Progress and perspectives of electrochemical CO₂ reduction to methanol

Abstract

The increasing emission of carbon dioxide (CO₂) has intensified global efforts toward its conversion and utilization. Electrocatalytic CO₂ reduction reaction (CO₂RR) has emerged as a promising sustainable strategy to address interconnected energy and environmental challenges. Among the various products of CO₂ reduction, methanol has attracted significant research attention as both an essential chemical feedstock and a promising renewable energy carrier. This review comprehensively summarizes recent advances in the electrocatalytic conversion of CO₂ to methanol, with systematic discussions on fundamental reaction mechanisms and pathways, innovative reactor configurations, diverse catalysts, and auxiliary optimization strategies. Particular emphasis is placed on categorizing and evaluating various catalysts, including mono-/bimetallic catalysts, molecular catalysts, enzyme catalysts, and carbon-based materials, while exploring their structure-activity relationships and performance enhancement strategies for improving methanol selectivity. Furthermore, the techno-economic viability of current processes is analyzed, assessing the cost-effectiveness and commercial potential of electrocatalytic methanol production. Finally, based on current research progress and existing challenges, key research directions are outlined to advance the development of commercially feasible electrocatalytic CO₂-to-methanol systems, providing practical guidance for future investigations.

Graphical abstract

Keywords

electrocatalysis / carbon dioxide / methanol / catalysts / electrolyte

1 Introduction

With accelerating global industrialization, a surging global population, and rapid civilization progress, energy demand has risen substantially. The large-scale use of fossil fuels has driven a sharp rise in CO₂ emissions [1]. The report “Global Carbon Balance 2024” indicates that global CO₂ emissions reached 41.6 billion tons in 2024, with fossil fuels emissions accounting for 37.4 billion tons. These excessive CO₂ emissions have triggered a series of severe environmental problems, including global warming, rising sea levels, and more frequent extreme weather events [2–4]. Concurrently, CO₂ has garnered increasing attention from researchers across diverse fields due to its characteristics of non-toxicity, easy availability, and high recyclability. Converting emitted CO₂ into valuable resources and chemicals through capture and reduction processes represents a promising strategy not only for addressing current energy challenges but also for mitigating global warming [5–7].

The electrocatalytic CO₂ reduction reaction (CO₂RR), is a process that mimicks artificial photosynthesis by converting CO₂ into value-added chemicals using renewable electricity, utilizes an applied potential between electrodes to drive CO₂ reduction [8]. Recognized as a promising strategy for achieving carbon neutrality, this technique has been extensively studied due to its dual advantages of economic feasibility and environmental sustainability [9]. The electrocatalytic CO₂RR involves the sequential transfer of multiple electrons and protons, enabling the production of diverse products ranging from C₁ products (e.g., carbon monoxide, formate, and methanol) to multi-carbon species such as ethylene, ethanol, propanol, and acetate (Fig. 1) [10–13]. However, this intrinsic product diversity, stemming from complex reaction pathways, presents a significant challenge in achieving high selectivity toward specific target products. Crucially, the reaction selectivity and efficiency are critically determined by the interplay of multiple factors, including catalyst composition and structure, electrolyte properties, cell configuration, and applied potential [14–16].

Among the many products derived from CO₂RR, methanol stands out as a vital chemical feedstock and clean energy carrier. With an energy density of 15.6 MJ/L, methanol serves as an efficient fuel for fuel cells, offering advantages such as high energy density and clean combustion. These properties position it as a leading candidate to replace conventional fossil fuels [17]. In industrial applications, methanol demonstrates extensive versatility, not only as a precursor for key organic compounds, including formaldehyde, acetic acid, methylamine, but also as critical building block for synthesizing olefins and gasoline-grade fuels [18]. At present, industrial methanol production primarily relies on fossil-derived syngas, a process that emits substantial CO₂ (approximately 2.6 tons of CO₂ per ton of methanol). While direct electrocatalytic conversion presents a transformative pathway for sustainable methanol synthesis, simultaneously achieving high selectivity, industrially relevant current densities (> 200 mA/cm²), and long-term operational stability remains a formidable challenge.

Catalyst design is crucial for electrochemical CO₂ reduction reactions, as material composition, structure, and mechanistic pathways dictate distinct catalytic performance profiles [19,20]. Under certain conditions, low-cost catalysts can facilitate the formation of valuable chemicals [21]. Strategic catalyst design thus enables cost-effective production of high-value products [22]. Currently, copper-based catalysts are the most widely studied and have received significant attention in electrocatalytic CO₂ to methanol research. Additionally, noble metal catalysts, molecular catalysts, enzyme catalysts, and homogeneous catalysts have emerged as promising alternatives owing to their unique advantages in this field [23].

This review encompasses the critical aspects of electrocatalytic CO₂ reduction to methanol, highlights recent research progress, identifies scientific challenges, and outlines future research directions toward commercial applications. It aims to provide the research community with a deeper understanding of the current state of the field, thereby fostering further innovation in electrocatalytic conversion of CO₂ to methanol.

2 Fundamentals of electrocatalytic CO₂RR to methanol

The electrocatalytic reduction of CO₂ to methanol is an electrically driven process that utilizes CO₂ as feedstock and involves a six-electron/six-proton transfer [24]. The overall reaction can be represented as:

{\mathrm{C}\mathrm{O}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to {\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}

Typically, this complex process can be briefly summarized in four key stages:

1) adsorption and activation of CO₂ molecules on the catalyst surface;

2) proton-coupled electron transfer (PCET) generating reaction intermediates;

3) transformation of these intermediates;

4) formation of methanol [25].

However, achieving efficient CO₂ reduction to methanol remains a significant challenge. First, the reduction potential of CO₂ is close to that of the hydrogen evolution reaction (HER), leading to strong competition between the two processes [26]. Second, the highly stable C=O bond in CO₂ requires substantial energy input to break. In addition, the entire reaction process involves multiple electron-transfer steps and complex intermediate transformations, making it difficult to kinetically control intermediates and enhance methanol selectivity [27].

Catalysts play a crucial role throughout the entire reaction process by forming specific chemical bonds with CO₂ molecules, thereby lowering the energy required for CO₂ activation. Different catalysts exhibit varying catalytic activities in the PCET. These variations, determined by the type and properties of the catalyst, influence CO₂ adsorption patterns, reaction rates, product selectivity, and the subsequent reaction pathways leading to methanol generation [28]. In CO₂ reduction research, both photothermal catalysis and electrocatalysis have demonstrated significant advances [29].

Figure 2(a) illustrates the generalized CO₂RR-to-CH₃OH pathway common to most catalysts. Following adsorption and activation, CO₂ undergoes a PCET step to form the *COOH intermediate. This *COOH intermediate then accepts an electron and a proton to formic acid or is converted to the *CO intermediate. *CO represents a very critical reaction intermediate, as subsequent reaction pathways determine the final product distribution. Specifically, *CO desorb as gaseous CO or undergo hydrogenation to *CHO or *COH intermediates, both of which favor CH₃OH and CH₄ formation, respectively [30]. The *CO and *COH intermediates are also susceptible to C−C coupling, which tends to produce a variety of multi-carbon products.

The typical pathway for methanol production involves the sequential hydrogenation of the *CO intermediate through intermediates such as *CHO, *CH₂O, and *CH₃O, ultimately leading to methanol formation via hydrogenation and desorption from the catalyst surface to obtain the final product. Taking the Co-Pc-PBBA catalyst (PBBA = 1,4-phenylene diboraic acid) as an example, the CO₂ reduction process can be analyzed from the perspective of thermodynamic energy barriers (Fig. 2(b)) [31]. The entire methanol formation process exhibits a thermodynamic tendency toward spontaneous progression. However, distinct energy barriers must be overcome for each reaction intermediate, and the formation of alternative products poses a major challenge. Consequently, optimal reaction conditions and catalyst design are required at every stage to ensure efficient progression throughout the entire process.

