Copper-based metal-organic frameworks for electrochemical reduction of CO<sub>2</sub>

Citation: Xiaomin Kang, Guodong Fu, Xian-Zhu Fu, Jing-Li Luo. Copper-based metal-organic frameworks for electrochemical reduction of CO₂[J]. Chinese Chemical Letters, 2023, 34(6): 107757. doi: 10.1016/j.cclet.2022.107757 shu

Copper-based metal-organic frameworks for electrochemical reduction of CO₂

English

Copper-based metal-organic frameworks for electrochemical reduction of CO₂

a.
Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China
b.
School of Mechanical Engineering, University of South China, Hengyang 421001, China
^* Corresponding authors.
E-mail addresses: xz.fu@szu.edu.cn (X.-Z. Fu)
Jingli.Luo@ualberta.ca (J.-L. Luo).
Received Date: 27 May 2022
Accepted Date: 16 August 2022
Revised Date: 22 July 2022
Available Online: 15 June 2023

Abstract: The electrochemical CO₂ reduction reaction (CO₂ER) is an emerging process that involves utilizing CO₂ to produce valuable chemicals and fuels by consuming excess electricity from renewable sources. Recently, Cu and Cu-based nanoparticles, as earth-abundant and economical metal sources, have been attracting significant interest. The chemical and physical properties of Cu-based nanoparticles are modified by different strategies, and CO₂ can be converted into multicarbon products. Among various Cu-based nanoparticles, Cu-based metal-organic frameworks (MOFs) are gaining increasing interest in the field of catalysis because of their textural, topological, and electrocatalytic properties. In this minireview, we summarized and highlighted the main achievements in the research on Cu-based MOFs and their advantages in the CO₂ER as electrocatalysts, supports, or precursors.

Key words:

1. Introduction

The increasing of CO₂ emission in the earth's atmosphere is nowadays an urgent and critical issue facing humanity [1]. It further causes the rapid accumulation of greenhouse gas, melting of floating icebergs, a rise in sea level and so forth, threatening the existence and development of human race. Thus, the electrochemical conversion of electricity and CO₂ into chemicals and fuels has drawn great attention [2,3]. Electrochemical CO₂ reduction reaction (CO₂ER) is energetically and atomically inefficient caused by large kinetic overpotentials and lack in selectivity of the chemical product, especially the competition with the hydrogen evolution reaction in aqueous solutions. The CO₂ molecules get to the catalytic sites through diffusion in the aqueous phase, and several valuable products can be generated, such as carbon monoxide (CO), ethylene (C₂H₄), formic acid (HCOOH), oxalic acid (H₂C₂O₄) and alcohols (CH₃OH, C₂H₅OH, etc.) [4]. In CO₂ER, the cathodic reaction is of general form:

(1)

while the anodic reaction in CO₂ER is oxygen evolution reaction (OER), like in water splitting in order to sum to the overall reaction [5]:

(2)

In other words, CO₂ + 2H⁺ + 2e⁻ → HCOOH, water is the only renewable and scalable source of electrons and protons [6]. Besides, the equilibrium potentials for CO₂ER have been listed in Table 1 as follows.

Table 1

Table 1. Electrochemical reactions with equilibrium potentials.

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All the CO₂ER standard potentials are calculated via the Gibbs free energy of reactions using gas-phase thermochemistry and the Henry law data from National Institute of Standards and Technology (NIST) for aqueous products. Furthermore, the CO₂ER products exhibit similar reduction potentials. This explains why CO₂ER and hydrogen-evolution reaction (HER) compete since their reduction potentials are similar [7,8].

Among all the catalysts for CO₂ER, Cu and its oxides have been well developed for the electroreduction of CO₂ to high-value hydrocarbons, fuels, alcohol products, and multicarbon chemicals [9-12]. In 1989, Hori et al. first analyzed the gaseous and liquid products of a CO₂ER catalyzed by polycrystalline metal electrodes (Table 2) [13]. Among various metal electrodes, Cu exhibits outstanding selectivity toward hydrocarbon, aldehyde, and alcohol products. Moreover, as a pure metal, Cu can reduce CO₂ to products that require the transfer of more than two electrons with substantial Faradaic efficiencies (FEs).

Table 2

Table 2. Summary of properties of various metals towards the electrochemical reduction of CO₂.

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Furthermore, metals are divided into four groups based on their selectivity, which depends on their binding energy to key CO₂ER and HER intermediates, including *H, *OCHO, *COOH, and *CO [14]. Specifically, Cu alone can reduce CO₂ to > 2e⁻ products [15]. This is attributed to its negative adsorption energy for *CO and positive adsorption energy for *H [16]. In addition, the multiple crystal facets, grain boundaries, and defects on the surface of metal catalysts influence their selectivity for CO₂ER products [17].

Nevertheless, various Cu(hkl) surfaces have different effects on the product distribution [18]. Thus far, according to experimental data, the surface crystal facet has been determined to have a slight effect on the catalytic activity. However, crystal defects have been shown to significantly impact the CO₂ER selectivity [19]. For instance, the Cu(111) surface shows considerably higher selectivity toward C₂ products than the Cu(751) surface, which has abundant highly undercoordinated sites [20]. Among the three Cu single-crystal facets with low indexes, i.e., Cu(111), Cu(110), and Cu(100), the Cu(100) surface exhibits the highest selectivity toward C₂H₆, while Cu(111) and Cu(110) exhibit the highest selectivity toward CH₄. The Cu(100) surfaces more easily form alcohols (CH₃OH, C₂H₅OH, etc.) compared to formic acid at −1.00 V vs. reversible hydrogen electrode (RHE), while the opposite is observed for Cu(111). The Cu(110) surface exhibits high selectivity toward alcohols and carboxylates at similar rates. The C₂H₄/CH₄ ratio is plotted as a function of the crystal orientation, and the results clearly demonstrate that steps can enhance the selectivity toward C₂H₄ formation in comparison with planar single crystals [21]. Further, defects enhance the selectivity toward ethanol (C₂H₅OH) and C₂H₄.

Hori et al. during their tests, found that the single-crystal electrode could be reduced and reconstructed, thus leading to the formation of defects. Specifically, the reconstruction on the surface of a Cu electrode would alter the activity and product selectivity [4]. To date, the mechanisms of the CO₂ER on Cu electrodes remain unclear. However, early experiments revealed that the reduction of CO on Cu results in similar product distribution as that for reduction of CO₂ (CO₂R) on Cu, and the following research on CO reduction on Cu further supported this proposal [22]. Therefore, CO is a key intermediate in the CO₂R to hydrocarbons, aldehydes, and alcohols on Cu [23]. In detail, *CO intermediates (adsorbed CO) are bound on the Cu surface and converted into alcohols by *CO dimerization or hydrocarbons via *COH or *CHO intermediates [24]. Furthermore, CO adsorption on Cu can suppress the competing HER because of site-blocking effects and the changes in the *H-binding energy. This explains the high FE for CO₂R in aqueous electrolytes [25]. Overall, Cu is a transition metal capable of catalyzing the conversion of CO₂ directly into various value-added products.

