Electrocatalytic CO 2 Reduction over Bimetallic Bi-Based Catalysts: A Review

Open AccessCCS ChemistryMINI REVIEWS28 Dec 2022Electrocatalytic CO2 Reduction over Bimetallic Bi-Based Catalysts: A Review Wei Chen, Yating Wang, Yuhang Li and Chunzhong Li Wei Chen Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yating Wang Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yuhang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 and Chunzhong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.022.202202357 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Electrocatalytic reduction of carbon dioxide (CO2) to valuable fuels is an up-and-coming approach. Owing to the low cost, environmental friendliness, and high selectivity to formate single product at low overpotentials, bismuth (Bi)-based catalysts have attracted extensive research attention. In this review, the reaction mechanisms of Bi-based catalysts are first introduced, and the bimetallic Bi-based catalysts synthesized by alloying, doping, and loading strategies are reviewed from the aspects of catalyst component, morphology, synthesis procedure, and performance optimization for electrocatalytic CO2 reduction. We provide an in-depth discussion of the existing challenges and an outlook for this highly promising kind of electrocatalysis. Download figure Download PowerPoint Introduction The energy crisis and environmental pollution have been ongoing significant issues and the focus of the international community. Over the past few decades, the strong dependence and overuse of fossil fuels has resulted in a rapid increase in the concentration of carbon dioxide (CO2) in the atmosphere.1,2 When the World Meteorological Organization released its latest Greenhouse Gas Bulletin, they pointed out that the current CO2 concentration in the atmosphere is 149% of preindustrial levels.3 The Bulletin additionally noted that greenhouse gases have increased by 47% through radiative forcing, with CO2 accounting for 80% of this increase from 1990 to 2020. The emission of an oversized quantity of CO2 and other harmful gases has resulted in global warming and environmental pollution, documented in numerous studies related to the energy and environment around the world. CO2 emission reduction is urgently needed. Electrocatalytic CO2 reduction reaction In order to curtail the global warming trend, scientists conduct in-depth discussions and research on the issue of reducing CO2 emissions. There are four primary strategies: (a) developing new clean energy technologies; (b) upgrading existing processes to eliminate and replace the low-efficiency sectors and equipment in traditional technologies;1 (c) afforestation and forestation; and (d) carbon capture, utilization and storage.4,5 Research on carbon capture, carbon storage, and carbon utilization has produced many advances and breakthroughs in CO2 storage and conversion. Among them, carbon capture and storage technology have certain limitations. First of all, the technology is expensive. Second, storage equipment may leak and cause a series of other hidden safety problems, such as local seawater acidification. In contrast, carbon capture and utilization technology, which reduce CO2 and convert it into usable chemical value-added fuels, possesses greater development prospects.6–8 Not only can this technology reduce high CO2 concentrations in the atmosphere, but it also produces renewable fuels to combat the energy crisis. In recent years, numerous catalytic conversion methods have been developed successively. The methods used for CO2 reduction chiefly include biological (enzyme) catalysis, photocatalysis, thermocatalysis, and electrocatalysis.9–12 At present, electrocatalysis as an emerging energy technology for CO2 emission reduction and production of value-added fuels receives a great deal of research attention.7 Compared with traditional industrial processes, electrocatalytic CO2 reduction reaction (CO2RR) can be carried out under milder environmental conditions, improving electrochemical stability and selectivity in CO2RR via selecting appropriate electrocatalysts to manipulate reaction-tailored products.13 In this regard, reports in the literature demonstrate the exploration of various electrocatalysts and electrode reaction mechanisms for CO2RR. CO2RR also utilizes renewable energy for catalytic reactions to achieve large-scale energy storage and production of high-energy products.14 Generally, CO2RR is in a position to produce a variety of reduction