Recent advances in electrolytic cells for synchrotron radiation characterization of electrocatalytic CO₂ reduction

Abstract

Carbon dioxide, as a greenhouse gas, is expected to be converted into other useful substances by the electrocatalytic CO₂ reduction reaction (CO₂RR) technology. The electrocatalytic cell, or electrochemical cell, used to provide the experimental environment for CO₂RR plays an irreplaceable role in the study of this process and determines the success or failure of the measurements. In recent years, electrolytic cells that can be applied to in-situ/operational synchrotron radiation (SR) characterization techniques have gradually gained widespread attention. However, the design and understanding of electrolyte systems that can be applied to in-situ/operational SR technologies are still not sufficiently advanced. In this paper, the electrocatalytic cells used to study the CO₂RR processes with in-situ/operando SR techniques are briefly introduced, and the types and characteristics of the electrolytic cells are analyzed. The recent advancements of in situ/operando electrolytic cells are discussed using X-ray scattering, X-ray absorption spectroscopy (XAS), light vibration spectroscopy, and X-ray combined techniques. An outlook is provided on the future prospects of this research field. This review facilitates the understanding of the reduction process and electrocatalytic mechanism of CO₂RR at the atomic and molecular scales, providing insights for the design of electrolysis cells applicable to SR technologies and accelerating the development of more efficient and sustainable carbon negative technologies.

Graphical abstract

Keywords

electrolytic cell / carbon dioxide reduction reaction (CO₂RR) / synchrotron radiation (SR) / in-situ / operando

1 Introduction

The rapid growth of the global economy is accompanied by skyrocketing energy consumption, intensifying environmental damage, and climate change. In 2019, a joint paper [1] by global scientists published in the journal Bioscience warned that “the planet is facing a climate crisis” and greenhouse gases are to blame, the main component of which is carbon dioxide (CO₂), accounting for 74.4%. However, nearly 87.0% of the CO₂ generated by human activities comes from energy use [2,3]. The continuous increase [4,5] in greenhouse gas emissions will lead to a rise in global average temperature, endangering human life and the living environment. The concept of “carbon neutrality” has emerged as a global consensus to address the increasingly severe weather problems and natural disasters by reducing CO₂ emissions. The specific technology of carbon neutrality mainly includes four aspects: carbon reduction technology, zero carbon technology, carbon negative technology, and carbon economy technology. The carbon negative technologies [6–8] include CO₂ conversion, carbon capture, utilization, storage, and carbon sequestration.

Nowadays, there are many CO₂ conversion technologies [9–11], including photocatalysis, electrocatalysis, thermal energy conversion, polymerization, mineralization, etc. Among these conversion technologies, the electrocatalytic CO₂ reduction reaction (CO₂RR) [12–15] is widely used due to its relatively simple reaction device, higher environmental compatibility, easier integration with other renewable energy sources (such as wind, tidal, and solar), and the ability to overcome the high redox potential of CO₂/CO²⁻. CO₂RR is a process that utilizes electrochemical methods to convert atmospheric CO₂ into valuable chemicals such as methane, methanol, and formic acid. During the electrocatalytic process, CO₂ molecules adsorb on the surface of the catalyst, forming an adsorbed state. The adsorbed CO₂ molecules undergo a reduction reaction by accepting electrons on the catalyst surface. The surface structure and electronic properties of catalysts can affect the formation and conversion of intermediates, thereby controlling the selectivity of the final product. Common catalysts include metals [16,17] (such as Cu, Ni, Co, Bi, and Sn), alloys, oxides, and carbon materials [18]. For example, free-standing porous stanene exhibited excellent electrocatalytic performance toward HCOO⁻ in CO₂RR [17]. In a 0.5 mol/L KHCO₃ aqueous solution, a high Faradaic efficiency (93% at −930 mV) can be achieved, mainly due to the edge sites on Sn (100). Nanosheet electrocatalysts derived from bismuth salts (MBi₂O₄, M = Ca, Cu, Zn) [19] undergo surface reconstruction, exposing more active Bi⁰ sites and forming abundant metal-modified edge defects, which are beneficial for regulating the charge state of adjacent Bi sites, improving the adsorption of *OCHO intermediates, and enhancing the selectivity of CO₂RR for HCOOH. A CuSiO_x amorphous nanotube catalyst with rich atomic Cu-O-Si interface sites was synthesized [20]. The strong interfacial interaction between Cu and silica made the Cu-O-Si interface sites super stable in the CO₂RR without any significant reconstruction, exhibiting high CO₂ to CH₄ selectivity (72.5%) and stability. Currently, more than 15 carbon-containing end products [21,22] of CO₂RR have been identified and experimentally confirmed. Increasing the specific surface area, enhancing the number of active sites, nanosizing catalysts, and combining multiple characterization techniques [23,24] to reveal the electrocatalytic mechanism in depth would be the key research directions for electrocatalytic CO₂RR in the future.

