MOF electrocatalysts in CO₂ conversion: critical analysis of research trends, challenges and prospects

Abstract

Achieving sustainable energy and a clean environment is a strong driving force behind the exploration of the electrocatalytic potential of MOFs for CO₂ conversion. The growing interest in the application of MOFs as electrocatalysts for CO₂RR has been attributed to their high surface area and excellent catalytic properties. MOFs have been deployed in their pristine form as catalysts, as porous cavity supports for the incorporation of catalytic active material, or as precursors to obtain single-atom catalysts, showing that they can reduce CO₂ into CO, formic acid and even hydrocarbons and alcohols. Despite these advantages and promising early results, they still have several challenges to overcome, such as poor electrical conductivity, stability and selectivity, and high overpotential, which would limit their practical application as electrocatalysts for CO₂ reduction. In this review, various strategies to improve the electrocatalytic performance of MOFs are highlighted and directions that future studies in this field may take identified.

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Oriyomi Ogunbanjo

Dr Oriyomi Ogunbanjo received his PhD in Chemistry from the University of Birmingham, United Kingdom, in 2024 under the supervision of Professors Paul Anderson and Paramaconi Rodriguez. He is currently working as a Battery Research Fellow in the School of Chemistry at the University of Birmingham. His research interests include MOF electrocatalysts for energy conversion, as well as battery recycling and upcycling.

Paramaconi Rodríguez

Prof. Dr Paramaconi Rodríguez is an Ikerbasque Professor at CIC energiGUNE, leading the Electrochemical Hydrogen Technologies group since 2023. He earned his Bachelor's degree in Chemistry from Simón Bolívar University in 2003 and obtained a PhD in Materials Science with a specialization in Electrochemistry from the University of Alicante in 2007, conducting research stays at Guelph University (Canada) and Bern University (Switzerland). Following his PhD, he was a postdoctoral researcher at Leiden University (Netherlands), where he received the prestigious VENI fellowship in 2010. In 2011, he joined the Paul Scherrer Institute (Switzerland) as a Scientific Officer. The following year, he was awarded a University of Birmingham Research Fellowship, where he later advanced to Associate Professor. His research focuses on the structure–reactivity relationships in electrode materials for electrochemical energy conversion, contributing significantly to advancements in sustainable energy technologies.

Paul Anderson

Paul Anderson is Professor of Strategic Elements and Materials Sustainability in the School of Chemistry at the University Birmingham. He qualified with a BA (Hons) in Natural Sciences from the University of Cambridge in 1987, and went on to study for a PhD in Chemistry before joining the Department of Organic, Inorganic and Theoretical Chemistry at Cambridge as a post-doctoral research fellow in 1990. In 1993, he took up a Royal Society University Research Fellowship at the University of Birmingham. He was awarded the Royal Society of Chemistry/Society of Chemical Industry/British Zeolite Association triennial Barrer award in 1996. His research has encompassed themes of zeolite chemistry, alkali metal chemistry, energy materials, elemental stewardship, recycling and sustainability, and in 2016 he founded with a colleague the influential Birmingham Centre for Strategic Elements and Critical Materials. Since 2018 he has been Principal Investigator of the Faraday Institution's ReLiB – Recycling and Reuse of Lithium Ion Batteries – project, which was the recipient of the Royal Society of Chemistry's 2024 Environment, Sustainability and Energy Horizon Prize: John Jeyes Prize.

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1. Introduction

Right from the start of industrialization to the present time, global energy demand has skyrocketed due to the attendant increase in the human population.^1,2 An effort to sustain this ever-increasing population and economic growth has resulted in an over-dependence on hydrocarbon-based fossil fuels such as petroleum, natural gas and coal which still account for over 80% of world energy consumption.³ This is in addition to the fact that the long-term availability of fossil fuels may not be guaranteed which could plunge the world into an energy crisis. However, the use of fossil fuels has been associated with global climate change which has been identified as the root cause of various environmental disasters ravaging the world in recent times.⁴ Pertinent among these environmental problems are increased atmospheric CO₂ level which is being projected to reach 450 ppm by 2050 and a rising global average temperature which is promoting extreme weather events and negatively impacting our ecosystem.⁵ The concern of continuous environmental disaster and the anticipated energy crisis has necessitated the urgent need to revolutionize technologies aimed at developing sustainable, clean and renewable energy to replace the use of fossil fuels.

To tackle this problem, various CO₂ capture and utilization technologies have been implemented.⁶ However, CO₂ capture technology does not just consume a great deal of energy and financial resource, but its sustainability over a long period of time cannot be guaranteed. For this reason, the electrochemical CO₂ reduction reaction (CO₂RR) has been suggested as a promising CO₂ utilization technology to convert waste CO₂ streams into useful chemicals.⁷ Unlike other chemical processes, electrochemical CO₂ reduction can be performed at ambient temperature using green renewable energy sources and water as a source of the protons making the overall operation a carbon neutral process.⁸ However, so far the electrochemical reduction of CO₂ still suffers from high overpotential (large energy requirement) and poor product selectivity, making the practical application of electrochemical CO₂ reduction still unfeasible from the industrial point of view. The high overpotential and poor product selectivity are the result of non-ideal adsorption energies of key reaction intermediates.⁹ The low faradaic efficiencies are due to the competition with the hydrogen evolution reaction (HER), which takes place in the same potential window as CO₂ reduction.¹⁰

Materials such as transition metals, noble metals, nanoporous carbon, oxides, sulfides and phosphides are notable state-of-the-art electrocatalysts for various electrochemical systems.^11–13 Despite the exceptional catalytic performance of these materials in some electrochemical systems, high cost, coupled with the scarcity of noble metals and the propensity to be poisoned of precious metal catalysts on exposure to some chemical compounds, has severely hindered their large-scale commercialization.¹⁴ Therefore, the focus of 21st-century researchers is to develop nanostructured porous electrocatalysts featuring, high activities, low cost, ease of synthesis, and widespread availability. In the past decades, electrocatalytic porous functional materials such as zeolitic materials, mesoporous silica, metal–organic frameworks, etc., have been the subject of intense research in the literature.^14–16 Among the numerous porous materials available in the literature, metal–organic frameworks (MOFs) as electrocatalysts have captured the attention of a wide variety of research scientists.

MOFs are a specific class of materials constructed by joining metal-containing units, termed secondary building units (SBUs), with organic linkers using strong bonds to create open crystalline frameworks with permanent porosity.¹⁷ The early foundations of metal–organic frameworks (MOFs) can be traced back to the establishment of coordination chemistry by Swiss chemist Alfred Werner in 1893.¹⁸ Werner proposed the arrangement of coordination complexes as structures comprising a central metal atom surrounded by organic or inorganic ligands, laying the groundwork for future developments. Over the following decades, research into porous supramolecular architectures gradually progressed, eventually leading to the creation of novel hybrid porous crystalline structures in the 1990s.¹⁹ These structures, formed by the coordination of inorganic metal centres with organic linkers, marked a significant milestone. The term “metal–organic framework” (MOF) was introduced in 1995 by Yaghi's group, whose pioneering work on the topic was first published in Nature.²⁰ This was followed by the synthesis of MOF-5, the first known porous MOF with permanent porosity in 1999 which was also reported in Nature.²¹ Since these important discoveries, the number of known MOF structures and published papers concerning MOFs has increased astronomically, attributed to the ever-expanding potential applications resulting from their unique properties and ease of synthesis.²²

In this review, our discussion starts by investigating the numerous opportunities and challenges in the electrocatalytic application of metal–organic frameworks (MOFs) in CO₂ reduction. Attention is focused toward addressing the conductivity and stability problems, and how MOF electrode preparation methods can affect the electrocatalytic performance of MOFs in respect of the CO₂RR. We include a critical view on the lack of consensus on the normalization of catalytic activity—and some practices that can be considered to be incorrect—and the repetitive lack of information regarding post-mortem analysis of the MOFs. Finally, recent progress in utilizing MOFs as electrocatalysts in CO₂ reduction is explored, and we briefly summarize the findings and proposed future outlook for this research field. It is anticipated that this review will serve as useful reference for scientists in chemistry and materials science and interdisciplinary researchers who are interested in electrocatalytic application of MOFs in CO₂ reduction.

2. MOFS as electrocatalysts for CO₂ reduction

The growing interest in the application of MOFs as homogeneous and heterogeneous electrocatalysts in the CO₂ reduction reaction has been attributed to their promising catalytic properties.^23–26 The exceptionally high surface area of MOFs provides plentiful catalytic active sites for surface-related reactions such as adsorption/desorption, insertion/de-insertion, surface redox reactions and heterogeneous catalysis.²⁷ The large pore volume commonly exhibited by MOFs facilitates mass transport of selected substrates to the active sites as well as providing a platform for loading guest species.²⁷ Catalytically active sites, and an understanding of where they originate from, are essential in MOF catalytic applications (Fig. 1). For pristine MOFs, catalytically active sites are often associated with coordinatively unsaturated metal sites, which act as Lewis acids, and dangling acid/base sites from the organic linkers.²⁸

Fig. 1 Metal–organic framework catalyst structure where: (A) is the connecting site for metal clusters and linkers in pristine MOFs, (B) is a site for grafting metal/non-metals in MOFs by modification, (C) is a site for encapsulated species in MOF composites and (D) represents sites generated in MOF derivatives upon thermal/chemical conversion.²⁸ Copyright 2019, Elsevier.

Secondly, the porous nature of MOFs makes it possible in practice to modify them post-synthetically with various catalytically active species, such as molecular catalysts, metal-based nanoparticles (NPs), enzymes, etc., which can create structural defects within the MOFs for efficient catalysis. Thirdly, the catalytically active sites of MOFs can be exposed to chemical/thermal treatment to obtain porous materials, including metal-based compounds, porous carbons and their composites. These MOF derivatives are promising candidates for catalytic application because of their exceptionally large surface area, tunable structures/composites and highly dispersed active sites. An interesting attribute of MOF-derived materials is their ability to retain, to a certain degree, the morphological architecture of the pristine MOF, which upon doping with heteroatoms, can improve the conductivity and the degree of graphitization, resulting in better performance in electrocatalytic applications.^28–30 Based on these merits—tunable porosity, high surface area and modifiable active sites—the electrocatalytic application of MOF-based nanocomposites has become the subject of intense discussion in recent years.^31–35 However, their role as electrocatalysts remains limited by critical challenges that hinder industrial viability. While MOFs provide well-defined catalytic sites, including coordinatively unsaturated metal centres and redox-active ligands, their inherently poor electrical conductivity severely limits charge transfer, requiring additional modifications such as guest-molecule doping or hybridization with conductive supports.

