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Metal–organic frameworks (MOFs) are used in catalysis due to their high specific surface area and porous structure. In situ electrochemical reduction is a mild and effective reduction method. For some unstable MOFs, the pretreatment process of electrochemical reduction is often accompanied by the reduction of metal ions and spontaneous aggregation to form metal nanomaterials, while the organic ligands or linkers are dissolved in the electrolyte. Compared to MOFs connected through relatively weak coordination bonds, metal nanoparticles connected by metallic bonds are significantly more conductive and stable, which effectively improves the catalyst activity and stability in ECO2RR. At the same time, compared with reducing agents, electrochemical reduction often retains some M-O species or organic ligands on the surface, which has an important impact on catalytic activity and stability. The structure of the MOF precursor also has an important impact on the morphology of the derived catalyst and the corresponding ECO2RR performance.
- metal–organic frameworks
- electrocatalysis
- CO2 reduction
1. Copper-Based Nanomaterials
The electrocatalytic activity of Cu catalysts can be significantly enhanced by reducing the particle size, especially in the particle size range of less than 10 nm. In addition, electrochemically reduced Cu nanoparticles (NPs) from copper oxide appear to be particularly suitable for ECO2RR and have been shown to be superior to both polycrystalline Cu and hydrogen-reduced Cu from copper oxide at high temperatures. A solvothermal deposition in the MOF method is adopted to deposit Cu2+ in Zr-MOF (NU-1000). Cu NPs are further formed in electrochemical reduction, while the size of NPs is effectively limited by the pore size of MOF, leading to a maximum FEHCOO− of 28% at −0.82 V in 0.1 M KClO4 electrolyte.[1]
By in situ electroreduction, the HKUST-1-covered Cu foam electrode is transformed into a HE-Cu electrode with the morphology of NSs. The step-like structure can be observed by high-resolution transmission electron microscopy (HRTEM) images, which is considered to be a Cu (211) surface. According to the DFT calculations, the stepped (211) surface of the Cu catalyst is more favorable to the formation of HCOOH than the stepped (111) and stepped (200) surfaces. Moreover, it has a suitable binding ability with *CO, being conducive to a further reduction to *COH/*CHO intermediates, followed by producing C2 products. Compared with the p-Cu without the stepped (211) surface, it has higher catalytic activity and HCOOH selectivity, reaching a FEHCOOH of 40.1% at −1.03 V, as well as the highest FEs for C2H4 and C2H6 of 2.93% and 2.58%, respectively at −1.19 V.[2]
2. Bismuth-Based Nanomaterials
As mentioned above, the SALE process will occur in the HCO3− electrolyte for Bi-carboxylate MOFs, which then derive into Bi2O2CO3. After further electrochemical reduction process, the Bi nanomaterial is obtained. In this regard, CAU-17 will derive into Bi2O2CO3 NSs in the HCO3− electrolyte, and the thickness of NSs is affected by the concentration of HCO3−, which influence the thickness of Bi NSs obtained in the next electrochemical reduction process. Bi NSs with thicknesses of 3.5 nm and 11 nm are obtained by 0.1 M and 1 M KHCO3 electrolyte and electrochemical reduction, respectively. Thinner Bi NSs provide more accessible active sites and exhibit faster electron transfer, exhibiting higher catalytic activity and formate selectivity. The 3.5 nm Bi NSs exhibit an FEHCOO− of 92% and partial current density of ~10.8 mA cm−2 at −1.1 V.[3]
Residual Bi-O species are inevitable in the process of electrochemical reduction of CAU-17 to obtain Bi NSs. Compared to the Bi NPs obtained by reduction with the strong reducing agent, e.g., sodium borohydride, the FEs and cathodic energetic efficiency (CEEs) for HCOOH of Bi NSs have shown obviously higher than that of Bi NPs without Bi-O species. The DFT calculations indicate that O atoms at the Bi-O surface of Bi NSs help to reduce the free-energy barrier for forming *OCHO intermediate, resulting in a high FEHCOOH of 97.4% with a current density of 133 mA cm−2 at −0.48 V in the flow cell.[4]