Acquiring the evolution of intermediates and catalytic active sites in the CO₂RR process is essential for understanding its mechanism. Therefore, it is necessary to monitor the CO₂RR process in real-time under in situ or operando condition. Based on synchrotron radiation (SR) sources, in-situ/operando techniques [25–30] are of paramount importance for characterizing the dynamics and structural reconstructions of catalysts and have already been widely performed in the study of electrocatalytic CO₂ reductions. For example, Han’s group [31,32] used X-ray absorption spectroscopy (XAS) to examine the electronic structure of single-atom catalysts (SACs) and small-angle X-ray scattering (SAXS) to resolve the fractal structure of precursors. Utilizing SR based characterization techniques, especially a combination of multiple techniques, to characterize the CO₂RR process requires careful implementation of the sample environment. Generally, the electrolytic cell of samples under real reaction conditions needs to be specially designed [33]. So far, a variety of in situ/operando electrolytic cells have been successfully used to monitor the CO₂RR process in real time. A reasonable design and convenient operation of the sample environment system are prerequisites to achieving the experimental purpose (). Here, this paper focuses on the advancements of electrolytic cells for in situ/operando electrocatalytic CO₂RR measurements. It is expected that this paper will be helpful for the design of in situ sample environmental systems and applications in SR-CO₂RR experiments.

2 Electrolytic environment systems for CO₂RR

For an in situ/operando characterization of the electrocatalytic CO₂RR process, the electrolytic cell of the sample is a key component that has a great influence on mass transfer. According to the number of electrodes, the electrocatalytic CO₂RR cells can be divided into two categories: the three-electrode system and the two-electrode system. The three-electrode systems include the H-type double-chamber electrolytic cell [34] shown in Fig.1(a) and the continuous flow electrolytic cell [35] shown in Fig.1(b). The two-electrode systems include the membrane electrodes assembled (MEA) electrolytic cells [36] shown in Fig.1(c) and the polymeric solid state electrolytes electrolytic cell [37,38]. The specific two-electrode electrolytic cell is shown in Fig.1(d). It illustrates the entry of CO₂ gas from the left side, which can reach the cathode through diffusion and be converted into products (such as CO). Anions (such as CO₃^2‒) are transported to the anode through an anion exchange membrane, where oxygen is produced [36]. The H-type electrolytic cell of the three-electrode systems is more commonly used in ordinary laboratories. It is composed of an ion exchange membrane, two separable rooms, working electrode (WE), reference electrode (RE), and counter electrode (CE). Its cathode and anode are separated by a proton exchange membrane. The H-type electrolytic cell [39] has been successfully applied in the CO₂RR. However, the large spacing between the cathode and anode, as well as the limited electrolyte concentration in the H-type electrolytic cell [40], can cause high Ohmic resistance, resulting in low energy efficiency. Additionally, the limited solubility [41] of CO₂ in the water-based electrolytes lowers the upper limit of the current density. Due to the limitations of the H-type electrolytic cells for in situ/operando measurements, a flow electrolytic cell with a gas diffusion electrode (GDE) as the WE was designed and developed, as shown in Fig.1(e). This electrolytic cell has a three-phase interface and a membrane electrode assembly (MEA) consisting of anode and cathode GDE on both sides of the polymer electrolyte membrane (PEM) [42], allowing more CO₂ molecules to reach the catalyst surface quickly. Nevertheless, the GDE [43] is only suitable for powder catalysts, and not for metal foil catalysts. This flow electrolytic cell is also three-electrode system, so it was also affected [44] by the Ohmic resistance caused by the ion-exchange membrane and the thickness of the electrolyte. In contrast, the MEA cell [42] can significantly reduce energy loss, as shown in Fig.1(f). The design of the PEM separates the cathode and anode, promoting ion flow while reducing product crossover. Its highly porous gas diffusion layer between the electrode and membrane increases contact between CO₂ molecules and the electrocatalyst during the reaction, accelerating the reaction rate.