Stability is another major limitation. Many MOFs degrade in electrochemical environments, particularly in aqueous bicarbonate electrolytes, leading to metal leaching or framework collapse. Strategies such as robust metal–ligand combinations and post-synthetic modifications improve durability but add complexity and cost. Moreover, while 2D MOFs, nanocages and defect engineering enhance active site accessibility, these modifications do not at present fully resolve conductivity and stability issues.

Despite their theoretical promise, MOFs in the CO₂RR largely serve as precursors or support materials rather than standalone catalysts. The field must shift toward MOF-derived materials or radically rethink MOF design to address fundamental limitations. Without breakthroughs in conductivity and stability, MOFs will remain an academic curiosity rather than a scalable CO₂RR solution.

2.1. Addressing conductivity challenges in MOFs

Based on the literature reports so far, conductivity in MOFs can range from 10⁻¹⁰ to 10³ S cm^−1 36,37 which is still, unfortunately, far below the electrical conductivity requirement for industrial application in an electrochemical system. The poor electrical conductivity of MOFs has been linked to: (i) the insulating character of the organic ligands, attributable to the lack of free charge carriers; and (ii) poor overlap between π orbitals of the organic ligands and the d orbitals of the metal ions, which present a barrier for charge transfer.⁸ Conductivity has become an important indicator to measure the suitability of MOFs for electrochemical application.³⁸ Since electrical conductivity (σ) in MOFs is dictated by the amount of charge density (n) and mobility (μ) of electrons (e) and holes (h), it is essential to maximize both charge density and mobility of charge carriers (eqn (I)) to achieve high conductivity.

σ = e(μ_en_e + μ_hn_h) (I)

The general requirement for constructing MOFs with high conductivity is to select building blocks with loosely bound charge carriers, otherwise regarded as materials with high charge density. The metal ions, organic ligands or guest molecules in a MOF could serve as sources of charge carriers. Facile charge migration is required for efficient MOF electrocatalysts; otherwise, the reaction would only occur at the catalytic centres in direct proximity to the electrode. Pathways for charge mobility within a MOFmaterial can be described by two important processes, namely: redox hopping and band transport.³⁹

In redox hopping, the charge carriers (such as redox centres) are usually localized at specific sites within the framework which often results in charge hopping between neighbouring sites in MOFs. However, in order to maintain electroneutrality during charge hopping, a counter ion usually must diffuse through the MOF structure, thereby limiting the charge transport.³⁹ Therefore, charge transport via redox hopping is evaluated based on diffusion phenomena where the measurable diffusion coefficient D_e (cm² s⁻¹) for an electron-hopping mechanism in MOFs is related to total redox-active linker concentration C⁰_P, the electron-exchange rate constant k_e (s⁻¹) and average hopping distance d via the following equation:⁴⁰

Hence to optimize charge transport via redox hopping, a MOF must be designed in such a way that its pore size is sufficiently large enough to allow smooth diffusion of the counter ion thereby enhancing MOF conductivity. One strategy to improve charge transport is via post-synthetic incorporation of guest redox molecules into the frameworks.^41,42 These guest molecules form charge transport pathways through guest to guest or guest to framework interactions. Iodine, redox-active molecules and some macromolecules or clusters are popular guest molecules that can be incorporated into the pores of MOFs to improve their conductivities.⁴² For example Kobayashi et al. reported a significant increase in conductivity of Cu[Ni(pdt)₂] from 10⁻⁸ to 10⁻⁴ S cm⁻¹ after I₂ vapor treatment.⁴³ The authors attributed the increase in conductivity to the partial oxidation of the Cu[Ni(pdt)₂] framework induced by I₂ doping. Talin et al. also reported that conductivity of a Cu₃(BTC)₂ MOF thin film improved drastically from ca. 10⁻⁸ S cm⁻¹ to 0.07 S cm⁻¹ when the MOF was infiltrated with tetracyanoquinodimethane (TCNQ) which is a redox-active molecule (Fig. 2).⁴⁴ Based on spectroscopic data and first principles modelling, the authors claimed that conductivity arose from TCNQ guest molecules bridging the binuclear copper paddlewheels in the framework, leading to strong electronic coupling between the dimeric Cu subunits. Unfortunately, the disadvantage of this strategy is the blockage of pore channels resulting in the reduction of the surface area which is a key property for electrocatalysis.⁴² Therefore, care must be taken to choose MOF and guest molecule to minimize these effects. Another early example of this strategy was reported by Kung et al., where their findings shows a great improvement in conductivity of NU-1000 from 9.1 × 10⁻¹² S cm⁻¹ to 2.7 × 10⁻⁷ S cm⁻¹ after loading nickel bis(dicarbollide) into the micropores of the MOF.⁴⁵ In more recent findings by Xin et al., the electrocatalytic activity of MOF-545-Co for the CO₂RR was greatly enhanced after the incorporation of cobaltocene which acts as a potential donor and carrier to enhance the electron density within the MOF structure.⁴⁶ While the pristine MOF-545-Co FE_CO was 55% at −0.8 V vs. RHE, the presence of the guest molecule is reported to improve the FE_CO up to 97% at −0.7 V vs. RHE. This was attributed to the increased conductivity which plays a crucial role in enhancing the electrocatalytic activity of the MOF.

Fig. 2 (a) Fabrication of electrically conductive TCNQ@Cu₃(BTC)₂ via post-synthetic incorporation (b) I–V curve showing the electrical conductivity of pristine Cu₃(BTC)₂ (red), and after infiltration with TCNQ (green), F4-TCNQ (gold) and H4-TCNQ (purple). Reproduced with permission from Talin et al.⁴⁴ Copyright from The American Association for the Advancement of Science, 2014.

The second and more efficient process that can be used in obtaining high charge transport and enhance conductivity in MOFs is through band transport.³⁸ This is achieved by increasing the extent of delocalization between the framework components by introducing mixed valent states in the node/linker, donor–acceptor type interactions, π–π stacking or π-conjugation into the framework which could lead to improved charge propagation.⁴⁷ Carefully selecting atoms with diffuse frontier orbitals, such as softer transition metal ions (Fe, Mn, Co, Ni and Cu) and linkers containing nitrogen or sulfur can result in construction of highly delocalized MOF frameworks.⁴⁸ Sun et al., demonstrated that when a linker 2,5-dihydroxybenzene-1,4-dicarboxylic acid was replaced with 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid in the construction of a Mn-MOF, infinite 1D metal–sulfur chains were created within the framework, prompting charge delocalization and resulting in a dramatic increase in conductivity.⁴⁹ The presence of extended π–π-stacking or π-conjugation in MOFs destined for electrocatalysis have been shown to create a pathway for the efficient delocalization of charge carriers within the framework.⁵⁰ The strategy of extended π–π-stacking was implemented by some researchers to design MOFs with functionalized TTF (tetrathiafulvalene) derivatives, with the π–π stacking of TTF moieties resulting in the formation of highly conductive charge transport columns within the framework, leading to improved conductivity.^51–53 In a similar strategy, incorporation of in-plane π-conjugation into MOF frameworks has been discovered to induce charge delocalization in 2D MOFs.^54–56 Of particular interest is the work of Zhu et al. where they reported that 2D MOF Cu-BHT (BHT = benzenehexathiol) displays great charge delocalization due to extended 2D π-conjugation, prompting conductivity as high as 1580 S cm⁻¹ at room temperature (Fig. 3).³⁶ Overall, findings have shown that π-conjugation and π–π stacking strategies have resulted in the largest increase in conductivity.^37,57

Fig. 3 (a) Electrically conductive Cu-BHT 2D MOFs designed by π-conjugation (b) electrical conductivity measurement of Cu-BHT film as a function of temperature ranging from 2 to 300 K. Reproduced with permission from Zhu et al.³⁶ Copyright Springer Nature, 2015.

The reports on conductive MOFs for electrocatalysis have thus been encouraging so far. However, opportunities still abound in developing conductive MOFs with optimal electrocatalytic performance by carefully selecting ligand, metal centre and guest molecule that will promote redox hopping and/or band transport within the MOFs. It is important to notice that the vast majority of the reports, if not all, have provided information on the conductivity of pristine or post-synthetic modified MOF catalysts, however, information on the conductivity of the catalyst after the electrochemical experiments is missing. Such information is key to understanding the changes in MOFs under operating conditions, in particular with regard to stability and changes in the product selectivity.

It is important to note that the correlation between the conductivity and stability of metal–organic frameworks (MOFs) and their performance in the CO₂ reduction reaction (CO₂RR) is a critical aspect for their practical application as electrocatalysts. Conductivity in MOFs, whether achieved via redox hopping, band transport or guest-promoted pathways, is essential for efficient electron transfer during electrocatalysis. Enhanced electrical conductivity reduces charge transfer resistance, facilitating the activation of CO₂ and stabilizing key reaction intermediates, thus improving faradaic efficiency and product selectivity. Stability, on the other hand, ensures that MOFs maintain their structural integrity and catalytic activity over extended operational periods. However, many MOFs suffer from degradation under electrochemical conditions due to framework dissolution, ligand hydrolysis, or metal node leaching. It is also important to mention that strategies such as post-synthetic modification, incorporation of conductive linkers, and hybridization with conductive supports have been explored to enhance simultaneously conductivity and stability. The challenge remains in designing MOFs that balance high electrical conductivity with robust chemical stability, ensuring their long-term viability for CO₂RR applications.