In electrocatalysis, the surface area of the catalyst significantly influences its CO₂ER activity. Cu-based catalysts have been fabricated into various shapes to enlarge their active surface areas. Among these Cu-based electrocatalysts, Cu-based metal-organic frameworks (MOFs), as porous crystalline materials consisting of a 3D framework of Cu ions or Cu clusters linked by organic linkers, have attracted significant interest for their application as electrocatalysts in CO₂ER [26-31].

Cu-based MOFs exhibit many intriguing properties, including high crystallinity and porosity, and strong metal-ligand coordination bonds; thus, they exhibit a high adsorption capacity for reaction intermediates [32-36]. Moreover, the formation of undercoordinated Cu sites in MOFs can be promoted to enhance the CO₂ER efficiency; This is attributed to the chemical tunability and the porosity/surface-topography controllability of MOFs. Thus, Cu-based MOFs can be explored as a catalytic material for better understanding the CO₂ER. However, owing to the textural properties and poor stability of MOFs, Cu-based MOFs are employed as active supports and catalyst precursors, which are either treated by pyrolysis or electrochemical reaction to afford stable and efficient electrocatalysts for the CO₂ER (Fig. 1).

Figure 1

Figure 1. Cu-MOF related catalysts engineering for CO₂ER.

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2. Cu-MOF related catalysts for CO₂ER

2.1 Cu-MOF as electrocatalysts

The combination of the catalytic properties of Cu with the unique structural features of MOFs has been explored in CO₂ER research (Table 3). In particular, Cu ions coordinate with organic ligands, forming 3D network-like structures. Subsequently, the Cu centers participate in electrochemical redox processes. The coordinated Cu²⁺ can shift into transient reduced states, i.e., Cu⁺ and Cu⁰, or transient oxidized states, i.e., Cu³⁺ or Cu⁴⁺.

Table 3

Table 3. Summary of electrochemical reduction of CO₂ (CO₂ER) performance of Cu-MOF catalysts.

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In 2010, Angamuthu et al. reported a dinuclear Cu(Ⅰ) complex as a reductant for CO₂ER (Fig. 2a). The Cu(Ⅱ) complex could be reduced to regenerate the Cu(Ⅰ) complex by treatment in acetonitrile with a soluble lithium salt at a relatively accessible potential [37].

Figure 2

Figure 2. Copper ions coordinated by organic ligands, resulting in 3D network structures. (a) Schematic overview of the formation and reactivity of the Cu complexes. Reproduced with permission [37]. Copyright 2010, Science. (b) Optimized structure of the models for Cu-MOF-5, and CO₂ adsorbed on Cu-MOF-5 (Ads). Reproduced with permission [40]. Copyright 2013, American Chemical Society. (c) Structural schematic diagram of GDE with Cu₃(BTC)₂ as CO₂ capture agent. Reproduced with permission [44]. Copyright 2018, American Chemical Society. (d) Molecular structures of three Cu-complex materials and their electrocatalytic performance for CO₂ reduction. Molecular structures of CuPc, HKUST-1, and [Cu(cyclam)]Cl₂. Reproduced with permission [46]. Copyright 2018, Springer Nature. (e) Hierarchical tuning of the performance of electrochemical carbon dioxide reduction using conductive two-dimensional metallophthalocyanine based metal-organic framework. Reproduced with permission [49]. Copyright 2020, American Chemical Society. (f) Cu₃(BTC)₂ metal-organic framework (Cu-MOF) and graphene oxide composite in electroreduction of CO₂ to formic acid. Reproduced with permission [51]. Copyright 2020, American Chemical Society.

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For the application of Cu-MOF as a catalyst for CO₂ER, which was firstly investigated in 2012, Hinogami et al. synthesized a copper rubeanate MOF (CR-MOF) to enhance the selectivity of the CO₂ER. At the onset potential of ~ 0.20 V, which was more positive than that for a Cu electrode in an aqueous electrolyte, formic acid was the only CO₂R product obtained on the CR-MOF electrode (FE ≈ 100.00%). However, the Cu electrode generated several products. Meanwhile, the quantity of products and the partial current of formic acid on the CR-MOF electrode were considerably greater than those on the Cu electrode [38].

Later in 2012, Kumar et al. prepared copper benzene-1,3,5-tricarboxylate (Cu-BTC) porous films, which exhibited excellent stability and uniform dispersion in an aqueous system as electrocatalysts loaded on glassy carbon electrodes in 0.10 mol/L KCl for CO₂ER. Cyclic voltammetric studies indicated that Cu(Ⅰ) would react in-situ with CO₂. Moreover, by adding N, N-dimethylformamide (DMF) to saturated CO₂, oxalic acid can be obtained (FE of ~ 51.00% and a current density of 19.00 mA/cm²). During the CO₂ER process, protons are taken from DMF through a two-electron pathway and dimerization process, thus reducing CO₂ to oxalic acid [39].

Maihom et al. further investigated the reaction mechanisms of CO₂ hydrogenation to formic acid over Cu-alkoxide-functionalized MOFs (Cu-MOF-5) by M06-L density functional calculations (Fig. 2b). The results revealed that the CO₂ER has two pathways: concerted and stepwise. Compared with the concerted mechanism, the stepwise mechanism firstly proceeds with hydrogen abstraction by CO₂ to yield a formate intermediate, after which another hydrogen is obtained to yield formic acid. The activation energies are approximately 24.20 and 18.30 kcal/mol for the first and second steps, respectively. These energies are considerably smaller than that for the concerted mechanism (67.20 kcal/mol). This result indicated that the stepwise mechanism was more efficient than the concerted mechanism. Moreover, the catalytic effect of Cu-MOF-5 was comparable to that achieved in a gas-phase uncatalyzed reaction with an activation energy of 73.00 kcal/mol. This further indicated that Cu-MOF-5 can be utilized as an electrocatalyst for the CO₂ER [40].

Following these pioneer works, Cu-based MOF-loaded gas-diffusion electrodes (GDE), including 1-(2-methyl-4-(2-oxopyrrolidin-1-yl)phenyl)-3-morpholino-5,6-dihydropyridin-2(1H)-one (HKUST-1) MOF, copper(Ⅱ)–adeninate–acetate coordination framework (CuAdeAce) MOF, copper bis-bidentate dithiooxamidate (CuDTA) mesoporous metal-organic aerogel (MOA), and CuZnDTA MOA, were employed in CO₂ER by Albo et al. These electrode materials exhibited excellent catalytic potentials because of their high surface areas and abundant, exposed Cu active centers, which were beneficial for CO₂ER. Furthermore, the production of methanol (CH₃OH) and C₂H₅OH in the liquid phase can be achieved with a high efficiency. The FEs for the CO₂ER on HKUST-1, CuAdeAce-, CuDTA- and CuZnDTA-based electrodes were 15.90%, 1.20%, 6.00% and 9.90%, respectively, at the current density of 10.00 mA/cm². Among the sample electrode materials, HKUST-1- and CuZnDTA-based electrodes exhibited outstanding and stable electrocatalytic performances [41].

