power and PSU output power should not be confused with reactor power when evaluating reactor performance. Instead, accurate measurement of the voltage and current, and subsequent calculations based on these, remain the most reliable approach. Additionally, it is important to recognize that the plasma system is not the only component consuming power in chemical conversion processes. Auxiliary units, such as external heating or cooling systems, also contribute to overall energy consumption and must be accounted for when evaluating the energy efficiency of the entire process. This comprehensive approach ensures that the energy efficiency of plasma-based processes is not overstated or overclaimed.

3.2.2 Optical diagnostics

Optical diagnostics play a crucial role in advancing our understanding of plasma chemistry and plasma catalysis. Numerous methods, originally developed in plasma physics research, are well-established for studying plasma-based gas conversion. In general, optical diagnostics fall into two main categories: imaging and spectroscopic methods. Time-resolved imaging has been used to study the characteristics and spatial and temporal development of plasma discharges. Many previous studies have reported the use of ICCD cameras with high sensitivity and fast gating to visually trace the microdischarges [162]. In the case of plasma-catalytic reactors, the distribution and dynamics of discharges on the catalyst surface can be investigated [163,164]. Furthermore, it could help to study the influence of different catalyst packings on the plasma, providing vital information to reveal the plasma-catalyst interaction [165]. The results can also be studied in combination of results from modeling [166] or other techniques for validation or compensation, including integration with a spectrometer for enhanced measurement of emission spectra [167]. While imaging techniques are relatively straightforward, they require careful attention to several factors in practice, such as a good optical path, particularly in the case of plasma-catalytic reactors. Additionally, the temporal and spatial characteristics of the fast discharge phenomenon need to be synchronized with the acquisition of the ICCD camera, with a certain resolution that is feasible for analysis. These considerations emphasize the need for careful equipment setting and reactor design in laboratory studies.

Spectroscopic techniques, including OES, laser-induced fluorescence, scattering, and absorption spectroscopy, provide valuable data on key plasma parameters such as gas temperature, electron density, electric field, and active species concentrations. This information is essential for understanding reaction mechanisms and plasma properties. These spectroscopic methods are well-established and have been reviewed extensively in the literature [168−175]; interested readers are encouraged to consult these sources for further details. In recent years, in situ/operando spectroscopic characterization of catalysts under plasma conditions has become increasingly important in plasma-catalysis research. Techniques such as in situ IR, Raman, and X-ray absorption fine structure (XAFS) spectroscopy have been applied to gain insights into the dynamic behavior of catalysts in a reactive plasma environment. Due to the highly reactive environment and complex plasma-catalyst interactions, measurement and analysis in this context present significant challenges, often requiring custom experimental set-ups. For instance, conventional plasma-catalytic reactors like coaxial DBD reactors with dense packing are frequently avoided in in situ IR studies due to limited optical access and difficulties in integrating optical components due to reactor size and operating conditions. Instead, custom-designed cells with small sample volumes and reduced electrode dimensions are used to focus measurements specifically on areas of plasma-catalyst interaction. Since these designs prioritize optical access and measurement control over reactor performance factors (such as plasma uniformity and gas residence time), the chemical performance of these in situ cells is not always comparable to that of reactors optimized for conversion and energy efficiency. However, connecting microscopic insights from in situ diagnostics with macroscopic chemical performance remains an important goal, and future efforts in reactor design are needed to achieve this.

In in situ set-ups, DBD and plasma jet sources are often used because they can operate at atmospheric pressure and relatively low temperatures. High-temperature sources like GA and microwave plasmas are less common due to the challenges of controlling temperature and preventing damage to cell windows and other sensitive components [171]. Moreover, the sampling methods for optical measurement influence the design of cell configuration, especially the arrangement of the electrodes. For example, IR absorption spectroscopy set-ups may vary between transmission IR (TIR), diffuse-reflectance IR Fourier transform spectroscopy (DRIFTS), and attenuated total reflection modes, each requiring different cell configurations. Fig.16 presents examples of two different plasma cells for in situ IR study. To ensure reliable measurements, custom cells must meet three criteria: (1) provide an unobstructed optical path across the catalyst-plasma interaction region; (2) enable effective plasma-catalyst contact, ensuring that plasma species can interact with the catalyst surface; (3) allow stable plasma operation with minimal interference to measurements, avoiding issues such as window overheating or electromagnetic noise affecting the measurement.