products through electrocatalysis. The corresponding reduction products are different as electron-transferred numbers change. Electrocatalytic CO2RR products are categorized as formic acid (HCOOH),12 carbon monoxide (CO), methane (CH4), methanol (CH3OH) as C1, and ethanoic acid (CH3COOH), ethanol (C2H5OH)15 as C2.16,17 The above reduction products are obtained by various electron-transfer mechanisms and half-reactions, as shown in Table 1.18,19 Table 1 | Electrochemical Potentials of Several CO2 Reduction Reactions CO2 Reduction Half-Reactions Electrode PotentialV (vs SHE) Electrode PotentialV (vs RHE) CO2 + 2H+ + 2e− → CO + H2O −0.52 −0.106 CO2 + 2H+ + 2e− → HCOOH −0.61 −0.250 CO2 + 4H+ + 4e− → HCHO + H2O −0.51 −0.070 CO2 + 6H+ + 6e− → CH3OH + H2O −0.38 0.016 CO2 + 8H+ + 8e− → CH4 + 2H2O −0.24 0.169 2CO2 + 12H+ + 12e− → C2H4 + 4H2O −0.34 0.064 2CO2 + 12H+ + 12e− → C2H5OH + 3H2O −0.33 0.084 During the electrocatalytic reduction process, not solely CO2RR but also other side reactions will occur, resulting in the complexity of the reaction. For instance, the hydrogen evolution reaction (HER) competes with CO2RR to generate H2, which decreases the performance of CO2RR20–22 in order to efficiently overcome the energy barrier of electron-transfer proton coupling, accelerate the catalytic reaction rate. and suppress the occurrence of side reaction processes. The development of ideal catalysts with remarkable CO2RR selectivity and activity is the focus of current research. Several parameters of electrocatalytic performance for CO2RR catalysts can primarily be evaluated, including Faradaic efficiency (FE), overpotential, current density, Tafel slope, stability, and so on.7,12,14 FE is the charge required as a percentage of the initial charge to cross the working electrode and facilitate the electrochemical reaction. In simple terms, FE is a significant indicator to measure the selectivity of electrocatalytic CO2RR products.23 Due to the complex reaction mechanism and sluggish kinetics, the actual reduction reaction working potential is more negative than the theoretical reduction potential. High reduction overpotentials lead to wasted energy and significant HER reactions. Therefore, overpotential is an important indicator in evaluating the electrocatalytic activity of CO2RR catalysts. Studies confirm that when current density goes above 300 mA cm−2, the production cost will be reduced as much as possible.24 The equation of Tafel (η = blgj + a) is able to directly reflect the rate of reaction dominated by kinetics. The smaller the Tafel slope of b in the equation, the faster the electrochemical reaction rate and the higher the catalytic activity, which is more favorable for the electrocatalytic reaction.25 Stability is an indicator of whether an excellent electrocatalyst possesses long-term stability and efficiency.26 The stability of electrocatalysts is usually effectively assessed with potentiostatic electrolysis or cyclic voltammetry. Bismuth-based electrocatalysts Recently, P-block electrocatalysts consisting of bismuth (Bi),27–30 tin (Sn),31–33 lead (Pb),34–36 and indium (In)37,38 have inarguably facilitated electrocatalytic CO2 reduction with remarkable selectivity for C1 products, especially formic acid or formate. The advantage of electrocatalytic generation of formate lies in the high selectivity and current density achieved by prohibiting competing side reactions. Compared with other value-added products from CO2 reduction, which are difficult to solely generate and low in yield, the FE of Bi-based CO2 toward formate can reach nearly 100%.39 In addition, formate is a liquid product with excellent chemical stability at room temperature for storage and transportation compared to gas-phase products such as CO. Experts evaluate various chemicals with economic viability in CO2RR and discovered that formate has considerable marketability.13 Moreover, converting CO2 toward formate is a 2-electron transfer process, leading to a low production cost of 1$US/0.59 kg, suggesting that CO2 electrolysis of formate is more cost-competitive than the C2 product production process.40 It may be difficult to attain widespread application of Pb and In metals on a marketable scale because of toxicity or low availability. However, Bi is a dramatic and promising electrocatalyst due to its low cost, nontoxicity, environmental safety, relatively single reduction product, and high formate generation activity. Bi-based catalysts can be traced back to the Bi electrocatalyst