During the progressive conversion of CO₂ to methanol, the adsorption energy of intermediates serves as a fundamental descriptor linked to catalytic activity, exerting a certain influence on methanol selectivity. For instance, a moderate *CO adsorption energy helps prevent desorption to CO while reducing the occurrence of C–C coupling that leads to multi-carbon products. However, the overall reaction pathway involves the transformation of multiple intermediates and multi-step PCET processes. Across different catalytic systems, the adsorption energies of reaction intermediates and the associated PCET energy barriers vary substantially. Furthermore, complicating factors such as solvent effects complicate quantitative predictions of methanol selectivity, making it difficult to identify universal descriptors applicable across diverse catalytic systems.

Currently, insights can be drawn from descriptors developed for other electrocatalytic processes. Among these, the d-band center is the most widely used descriptor, as it quantitatively captures interactions between the catalyst surface and adsorbates. By modulating the d-band center of active metal sites through strategies such as alloying or introducing defects, it is possible to tailor the electronic structure of the catalyst and enhance its performance. Additional types of descriptors include intrinsic property descriptors, which use readily accessible parameters such as electronegativity, coordination environment, and atomic radius to develop predictive models of catalytic activity. Moreover, spin property descriptors and multi-feature descriptors have shown promise in studies of reactions such as OER and HER [32]. These approaches offer valuable insights for developing suitable descriptors in the electrocatalytic conversion of CO₂ to methanol.

A comprehensive understanding of the electrocatalytic reaction process and mechanism enables a holistic view of the CO₂ conversion pathway. The formation of the key *CO intermediate and the precise control of subsequent reaction pathways will remain major focal points for future research.

3 Electrochemical cell configurations of CO₂RR to methanol

To realize an efficient CO₂RR process, researchers have developed various electrochemical cell configurations, primarily including H-type cells, gas diffusion electrolyzers, and membrane electrode assemblies (MEAs) [33]. Each configuration exhibits distinct structural, operational, and performance characteristics, making them suitable for different research or industrial applications. This section provides a brief introduction to these systems.

Schematic diagrams of typical electrochemical cell configurations are shown in Fig. 3. As depicted in Fig. 3(a), the H-type cell is a conventional device consisting of two separate chambers, the cathode chamber and the anode chamber, connected by an ion-exchange membrane or salt bridge. The cathode chamber is typically filled with a CO₂-saturated electrolyte solution (e.g., aqueous KHCO₃), which is in direct contact with the electrode. The anode chamber contains an electrolyte primarily used for the oxidation reaction of water (OER). During the CO₂RR process, CO₂ is first dissolved in the cathodic electrolyte and diffuses toward the cathode surface, where it is subsequently reduced to products such as methanol. Concurrently, OER occurs in the anode chamber, generating O₂ and H⁺. The generated H⁺ then migrates through the ion-exchange membrane or salt bridge to the cathode chamber, where it participates in the CO₂RR.

The advantages of the H-type cell lie in its simple structure and ease of operation, making it well suited for fundamental laboratory-scale research. However, its limitations include low CO₂ solubility in the electrolyte, poor mass transfer efficiency, and limited product selectivity.

The gas diffusion electrolyzer addresses the problem of low CO₂ solubility by delivering gaseous CO₂ directly to the electrode interface via a gas diffusion electrode (GDE) (Fig. 3(b)). The GDE consists of a porous conductive substrate, a gas diffusion layer, and a catalyst layer, forming an efficient three-phase (gas–liquid–solid) reaction interface [34]. During the CO₂RR process, gaseous CO₂ is directly transported through the porous structure of the GDE to the catalyst surface, where it subsequently reduced into products such as methanol. The anode chamber functions to the H-cell, where the OER occurs to produce H⁺ ions. The generated H⁺ then migrates through the membrane to the cathode, participating in the CO₂RR. Compared with the H-type cell, this configuration offers high CO₂ mass transfer efficiency and can achieve higher current densities. However, the electrode structure is more complex and expensive, and the reaction requires optimization of electrode hydrophobicity and stability to achieve selective production of target products.

MEAs integrate the cathode, anode, and proton exchange membranes (e.g., Nafion) into a compact electrochemical reactor (Figs. 3(c) and 3(d)) [35]. At the cathode, catalysts are usually loaded onto the electrode surface to catalyze CO₂RR. At the anode, the water oxidation reaction occurs, providing the H⁺ required for the entire cycle. The proton exchange membrane facilitates H⁺ transport while physically separating the cathode and anode compartments. During operation, humidified CO₂ is continuously fed into the cathode chamber. The GDE then delivers a high concentration of CO₂ to the cathode surface, where it is subsequently reduced to products such as methanol. Concurrently, water oxidation at the anode generates H⁺, which migrates through the proton exchange membrane to the cathode to participate in the reduction reaction. This integrated configuration offers advantages of compactness and energy efficiency, making it suitable for large-scale applications and continuous operation. However, the high cost of membrane materials and their susceptibility to contamination remain challenges. Further in-depth studies into the interfacial properties and mass transfer between the catalyst and membrane are essential to enhance overall reaction efficiency.

In electrocatalytic systems, the formation of precipitates during cathodic reactions that readily adsorb onto electrode surfaces can significantly compromise system stability and catalytic efficiency. At elevated cathode potentials, cations such as potassium and sodium in the electrolyte readily combine with anions like carbonate and bicarbonate to form insoluble salts. These precipitates adsorb onto the electrode surface, blocking the active sites of the catalyst and impeding mass transfer during the reaction. Consequently, optimizing the catalytic system is essential to suppress cathodic precipitation.

Currently, the industrial-scale implementation of electrocatalytic CO₂ reduction to methanol remains constrained by high costs and energy requirements. Similarly, photocatalytic CO₂ conversion faces scalability challenges due to low quantum efficiency and limited stability, although it has demonstrated promising progress toward industrialization. The cross-scale construction of photothermal co-catalytic systems offers valuable engineering paradigms for industrial applications such as methane reforming and water splitting [36]. Through optimization of reactor structure (e.g., tower concentrators and flat-plate membrane integrations) and system coupling, photothermal catalysis has achieved engineering demonstrations ranging from laboratory scale to installations exceeding hundred square meters. However, critical challenges persist in maintaining long-term stability and improving photothermal synergy efficiency requiring, both of which require further enhancement [37].

4 Recent progresses in electrocatalytic CO₂RR to methanol

4.1 Catalyst

With the continuous deepening of catalyst research, an increasing number of catalysts have become available for the electrocatalytic reduction of CO₂ to methanol. Based on the phase state of the catalyst and the reactants, these catalysts can be broadly classified into two categories: heterogeneous catalysts and homogeneous catalysts (Fig. 4) [38]. The following section outlines current research advances in various types of catalysts.

In general, Cu based catalysts remain the predominant choice for the electrocatalytic reduction of CO₂ to methanol. For the preparation of {\mathrm{C}}_{2^{+}} products, non-copper catalysts (such as silver, tin, and molecular catalysts) and other carbon-based materials employ distinct enrichment strategies. Non-copper catalysts such as Ag and Sn are primarily employed to produce C₁ products like carbon monoxide (CO) and formic acid (HCOOH). These catalysts can be coupled with Cu through interfacial engineering or tandem catalysis strategies to achieve surface enrichment of CO, thereby altering the reaction pathway [39].