2.2. Addressing chemical stability challenges in MOFs

Stability of MOFs during electrochemical operation is another crucial factor for their use in electrocatalytic applications.⁵⁸ During electrocatalysis, the voltage applied induces diffusion of ions/molecules onto the MOF's surface to establish an electrical double layer (EDL) (Fig. 4(a)). This charged MOF surface creates a concentration gradient of ions (H⁺ in this case), causing the local pH in the EDL to drift to higher or low values resulting in complete or partial destruction of the MOF structure (Fig. 4(b) and (c)).⁵⁹ Therefore, one of the main challenges to overcome in the CO₂RR on MOFs is to deliver materials that are stable in neutral and strongly alkaline media. Additionally, during potential cycling (Fig. 4(d)) chemical gradients of cation/anion at the MOFs electrochemical interface can result in degradation of the metal nodes and organic linkers (Fig. 4(e)) resulting on transformation of the MOFs into metal oxides, hydroxides or oxyhydroxides during catalysis.^58,60,61

Fig. 4 (a) Established EDL structure on the surface of a MOF particle. (b) Diffusion of ions/molecules into a MOF framework in the established EDL. (c) Cross-sectional illustration of a MOF-coated electrode in neutral electrolyte. (d) EDL formation and ion diffusion on the surface or inside the pores of MOFs (right) due to the dynamic potential sweeping (left). (e) Structural destruction of MOFs due to the redox reaction of metal nodes and/or organic linkers. Reproduced with permission from Zheng et al.⁶² Copyright American Chemical Society, 2021.

While it might be challenging to address both the chemical and electrochemical stability of MOFs without some form of trade off, research has progressed over the years on how to improve MOF stability in electrocatalytic applications.^63,64 There remain questions which are still the subject of discussion regarding the role of MOFs during electrocatalysis: are MOFs the real catalysts? Or are the MOFs merely the precursors to active phases produced during electrochemical treatment? It is therefore important to understand the reactivity of MOFs in the chemical and electrochemical environment, and confirm, either in situ or post mortem, their chemical and structural integrity in order to design and construct stable MOFs for electrocatalytic application. The chemical environment of MOFs during electrochemical reaction comprises water, counter ions, reactants and products from electrocatalysis, as well a flow of electrons owing to the applied potential. Generally, water has the tendency to interact with metal-linker coordination sites, prompting the decomposition of the framework, producing hydrated/hydroxide metal species (in basic electrolytes) and/or protonated linkers (in acidic electrolytes) that further diffuse into the electrolyte.^65,66 Aside from hydrolysis of MOFs, the effect of counter ions has also been documented to be destructive. For example, phosphate-buffered (PB) solution contains PO₄³⁻ anions (hard Lewis base) which can strongly interact with high valency metal ions. An example of this effect has been found when using the UiO-66 framework, which disintegrates within a few minutes in PB solution as a result of the strong interaction of PO₄³⁻ anions with the Zr⁴⁺ nodes of the UiO-66 framework.⁶⁷ Zeolitic imidazolate frameworks (ZIFs) are a class of porous materials characterized by their zeolite-like topology and imidazolate linkers. Mirsaidov's group demonstrated the lack of stability of ZIF-8 in Co²⁺-containing solution resulting in the formation of ZnCo(OH)_x.⁶⁸ The products from electrocatalysis have not been given much attention as a possible cause of instability of MOFs. For example, it has been shown that nanobubbles can be generated at the active sites during gas evolution reactions which can pose both mechanical and chemical challenges to the structural integrity of MOFs.^69,70 Other products, such as formate/CH₃OH from the CO₂ reduction reaction (CO₂RR) and NH₄⁺ from the nitrogen reduction reaction (NRR), may pose a similar threat.⁶²

Due to the poor stability, some MOFs could show poor performance upon exposure to moisture which can attack the metal centre within the framework, causing decomposition and phase transformation of the MOF. Some reports have anticipated that the bond strength between the metal cation and the organic ligands is one of the main criteria that influences stability.^71–74 Therefore, the chemical stability of a MOF can be intrinsically improved by judicious selection of metal nodes and design of linkers.⁷⁵ De novo synthesis of MOFs by linking carboxylate-based ligands (hard bases) with high-valent metal ions (hard acids) has been explored in search of MOFs with improved stability. Based on the Hard–Soft Acid–Base Principle (HSAB) principle, high-valent metals with high charge density (hard acids), such as Zr⁴⁺, Cr³⁺, Al³⁺, Fe³⁺, etc., tend to form strong coordination bonds with O donor ligands (hard bases) (Fig. 5) thereby presenting MOFs with good chemical stability.^76–79 Interestingly, the strong coordination bonds and the low pK_a of carboxylate linkers endow the high-valent metal ion-containing MOFs with a good level of stability in acid.

Fig. 5 Some representative O donor ligands, with their pK_as. Reproduced with permission from Ding et al.⁶³ Copyright Royal Society of Chemistry, 2019.

On the other hand, low-valent metal ions, including Zn²⁺, Co²⁺, Ni²⁺, Fe²⁺ and Ag⁺, which are considered as soft acids can form strong coordination bonds with suitable N-containing linkers (soft bases) (Fig. 6) producing a highly stable MOFs.^80–82 The high pK_a of azoles and the strong coordination bonds usually bestows remarkable stability in basic solutions on these MOFs. Generally speaking, MOFs built from high-valent metal ions and carboxylate ligands tend to be stable in acids and decompose in basic solutions while MOFs constructed using low-valent metals and azolate linkers tend to be stable under alkaline conditions while being readily hydrolysed by acids.⁸³ The introduction of hydrophobic/rigid ligands, and phosphonate/carboxyphosphonate ligands into MOF synthesis has also been suggested as a way to improve MOF stability.^84,85 Surface hydrophobicity prevents the adsorption of water into pores and/or the condensation of water around the metal clusters, which enhances the MOF stability in the presence of water. Phosphonate/carboxyphosphonate MOFs have been shown to exhibit robust structural tolerance in harsh chemical environments,⁸⁶ which has been attributed to the stronger metal–ligand bonds formed when the three oxygen atoms from phosphonic acid coordinate to metal ions compared to the corresponding carboxylate MOFs.

Fig. 6 Some representative N donor ligands, with their pK_as. Reproduced with permission from Ding et al.⁶³ Copyright Royal Society of Chemistry, 2019.

The challenge of electrochemical stability can be approached by optimizing the various electrocatalyst parameters. For example non-aqueous electrolytes such as tetrabutylammonium hexafluorophosphate (TBAPF₆) in acetonitrile or DMF, have been used as substitutes in studying the catalytic activity of MOFs that are unstable in aqueous electrolytes.⁸⁷ The implementation of organic solvents as electrolytes can also be linked to their ability to increase the solubility of CO₂ which can, as a result, reduce mass transport limitations.⁸⁸ In addition, the low proton concentration inhibits the competing process of the hydrogen evolution reaction which is prevalent in aqueous electrolyte. Nevertheless, aqueous electrolytes remain the electrolytes of choice as water is a green and abundant solvent that can be used in large scale processes.⁸⁹ Therefore, the use of catalytically active MOFs stable in the presence of bicarbonate ions is necessary for future applications. MOFs constructed with nitrogen-containing linkers such as ZIFs have been found to be highly stable in water over a wide pH range and this family of MOFs has consistently shown promising activity towards the CO₂RR in aqueous electrolytes.^80,90,91 On a final note, MOFs under electrochemical conditions must meet two additional stability requirements: (i) maintaining physical attachment to the electrode surface, and (ii) remaining structurally intact under reducing and/or oxidizing conditions during electron flow through the framework.⁸³ The likelihood of meeting the first stability challenge—the potential for delamination or generally loss of MOF from an electrode surface—depends to a large degree on how the electrode is prepared. The second stability issue of resisting degradation under electron flow while in the presence of substrate is a much more MOF-specific challenge. MOFs constructed from ligands that are able to give reversible electrochemical responses are often considered to be stable under electrochemical conditions. However, a significant number of reports have not performed adequate and exhaustive characterization of the MOF after electrochemical experiments. It should be expected and common practice to evaluate the structural and compositional integrity of the MOF catalyst after electrochemical experiments including: high-resolution scanning transmission electron microscopy (HR-STEM), energy-dispersive X-ray spectroscopy (EDX) and, if possible, in situ X-ray absorption spectroscopy XAS experiments to confirm the oxidation state and coordination of the metal centres. Numerous electrochemically stable MOFs have been successfully built from ligands based on a naphthalene diimide core,^92–94 a moiety that seems to be stable when reduced. For more in-depth discussion, readers are directed to a recent review by Liu et al.⁹⁵ Future studies focusing on the structural evolution of MOFs under CO₂RR conditions are fundamental to establish structure–activity correlations and to understand the different effects that contribute to the stability of MOF-based catalysts.

2.3. Addressing the product selectivity in MOFs

Achieving precise control over product selectivity in the CO₂ reduction reaction (CO₂RR) on MOF-based electrodes is essential for optimizing their practical applications in carbon-neutral fuel and chemicals production. While MOFs inherently offer tunable catalytic environments, their effectiveness in selectively generating desired products depends on a combination of structural modifications, electronic tuning and reaction environment optimization. However, as observed in other CO₂RR catalysts, product selectivity is highly sensitive to mass transport phenomena, including CO₂ diffusion, intermediate adsorption, and proton availability at the electrode interface.^96–98 The interplay between catalyst structure and reaction conditions must be carefully controlled to mitigate transport limitations that could otherwise lead to competing side reactions, such as hydrogen evolution. Additionally, the intrinsic porosity of MOFs plays a crucial role in governing mass transport, as pore size and connectivity dictate the diffusion of CO₂ molecules and reaction intermediates to active sites. While highly porous MOFs offer increased surface area and active site accessibility, excessive porosity or narrow pore channels can hinder CO₂ diffusion, leading to local variations in reactant concentration and altering selectivity.

Regulating the selectivity of products in the CO₂RR process on MOF electrodes requires a combination of material design and reaction environment optimization. One of the most effective strategies involves tailoring the organic ligands that coordinate with the metal centres. The choice of ligands influences the electronic properties and coordination environment, which in turn affects the binding strength of CO₂ and its reaction intermediates.² For example, 2-methylimidazole coordinated to Zn²⁺ in ZIFs has been shown to favor CO₂ reduction to CO, while other ligands may steer the reaction toward formate or hydrocarbons.⁹¹

Another powerful approach is doping MOFs with electron-donating molecules, such as 1,10-phenanthroline or metallocenes.⁹⁰ These additives enhance CO₂ adsorption and reduce the Gibbs free energy barrier for key reaction steps, making it easier to direct the reaction pathway toward a specific product. Similarly, structural modifications to MOFs can significantly impact selectivity. By reducing MOFs to 2D nanosheets, a larger proportion of active metal sites are exposed, improving electron transfer and influencing reaction intermediates.^99,100 Introducing strain or defects further modifies the electronic structure, fine-tuning the catalytic behaviour and favouring certain products over others.