Other practical considerations include aligning the cell and light path precisely, focusing the beam at the plasma-catalyst interface, and adjusting beam intensity to optimize measurement sensitivity. Weak IR signals, a common challenge due to low transmission through catalyst samples, are often addressed by using KBr as a sample carrier due to its minimal IR absorption. Moreover, for DRIFTS or TIR, the gas species which are sensitive to IR, could introduce interference to the measurement of IR signals from the surface species. Hence it is beneficial to consider surface-specific techniques such as polarization-modulation IR reflection-absorption spectroscopy (PM-IRAS). PM-IRAS can differentiate IR absorption between the s- and p-linearly polarized light for surface species, providing a high surface sensitivity. Lee et al. [176] have demonstrated the use of PM-IRAS in their study of plasma-catalytic conversion of CH₄. The author developed a multimodal spectroscopic diagnostic system which combines PM-IRAS, OES and MS to correlate reactions in both gas phase and on the surface with the products. While interesting results have been achieved for the analysis of reaction mechanisms, the authors also stated the limitations of their diagnostic system, such as suitability only for low-surface-area materials and the requirement that surface species have a perpendicular dipole moment. Nonetheless, this approach offers valuable insights and could inspire future multimodal systems for deeper study of plasma-catalyst interactions.

Raman spectroscopy is often considered as a complementary to IR for catalyst characterization. Common catalyst supports like alumina and silica are not Raman active, facilitating the detection of active species adsorbed on their surfaces [175]. Till now, there is limited research using in situ/operando Raman spectroscopy in plasma-catalysis studies. However, an example can be found in the study of Saito et al. [179] who investigated the CO₂ reduction with CH₄ and H₂ in a DBD reactor with copper nanoparticles supported on zinc oxide as the catalysts. The raman peaks measured were compared with literature and results from density function theory calculations. Important information was gained to analyze the surface species and their reaction pathways including CH₃CHO and CH₃COO formation, and the role of activated H₂ and H₂O. Besides, results from in situ Raman could also assist the study of possible structural changes of the catalysts during reaction under plasma exposure. This has been demonstrated by the work of Sun et al. [180] who studied the reaction-driven transformation of Pd-ZnO interface during plasma-catalytic conversion of CO₂ via reverse water gas shift reaction.

The change of structure can also be monitored by in situ XAFS. This was demonstrated in the study by Gibson et al. [181]. In their work of plasma-catalytic oxidation of CH₄, significant structural changes of the catalysts used (Pd/Al₂O₃) did not occur during the plasma exposure, but the temperature increase of Pd nanoparticles was observed, although the temperature was insufficient for thermal reaction to occur. The temperature of surface Pd was estimated from the changes in mean squared thermal disorder parameter based on premade calibration data. The authors also noted that the XAFS provides bulk averaging results which may not indicate possible minor changes below the detection level.

3.2.3 Other methods and techniques

In addition to the diagnostic and analytical methods mentioned above, there are other approaches reported in literature. A notable example is isotope tracing, which is a well-established technique in chemical reaction studies that has been effectively adapted to investigate reaction mechanisms within plasma environments. For example, isotopes of carbon and oxygen can be used in plasma reactors to trace the behavior of reactants such as CO₂. Among carbon isotopes, ¹²C and ¹³C are stable, while ¹⁴C is used primarily for radiolabelling; for oxygen, the stable isotopes are ¹⁶O, ¹⁷O, and ¹⁸O. Navascués et al. [182] applied isotope tracing in a DBD reactor for DRM using ¹³CO₂ and ¹²CH₄ as reactants. Their analysis of reaction products revealed that C₂ and C₃ hydrocarbons contained only ¹²C, while carbon monoxide included both ¹²C and ¹³C, suggesting parallel reaction pathways for CO₂ and CH₄ with minimal interaction between the two reactants in plasma conditions. Morillo-Candas et al. [183] investigated fast oxygen atom exchange in CO₂-CO-O₂ mixture with RF plasma. Isotopic tracing using ¹⁸O₂ was performed and the author developed a fitting algorithm for 12 isotopologues of CO₂ and 6 of CO in gas mixtures. They observed the incorporation of ¹⁸O into CO and CO₂ and analyzed the mechanism with an emphasis on excited oxygen atoms O (1D). Besides the gas phase reaction, examples of isotope tracing can be found in reported studies on plasma-based surface processing [184,185], and surface reaction in plasma-catalytic CO₂ conversion [186]. This method shows significant promise for uncovering detailed reaction pathways and mechanisms, offering a valuable avenue for future research in plasma catalysis.