Molecular catalysts enable precise control over their structure and environment, utilizing confinement effects to enrich carbon sources or reaction intermediates [40]. Carbon-based materials, characterized by high specific surface area and excellent conductivity, can be engineered through nitrogen doping or vacancy introduction to modulate their electronic structure. This modification enhances *CO adsorption, thereby promoting the kinetics of CO₂ reduction [41].

4.1.1 Heterogeneous catalysts

Heterogeneous catalysts are the most widely used in CO₂RR, and significant progress has been made in their application for the electrocatalytic reduction of CO₂ to methanol. As summarized in Table 1, the most extensively studied catalyst types include Cu-based catalysts, noble metal catalysts, molecular catalysts, and enzyme catalysts. Representative examples of each type will be discussed in detail below.

4.1.1.1 Cu-based catalysts

Among the many catalysts for the electrocatalytic reduction of CO₂ to methanol, Cu-based catalysts are the most extensively studied. This is because Cu exhibits a moderate adsorption capacity for both CO₂ and reduction intermediates, enabling the production of variety of CO₂ reduction products [42]. This property not only effectively adsorbs CO₂ molecules, enriching them on the surface of the catalyst to enhance reaction probability, but also allows reduction products to desorb at the appropriate time. This prevents product over-adsorption and the consequent occupation of active sites, which will otherwise impede subsequent reaction steps.

Cu commonly exposes three crystal facets: Cu(100), Cu(110), Cu(111) [43]. These facets exhibit distinct catalytic and adsorption behaviors (Figs. 5(a) and 5(b)). The Cu(100) lattice structure facilitates CO₂ bending activation and promotes efficient *CO dimerization, favoring C–C bond formation and C₂ product generation. In contrast, the grooved Cu(110) surface exhibits strong *CO adsorption that impedes intermediate conversion and induces high surface coverage. The Cu(111) facet, characterized by a hexagonal close-packed compact structure, shows weak adsorption strength for CO₂ and *CO but moderate adsorption for key reaction intermediates such as *CHO and *CH₂O. Due to the high coordination number of surface atoms, Cu(111) inhibits C–C coupling while promoting stepwise hydrogenation toward methanol. This pathway initiates with CO₂ activation to form *COOH, followed by reduction to *CO. Sequential hydrogenation proceeds through *CHO, *CH₂O, and *CH₃O intermediates before final protonation yields CH₃OH.

In a representative study, Gao et al. employed isotope labeling techniques to investigate the electrocatalytic activity of two specific sites on Cu. They identified {\mathrm{C}\mathrm{u}}_{{\mathrm{C}\mathrm{O}}_2} as corresponding to the Cu(111) site and Cu_CO as a defective site. Both sites facilitate CO₂RR, with the {\mathrm{C}\mathrm{u}}_{{\mathrm{C}\mathrm{O}}_2} site primarily promoting CO₂RR to CO, and the Cu_CO site favoring further reduction of CO to {\mathrm{C}}_{2^{+}} products. Notably, the activity for CO adsorption leading to {\mathrm{C}}_{2^{+}} product formation at the Cu_CO site was found to be at least 6-fold higher than that at the {\mathrm{C}\mathrm{u}}_{{\mathrm{C}\mathrm{O}}_2} site [44].

Cu-based catalysts can be optimized to promote CO₂RR toward methanol by tuning their oxidation states and compositions, which in turn alters their electronic structures and surface properties. The overall methanol yield depends on factors such as the density of active sites, metal-support interactions and the functional additives [45]. Notably, adjusting the oxidation state of Cu has been shown to promote methanol formation. Leveraging the properties of Cu oxides, Roy et al. [47] fabricated Cu₂O/CuO electrocatalysts on porous nickel foam via electrodeposition followed by annealing. Catalysts annealed in air at 300 °C for 2 h exhibited optimal performance, achieving a current density of 46 mA/cm², although the Faradaic efficiency (FE) for methanol remained low at approximately 6% [46]. This enhanced CO₂RR activity was attributed to improved charge transfer between the electrode and catalyst, facilitated by the growth of the mixed Cu phase on the nickel foam. During CO₂RR, adsorbed CO molecules on the Cu₂O/CuO surface play a critical role in forming *HCO intermediates, which undergo PCET to generate *CH₃O adsorbate, ultimately yielding methanol.

Another strategy to enhance methanol production involves forming Cu alloys. By tuning the alloy composition, the CO₂RR pathway can be modulated. To explore this, Roy et al. [47] conducted density functional theory (DFT) calculations based on the standard hydrogen electrode model to investigate CO₂RR on Cu-based alloys (Cu₃X). Figure 5(c) compares the onset potentials for CO₂RR on pure Cu and eight Cu-based alloy surfaces. These results, combined with step potentials for CO₂RR to formic acid, indicate that formic acid formation is more favorable on Cu₃Pt, Cu₃Ni, Cu₃Co and Cu₃Rh surfaces. However, competing HER and the excessive accumulation of OH* species, leading to OH* poisoning, negatively impact catalyst activity [48]. Analysis of OH* and H* binding energies on these surfaces (Fig. 5(d)) reveals that methanol formation is more favorable on Cu₃Pd and Cu₃Pt surfaces. Crucially, methanol selectivity depends on creating favorable conditions for the formation of the CH₂OH* intermediate, which occurs when protonation preferentially happens at the C atom rather than the O atom.

In addition to tuning the oxidation state and composition, carbon cladding also significantly influences the properties of Cu-based catalysts and offers multiple advantages. The carbon layer prevents Cu particles from sintering and agglomerating under high temperatures or reactive conditions, thereby stabilizing the catalytic structure. Additionally, the carbon layer facilitates electron transport due to its excellent electrical conductivity, significantly enhancing catalytic activity and efficiency. Furthermore, the surface properties of the catalyst can also be flexibly modulated through carbon coating, enabling Cu-based catalysts to operate effectively in more complex reaction environments.

In a recent study, Yu et al. [49] synthesized carbon-coated Cu materials and observed the formation of key intermediates (*COOH and *OCHO) during CO₂RR. They demonstrated that the surface charge of the catalysts modulates the reaction intermediates, leading to different reaction pathways (Fig. 6(a)) [50]. As illustrated in Fig. 6(b), catalysts with different electron densities yield different products via these pathways. Specifically, Cu₂Se and Cu_1.8S catalysts with lower electron densities on the carbon-coated surfaces predominantly generate formate, while Cu₃P catalysts with higher electron densities on the carbon-coated surfaces favor methanol production. The Cu₃P catalysts demonstrated high methanol selectivity in both H-type cell and the flow cell systems. At −0.36 V vs. RHE, the Cu₃P@C catalyst achieved a methanol FE of 59.2% in the H-type cell. When operated in a flow cell with 1 mol/L KOH electrolyte, the FE of methanol reached 61.2% at 0.76 V vs. RHE, with a partial current density of 130 mA/cm², outperforming many reported electrocatalysts for the methanol production (Figs. 6(c) and 6(d)). The energy level diagram in Fig. 6(e) further confirms that the carbon-coated Cu₃P surface thermodynamically favors the methanol formation pathway.

In summary, copper-based catalysts remain central to the electrocatalytic conversion of CO₂ to methanol by virtue of advanced optimization strategies such as oxidation state tuning, alloying, and carbon coating. Nonetheless, maintaining catalytic efficiency over prolonged operation continues to be a critical challenge for their practical application.

4.1.1.2 Noble metal catalysts

In the electrocatalytic CO₂RR for methanol production, noble metal catalysts demonstrate unique advantages by significantly enhancing methanol selectivity. Noble metals such as platinum (Pt), Ruthenium (Ru), rhodium (Rh), and palladium (Pd) are commonly employed as primary active components due to their excellent electronic conductivity and distinctive electronic structures. These properties provide abundant active sites for CO₂RR, reducing the activation energy of the reaction, thereby exhibiting high catalytic activity.