The way MOFs are integrated into electrodes also plays a crucial role in selectivity. Growing MOFs directly on conductive substrates improves charge transfer efficiency, ensuring more effective electron flow to the catalytic sites. Additionally, combining MOFs with conductive additives such as carbon black or graphene oxide further enhances charge transport and helps control product distribution. However, mass transport limitations in porous MOFs must be carefully managed, as restricted CO₂ diffusion within the framework can lead to selectivity shifts, favouring hydrogen evolution over CO₂RR. Pore engineering strategies, such as optimizing pore size distribution or introducing hierarchical porosity, can help balance CO₂ diffusion rates and reactant accessibility, preventing local reactant depletion or oversaturation, both of which affect selectivity. Studies have shown that using imidazolium-based ionic liquids in electrolytes can promote the formation of methane,¹⁰¹ while other electrolytes may favor CO or formic acid production.^102–104

Finally, the construction of heterogeneous MOF structures, such as MOF heterostructures or composites with metal oxides, introduces synergistic effects that influence reaction pathways. By carefully designing these hybrid systems, it is possible to steer CO₂ reduction toward higher-value products like hydrocarbons or alcohols. Together, these strategies offer a comprehensive approach to improving product selectivity in CO₂ reduction on MOF electrodes, opening new possibilities for efficient and sustainable carbon conversion.

3. Assessing the performance of various methods of MOF electrocatalyst preparation

Electrochemical evaluation of MOFs requires interfacing the MOF with an electrode. Finding an effective method of preparing MOF-modified electrodes play an important role in the attachment of the MOF catalyst to the electrode and by extension determines the MOF electrochemical response, specifically regarding the rate of electron transfer between the electrode and the first MOF layer as well as how much of the bulk MOF is electrochemically accessible.⁸³ The type of MOF-electrode attachment can also have a large impact on the strength of mechanical adhesion, with poor adhesion resulting in delamination.^83,105 Two approaches have generally been adopted in preparing MOF-modified electrodes for the CO₂RR and other applications. The first entails immobilizing the MOF on a conductive substrate via physical interactions or through the formation of chemical bonds between the substrate and the MOF. The second method involves synthesizing MOFs directly on the electrodes. MOFs for electrochemical studies are generally prepared by four methods: drop-casting, direct solvothermal growth on a bare electrode surface, self-assembled monolayer deposition on the electrode surface or electrochemical methods. Each of these methods has different characteristics of mechanical stability, preparation time, and ease of electron transfer between the MOF and conductive electrode substrate. Despite the diversity of synthetic approaches used for preparing MOF-modified electrodes, some of them, like direct growth on conductive substrates are yet to be fully explored for preparing MOF electrodes for the CO₂RR unlike other electrochemical reactions such as the OER and ORR.¹⁰⁶

3.1. Drop casting

Drop casting is one of the facile methods for immobilizing MOFs on a conductive electrode for electrochemical studies. In this case, a catalyst ink is usually prepared by dispersing (typically by sonication) MOF powder into a solvent or solvent mixture. The electrode is either left to dry in open air or oven dried at 60 °C, leaving behind a thin film of the MOF dispersed on the electrode for electrochemical analysis (Fig. 7(a)). Additives such as carbon black and graphite can be added to the catalyst ink to increase the MOF electrical conductivity,^107–109 while polymeric binders such as Nafion can be added to improve catalyst immobilization on the electrode.¹¹⁰ However, it is also possible for the electrochemical system to be contaminated by these additives, thereby increasing resistance and possibly blocking some MOF active sites, leading to a decrease of the desired catalytic activity or even change in selectivity.¹¹¹ Common working electrode substrates, where the drop casting takes place, include glassy carbon (GC), carbon paper and fluorine-doped tin oxide (FTO).^{90,100,110,112–114} However, despite the simplicity of this method of preparing MOF electrodes, controlling the resulting films’ thickness and morphology is challenging.⁸³ As a result of their poor stability, MOF films prepared by drop casting can only be used as electrodes for fundamental studies and are not suitable for use in electrolysers. Alternatively, MOF-based gas diffusion electrodes (GDEs) prepared by airbrushing catalyst ink on porous carbon paper have been proposed to overcome the above challenges (Fig. 7(b)).^115,116 In GDEs, since CO₂ gas is fed directly on one side of the electrode, the possible hindrance caused by the low solubility of CO₂ in the electrolyte is reduced. The work of Albo et al.¹¹⁷ and Sikdar et al.¹¹⁵ have used MOF-based GDEs in the CO₂RR with some promising results in terms of high selectivity towards hydrocarbon formation.

Fig. 7 MOF-based electrode construction by (a) drop casting (b) spray coating methods.

3.2. Direct solvothermal synthesis

In this method, direct growth of MOFs on conductive substrates is achieved by placing the electrode in a solvothermal reactor with the standard MOF precursors and heating for the reaction to take place (Fig. 8).^83,118 The direct solvothermal synthesis method has a significant advantage over drop casting which is that the resulting MOF is directly attached to the electrode surface without the need for extra additives that might interfere with the electrochemical performance. During synthesis, the rugosity of the surface of the electrode itself provides nucleation sites for MOF crystallization. To achieve effective MOF binding, the electrode is usually pre-treated in dilute linker, creating a monolayer, which provides the binding sites for the first secondary building unit (SBU) layer and subsequently initiate further growth.¹¹⁹ Similarly to the drop casting method, GC, FTO glass and metal foams are commonly used as electrode substrates.

Fig. 8 MOF-based electrode construction by direct solvothermal growth.

Before a MOF is grown on a carbon electrode, the electrode is usually pre-treated to create functional groups such as carboxylate groups that facilitate nucleation and subsequent MOF growth. As for FTO and ITO substrates, the metal oxides of the electrode provide centres for the nucleation of MOF precursors, facilitating MOF growth on the FTO and ITO, resulting in a uniform MOF thin film of nanocrystals.¹²⁰ The main drawback of these substrates, apart from the cost, is their reduced conductivity when compared with carbon electrodes.⁸

3.3. Liquid phase epitaxy (LPE)

The liquid phase epitaxy method for the preparation of MOF electrodes was first proposed by Shekhah et al.¹²¹ The first step of the method involves the pre-coating of the substrate with self-assembled monolayers (SAMs) of organic molecules bearing carboxylate or pyridine moieties, which later will serve as nucleation sites.¹²² Then the substrate is immersed sequentially into different solutions of MOF precursors in a layer-by-layer fashion, producing a highly orientated and homogeneous MOF thin film directly over the support (Fig. 9).¹¹⁸ This strategy is also known as SURMOF (surface-bound metal–organic framework) in the literature.

Fig. 9 MOF-based electrode construction by liquid phase epitaxy (LPE).

Although this technique has useful advantages like control of orientation, tunable thickness and strong adhesion to the substrate surface,^123,124 the resulting SAM could also constitute an insulating layer between the electrode surface and the MOF, resulting in an increase of the electrical resistance, thus diminishing the electrochemical performance of the MOF-modified electrode.¹²⁵ Since its inception over a decade ago, there has been extensive research work on surface-mounted metal–organic frameworks (SURMOFs) and related epitaxial growth methods,^126,127 however, few SURMOFs prepared by LPE have been tested as electrocatalysts.¹²⁴ This, however, is a synthesis method that could be further explored since the strong bond between the MOF and the substrate could be advantageous for long-term electrolysis. It may also be possible to leverage the highly oriented nature of SURMOFs to tune their morphology and thickness in order to promote mass and charge transport through the MOF and possibly enhance catalytic performance.

3.4. Electrochemical synthesis

Two approaches have been identified for preparing MOF thin films on substrates for electrochemical application: direct and indirect electrosynthesis. With direct electrosynthesis, the process can be anodic or cathodic. On the other hand, electrophoretic deposition is a popular indirect electrosynthesis method used in depositing MOF thin films on conductive substrates (Fig. 10). The characteristics of these methods are summarized in Table 1.

Fig. 10 MOF-based electrode construction by electrophoretic deposition. Reproduced from Liu et al.¹²⁸ Copyright 2021, Wiley-VCH.

Table 1 Summary of electrochemical synthesis methods

Direct electrosynthesis		Indirect electrosynthesis
Anodic electrosynthesis	Cathodic electrosynthesis	Electrophoretic deposition
• Anode electrode is immersed in an electrolyte which contains the ligand	• Anode and cathode electrodes are immersed in an electrolyte which contains MOF precursors	• Two similar electrodes acting as the anode and the cathode are immersed in a cell containing MOF suspension and supporting electrolyte
• As potential is applied, electrode (metal) is oxidized to metal ions	• MOFs are formed directly on the cathode electrode surface by an electrochemical reaction^129	• After applying a potential, the MOF diffuses into the electrolyte and becomes deposited onto the oppositely biased electrode surface depending on the partial charges on the MOF (Fig. 13)^130
• Metal ions react with ligand to form a thin-film MOF on the anode electrode surface	• The electrodes merely act as a source of electrons and surface for nucleation and MOF growth
• MOF properties are independent of the metal precursor since no metal salts are used^131	• MOFs are generated at lower potential (1.5–10 V) and within few minutes at room temperature	• Higher applied potential and longer time (typically 50–100 V during 1–3 h) are required to produce MOF films
• Different metal oxidation states can be generated at different potentials which lead to a better control over the MOF properties^129	• Two different electrodes act as anode and cathode	• Longer deposition time is advantageous as it allows control of MOF electrode properties e.g. MOF film thickness and particle–particle connectivity at FTO surfaces^130
• MOFs are generated at high potential (12–19 V) and longer time

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The first MOF synthesized by anodic dissolution was HKUST-1. In this method, a potential of 12–19 V was applied for 150 min to a Cu anode which oxidized to Cu²⁺ ions in a BTC linker containing solution (Fig. 10). The chemical reaction between the Cu ions and the BTC linker resulted in the formation of an HKUST-1 layer on the electrode surface.¹²⁹ Since its first deployment for HKUST-1 synthesis, many other MOFs have been produced using electrochemical synthesis.^132–136 Dinca et al. reported the first MOF synthesized by cathodic deposition producing Zn₄O(BDC)₃ (MOF-5).¹³⁷ The synthesis was carried out by immersing FTO electrode in MOF-5 precursors and (NBu₄)PF₆ as conducting electrolyte (Fig. 11). As a constant cathodic potential of −1.6 V was applied for 15 min at a current density of −6.6 mA cm⁻², the probase (P) became reduced to a base equivalent (B) which increased the local pH near the electrode surface.¹³⁸ This change in pH induced deprotonation of the ligand (H₂BDC), which further coordinated with metal ions and eventually triggered precipitation of MOF-5 crystals.