In 2016, Rungtaweevoranit et al. utilized Zr-based MOFs to promote the activity and selectivity of a Cu catalyst. Specifically, an 18.00 nm-Cu nanocrystal was encapsulated in a UiO-66 single crystal, and it exhibited strong interaction with the Zr oxide of the MOF support. The Cu⊂UiO-66 structure induced an 8-fold enhancement in the yield and 100.00% selectivity for CH₃OH. This indicated that Cu nanocrystals embedded in MOFs can be applied to achieve high catalytic activities [42].

Kung et al. reported a method to anchor Cu nanoparticles (NPs) on NU-1000 thin films by installing single-site Cu(Ⅱ) into the NU-1000 thin film. In the subsequent electrochemical reduction reaction, Cu(Ⅱ) was reduced to metallic Cu. Furthermore, the Cu nanoparticles exhibited moderate electrocatalytic activity, with an HCOO⁻ FE of 28.00% at −1.20 mA/cm² and −0.82 V vs. RHE [43].

Moreover, Qiu et al. utilized Cu₃(BTC)₂ (Cu-MOFs) as electrocatalysts for CO₂ capture. This material was employed in the fabrication of a GDE (Fig. 2c). The FEs of CH₄ on the GDE with Cu-MOFs were 2–3 fold higher than those on the GDE without Cu–MOFs at negative potentials (−2.30 ~ −2.50 V vs. saturated calomel electrode (SCE)). Meanwhile, the FE of the competitive HER was reduced by 30%. The enhanced CO₂ER performance on the GDE with Cu-MOFs was attributed to the accumulation of CO₂ on the interface of the GDE and the electrolyte, which was attributed to the CO₂-capture properties of Cu-MOFs [44].

In 2019, An et al. put forward a catalyst composed of cooperative Cu(Ⅰ) sites on a Zr₁₂ cluster of a MOF for CO₂ER to C₂H₅OH. By introducing the alkali cation, the spatially proximate Zr₁₂-supported Cu(Ⅰ) centers activate hydrogen via bimetallic oxidative addition and promote C-C coupling to yield C₂H₅OH. Notably, Cs⁺-modified MOF catalysts yielded C₂H₅OH with > 99.00% selectivity after a 10 h reaction. This demonstrated that the catalyst can be employed as a tunable platform to design non-precious metal catalysts for future CO₂ER [45].

Apart from the above Cu-MOFs, three Cu complex materials for CO₂ER were investigated by Weng et al. (Fig. 2d). Among these three materials, copper(Ⅱ) phthalocyanine exhibited the highest selectivity toward CH₄ with a FE of ~ 66.00% at 13.00 mA/cm² and −1.06 V vs. RHE. In-situ X-ray absorption spectroscopy analysis highlighted that copper(Ⅱ) phthalocyanine underwent reversible structural and oxidation state changes to form 2.00 nm-metallic Cu clusters. Density functional calculations further confirmed the molecular structure of the metal-ion ligand coordination and the dispersed Cu clusters, which contributed to the CO₂ER selectivity toward CH₄ [46].

Nevertheless, Xia et al. utilized Cu^Ⅱ/adeninato/carboxylato metal-biomolecule frameworks (Cu^Ⅱ/ade-MOFs) for CO₂ER toward hydrocarbon (CH₄, and C₂H₄) generation. The cathodized Cu^Ⅱ/ade-MOFs exhibited excellent catalytic performance with a total hydrocarbon FE of over 73.00%. C₂H₄ was collected and measured with a maximum FE of 45.00% at 8.50 mA/cm² and −1.40 V vs. RHE, while CH₄ was obtained with a FE of 50.00% at ~15.00 mA/cm² and −1.60 V vs. RHE. This indicated that the reconstruction of cathodized Cu^Ⅱ/ade-MOFs and the formed Cu nanoparticles functionalized by N-containing ligands contributed to the excellent CO₂ER performance [47].

In 2019, Wu et al. utilized Cu(Ⅱ) porphyrinic MOF nanosheets for CO₂ER and compared the results with those obtained with CuO, Cu₂O, Cu, a porphyrin-Cu(Ⅱ) complex, and a CuO/complex composite. Among them, Cu-MOF nanosheets exhibited excellent formate selectivity with a FE of 68.40% at −1.55 V vs. Ag/Ag⁺. Furthermore, the C-C coupling product, acetate, and formate were obtained in a rather wide voltage range from –1.40 V to −1.65 V with the total liquid product FE ranging from 38.00% to 85.20%. Thus, the high selectivity and catalytic activity of Cu-MOFs for CO₂R to yield formate and acetate should be attributed to the synergistic enhancement of the porphyrin-Cu(Ⅱ) complex [48].

In addition to the above structures, four systematic structural analogs of conductive 2D MOFs composed of metallophthalocyanine (MPc) ligands linked by Cu nodes for CO₂ER to CO were investigated by Meng et al. (Fig. 2e). The catalytic activity and selectivity of the MOFs were determined hierarchically by two structural factors: the metal within the MPc (M = Co vs. Ni) catalytic subunit and the identity of the heteroatomic cross-linker (X) between the subunits (X = O vs. NH). Furthermore, these properties were governed by the metal within the MPcs and further modulated by the heteroatomic linkages. Among these MOFs, CoPc-Cu-O exhibited the highest selectivity toward CO (as a composite with carbon black in 1:1 mass ratio; FE_CO = 85.00% at the current density of −17.30 mA/cm²) at −0.63 V. Without any conductive additives, CoPc-Cu-O, when employed directly as an electrode material, exhibited excellent performance at a current density of −9.50 mA/cm² with a FE_CO of 79.00% [49].

In 2021, copper tetrahydroxyquinone (Cu-THQ), which is a 2D Cu-based conductive MOF with an average lateral size of 140 nm, was fabricated by Salehi-Khojin et al. for aqueous CO₂ER at low overpotentials. It exhibited a high current density of ~173.00 mA/cm² at −0.45 V vs. RHE, an average FE of ~ 91.00% toward CO production, and an excellent turnover frequency of ~20.82 s⁻¹. When Cu-THQ was employed for CO₂ER to yield CO, it exhibited 35 and 25 times higher activities than those of state-of-the-art MOFs and MOF-derived catalysts, respectively. The experimental and DFT results both revealed the existence of reduced Cu (Cu⁺) during the CO₂ER, which reversibly converts into Cu²⁺ after the reaction [50].

