Copper-Based Metal–Organic Frameworks Applied as Electrocatalysts for the Electroreduction of Carbon Dioxide (CO₂ER) to Methane: A Review

Abstract

The electrochemical reduction of carbon dioxide (CO₂) to methane (CH₄) holds tremendous potential in mitigating greenhouse gas emissions and producing renewable fuels. Thus, this review provides a comprehensive overview of the utilization of copper-based metal–organic frameworks (Cu-MOFs) as catalysts for this transformative process. Diverse key aspects of Cu-MOFs that make them ideal candidates for CO₂ reduction are discussed, including their high surface areas, tunable pore sizes, and customizable active sites. Furthermore, recent advances in the design and synthesis of Cu-MOFs tailored specifically for enhanced catalytic activity and selectivity towards CH₄ production are highlighted. Additionally, mechanistic insights into the CO₂ reduction process on Cu-MOF catalysts are examined. Moreover, the recent application of diverse Cu-MOFs and derived materials in electrochemical reduction systems is discussed, and future research directions and potential applications of Cu-MOFs in sustainable energy conversion technologies are outlined. Thus, this review provides valuable insights into the current state of the art and the prospects for utilizing Cu-MOFs as efficient catalysts for the electrochemical conversion of CO₂ to CH₄, offering a pathway towards a greener and more sustainable energy future.

1. Introduction

Carbon dioxide (CO₂) is a colorless, odorless gas that plays a significant role in the Earth’s atmosphere and global climate [1]. Human activities, particularly the burning of fossil fuels and deforestation, have dramatically increased the concentration of CO₂ in the atmosphere since the industrial revolution. CO₂ is a greenhouse gas, meaning it absorbs and emits infrared radiation, trapping heat in the Earth’s atmosphere and contributing to the greenhouse effect. This phenomenon leads to global warming and climate change, resulting in rising temperatures, shifts in weather patterns, melting ice caps, increase of sea levels, and extreme weather events [2]. The excessive accumulation of CO₂ in the atmosphere is widely recognized as the primary driver of anthropogenic climate change, posing significant environmental, economic, and societal challenges on a global scale.

Efforts to mitigate the environmental impact of CO₂ emissions are critical for addressing climate change and ensuring a sustainable future. Various strategies have been proposed and implemented to reduce CO₂ emissions, including transitioning to renewable energy sources, improving energy efficiency, implementing carbon capture and storage (CCS) technologies, and promoting reforestation and afforestation initiatives [3,4]. Additionally, advancements in carbon capture and utilization (CCU) technologies aim to capture CO₂ emissions from industrial processes and convert them into valuable products, such as fuels and chemicals, thereby reducing greenhouse gas emissions while creating economic opportunities [5].

Among the plethora of potential solutions related to the reuse of CO₂, the conversion of CO₂ into methane (CH₄) stands out as a promising avenue. Not only does this process offer a means of recycling CO₂ emissions, but it also yields a valuable fuel with diverse industrial applications [6]. As widely reported, CO₂ conversion can be carried out through diverse processes such as thermocatalytic conversion, photocatalytic reduction, and electrochemical reduction. The first process requires the interaction of CO₂ with hydrogen at high temperature in the presence of certain transition metal catalysts (Fe, Ni, Zr) to produce methane [7,8,9,10,11,12]. Depending on the reaction conditions, thermocatalytic conversion may present diverse side reactions that yield important by-products such as carbon monoxide or that consume the H₂ intake, reducing the expected reaction yield [13]. Challenges include achieving efficiency, managing energy requirements, ensuring catalyst stability, and securing hydrogen supply.

On the other hand, photochemical reduction of CO₂ involves the use of a light source (with a specific wavelength) and photocatalysts to convert CO₂ into methane. In this process, the photocatalysts (typically titanium dioxide-based composites) absorb photons from light sources, which excites electrons from the valence band to the conduction band creating electron-hole pairs [14]. The excited electrons then reduce CO₂ at the catalyst surface, while the holes oxidize water or other sacrificial agents to provide protons (H⁺). Challenges include achieving high efficiency and selectivity, managing energy conversion rates, ensuring photocatalyst stability, and optimizing light absorption.

Finally, electrochemical reduction requires the use of electrical energy to drive the reduction of CO₂ at the cathode of an electrochemical cell in the presence of a specific electrocatalyst, typically copper-based materials. Although some challenges include the accurate control of by-products, the improvement of the selective production of CH₄, and the need of low reduction potentials, this process stands out as a promising avenue for mitigating carbon emissions and producing valuable chemical feedstocks [15,16,17]. As a result, electrochemical reduction has recently garnered significant attention due to its potential to facilitate the efficient conversion of CO₂ into a valuable hydrocarbon fuel. Unlike traditional methods of methane production, which often involve fossil fuel extraction and contribute to carbon emissions, electrochemical reduction offers a more sustainable alternative by utilizing renewable energy sources and recycling CO₂ from industrial processes or the atmosphere [18]. While thermocatalytic methods typically achieve higher yields and are more efficient, electrochemical reduction operates at low temperatures, making it less energy-intensive and more adaptable to intermittent renewable energy sources. Compared to photocatalytic reduction, electrochemical reduction offers better control over reaction conditions and thus leads to higher selectivity. This method also benefits from simpler system design and scalability, presenting a promising alternative for sustainable CO₂ conversion.