Alinejad et al. [45] compared the H-type electrolytic cell and the flow electrolytic cell with a zero-gap GDE device for studying the performance of gold catalysts. Differences in electrocatalytic performance were found for the same catalyst in different environments. This indicates that the sample environments provided by the electrolytic cells play an important role in CO₂RR experiments. For in situ/operando characterization of the CO₂RR process, designing an electrolytic cell suitable for real sample environment is even more challenging. So far, there is no fixed standard for the experimental method and experimental setup of electrocatalytic CO₂RR. Different research groups have selected and optimized their respective measurement setups according to varying experimental requirements. Referring to other researchers’ design ideas [46] about in-situ/operando electrolytic cells is very helpful for constructing in-situ/operando electrolytic cells suitable for specific experimental environments.

Electrolytic sample cells can be used not only to measure the electrolytic performance, but also to characterize the structures of electrolytic catalysts. This paper focuses mainly on the electrolytic sample cells used for structural characterization, especially those utilized for in situ/operando SR-based characterization of structures. The common structural characterization techniques primarily include electron microscopies [47], light scattering techniques (vibrational spectroscopy) [22], and X-ray characterization techniques [48]. Vibrational spectroscopic characterization includes infrared [49] and Raman [50] spectroscopy. The widely used X-ray techniques comprise X-ray diffraction (XRD) [51], SAXS [52], and X-ray absorption fine structure (XAFS) [53], among others. As seen in Tab.1, SR technologies have many advantages, but there are also some challenges and limitations in practical applications. XRD can be used to identify and characterize the long-range ordered crystallographic structure and phase transformation, as well as to determine possible impurities or defects. SAXS can provide valuable insights into nanoparticle morphology, nanoporosity, specific surface area, and interactions between particles in nanoscale materials. XAFS is a powerful tool for detecting local atomic structures, electronic structure, and element valence state of materials. Different structural characterization techniques probe different scales of structures in materials. Importantly, different structural characterization techniques have distinct detection modes, measurement setups, and experimental requirements. For example, XAFS spectra need to acquire the absorption coefficient as it changes with incident X-ray energy, while XRD and SAXS patterns need to record the scattering intensity as it varies with scattering angle or scattering vector. The different detection methods and experimental requirements determine the various measurement setups, which in turn limit the design of electrolytic cells suitable for different measurement conditions. Therefore, a reasonable design of electrolytic cell and optimal coupling of reaction conditions are essential for the in situ/operando tracking of the CO₂RR process, the acquisition of the reaction mechanism, and a comprehensive understanding of the material structure-activity relationship. In situ/operando SR-based characterization techniques can trace the CO₂RR process, verify the stability of electrocatalysts in the reaction environment, identify key intermediates and their conformations, determine catalytic active sites, clarify the preferred pathway and selectivity of the reaction, and reveal the influence of the reaction environment on the experiment. The following sections will be a brief introduction and discussion of the existing electrolytic cells for CO₂RR [54–56] based on different SR-based characterization techniques.