The design noble metal catalysts for electrocatalytic CO₂RR can be inspired by principles from thermocatalytic CO₂ hydrogenation. In pioneering work, Kothandaraman et al. [51] demonstrated the use of polyamine-supported homogeneous Ru catalysts to convert atmospheric CO₂ into methanol, achieving a remarkable methanol conversion rate of 79%.

This achievement represents a significant milestone toward realization of a future “methanol economy”. More recently, Chen et al. [52] identified the formate pathway as the optimal route for CO₂ hydrogenation to methanol on Rh cluster-loaded In₂O₃ (111) surface among three proposed pathways. In this mechanism, CO₂ first adsorbs onto the catalyst surface and reacts with active hydrogen atoms to form HCOO* intermediate, which then undergoes sequential hydrogenation steps to produce methanol. The maximum conversion efficiency observed for this pathway reached 3.02 × 10⁻⁵ s⁻¹. This high methanol conversion efficiency is attributed to Rh clusters promoting both CO₂ adsorption and hydrogen dissociation, thereby greatly enhancing catalyst performance. In addition, studies on thermocatalytic CO₂ hydrogenation to methanol utilizing other noble metals such as Pd and Pt have also yielded promising results [53,54].

Findings from thermocatalytic CO₂ hydrogenation reveal that noble metal catalysts such as Ru and Rh demonstrate high stability and catalytic activity for the CO₂RR to methanol. Their high efficiency in converting CO₂ to methanol is mainly attributed to their unique electronic structures and catalytically active sites. Specifically, the electron cloud distribution around Ru atoms enables effective adsorption and activation of CO₂ molecules, which reduces the activation energy of CO₂ and facilitates electron transfer, thereby accelerating methanol generation.

Building on this principle, Zhang et al. [55] designed a Pd/SnO₂ catalyst with a high CO₂ adsorption capacity. Their study revealed that the resulting Pd-O-Sn interface is particularly favorable for the reduction of CO* intermediates to methanol, providing a novel approach for enhancing the electrocatalytic CO₂RR to methanol via the strategic construction of metal oxide interfaces.

In a recent study, Zhu et al. [56] synthesized MnO₂ nanosheets via a hydrothermal method and subsequently deposited Pd nanoparticles onto MnO₂ nanosheets using solution-phase deposition, creating composite catalysts with varying Pd loadings (denoted as Pd NPs/MnO₂ NSs). The morphology of the Pd_1.80%/MnO₂ catalyst is characterized and shown in Figs. 7(a)–7(e). Its CO₂RR performance was evaluated in both flow cell and a MEA electrolyzer. In the flow cell with a 1.0 mol/L KOH electrolyte, Pd_1.80%/MnO₂ exhibited the highest CO₂RR catalytic activity among tested loadings, achieved a methanol FE of 80.9 ± 1.5% and a high partial current density of 243.5 ± 4.3 mA/cm² at −0.6 V vs. RHE (Figs. 7(f)–7(g)). In the MEA electrolyzer, the same catalyst achieved a methanol FE of 77.6 ± 1.3% at 3.2 V vs. RHE, with a methanol partial current density of 250.8 ± 4.3 mA/cm² and a full-cell energy efficiency of 29.1 ± 1.2%. This excellent catalytic performance is attributed to the Pd nanoparticles modulating the electronic structure of MnO₂ and inducing oxygen vacancies, which facilitate CO₂ activation and intermediate conversion to methanol (Fig. 7(h)). In addition, palladium doping has also shown excellent catalytic performance for CO₂ reduction to {\mathrm{C}}_{2^{+}} products [57].

In summary, ongoing exploration of noble metal catalysts has significantly advanced research in the electrocatalytic conversion of CO₂ to methanol, contributing substantially to resource conservation and environmental protection. However, challenges such as high production costs and susceptibility to environmental factors that degrade catalytic performance [58] limit the large-scale deployment of noble metal catalysts in the foreseeable future.

4.1.1.3 Molecular catalysts

Unlike heterogeneous catalysts, molecular catalysts provide distinct advantages in precise active site identification, reaction mechanism elucidation, and structural tunability [59,60]. Their well-defined active sites, typically metal centers coordinated to specific ligands, enable precise control over catalytic active units and a deeper mechanistic understanding. By tuning the electronic property and spatial resistance of the ligand, the electron density at metal centers can be finely adjusted to optimize reaction pathways. In addition, metal-centered engineering via substitution or modification allows targeted regulation of substrate and intermediates interactions, thereby lowering the energy barrier of rate-determining steps. Currently, molecular catalysts have demonstrated significant progress in diverse fields, including CO₂RR [61,62], alkane dehydrogenation [63], hydrogen preparation [64], and fuel cell research [65].

Metal-organic frameworks (MOFs) represent a promising class of molecular catalysts that have demonstrated highly efficient electrocatalytic activity for methanol production via CO₂RR [66,67]. For example, MOF-derived nickel nanoparticles have exhibited catalytic activity for CO₂ reduction over a wide potential window [68]. In a representative study, Albo et al. [69] loaded four MOFs, i.e., HKUST-1, CuAdeAce, CuDTA mesoporous metal-organic aerogel (MOA), and CuZnDTA MOA, onto a GDE. They observed that the MOF-modified GDE enhanced CO₂RR performance, producing liquid-phase products including methanol with significant efficiency. Among these, the HKUST-1-based electrode achieved a FE of 5.6% for CO₂-to-methanol conversion. Similarly, Zhao et al. [70] utilized a Cu/C-derived MOF electrode and achieved a remarkable methanol FE of 43.2% under optimal conditions.

The discovery and development of cobalt phthalocyanine (CoPc) and its derivatives represent significant progress in the electrocatalytic CO₂RR for methanol production. CoPc catalysts offer notable advantages such as simple preparation methods, relatively low catalyst loading, and the ability conversion of CO₂ to CO [71]. Notably, the subsequent reduction of CO to methanol under acidic conditions, especially in the presence of alkali metal cations, significantly enhances the overall CO₂-to-methanol conversion rate [72].

In 2019, Wu et al. [73] first demonstrated the potential of CoPc by loading it onto carbon nanotubes (CoPc/CNT) and investigating its catalytic performance for methanol synthesis via CO₂RR. It was found that the reaction proceeds via a distinct domino mechanism with CO as a key intermediate. Remarkably, the CoPc/CNT catalyst achieved a FE for methanol exceeding 40% at 0.94 V versus RHE in a near neutral electrolyte.

Subsequent research highlighted the critical role of CoPc dispersion on the CNT support. Rooney et al. [74] showed, supported by mechanistic studies, that the molecular dispersion state of CoPc on the CNT surface is crucial for enabling rapid electron transfer to active sites and efficient multi-electron CO₂ reduction. Therefore, CoPc on CNTs typically exists in two distinct states: well-dispersed and aggregated. These configurations exhibit distinct structural and performance characteristics, with aggregated CoPc/CNT primarily producing CO, whereas the well-dispersed CoPc/CNT favors methanol formation [75].

Despite significant catalytic activity, CoPc/CNT catalysts suffer performance degradation over time due to the detrimental reduction of phthalocyanine ligands. This deactivation can be mitigated by introducing electron-donating amino substituents onto the phthalocyanine ring, yielding CoPc-NH₂/CNT catalysts with enhanced stability, activity, and selectivity for CO₂-to-methanol conversion [73]. Remarkably, CoPc-NH₂/CNT catalyst achieves a methanol FE of 32%, maintaining 28% FE after 12 h of operation. Further improvements were realized in a flow cell configuration, where a methanol FE of a 43% was reported [76]. Furthermore, CoPc-NH₂/CNT catalysts effectively convert carbon monoxide to methanol, achieving an 85% FE under optimized conditions with pH-independent behavior [77]. Notably, the *CO intermediates generated during CO₂ and CO reduction exhibit distinct adsorption structures, influencing their reaction pathways [78].