In the intricate realm of electrochemical processes for CO₂ electroreduction (CO₂ER), significant energy inputs are typically required. This is because the activation of the relatively stable CO₂ molecule requires a favorable interaction with the catalyst surface to mitigate the inertness of the CO bond [19]. In essence, the process demands a delicate interplay between the catalyst and CO₂ molecules, wherein the catalyst serves as a facilitator for breaking down the molecular bonds of CO₂. This activation step is crucial for initiating the reduction reaction, which ultimately leads to the formation of valuable chemical products. Thus, understanding and fine-tuning the dynamics of this interaction is pivotal for advancing the efficiency and efficacy of CO₂ conversion technologies, offering promising solutions for carbon management and sustainable resource utilization.

Achieving high selectivity towards methane in the electrochemical reduction of CO₂ represents a formidable challenge. Conventional catalysts such as metal nanoparticles and metal oxides often suffer from poor selectivity, leading to the formation of undesired by-products such as methanol and limiting overall the efficiency. In this context, the advent of metal–organic frameworks (MOFs) and MOF-derived materials has sparked considerable interest due to their unique structural and chemical properties.

These materials offer exceptional control over pore size, surface area, and chemical functionality, rendering them highly versatile for catalytic applications [20,21]. In this regard, MOFs have been extensively used as electroactive materials in energy storage devices [22], batteries, and fuel-cell technologies [23] and have recently been widely explored as electrocatalysts in water splitting [24] and electrochemical sensors [25]. The increase in the use of these materials for electrochemical applications is because MOFs can be precisely engineered to modulate the coordination environment of catalytic sites. Moreover, the presence of pores with a specific size allows the adsorption of certain molecules inside the framework without collapsing the structure. As a result, the electrocatalysis of certain interesting molecules can be selective and can be controlled with accurate electrochemical methods. In the case of CO₂ electroreduction, MOFs provide selectivity towards methane production while minimizing side reactions. Results with MOFs have demonstrated their great applicability on the selective production of methane via CO₂ER with great Faradaic efficiencies.

Thus, in this review, we discuss the key role of MOFs in CO₂ER, the factors affecting their catalytic performance, and the recent applications of these compounds and their derived materials in the selective production of methane.

Through a comprehensive analysis of the literature, we endeavor to provide insights into the opportunities and limitations of Cu-based MOFs in electrochemical CO₂ reduction, offering perspectives on future research directions and technological developments. By harnessing the unique properties of MOFs, we envision a pathway towards efficient and selective CO₂-to-methane conversion, contributing to the realization of a sustainable energy landscape and addressing the imperative act of mitigating CO₂ emissions in the fight against climate change.

2. Metal–Organic Frameworks: An Overview

Metal–Organic Frameworks (MOFs) are a class of inorganic polymeric materials composed of metal ions or clusters connected by organic linker molecules (Figure 1), forming two- or three-dimensional networks with diverse topologies [26,27,28]. Most of the reported MOFs contain transition metals with square-planar, tetrahedral, or octahedral geometries, whereas lanthanide-based MOFs can pose a heptacoordinated or nonacoordinated environment, highlighting the diverse coordination chemistry within this fascinating class of materials [20,21,29].

The growing interest in the research of MOF materials is focused on their incredible porosity, which underpins much of their utility and fascination in various scientific and technological applications [30,31,32]. These pores can be tuned in size, shape, and connectivity, offering a vast range of possibilities for accommodating guest molecules or ions without collapsing [21].

As illustrated in Figure 2, the variety of pore shapes and sizes is a function of the number of binding sites in ligands and the coordination geometry adopted by the metal ions. This modular design allows for precise control over pore size and shape by selecting appropriate metal ions and linkers during synthesis [28,33]. The high surface area associated with MOF porosity is particularly noteworthy. MOFs can possess extremely large internal surface areas, often surpassing those of traditional porous materials such as zeolites and activated carbons. This immense surface area provides ample space for adsorption and storage of gases, liquids, or even small molecules, making MOFs promising candidates for applications such as gas storage and separation [34,35,36], catalysis [37,38,39], and lately drug delivery [40].