3 Electrolytic cells for X-ray scattering

The X-ray scattering technique generally uses a single-energy X-ray beam to extract structural information in real space by detecting scattering signals in reciprocal space. X-ray scattering techniques include XRD, wide-angle X-ray scattering (WAXS) or diffraction (WAXD), SAXS, and others, depending on the range of scattering angles or scattering vectors being probed. When in-situ/operando X-ray scattering techniques are used to study the CO₂RR process in transmission mode, the corresponding design of electrolytic cells is not substantially different from that used for transmission XAFS measurements. However, X-ray scattering techniques in reflection mode typically require the sample surface to be at or near a horizontal attitude, which results in a design of electrolytic cells that is completely different from that used for transmission X-ray scattering techniques. The SAXS technique [57–59] provides information on the size, shape, morphological evolution of nanoscale structures [60], enabling insights into the dynamic structural evolution of catalysts and the interaction between particles. Grazing incidence small angle X-ray scattering (GISAXS) is a typical X-ray scattering technique in reflection mode. By adjusting the incidence angle, the GISAXS technique [61,62] can be used to study the nanostructural evolution on the sample surfaces or buried beneath them, allowing for layer sequence analysis. Bogar et al. [61] designed an electrolytic cell, specifically for in situ GISAXS studies on the evolution of Pt nanoparticles at different potentials, as shown in Fig.2(a)−Fig.2(d). A sample of nanometer thickness (~9 nm) was prepared by spraying electrolyte onto the substrate. When GISAXS measurements are performed, the incident X-ray beam typically hits the sample at a grazing incidence angle of less than one degree, resulting in a long footprint of the incident beam on the substrate along the direction of the incident beam. Therefore, the size of the substrate along the beam direction needs to be large enough, and the sample cell needs to be designed to leave sufficient space without interfering with the X-ray path. Considering the absorption of X-rays, the incoming and outgoing window materials of the sample cell are generally low-Z materials. Beryllium film can be used as a window material, but beryllium is hazardous. Kapton film is often preferred as a window material because of its lower scattering background, high chemical stability, electrical insulation, and low cost. Combining in situ GISAXS technique with electrochemical atomic force microscopy (EC-AFM) and inductively coupled plasma mass spectrometry, Khalakhan et al. [63] investigated in depth the dynamic reconfiguration morphological changes of Pt-Ni bimetallic catalysts during dynamic cycling of potentials under acidic aqueous electrolyte conditions. This study used a silicon (SiO₂/Si(111)) wafer as the substrate. A carbon layer with a thickness of 10 nm was deposited on the silicon wafer and used as the WE. The electrolytic cell adopted a simple three-electrode design. The CE and RE were placed on both sides of the cell without affecting the X-ray path, while the WE was placed at the bottom of the cell in contact with the catalyst, as shown in Fig.2(e). After penetrating the entrance window of the electrolytic cell and the first half of the electrolyte, the incident X-ray beam hits the sample at a grazing incidence angle. The signal scattered by the sample then reaches the detector after passing through the back half of the electrolyte and the outgoing window. When designing such electrolytic cells, it is essential to ensure that the intensity of the outgoing signal has sufficient statistics, and special attention should be paid to the thickness of the electrolytic cell and the X-ray absorption by the electrolyte.

4 Electrolytic cells for X-ray absorption

The SR-based XAFS technique can be used for various sample systems [64], except for single-atom gases, to study the local atomic structure, species, and/or valence state of the central atoms. In recent years, in situ/operando SR-based XAFS techniques [65] have been widely employed to study the local structure evolutions of CO₂RR catalysts to elucidate the electrocatalytic activity, coordination configuration, oxidation valence, and catalytic mechanism under reaction conditions. For transition metal or heavier elements, the two main measurement modes of XAFS spectra are the transmission mode and the fluorescence mode. The former is suitable for high concentration samples, with the transmitted X-ray intensity collected behind the sample. The latter is appropriate for low concentration samples, with the fluorescence X-ray intensity collected from the side of the sample. Usually, the collected fluorescence X-ray beam and the incident X-ray beam are positioned on the same side of the sample, with both incident and fluorescence X-ray beams arranged symmetrically at a 45° angle to the normal direction of the sample plane. Due to the difference in XAFS measurement modes, the designs of the corresponding electrolytic cells vary. The electrolytic cell used for fluorescence XAFS measurements adopts a reflection geometry. Theoretically, the sample thickness is not limited; however, the fluorescence XAFS signal is primarily derived from the surface layer of the sample, depending on the X-ray penetration depth. The sample surface should be as close as possible to the outer wall of the electrolytic cell. The incident X-ray beam and the fluorescence X-rays emitted toward the 4π solid angle pass in and out through the same X-ray window; therefore, the X-ray window of the electrolytic cell needs to be large enough. The electrolytic cell used for transmission XAFS measurements adopts the transmission geometry, and its design is relatively simple. However, the sample thickness (thickness (d) is limited, and the absorption length (ion length (μ_bd) of the solvent or matrix should be minimized to reduce background absorption. The optimal sample thickness is the one where the edge jump (edge jump (Δμd) of the sample absorption spectrum equals 1. In addition to the above requirements, the electrolytic cell design for fluorescence and transmission XAFS measurements should also pay special attention to electrode placements.