Beyond amino-functionalized derivatives, other cobalt-based molecular catalysts show promise for electrocatalytic CO₂-to-methanol conversion, including CoPc loaded onto multi-walled carbon nanotubes (CoPc/MWCNT), Co(II) 2,9,16,23 - tetrakis(amido)phthalocyanine (CoTAPc) composites, and tetramethylcobalt(II) tetrapyridylpyrazine (CoTmTPyPz) [79–81].

To further enhance methanol selectivity, Li et al. [82] recently designed a dual-site catalyst by co-loading nickel tetramethoxyphthalocyanine (NiPc-OCH₃) and CoPc-NH₂ onto multiwalled carbon nanotubes (MWCNTs). This dual-site system achieves a methanol FE of 50% with a partial current density of 150 mA/cm², significantly outperforming the single-site CoPc-NH₂/CNT catalyst. This enhancement is attributed to CO spillover from NiPc-OCH₃ sites to the methanol-active CoPc-NH₂ sites, which dramatically accelerates methanol production.

In another approach, Song et al. [83] synthesized a novel covalent organic nanosheet (CON) with an ultrathin layered structure (Fig. 8(a)) and increased Co active sites by utilizing the spatial site resistance from tert-butyl groups and electrostatic effects. Electrocatalytic testing showed that these iminium-CONs exhibit promising CO₂-to-methanol conversion efficiencies (Figs. 8(b) and 8(e)). Given the multi-step PCET mechanism involved, factors such as CO₂ partial pressure and mass transfer rates critically influence methanol yields [84–86].

In summary, molecular catalysts underscore the importance of precisely regulating the oxidation state and coordination environment of the metal centers for efficient CO₂-to-methanol conversion. This intrinsic tunability aligns closely with the fundamental properties of molecular catalysts, establishing them as a key focus for ongoing research.

4.1.1.4 Enzyme catalysts

Enzyme catalysts are distinguished by their high efficiency, enabling significantly faster reaction rates compared to conventional catalysts. This high catalytic performance stems from several inherent advantages of enzymes. First, each enzyme exhibits strict substrate specificity, allowing precise regulation of the reaction pathway. Second, enzyme possesses stereo-specificity, enabling precise control over the stereochemistry of the reaction to form desired enantiomeric products. Due to these characteristics, enzymes play crucial roles across diverse fields including biocatalysis [87], organic synthesis [88], fuel preparation [89,90], carbon dioxide fixation, and organic matter degradation [91,92]. However, a notable limitation is that enzyme-catalyzed reactions require strictly maintained mild conditions, particularly specific temperature and pH ranges.

The enzymatic reduction of CO₂ to methanol proceeds via a cascade of reactions involving key enzymes such as formate dehydrogenase (FateDH), formaldehyde dehydrogenase (FaldDH), and alcohol dehydrogenase (ADH). This process requires substantial coenzyme consumption to provide protons and energy, with nicotinamide adenine dinucleotide (NADH) serving as the predominant cofactor. To address NADH depletion, El-Zahab et al. innovatively incorporated glutamate dehydrogenase (GDH) into the system, enabling in situ NADH regeneration and significantly enhancing catalytic cycling efficiency (Fig. 9(a)) [93]. Currently, NADH regeneration has become a focal point of research, with major strategies including enzymatic, electrochemical, and photochemical regeneration methods [94]. These regeneration strategies are detailed below.

Enzyme regeneration: Enzyme catalysts are increasingly utilized in bioelectrocatalytic systems for CO₂RR to methanol [95]. Schlager et al. [96] demonstrated this conversion by directly injecting electrons from the electrode into the immobilized enzyme, thereby avoiding the use of the coenzyme NADH (Fig. 9(b)). Their reactor employed carbon felt as the electrode and a mixed alginate-silicate gel matrix to immobilize formate dehydrogenase (FDH), formaldehyde dehydrogenase (FaldDH), and alcohol dehydrogenase (ADH). Electrolysis at −1.2 V (vs. Ag/AgCl) for 4 h produced approximately 0.15 ppm of methanol in a CO₂-saturated system, corresponding to a methanol FE of 10%. In some trials, methanol FE reached up to 30%. Control experiments confirmed no methanol production occurred under enzyme-free or N₂-saturated conditions, verifying the essential catalytic role of the enzyme. This study validates the feasibility of conducting CO₂ electrosynthesis without exogenous coenzymes.

Electrochemical regeneration: Zhang et al. [97] successfully constructed a novel bioelectrocatalytic system by embedding relevant enzymes into the metal-organic framework ZIF-8 and grafting a Rh complex (Cp*Rh(2,2-bipyridyl-5,5-dicarboxylic acid)Cl₂) onto the electrode for sustainable NADH regeneration. This approach overcame challenges of the poor CO₂ solubility in water and the high cost of NADH. Methanol production efficiency was significantly enhanced when the enzyme was immobilized within ZIF-8, reaching 0.320 mmol/L after 3 h, compared to much lower yields in conventional free-enzyme catalytic systems. Furthermore, the heterogeneous NADH regeneration enabled by the grafted Rh complex further boosted methanol production to 0.742 mmol/L, corresponding to a production rate of 822 μmol/(g·h), approximately 12 times higher than the conventional system. These results fully demonstrate the high efficiency and stability of this novel catalytic approach.

Photochemical regeneration: In addition to enzyme regeneration and electrochemical regeneration, photochemical regeneration is another effective method for NADH recovery. Ma et al. [98] designed a photoelectrocatalytic system combining alcohol dehydrogenase (ADH) from brewer’s yeast (Saccharomyces cerevisiae) with the CO₂ reduction products generated by a photocatalytic cell (PEC). In this system, ADH reduced formaldehyde in solution to extremely low micromolar concentrations. The addition of ADH to PEC products increased methanol yield rapidly by 3 to 4 times, primarily attributed to the absence of reverse reactions. This study highlights the powerful potential of combining biocatalysts with synthetic photosystems to ultimately improve the efficiency of producing liquid fuels from carbon dioxide and water (Fig. 9 (c)).

In summary, the high efficiency and specificity of enzymes have drawn significant attention in catalysis. However, the stringent operational conditions required by enzymes limit their broader application in electrocatalytic CO₂ reduction. With advances in scientific techniques and instrumentation, research progress in enzyme catalysts continues to accelerate, promising new breakthroughs in the near future.

4.1.2 Homogeneous catalysts

Compared to heterogeneous catalysts, homogeneous catalysts are typically molecules or ions species uniformly dispersed within the reaction medium. This molecular-level dispersion maximizes catalyst-reactant contact and dispersion, thereby lowering reaction activation energy barriers and enhancing overall reaction rates. To date, homogeneous catalysts have played an important role in electrocatalytic CO₂RR, particularly for methanol production [99]. Their ability to facilitate efficient electrons transfer between electrodes and CO₂ molecules promotes both activation and subsequent reduction of CO₂, thereby improving reaction efficiency and methanol yield.