A critical factor of MOFs is their chemical and electrochemical stability. Chemically, MOFs are resistant to degradation when exposed to solvents due to their high insolubility. Moreover, their exposition to gases and other environmental conditions such as acidic and alkaline media does not affect their structural integrity and porosity, which is often achieved through careful selection of metal nodes and organic linkers. Electrochemical stability, on the other hand, pertains to a MOF’s ability to maintain its structural and functional properties under the influence of an applied electric field or during redox cycling [41]. Certain metal-coordinated materials such as Schiff base complexes or organometallic complexes suffer leaching and degradation when exposed to a potential window in electrochemical experiments [42,43,44]; however, MOFs have demonstrated superior behavior in electrochemical systems, where the only changes are derived from the redox state of their metallic centers, which is beneficial to mediate redox reactions [20,21,29,45].

Factors such as the choice of metal center, linker stability, and the overall framework design play significant roles in enhancing the electrochemical stability of MOFs, ensuring they retain their performance over prolonged periods and under rigorous conditions.

During the last decade, the interest in applying MOFs in electrochemical systems has increased considerably due to their high surface area (>6000 m²/g), high thermal and chemical stability, and the redox-active behavior of their ions [46]. However, their direct application is limited due to their low electron conductivity. According to Morozan [47,48], this effect is the result of poor overlap between the frontier orbits and the electronic states of ligands and metal ions. To face this problem, MOFs are combined with conductive nanomaterials to create MOF-based nanocomposites with enhanced electron conductivity that have been extensively applied in solid-state capacitors [49], energy storage devices [22], electrocatalysis [23], and electrochemical sensors [50]. In this last area, MOF-based nanocomposites serve as efficient catalyst supports or even active catalytic components themselves. Their enhanced electron conductivity facilitates electron transfer at the catalyst interface, thereby enhancing reaction kinetics and overall catalytic efficiency, whereas some metallic ions in MOFs such as Zn(II), Fe(II), Co(II), Ni(II), and Cu(II) exhibit electrocatalytic activity in the presence of alkaline conditions where a redox-active pair M(II)/M(III) can be easily obtained [20,44]. This redox pair mediates the electro-oxidation or reduction of different kinds of molecules according to the Fleischman mechanism based on an M(III) species [51]. This makes MOF-based nanocomposites particularly attractive for applications such as fuel cells, water electrolysis, and CO₂ reduction, where electrocatalytic processes play a crucial role in energy conversion and environmental remediation.

3. Electrochemical Techniques for CO₂ Electroreduction and Instrumental Setup

The MOFs presented in this work are directly utilized to modify electrodes for CO₂ electroreduction (CO₂ER) in alkaline media. These MOFs can be employed either as pristine materials or as sacrificial materials to create highly active composites. These composites are then deposited on the surface of a working electrode, typically a glassy carbon electrode (GCE) or a metallic electrode, by a drop-coating process.

The experimental setup to conduct basic research on the heterogeneous electrochemical reduction of CO₂ involves the use of an H-type sealed cell provided with a three electrode system (Figure 3) consisting of a reference electrode (an Ag/AgCl electrode) which controls the imposed potential, a working electrode (a modified electrode containing the electrocatalyst) where electrochemical reduction occurs, and a counter electrode (commonly a platinum foil or wire) that carries out an opposite electrochemical process, in this case, an oxidation reduction. The electrodes are connected in an electrochemical workstation (potentiostat). The support electrolyte used for the electroreduction process is KHCO₃, which mediates the transportation of CO₂ to the electrodes via HCO₃⁻ ions [52,53]. Prior to any measurement, each cell compartment is filled with a controlled flow of CO₂ in order to generate a CO₂-satured KHCO₃ solution. Then, to evaluate the electrocatalytic activity of a given catalyst, a linear sweep voltammetry is performed in the absence (argon-saturated solution) and presence of CO₂. The argon-saturated solution serves as a control to establish a baseline current response, while the CO₂-saturated solution is used to investigate the electrocatalytic reaction, changes in the current density, and the best reaction conditions. Observing hydrocarbon products in the CO₂-saturated solution, but not in the argon-saturated solution, confirms the electrocatalytic activity of the MOFsystem. Moreover, gas chromatography (GC) quantifies gaseous products and determines the conversion rate and Faradaic efficiency which is essential for evaluating the selectivity and effectiveness of MOFs, while liquid products are analyzed using nuclear magnetic resonance (NMR).

4. The Key Role of MOF Materials in the Production of Methane via CO₂ER

As aforementioned, carbon dioxide electroreduction (CO₂ER) has emerged as a promising strategy for producing valuable chemicals, reducing the environmental impact of CO₂. This electrochemical process involves converting CO₂ into useful products such as carbon monoxide, methane, ethylene, and ethanol using electricity as the main force.

