Br-doped CuO promotes water dissociation and enables efficient asymmetric C-C coupling during electrochemical reduction of CO2 to ethylene.

Tin oxides (SnO2) have been widely utilized in electronics, nanolithography, and catalysis. As the atomically precise models of SnO2, tin-oxo clusters (TOCs) not only provide opportunities for mech...

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 [email protected] 0.5 M NaHCO3 Formate 95 (vs RHE) 15 12 45 NSs 0.5 M KHCO3 Formate −0.86 (vs RHE) 8 90 0.5 M KHCO3 Formate −0.8 (vs RHE) 0.5 M KHCO3 Formate −0.8 (vs RHE) 10 [email protected] 0.5 M KHCO3 Formate (vs RHE) 10 M Formate (vs RHE) 34 Bi The is one of the methods to enhance the electrocatalytic performance of Bi-based effects facilitate the catalytic reaction by a between two different metal elements. P-block catalysts have CO2 conversion that is highly selective for Among them, Bi-Sn bimetallic is the most is a and method for the preparation of The method a of two metallic elements. Bimetallic obtained via are or and possess such as high density and can improve the catalytic activity of the catalyst Li et have two Bi and on the an In this different electrode The at different of This structure is to the surface and more catalytic When the of metal Bi and the of metal Sn 60 the catalytic performance the as the of −1.0 with mA partial current density, the FE of formate it excellent formate yield, which superior to most electrocatalysts. The of bimetallic is to the OCHO* intermediate and suppress the HER process. Li et have also electrocatalysts on and discovered that the FE of formate enhanced by the Bi content in the BixSny/Cu electrode. is an emerging in electrocatalytic due to their structure and the of the catalyst, contributing to the generation of more catalytic Bimetallic prepared by the method it to compared with methods such as or solution et have prepared Bi-Sn bimetallic under with and abundant Bi and Sn the of via controlling the ratio. with with a electron as shown in Figure abundant of that favorable for and abundant active sites during electrocatalysis. The of the Bi-Sn aerogel well with of Sn and Bi suggesting that the obtained aerogel is a Bi-Sn The that the are and corresponding to the and of Bi and This that the Sn and Bi are and more reaction sites are during the catalytic process. The the of Sn and Bi in the The electrochemical performance displayed compared with Sn and Bi catalysts, Bi-Sn aerogel excellent performance for formate with FE as high as Density that electronic between Bi and Sn the energy barrier for formate, the electrocatalytic performance In can indicate the reaction pathway of Bi-Sn aerogel bimetallic catalyst in CO2RR. shown in Figure the at at V and more as the potential increased from to V. This is to the that in the formate intermediate HCOO* is in suggesting that the synergistic effects between Bi-Sn bimetals can the generation of Figure 2 | (a) of the synthesis of Bi-Sn (b) of (c) of (d) and the corresponding of the catalytic mechanism of the of Bi-Sn with various with from Download figure Download PowerPoint is a and catalyst in the field of electrocatalysis. The product selectivity of catalysts is C1 products such as formate and CH4 and value-added fuels such as The of the Bi and to form a Cu-Bi usually formate conversion and the HER The reaction intermediates with of the between bimetallic Cu-Bi facilitate the catalytic reaction. et have synthesized bimetallic Cu-Bi electrocatalysts with Owing to the difference between Bi and the of bimetallic defect sites with high density, leading to the formation of more catalytic Cu-Bi exhibit lower current for HER and CO evolution thus the higher selectivity of Cu-Bi for formate. At −0.8 the FE of the formate product In to materials obtained by have numerous and catalytic activity will be Through the of bimetallic catalysts, the of reaction sites of electrocatalysts are increased as much as and the electrochemical of product selectivity is For instance, et have synthesized bimetallic Cu-Bi electrocatalysts by at room temperature In the by the of CuBi-100 the electrochemical performance At the potential from −0.8 V to CuBi-100 excellent selectivity for formate and FE of more than The supreme at the potential of −1.0 electron that CuBi-100 on carbon and a The of CuBi-100 has