Jeong et al. designed a Cu-based MOF (Cu-benzene-1,3,5-tricarboxylic acid) along with graphene oxide (GO) for CO₂ER. The Cu-MOF/GO electrocatalyst was employed in different supporting electrolytes, including KHCO₃/H₂O, tetrabutylammonium bromide (TBAB)/DMF, KBr/CH₃OH, CH₃COOK/CH₃OH, TBAB/CH₃OH, and tetrabutylammonium perchlorate (TBAP)/ CH₃OH (Fig. 2f). After conducting CO₂ER at various polarization potentials, the results showed that HCOOH was the main product. The highest concentrations of HCOOH formed were 0.14 mmol/L (at −0.10 V), 66.57 mmol/L (at −0.60 V), 0.27 mmol/L (at −0.50 V), 0.24 mmol/L (at −0.50 V), 0.78 mmol/L (at −0.40 V), and 0.31 mmol/L (at −0.45 V) in KHCO₃/H₂O, TBAB/DMF, KBr/CH₃OH, CH₃COOK/CH₃OH, TBAB/CH₃OH, and TBAP/CH₃OH supporting electrolyte systems, respectively. A high FE of 58.00% was obtained in 0.10 mol/L TBAB/DMF, whereas with Cu-MOF alone, the efficiency was 38.00%. Jeong et al. demonstrated that the synergistic effect of GO sheets at a concentration of 3.00 wt% and the Cu⁺-OH⁻ interaction both contributed to the formation of formic acid in various electrolytes [51].

Recently, the facile electrodeposition of an HKUST-1 thin film on carbon paper was investigated by Wang et al. Their findings indicated that the CO₂ER activity was highly affected by the structural reconstruction of HKUST-1. The HKUST-1 film was reconstructed into two structures during the CO₂ER process. Three-dimensional nanospheres (numerous small fragments) and 3D nanonetworks (cross-linked nanobelts in different directions) were formed during the CO₂ER over time. The 3D nanosphere structure exhibited enhanced catalytic activity, attributed to its abundant active sites, relatively low charge-transfer resistance, and high Cu⁺/Cu⁰ ratio. The FEs of C2 product (C₂H₄ and C₂H₅OH) were 58.60% at −0.98 V vs. RHE [52].

Studies have shown that the doping amount of Cu plays a key role in the catalyst structure and catalytic activity for CO₂ER. Thus, a series of Cu-doped ZIF-8 catalysts with different Cu²⁺ doping amounts, obtained by solvothermal synthesis method, was investigated by Iqbal et al. Among the various Cu-ZIF-8 catalysts, Cu30%ZIF-8 exhibited the highest current density of −40.00 mA/cm² at −2.10 V vs. Ag/AgCl and selectivity toward CH₄ and CO compared to previously obtained results on Cu electrodes. The excellent CO₂R performance of the as-prepared catalyst was attributed to its crystalline nanostructure with abundant Cu active metal sites and N, micro‑meso dual porosity, and the broad surface area of the ZIF structure [53].

Recently, Peng et al. investigated semiconductive MOF-Cu₃(HITP)₂ (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) as a catalyst for the CO₂ER. In comparison with the MOF alone, the combination with Ketjen Black would promote the selectivity toward C₂H₄ with FEs of 60.00%-70.00% in a wide potential range. Multicrystalline Cu nanocrystallites were induced in the reconstructed MOF and stabilized by the conducting support via current shock and charge delocalization. The action mechanism of this catalyst is similar to that of dendrite prevention through conductive scaffolds in metal-ion batteries. DFT calculations revealed that the multifacets and rich grain boundaries both promoted C-C coupling while suppressing the HER [54].

Above all, Xiao et al. investigated an electrocatalyst derived from a helical Cu-porphyrinic MOF meso‑tetra (4-carboxyphenyl) porphyrin (TCPP) on Cu(OH)₂ nanoarrays (H-CuTCPP). The catalysts exhibited an extraordinary excellent acetic acid production whose FE was of 26.10% at −1.60 V vs. Ag/Ag⁺, which was much higher than that of non-helical CuTCPP whose FE was of 19.80%. The high efficiency was attributed to the efficient utilization of the space derived from helical MOF nanoarrays. It contributed to the large amount of the active catalytic sites. Besides, the results also indicated that H-CuTCPP exhibited stronger CO linear adsorption which was in favor of producing acetic acid [55].

2.2 Cu-MOF as electrocatalyst precursors

Attributed to their poor stability, particularly at high negative potentials, Cu-MOF, as precursors, undergo further treatment (pyrolysis or electrochemical reaction) to fabricate more stable and efficient electrocatalysts (Table 4).

Table 4

Table 4. Cu-MOF as electrocatalyst precursors.

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2.2.1   Cu-MOF treated by pyrolysis

Cu-MOF can be pyrolyzed to fabricate more stable and active electrocatalysts. In 2017, Zhao et al. utilized a Cu-based MOF (HKUST-1) as a precursor, and after a facile carbonization process, oxide-derived Cu/carbon (OD Cu/C) catalysts were obtained (Fig. 3a). The catalysts exhibited high selectivity toward alcohols in the CO₂ER with a total FE of 45.20%-71.20% at −0.10 ~ −0.70 V vs. RHE. This indicated that the onset potential for C₂H₅OH formation was approximately −0.10 V vs. RHE, which is among the lowest overpotentials reported. Thus, the enhanced activity and selectivity of the oxide-derived Cu/carbon might be attributed to the synergistic effect between the highly dispersed Cu and the porous carbon matrix [56].

Figure 3

Figure 3. Cu-MOF as precursor under pyrolysis treatment in CO₂ER. (a) The synthesis process of oxide-derived Cu/carbon catalysts. Reproduced with permission [56]. Copyright 2017, American Chemical Society. (b) Comparison of paddle-wheel structured HKUST-1 vs. CuAc. Structural investigations of as-fabricated HKUST-1 by SEM. Reproduced with permission [57]. Copyright 2018, American Chemical Society. (c) Illustration of the synthesis procedure of the Cu-MOF/NP catalysts. Reproduced with permission [59]. Copyright 2019, American Chemical Society. (d) Schematic illustration fabrication process for the Cu-NC composite. Reproduced with permission [61]. Copyright 2019, Royal Society of Chemistry. (e) Schematic illustration of the catalyst synthesis process (red oxygen atoms relate to CuO, pink oxygen atoms relate to Cu₂O). Structural investigations of H-265. Reproduced with permission [63]. Copyright 2020, Royal Society of Chemistry. (f) Schematic of the procedure to synthesize the Cu-N-C-T catalysts. Cu(BTC)(H₂O)₃ MOF precursor. Cu-N-C-800 synthesized by pyrolysis of Cu(BTC)(H₂O)₃ MOF and dicyandiamide at 800 ℃, which favors the CO₂ER toward C₂H₄. Cu-N-C-900 derived from the pyrolysis of Cu(BTC)(H₂O)₃ MOF and dicyandiamide at 900 ℃, which favors the CO₂ER toward CH₄. Reproduced with permission [64]. Copyright 2020, American Chemical Society.

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Later, Nam et al. designed a strategy that involved the formation of MOF-regulated Cu clusters, which shifted the CO₂ER selectivity toward multiple-carbon-atom-containing products. The symmetrical Cu dimer was distorted into an asymmetric motif by the separation of adjacent benzene tricarboxylate moieties under thermal treatment (Fig. 3b). Cu clusters with low coordination numbers were formed from distorted Cu dimers in HKUST-1 during the CO₂ER, thus resulting in a FE of 45.00% for C₂H₄ [57].