Another example is the work by Parastaev et al. [186], who developed a technique called temperature-programmed plasma surface reaction (TPPSR) to investigate the plasma-catalytic hydrogenation of CO₂. This method consists of four key steps: (1) adsorbing the reactant, CO₂ in this case, onto the catalyst surface at ambient temperature; (2) flushing the reactor to remove any gas-phase CO₂; (3) exposing the catalyst to plasma at ambient temperature to eliminate weakly adsorbed species; (4) ramp the temperature by gradually heating the catalyst at a constant rate in the presence of plasma while monitoring the gas-phase products. According to the authors, the TPPSR method enables the study of reactions primarily occurring on the catalyst surface, with minimal interference from gas-phase reactions. This approach helps to decouple surface reactions from gas-phase processes, addressing a fundamental challenge in understanding reaction mechanisms within plasma-catalyst systems. However, the authors also acknowledge certain limitations of the TPPSR technique. These include the need to avoid extensive gas-phase conversion of the reactant, the necessity for at least one reactant to exhibit reasonable adsorption on the catalyst surface, and the unclear effects of plasma-induced heating on reaction dynamics.

3.3 Key performance indicators

The testing of various process parameters in experiments forms a fundamental approach for examining the influence of these variables and comparing the performance of different reactors and systems. Beyond directly controlled parameters such as flow rate, gas composition, and discharge power, a key process parameter frequently used in the analysis is the specific energy input (SEI, sometimes referred to as SIE). SEI represents the amount of energy delivered per mole or per unit volume of gas treated by plasma. In the case of a flow reactor, SEI is calculated as a ratio of the plasma power to the total feed gas flow rate, as shown in Eq. (5). Typically, it is expressed in units such as J·L^–1 or eV·molecule^–1 by considering the volume at specific temperature and pressure with Avogadro number. SEI is a critical factor which has been extensively used in analysis in many reported studies. However, it cannot be adjusted directly, but through changing the power or flow rate. As the same SEI can be achieved under different conditions (e.g., high power with high flow rate or low power with low flow rate), discussions regarding the effect of SEI should be done carefully.

Given the broad diversity and interdisciplinary nature of research in plasma, establishing a standard for results reporting and validation poses a significant challenge. Alves et al. addressed this challenge in their review paper, focusing on laying the foundation for plasma standards to enhance reproducibility, transparency, and collaboration [187]. In the case of gas conversion, taking CO₂ as an example, terms and key performance indicators such as conversion, selectivity, and energy efficiency are frequently discussed. Previous review papers have provided detailed descriptions, such as relevant equations referenced in [50,59]. Tab.3 contains examples of equations used in the case of plasma-based CO₂ conversion.

These energy-related metrics are calculated based on power, necessitating particular attention to accurate power measurement. As discussed in Section 3.2.1(b), it is crucial to determine which power measurement should be utilized—whether it is the reactor power, the plug power, or the total power consumption of the entire process, including other process units. When calculating energy efficiency, Eqs. (4), (5) and (7), based on the reaction enthalpy and the SEI, serves as a fundamental principle and can be easily applied to cases such as CO₂ splitting. However, in more intricate scenarios like DRM and CO₂ hydrogenation, this approach becomes impractical. Consequently, an alternative method utilizing the lower heating value, outlined in Eqs. (4), (5) and (8), is often employed. The definition of energy consumption or cost varies in literature depending on the process’s focal point. Equations (4), (5) and (9), and (4), (5) and (10) are utilized if the cost pertains to the energy required per unit of CO₂ conversion or total reactants, whereas Eqs. (4), (5) and (11) quantifies the energy necessary to produce per unit of product.

It is crucial for these aspects to be clearly defined to ensure accurate analysis and reporting of relevant performance indicators. Bart et al. [49] conducted a critical review that summarized correct terms and definitions while highlighting common errors in the literature. The authors addressed the change in volumetric flow rate as a prevalent issue leading to inaccurate analysis, and they discussed corrected methods to improve data comparability, such as the use of a flux ratio as a potential solution. The flux ratio can be determined using a stoichiometrically derived expression in straightforward cases, such as CO₂ splitting. However, in more complex scenarios, like DRM, this approach becomes impractical due to the formation of by-products. In such cases, alternative methods are required, such as utilizing a flow meter or introducing a standard reference component at the reactor outlet.

In the exploration of the synergy between plasma and catalysts, prior research by Wang et al. introduced the “synergetic factor”. This factor is calculated as the ratio between the conversion achieved using the plasma-catalyst system and the sum of conversions achieved separately using plasma alone and catalysts alone [126]. It is essential to carefully maintain consistent reaction conditions when discussing this factor. The mutual influence of plasma on catalysts and vice versa may lead to variations in conditions, such as discharge properties and catalyst bed temperature, under which these conversions are attained.