Fig. 11 (a) General scheme for the cathodically induced electrochemical deposition of MOFs, involving the reduction of probase (P), the generation of base equivalents (B), the deprotonation of ligands (H₂BDC), and MOF crystallization from BDC²⁻ and metal ions (Zn²⁺). (b) Schematic illustration of the formation of a biphasic mixed film. Reproduced from Li et al.¹³⁸ Copyright 2014, Royal Society of Chemistry.

It has been proposed that electrochemical synthesis has the potential to produce more CO₂RR efficient MOF-modified electrodes. For example, Kang et al.¹³⁹ synthesized MFM-300 on indium foil electrodes by immersing in a solution containing the organic linker and supporting electrolyte, and the MOF-modified electrode system was found to be catalytically active for the CO₂RR, exhibiting a current density of 46.1 mA cm⁻² at applied potential of −2.15 V vs. Ag/Ag⁺, demonstrating an faradaic efficiency of 99.1% for the formation of formic acid. Similarly, Hod et al.¹²⁵ deployed the electrophoretic deposition method to obtain a Fe–porphyrin MOF thin film on an FTO electrode which showed unprecedented catalytic activity towards the CO₂RR, generating a mixture of CO and H₂ at 100% faradaic efficiency. However, despite these promising results, not many MOF catalysts for the CO₂RR prepared by electrochemical synthesis have been reported, and this is an apparently promising path that remains largely unexplored.

Table 2 provides a concise summary of the various MOF-based electrode preparation methods discussed in the manuscript. It highlights their key characteristics and associated remarks, offering a comparative perspective on their advantages and limitations for CO₂RR applications.

Table 2 Summary of MOF-based electrode preparation methods, highlighting their key characteristics and associated considerations for CO₂RR applications

Method	Characteristics	Remarks
Drop casting	Simple, fast, limited control over film thickness	Prone to delamination, poor stability
Gas diffusion electrode (GDE)	Improved CO2 mass transport, better selectivity	Improved dispersion on the support
Solvothermal growth on electrode	Strong adhesion, high crystallinity	Ensures better electron transfer and durability
Electrophoretic deposition	Uniform coverage, good mechanical stability	Enables precise control over film thickness, enhanced conductivity
Thermal treatment (MOF-derived catalysts)	Generates defect-rich, conductive structures	Improved conductivity, but may alter active sites
Electrochemical synthesis	Direct deposition, controlled morphology	Improved stability

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4. Critical review of activity metrics for benchmarking electrocatalyst performance

The unprecedented growth witnessed in the field of MOFs has resulted in a rise in papers directly using MOFs in electrocatalysis.^140,141 Despite the increasing number of reports, a standardized method for determining the electrocatalytic performance and true electrochemical surface area of MOFs has not been fully established.¹⁴² Many researchers on MOF electrocatalysts summarize and compare their results, however, the comparison of different experimental data from different laboratories remains problematic owing to a lack of standardization in the measurement procedure and reference system.^143,144 Therefore, a correct and convenient activity metric that can reasonably normalize the electrocatalytic performance is needed, to provide a basis for comparing the performance of different catalysts.

Several activity metrics have been utilized to report selectivity and electrocatalytic activity. One commonly used metric for selectivity is faradaic efficiency, which is defined as the fraction of faradaic charge utilized to produce a given product. Despite its utility in describing selectivity, faradaic efficiency is problematic when comparing catalysts with different activities. As an example, one might conclude that a catalyst that produces a specific product more selectively is also more active in producing that product. However, it is possible for selectivity toward a product to increase without increase in production rates of that product. Therefore, true differences between two catalysts can be obscured if they are only compared on the basis of faradaic efficiencies.¹⁴⁵ The rate of product generation, which is proportional to its partial current density, is a much less ambiguous descriptor of catalytic activity.

As for electrocatalytic activity, the most important activity metric used to benchmark the performance of a catalyst is exchange current density (j₀).¹⁴⁶ However, this must be normalized to provide a common ground for comparing the performance of catalysts. Normalized current densities may be described as: (i) geometric activity i.e. the current density normalized to the geometric area of electrode (mA cm_geo⁻²), (ii) specific area activity i.e. the current per surface area of electrocatalyst (mA cm_catalyst⁻²) and (iii) mass activity i.e. the current per loading mass of electrocatalysts (A g_catalyst⁻¹).

In geometric normalization, the current is normalized to the geometric area of electrode. However, geometric activity does not necessarily reflect a catalyst's intrinsic activity. The geometric activity fails to consider that the electrocatalytic reaction is a surface process, where only the surface active sites participate in the reaction.¹⁴⁷ On the other hand, the geometric activity is largely dominated by the loading mass of catalysts. It has been commonly observed that with more catalyst loading, the overpotential to reach the given geometric surface area normalized current density (i_geo) certainly becomes smaller.^148,149 Most electrocatalysts studied these days are nanostructured and have a large surface area with very high roughness factors, thus current normalization based on the geometrical area of the electrode is not appropriate and should ideally be avoided.¹⁵⁰

Since electrocatalysis is a surface reaction where the adsorption/desorption of reactants/products take place only at the surface of the catalyst, the intrinsic electrocatalytic activity is best described using specific activity which is defined as the current divided by the surface area of the catalyst (mA cm⁻² catalyst).¹⁵¹ Specific activity is estimated by normalizing the current density by either the number of catalyst active sites or electrochemically active surface area.¹⁴⁷ While normalization to the number of active sites is the preferable metric,^142,152,153 in heterogeneous systems such as MOFs, it can be difficult to identify what actually the active site is. Extensive and detailed in situ spectroscopic studies will be required for such electrocatalysts. Therefore, normalizing the measured activity by the electrochemically active surface area is a more rational way to normalize catalytic activity.¹⁴⁴ Previous reports have proposed that the electrochemically active surface area (ECSA) of the catalyst can be determined by measuring the double-layer capacitance of the electrode–electrolyte interface. The double layer capacitance can be measured by conducting cyclic voltammetry (CV) in a potential range where no faradaic processes occur, typically a 100 mV window centred at the open circuit potential (OCP). In this potential region, any measured current can be ascribed to the non-faradaic process of charging the electrochemical double layer. The obtained C_dl is divided by the specific capacitance (C_s) of the material studied, which is not usually determined as part of the same experiment but taken from literature sources.¹⁴² Unfortunately, poor practice of this method has led to misuse of the specific capacitance of the material. To determine the specific capacitance of a material, the C_dl of the material should be measured on an electrode with a well defined geometrical area in order to obtain a value of C cm⁻². As this is not usually the case, researchers are commonly tempted to use a generic value of C_s which inevitably causes a major error in the calculation of the ECSA.

Mass activity may serve as a reasonable parameter for evaluating intrinsic performance of different electrode materials in bulk chemical processes, such as lithium ion battery reactions, where the Li⁺ diffuses deeply into the material and the original micro-structure cracks on charge/discharge cycles.¹⁵⁴ However, for the surface chemistry processes such as electrocatalysis, where the reaction occurs only at/near the surface of the electrode, the mass activity does not represent the intrinsic activity.¹⁵¹ The definition of mass activity assumes that all atoms within each particle are electrocatalytic active sites, which is in conflict with the fact that the bulk atoms do not contribute to the electrocatalytic process. As a result, the mass activity is largely dependent on the particle size (or equivalently, surface area of catalyst), which reflects the fraction of surface atoms. Usually, catalysts of smaller particle size give higher mass activity, because smaller sized particles possess a larger ratio of surface atoms to the total atoms per mass and give a larger number of electrocatalytic active sites. As the particle size is decreased, most of the catalyst surface becomes active and the normalization by mass becomes more accurate. In addition, normalizing the catalytic activity per unit mass of catalyst could provide the end-users with a cost of operation. In this regard, the normalization can also be done per weight of the metal centre e.g., Cu, Ni or Zn.

Many claims have been made regarding the performance of MOF electrocatalysts for the CO₂RR which have been subject to criticism, especially by more experienced electrochemists. It is argued that comparing the activity metric of one MOF catalyst to another without recourse to a method of normalization is not only misleading but can result in over-rating catalyst performance. A survey of MOF electrocatalysts for the CO₂RR has been carried out in an effort to identify the method of current normalization and expose errors in comparison of the electrocatalytic activities of different MOFs (Tables 3–5). First, some authors failed to clearly present the method they used to normalize their current, as they compared the activity metric of their catalyst with other catalysts from different laboratories, for example,^112,155,156 presenting the performance of their MOF electrocatalyst for CO₂ reduction with no clear idea of the type of surface area used in normalizing the catalyst current. Misinformation of this sort has made it impossible to compare the performance of these catalysts objectively with other MOF electrocatalysts for CO₂ reduction.