The corresponding Bi NPs can be obtained by CAU-7 with BTB as an organic linker. However, compared to Bi-BTC, the larger BTB makes Bi3+ too far apart from each other, leading to unstable dissociative Bi3+ during the electroreduction dissociation process. Finally, only aggregated Bi NPs can be obtained. The NSs structure derived from Bi-BTC is more conducive to adhesion to the surface of the carbon cloth, which promotes the charge transfer between the substrate and the catalyst, and thus exhibits better catalytic activity.[5]
Bi-MOF-derived nanosheets (MNS) constructed from BDC can also be obtained by electrochemical reduction.[6] In this process, metal coordination bonds break, and violent structural recombination occurs. The main components of BiMNS are Bi and a small amount of Bi2O3. According to in situ Raman spectra, the organic-associated peaks disappear with cyclic voltammetry (CV) cycles increasing, demonstrating the disappearance of the organic linkers. Compared with Bi NSs synthesized by conventional wet chemicals, BiMNS shows better stability because the residual ligands on the surface inhibit the dissolution and re-deposition of Bi atoms on the surface, preventing the deactivation of the active sites during long-term electrocatalysis. This modification strategy can also be applied to synthesize indium-based catalysts, which also show suitable activity and durability.
Metallenes have also received attention due to their high atomic utilization rate. Bi metal–organic layers (MOLs) constructed by 4,5-imidazoledicarboxylic acid (IDC) with unique bilayer structures and a tendency to grow in thin layers can be converted into bismuthene (Bi-ene) with fewer layers by electrochemical reduction. It has an ultra-thin NSs morphology similar to graphene, and the thickness is only 1.28–1.45 nm, which is the thinnest Bi NSs reported so far. The phase transition involves not only the in situ electrochemical reduction of Bi3+ in Bi-MOLs but also the migration of Bi atoms due to the gradual loss of organic ligands, and the dissociated ligands may serve as soft templates to control the final morphology of Bi-ene. The catalyst reaches FEHCOO− of nearly 100% in the potential range of −0.83 to −1.18 V and maintains this formate selectivity in the flow cell at the current density of 200 mA cm−2. The in situ attenuated total reflection-infrared (ATR-IR) spectra suggest that the *OCHO intermediate (the upward peaks at 1403 cm−1) is formed while the absorbed HCO3− groups (the downward peaks at 1352 cm−1 and 1300 cm−1) are consumed. Surprisingly, a similar phenomenon is also observed in in situ ATR-IR spectra under Ar condition, while formate is detected after electrolysis in Ar-saturated KHCO3. A new mechanism is proposed: when the applied overpotential is low, some easily absorbed HCO3− groups can directly participate in the formation of formate. With the increase in the applied overpotential, formate is produced mainly through the reduction of CO2 in the feed gas and the dissociation of adsorbed HCO3− groups.[7]
In addition, it has also been reported that Bi NSs clusters are usually dense, and the atomic utilization rate of Bi NSs is relatively low because only the surface Bi atoms are active. Highly dispersed Bi NPs derived from Bi-BTB MOF have FEHCOO− of 95% at −0.97 V. More importantly, the strategy of using MOF as a pre-catalyst introduces a low load of metal atoms, so the Bi-BTB-derived electrocatalyst achieves a Bi mass activity of 158 A g−1 at −0.97 V, which is superior to most advanced bismuth-based catalysts reported for ECO2RR to formate.[8]
3. Lead-Based Nanomaterials
Pb-MOF with 5-Chloronicotinic acid (CA) as the organic linker is effectively dispersed on the Pb electrode, and the nanocolumn structure of MOF collapses under the condition of electrochemical reduction. In this process, the inner layer of Pb2+ is reduced to Pb0, while the outer layer of Pb2+ is etched by HCO3− and undergoes the insertion of OH− to form hydrocerussite [Pb3(CO3)2(OH)2] thin film (ER-HC). The formed hydrocerussite film is uniformly and closely attached to Pb and has abundant surface defects, which can improve the activity of ECO2RR and selectivity for HCOOH while effectively inhibiting the HER of Pb, which is originally strong. At −0.88 V, the optimal FEHCOOH is 96.8%, with a current density of 2.0 mA cm−2.[9]
This entry is adapted from the peer-reviewed paper 10.3390/catal13071109
References
- Chung-Wei Kung; Cornelius O. Audu; Aaron W. Peters; Hyunho Noh; Omar K. Farha; Joseph T. Hupp; Copper Nanoparticles Installed in Metal–Organic Framework Thin Films are Electrocatalytically Competent for CO2 Reduction. ACS Energy Letters 2017, 2, 2394-2401, .