Although the production of methane (0.17 V vs RHE) as a main product of the CO₂ER process is thermodynamically more viable compared with other compounds such as formate (−0.12 V vs RHE), carbon monoxide (−0.10 V vs RHE), or methanol (0.03 V vs RHE) [54], strict control on selectivity remains an important challenge because it requires a multi-electron transfer (8 e⁻) process which is kinetically limited in comparison with the 2 e⁻ reduction to CO or HCOO⁻ [55] that is more predominant during CO₂ reduction [56]. In this respect, the design and exploration of accurate catalysts that allow a precise control of electron transfer is a current topic that has attracted the attention of inorganic and material chemistry researchers.

The mechanism of CO₂ER to produce CH₄ has been extensively investigated via in silico methods and several proposals [54,57,58,59] agree that CO₂ is initially hydrogenated to produce *COOH, which is then converted into *CO through a proton transfer process as illustrated in Figure 4. Then, the hydrogenation of *CO to produce CH₄ can follow the *COH or *CHO path. In the first path the *COH species is reduced to carbon, which is absorbed in the electrode surface. Then, four protons are involved in the process until formation of CH₄. The other path is quite similar; however, the interaction between the electrode surface and *CHO is direct with the oxygen moieties. Although the protonation requires less steps, it generates two other products (HCOH and CH₃OH) prior to obtaining CH₄. According to Nie and coworkers [60], the production of methane in this route is not favorable because its formation barrier is higher (1.2 eV). As a result, the selective formation of methanol (0.15 eV) is preferred.

It has been demonstrated that only copper-based materials can reduce CO₂ to hydrocarbon products in an efficient way obtaining considerable Faradaic efficiencies (FE) due to their adequate binding strength to intermediates [61]. At present, different types of copper-based catalysts such as alloys, oxides, nanoparticles, nanosheets, or nanocomposites have been widely evaluated, reporting FEs of around 70% [15,56,62]; however, their selectivity in the production of methane and their low current densities (<20 mA) are the main challenges to solve. Therefore, in order to increase the selectivity on methane production and the current density, efforts have been made to design new, highly active catalysts. In this line, the research in coordination and organometallic chemistry has played an important role in providing several copper materials based on phthalocyanines [56,63], Schiff bases [64], and pincer ligands [65] that have displayed interesting results; however, the exploration of methane production with Cu-MOFs has demonstrated promising FEs above 50%. In this scenario, Cu-MOFs focused for CO₂ electroreduction applications are designed to contain the same active centers in all the framework structures, which enhances the selectivity and the current density. Conversely, some problems presented in other materials are the presence of multiple catalytic species (Cu, Cu₂O, Cu(OH)₂) in their surfaces, which in consequence generate other products during the catalysis, leading to a low selectivity on the production of methane. As a result, MOFs offer a controlled catalysis environment due to their well-defined structures, the tunability of their pores, their d-orbitals, and oxidation states. These features help optimize the binding energy between intermediates and MOF active sites, thereby adjusting the final product composition, and optimizing the catalytic performance in the CO₂ electroreduction processes [55]. This selectivity is mediated through electron-donating and electron-withdrawing effects.

Moreover, the incorporation of various functional groups into the MOF structure can further enhance the interaction between the catalyst and the CO₂ molecules, thereby increasing the overall efficiency of the reduction process. This functionalization strategy also provides a platform for tailoring the electronic environment of the active sites, which is crucial for achieving higher selectivity towards methane production.

On the same line, Pei-Qin and colleagues assessed the impact of Cu center geometry in different MOFs [66,67,68]. In this respect, Cu-THQ (tetrahydroquinoline copper-based MOF) and Cu-HHTP (hexahydroxytriphenylene copper-based MOF) possess square-planar CuO₄ coordination, while Cu-DBC features square-pyramidal CuO₅ coordination. Cu-DBC (benzene dicarboxyl copper-based MOF) presented an optimal geometric configuration and electronic state for CO₂ reduction due to higher energy levels of the d_z², d_xz, and d_yz orbitals of the CuO₅ node compared with CuO₄, resulting in stronger π-back-bonding with CO. Consequently, the CuO₅ active sites facilitated the hydrogenation of *CO into *CHO, eventually leading to the production of CH₄ instead of CO.

The effect of the structural conformation of the active site for Cu ions in MOFs also significantly affects the performance and selectivity of reactions. Several factors influence the structural conformation of the active site, including the type of ligand. For instance, in HATNA-Cu-MOF [69], the favorable π-π stacking of HATNA (hexaazatrinaphthylene) ligand enhances the material’s stability and selectivity, preserving its structure and increasing methane selectivity. Similarly, in Cu₄-MFU-4l [68], the influence of adjacent aromatic hydrogens through a strong non classical C−H···O hydrogen bond from the ligand plays a crucial role in stabilizing key intermediates of CO₂ reduction and inhibiting the hydrogen evolution reaction.