synthesized by Komatsu's team in 1995.41 In the following decades, research on Bi-based catalysts continued to deepen. Monometallic catalysts, including metallic Bi, are currently a hot topic in the field of electrocatalysis. Recently, with in-depth exploration and development of synthesis techniques, various nanostructured monometallic Bi catalysts have been designed, such as nanoparticles, nanowires, nanotubes, nanosheets, nanodendrites, and so on, in multidimensional aspects. However, monometallic Bi catalysts may be undesirable for the breakthrough of formate electrosynthesis, owing to the limited active sites on the catalyst surface. This means that monometallic Bi catalysts usually require high overpotential to achieve high formate FE and partial current density. Compared with the monometallic Bi catalysts that use advanced synthetic strategies or tedious structural optimization to enhance their performance, bimetallic Bi-based catalysts will further involve synergistic effects. The synergistic interaction between bimetals gives the bimetallic electrocatalyst a superior catalytic performance.42 The synergistic effects in bimetallic Bi-based catalysts can be broadly viewed as Bi acting as the active site and the other metals mainly playing three roles: (1) tailoring the electronic structures of Bi sites, (2) regulating the adsorption states of the key intermediates, and (3) generating interfacial active sites to further enhance performance. Through the electronic structure modulations by the second metal, the bimetallic Bi-based catalysts will boost the formation of the key intermediate OCHO*, thus improving the performance of CO2 electroreduction to formate. The activity, selectivity, and stability will be further improved via preparing bimetallic Bi-based materials through strategies such as alloying, surface doping, defect introduction, and nanoengineering. Here, in this review, the representative reaction pathways of Bi-based electrocatalysts are first introduced, and then the reaction mechanisms of bimetallic Bi-based heterogeneous CO2RR electrocatalysts are summarized with examples from the perspective of reaction pathways. Afterward, based on the Bi-based electrocatalysts in recent development, we divide bimetallic Bi-based catalysts into three categories: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. For each category, we describe in detail its performance-enhancing strategies and provide examples of catalysts, including descriptions of their preparation process, composition, morphology, catalytic activity, and product properties. Finally, we provide an in-depth analysis of the existing challenges and the current outlook for this field. Reaction Mechanisms of the Bi-Based Electrocatalysts Bi-based catalysts have high efficiency, selectivity, and stability to form formate via electrocatalytic CO2RR in aqueous solutions. They also possess the capacity to generate CO, according to some reports.43 The pathways of the formate and CO products are comparatively simple compared to that of other CO2RR products, and the essential difference is the intermediate products. An in-depth study of the electrocatalytic CO2RR process on the surface of Bi-based catalysts is required for a good understanding of the catalytic mechanism of Bi metals. In general, there are three types of steps involving the generation of products theoretically, consisting of: (1) reactant adsorption on the electrocatalytic surface, (2) transfer of electrons and protons to the reactant, and (3) the products desorption from the electrocatalyst surface.44 Research demonstrates that the first proton coupling determines the selectivity for a catalyst, which takes place at the C or O in CO2*− radical anion. Three reaction pathways for electrocatalytic CO2RR over Bi-based catalysts are displayed in Figure 1: (a) Generally, CO2 comes into contact with the catalyst via carbon or oxygen atoms. If the carbon atom binds to the catalyst electrode surface first, *COOH intermediate will be formed, which is the first intermediate for CO2 activation in this pathway. However, *COOH intermediate will have multiple pathways in the second proton coupling electron transfer (PCET) process, which is not conducive to promoting highly selective formate production. *COOH can lose H2O to form CO, or be reduced to form HCOOH. (b) Compared to pathway a, pathway b is different in that the oxygen is bound to the electrode surface, by which CO2* − hydrogenation forms the HCOO* intermediate.19 Based on the reaction mechanism of Bi-based catalysts, the second PCET process of the HCOO* intermediate can only generate HCOOH.