Open AccessCCS ChemistryCOMMUNICATIONS1 Jul 2022BiO2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO2 Reduction to Formate Zhonghao Tan, Jianling Zhang, Yisen Yang, Yufei Sha, Ran Duan, Jiajun Zhong, Buxing Han, Jingyang Hu and Yingzhe Zhao Zhonghao Tan Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 , Jianling Zhang *Corresponding author: E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 , Yisen Yang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 , Yufei Sha Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 , Ran Duan Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Jiajun Zhong Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100190 , Buxing Han Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 , Jingyang Hu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 and Yingzhe Zhao Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100190 https://doi.org/10.31635/ccschem.022.202202068 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail To enhance the activity and selectivity of electrocatalytic CO2 reduction to formate is of great importance from both environmental and economical viewpoints. Herein, the BiO2-x nanosheets with surface electron localizations were constructed and utilized for the efficient CO2-to-formate conversion. The formate Faraday efficiency reaches 99.1% with current density of 12 mA cm−2 at −1.1 V versus the reversible hydrogen electrode (RHE) in an H-type cell while those in the flow cell are 91.3% and 319 mA cm−2 at −1.0 V versus RHE, respectively. Theoretical calculations indicate that the electron localization presenting in the BiO2-x nanosheet favors OCHO* intermediate stabilization and suppresses H* intermediate adsorption, thus improving the CO2-to-formate efficiency. The BiO2-x electrocatalyst is nondopant, easily prepared, low-cost, highly active and selective for CO2RR to formate, which has demonstrated potential for application in the Zn-CO2 battery. The maximum power density can reach 2.33 mW cm−2, and the charge/discharge cycling stability is >100 h (300 cycles) at 4.5 mA cm−2. Download figure Download PowerPoint Introduction The high CO2 concentration in the atmosphere caused by excessive consumption of fossil fuels has led to a series of environmental problems and disrupted the natural carbon cycle.1–4 CO2 reduction reaction (CO2RR) to value-added chemicals using electricity is a promising way to reduce CO2 concentration in the atmosphere and achieve artificial carbon cycles.5–15 In particular, the electrochemical CO2RR to formate is very important because formate is a high-value liquid product, widely used in chemical production and fuel cells.16–18 Compared with the CO2RR to other carbon-containing products (e.g., methane, methanol, ethylene, ethanol, etc.), the CO2RR to formate has been regarded as the most economically viable route due to the need for only two electrons and its low equilibrium potential.19–25 Despite these advantages, the CO2RR to formate is restricted by the high energy barrier for CO2 conversion to OCHO* and the competition with the hydrogen evolution reaction (HER) in aqueous solution.26,27 Up to now, various metals such as Sn,28 In,29 Bi,30 Pb,31 and Co32 have been utilized for electrocatalytic CO2RR to formate. Among these electrocatalysts, the Bi-based materials have attracted much attention because Bi has advantages of being nontoxic, inexpensive, and abundantly available in the earth.33,34 Diverse kinds of Bi-based catalysts have been synthesized for the electrocatalytic CO2 conversion to formate, including Bi,35 Bi-Sn,36 Pd3Bi,37 Bi2Te3,38 Bi2WO6,39 and their composites with a secondary phase, like Bi/CeOx,40 [email protected] carbon nanotubes,41 S-Bi2O3/carbon nanotubes,42 Bi2O3@carbon,43 Bi2O3/carbon nanofiber,44 and so on. The formate Faraday efficiencies (FEs) can reach values >90.0%, but the current densities mostly remain low. It is still a challenge to develop Bi-based catalysts with both high formate selectivity and