On the other hand, the presence of strong cuprophilic interactions, characterized by short Cu-Cu distances inside MOF clusters, stabilizes Cu(I) ions and enhances CO₂ER, resulting in several significant effects. The amount and proximity of cuprophilic interactions directly influence, with more interactions and shorter Cu(I)-Cu(I) distances enhancing the FE (%) of CH₄ [70]. Additionally, cuprophilic interactions reduce the Gibbs free energy change of the potential determining step (*H₂COOH → *OCH₂), lowering the energy barrier for CO₂ to CH₄ conversion and increasing efficiency [71]. Catalytic durability is also improved, as stable Cu(I) active sites with cuprophilic interactions ensure sustained performance over time.

Regarding mechanistic insights about CO₂ER catalyzed by MOFs, an interesting proposal was presented for the MOF Cu₄ II-MFU-4l (Figure 5) [72]. The proposed mechanism involves several key steps. Initially, the CO₂ molecule is adsorbed into the Cu(I)–nitrogen active sites. CO₂ is then reduced through the addition of protons and electrons, forming the CO intermediate (*CO). The Cu(I)–nitrogen active site has a high affinity for CO, likely due to σ−π back-bonding between Cu(I) and CO. CO is hydrogenated to form a carbonyl intermediate (formyl groups, *CHO), which undergoes further reduction and hydrogenation, ultimately producing methane (CH₄) that is released from the catalyst’s surface.

Although diverse works reporting the use of MOFs for CO₂ER have been recently published, and the factors affecting the performance related to the reduction process and the control of the final products are widely known, mechanistic insights related to the structural interactions and the changes in the coordination geometry are not fully understood yet, and not all authors reported a proposed mechanism; thus, more computational studies are required to fully understand the interaction of MOFs with CO₂.

Understanding the mechanistic aspects of CO₂ER is essential for optimizing catalyst design and reaction conditions. The electrochemical reduction of CO₂ involves multiple intermediate steps, including the adsorption of CO₂ onto the catalyst surface, followed by the transfer of electrons and protons to form intermediate species and, finally, the desorption of product molecules. Elucidating these reaction mechanisms through theoretical modeling, spectroscopic techniques, and electrochemical measurements is critical for rational catalyst design and process optimization. Moreover, tailoring the catalysts’ surface structure, composition, and electronic properties can lead to improved selectivity towards valuable products.

5. Application of MOFs in CO₂ER

5.1. Pristine MOFs

The use of pristine MOFs for direct electrochemical application is scarce due to their low conductivity; however, for CO₂ER, their direct applications have demonstrated promising results on the conversion of CO₂ to methane. Xia and coworkers [68] proposed a Cu-MOF based on adenine (Cu^II-ade) (Figure 6a) whose N-containing groups contributed to proton exchange during CO₂ electroreduction. The proposed MOFs were obtained as nanosheets (sCu^II-ade) (Figure 6b), nanoplates (pCu^II-ade), and nanocuboids (cCu^II-ade). The thicknesses of the obtained MOFs played an important role in enhancing electrochemical performance. In this respect, the best current intensity (15 mA cm⁻²) and Faradaic efficiency (FE = 50%) in the production of CH₄ were obtained with sCu^II-ade (Figure 6c). According to the authors, this behavior was presented as a result of modifying the electroactive chemical surface (ECSA) of the electrodes, where the use of sCu^II-ade provided more active sites compared with the other structures.

Other factors such as work potential highly influence the Faradaic efficiency of the production of methane when using pristine MOF. For example, Wang and coworkers [73] found that the FE of methane production with 2D-vc-MOF was decreased from 65% to 32% when the potential was shifted to −1.2 to −1.4 V. This behavior was related to structural changes in CuO₄ at different potentials which directly affected catalyst performance.

The presence of Cu(I) in MOFs plays a pivotal role in mediating the hydrogenation of final products in the electroreduction process. Interesting results were obtained with the MOF Cu-BTDD (bis(1,2,3-triazolo-[4,5-b],[4′,5′-i])dibenzo-[1,4]-dioxin) [72] that was synthesized by replacing Zn ions from the original reported Zn MFU-4l. The Cu-MOF displayed a FE of 81% at −1.2 V in neutral conditions. The catalytic performance was attributed to the direct interaction of the trigonal Cu (I)-N active sites, where the Cu (I) and N-containing ligands controlled the hydrogenation of intermediates in CO₂ER, resulting in an almost selective production of methane.

Similar results were obtained by Qian Lan group [70] with two coordination polymers (NNU-32 and NNU-33) based on an H₂bptb (2-(1H-1,2,4-Triazol-3-yl)pyridine) ligand. The NNU-33 exhibited an enhanced current density of 391 mA cm⁻² and a Faradaic efficiency of 82% at −0.9 V. In this material, the presence of –OH groups in the ligand and the cuprophilic interactions mediated the final product composition in function of the amount and distance of these interactions. Moreover, no changes on the oxidation state of copper were observed, suggesting NNU-33 to be a highly stable catalyst.