4 Trends and new opportunities

Research in plasma-based gas conversion is expected to grow continuously, with current directions, such as exploring reaction mechanisms and plasma-catalyst interactions, remaining key focal areas. Significant advancements can also be anticipated in innovative catalyst material development and reactor design. Beyond these established approaches and topics, there are emerging opportunities and new trends which hold great potential to further advance the technology and broaden the scope of research in this field.

4.1 AI as a research tool

AI is rapidly emerging as a transformative tool in scientific research, revolutionizing methodologies, strategies, and the perspectives of researchers across various disciplines, including plasma science. With a strong focus on machine learning, AI offers new possibilities in modeling, simulation, and process control, with applications spanning from low-temperature plasmas to fusion research. In areas such as plasma physics, plasma chemistry, and plasma-surface interactions, AI has been utilized to accelerate simulations, enhance predictive capabilities, and reduce the need for costly experimental work. Comprehensive reviews in literature provide insights into the fundamentals of machine learning, its applications, as well as its strengths, limitations, and challenges [188−192]. In the context of plasma-based gas conversion, the integration of AI is still in its early stages. However, several pioneering studies have begun exploring its potential, demonstrating promising results.

4.1.1 Machine learning for analysis, prediction and optimization

There are already published works which demonstrated the application of machine learning to analyze, predict the experimental results and optimize the process [193−197]. Most of those cases use feed-forward artificial neural networks (ANNs) to predict macroscopic properties such as conversion, energy efficiency or selectivity as the output. The input can be the operational parameters such as discharge power, flow rate or gas composition, and reactor or catalyst properties such as the size and geometry. A general structure of an ANN is shown in Fig.17(a), it typically consists of minimally 3 layers: an input layer, one or more hidden layers and an output layer, each containing several neurons. Each neuron in the input layer (n^{\mathrm{I}\mathrm{L}}) represents an input variable of the ANN model, such as electrode spacing or input potential. The neurons in the output layer (n^{\mathrm{O}\mathrm{L}}) represent output parameters such as energy efficiency or conversion. The input neurons are multiplied by a weight (w_{i,n}^{\mathrm{I}\mathrm{L}}) and incremented with a bias (b_i^{\mathrm{I}\mathrm{L}}). The result is modified by a transfer function (F_i^{\mathrm{I}\mathrm{L}}) and values for all the input neurons are summed. The result is a neuron in the hidden layer (n^x), which has no relevant physical meaning on its own. A hidden layer can have multiple neurons, each with its own weights and biases. This process can be repeated multiple times, with the neurons of the hidden layer becoming the input for the next hidden layer until the output layer is reached. The number of hidden layers, the number of neurons per hidden layer and the type of transfer function are design choices when setting up the ANN. In the evaluated works most often, a single hidden layer is used containing around 10 neurons.

For the training of an ANN, a large data set is required. This data set is divided into at least two subsets: a training set and a validation set. To obtain the values of the weights and biases, the training set is used. A loss function describes the difference between the solution of the neural network and the experimental data from the training set. This error can be expressed for example as the mean squared error or the R² value. During training, the value of the loss function is reduced by adjusting the values of the weights and biases via algorithms such as the Levenberg-Marquardt, Broyden-Fletcher-Goldfarb-Shanno Bound or the backpropagation algorithm. After training, the validation data set is used to determine the reliability of the ANN.

In the study of Cai et al. [194], prediction and optimization of the plasma catalytic DRM process were conducted with a hybrid model combining ANN, support vector regression, and regression trees to handle limited data sets and complex parameter interactions. The model provided insights into the relative significance of different parameters, revealing that the total flow rate had the most significant impact on CO₂ and CH₄ conversion, while Ni loading on the catalyst had a relatively minor effect on the overall plasma process performance. Another study by Chen et al. [198] used an ANN to screen for an optimal catalyst for NO_x reduction. They compiled a data set from literature and their own experimental measurements to predict NO_x conversion consisting of information about catalyst elemental composition, catalyst structure and morphology, the catalyst synthesis method and the NO_x reduction conditions. The authors succeeded in developing a low-cost catalyst with a broad temperature operation window. In plasma catalyst design, ANNs can accelerate catalyst development compared to traditional trial-and-error methods. Additionally, multiple synthesis conditions can be optimized simultaneously, offering a more efficient pathway to improved catalyst performance.