Table 3 Summary of pristine MOF materials as electrocatalysts for CO₂ reduction

Electrocatalyst	Synthetic method	Electrode potential	FE (%)	Peak jtotal (mA cm^−2)	Current normalization	Main product	Electrolyte	Ref.
CR-MOF	Solution self-assembly	−0.78	98	∼7.1	GSA	HCOOH	0.5 M KHCO3	102
HKUST-1	Room temperature synthesis	−2.5 vs. Ag/Ag^+	51	19	N/A	Oxalic acid	0.01 M TBATFB in DMF	112
Al2(OH)2TCPP-Co	Microwave	−0.7 V vs. RHE	76	∼1	N/A	CO	0.5 M KHCO3	103
Fe_MOF-525	Electrophoretic deposition	−1.3 vs. NHE	50	4	GSA	CO	1 M TBATF6 in DMF	125
Cu2(CuTCPP)	Vigorous stirring at temperature of 85 °C	−1.55 V vs. Ag/Ag^+	68.4	1.0	GSA	Acetate	CH3CN with 1 M H2O and 0.5 M EMIMBF4	100
Cu2(CuTCPP)	Vigorous stirring at temperature of 85 °C	−1.40 V to −1.65 V	38.8% to 85.2%	3.5	GSA	Formate	CH3CN with 1 M H2O and 0.5 M EMIMBF4	100
HKUST-1	Electrophoretic deposition	−2.2 vs. Ag/Ag^+	80	3	ECSA	CH4	BMIMBF4	157
HKUST-1	Solvent-free synthesis by oven heating	−0.9 vs. Ag/Ag^+	15.9	10	GSA	CH3OH and C2H5OH	0.5 M KHCO3	158
ZIF-8	Room temperature synthesis under vigorous stirring	−1.14 vs. SCE	65	∼3	N/A	CO	0.5 M NaCl	155

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Table 4 Summary of MOF composite as electrocatalytic for CO₂ reduction

Electrocatalyst	Synthetic method	Electrode potential	FE (%)	Peak jtotal (mA cm^−2)	Current normalization	Main product	Electrolyte	Ref.
Ligand doped ZIF-8	Room temperature synthesis	−1.2 V vs. RHE	90	10.1	N/A	CO	0.1 M KHCO3	156
Cu^II/ade-MOFs	Room temperature synthesis	−1.4 V vs. RHE	45	8.5	ECSA	C2H4	0.1 M KHCO3	159
Cu^II/ade-MOFs	Room temperature synthesis	−1.4 V vs. RHE	50	15	ECSA	CH4	0.1 M KHCO3	159
2D c-MOF	Solvothermal synthesis	−0.7 V vs. RHE	88	8	N/A	CO	0.1 M KHCO3	160
Re-SURMOF/FTO	Liquid-phase epitaxy	−1.6 V vs. NHE	93 ± 5	>2	N/A	CO	0.1 M TBAH in CH3CN + 5% CF3CH2OH	124
M-PMOF	Hydrothermal	−0.8 V vs. RHE	98.7	38.9	GSA	CO	0.5 M KHCO3	104
FeTCPP-UiO-66	Thermal treatment	0.45	100	1.3	ECSA	CO	0.1 M KHCO3	161
Cu2O@Cu-MOF	Hydrothermal	−1.71	63.2	−14	ECSA	CH4	0.1 M KHCO3	162
Cu-SIM NU-1000	Heat treatment	−0.82	28	−1.2	GSA	HCOO^−	0.1 M NaClO4	163
Ag2O/layered ZIF composite	Refluxed at high temperature	−1.2 V vs. RHE	80.5	26	ECSA	CO	0.25 M K2SO4	164
TCPP(Co)/Zr-BTB	Thermal treatment	−0.769 vs. RHE	85.1	6	GSA	CO	0.5 M KHCO3	165
PCN-222(Fe)	Thermal treatment	1.2	91	−0.6	GSA	CO	0.5 M KHCO3	107
HKUST-1 mediate Cu	Thermal treatment	−1.07 V vs. RHE	45	262	GSA	C2H4	1 M KOH	166

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Table 5 Summary of MOF-derived materials as electrocatalytic for CO₂ reduction

Electrocatalyst	Electrode potential	FE (%)	Peak jtotal (mA cm^−2)	Current normalization	Main product	Electrolyte	Ref.
OD Cu/C-1000(Cu-based HKUST-1)	−0.1 V vs. RHE	45.2–71.2	∼8.9	GSA	CH3OH and C2H5OH	0.1 M KHCO3	167
ZIF/MWCNT	–0.86 V vs. RHE	100	7.7	ECSA	CO	0.1 M NaHCO3	168
ZIF-8 to Ni SAs/N-C	−1.0 V vs. RHE	71.9	10.48	GSA	CO	0.5 M KHCO3	169
Co/Zn ZIFs derived Co-N2	−0.8 V vs. RHE	94.7	18.1	N/A	CO	0.5 M KHCO3	170
C-AFC©ZIF-8	−0.43 V vs. RHE	93	2.8	N/A	CO	0.1 M KHCO3	171
Ni SAs/NCNTs	−1.0 V vs. RHE	95	57.1	GSA	CO	0.5 M KHCO3	172
ZIF-8 derived NC	−0.5 V vs. RHE	95.4	1	N/A	CO	0.5 M KHCO3	173
ZIF-8 derived NC	−0.93 vs. RHE	78	1.1	N/A	CO	0.1 M KHCO3	174
ZIF-8 derived Fe–N–C	−0.6 V vs. RHE	91	17.8	N/A	CO	0.5 M KHCO3	175
ZIF-8 derived Fe-N	−0.4 vs. RHE	90	6.8	GSA	CO	0.5 M NaHCO3	176

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In another scenario, some reports have wrongly compared the catalytic activity of MOF electrocatalysts normalized by geometrical area with other reports whose catalytic activity was normalized by ECSA, and vice versa.^{102,124,125,161,169,177–179} In addition to reports on MOFs catalyst normalized per geometrical area, information on the number of active sites per catalyst loading were not stated in most cases, making it difficult to make a comparative judgement of the activity of the catalysts. For instance, it has been claimed that polyoxometalate-based MOFs (PMOFs) show excellent catalytic activity towards the CO₂RR, however, the catalyst was normalized by geometrical area and quantitative information on the active site loading per surface area was not provided as the entire catalyst cannot be assumed to be active toward CO₂ reduction.¹⁰⁴

5. Recent progress in utilizing MOFs for electrocatalysts for CO₂ reduction

5.1. Pristine MOFs as electrocatalysts for CO₂ reduction

The use of pristine MOFs in catalysing CO₂ reduction was pioneered by Hinogami et al.,¹⁰² who reported the use of a Cu rubeanate framework (CR-MOF) containing coordinatively unsaturated metal sites as catalyst for the CO₂RR to formic acid at 98% selectivity and 30% faradaic efficiency. Kumar et al.¹¹² reported the electrochemical conversion of CO₂ to oxalic acid (H₂C₂O₄) at 90% selectivity, 51% faradaic efficiency on a Cu₃(BTC)₂ (1,3,5-benzenetricarboxylic acid [BTC]) thin film catalyst deposited on a glassy carbon electrode (Fig. 12). In Fig. 12(a), the cyclic voltammogram shows distinct reversible oxidation and reduction of Cu(II) to Cu(I) at 0.14 and 0.02 V vs. SCE, respectively. A reduction peak at 0.5 V vs. SCE and a sharp oxidation peak at 0.102 V vs. SCE indicate the presence of the Cu(0) to Cu(I) redox couple. Cu₃(BTC)₂ exhibited a cyclic voltammetric response indicative of copper being in the Cu²⁺ ionic state, whereas Cu electrodes in 0.1 M KCl solution did not exhibit such behaviour (Fig. 12(a) inset). Fig. 12(b) shows the redox activities of a GCE and Cu₃(BTC)₂ coated GCE in electrolyte without CO₂ bubbling (1 and 3) and in CO₂-saturated 0.01 M TBATFB/DMF (2 and 4), indicating that the direct reduction potential of CO₂ starts at a potential more positive on a Cu₃(BTC)₂ coated GC electrode (−1.12 V vs. Ag/Ag⁺) than a bare GC electrode (−1.75 V vs. Ag/Ag⁺), and also showing that cathodic evolution current density is increased from 2.27 mA cm⁻² to 19.22 mA cm⁻². These two pioneering studies have proven the importance of nature and density of active site (metal centre) and the electronic environment (ligand) in deciding the product selectivity and efficiency.

Fig. 12 (a) CV readings of Cu₃(BTC) + GCE (b) CV scan showing redox activities of (1) GCE, (2) GCE in presence of CO₂, (3) Cu₃(BTC)₂ + GCE and (4) Cu₃(BTC)₂ + GCE in CO₂-saturated 0.01 M TBATFB/DMF. Reproduced from Kumar et al.¹¹² Copyright 2012, Elsevier.

In situ growth of MOFs into a thin layer has been recently explored to control MOF thickness which can enhance mass and charge transfer, thereby promoting higher catalytic activity towards CO₂ reduction. Kornienko et al.¹⁰³ demonstrated this idea by synthesizing a thin-film of cobalt–porphyrin MOF (Al₂(OH)₂TCPP-Co) via atomic layer deposition (ALD) on a transparent conducting fluorine-doped tin oxide (FTO) support. An analysis of this MOF's voltammogram trace showed that it displayed an enhanced current density under CO₂-saturated solutions relative to argon-saturated solutions and an irreversible catalyst peak, producing CO and H₂, with selectivity for CO reaching up to 76% at −0.7 V vs. RHE (Fig. 13(a) and (c)). The stability of the MOF catalyst was tested over an extended period of 7 h at −0.7 V vs. RHE in CO₂-saturated aqueous bicarbonate buffer, reaching a stable current density after several minutes, generating 16 mL of CO with estimated turnover number of 1400 (Fig. 13(d)). According to the author, the thin-layer configuration of Al₂(OH)₂TCPP-Co MOF was observed to enhance the reduction of CO₂ to CO. In another finding by Wu et al.,¹⁰⁰ layering of copper porphyrin MOF (Cu₂(CuTCPP) into nanosheets was reported to enhance the catalytic conversion of CO₂ into formate (FE = 68.4% at −1.55 V) and acetate (Fig. 14). Furthermore, spectroscopic studies showed that Cu₂(CuTCPP) was converted into copper clusters (CuO, Cu₂O and Cu₄O₃) which in turn catalysed the transformation of CO₂ to formate and acetate.

Fig. 13 (a) CV scan of the Al₂(OH)₂TCPP-Co MOF catalyst showing a current increase in a CO₂ environment relative to an argon-saturated environment. (b) Varying scan rate with corresponding redox potentials (c) plot of the faradaic efficiency at the potential range of −0.5 to 0.9 V vs. RHE; (d) MOF stability test with corresponding FE measurement. Reproduced from Kornienko et al.¹⁰³ Copyright 2015, American Chemical Society.

Fig. 14 (a) Crystal structure of Cu₂(CuTCPP) MOF nanosheets; (b) schematic of electrochemical system showing electrocatalytic reduction of CO₂ on Cu₂(CuTCPP) nanosheets. Reproduced with permission from Wu et al.¹⁰⁰ Copyright 2019, Royal Society of Chemistry.