In addition to Cu-MOF, which are applied alone as precursors, porous porphyrinic triazine frameworks with atomically isolated M-N₄ (M = Ni, Cu, Fe and Co) sites were fabricated via pyrolysis for the CO₂ER to CO. Among them, Ni single atoms/porphyrinic triazine framework (NiSAs/PTF) exhibited the highest FE of 98.00% at a mild potential of −0.80 V vs. RHE. Moreover, the mechanisms of the catalytic CO₂-CO conversion reaction over the different active M-N₄ sites were unraveled by the combination of DFT calculations and experiments [58].

Inspired by previous studies, Liu et al. designed a Cu-MOF-derived nanoparticle as a catalyst for CO₂ER (Fig. 3c), in which Cu/Cu₂O particles formed a porous octahedral structure containing tunable Cu and Cu^I catalytic active sites. The CO₂ER could be realized with a high current density of 25.15 mA/cm² at −0.79 V vs. RHE, which is attributed to the high surface area derived from the porous structures with exposed metal ions. Moreover, a new flow electrochemical reactor integrated with a membrane electrode assembly was designed to reduce the applied potential by approximately 200.00 mV and promote the sensitivity of the reactor for identifying and quantifying products [59].

Guo et al. introduced a method for tuning the selectivity of the CO₂ER to yield CO, using a MOF-derived bimetallic oxide catalyst. MOF-derived In-Cu bimetallic oxides were synthesized through the pyrolysis of an In-Cu bimetallic MOF. By altering the In/Cu ratio, the FE of CO could reach up to 92.10% at a current density of ~ 11.20 mA/cm². Guo et al. believed that the outstanding performance of the catalyst was attributed to the strong CO₂ adsorption, high electrochemical surface area, and low charge-transfer resistance of the bimetallic catalyst [60].

Furthermore, Wei et al. developed a MOF-derived Cu@NC catalyst by the calcination of N-containing benzimidazole-modified Cu-BTC MOFs (BEN-Cu-BTC) at various temperatures (Fig. 3d). The Cu-NC samples exhibited N-species- and content-dependent catalytic activities for the highly selective electroreduction of CO₂ to C₂ products. The results indicated that the high content of the pyrrolic-N and Cu-N species formed by the pyrolysis process at 400 ℃ can facilitate the production of C₂ on the Cu surface, with reaction rates and FEs for C₂H₄ and C₂H₅OH of r_CH = 5.38 µmol m⁻² s⁻¹ (FE = 11.20%) and r_C = 8.83 µmol m⁻² s⁻¹ (FE = 18.40%) at −1.01 V (vs. RHE), respectively. This further demonstrated that the catalytic activity can be controlled by N-type species, which are affected by the corresponding annealing temperature. Furthermore, the Cu-NC catalysts exhibited excellent electrochemical stability for over 8 h [61].

Inspired by previous studies, a layer-stacked, bimetallic 2D conjugated MOF (2D c-MOF) was designed with copper phthalocyanine as the ligand (CuN₄) and zinc-bis(dihydroxy) complex (ZnO₄) as the linker (PcCu-O₈-Zn). The PcCu-O₈-Zn complex exhibited a high CO selectivity of 88.00%. The catalytic process followed a synergistic mechanism, in which ZnO₄ complexes acted as CO₂ER catalytic sites, while CuN₄ centers promoted the protonation of adsorbed CO₂ during the CO₂ER [62].

Furthermore, a catalyst derived from Cu-MOFs (Fig. 3e) for CO₂ conversion into C₂H₄ conversion was proposed by Yao et al., which exhibited enhanced performance because of its porous morphology, complex oxidation states, and strong lattice strain. The Cu@Cu_xO core-shell structure promoted the activation of CO₂, attributed to the formation of Cu⁺/Cu⁰ interfaces via stabilized Cu⁺to facilitate *CO-CO dimerization. This accelerated the conversion of CO₂ into C₂ products but suppressed the conversion to C₁ products. The catalyst exhibited a 51.00% FE for C₂H₄ with a current density of 150.00 mA/cm² in a flow cell and a 70.00% FE for C₂ products in an H-cell with excellent stability [63].

Inspired by the significant research interest received by single-atom catalysis, a single-atom Cu catalyst dispersed on N-doped carbon by a N-coordination strategy was proposed by Guan et al. They highlighted that Cu-N_x configurations have a high catalytic activity for CO₂ER. Furthermore, the Cu-doping concentration and Cu-N_x configurations were adjusted as a function of the pyrolysis temperature (Fig. 3f). When the Cu concentration reached 4.90%, the distance between neighboring Cu-N_x species decreased, further promoting the C-C coupling and the production of C₂H₄. However, the concentration of Cu was lower than 2.40%, and the distance between the Cu and N_x species was large, which led to the formation of CH₄. DFT results further verified that two CO intermediates binding on two adjacent Cu-N₂ sites promoted the production of C₂H₄, while the isolated Cu-N₄, neighboring Cu-N₄, and isolated Cu-N₂ sites facilitated the formation of CH₄ [64].

Moreover, Cao et al. fabricated a N-doped Cu-NC composite electrocatalyst using a N-rich Cu-BTT MOF precursor (H₃BTT = 1,3,5-benzenetris tetrazolate) by pyrolysis. Among the various pyrolysis temperatures applied, 1100 ℃ was the optimal temperature at which Cu-NC (Cu-NC@1100 ℃) exhibited the best catalytic activity for CO (−0.60 V vs. RHE, j_CO = 0.40 mA/cm²) and HCOOH production (−0.90 V vs. RHE, j_HCOOH = 1.40 mA/cm²). This abnormal activity might be due to the synergistic effect of the low-density-dispersed Cu nanoparticles (~ 27 nm), large porous volume, rich pyrrolic-N and Cu-N_x active sites, and stable CO₂ adsorption [65].

Furthermore, Teng et al. prepared an Fe-Cu-BTT composite by introducing the Fe ion into a microporous N-rich MOF, and they employed it as a precursor. After the pyrolysis process, the Fe_XCu-NC catalyst exhibited an excellent selectivity toward CO. The FE of the catalyst pyrolyzed at 800 ℃ reached 48.50%, which was attributed to the Brunauer-Emmett-Teller surface area, total pore volume, Fe-N_x sites, and the low density of the Cu nanoparticles in the carbon matrix [66].

Moreover, a Cu/Cu₂O nanocomposite loaded on the surface of carbon derived from ZIF-L coated vertically on GO (Cu-GNC-VL) by pyrolysis was proposed by Zhang et al. The catalyst exhibited an excellent FE of 70.52% for C₂H₅OH production at a current density of 10.40 mA/cm² and −0.87 V vs. RHE, which benefited from the synergy between the CO₂ asymmetric chemical adsorption on Cu(111) and the suitable kinetics and thermodynamics of the C–C coupling on Cu₂O(111) [67].