It is important to note that ANN models are data-driven and can produce predictions that violate physical laws. The performance of ANNs relies heavily on the quality and diversity of their training data sets, necessitating caution when extrapolating results to scenarios that significantly deviate from the training data. For example, an ANN trained on data from a single plasma gas conversion set-up is unlikely to accurately predict performance for different set-ups. Developing robust and broadly applicable ANNs requires large, diverse data sets that encompass various reactor geometries and operational parameters. However, such comprehensive data sets remain scarce in plasma gas conversion research. A 2022 report highlighted this gap, attributing it to inconsistent definitions, unstructured metadata, and a lack of standardized formats [192]. To address these limitations and fully integrate machine learning techniques into plasma system modeling, the plasma research community must adopt standardized reporting practices aligned with the FAIR data principles (Findable, Accessible, Interoperable, and Reusable) [199]. A current debate concerns that if large data sets are necessary to build predictive models, how machine learning can still contribute to reducing practical experimental efforts. This remains a question to be answered by further exploration.

4.1.2 Large language models (LLMs)

The recent heat on LLMs has had a profound impact on both daily life and scientific research. Tools like ChatGPT, which garnered significant attention following its release in November 2022, have introduced transformative changes across various fields, including plasma-relevant research. As von Keudell discussed [201], these tools bring both substantial benefits and challenges to scientific workflows.

At their core, LLMs like ChatGPT operate by analyzing vast amounts of text data and generating responses based on the statistical likelihood of word sequences.

Owing to its sophisticated deep learning algorithm known as the transformer, and training with massive text data from various sources, LLMs can provide answers with a certain degree of accuracy for common questions and recurring topics. They also excel in simplifying explanations and summarizing conclusions. In research, LLMs can assist in multiple ways, such as conducting literature reviews, analyzing and visualizing data, designing experiments, and supporting scientific writing. For instance, Bai et al. [200] demonstrated the use of GPT-4 throughout various stages of their research on plasma-based nitrogen fixation (see Fig.17(b)). In their work, GPT-4 went beyond merely retrieving information for literature analysis—it also analyzed data, explained observed phenomena, provided insights, and suggested strategies. Additionally, LLMs can support programming by aiding in coding tasks, which could be especially useful for modeling and simulation in plasma and chemical processes.

However, as with any new research tool, LLMs come with limitations and challenges that require careful consideration. Three key issues are particularly relevant:

(1) LLMs do not inherently understand physical and chemical phenomena. As a result, they may generate incorrect or misleading statements. Furthermore, correct answers are often oversimplified, which is problematic in interdisciplinary fields like plasma-based gas conversion, where nuanced and highly specialized information is critical.

(2) While LLMs can perform well with general queries, they struggle with complex mathematical, physical, or chemical problems, especially those at the cutting edge of research. This limitation underscores the need for expert validation of their output.

(3) Since LLMs are trained on pre-existing data and have limited capacity to incorporate recent developments, they may not capture the latest advancements, especially in the field like plasma-based gas conversion which has been evolving rapidly in recent years.

These limitations highlight the importance of human oversight when integrating LLMs into research workflows. Additionally, concerns about data security and integrity must be addressed, as well as risks related to plagiarism, originality, and reliability in scientific writing. Adhering to ethical guidelines and maintaining transparency are essential when using such tools. In summary, LLMs serve as valuable assistants with extensive general knowledge and rapid response capabilities, but they are not replacements for your experienced colleagues, supervisors, or collaborators.

4.2 Plasma-material systems

Beyond plasma-catalyst interactions, which have been extensively studied, recent years have seen growing interest in exploring other plasma-material systems. A prominent example is the plasma-sorbent system, which has shown promise in applications of CO₂ conversion and ammonia synthesis. For instance, Rouwenhorst et al. [202] demonstrated the use of zeolite 4A to enhance the energy yield of plasma-based ammonia synthesis. In their study, the in situ adsorption of synthesized ammonia on zeolite effectively suppressed ammonia decomposition by the plasma, resulting in a twofold improvement in energy yield. Furthermore, Li et al. [146] highlighted the potential of plasma-sorbent systems for CO₂ capture and conversion within a single reactor. Using a DBD reactor packed with hydrotalcite, the authors implemented a cyclic operation in which CO₂ was first captured from air or flue gas by the sorbent material, followed by plasma-induced desorption and conversion, as illustrated in Fig.18. During plasma exposure, CO₂ desorption occurs due to electron-stimulated desorption, plasma-induced heating, or interactions with reactive plasma species. Simultaneously, CO₂ undergoes splitting, either in the gas phase or on the sorbent surface. This dual function of plasma—driving chemical reactions and inducing desorption—addresses one of the most energy-intensive steps in conventional CO₂ capture processes. Furthermore, this plasma-sorbent system can be extended to reactions involving CO₂, such as hydrogenation or DRM, as demonstrated by Vertongen et al. [203]. Apart from solid sorbents, a similar concept has also been demonstrated in systems that combine plasma with ionic liquids. For example, Attri et al. [204,205] reported a system using the ionic liquid 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) to capture CO₂, followed by plasma treatment to convert it into CO. Although research in this field is still in an early stage, the potential for such systems to enable efficient process integration has garnered significant attention.