The choice of electrolyte has also been shown to enhance performance and product selectivity of catalytic MOFs toward the CO₂RR. For instance, Kang et al.¹⁵⁷ observed that ionic liquids (ILs) used as the electrolyte were able to influence the selective conversion of CO₂ to CH₄ using Zn-MOF catalyst. Imidazolium based ILs can interact with CO₂ by physical absorption,¹⁸⁰ thereby serving as both robust electrolytes and CO₂ activation promoters.¹⁸¹ The cyclic voltammetry profile shows that the CO₂ reduction activities of Zn-MOF in CO₂-saturated saturated IL 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) gives a current density of 3 mA cm⁻² at reduction peak of about −2.2 V vs. Ag/Ag⁺ while the current density of the N₂-saturated system was negligible (Fig. 15(a)). After 2 h of electrolysis, CH₄ was the dominant product (FE = 80% at −2.2 V vs. Ag/Ag⁺), showing no decrease in current density with time, suggesting that the Zn-MOF electrode and the IL were stable (Fig. 15(b) and (c)).

Fig. 15 (a) CV scans in CO₂-saturated and (b) stability test of different Zn-MOF/CP cathodes. (c) Amount of CH₄ (A_CH4, volume at standard temperature and pressure) generated in 2 h under different potentials. (d) Tafel plot for CH₄ of the Zn-MOF/CP cathode using Zn-MOF synthesized at x = 0.38. Reproduced from Kang et al.¹⁵⁷ Copyright 2016, Wiley.

Downsizing the dimensionality of MOFs into 2D nanosheets destined for catalytic application can increase their surface area thereby exposing more metal active sites and improve electrical conductivity by decreasing the electron transfer distance from the ultrathin nanosheets to the current collector of the electrodes.¹⁸² The use of 2D MOFs as electrocatalysts for the CO₂RR was demonstrated by Yang et al.¹⁵⁹ where they attributed the significant catalytic activity and product affinity of their Cu-MOF nanosheets to the cathodized restructuring of the framework. Similarly, Zhang et al.¹⁶⁵ showed that using 2D porphyrin-based MOF (TCPP(Co)/Zr-BTB) to catalyse the CO₂RR resulted in the production of CO with satisfactory faradaic efficiency and selectivity.

5.2. MOF composites as electrocatalysts for CO₂ reduction

Another strategy to obtain catalytically active MOFs is by incorporating catalytic/electrically conducting molecules which can be achieved either via in situ synthesis or post-synthetic modification.¹⁴¹ Since an in situ synthesis strategy is very difficult to achieve owing to kinetic and steric challenges, and the chemical effect that guest species may have on the reaction mixture, post synthetic modification has been more explored to incorporate molecules such as metal ions, metal nanoparticles or organic linker-based functional groups into the framework.¹⁸³ Findings reveal that doping of MOF ligands could improve their electron-donating ability thus affecting their electrocatalytic activity towards the CO₂RR.⁶ For example, Dou et al.¹⁵⁶ found that doping ZIF-8 (zeolitic imidazolate framework) with 1,10-phenanthroline molecules enhanced the charge transfer ability of the MOF, thereby promoting the catalytic conversion of CO₂ into CO (FE_CO = 90.47% and j_CO = 10 mA cm⁻² at the −1.1 V vs. RHE) compared to pristine ZIF-8 (FE_CO = 50%) (Fig. 16).

Fig. 16 (a) LSV curves of ZIF-8 activated and 1,10-phenanthroline ligand doped product (ZIF-A-LD) in N₂- and CO₂-saturated 0.1 M KHCO₃ solution in comparison with ZIF-A-LD/CB (b) FE_CO. Reproduced from Dou et al.¹⁵⁶ Copyright 2019, Wiley.

Functionalizing the metal component of MOFs (active sites) with catalytic metals/NPs can create more coordinatively unsaturated sites (CUSs) within MOFs thereby boosting catalytic activities for the CO₂RR.¹⁴¹ Feng et al.¹⁶⁰ demonstrated this strategy by using a MOF constructed from zinc-bis(dihydroxy) complex as node and copper–phthalocyanine as ligand to drive the CO₂RR producing syngas. In situ XAS was able to confirm that ZnO₄ being the active sites was responsible for the CO₂RR while CuN₄ act as supports to promote proton and electron transport. In this system, the authors claim that the CuN₄ might be responsible for drawing electrons and H₂O to produce protons which are reduced to molecular H₂. On the other hand, the adsorbed CO₂ on ZnO₄ complexes is reduced to *COOH by coupling with protons/electrons from the CuN₄ sites and electrode/electrolyte, releasing CO as the final product (Fig. 17). The possibility of having dual metallic sites in a catalyst is highly desirable, considering the overall effect it will have on electrocatalytic performance towards the CO₂RR. However, this strategy has not been widely explored and therefore presents exciting opportunities for further research.

Fig. 17 (a) Zn K-edge Fourier transform EXAFS spectra of Zn foil, ZnO and PcCu-O₈-Zn samples. (b) Cu K-edge Fourier transform EXAFS spectra of Cu foil, CuO and PcCu-O₈-Zn samples (c) schematic HER and CO₂RR reaction processes for PcCu-O₈-Zn (d) faradaic efficiency of CO and H₂ for PcCu-O₈-Zn/CNT, PcCu-O₈-Cu/CNT, PcZn-O₈-Zn/CNT and PcZn-O₈-Cu/CNT at −0.7 V vs. RHE. Reproduced from Feng et al.¹⁶⁰ Copyright 2020, SpringerNature.

It has been suggested that incorporating conductive materials (e.g. carbon black or graphene oxides) or catalytic species into MOFs via guest–host interaction is a plausible way to improve the performance in MOFs.¹⁴¹ Ye et al.¹²⁴ adopted this strategy to enhance the catalytic performance of HKUST-1 MOF by grafting into its framework a catalytically active carboxylate-based complex-ReL(CO)₃Cl (L = 2,2′-bipyridine-5,5′-dicarboxylic acid) (Fig. 18). The author reported that a thin film of this catalyst on FTO achieved selectivity towards CO formation of up 93 ± 5%, in comparison to just Re-linkers as catalysts. According to the authors, the higher % FE_CO of this Re-SURMOF was attributed to its highly oriented structure on the electrode surface.

Fig. 18 (a) Schematic description of the preparation of a Re-SURMOF on fluorine-doped titanium oxide (FTO) electrode (b) CV scan and (c) FE of the Re-SURMOF and Re-linker at different applied potentials. Reproduced from Ye et al.¹²⁴ Copyright 2016, Royal Society of Chemistry.

Wang et al.¹⁰⁴ proposed the implementation of a polyoxometalate MOF (PMOF) catalyst prepared from reduced polyoxometalates (POMs) and metalloporphyrin (M-TCPP). Among the metal polyoxometalate MOFs tested, Co-PMOF exhibited superior performance (99% FE_CO at 38.9 mA cm⁻²) over metal polyoxometalate MOFs. The performance of this Co-PMOF was attributed to synergistic cooperation between Co-porphyrin and POMs.

It is important to note again the lack of standards in the normalization process of electrochemical activities as described in Section 1.4. While comparisons in catalytic activity are made across different experiments and materials, those comparisons, on occasion “benchmarking” using geometrical areas, surface areas measured from BET curves or electrochemical surface areas from capacitive curves, are often not only unreliable but also misleading.

5.3. MOF-derived materials as electrocatalysts for CO₂ reduction

Despite the popularity of research on the use of pristine MOFs as electrocatalysts for the CO₂RR, lack of electrical conductivity and structural stability are still major issues for most MOFs,⁸ and have cast doubt in the minds of many among the electrochemical community on the use of MOFs as electrocatalysts. While significant amount of reports on MOFs electrocatalysts claim to exhibit long term stability, few, if any, of these works, have performed an appropriate post-reaction analysis of the catalyst to justify these claims.¹⁸⁴ A few reports have suggested that upon exposure of the MOF or MOF-composite to electrochemical conditions, the integrity of the MOF may be compromised resulting in the “segregation” of the metal centre and the formation of active metal clusters supported on organic matrix.⁵⁸ With this in mind, some groups have proposed the use of MOFs as sacrificial “templates” to generate more stable carbon-supported catalysts for efficient CO₂ reduction. MOFs, upon calcination/pyrolysis at various temperatures, can generate carbon-derived metal catalysts with inherited porosity and surface area. These MOF-derived materials are claimed to possess improved physicochemical and electrocatalytic properties including large surface area, better electronic conductivity (in comparison to MOFs), evenly dispersed single-atom active sites, clusters or NPs.¹⁸⁵ One example of this strategy is given by Zhao et al.¹⁶⁷ who pyrolysed HKUST-1 to produce an oxide-derived Cu/carbon (OD Cu/C) catalyst. During the CO₂RR, the OD Cu/C showed formation of alcohols at potentials as low as −0.1 V vs. RHE. The improvement in selectivity and activity of this catalyst was linked to the mutual interaction between the matrix of porous carbon and the highly dispersed copper. Nitrogen-doped carbon materials (N-Cs) obtained from pyrolysis of MOFs have been described as electrocatalyst materials with good electrical conductivity and sufficient mesoporous surface for effective transport.¹⁸³ The work of Guo et al.¹⁶⁸ demonstrated the effectiveness of using hybrid material obtained from pyrolysis of zeolitic imidazolate frameworks (ZIFs) and multi-walled carbon nanotubes (MWCNTs) to catalyse the CO₂RR, producing CO at almost 100% efficiency, overpotential of 740 mV and a current density of 7.7 mA cm⁻² (Fig. 19). Upon adding Fe into the composite, higher FE_CO of 97% was observed at low overpotentials. The excellent CO₂RR activities of this material were attributed to synergistic cooperation between the inherent active sites in the hybrid material and the carbon nanotube (CNT) support which promotes electron and mass transport.

Fig. 19 (a) Schematic showing the pyrolysis of ZIFs on multi-walled carbon nanotubes (MWCNT) which aid interparticle conductivity and enhance the mass transport in CO₂RR. (b) FE_(CO) and (d) partial CO current for different catalysts at different applied potentials. Reproduced with permission from Guo et al.¹⁶⁸ Copyright 2017, Royal Society of Chemistry.