(c) In the pathway c, CO2 forms the OCHO* intermediate during the first PCET when only one oxygen molecule is bound to the surface electrode. Formate is the only product obtained via a subsequent second PCET process. Theoretical analyses indicate that the formation energy barrier of OCHO* intermediate is lower than that of *COOH and HCOO* intermediates, leading to the importance of OCHO* intermediates in the Bi-based electroreduction process.45 Figure 1 | Possible electrochemical reaction pathways of CO2 over Bi-based catalysts. Download figure Download PowerPoint Theoretical calculations confirm that the CO2RR process of Bi-based catalysts follows the key intermediate of OCHO* from a PCET mechanism, facilitating highly selective formate production. The specific mechanism equations (1–5) for reducing CO2 pathway in solution to HCOOH are summarized as follows:46 CO 2 ( g ) → CO 2 * (1) CO 2 * + e − → CO 2 * − (2) CO 2 * − + e − + H + → OCHO * − (3) OCHO * − + e − + H + → HCOOH * (4) HCOOH * → HCOOH ( aq ) + * (5)where * denotes the catalytic surface or adsorption site, initially the CO2 molecules are dissolved in the solution and contact the electrode surface to form adsorbed CO2*. Afterwards, single electron is transferred to CO2* that forms the CO2* − radical anion. According to HCO3 − ↔ H+ + CO32−, electron transfer and proton coupling form OCHO* − intermediates. Ultimately, OCHO* − forms formic acid solution by PCET. Since the slow kinetics of the electrocatalytic CO2RR process, HER side reactions inevitably generate H2, and some reduction reactions are accompanied by CO generation, which negatively affects the highly selective production of formate from Bi-based materials. For the purpose of obtaining more formate, the production of H2 and CO is reduced as much as possible. The catalytic mechanism may be diametrically different in the same Bi-based alloy, depending on the content of two metal elements. Therefore, the reaction mechanism is manipulated via controlling different proportions of the two metals in bimetallic Bi-based electrocatalysts. For instance, Zhang et al.47 have developed CuO/Bi(OH)3 decorated on carbon nanotubes for CO2 electroreduction. CuBi#8 and CuBi#4 are bimetallic nanoparticles obtained via a two-step hydrolysis method and adjusted the Cu/Bi ratio. CuBi#8 and CuBi#4 exhibit CO and formate FE of 96% and 60% at −0.99 V versus reversible hydrogen electrode (RHE) (VRHE), respectively. It is found that with Bi content increasing, HER is well suppressed, and the products are mainly CO. By further increasing Bi content, FEHCOOH rapidly increases accompanied by the rapid decrease of FECO. FEHCOOH reaches the maximum of 96% at 12.5 mA cm−2. The conversion of intermediates from *COOH to OCHO* via increasing Bi content further illustrates the high selectivity of OCHO* to formate. Adjusting the optimal Cu/Bi ratio suppresses the production of CO and H2, leading to efficient production of formate. In addition, defects such as oxygen vacancies or doped atoms significantly improve the catalytic performance.12 Li et al.48 have prepared Sn atom-doped Bi2O3 nanosheet (NS) electrocatalysts by constant electrolysis. Three products can be detected during the electrolysis, including H2, CO, and HCOOH. The Sn-doped Bi2O3 NSs current density is significantly increased compared to the undoped Bi2O3 NSs. The 2.5% Sn-doped Bi2O3 NSs exhibit high selectivity for formate, obtaining a supreme FE of 93.4% at the potential of −0.97 V. The HER inhibition effect is significantly enhanced compared to the undoped Bi2O3 NSs. Moreover, the catalytic capacity is optimized by coping with significant HER and expanding its specific surface area. For the first time, metallic aerogel is a three-dimensional (3D) material that has attracted enormous attention due to its abundant specific surface area, contributing to the generation of more catalytic centers.49 In addition, adjusting the partial pH can also improve the selectivity of CO2RR, and proper control of pH into acidity facilitates the formation of formate.50 Advanced Bi-Based Electrocatalysts for CO2 Reduction In recent decades, various Bi-based CO2 reduction electrocatalysts have been exhaustively studied, mainly with formate as the end product. Especially, the preparation of bimetallic catalysts via different synthetic methods is the focus of most current studies. Bimetallic