The operando XAFS technique [66] was used to study the evolution of the oxidation state and the local atomic coordination structure of SnO₂ nanoparticles during the electrochemical reduction of CO₂ to formate. An electrochemical cell for fluorescence XAFS measurements was employed, as shown in Fig.3(a), where, SnO₂ nanoparticles supported on carbon spheres (SnO₂/C-n, n = 1, 2, and 3) served as the catalyst and the catalyst-coated carbon paper was used as the WE. Ag/AgCl was used as RE, while platinum wire acted as the CE. A 0.5 mol/L KHCO₃ was used as electrolyte, which was constantly filled with flowing CO₂. The operando XAS measurement revealed the changes of SnO₂ under open circuit voltage and different applied negative potentials, indicating that SnO_x was the active site for CO₂-to-formate conversion.

In 2020, Liang et al. [44] reviewed different types of electrolytic cells reported in literatures for CO₂ electroreduction. In 2017, Ishihara et al. [67] developed a spectro-electrochemical cell for element-specific florescence X-ray absorption and X-ray emission spectroscopy in the soft X-ray region. This electrochemical cell featured a 15-nm-thick Pt layer deposited on a 150-nm-thick film window (SiC), with an adhesive 3-nm-thick Ti layer serving as both the WE and the separator window between vacuum and a sample liquid under atmospheric pressure. A platinized Pt wire and a commercial Ag/AgCl electrode were, respectively, used as the CE and RE. To elucidate the electrocatalytic CO₂RR over a model nickel SAC, a three-electrode cell [68] was used for operando X-ray absorption near-edge structure (XANES) spectroscopy, Raman spectroscopy, and near-ambient X-ray photoelectron spectroscopy (XPS) measurements. A glassy carbon disc with a diameter of 5 mm served as the WE, containing a 0.1 mg/cm² catalyst. A Pt foil of 1 cm × 2 cm was used as the CE, while a saturated calomel (SCE, Hg/Hg₂Cl₂) functioned as the RE. To study the kinetics of the oxygen evolution reaction (OER), ambient-pressure X-ray photoelectron spectroscopy (APXPS) [69] was performed on a Ni−Fe electrocatalyst. The three-electrode electrochemical cell was also used for operando XPS to probe the solid-gas interface. The WE was prepared by first evaporating a 4 nm-thick Ti film onto a 6 mm × 40 mm glass slide, followed by the deposition of a 10 nm-thick Au film on top of the Ti film. Finally, the Ni−Fe electrocatalyst film was electrodeposited onto a conductive substrate from a solution containing 0.01 mol/L nickel sulfate hexahydrate and 0.01 mol/L iron sulfate heptahydrate in ultrapure water. Pt foil was used as the CE while a leak-free Ag/AgCl (eDAQ ET072-1) was used as the RE. Operando APXPS data revealed that the 7 nm-thick Ni−Fe (50% Fe) film existed in both metallic and oxidized states and was further oxidized during the electrochemical oxidation-reduction cycles. Fe was oxidized to Fe³⁺ while Ni to Ni^2+/3+.

In situ XAFS technique [70] was employed to investigate the synergistic interactions of a Co−Mn oxide catalyst, which exhibited impressive oxygen reduction reaction (ORR) activity in alkaline fuel cells. A Teflon electrochemical cell was designed for in situ transmission XAFS measurements, as illustrated in Fig.3(b). The thickness of the electrolyte was controlled to be less than 200 μm. A 40 μm-thick catalyst layer was sprayed/deposited onto a 200 μm-thick carbon paper, serving as the WE. A carbon rod immersed in 1 mol/L KOH solution was used as the CE, while the RE was connected to the cell by a salt bridge. The cell window was sealed with glass or Kapton film. This electrochemical cell has been successfully used to determine the catalytic active sites for ORR in conjunction with in situ XAFS technique under real-time electrochemical conditions, providing favorable insights into the reaction mechanism of polymetallic electrocatalysts.