Pyridine and pyridazine are representative homogeneous catalysts widely studied for electrocatalytic CO₂RR to methanol. Protonated pyridine (pyridinium) notably promotes CO₂ conversion to methanol and has been studied extensively in both photocatalytic and electrocatalytic systems [100]. As early as 1993, Seshadri et al. [101] demonstrated that pyridinium ions enabled efficient CO₂RR to methanol, achieving approximately 30% FE at palladium hydride electrodes despite competitive hydrogen evolution reactions. This system operates effectively at low overpotentials, highlighting its practical advantage. Subsequent mechanistic studies established a general reaction pathway for pyridinium-catalyzed CO₂ reduction to methanol (Fig. 10(a)) [102]. Interestingly, substituting pyridine with 4-tert-butylpyridine significantly reduced methanol yield and eliminated formic acid production. This reduction is attributed to diminished catalyst charge-transfer capacity and energy-supply efficiency of the modified catalyst.

Portenkirchner et al. [103] further compared the CO₂RR performance of protonated pyridine and pyridazine at a Pt electrode, reporting methanol FEs of 14% ± 1.5% and 3.6% ± 0.5%, respectively. In addition, Rybchenko et al. [104] investigated pyridine-mediated CO₂RR to methanol conversion at elevated CO₂ pressure (55 bar), achieving up to 10% methanol FE. However, methanol concentration plateaued despite increased charge passed, suggesting limitations in sustained production. Cyclic voltammetry (CV) analysis showed that hydrogen precipitation dominated the electrode reaction and that CO₂ reduction to methanol was a transient process, decoupled from continuous electrode charge transfer.

Beyond these established systems, recent research has explored novel homogeneous catalysts for electrocatalytic CO₂RR to methanol. Giesbrecht et al. [105] investigated the electrochemical reduction of dihydropyridines such as 1, 2-dihydrophenanthridine and 9, 10-dihydroacridine, confirming that these species can convert CO₂ to methanol, with the electroreduction mechanism exhibiting strong electrode-surface dependence (Fig. 10(b)). In recent years, 6, 7-dimethyl-4-hydroxy-2-mercapto pteridine (PTE) has attracted attention for this application [106,107]. However, these studies reported unsuccessful CO₂ reduction to methanol under tested conditions, indicating limited catalytic efficacy for PTE in this reaction.

Bi et al. [108] investigated an iron complex, [Fe(PP₃)(MeCN)₂](BF₄)₂, featuring a tetradentate phosphine ligand a homogeneous catalyst for electrocatalytic CO₂RR in acetonitrile (Figs. 10(c) and 10(d)). In the absence of amine additives, this iron complex selectively reduced CO₂ to formate with a high FE of 97.3%. Remarkably, introducing diethylamine as a co-catalyst shifted product selectivity toward methanol (Fig. 10(e)), achieving a methanol FE of 68.5% under optimized conditions (Fig. 10(f)). Mechanistic studies revealed that diethylamine first reacts with CO₂ to form a carbamate intermediate, which is subsequently reduced by [Fe(PP₃)](BF₄)₂ through sequential steps to formamide and ultimately methanol (Fig. 10(g)). This study highlights a novel cooperative catalytic strategy leveraging amine additives for selective CO₂-to-methanol conversion.

In summary, different catalysts have been employed for the electrocatalytic CO₂ reduction to methanol, employing various enrichment strategies to enhance CO₂ adsorption and intermediate stabilization, as summarized in Table 2. For instance, Cu-based catalysts such as Cu₃P@C utilize electronic modulation and porous structures to improve CO₂ uptake and maintain a stable current density with a FE of approximately 60% for methanol over 24 h. Noble metal catalysts like Pd_1.80%/MnO₂ introduce oxygen vacancies and specific atomic configurations to enrich CO₂ and stabilize intermediates, showing minimal activity loss over 60 h. Molecular catalysts such as CoPc-NH₂/CNT utilize in situ CO generation to activate deep reaction sites, though a gradual decline in FE of MeOH is observed within 100 h. Enzyme catalysts leverage encapsulation in porous frameworks like ZIF-8 for synergistic CO₂ and cofactor enrichment, significantly improving long-term stability compared with free enzymes. Homogeneous catalysts including Fe(PP₃)(MeCN)₂ employ amine additives to form carbamate intermediates, enabling stable current and increased methanol FE over 12 h. Collectively, these approaches contribute to enhancing both the efficiency and durability of methanol production from CO₂.

4.2 Electrolyte assisted CO₂RR to methanol

Using complementary strategies can significantly improve the electrochemical properties of materials [109]. Among these, electrolytes are essential components in electrochemical systems, serving as the reaction medium and profoundly influencing reactant distribution, mass transport, and electrode surface processes [110]. In the electrocatalytic CO₂RR system, selecting an appropriate electrolyte can regulate reaction pathways and product distribution by enhancing CO₂ solubility, thereby improving reactant-catalyst contact and overall catalytic efficiency.

Current research employs diverse electrolytes, including aqueous solutions, organic solvents, and ionic liquids (ILs), for electrocatalytic CO₂RR [111,112]. Among these, IL-based electrolytes have garnered particular attention due to their exceptional CO₂ solubility and high ionic conductivity, making them promising candidates for efficient electrocatalytic CO₂ conversion [113].

In 2011, Rosen et al. [114] reported a breakthrough in IL electrolytes for CO₂RR by demonstrating that 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF₄) significantly reduces the energy barriers for CO₂ reduction intermediates (Fig. 11(a)). Since then, ILs have been widely applied in electrocatalytic CO₂RR to methanol with notable success.

For example, Sun et al. [115] developed a Mo-Bi bimetallic chalcogenide catalyst (Mo-Bi BMC), where Bi promotes CO₂-to-CO conversion while Mo generates H₂ for subsequent reactions, resulting in synergistic catalytic enhancement. Crucially, this system achieved a methanol FE of 71.2% in a 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim]BF₄/acetonitrile electrolyte, substantially outperforming conventional electrolytes.

Similarly, Lu et al. [116] systematically studied Pd‒Cu aerogel electrocatalysts, identifying Pd₈₃Cu₁₇ as the optimal electrocatalyst. When using an aqueous [Bmim]BF₄ electrolyte, it achieved a methanol FE of 80.0% at 31.8 mA/cm². Control experiments in NaHCO₃ or Na₂SO₄ electrolytes yielded primarily hydrogen and trace formate, confirming the critical role of [Bmim]BF₄ in facilitating methanol formation.

This electrolyte dependence was further demonstrated using Sn-doped defective CuO (Sn₁/V_o-CuO-90) catalysts [117]. In a [Bmim]BF₄/H₂O (molar ratio 1:3) electrolyte, this system achieved an impressive methanol FE of 88.6% at 67.0 mA/cm². The high performance arises from synergistic effects between atomic Sn sites, CuO carriers with adjacent oxygen vacancies, and the IL electrolyte.

In a recent breakthrough, Li et al. [118] developed in situ dual-doped electrocatalysts (Ag,S-Cu₂O/Cu) and systematically optimized their electrolyte environment (Fig. 11(b)). Through comprehensive screening of electrolyte compositions, they demonstrated that a [BmimBF₄]/H₂O (1:3 molar ratio) binary system maximizes methanol production (Fig. 11(c)).

This optimal electrolyte synergizes with the catalyst’s unique architecture to achieve a methanol FE of 67.4% at current density up to 122.7 mA/cm² (Fig. 11(d)). Beyond binary systems, ternary electrolytes have emerged as advanced media for selective electrocatalytic CO₂RR to methanol. Yang et al. [119] pioneered a [Bmim]PF₆/CH₃CN/H₂O (30 wt%/65 wt%/5 wt%) ternary electrolyte coupled with a Cu_1.63Se electrocatalyst, achieving a methanol FE of 77.6% at 41.5 mA/cm² (Fig. 11(e)). These works indicate that the enhanced performance is attributed to IL’s dual role in promoting CO₂ mass transport via increased solubility and stabilizing key reaction intermediates, coupled with water-mediated proton delivery.