- Da Wang; Jinli Xu; Ying Zhu; Lingsha Wen; Jiexu Ye; Yi Shen; Tao Zeng; Xiaohui Lu; Jun Ma; Lizhang Wang; et al. HKUST-1-derived highly ordered Cu nanosheets with enriched edge sites, stepped (211) surfaces and (200) facets for effective electrochemical CO2 reduction. Chemosphere 2021, 278, 130408, .
- Dazhi Yao; Cheng Tang; Anthony Vasileff; Xing Zhi; Yan Jiao; Shi‐Zhang Qiao; The Controllable Reconstruction of Bi‐MOFs for Electrochemical CO 2 Reduction through Electrolyte and Potential Mediation. Angewandte Chemie International Edition 2021, 60, 18178-18184, .
- Jian Yang; Xiaolin Wang; Yunteng Qu; Xin Wang; Hang Huo; Qikui Fan; Jin Wang; Li‐Ming Yang; Yuen Wu; Bi‐Based Metal‐Organic Framework Derived Leafy Bismuth Nanosheets for Carbon Dioxide Electroreduction. Advanced Energy Materials 2020, 10, 2001709, .
- Bo Zhang; Shuyan Cao; Yunzhen Wu; Panlong Zhai; Zhuwei Li; Yanting Zhang; Zhaozhong Fan; Chen Wang; Xiaomeng Zhang; Jungang Hou; et al. Metal‐Organic‐Framework‐Derived Bismuth Nanosheets for Electrochemical and Solar‐Driven Electrochemical CO 2 Reduction to Formate. Chemelectrochem 2021, 8, 880-886, .
- Nanhui Li; Ping Yan; Yuanhao Tang; Jianghao Wang; Xin-Yao Yu; Hao Bin Wu; In-situ formation of ligand-stabilized bismuth nanosheets for efficient CO2 conversion. Applied Catalysis B: Environmental 2021, 297, 120481, .
- Changsheng Cao; Dong‐Dong Ma; Jia‐Fang Gu; Xiuyuan Xie; Guang Zeng; Xiaofang Li; Shu‐Guo Han; Qi‐Long Zhu; Xin‐Tao Wu; Qiang Xu; et al. Metal–Organic Layers Leading to Atomically Thin Bismuthene for Efficient Carbon Dioxide Electroreduction to Liquid Fuel. Angewandte Chemie International Edition 2020, 59, 15014-15020, .
- Paolo Lamagni; Matteo Miola; Jacopo Catalano; Mathias S. Hvid; Mohammad Aref H. Mamakhel; Mogens Christensen; Monica R. Madsen; Henrik S. Jeppesen; Xin‐Ming Hu; Kim Daasbjerg; et al. Restructuring Metal–Organic Frameworks to Nanoscale Bismuth Electrocatalysts for Highly Active and Selective CO 2 Reduction to Formate. Advanced Functional Materials 2020, 30, 1910408, .
- Da Wang; Shiwen Dong; Lingsha Wen; Weiting Yu; Zhiqiao He; Qingqing Guo; Xiaohui Lu; Lizhang Wang; Shuang Song; Jun Ma; et al. Highly selective electrocatalytic reduction of CO2 to HCOOH over an in situ derived hydrocerussite thin film on a Pb substrate. Chemosphere 2021, 291, 132889, .
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