current density that can meet the industrial requirement of being >300 mA cm−2. For the first time in this work, we constructed BiO2-x nanosheets with surface electron localizations that exhibit high selectivity and activity for electrochemical CO2RR to formate. The formate FE can reach 99.1% at −1.1 V versus a reversible hydrogen electrode (RHE) in a H-type cell. The current density is up to 319 mA cm−2 with formate FE of 91.3% at −1.0 V versus RHE in flow cell. Density functional theory (DFT) calculations reveal that the electron localization of BiO2-x lowers the energy barrier for the production of OCHO* intermediates, making it easier to generate than H* intermediates. Compared with the widely adopted heteroatom doping method to induce electron localization of catalyst for boosting electrochemical CO2RR,45–47 the BiO2-x electrocatalyst is nondopant, easily prepared, and low-cost. To explore its potential application, a Zn-CO2 battery using the BiO2-x catalyst for cathode was assembled, which exhibits high power density of 2.33 mW cm−2 with a long-term stability >100 h (300 cycles) at 4.5 mA cm−2. Experimental Section Chemicals Ascorbic acid and sodium hydroxide (purity, 96.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium bicarbonate (purity, 99.5%) and potassium hydroxide (purity, 85.0%) were provided by Aladdin Reagent Co., Ltd. (Shanghai, China). Zn plate, sodium bismuthate dihydrate (purity, 80.0%), Nafion D-521 dispersion, Nafion N-117 membrane, and Toray carbon paper (TGP-H-060) were supplied by Alfa Aesar Reagent Co., Ltd. (Shanghai, China). The gas diffusion layer (YLS-30T), Fumasep FAA-3-PK-130 membrane, and Fumasep FBM-PK membrane were obtained from Suzhou Sinero Technology Co. Ltd. (Suzhou, China). D2O (purity, 99.9%) and dimethyl sulfoxide (DMSO; purity, 99.0%) were bought from Innochem Reagent Co., Ltd. (Beijing, China). High purity CO2 gas (99.999%), high purity Ar gas (99.999%), and deionized water were provided by Beijing Analytical Instrument Company (Beijing, China). All reagents were used directly without further treatment. Synthesis of p-BiO2-x p-BiO2-x was synthesized by a hydrothermal method. First, 3.0 g sodium bismuthate dihydrate and 2.4 g sodium hydroxide were dissolved in 60 mL deionized water and then stirred vigorously for 0.5 h. The above solution was transferred into a 100 mL Teflon-lined autoclave and heated to 180 °C for 5 h. After naturally cooling down to room temperature, the solid product was separated by centrifugation and washed three times by deionized water and dried in vacuum at 80 °C for 6 h. Synthesis of m-BiO2-x m-BiO2-x was synthesized by a solid-phase grinding method for p-BiO2-x. A mixture of p-BiO2-x and ascorbic acid with a mass ratio of 1:1 was vigorously ground in an agate mortar for 0.5 h. Then the solid was washed six times by deionized water to remove ascorbic acid and dried in vacuum at 80 °C for 6 h. Characterizations X-ray diffraction (XRD) patter was determined by a Rigaku D/max-2500 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was determined by a Thermo Fisher Scientific ESCALAB 250 Xi (Thermo Fisher Scientific, Waltham, MA, USA) using 200 W Al Kα radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained from a HITACHI S-4800 scanning electron microscope (Hitachi, Tokyo, Japan) and a JEOL-1011 field-emission transmission electron microscope. High-resolution TEM (HRTEM) image was obtained from a JEOL-2100F field-emission transmission electron microscope (JEOL, Tokyo, Japan). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and STEM electron energy loss spectroscopy (EELS) were characterized by Cryo-TEM (Thermo Scientific Themis 300; Thermo Fisher Scientific, Waltham, MA, USA). X-ray absorption fine structure (XAFS) data were collected at the 1W2B station at the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). Raman spectra were determined by a Horiba LabRAM HR Evolution Raman microscope (HORIBA Scientific, Paris, France; 512 nm). Electrocatalytic CO2 reduction All electrochemical tests were performed using a CHI-660E electrochemical workstation equipped with a high current amplifier CHI 680c in a H-type