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 electrocatalytic CO2 reduction reaction (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.
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 ECO
2RR 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 Cu
2+ 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 FE
HCOO− of 28% at −0.82 V in 0.1 M KClO
4 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 C
2 products. Compared with the p-Cu without the stepped (211) surface, it has higher catalytic activity and HCOOH selectivity, reaching a FE
HCOOH of 40.1% at −1.03 V, as well as the highest FEs for C
2H
4 and C
2H
6 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 HCO
3− electrolyte for Bi-carboxylate MOFs, which then derive into Bi
2O
2CO
3. After further electrochemical reduction process, the Bi nanomaterial is obtained. In this regard, CAU-17 will derive into Bi
2O
2CO
3 NSs in the HCO
3− electrolyte, and the thickness of NSs is affected by the concentration of HCO
3−, 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 KHCO
3 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 FE
HCOO− 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 FE
HCOOH 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 Bi
3+ too far apart from each other, leading to unstable dissociative Bi
3+ 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 Bi
2O
3. 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 Bi
3+ 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 FE
HCOO− 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 HCO
3− 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 KHCO
3. A new mechanism is proposed: when the applied overpotential is low, some easily absorbed HCO
3− groups can directly participate in the formation of formate. With the increase in the applied overpotential, formate is produced mainly through the reduction of CO
2 in the feed gas and the dissociation of adsorbed HCO
3− 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 FE
HCOO− 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 ECO
2RR 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 Pb
2+ is reduced to Pb
0, while the outer layer of Pb
2+ is etched by HCO
3− and undergoes the insertion of OH− to form hydrocerussite [Pb
3(CO
3)
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 ECO
2RR and selectivity for HCOOH while effectively inhibiting the HER of Pb, which is originally strong. At −0.88 V, the optimal FE
HCOOH 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