In addition, the incorporation of dicopper (I) active sites in a MOF was explored by the same group. They reported nanolayers of MOF Cuobpy [66] provided with dicopper sites that were evaluated in CO₂ER for methane production. The electrochemical experiments were performed considering diverse thicknesses. The single thickness (1.1 nm—Cuobpy-SL) exhibited the best catalytic performance (FE = 82 %) compared with the bulky Cuobpy material. This result is due to the more exposed active sites in a single layer in comparison with the more confined active sites in a bulky material.

As observed, all the pristine MOFs evaluated have N-containing ligands which are useful in the critical steps for CO₂ER that require proton exchange. Furthermore, by comparing the results obtained with Cu(II)- and Cu(I)-based MOFs significant differences are observed. In this respect, Cu(I)-based MOFs have shown to be excellent candidates with FEs above 80% due to their strong interaction with intermediates that play an important role in stabilizing them and avoiding the hydrogen evolution reaction [74,75]; however, Cu(I) is unstable in air, and its derived catalyst must be handled under inert atmosphere to avoid poisoning, which limits its practical application at the industrial level.

Until now, the exploration of pristine MOFs for CO₂ER in the selective production of methane is in its infancy. Although the observed advances are promising, there is still a long way to go. Thus, the development of selective catalysts based on MOFs requires study of the effects of numerous parameters during CO₂ conversions and the use of computational tools to help us determine the optimum conditions to carry out successful high-yield conversions. Moreover, the electrochemical measurements require controlled atmospheres and the use of chromatographic techniques to analyze the obtained products in real time, which in turn involves specific equipment and limits the research in this area. As a result, there have been only a few papers published in the last 6 years; the catalytic parameters of these papers are summarized in

Table 1. Catalytic parameters of diverse Cu-MOFs applied in the obtention of methane via CO₂ER.

Catalyst	Reduction Potential (V)	Maximum Current Intensity (mA cm−2)	Faradaic Efficiency (%)	Ref.
p−CuII/ade−MOF	−1.6	---	24	[68]
c−CuII/ade−MOF	−1.6	---	22	[68]
s−CuII/ade−MOF	−1.6	15	50	[68]
HATNA-Cu−MOF	−1.5	8.2	78	[69]
Cu4-MFU-4l	−1.2	9.8	81	[72]
NNU-32	−1.0	384	55	[70]
NNU-33(H)	−0.9	391	82	[70]
NNU-50	−1.0	398	66	[71]
Cu-THQ	−1.4	---	<2	[67]
Cu-HHTP	−1.4	---	<2	[67]
Cu-DBC	−1.4	11.4	56	[67]
2D-vc-MOF(Cu)	−1.4	7.5	65	[73]
Cuobpy (Bulk)	−1.4	12	51	[66]
Cuobpy-SL	−1.4	82	82	[66]
Cu-TCPP	−1.4	---	2	[76]
Cu-TCPP/Ag	−1.4	50	73	[76]

HATNA: hexaazatrinaphthylene ligand. MFU: MOF initials denoted on behalf of Materials of Freiburg University. NNU: MOF initials denoted on behalf of Nanjing University. THQ: tetrahydroquinoline ligand. HHTP: hexahydroxytriphenylene ligand. BDC: benzene dicarboxylic acid ligand. Obpy: 4,4′-Bipyridine ligand. TCPP: tetrakis(4-carboxyphenyl) porphyrin ligand.

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5.2. MOFs as Sacrificial Materials

MOFs as sacrificial materials stand out as a novel and promising concept with significant implications, particularly in the realm of electrochemistry. Sacrificial materials are those that undergo controlled degradation or transformation during a chemical process to facilitate the synthesis or manipulation of other materials such as metal oxides, porous carbons, and nanoparticles. In electrochemistry, MOFs are increasingly recognized for their potential as sacrificial templates or sacrificial components in electrode fabrication, offering unique advantages in terms of versatility, tunability, and functionality.

Cu-based catalysts are often composed of multiple species (Cu, Cu₂O, CuO, or Cu(OH)₂) which often leads to different types of products. Rong Cao group [77] suggested the production of Cu-based electrocatalysts with single-type active sites, where MOFs emerged as an efficient tool. Hence, they fabricated uniform Cu₂O(111) quantum dots that were deposited on CuHHTP (synthesized by solvothermal methods using copper acetate and the conjugated tricatecholate ligand HHTP) (Figure 7a) by electrochemical treatment of the conductive Cu-MOF containing CuO₄ nodes. The Cu ions in CuHHTP were later partially reduced to single-type Cu₂O sites by electrochemical methods and distributed uniformly on the remaining framework to obtain the Cu₂O@CuHHTP electrocatalyst which exhibited outstanding CO₂ER performances with 73% Faradaic efficiency in the conversion of CO₂ to CH₄ and a partial current density of 10.8 mA cm⁻² at −1.4 V (vs. reversible hydrogen electrode (RHE)) (Figure 7b).