cell and flow cell, respectively. All electrode potentials were transformed into standard RHE potentials by using the following formula: E ( RHE ) = E ( Ag / AgCl ) + 0.197 + 0.0591 × pH The iR compensation was conducted in flow cell experiment at each potential. In a H-type cell, 0.1 M KHCO3 aqueous solution and Nafion-117 membrane were used as electrolyte and proton exchange membrane, respectively. Before each electrochemical experiment, 0.1 M KHCO3 aqueous electrolyte was saturated with CO2 for at least 0.5 h. The hydrophobic carbon paper (TGP-H-060, 1 × 1 cm−2) coated with catalyst was used as the working electrode. Ag/AgCl electrode and Pt net (1 × 1 cm−2) were used as reference electrode and counter electrode, respectively. For the preparation of the working electrode, 1 mg catalyst powder and 10 μL Nafion were distributed into 200 μL ethanol to form a homogeneous catalyst ink, which was then spread onto carbon paper. The flow cell was obtained from Gaoss Union (Tianjin) Photoelectric Technology Company (Tianjin, China). It was constructed with an anode chamber and a cathode chamber. An anion exchange membrane (Fumasep FAA-3-PK-130) was used to separate the anode and cathode chambers. In the flow cell, 1 M KOH aqueous solution and the Fumasep FAA-3-PK-130 membrane were used as electrolyte and anion exchange membrane, respectively. The gas diffusion layer (YLS-30T, 0.5 × 2 cm−2) coated with catalyst was used as a gas diffusion electrode. Ag/AgCl electrode and Pt net (0.5 × 2 cm−2) were used as reference electrode and counter electrode, respectively. To prepare the gas diffusion electrode, 5 mg catalyst powder and 50 μL Nafion were dispersed into 750 μL ethanol to form a homogeneous catalyst ink. Then 160 μL catalyst ink was dropped on the gas diffusion layer with the eventually loading mass of ∼1 mg cm−2. The flow rate of cathode peristaltic pump and anode peristaltic pump were adjusted to be 30 mL min−1, and the flow rate of CO2 was controlled at 20 sccm by gas flow meter. Product analysis The gas product of electrochemical experiment was collected by a gas bag (each gas bag was collected for 2000 s). The gas product was detected by gas chromatography (Agilent 8890; Agilent Technologies Inc., CA, USA) with a thermal conductivity detector (TCD) detector using high purity argon as carrier gas. The liquid product was determined by 1H NMR (Bruker AVANCE III 400 HD; Bruker, Germany). After each electrochemical test, 200 μL reaction electrolyte was mixed with 200 μL D2O and 100 μL 6 mM DMSO solution, and then detected by 1H NMR. The Faraday efficiency was calculated by the following formulation: FE = Moles of product Q / n F × 100 % where Q: charge (C); F: 96485 C/mol; n: number of electrons required to generate the product. Zn-CO2 battery The Zn-CO2 battery test was performed in the flow cell. The gas diffusion layer (YLS-30T, 0.5 × 2 cm−2) coated with m-BiO2-x and the polished Zn plate were used as battery electrodes. The 1 M KHCO3 aqueous solution, and 2 M KOH/0.02 M Zn(CH3COO)2 aqueous solution served as electrolytes for cathode and anode, respectively, which were separated by a bipolar membrane (Fumasep FBM-PK). CO2 flow rate was controlled at 20 sccm during the test. The galvanostatic discharge curves were measured by galvanostatic discharge at 1.5, 3.0, and 4.5 mA cm−2. For charge/discharge cycles, the current density was set to 4.5 mA cm−2. In situ Raman spectroscopy The in situ Raman experiment was conducted by using a Horiba LabRAM HR Evolution Raman microscope (HORIBA Scientific, Paris, France). The laser wavelength was controlled at 785 nm. 0.1 M KHCO3 aqueous solution was used as electrolyte. Carbon paper coated with catalyst, a Ag/AgCl electrode, and a carbon rod were used as working electrode, reference electrode, and counter electrode, respectively. The in situ Raman electrolytic cell was purchased from Gaoss Union (Tianjin) Photoelectric Technology Company (Tianjin, China). Computational method All the computations were performed by using the Vienna ab initio simulation package.48,49 The ion-electron interactions were described by the projector augmented wave method,50 and the