Recently, Schuhmann et al. developed a series of Cu_xO_yC_z electrocatalysts obtained from a Cu-based MOF as a porous self-sacrificial template. By modifying the GDEs with Polytetrafluoroetylene (PTFE), 25.00–50.00 wt% teflonized GDEs were obtained, which exhibited a FE of 54.00% at −80.00 mA/cm² for C₂ products. The local OH⁻ activity of the PTFE-modified GDEs was evaluated using a closely positioned Pt nanoelectrode. The OH⁻/H₂O activity ratio increased with the current density because of the locally generated OH⁻ ions, irrespective of the PTFE amount [68].

As mentioned above, single-atom catalysis of CO₂ER has been widely investigated. Lu et al. fabricated atomically dispersed Cu species on N-doped carbon nanosheet composite materials (Cu-NC) obtained by MOF derivatization. The Cu-NC materials exhibited better catalytic performance for the synthesis of methyl N-phenylcarbamate in 71.00% yield, compared with the results obtained with traditional Cu bulk electrodes at ambient temperature and normal pressure. The as-prepared catalyst exhibited excellent stability, and its catalytic activity did not decrease even after 10 consecutive cycles. Furthermore, with the Cu-NC material, a variety of amines could be turned into corresponding carbamates smoothly after the reaction [69].

By incorporating alloys into the structure of Cu-MOFs, it is possible to enhance their catalytic properties. Thus, Zhang et al. fabricated a Cu-In/C bimetallic catalyst by the pyrolysis of Cu-In MOF materials. The Cu-In/C catalyst with a molar ratio of 9:1 exhibited a high CO selectivity of up to 85.00% at −0.75 V vs. RHE, compared with the 3.10% and 10.80% achieved on Cu/C and In/C, respectively. The excellent performance was attributed to the formation of Cu₄In derived from In distributed on the surface of Cu nanoparticles, which changed the geometric and electronic structures of the Cu surface. These modifications affected the adsorption properties of the catalyst for *H, CO₂, and the intermediates during the CO₂ER; thus, the electrochemical conversion of CO₂ to CO was accelerated, and hydrogen evolution was suppressed. The large electrochemically active surface area and accelerated charge-transfer and reaction kinetics of this Cu-In bimetallic catalyst also promoted the CO₂ER activity [70].

Apart from the Cu-In/C bimetallic catalyst, a CuZn bimetallic material embedded on carbon electrocatalysts through a one-step MOF carbonization was fabricated and investigated by Rungtaweevoranit et al. The CuZn alloy exhibited the highest improvement (5-fold) in FE for the C₂ products (C₂H₄ and C₂H₅OH) at −1.00 V vs. RHE, compared with that for its Cu counterpart, far outperforming the catalyst with segregated Cu and Zn phases. This excellent performance was attributed to the influence of Zn on the electronic structure of the Cu sites [71].

Peng et al. designed a Cu₂O/Cu@NC catalyst derived from a Cu-NBDC MOF (a Cu-based MOF synthesized using 2-aminoterephthalic acid (NBDC) as the ligand) by annealing at different temperatures. The results indicated that Cu₂O/Cu@NC exhibited better CO₂R activity and multielectron product selectivity than Cu₂O/Cu@C. The FE increased with increasing temperature, while those of C₂H₄ and CH₄ decreased with increasing temperature. Upon pyrolysis at 400 ℃, the FE of Cu₂O/Cu@NC-400 was over 86.00% at −1.40 and −1.60 V vs. RHE, including a FE_C2H4 of 20.40% at −1.40 V vs. RHE and FE_CH4 of 23.90% at −1.60 V vs. RHE. However, the FE for CH₄ (−1.60 V vs. RHE) of Cu₂O/Cu@C-400 without N doping was only approximately 2.33%, and no C₂H₄ was detected. These differences in the catalytic behavior were derived from the fact that Cu-N was conducive for the stable adsorption of the *CH₂ intermediate during the CO₂ER, thus inhibiting the evolution of H₂ [72].

Yang et al. proposed a novel Cu@Cu₂O NC electrocatalyst which exhibited excellent performance in converting CO₂ into methanol derived from Cu-BTC after pyrolysis at 400 ℃. The optimized catalyst exhibited excellent FE of 45.00% at −0.70 V vs. RHE. It was attributed to the synergistic effect between Cu⁰ and Cu⁺ ions on the surface which contributed to the adsorption of CO₂ and the following formation of methanol [73].

A Cu/Bi bimetal catalyst was obtained by calcination of the MOF precursor under inert atmosphere. The Bimetal structure contributed to the stronger adsorption for CO₂ intermediate in comparison with Bi/Bi₂O₃@C without Cu species. The Cu/Bi catalyst exhibited extraordinary selectivity towards HCOOH with FE of about 93.00% at −0.94 V vs. RHE [74].

Sikdar et al. developed an Ag/Cu bimetallic catalyst via a redox replacement process on the Cu/C catalyst which was fabricated by pyrolysis of Cu-MOF. The catalyst exhibited good performance in producing C2 product (C₂H₄, C₂H₆, C₂H₅OH, C₃H₇OH) with FE of 21.00% and HCOOH with FE of 40.00%. The results in the work indicated that the formation of adsorbed *CO followed by consumption of CO in the successive cascade steps contributed to yielding HCOOH and C-C coupled products [75].

2.2.2   Cu-MOF treated by an electrochemical reaction

Apart from applying the Cu-MOF as a precursor to yield the Cu-NC catalyst via further pyrolysis, Cu-MOFs can also be converted into Cu-nanoparticle-embedded carbon catalysts via an electrochemical reaction. Kim et al. proposed a method using Cu-based MOF-74 as the precursor to produce Cu nanoparticles via an electrochemical reduction process (Fig. 4a). The MOF, due to its porous structure, acted as a template for the fabrication of isolated Cu-nanoparticle clusters with high catalytic activities and high efficiencies for the CO₂ER to CH₄. The MOF-derived Cu nanoparticles demonstrated high FEs of > 50.00% for CH₄ at −1.30 V vs. RHE [76].

Figure 4

Figure 4. Cu-MOF as precursor undergoes electrochemical treatment in CO₂ER. (a) Schematic illustration of the hydrothermal synthesis of Cu-MOF-74 and preparation of Cu NPs from Cu-MOF-74 by electroreduction. Reproduced with permission [76]. Copyright 2019, Elsevier. (b) Electrodeposited hollow metal organic framework mediated in-situ synthesis of copper dendrites with abundant active sites show outstanding performance for electroreduction of CO₂ to formate. Reproduced with permission [77]. Copyright 2020, John Wiley and Sons. (c) The preparation and schematic CO₂R of Cu/Cu₂O@NG from MOF-199 and NG. Reproduced with permission [78]. Copyright 2021, Elsevier. (d) Schematic diagram of electrochemical CO₂R of HE-Cu and p-Cu. (Catholyte: 0.10 mol/L CO₂ bubbled KHCO₃, anolyte: 0.50 mol/L NaOH and 25 ℃, CO₂R activity and HCOOH selectivity potential: −1.03 V, C₂ products selectivity and CH₄ selectivity potential: −1.19 V). Reproduced with permission [80]. Copyright 2021, Elsevier.