In summary, the electrolyte-catalyst co-optimization strategy establishes a promising route for efficient CO₂RR conversion, with IL-based systems demonstrating unparalleled methanol productivity under industrially viable conditions.

4.3 Photo-assisted CO₂RR to methanol

Photo-assisted strategies can enhance reactivity, selectivity, and energy utilization efficiency by synergizing light energy with thermal/electrical inputs or catalytic systems. Peng et al. [120] developed a zirconium-tungsten oxide heterostructure catalyst (Pt/(Zr-W)O_x) (Fig. 12(a)). This catalyst features three functional units that facilitate H₂O dissociation to generate active hydrogen species, CO production, and C–C coupling. These sites synergistically drive CO₂ conversion to diverse products. Experimental testing revealed that the yield of C₂H₄ reached 242 μmol/g after 0.5 h of concentrated solar irradiation, with 83.9% electron selectivity and 1.17% solar-to-chemical efficiency. This represents a 90-fold enhancement compared to non-concentrated conditions (Figs. 12(b) and 12(c)). The study proposed a mechanism of photothermal tandem catalysis, providing a new concept for designing efficient tandem photocatalysts (Fig. 12(d)). Complementarily, Si et al. [121]’s Ni single-atom-induced dioxygen vacancy TiO₂ photocatalyst also achieved efficient conversion of CO₂ to C₂H₄. This catalyst has a microstructure consisting of Ti-O_v(2f)-Ni-O_v(3f)-Ti, where the dioxygen vacancies work together to promote the involvement of *CO in C−C coupling. Additionally, CO₂ reduction to carbon monoxide and methane has been effectively achieved by constructing composites, depositing Pt and Au, and loading Ni [122–124].

In electrocatalytic CO₂RR to methanol, photo-assisted electrocatalysis shows unique advantages in energy conversion by combining light and electric energy [125]. This approach significantly reduces overpotentials while enhancing methanol efficiency and selectivity. Compared to conventional electrocatalysis, photoelectrocatalytic systems exhibit superior competitiveness in energy utilization, operational conditions, and product diversity, offering a viable pathway toward sustainable methanol production [126].

Currently, significant advances have been made in photoelectrocatalysis using CuInS₂ and Cu₂O materials. Yuan et al. demonstrated methanol production on CuInS₂ thin film electrodes, revealing that the methanol yield depends critically on crystal size and composition of CuInS₂ [127]. Subsequently, they developed an electrodeposited Cu₂O electrode [128], which exhibited strong light absorption in the visible light range of 400–550 nm (Fig. 13(a)). Studies show that the catalytic effect correlates directly with the Cu₂O crystal surface, with methanol FE reaching 29.1% under optimal conditions.

While CuInS₂ and Cu₂O semiconductors have excellent optical properties, their inherent catalytic activity remains limited. To address this, composite electrodes have been developed for photo-assisted CO₂RR to methanol. Yuan et al. [129] engineered CFO/CIS thin-film photoelectrodes by depositing CuFeO₂ nanoparticles onto CuInS₂ via differential pulse voltammetry (DPV). This heterostructure demonstrated superior CO₂RR performance, yielding methanol at rates three times higher than bare CuInS at an overpotential of 0.17 V, while also co-producing ethanol. The enhanced activity originates from surface Cu enrichment and suppressed electron-hole recombination.

In another study, Foster et al. [130] proposed an innovative ‘catalytic mismatch’ strategy by integrating a photocatalyst and an electrocatalyst to prepare a CuInSe₂/Ni₃Al + TiO₂ composite electrode (Fig. 13(b)). This composite achieved a FE of 25% for methanol production under optimal conditions (Fig. 13(c)), which is 25 times higher than pure Ni₃Al catalyst. Kang et al. [131] further identified the kinetic processing window for single-phase Cu₂O synthesis through theoretical modeling. Guided by these calculations, they synthesized a layered photoelectrode consisting of a Cu₂O thin-film substrate, single-phase Cu₂O nanofibers, and a TiO₂ passivation layer. This structured photoelectrode demonstrated high CO₂ reduction and methanol selectivity in aqueous electrolyte.

Si and Au have demonstrated significant promise in photoelectrocatalysis due to their unique optoelectronic properties. Kang et al. [131] prepared a Co phthalocyanine-modified Si photoelectrode (STA-GO/CoPc) (Fig. 13(d)), consisted of p-type silicon, TiO₂, (3-aminopropyl) triethoxysilane, and GO/CoPc (Fig. 13(e)). This pioneering molecular-modified photoelectrode achieved CO₂RR-to-methanol conversion. Further optimization of semiconductor/catalyst interfaces led to the development of a micropillar array and coated with a superhydrophobic fluorocarbon layer (Figs. 13(f) and 13(g)) [132,133]. This architecture achieved a FE of approximately 20% under simulated sunlight by enhancing mass transport and light trapping.

In another study, Lu et al. [134] exploited the excellent electrocatalytic and light absorption properties of Au nanoparticles (Au NPs) for efficient photoelectrocatalytic CO₂RR to methanol using a surface plasmon resonance (SPR)-mediated method (Fig. 13(h)). They systematically investigated how Au NPs size influences current density and FE, alongside the influence of light wavelength and intensity on methanol selectivity. Optimal performance was achieved with 20.2 nm Au NPs on glassy carbon electrodes under 520 nm illumination (120 mW/cm²), yielding a methanol FE of 52% at −0.8 V vs. RHE. Mechanistic analysis confirmed that plasmon-derived high-energy electrons critically enhance CO₂ activation, driving both conversion efficiency and methanol selectivity. To date, a diverse range of high-performance photoelectrode materials have demonstrated efficacy for efficient CO₂RR to methanol [135,136].

Photo-, electro-, and thermal multi-energy assistance strategies can enhance electrochemical methanol production by overcoming the thermodynamic limitations of single electrochemical approaches, while also exhibiting promising efficiency and selectivity. Nevertheless, exploring the theoretical efficiency limits of methanol synthesis under multi-field coupling remains a fundamental challenge. Establishing dynamic efficiency models faces several obstacles, including the complex and unquantified synergies among different energy forms, the absence of a unified theoretical framework across distinct catalytic systems, and the need to bridge microscopic charge carrier behavior with macroscopic reactor performance. Therefore, establishing an “energy input-charge carrier migration-catalytic site activation” framework will be essential to delineate the theoretical efficiency boundaries of methanol synthesis under multi-field coupling conditions. Advancing research on multi-energy-assisted strategies and constructing dynamic efficiency models have thus become key priorities for future studies.

5 In situ research methods

In studying structure-activity relationships, in situ research methods are indispensable. In situ Raman spectroscopy, for instance, enables effective detection of dynamic changes in surface species and reaction intermediates on catalysts. Based on the principle of inelastic scattering of molecular vibrations, this technique is applicable in aqueous solutions and achieves signal amplification through surface-enhanced Raman scattering (SERS) technology. In the context of CO₂ reduction, it plays a crucial role in identifying key intermediates such as *CO, *CHO, and *CH₃O, thereby clarifying reaction mechanisms and pathways. Typically, CO₂ undergoes initial activation through electron acquisition at the catalyst surface, followed by a series PCET steps that generate various intermediates. Furthermore, this technique offers some capacity to monitor changes in the oxidation state of the catalyst. However, it remains limited by sensitivity and spatial resolution constraints, which represent key challenges for future technological advancement [137,138].