general gradient approximation in the Perdew–Burke–Ernzerhof form was used.51,52 A cutoff energy of 450 eV for the plane-wave basis set was adopted. During structural relaxation, the convergence criterion was set to be 0.03 eV/Å and 10−5 eV for the residual force and energy, respectively. A 3 × 3 × 1 BiO2-x (111) slab was used as the model, and the Brillouin zone was sampled by a Monkhorst–Pack 3 × 3 × 1 k-point grid. To avoid the interaction between two periodic units, a vacuum space of 15 Å was employed. The free energy change (ΔG) of each elementary reaction was calculated as Δ G = Δ E + Δ E ZPE − T Δ S where ΔE, EZPE, T, and S are reaction energy difference, zero-point energy, temperature, and entropy, respectively. Results and Discussion The BiO2-x electrocatalyst was prepared by a two-step method. First, the normal BiO2-x was synthesized by a hydrothermal route for sodium bismuthate dihydrate and sodium hydroxide at 180 °C for 5 h (see its characterizations in Supporting Information Figures S1–S3). Then a surface modification step was applied to the above BiO2-x by grinding it with ascorbic acid for 0.5 h. The pristine BiO2-x synthesized in the first step and the modified BiO2-x were named as p-BiO2-x and m-BiO2-x, respectively. The XRD pattern of m-BiO2-x coincides completely with that of BiO2-x (JCPDS PDF#47-1057) and p-BiO2-x (Figure 1a). This indicates that the crystalline structure of m-BiO2-x remains unchanged after modification from p-BiO2-x. Notably, the crystallinity of m-BiO2-x decreases compared with that of p-BiO2-x, which might be attributed to the generation of oxygen defects as discussed in following.53 SEM and TEM images reveal that m-BiO2-x keeps the nanosheet morphology of p-BiO2-x (Figure 1b,c). The HRTEM image shows lattice spacings of 0.317 and 0.119 nm (Figure 1d), corresponding to the (111) and (220) crystal planes of BiO2-x, respectively. Bi M4,5-edge EELS characterization displays that the M4,5-edge gradually shifts to higher energy areas by 12 eV from spot 1 to spot 4 (Figure 1e,f). This suggests the gradual increase of oxygen content from surface to bulk, which is indicative of a large number of surface oxygen defects in m-BiO2-x.54 In contrast, the M4,5-edge of p-BiO2-x has a shift of 1 eV from spot 1 to spot 4 ( Supporting Information Figure S4). The Brunauer–Emmett–Teller surface areas of m-BiO2-x and p-BiO2-x are 7.73 and 6.37 m2 g−1, respectively, as determined by the N2 adsorption–desorption method ( Supporting Information Figure S5). Figure 1 | Morphology characterizations of m-BiO2-x. (a) XRD patterns of p-BiO2-x and m-BiO2-x. (b) SEM image, (c) TEM image, (d) HRTEM image, (e) STEM-EELS sampling region, and (f) EELS spectra of m-BiO2-x. Scale bars: 200 nm in (b), 400 nm in (c), 5 nm in (d) and 100 nm in (e). Download figure Download PowerPoint The Raman spectrum of m-BiO2-x exhibits a lowered intensity and a red shift compared with that of p-BiO2-x (Figure 2a). This indicates a smaller Bi-O bond force constant in m-BiO2-x according to Hooke's law, which is consistent with the large number of oxygen defects in m-BiO2-x.55 By electron paramagnetic resonance (EPR), the signal of m-BiO2-x is much stronger than that of p-BiO2-x (Figure 2b), indicating a higher defect degree. The g-factor (g) values for m-BiO2-x and p-BiO2-x were determined to be 2.007 and 2.004, which are typical oxygen defect signals.56 XPS was utilized to further characterize p-BiO2-x and m-BiO2-x ( Supporting Information Figure S6). For high-resolution O1s XPS spectra (Figure 2c), the three peaks represent lattice oxygen (O1, 529.2–529.5 eV), defect oxygen (O2, 531.1–531.7 eV), and absorbed oxygen (O3, 533.4–533.6 eV), respectively. The content of defect oxygen relative to the total oxygen in m-BiO2-x was calculated to be 62.2%, which is much higher than that of p-BiO2-x (25.5%, Supporting Information Table S1). For high-resolution Bi 4f XPS spectra, the Bi 4f7/2 and Bi 4f5/2 peaks present at 158.15 and 163.40 eV for m-BiO2-x and 158.30 and 163.55 eV for p-BiO2-x, respectively (Figure 2d). The results suggest a higher electron density around Bi atoms of m-BiO2-x than p-BiO2-x. Figure 2 | Structural characterizations of m-BiO2-x. (a) Raman spectra, (b) EPR spectra, (c) O1s XPS spectra, (d) Bi 4f XPS spectra, (e) Bi L3-edge XANES spectra, and (f) Bi L3-edge EXAFS spectra of p-BiO2-x and m-BiO2-x. Download figure Download PowerPoint The fine structure of m-BiO2-x was investigated by Bi L3 edge X-ray absorption near edge structure (XANES). Commercial Bi, Bi2O3, and p-BiO2-x were used as contrast samples. The XANES spectra of m-BiO2-x and p-BiO2-x are roughly similar but different from those of commercial Bi and Bi2O3 powder (Figure 2e). Compared with p-BiO2-x, the white line peak of m-BiO2-x is weaker and shifts to a lower energy region, indicating the weaker oxidation state in m-BiO2-x. From extended X-ray absorption fine structure (EXAFS), the Bi-O bond lengths of p-BiO2-x and m-BiO2-x are 1.56 Å while that for commercial Bi2O3 is 1.65 Å (Figure 2f). According to EXAFS fitting curves ( Supporting Information Figure S7 and Table S2), the Bi coordination numbers of p-BiO2-x and m-BiO2-x were determined to be 4.4 and 3.1, respectively. The smaller coordination number of m-BiO2-x confirms the presence of a large number of oxygen defects. Further, the wavelet-transform EXAFS (WT-EXAFS) gives support that m-BiO2-x has an unsaturated coordination environment ( Supporting Information Figure S8). To investigate the catalytic activity of m-BiO2-x for CO2RR, linear sweep voltammetry (LSV) measurements were carried out. A H-type cell was used with 0.1 M KHCO3 aqueous solution as electrolyte. Obviously, the current densities in CO2-saturated electrolyte are higher than those in electrolyte (Figure This indicates that m-BiO2-x is active for CO2RR than Compared with p-BiO2-x, m-BiO2-x exhibits current density at the reduction higher CO2RR activity of m-BiO2-x. Figure 3 | CO2 reduction measurements in H-type cell. (a) curves of p-BiO2-x and m-BiO2-x in CO2-saturated 0.1 M KHCO3 aqueous electrolyte. (b) Formate FE values at different applied potentials in CO2-saturated 0.1 M KHCO3 aqueous electrolyte. (c) Formate current densities at different applied (d) current density different (e) for p-BiO2-x and m-BiO2-x. (f) test of m-BiO2-x at −1.1 V versus Download figure Download PowerPoint The of m-BiO2-x for electrocatalytic CO2RR was The gas and liquid products were determined by gas chromatography and 1H respectively. Formate was detected as the product with the production of of and The formate FE values in an electrochemical from −1.0 to V versus RHE (Figure −1.1 V versus RHE, the maximum formate FE can reaches indicating the efficient of and production ( Supporting Information Figure Compared with p-BiO2-x, the selectivity to formate is by m-BiO2-x as as the formate current density Figure To a into the catalytic activity of m-BiO2-x, voltammetry various sweep were conducted ( Supporting Information Figure The of p-BiO2-x and m-BiO2-x were determined to be and cm−2, respectively (Figure This indicates that m-BiO2-x has a electrochemical surface and active To the of p-BiO2-x and m-BiO2-x, the curves Ar were ( Supporting Information Figure It is that m-BiO2-x has lower The electrochemical spectra of p-BiO2-x and m-BiO2-x were measured for electron From the m-BiO2-x has much smaller electron than p-BiO2-x (Figure the of active and smaller electron in m-BiO2-x are for The stability of m-BiO2-x was investigated at −1.1 V versus RHE for 15 h (Figure The current density can at mA cm−2 during the test while formate FE to 10 h and remains up to 15 h. the flow cell was used as an to the above H-type cell for electrocatalytic CO2RR to the current density to meet the requirement of commercial application ( Supporting Information Figure 1 M KOH aqueous solution was used as electrolyte in with the From the curves (Figure we can that the current densities are higher than those in the H-type cell (Figure 2a). For a current density of 400 mA cm−2 was at V by m-BiO2-x, which is higher than that by p-BiO2-x mA The electrocatalytic CO2RR was carried in a flow cell at potentials from to −1.0 V versus by m-BiO2-x, the maximum formate FE reaches at V versus RHE and the current density is mA cm−2 (Figure The current density