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Moreover, Han et al. reported a strategy for fabricating heterogeneous electrocatalysts composed of 3D hierarchical Cu dendrites (Fig. 4b). Traditional electrode materials (i.e., Sn and In) often suffer from a low activity for CO₂ conversion into formate. Moreover, the CO₂ER yields a single product at high current densities and efficiencies. By an in-situ electrosynthesis method, a hollow-structured Cu-MOF film could be obtained in only 5 min. This indicated that more active sites could be exposed using this synthesis strategy, which is beneficial for the reduction of CO₂ to formate. Notably, the current density could reach 102.10 mA/cm² with a selectivity of 98.20% in an ionic-liquid-based electrolyte [77].

Apart from the in-situ electrosynthesis method, Zin et al. reported a simple MOF-derivatization strategy to fabricate electrocatalysts (Fig. 4c). Using MOF-199 and N-doped graphene (NG) as precursors, they carried out electroreduction to obtain Cu/Cu₂O@N-doped graphene (Cu/Cu₂O@NG) materials for efficient CO₂ER to produce C₂-C₃ products (C₂H₄, C₂H₅OH and n-propanol). A series of Cu/Cu₂O@NG materials, particularly Cu/Cu₂O@NG-2 with a FE of 56.00% and a current density of 19.00 mA/cm², were produced to afford C₂-C₃ products. The excellent performance was ascribed mostly to the synergistic effect between Cu/Cu₂O and NG, which resulted in good CO₂ adsorption, rapid mass transfer, and abundant active sites [78].

Moreover, an electrochemical oxidation–reduction method to prepare Cu clusters from MOFs was put forward by Zeng et al. The as-prepared Cu clusters exhibited a FE of 51.20% for CH₄ at a current density of > 150.00 mA/cm². These results suggested that the distinctive CH₄ selectivity is attributed to the sub-nanometer size of the derived materials; moreover, the stabilization of the clusters by the residual ligands of the pristine MOF also contributed to the distinctive selectivity [79].

A Cu-nanosheet-structured catalyst derived from the HKUST-1 MOF by an in-situ electrochemical reaction was proposed by Song et al. (Fig. 4d). The as-prepared electrochemical derivation from HKUST-1catalyst (HE-Cu) exhibited a superior FE of 56.00% compared with that of the pristine Cu foil (p-Cu, FE of 32.30%) at −1.03 V vs. RHE. HE-Cu exhibited a relatively low CO₂R onset potential, a higher CO₂ adsorption capacity (1.58-fold), and a larger electrochemical active surface area (1.24-fold) compared to those of p-Cu. The FEs of HCOOH and the C₂ products for HE-Cu increased by 1.57-fold and 10.6-fold at −1.19 V vs. RHE, respectively, compared with those of p-Cu. The excellent CO₂R activity and HCOOH and C₂ product selectivity for HE-Cu were attributed to its stepped Cu(211) surfaces, (200) facets, and Cu edge atoms [80].

2.2.3   Cu-MOFs as active supports

Apart from direct application as electrocatalysts, Cu-MOFs can also be used as active supports for CO₂ER, attributed to the textural properties and active sites of MOFs (Table 5). In 2019, Tan et al. prepared a catalyst by the in-situ etching of a Cu₂O sphere with H₃BTC to obtain a Cu-MOF shell as a support to further synthesize a Cu₂O@Cu-MOF electrocatalyst (Fig. 5a). The catalyst exhibited outstanding performances for the formation of hydrocarbons from CO₂, with a FE of 79.40% for CH₄ and C₂H₄. Moreover, the FE for CH₄ was determined to be up to 63.20% at −1.71 V [81]. As shown in Fig. 5b, Cu-MOFs used as supports contribute to the productivity of and selectivity toward CH₄ and other valuable products.

Table 5

Table 5. Cu-MOF as active support.

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Figure 5

Figure 5. Cu-based MOF as active supports. (a) Schematic illustration of the process to synthesize Cu₂O@Cu-MOF. Reproduced with permission [81]. Copyright 2019, American Chemical Society. (b) FEs of CH₄ and C₂H₄ and the ratio of CH₄ to C₂H₄ for Cu₂O@Cu-MOF, Cu-MOF, and Cu₂O at −1.71 V vs. RHE in CO₂-saturated 0.10 mol/L KHCO₃ solution. Reproduced with permission [81]. Copyright 2019, American Chemical Society. (c) Schematic illustration of preparation and CO₂ER of M-TCPP@Cu electrode. Reproduced with permission [82]. Copyright 2021, American Chemical Society. (d) Possible reaction route of the CO₂ER to C₂H₄. Reproduced with permission [82]. Copyright 2021, American Chemical Society.

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In 2021, Sun et al. proposed a molecular encapsulation strategy to enrich intermediates to facilitate the electrochemical conversion of CO₂ to C₂H₄. They employed metal (FeCl, Co and Ni) tetrakis(4-carboxyphenyl) porphyrin (M-TCPP) with a Cu-MOF to create a series of metalloporphyrin-decorated Cu catalysts with a coral-like shape (Fig. 5c). As shown in Fig. 5d, the possible reaction route suggests that the M-TCPP in the catalysts provides more CO intermediates to the Cu sites, enhancing the selectivity toward C₂H₄ with a FE of 33.42% at −1.17 V vs. RHE on the Fe-TCPP@Cu electrode compared to that on the Cu electrode (16.85%, at −1.27 V vs. RHE). In addition, the metalloporphyrin-decorated Cu catalysts exhibited better performance than the physical mixture of Cu-MOFs and metal (FeCl, Co, and Ni) 5,10,15,20-tetraphenylporphyrin (M-TPPs) [82].

Sun et al. prepared a novel catalyst using CuO NPs anchored on Cu-MOF nanosheets via a facile solvothermal method. The catalyst delivered excellent performances with a FE of 50.00% for C₂H₄ at −1.10 V vs. RHE and outstanding stability even after long-term experiment. The marvelous performances can be attributed to the interface between CuO and Cu-MOF which could adsorb and activate CO₂ molecules [83].

The atomic layer infiltration (ALI) technique was used to fabricate an HKUST-1–5C-Zn catalyst which used Cu-MOF as active support with highly dispersed Zn atoms. Attributed to the uniform distribution of Zn-O-Zn sites on HKUST-1 which contributed to the CO₂ adsorption and formation of COOH*, it exhibited excellent performance in production of CO with a FE of ~80.00% at −1.80 V [84].

3. Conclusions and outlook

As a promising technology, the CO₂ER is essential in solving the global climate and environmental problems caused by the increasing CO₂ emissions. As interesting electrochemical catalysts, Cu-based MOFs have been receiving widespread interest from scholars owing to their porous structures and properties, and recently, the research interest and number of studies have been increasing.