Bi-based catalysts can mainly be classified into three types: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. Detailed CO2RR performances of bimetallic Bi-based electrocatalysts are summarized in Table 2. Table 2 | Performance of Bimetallic Bi-Based Catalysts in Electrocatalytic CO2RR Catalyst Electrolyte Major Products FE (%) Potential at FEMax (V) Current Density (mA cm−2) Stability (h) References Bi5Sn60 0.1 M KHCO3 Formate 94.8 −1.0 (vs RHE) 34 20 52 BixSny/Cu 0.1 M KHCO3 Formate 90.4 −0.84 (vs RHE) 30 12 53 Bi-Sn aerogel 0.1 M KHCO3 Formate 93.9 −1.0 (vs RHE) 9.3 10 57 Cu-Bi 0.1 M KHCO3 Formate 90 −0.8 (vs RHE) >2 — 60 CuBi-100 0.5 M KHCO3 Formate 94.7 −1.0 (vs RHE) 12.8 8 61 CuBi 0.5 M KHCO3 Formate 94.4 −0.97 (vs RHE) 38.5 — 62 CuBi 0.5 M KHCO3 Formate 98.3 −1.07 (vs RHE) 56.6 — 62 Bi/Cu 0.5 M KHCO3 Formate 95 −0.9 (vs RHE) 59.7 12 64 CuBi75 0.5 M KHCO3 Formate 100 −0.77 (vs RHE) 33.65 24 66 Cu1-Bi/Bi2O3@C 0.5 M KHCO3 Formate 93.4 −0.94 (vs RHE) 10.1 10 67 Pd3Bi-IMA 0.1 M KHCO3 Formate >90 −0.35 (vs RHE) 3 8 68 a-NPSB 0.1 M KHCO3 Formate 88.4 −1.15 (vs RHE) 21.2 18 70 Bi–Pt complex 0.1 M TBAPF6/THF CO 82 −1.25 (vs NHE) 0.125 — 71 Mo-Bi BMC/CP 0.5 M [Bmim]BF4 CH3OH 71.2 −0.7 (vs SHE) 12.1 — 72 Sn-doped Bi2O3 NSs 0.5 M KHCO3 Formate 93.4 −0.97 (vs RHE) 24.3 8 48 Bi/Bi(Sn)Ox NWs 1 M KOH Formate ∼100 −0.7 (vs RHE) 301.4 20 77 Cu-Bi2Se3 0.5 M NaHCO3 Formate 65.31 −1.3 (vs RHE) 24.1 24 78 Ce–[email protected]x/C 0.5 M KHCO3 Formate 96 −1.7 (vs SHE) 15.2 10 79 BiIn5[email protected] 0.5 M KHCO3 Formate 97.5 −0.86 (vs RHE) 13.5 15 81 Bi-Sn/CF 0.5 M KHCO3 Formate 96 −1.1 (vs RHE) 45 100 82 Sn0.80Bi0.20@Bi-SnOx 0.5 M KHCO3 Formate 95.8 −0.88 (vs RHE) 74.6 — 83 Bi-SnO/Cu 0.1 M KHCO3 Formate 93 −1.7 (vs Ag/AgCl) — 30 84 Cu [email protected] NW 0.5 M NaHCO3 Formate 95 −0.69 (vs RHE) 15 12 45 Cu NWs-Bi NSs 0.5 M KHCO3 Formate 87 −0.86 (vs RHE) 16.7 8 90 Bi-NAs 0.5 M KHCO3 Formate 94 −0.8 (vs RHE) 30.9 6 86 BiCu/CF 0.5 M KHCO3 Formate 94.2 −0.8 (vs RHE) 856 10 91 [email protected] nanocone 0.5 M KHCO3 Formate 96.9 −0.95 (vs RHE) 31.9 10 92 Bi/CeOx 0.2 M Na2SO4 Formate 98 −1.2 (vs RHE) 149 34 87 Alloyed Bi The alloying strategy is one of the vital methods to enhance the electrocatalytic performance of Bi-based catalysts.51 Synergistic effects better facilitate the catalytic reaction by forming a dual-metallic alloy between two different metal elements. P-block transition catalysts have demonstrated CO2 conversion that is highly selective for C1. Among them, Bi-Sn bimetallic alloy is the most suitable. Electrodeposition is a low-cost and high-efficiency method for the preparation of alloys. The electrodeposition method promotes a better combination of two metallic elements. Bimetallic alloys obtained via electrodeposition are rarely grain-size-limited or shape-limited and possess advantages such as high density and poor porosity. These can potentially improve the intrinsic catalytic activity of the catalyst itself. Li et al.52 have deposited two metals, Bi and Sn, on the Cu mesh using an electrodeposition strategy. In this work, employing different deposition times tuned easily electrode morphology. The electrodes at different lengths of deposition periods showed similar pine needle-like dendritic microstructures. This structure is beneficial to extend the surface area and develop more catalytic centers. When the deposition time of metal Bi was 5 min, and the deposition time of metal Sn was 60 min, the catalytic performance was the best, expressed as Bi5Sn60. Under the condition of −1.0 VRHE with 34.0 mA cm−2 partial current density, the FE of formate reached 94.8%. More interestingly, it exhibited excellent 634.3 μmol cm−2 h−1 formate yield, which was superior to most reported electrocatalysts. The metastable interface of bimetallic deposition is beneficial to stabilize the OCHO* intermediate and suppress the HER process. Li et al.53 have also electrodeposited BixSny electrocatalysts on Cu foams electrodes and discovered that the FE of formate was enhanced by raising the Bi molar content in the BixSny/Cu electrode. Aerogel catalysis is an emerging branch in electrocatalytic systems, due to their high-density porous structure and the distinct nanoscale properties of the catalyst, contributing to the generation of more catalytic centers.49,54,55 Bimetallic alloys prepared by the sol–gel method make it easier to obtain rich nanostructures compared with methods such as electrodeposition, annealing, or solution synthesis.56 Wu et al.57 have prepared