Another emerging area of research in plasma-material systems is the integration of plasma with membranes in a single reactor. Two distinct approaches have been explored: (1) employing membranes as reactant distributors or as supports for catalysts within the reactor; (2) using membranes to separate products from plasma-induced reactions, thereby minimizing back-reactions and shifting the reaction equilibrium.

For the first approach, an example is the study by Veng et al. [206], who developed a membrane-DBD reactor for ammonia synthesis. The reactor design, illustrated in Fig.19(a), utilized an alumina membrane as both the dielectric barrier and a distributor for H₂. This innovative design allowed hydrogen to be transported radially while N₂ flowed axially, increasing the residence time of N₂ in the plasma zone to promote the production of atomic nitrogen. Simultaneously, H₂ which requires less energy for dissociation, was evenly distributed. This configuration achieved an energy yield of 0.25 g-NH₃·kWh^–1, which is three times higher than conventional DBD operation. The authors also demonstrated that both ammonia production and power consumption were inversely proportional to the product of membrane pore size and porosity.

In the second approach, the in situ separation of products significantly enhances reactor performance and facilitates process integration. For instance, Hayakawa et al. [207] investigated hydrogen production from ammonia decomposition in a plasma-membrane reactor combining a DBD plasma with a Pd-Cu alloy membrane. By integrating the membrane, the maximum hydrogen production rate increased from 13% to 24.4%, compared to a conventional plasma reactor. The authors proposed a plasma-driven permeation mechanism, as depicted in Fig.19(b). Ammonia was decomposed in plasma to produce H radicals, which were adsorbed on the membrane surface. These radicals diffused through the membrane and recombined on the opposite side to form H₂. This study highlighted the synergy between plasma and membranes, where plasma not only drives the reaction but also facilitates permeation. The contribution of plasma can occur through the generation of excited species or heat, which improves membrane performance by enhancing separation or enabling operation under milder conditions. For example, Zheng et al. [208] studied oxygen permeation in a plasma-membrane reactor composed of an oxygen-permeable membrane (La₀.₆Sr₀.₄Co₀.₂Fe₀.₈O_3−δ) and a DBD plasma. The proposed permeation mechanism, shown in Fig.19(c), demonstrated the role of plasma in activating O₂, which enhanced the dissociation of O₂ and the ionization of O that reached the membrane surface, facilitating permeation. Their results showed a remarkable 30-fold increase in oxygen flux at 600 °C with the application of plasma with 15 W power. Since high temperatures are favorable for the optimal performance of perovskite membranes, it may be beneficial to consider the use of warm plasma in this context. A study conducted by Chen et al. [209] implemented microwave plasma in combination with an La₀.₆Ca₀.₄Co₀.₂Fe₀.₈O_3−δ hollow fiber membrane, achieving an oxygen permeation rate of up to 4 mL·min^–1·cm^–2). The membrane also demonstrated excellent chemical resistance toward CO₂ and CO₂ plasma. Such a dual-function membrane reactor represents a novel approach, with several promising research directions worth exploring, including membrane material development and plasma–membrane reactor design [210].

In summary, the development of novel plasma-material systems presents exciting opportunities for process intensification and integration. However, further studies are essential to understand the interaction mechanisms, design or select optimal combinations of materials and plasma, and develop tailored materials or plasma reactors for such applications. Additionally, the operation of these systems and their experimental methodologies may need to be adapted. For instance, plasma-sorbent systems often employ cyclic operations involving adsorption and desorption steps, which differ from the continuous operations commonly used in plasma-based gas conversion studies. Furthermore, plasma–membrane systems often require careful coupling of operating conditions, as optimal temperature and pressure differences across the membrane are typically necessary for effective permeation. This may necessitate operating the plasma under modified conditions to ensure compatibility with membrane performance requirements.