In this minireview, we summarized the recent works on Cu-based MOFs and their derivatives as electrocatalysts for the CO₂ER. Cu-MOF catalysts offer several advantages for the CO₂ER process, which are attributed to their facile synthetic route, porous structure, and multiple active sites. When Cu-MOFs are directly used as catalysts, the discrete Cu atoms can provide active sites, and organic linkers can be modified to form active sites or charge-transfer agents. Further, their porous structure, formed by metal particles and the organic phase, facilitates the reaction with CO₂ or their application as catalyst supports. In addition, carbon-based catalysts with more highly dispersed metal particles can be obtained, using Cu-MOFs or their derivatives as precursors. The metal active sites inherited from Cu-MOFs maintain their catalytic activity and conductivity, facilitating the realization of efficient single-point catalysis. The MOF structure, which is tunable, can be employed to further construct bimetallic or even polymetallic structures, which would enable the facile synthesis of metal alloys and the design of efficient catalysts for the CO₂ER.

However, Cu-MOFs catalysts, similar to traditional MOF catalysts, still suffer from poor chemical stability. Thus, further improvement is required, as it is still difficult to conduct detection and analysis processes after the reaction. The Cu-MOF catalysts obtained by pyrolysis and the electrochemical method have significantly stable structures and exhibit excellent catalytic activities. Future studies are suggested to address these properties in literatures. In addition, new methods and strategies are recommended to fabricate Cu-MOF with more stable structures and excellent chemical activity. For instance, incorporated with highly conductive and active materials (graphene, carbon-based catalysts, etc.) will help enhance the electrochemical activity and stability. Besides, due to the complexity and diversities of the Cu-MOF based structures, machine learning can be applied to design efficient Cu-MOF and its derivatives as electrocatalysts for CO₂ER. These properties and recommendations highlight their significant untapped potential for future application in environmental remediation.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 22003041 and 51902204) and Shenzhen Innovative Research Team Program (No. KQTD20190929173914967).
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Figure 1 Cu-MOF related catalysts engineering for CO₂ER.

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Figure 2 Copper ions coordinated by organic ligands, resulting in 3D network structures. (a) Schematic overview of the formation and reactivity of the Cu complexes. Reproduced with permission [37]. Copyright 2010, Science. (b) Optimized structure of the models for Cu-MOF-5, and CO₂ adsorbed on Cu-MOF-5 (Ads). Reproduced with permission [40]. Copyright 2013, American Chemical Society. (c) Structural schematic diagram of GDE with Cu₃(BTC)₂ as CO₂ capture agent. Reproduced with permission [44]. Copyright 2018, American Chemical Society. (d) Molecular structures of three Cu-complex materials and their electrocatalytic performance for CO₂ reduction. Molecular structures of CuPc, HKUST-1, and [Cu(cyclam)]Cl₂. Reproduced with permission [46]. Copyright 2018, Springer Nature. (e) Hierarchical tuning of the performance of electrochemical carbon dioxide reduction using conductive two-dimensional metallophthalocyanine based metal-organic framework. Reproduced with permission [49]. Copyright 2020, American Chemical Society. (f) Cu₃(BTC)₂ metal-organic framework (Cu-MOF) and graphene oxide composite in electroreduction of CO₂ to formic acid. Reproduced with permission [51]. Copyright 2020, American Chemical Society.

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Figure 3 Cu-MOF as precursor under pyrolysis treatment in CO₂ER. (a) The synthesis process of oxide-derived Cu/carbon catalysts. Reproduced with permission [56]. Copyright 2017, American Chemical Society. (b) Comparison of paddle-wheel structured HKUST-1 vs. CuAc. Structural investigations of as-fabricated HKUST-1 by SEM. Reproduced with permission [57]. Copyright 2018, American Chemical Society. (c) Illustration of the synthesis procedure of the Cu-MOF/NP catalysts. Reproduced with permission [59]. Copyright 2019, American Chemical Society. (d) Schematic illustration fabrication process for the Cu-NC composite. Reproduced with permission [61]. Copyright 2019, Royal Society of Chemistry. (e) Schematic illustration of the catalyst synthesis process (red oxygen atoms relate to CuO, pink oxygen atoms relate to Cu₂O). Structural investigations of H-265. Reproduced with permission [63]. Copyright 2020, Royal Society of Chemistry. (f) Schematic of the procedure to synthesize the Cu-N-C-T catalysts. Cu(BTC)(H₂O)₃ MOF precursor. Cu-N-C-800 synthesized by pyrolysis of Cu(BTC)(H₂O)₃ MOF and dicyandiamide at 800 ℃, which favors the CO₂ER toward C₂H₄. Cu-N-C-900 derived from the pyrolysis of Cu(BTC)(H₂O)₃ MOF and dicyandiamide at 900 ℃, which favors the CO₂ER toward CH₄. Reproduced with permission [64]. Copyright 2020, American Chemical Society.

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Figure 4 Cu-MOF as precursor undergoes electrochemical treatment in CO₂ER. (a) Schematic illustration of the hydrothermal synthesis of Cu-MOF-74 and preparation of Cu NPs from Cu-MOF-74 by electroreduction. Reproduced with permission [76]. Copyright 2019, Elsevier. (b) Electrodeposited hollow metal organic framework mediated in-situ synthesis of copper dendrites with abundant active sites show outstanding performance for electroreduction of CO₂ to formate. Reproduced with permission [77]. Copyright 2020, John Wiley and Sons. (c) The preparation and schematic CO₂R of Cu/Cu₂O@NG from MOF-199 and NG. Reproduced with permission [78]. Copyright 2021, Elsevier. (d) Schematic diagram of electrochemical CO₂R of HE-Cu and p-Cu. (Catholyte: 0.10 mol/L CO₂ bubbled KHCO₃, anolyte: 0.50 mol/L NaOH and 25 ℃, CO₂R activity and HCOOH selectivity potential: −1.03 V, C₂ products selectivity and CH₄ selectivity potential: −1.19 V). Reproduced with permission [80]. Copyright 2021, Elsevier.

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Figure 5 Cu-based MOF as active supports. (a) Schematic illustration of the process to synthesize Cu₂O@Cu-MOF. Reproduced with permission [81]. Copyright 2019, American Chemical Society. (b) FEs of CH₄ and C₂H₄ and the ratio of CH₄ to C₂H₄ for Cu₂O@Cu-MOF, Cu-MOF, and Cu₂O at −1.71 V vs. RHE in CO₂-saturated 0.10 mol/L KHCO₃ solution. Reproduced with permission [81]. Copyright 2019, American Chemical Society. (c) Schematic illustration of preparation and CO₂ER of M-TCPP@Cu electrode. Reproduced with permission [82]. Copyright 2021, American Chemical Society. (d) Possible reaction route of the CO₂ER to C₂H₄. Reproduced with permission [82]. Copyright 2021, American Chemical Society.

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Table 1. Electrochemical reactions with equilibrium potentials.

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Table 2. Summary of properties of various metals towards the electrochemical reduction of CO₂.

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Table 3. Summary of electrochemical reduction of CO₂ (CO₂ER) performance of Cu-MOF catalysts.

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Table 4. Cu-MOF as electrocatalyst precursors.

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Table 5. Cu-MOF as active support.

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