Fig.4(a) and Fig.4(b) show the schematics [71] of a laboratory-made polymer electrolyte fuel cell (PEFC) designed for in situ XAFS measurements and the setup for in situ time-resolved quick XAFS (QXAFS), respectively. This MEA electrolytic cell was used for operando time-resolved XAFS studies of surface events on a Pt₃Co/C cathode catalyst in a PEFC during voltage-operating processes. In the cell shown in Fig.4(a), two sets of gas diffusion layer, gasket, slotted carbon separator, gold-plated copper current corrector, epoxy resin insulation layer, and heater were symmetrically arranged on both sides of the MEA. The X-ray path, associated equipment, and setup for in-situ time-resolved (500 ms) QXAFS measurements are detailed in detail in Fig.4(b). Utilizing this electrochemical cell, the QXAFS measurements elucidate the reaction mechanism, rate constants, bond breaking and reformation, change of charge density. In addition, EXAFS can also achieve high time-resolution (100 ms), with the primary difference from QXAFS being in the optical path system and image data processing method [72,73].

Pt is recognized as the most durable metal catalyst [74–76] and is widely used in different fields, including sustainable energy conversion. The two electrolytic cells [65] were employed for this study: one suitable for transmission XAFS/XRD combined measurements and the other for fluorescence XAFS measurements. These cells were utilized to investigate the oxidation and dissolution mechanisms of Pt and monolayer Pt catalysts within a potential range of 0.4 to 2.6V. Electrolytic cell-I was an improved version of the design presented by McBreen et al. [77]. It consisted of a carbon cloth loaded with catalyst, sandwiched between a proton exchange membrane and two polytetrafluoroethylene (PTFE) spacers. The PTFE spacers were embedded in the middle of the cell and made of plexiglass. Both sides of the cell featured optical windows bonded with acrylic film. Copper foil was used as the CE on the thicker side of the cell, and two capillary pathways were incorporated: one for adding electrolyte and the other for Ag/AgCl RE. The operating principle of this electrolytic cell was similar to that of the H-type electrolytic cell. Electrolytic cell-II, on the other hand, was specifically designed for fluorescence XAFS measurements and only a single side window is required for photon entry and exit, which was sealed with polyimide film. A carbon paper [78] deposited with catalyst at the lower end was used as the WE positioned near the X-ray window. The cell body was made of plastic, and Pt wires were wound around the RE. Cell-II can accommodate a wider range of applied potentials [79,80] during testing.

5 Electrolytic cells for vibration spectroscopy

In situ EIRS [81–83] is a powerful technique for analyzing molecules, adsorbates, and reaction intermediates in reaction processes. The first measurement of in situ EIRS was reported in 1980 [84]. Since then, numerous researchers [85–87] have developed related methodologies.

Fig.5(a) depicts the measurement principle [83] of in situ EIRS using an FTIR spectrometer with both conventional and synchrotron lights sources. The in situ IR technique for electrocatalysis is among the most important tools for probing dynamic electrocatalytic reaction processes at the molecular level.

Fig.5(b) illustrates a typical electrolytic cell [88] used for in situ EIRS measurements in transmission mode. Traditional electrolytic cells use a gold grid as the WE, but this can obstruct infrared radiation signals during an in situ ERIS measurements. Therefore, the gold grid is replaced with an infrared transparent born-doped diamond as the WE.

Two electrolytic cells [83] for in situ EIRS measurements in the reflectance mode are shown in Fig.5(c) and Fig.5(d), which are suitable for thin-layer external reflection configuration and thick-layer internal reflection or attenuated total reflection (ATR) configurations, respectively.

As early as 2004, Mucalo’s group reported an external reflection setup [89,90] for IR measurements. However, the complex optical path led to irradiation loss and a poor signal-to-noise ratio, severely limiting its in situ application. The internal reflection method addressed these issues by positioning the WE so that the incident beam directly reached its surface and reflected back to the detector. Unlike the external reflection method, the internal reflection method allows for a thick solution configuration, effectively mitigating the disadvantages related to light absorption by the electrolyte solution and the weak signal from intermediates on the catalyst surface during coupling testing.

Raman spectroscopy is a valuable tool for providing structural information [91–93] about surface species of catalysts, intermediates, and molecules in the reaction process of multiphase catalytic systems. This technique is particularly sensitive to the vibrations [94,95] associated with changes in molecular polarity, making it well-suited for studying electrochemical CO₂RR systems. In addition, in-situ Raman spectroscopy [96–99] offers time resolution on the order of seconds or better, which is superior to that of in-situ XAFS techniques, allowing it to effectively study rapidly changing catalyst systems.