Nanostructured Cu-based electrodes have shown to be an effective way to achieve better CH₄ production in CO₂ER because multi-electron transfer in electrochemical reactions is highly influenced by the particle size of the material. In this sense, studies on isolated Cu atoms and Cu nanoparticles (NPs) in the production of methane in CO₂ER exhibited higher Faradaic efficiency (FE) and suppressed ethylene gas formation. However, CuNPs tended to easily agglomerate leading to an undesired increment in FE for unwanted C₂ and C₃ products.

Jeong et al. [78] proposed the fabrication of nanosized and sufficiently isolated Cu-based electrocatalysts to effectively promote CH₄ production on copper catalytic sites. Cu-based MOF (MOF-74) was chosen as the precursor, which was electrochemically reduced to obtain isolated CuNPs (Figure 8) to prevent agglomeration of the reduced Cu clusters. The MOF-derived CuNPs were less aggregated and demonstrated high FE (>50%) for CH₄ production with suppressed C₂ production attributed to the restraint of C-C coupling for C₂ product formation as a result of the isolated NP clusters. Higher methanation activity (2.3-fold) was also observed at −1.3 V (vs. RHE) compared to commercial CuNPs which confirmed results from other groups that selectivity of the methanation reaction on CuNPs was closely related to the extent of aggregation of the particles at nanoscales. The results were in line with predicted theoretical calculations indicating that Cu nanoclusters improve methane selectivity.

Rong Xu and coworkers [79] also studied the way to stabilize active, under-coordinated nanoparticles or clusters prone to oxidation and agglomeration often resulting in low catalyst stability and selectivity degrading. To address these issues, electronic metal–support interactions were tested for both, tuning the electronic structures of the metal and stabilizing it. The precursor Cu-MIL was synthesized from a solid-state mixture of Cu(NO)₃·3H₂O and terephthalic acid upon heat treatment, then Cu-MIL was electrochemically activated in the CO₂ER environment to create metal–support interactions during which Cu-MIL underwent a decomposition/redeposition process to obtain the Cu/C catalyst. The electronic metal–support interactions between Cu and C atoms enabled selective and stable CO₂ electroreduction to CH₄ with a FE of 53–54% at applied potentials between −1.35 and −1.45 V (vs. RHE).

As mentioned above, selectivity and low current density towards one specific product remains as the main challenge of methane production in CO₂ER. Other strategies for improving CO₂-to-CH₄ electro-conversion have been the use of copper-doped materials. In this line, Wei-Yin Sun et al. [80] proposed the use of cerium oxide (CeO₂) due to its remarkable properties that can improve molecular activation, dispersion, and catalytic performance by the formation of metal CeO₂ nanointerfaces. The strategy consisted of the creation of copper-doped cerium oxide composites (Cu/CeO₂@C) via one-pot pyrolysis of MOF precursors to modify the surface structure and suppress the reduction of Cu-based catalysts thus improving selectivity to methane with a high current density. The FE achieved for Cu/CeO₂@C was 80.3% with a methane partial current density of 138.6 mA cm⁻². Further studies of the possible mechanism demonstrated that indeed a synergic effect of carbon encapsulation and the Cu/CeO₂ active component optimized CO₂ and intermediate adsorptions, accelerating the charge-transfer and reaction kinetics and preventing the reduction of Cu active sites, resulting in better activity and selectivity in CO₂-to-methane electro-conversion.

Another example of doped materials is that reported by Gengfeng Zheng et al. [81]; they proposed the use of nitrogen to achieve good dispersion and attachment of Cu species on nitrogen-doped carbon frameworks with Cu-N_x configurations. They developed a CuN-doped carbon nanosheet structure obtained by calcination of Cu(BTC)(H₂O)₃ MOF (BTC: 1,3,5-benzenetricarboxylic acid) mixed with dicyandiamide at high temperatures. The products obtained were designed as Cu-N-C-T (where T denoted the calcination temperature used, 800, 900, 1000, and 1100 °C, respectively) and similar undoped samples were synthesized for comparison. The effect of the pyrolysis temperatures on CO₂ electroreduction selectivity revealed high selectivity to C₂H₄ for Cu-N-C-800 while the other catalysts tended to produce CH₄ as the main product. Additionally, they found that Cu concentrations influenced the distance between neighboring Cu-N_x species; at high concentrations the distance was close enough to enable C-C coupling and produce C₂H₄; conversely, lower Cu concentrations produced large distances between Cu-Nx species, favoring the formation of CH₄ as C₁ products and demonstrating the possibility of obtaining different CO₂ electroreduction products by tuning active Cu sites by temperature.