4.3 Process level considerations

Research on the process development of non-thermal plasma technology for the conversion of N₂, CO₂, and CH₄ remains relatively scarce, primarily due to the low TRL of this field. Naturally, there is significant uncertainty which needs to be clarified for process development. However, process-level consideration at this stage could provide a critical understanding of the requirements and guide further research toward practical applications. In our previous work, several critical gaps were identified in the process development required to transition this technology from laboratory research to industrial implementation [211]. A key point that needs attention is the unique characteristics of plasma technology, which must be factored into process design and conceptualization. For instance, in the discussion of centralized versus modular process concepts, plasma technology could offer distinct advantages for modular systems. Its inherent flexibility and compatibility with distributed renewable electricity make it particularly suitable for decentralized set-ups. This contrasts with the centralized processes commonly used in conventional chemical production, offering an opportunity for innovative deployment in distributed systems. Similarly, the integration of plasma reactors with other process units requires careful consideration of plasma technology’s unique operational features. A widely touted advantage of plasma reactors is their instant response to intermittent renewable electricity, enabling dynamic operation with “turnkey” functionality. However, other process units that work with plasma reactors may not operate as efficiently under such fluctuating conditions. This discrepancy necessitates special attention to ensure compatibility and efficiency in the overall process design.

4.3.1 Scaling up of the non-thermal plasma system

Scaling up is a critical step in advancing the TRL of plasma technology. For thermal plasma reactors, upscaling is relatively straightforward and can be achieved through sizing up the reactor. This is largely due to the ionization-overheating stability of thermal plasmas, which remain stable despite fluctuations in temperature, density, and the reduced electric field [212]. However, in non-thermal plasma reactors, the instability of plasma properties becomes a limiting factor, particularly in terms of power density and pressure conditions. As a result, the numbering-up approach, which involves deploying multiple reactors in parallel, is often more suitable.

Nonetheless, even with numbering-up, laboratory-scale reactors (typically with discharge gaps in the millimeter range or a few centimeters) need to be scaled to a larger size (e.g., tens of centimeters) before considering parallel operation. Practical examples of scaling up plasma reactors for environmental and energy applications have been reviewed in the work of Okubo [213]. Industrial ozone production, which employs hundreds of DBD reactor tubes operating in parallel, is frequently cited as a classic example of scaling up non-thermal plasma reactors. However, while this provides a useful reference, the energy efficiency of DBD reactors remains generally too low for CO₂, N₂ and CH₄ conversions, hence significant improvement is required before upscaling.

On the other hand, warm plasma reactors, such as GA systems, exhibit favorable energy efficiency and robustness, making them strong candidates for industrial applications. The transitional nature of warm plasmas, which exhibit characteristics of both thermal and non-thermal plasmas, presents unique opportunities and challenges for scaling up. Specifically, achieving high energy efficiency in warm plasma reactors hinges on maintaining operation within the “warm” plasma regime, as transitioning to the thermal regime often results in reduced performance. Additionally, the heat management required for thermal plasmas—such as active cooling—can increase maintenance requirements and overall costs. Therefore, retaining the warm plasma regime during scale-up is crucial. Directly increasing the size and power input of GA reactors can inadvertently push the system into the thermal plasma regime, requiring careful attention during the scaling-up process. Rabinovich et al. [212] demonstrated the scaling-up of a GA reactor operating at a power level of 2–3 kW. Their analysis indicated maximum current and power per unit length for non-equilibrium GAs at 1–2 A and 0.15 kW·cm^–1, respectively. Furthermore, their findings suggested a physical limit, with the maximum expected power per unit length at approximately 0.5 kW·cm^–1—nearly 20 times lower than for thermal arc discharges. Tsonev et al. [214] investigated the geometric effects on scaling up GA reactors for nitrogen fixation. Their study compared two reactors with different sizes but identical pin-to-pin configurations. Results showed that increasing reactor size and flow rate without proportionally increasing plasma volume significantly reduced reactor performance. This issue stems from the contracted nature of plasma discharges, which limits the gas flow’s interaction with plasma, thereby diminishing overall efficiency. To address this, the authors implemented a torch configuration, which created a longer plasma volume and enhanced gas flow coverage. This design achieved higher production rates and reduced energy consumption compared to the pin-to-pin configuration, underscoring the importance of reactor design in scaling-up efforts.

When scaling beyond the limitations of size increases, the numbering-up approach becomes essential for systems such as GA reactors. O’Modhrain et al. demonstrated the parallelization of GA plasmatron reactors housed within a single unit, as in Fig.20(a) [215]. Their design preserved the performance observed at the laboratory scale with a single reactor while significantly increasing CO₂ conversion rates, demonstrating the feasibility of numbering up. Further improvements can be achieved by increasing the number of reactors and the flow rate while maintaining optimal SEI.