An earlier electrochemical/Raman-spectroscopic measurement setup [99] is schematically illustrated in Fig.6(a), showing the laser path, optical window, and the three-electrode arrangement. This setup utilized a thick optical window and ultrathin electrolyte layer, but it was not well-suited for the new generation of confocal Raman microscopy systems.

A modified electrolytic cell adopted a thin layer of electrolyte (ca. 0.20 mm) and a quartz window with a thickness of about 1 mm. However, it left an air layer between the WE and the microscope objective, which diminished the overall detection sensitivity of the system, as shown in Fig.6 (b(i)).

To enhance the design and eliminate the air layer, a new electrochemical/Raman-spectroscopic cell [91] employed a water-immersed Raman objective, extending the working distance to 2.8 mm and increasing the electrolyte layer to about 2.0 mm, as shown in Fig.6(b(ii)). Another improved version [91] of the electrolytic cell replaced the thick quartz window with a thin PTFE film (~13 μm thick), as shown in Fig.6(c). Additionally, the common rod-shaped WE was substituted with a disc-shaped WE embedded in the PTFE casing.

This improvement is particularly advantageous for combined characterization techniques that utilize both laser and X-ray beams as probes, as the thin window reduces X-ray absorption and enhances the signal-to-noise ratio of X-ray signals. For complex CO₂RR systems, it is hard for a single characterization technique to effectively monitor the changes in multi-scale structural information.

6 Electrolytic cells for combined characterization technique

During material synthesis, crystallization, or chemical reaction, structural evolution can occur hierarchically. The CO₂RR process is usually analyzed by integrating various structural information obtained independently through multiple characterization techniques. However, the structural information from different techniques sometimes can complicate ensuring consistent sample detection positions, accurate time sequences, and the identical experimental conditions.

To better understand the structural evolution and the dynamics of these processes, several combined techniques have been developed [100,101], allowing for the simultaneous acquisition of multiple-level or hierarchical structural information form the same location. For example, a SAXS/XAFS combined setup [101] was developed at the Swiss Light Source. This system features two ion chambers for XAFS measurements and a flight tube for SAXS measurements, all mounted in parallel on a moveable table that can shift perpendicular to the X-ray beam direction. XAFS and SAXS measurements can be performed alternately by repositioning the ion chambers or the flight tube along the X-ray path.

A transmission-mode electrochemical cell is placed in front of the movable table, which does not require movement during measurements. This electrochemical cell is designed as a liquid electrolyte flow cell, with its bottom and upper ends connected to the electrolyte reservoir and syringe pump through 1 mm diameter PTFE tubes. The cell is connected to a potentiostat and housed in a poly(methyl methacrylate) (PMMA) box. The cover of the box features two slits for the incoming X-ray beam and the outgoing X-ray beams, along with two additional lateral holes for introducing warm air from a heat gun, and connecting tubes and cables such as a thermocouple.

In addition, a vacuum degasser is included to remove any dissolved gases in the electrolyte before it reaches the heated cell. The WE is prepared as follows: First, the Pt catalyst is diluted in a mixture of ultrapure water, isopropyl, and ionomer. This mixture is then ultrasonically mixed for 15 min in glass vials to produce an ink for spray coating. Next, a 100 nm gold layer is sputter-coated on a carbon-coated Kapton foil in a thickness of 50 μm to enhance conductivity, while a 4 mm diameter region in the center is shielded with a mask to prevent gold sputtering. The Pt catalyst ink is manually spray-coated stepwise onto the central region of the substrate and allowed to dry for at least 12 h to finally obtain the WE.

The pre-cut, gold-sputtered Kapton substrate is spray-coated with an ink of Black Pearls 2000 carbon to serve as the carbon-based CE. A low-leak, 2 mm diameter Ag/AgCl electrode (Harvard Apparatus) is used as the RE. and 0.1 mol/L HClO₄ is employed as the electrolyte. The SAXS and XAFS signals collected by this combined technique originate from the same sample location, albeit at different time intervals.

Recently, a SAXS/XRD/XAFS combined technique [100] was developed at Beijing Synchrotron Radiation Facility, as shown in Fig.7. This innovative technique adopts a high-frequency sampling scheme that avoids switching between SAXS/XRD and XAFS measurement modes. Two miniature detectors, a diamond micro-detector [102] and silicon PIN photodiode [103]), were developed for XAFS measurements. By coupling these miniature detectors into the beamstop of SAXS, the need to switch measurement modes is eliminated, facilitating the implementation of the combined technique.