Bi-Sn bimetallic aerogels under ambient conditions with 3D morphology and abundant interfaces (Figure 2a). Bi and Sn influence the nanostructure of engineered nanomaterials via controlling the molar ratio. Nanowires with random mini diameters containing porous nanostructure were measured with a transmission electron microscope (TEM) as shown in Figure 2b. These nanowires randomly formed abundant pores of network morphology that was favorable for electrolyte transport, and exposed abundant active sites during electrocatalysis. The X-ray diffraction (XRD) peaks of the designed Bi-Sn aerogel matched well with those of Sn and Bi (Figure 2c), suggesting that the obtained aerogel is a coexisting Bi-Sn alloy. The high-resolution TEM (HRTEM) images show that the interplanar spacings are 0.33 and 0.29 nm, corresponding to the (012) and (200) planes of Bi and Sn, respectively (Figure 2d). This reveals that the Sn and Bi interfaces are abundant, and more reaction sites are generated during the catalytic process. The energy-dispersive X-ray spectroscopy (EDX) image (Figure 2e) confirmed the uniform distribution of Sn and Bi in the as-prepared Bi-Sn. The electrochemical performance displayed that, compared with pure Sn and Bi catalysts, Bi-Sn aerogel exhibited excellent performance for formate with FE as high as 93.9%. Density functional theory (DFT) reveals that electronic interactions between Bi and Sn optimize the energy barrier for formate, enhancing the electrocatalytic performance (Figure 2f). In situ attenuated total reflectance-Fourier transformation infrared (ATR-FT-IR) spectroscopy can indicate the reaction pathway of Bi-Sn aerogel bimetallic catalyst in CO2RR. As shown in Figure 2g, the characteristic peak at 1390 cm−1 occurred at −0.6 V and became more pronounced as the potential gradually increased from −0.6 to −1.2 V. This is attributed to the fact that O–C–O in the dioxygen-bridged formate intermediate HCOO* is in vibration, suggesting that the synergistic effects between Bi-Sn bimetals can stabilize the generation of HCOO*. Figure 2 | (a) Illustration of the synthesis of Bi-Sn aerogel. (b) TEM image of Bi-Sn. (c) XRD patterns of Bi-Sn. (d) HRTEM images and (e) the corresponding elemental mappings. (f) Illustration of the catalytic mechanism of the designed Bi-Sn. (g) ATR-FT-IR spectra of Bi-Sn with various applied potentials. Reproduced with permission from ref 57. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint Metal Cu is a vital and attractive catalyst in the field of electrocatalysis. The product selectivity of Cu-based catalysts is various, producing C1 products such as formate and CH4 and multicarbon value-added fuels such as C2H4.16,58 The introduction of the main group elements Bi and Cu to form a Cu-Bi alloy usually promote formate conversion and inhibit the HER reaction.59 The reaction intermediates with adjustment of the relative strength between bimetallic Cu-Bi facilitate the catalytic reaction. Hoffman et al.60 have synthesized dendritic bimetallic Cu-Bi electrocatalysts with nanosized grains. Owing to the lattice difference between Bi and Cu, the dendritic nanostructures of bimetallic alloys show defect sites with high density, leading to the formation of more catalytic centers. Cu-Bi dendrites exhibit lower current densities for HER and CO evolution reactions, thus reflecting the higher selectivity of Cu-Bi dendrites for formate. At −0.8 VRHE, the FE of the formate product reached 90%. In contrast to crystalline materials, amorphous materials obtained by amorphization have numerous abnormal atomic structures, and catalytic activity will be improved. Through the construction of amorphous bimetallic catalysts, the number of reaction sites of electrocatalysts are increased as much as possible, and the electrochemical window range of target product selectivity is improved. For instance, Xiong et al.61 have synthesized bimetallic Cu-Bi alloy electrocatalysts by electrodeposition at room temperature (Figure 3a). In the samples acquired by managing the time of electrodeposition, CuBi-100 exhibited the best electrochemical performance (Figure 3e). At the potential from −0.8 V to −1.2 VRHE, CuBi-100 showed excellent selectivity for formate and maintained FE of more than 85%. The supreme FEformate was 94.7% at the potential of −1.0 VRHE. Scanning electron microscopy (SEM) images clearly showed that CuBi-100 grows on carbon paper and exhibits a dendritic nanostructure (Figure 3b). The HRTEM image of CuBi-100 has amorphous struc