For multi-reactor systems, the connection strategy of individual reactors is critical to overall performance. Van Raak et al. [216] investigated series and parallel connection methods (as shown in Fig.20(b)) in their study on NO_x synthesis. Their results showed that the series connection of two reactors enhanced both NO_x production and energy efficiency compared to other configurations, including parallel reactor connections and a single reactor with a doubled discharge gap. This enhancement was attributed to longer residence times, altered discharge behavior, and only a moderate increase in power.

The power supply system is a crucial component of numbering-up strategies for plasma reactors. Van Raak et al.’s study [216] utilized reactors electrically connected in series and powered by a single power PSU. However, because GA reactors behave as dynamic loads, their interconnected operation affects discharge characteristics and increases the complexity of the overall electrical system. This is reflected by the different voltage and current characteristics observed as shown in Fig.20(c). By contrast, O’Modhrain et al. [215] employed individual PSUs for each reactor in their numbering-up approach. The authors believe this strategy avoided issues such as power division in centralized PSUs and allowed for optimized individual PSU performance, leading to cost advantages over centralized systems, where a single, larger PSU distributes power to all reactors. To further scale plasma systems to power outputs ranging from hundreds of kilowatts to megawatts, future research should also focus on optimizing PSU designs for stability, durability, and safety. Moreover, it needs to be noted that the cost of power supplies is often a significant factor influencing the capital expenditure of the entire system and, ultimately, the economic feasibility of plasma processes. Hence, cost-efficiency is a key consideration in the design of the PSU.

4.3.2 Process evaluation

Apart from technological development, another significant challenge is understanding the economic feasibility and environmental impact of plasma-based processes. To address this, TEA and life cycle assessment (LCA) are crucial for evaluating the potential of plasma technologies. Currently, there are limited publications on the TEA and LCA of plasma processes, mainly due to the low technological maturity and the scarcity of data for comprehensive analysis. Nevertheless, perspective assessments can guide the advancement of plasma technology in its early stages and assist in the practical development and optimization of plasma processes. This includes three major benefits of such assessments:

(1) Establishing performance targets: By comparing plasma technologies with conventional methods or other innovative approaches, a clear understanding of the current status of plasma-based processes can be developed. This comparison helps define technological performance targets for future research. For example, Winter et al. [217] analyzed the economic and environmental competitiveness of plasma-assisted ammonia synthesis. They found that for modular ammonia production powered by renewable energy, plasma processes would need a 6-fold improvement in energy efficiency to become competitive. Furthermore, they highlighted that if hydrogen (H₂) is produced via steam methane reforming and powered by natural gas, the plasma process would emit more CO₂ than the Haber-Bosch process unless its energy efficiency improves by a factor of 28.5.

(2) Understanding process integration: Exploring potential process designs provides valuable insights into the overall plasma process, particularly its integration with other process units. Van Rooij et al. [218] evaluated a microwave plasma process for CO production from CO₂, integrating CO₂ removal, recycling via the Benfield process, and purification through pressure swing adsorption. Their analysis showed that separation costs were the dominant expense, underscoring the need for better plasma conversion efficiency to reduce costs. Similarly, Kaufmann et al. [219] studied power-to-liquid plants that integrate plasma CO₂ splitting with oxygen separation through gas diffusion electrodes (GDE), along with hydrogen production and Fischer-Tropsch synthesis. Their study found that GDE performance significantly impacted production costs, accounting for 30%–70% of capital expenditures. Both studies emphasize the importance of a holistic view and process integration for plasma-based processes.

(3) Identifying viable application scenarios: Assessing different application scenarios helps identify the windows of opportunity for deploying plasma-based processes. For example, Delikonstantis et al. [220] conducted a study on plasma-assisted ethylene production from methane. Their analysis indicated that under current industrial electricity prices (ranging from 50 to 120 USD·MWh^–1), the process was not economically viable. To achieve break-even, electricity prices would need to drop to 23–35 USD·MWh^–1.

In summary, the exploration of plasma-based chemical production continues to uncover new opportunities and challenges. The success of this technology hinges on bridging existing knowledge gaps, responsibly leveraging advanced methodologies, and designing processes that align with industrial and environmental demands. Future research is expected to focus on these critical areas, striving for higher energy efficiency and yield while facilitating the transition from laboratory-scale innovations to impactful industrial solutions

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