When properly designed, an electrolytic cell in transmission mode can enable the synchronous collection of SAXS, XRD, XAFS signals form the CO₂RR process at the same sample position. As early as 1996, Haubold et al. [104] designed a plexiglass electrochemical cell for combined measurements of SAXS/XAFS data in transmission-mode. This electrochemical cell featured a simplest single-chamber structure. The Pt electrocatalysts were prepared on the surface of fine carbon particles, which were then mixed with approximately 30 wt% PTFE powder (wt% represents mass fraction) and highly dispersed onto porous carbon paper with a thickness of 0.15 mm. The carbon paper, loaded with the electrocatalysts, served as the WE and was positioned at the X-ray inlet end of the electrochemical cell, while the CE was placed at the X-ray outlet end, approximately 2‒5 mm away.

The porous structure of the WE provided the necessary gas‒liquid‒solid interface for the electrocatalysis. The acquired XANES and anomalous SAXS (ASAXS) data revealed that the Pt catalytic particles formed a core-shell structure, consisting of a Pt metal core and a Pt oxide shell with a shell thickness of about 1 nm.

Apparently, SR technologies play an important role in the CO₂RR field and have also found wide applications in other fields, as shown in Tab.1, due to its advantages of high intensity, high resolution, and adjustable energy. SR technologies can monitor the dynamic changes of catalysts and reaction intermediates in CO₂RR process in real time, providing key evidence for revealing the reaction mechanism. By integrating SR technology, researchers can design and optimize CO₂RR catalysts, improve their activity, selectivity, and stability, thereby promoting the commercialization of CO₂RR technology.

7 Future prospects

Recently, the study of CO₂RR process has received increasing attention, driven not only by the need to address environmental pollution but also by the potential for new applications through CO₂ conversion. To elucidate the catalytic mechanism of the CO₂RR process, directly probing intermediate products and monitoring the CO2RR dynamics through experimental methods remains a fundamental research strategy. Various advanced structural characterization techniques based on SR sources have been widely extensively employed to study the CO₂RR process. Research has evolved from offline static measurements to in situ/operando dynamics, transitioning from single structural characterization techniques to combined measurements involving multiple structural characterization techniques.

As knowledge of the CO₂RR process deepens, there is a growing interest in its comprehensive study. In situ/operando characterization of CO₂RR process has become the primary research direction, leading to the development of various electrolytic cells or electrochemical cells. This paper introduces the electrolytic cells developed in recent years for in situ structural characterization using SR techniques.

Due to the different detection techniques employed, the design of these electrolytic cells varies significantly. Broadly, existing electrolytic cells can be categorized into two types: transmission and reflection. It is crucial to emphasize that the design of the WE and its compatibility with characterization techniques are two important considerations. In addition, attention must be paid to other auxiliary equipment that supports in site experimental conditions. A well-designed WE must be tailored to specific experimental objectives.

To ensure high-quality experimental data, special consideration should be given to the selection of window materials, optimization of the sample thickness, and prevention of signal occlusion. Combined measurements using multiple structural characterization techniques can provide multi-scale or hierarchical structure information simultaneously, representing a significant direction for future development. With advancements in detector technology, breakthroughs in dynamic time resolution may also be achievable.

However, these potential advancements rely heavily on the successful implementation of appropriate sample environmental conditions, including those provided by electrolytic or electrochemical cells. Currently, suitable sample cells for multi-technique combined detection are relatively scarce and require continuous development and refinement. This limitation is due to the challenges posed by combined techniques and the stringent design requirements for sample environment that accommodate multiple techniques.

From a research perspective, a sample system often necessitates a uniquely designed sample cell. Even for the same sample system with varying experimental requirements, it can be challenging to meet all experimental conditions with a single design. Thus, while the development of a general sample cell with a wide range of applications faces many difficulties, it remains a promising direction for future research. The establishment and advancement of end-stations for combined technique, particularly in specialized research fields utilizing advanced SR sources, are expected to not only enhance the design and development of versatile sample environmental systems but also significantly promote breakthroughs in the research fields such as CO₂RR.

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