As noted, the Faradaic efficiency and current intensities obtained with some MOF-derived materials recently published (

Table 2. Catalytic parameters of diverse Cu-based materials derived from MOFs used in the obtention of methane via CO₂ER.

Catalyst	Reduction Potential (V)	Maximum Current Intensity (mA cm−2)	Faradaic Efficiency (%)	Ref.
Cu2O@CuHHTP	−1.4	−10.8	73	[77]
Cu-MOF-74	−1.4	−10.9	50	[78]
Cu-MIL derived Cu/a-C	−1.35	−13.4	53.1	[79]
Cu/CeO2@C	−1.5	−138.6	80.3	[80]
Cu-N-C-800	−1.4	−3.83	13.9	[81]
Cu-N-C-900	−1.6	−14.8	38.6	[81]

) are lower compared with those results obtained with pristine Cu(I)-MOFs, probably as a result of non-controlled layers inside the catalyst and the combination of different Cu oxidation states. Moreover, the absence of ligands did not support the stabilization of intermediates and allowed the formation of not-expected products. However, these derived materials represent a stable alternative compared with Cu(I)-MOF that tends to undergo poisoning under open atmosphere. In addition, recent advancements have shown that modification of Cu-MOFs with additional metals can create synergistic effects that improve catalytic activity [82]. For instance, bimetallic materials derived from Cu-MOFs as presented by Jeong group [77] or Wei-Yin and coworkers [80] exhibited the best performances compared to their monometallic counterparts due to improved electron transfer capabilities and better CO₂ adsorption properties. Therefore, although the research in bimetallic MOFs and MOF-derived composites is still relatively new, the results exhibited by some bimetallic examples could be considered an excellent starting point to produce innovative bimetallic catalysts for CO₂ electroreduction purposes.

6. Conclusions and Future Scope

The utilization of Cu-based MOFs and their derived materials in the electrochemical reduction of carbon dioxide to methane represents a promising avenue towards sustainable energy conversion and environmental remediation. Through meticulous design and engineering, Cu-based MOFs offer unparalleled catalytic activity, selectivity, and stability in the CO₂ reduction process, making them excellent candidates for small-scale implementation.

By harnessing renewable energy sources such as solar or wind power to drive the electrochemical conversion of CO₂ into methane, society can move closer to achieving carbon-neutral or even carbon-negative energy systems. With the use of MOFs as enablers of this transition, it can be obtained a versatile platform for the development of efficient and environmentally benign CO₂ conversion technologies.

Comparing CO₂ electroreduction to methane with other CO₂ utilization techniques, such as thermocatalytic and photocatalytic methods, highlights the unique advantages and challenges. While thermocatalytic methods typically achieve higher yields and efficiency, they operate at high temperatures and consume significant energy. Photocatalytic reduction, on the other hand, requires specific light sources and faces challenges in achieving high efficiency and selectivity. Thus, electrochemical reduction stands out due to its ability to operate at low temperatures and its adaptability to intermittent renewable energy sources, despite challenges in controlling by-product formation and achieving selective methane production.

Furthermore, when compared to other energy devices, the electrochemical CO₂ER process aligns well with the goals of sustainable energy systems by converting CO₂ into a valuable fuel using renewable energy, thereby contributing to a closed carbon cycle. This integration with renewable energy sources enhances the overall sustainability and economic viability of the process.

The importance of MOFs on a commercial scale cannot be overstated. MOFs provide a platform for precise control over the coordination environment of catalytic sites, optimizing performance for selective methane production and minimizing the generation of side products such as hydrogen and carbon monoxide. However, several challenges remain before widespread adoption of MOF-based electrochemical CO₂ reduction technologies can occur. These include further optimization of catalytic activity and selectivity, scalability, and integration with renewable energy sources. Currently, the best results have been obtained in laboratory settings, and replicating these conditions in commercial and industrial processes poses significant challenges.

Addressing these challenges will require interdisciplinary collaboration among scientists and engineers to overcome technical barriers and accelerate the deployment of MOF-enabled CO₂ reduction technologies. Future research should focus on enhancing the efficiency of MOFs, developing scalable synthesis methods, and integrating these materials into commercial applications. Additionally, exploring the synergistic effects of bimetallic MOFs and MOF-derived composites could lead to the development of innovative catalysts with improved performance.

Looking ahead, the continued advancement of MOFs and MOF-derived materials in CO₂ER holds promise for realizing a sustainable energy future. By leveraging the unique properties of MOFs and by harnessing the power of electrochemistry, researchers can contribute to the global effort to combat climate change and to transition towards a low-carbon economy. With concerted efforts and innovative solutions, the vision of converting CO₂ emissions into valuable fuels using MOFs may soon become a reality, paving the way for a greener and more sustainable world.