Promising electrocatalytic performance exhibited by MOF-derived catalysts has frequently been ascribed to “single atom catalysts” (SACs),¹⁸⁶ through direct evidence that the active sites are in fact single atoms is generally lacking. Zhao et al.¹⁶⁹ reported Ni-single atoms dispersed in nitrogen-doped porous carbon (Ni-SAs/N–C) prepared from ZIF-8 as precursor. Ni-SAs/N–C successfully catalysed CO₂ to CO at 71.9% faradaic efficiency, current density of 10.48 mA cm⁻², overpotential of 0.89 V and high TOF of 5273 h⁻¹, performing better than Ni foam and Ni NPs (Fig. 20). The clearly improved CO₂ reduction performance of Ni SAs/N–C may be attributed to the increased number of surface-active sites, lower adsorption energy of CO over active sites, and improved electronic conductivity.¹⁸⁷ In a similar report, Wang et al.¹⁷⁰ used ZIF-8 as sacrificial template to obtained a cobalt metal catalyst anchored on nitrogen-doped carbon (Co-SAs/N_x–C) which performed excellently catalysing CO₂ to CO, achieving FE of 94%, high current density of 18.1 mA cm⁻² and TOF of 18 200 h⁻¹. In comparison to the work of Zhao et al., it was reported that regulating the coordination number of the dispersed metal species obtained from ZIF-8 can enhance its CO₂RR activity. Some relevant works^171–176 have also demonstrated the use of the pyrolysis approach to obtain metal–N–C catalyst materials which have shown promising performance towards CO₂ reduction in terms of selectivity and activity.

Fig. 20 (a) LSV curves in the N₂-saturated (dotted line) or CO₂-saturated (solid line) 0.5 M KHCO₃ electrolyte at a scan rate of 10 mV s⁻¹. (b) FEs of CO and (c) partial CO current density plots and TOFs of Ni SAs/N–C and Ni NPs/N–C at different applied potentials. (d) Stability of Ni SAs/N–C at a potential of −1.0 V vs. RHE for Ni SAs/N–C during 60 h. Reproduced from Zhao et al.¹⁶⁹ Copyright 2017, American Chemistry Society.

6. Summary and future outlook

Achieving sustainable energy and a clean environment is the driving force behind a rapid surge of interest in the electrocatalytic potential of MOFs for CO₂ conversion fuelled by their excellent properties. In the last decade, the drive to obtain a highly competitive, viable and cost-effective alternative catalyst for the CO₂RR has created unprecedented efforts to harness electrocatalytic potential of MOFs. Despite this development, there are still some challenges preventing the industrial application of MOFs as electrocatalysts for CO₂ reduction.

Fig. 21 provides a comparative analysis of the CO₂RR performance across pristine MOFs, MOF composites, and MOF-derived materials, highlighting key parameters such as faradaic efficiency, overpotential, current density, stability and conductivity. This visualization underscores the fundamental trade-offs between these material classes, demonstrating how MOF modifications and derivatization significantly enhance catalytic activity and durability. While pristine MOFs benefit from high surface areas and tunable active sites, their poor conductivity and limited stability hinder their practical application. MOF composites offer improved charge transport and catalytic synergy, while MOF-derived materials, particularly carbonized or metal-based derivatives, exhibit the highest conductivity and long-term stability. This comparison reinforces the need for continued material innovation, particularly in optimizing conductivity and structural integrity, to unlock the full potential of MOF-based catalysts for CO₂ electroreduction.

Fig. 21 Summary of the overall performance metrics of the three types of MOF discussed in the review.

One key issue underlying the poor performance of MOFs compared to conventional inorganic catalysts is electrical conductivity. Therefore, building electrical conductivity into MOFs can push their limit, in terms of electrocatalytic performance, beyond the current catalysts. The general requirement for constructing MOFs with high conductivity is to select building blocks with loosely bound charge carriers that are capable of creating conducting pathways for charge transport via redox hopping and/or band transport. Carefully selecting building components which include mixed-valence metal ions, ligands with conjugated double bonds or nitrogen- or sulfur-containing ligands, can lead to the fabrication of MOFs with higher intrinsic conductivity. Taking advantage of the porosity, a guest-based conductive MOF can be built by way of incorporating conducting materials such as metallic species, redox-active molecules, or conductive polymers into the MOF framework.

Another major issue hindering the electrocatalytic performance of MOFs is stability, as many MOFs collapse into metal oxides/clusters during electrochemical reaction.⁵⁸ Because the metal–carbon bond in most MOFs is rather weak, they easily become degraded, losing their structural integrity, thereby limiting their operability in electrocatalysis. A few measures have been suggested to address MOF stability problems, such as de novo synthesis of MOFs by linking carboxylate-based ligands (hard bases) with high-valent metal ions (hard acids) or linking azolate ligands (soft bases) with low-valent metal ions (soft acids). The introduction of hydrophobic/rigid ligands, and phosphonate/carboxyphosphonate ligands into MOF synthesis have also been suggested as a way to improve MOF stability.

The stability of MOFs can also be affected by the strength of their attachment to the electrode surface. Finding an effective method for preparing MOF-modified electrodes could play an important role in creating a strong attachment of the MOF catalyst to the electrode and by extension enhance the MOF electrochemical response. Drop casting is one facile method for immobilizing MOFs on conductive electrodes for electrochemical studies, however, the ease of delamination of the catalyst from the substrate during electrochemical studies is a major drawback. Additionally, controlling the resulting MOF film's thickness and morphology is challenging. As a result, it is highly desirable to use alternative preparation methods such as direct solvothermal synthesis, electrochemical synthesis etc., which result in strong chemical bonds between the MOF and the substrate.

Having strong MOF attachment to the electrode surface is not sufficient to assure long-term stability as the supporting electrolyte and current flow can also affect the MOF structure. Considerable attention should be paid to the stability of the MOF in aqueous electrolytes and in the presence of bicarbonate ions, which is the preferred medium to carry out CO₂ electrolysis. Therefore, the development of stable MOFs which will be tolerant to electrochemical environments is vital for future CO₂RR electrocatalysis applications.

To complement the stability of MOFs, studies of catalytic performance should be accompanied by extensive material characterization, either during or after reaction, to ensure the MOF structure is maintained under reaction conditions. The method of catalyst normalization is crucial in rating and benchmarking the performance of catalysts, as comparing the activities without due consideration of the method of normalization is not only misleading but can lead to overrating or underrating the catalyst performance. While some authors failed to clearly present the method used in normalizing their current, others erroneously compared the activity metric of MOF electrocatalysts normalized geometrically to those normalized electrochemically. It is therefore necessary to be cautious in comparing the performance of catalysts without taking into account the method of catalyst normalization.

In addition, to confirm the stability of the materials, further post mortem analysis should be performed. Post-reaction analysis techniques play a crucial role in understanding the active sites and reaction mechanisms of electrocatalysts, particularly for CO₂RR. For instance, in situ X-ray absorption spectroscopy (XAS) has been widely used to monitor the oxidation state and coordination environment of metal centres during CO₂RR, providing insights into the dynamic transformations of catalysts, such as the reduction of Cu oxides into metallic Cu species, which directly influences product selectivity.¹⁸⁸ High-resolution transmission electron microscopy (HRTEM) has been instrumental in visualizing structural changes at the atomic level, as seen in the formation of defect-rich structures in carbon-supported single-atom catalysts, which enhance CO₂ adsorption and conversion efficiency.^188,189 Similarly, operando Raman spectroscopy has been used to track reaction intermediates in metal–nitrogen–carbon (M–N–C) catalysts, revealing the role of nitrogen coordination in stabilizing CO₂RR intermediates.¹⁹⁰ These techniques have been successfully applied to transition metal oxides, single-atom catalysts and bimetallic systems, but remain underutilized in MOF-based electrocatalysis. Implementing these methods in MOF-based CO₂RR research would provide real-time insights into MOF stability, electronic structure evolution, and potential catalyst degradation, paving the way for the rational design of highly selective and stable MOF-based CO₂RR electrocatalysts.

An overview of research on MOFs as electrocatalysts reveals that they have been deployed in pristine form as catalysts, as porous supports for the incorporation of catalytically active material or as precursors for dispersed metal catalysts, showing that they can reduce CO₂ into CO, formic acid and even hydrocarbons and alcohols. Several strategies have been attempted to improve the catalytic performance of MOFs for the CO₂RR.

In situ MOF growth into a thin layer has been reported to control MOF thickness which can enhance mass and charge transfer. Downsizing the dimensionality of MOFs into 2D nanosheets can increase the exposure of metal active sites and improve electrical conductivity by decreasing the electron transfer distance from the ultrathin nanosheets to the current collector of the electrodes. Compositing MOFs with conductive materials (such as carbon black or graphene oxides) or catalytic species via guest–host interaction has been demonstrated as a plausible means of inducing catalytic activity in MOFs.

Research involving the use of different MOF morphologies such as ultrathin 2D nanosheets, nanocages and nanowires for CO₂ reduction remains a tiny subset of the total and therefore channelling effort in this direction could yield results in producing efficient electrocatalysts that can promote mass transport and electronic transfer during electrocatalysis.

For the development of optimal catalysts, future studies should explore the tunability of MOFs to understand the role of the active metal centres, pore size or substituents on the linkers. Furthermore, theoretical studies can help understand the effect of metal centre and linker on the strength of interaction between the catalysts and reaction intermediates. This information is essential to elucidate and understand the reaction mechanisms, identify the rate-limiting step and to predict optimal active sites.

Additionally, using MOFs with multi-metal sites present an opportunity for further tuning their catalytic properties. Although, MOF-derived dispersed metal catalysts have recently emerged as a new frontier in catalysis science and have attracted extensive research attention due to their excellent activities toward CO₂ reduction, there remains a need to devise more efficient routes for the synthesis of catalysts having flexible structures and active sites, and for much better characterization of the latter.

Author contributions

All authors have contributed substantially and given approval to the final version of the manuscript.

Data availability

We would like to confirm that no primary research results, software or code have been included in this work, and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare that they have no competing financial interests or personal relationships that could have influenced, or appeared to influence, the contents of this paper.

Acknowledgements

The authors acknowledge the Petroleum Technology Development fund (PTDF), Nigeria for funding this research work by awarding a scholarship to Oriyomi Ogunbanjo (PTDF/ED/PHD/OOO/1553/19).

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