A variety of other in situ techniques are also widely employed to complement mechanistic understanding. FTIR offers detailed observation of reaction intermediates by detecting their characteristic vibrational modes, which is essential for identifying transient species under operational conditions. X-ray photoelectron spectroscopy (XPS) provides quantitative analysis of surface composition and chemical states of catalysts, allowing researchers to track oxidation state changes and adsorbate binding during reaction. XAS, including both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), offers element-specific insights into the local electronic structure and coordination environment of active sites. Furthermore, complementary methods such as online mass spectrometry (MS) enable real-time gas and liquid product analysis, electrochemical impedance spectroscopy (EIS) probes interfacial charge transfer and reaction kinetics, and in situ electron microscopy provides visual assessment of morphological and structural changes at the nanoscale. The integrated application of these techniques allows for a multidimensional understanding of catalytic mechanisms and material behavior under realistic working conditions.

6 Techno-economic analysis (TEA)

Comprehensive techno-economic assessments of electrocatalytic CO₂-to-methanol conversion reveal substantial commercialization barriers [139–142]. In addition to conventional routes, Adnan et al. systematically evaluated three emerging power-to-methanol pathways: single-step CO₂ electrolysis to methanol; two-step synthesis via water electrolysis coupled with CO₂ hydrogenation; and three-step synthesis combining water electrolysis, CO₂-to-CO electrolysis, and methanol synthesis (Fig. 14(a)) [139]. Their analysis demonstrates that, under current conditions, none of these routes are economically viable, exhibiting levelized costs of $860−1585 per ton of methanol, approximately 2 to 4 times higher than prevailing market prices ($300−500/ton). However, future scenarios with electricity costs below 3 cents/kWh could reduce production costs to $430−435 per ton, making them competitive. Crucially, cradle-to-gate lifecycle analysis indicates that climate benefits only materialize when grid emission intensity falls below 130 g CO₂/kWh. When powered exclusively by wind or nuclear energy, all three pathways achieve annual net-negative emissions of 170000–195000 tons of CO₂.

While economically promising under optimistic projections, only the two-step process currently possesses the technical maturity for industrial-scale implementation. Consequently, critical performance targets are established for CO₂ electrolysis under future low-cost electricity scenarios (3 cents/kWh): current densities exceeding 130 mA/cm² for CO₂-to-methanol and 360 mA/cm² for CO₂-to-CO conversion, alongside energy efficiencies above 40%. Meeting these thresholds would enable commercial viability for single-step and three-step pathways.

Complementing this analysis, Chang et al. [140] developed a comprehensive lifecycle assessment framework for ionic liquid-mediated CO₂ electrolysis to methanol. The generally proposed industrial route for electrocatalytic CO₂RR to methanol, applicable to both studies, is shown in Fig. 14(b). Their sensitivity analysis identified FE, electricity costs, and cell voltage as pivotal economic determinants. The study confirms dual advantages over conventional coal-derived methanol: an 11.67% cost reduction under optimal parameters, and net-negative emissions when renewable-powered-sequestering 1.29 kg CO₂ per kg methanol produced.

Currently, industrial methanol production relies predominantly on syngas conversion. As illustrated in Fig. 15(a), raw materials such as coal and natural gas are used, with a gasifier generating syngas at high temperatures. This syngas is subsequently converted to crude methanol in a synthesis tower. After impurities are removed by a purification unit, the methanol is further refined by a separator and a distillation tower before being stored.

Analysis of conventional methanol production costs (Fig. 15(b-I)) is based on current industry data. Energy cost ranges from $115.9 to $154.6 per ton, calculated from coal prices of $82.8− 96.6 per /ton at a consumption rate of $1.4−$1.6 tons of coal per ton of methanol. Raw material cost falls between $29.9 and $39.3 per ton. Maintenance and labor expenses amount to $24.0−31.5 per ton, based on a 10000-ton production line over 20 years, plus labor costs. The catalyst cost is estimated at $16.0−20.9 per ton, based on a catalyst price of $11000 per ton and an amortization period of 3 to 5 years. Altogether, these components yield a total production cost of $185.8−246.3 per ton.

For electrocatalytic CO₂-to-methanol conversion, cost projections were derived from optimal performance data of H-cells and flow cells in Table 1 (Fig. 15(b-II, b-III)) [82]. Both systems assume an industrial electricity price of $0.07 per kWh. Considering their respective current densities, operating voltages, and methanol FE, electricity costs were calculated at $1.65 per kg and $1.54 per kg, respectively. Industrial CO₂ capture costs range from $30−50 per ton, and each kilogram of methanol produced consumes about 1.375 kg of CO₂, with a feedstock cost of $0.04−0.07 per kg. Maintenance and labor costs, including equipment investment, maintenance, operation, and supervision, are estimated to be $0.47 and $0.35 per kg. The resulting catalyst costs were estimated at $50 per g, with usage amounts and lifetimes varying between the two cell types. Including membrane costs, this amounts to $0.56 and $0.33 per kg. Consequently, total estimated cost reach $2.75 and $2.29 per kg for these electrocatalytic routes.

Comparative analysis demonstrates that while electrocatalytic CO₂ conversion offers environmental advantages, its production cost ($2290–2750 per ton) substantially exceeds conventional syngas methods ($186–246 per ton). This economic disparity currently limits the technology to laboratory-scale and specialized applications. Future breakthroughs in catalyst efficiency, system lifetime, and reductions in green power cost are essential to make the electrocatalytic route commercially viable.

7 Summary and outlook

Compared to traditional syngas-based methanol production, electrocatalytic CO₂RR to methanol represents a promising research direction aligned with environmental protection and resource sustainability goals. This approach utilizes excess CO₂ to mitigate global warming while producing methanol through a green, sustainable process. This review summarizes the current status of this technology, covering fundamental reduction pathways, innovative reactor designs, and catalytic systems, including mono-/bimetallic, molecular, and carbon-based materials, with a particular emphasis on structure-activity relationships governing methanol selectivity. It also discusses performance-enhancing strategies such as IL electrolytes and photo-assisted activation, alongside techno-economic analyses. Nevertheless, further research is essential in this promising field, as highlighted below.

Catalysts play a pivotal role in electrocatalytic CO₂RR to methanol. Both metallic and molecular catalysts demonstrate promising methanol selectivity. However, their activity and selectivity remain far below industrial requirements. Moreover, maintaining long-term catalyst stability under operational conditions is a significant challenging. Future research should focus on designing sophisticated bimetallic and molecular composite catalysts to enhance reaction kinetics and durability overall performance.

IL electrolytes offer significant advantages for electrocatalytic CO₂RR to methanol. By tailoring anions-cation combinations, ion mobility and catalytic efficiency can be enhanced. The addition of co-catalysts such as pyridine further reduces overpotential and enhances methanol selectivity. Most current studies on CO₂RR-to-methanol utilize commercially available ILs. Future research should focus on exploring functionalized, task-specific ILs to develop high-performance, stable electrolyte systems.

Photo-assisted electrocatalysis leverages light energy to enhance CO₂RR efficiency. Materials such as CuInS₂, Cu₂O, Si, and Au have demonstrated promising methanol conversion efficiencies. Future research priorities include exploring low-cost, earth-abundant alternatives, enhancing photoelectrode durability, and optimizing their integration with other system components.

In situ characterization techniques are essential for understanding structure-activity relationships. In situ FTIR enables detailed monitoring of reaction intermediates while techniques like in situ XRD, XPS, and XAS provide insights into catalyst surface composition and local structure. Additionally, complementary in situ techniques such as mass spectrometry (MS), electrochemical impedance spectroscopy (EIS), and electron microscopy are also strategically employed to gain a multidimensional understanding of catalytic mechanisms.

In summary, this review provides a comprehensive overview of recent advances in electrocatalytic CO₂ reduction to methanol, offering a holistic perspective to guide future research. To realize commercially viable electrocatalytic methanol synthesis, future efforts should focus on advancing the rational design of novel catalysts and electrolytes, the development of innovative multi-energy assistance strategies, and in-depth mechanistic studies.

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