can be to 319 mA cm−2 at −1.0 V versus RHE, with formate FE of In contrast, the formate FE and current density p-BiO2-x at −1.0 V versus RHE are and mA cm−2, respectively. In the values by m-BiO2-x are higher than those by p-BiO2-x (Figure The stability of CO2RR m-BiO2-x in the flow cell was investigated at V versus 10 h the formate selectivity and current density high values of and mA cm−2, respectively (Figure Compared with the for CO2RR to formate, m-BiO2-x exhibits both selectivity and activity ( Supporting Information Table Figure 4 | CO2 reduction measurements in flow cell. (a) curves of p-BiO2-x and m-BiO2-x in 1 M KOH aqueous electrolyte in flow cell. (b) Formate FE values at different applied potentials in 1 M KOH aqueous electrolyte. (c) Formate current densities at different applied (d) test of m-BiO2-x at V versus Download figure Download PowerPoint The reaction of CO2 to formate on m-BiO2-x was investigated by an in situ Raman It was carried in an in situ Raman electrolytic cell, using 0.1 M KHCO3 aqueous solution as electrolyte. in Figure are two peaks at and which to the OCHO* intermediate and the the gradual increase of potential from to V versus RHE, the intensity of the peak at It indicates that the CO2 by the active electrons to form the peak at stronger with the of intermediate to form The in situ Raman spectra of p-BiO2-x were which a similar with that of m-BiO2-x ( Supporting Information Figure This indicates the reaction the two Figure 5 | of the electrochemical CO2-to-formate conversion on p-BiO2-x and m-BiO2-x. (a) In situ Raman spectra of m-BiO2-x at different applied (b) energy for CO2RR and on p-BiO2-x and m-BiO2-x. (c) of Bi of p-BiO2-x and m-BiO2-x. (d) density for the of OCHO* intermediate on p-BiO2-x and m-BiO2-x. is set to be and the charge and are in and respectively. and of p-BiO2-x and m-BiO2-x. Download figure Download PowerPoint calculations were applied to investigate the catalytic of m-BiO2-x for CO2RR to formate. The free energy of CO2 reduction to formate and the on m-BiO2-x and p-BiO2-x were The are in Supporting Information Figures and For CO2 conversion into formate, the first elementary that reduction of CO2 into is the potential in Figure the reaction free energy m-BiO2-x was calculated to be which is much lower than that on p-BiO2-x The free energy m-BiO2-x suggests that the intermediate OCHO* can be thus improving the selectivity for formate For the calculated free energy for hydrogen on m-BiO2-x is higher than that on p-BiO2-x (Figure This that the hydrogen production can be on m-BiO2-x, which is in with the density of were performed to the electron structure of m-BiO2-x. The of m-BiO2-x was calculated to be eV while that for p-BiO2-x is eV (Figure the of m-BiO2-x is much to the energy than that of p-BiO2-x. This the localization of Bi electrons in which is for the of between the active and the OCHO* intermediate and thus the OCHO* intermediate for The charge density for the of OCHO* intermediate on the two catalysts were calculated (Figure Compared with p-BiO2-x, electron between m-BiO2-x and OCHO* can be This stronger electron interaction between m-BiO2-x and From the electron localization it is that the electrons around the

Cobalt phthalocyanine promoted copper catalysts toward enhanced electro reduction of CO2 to C2: Synergistic catalysis or tandem catalysis?

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Electrocatalytic CO2 reduction reaction (CO2RR) has been extensively considered as a potential carbon cycle technology because it can use CO2 as resources to produce high value-added fuels and chemicals under mild conditions. Therefore, developing highly efficient CO2RR catalysts is extremely significant. Here, we designed a series of TM-N2O2Cx (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) single-atom catalysts and investigated their catalytic activities for CO2RR by using the density functional theory. Interestingly, these TM-N2O2Cx catalysts exhibit excellent CO2RR selectivity towards three kinds of different products, including CH4, CO, and HCOOH at relatively low overpotentials, where the CO2RR product of Sc-N2O2Cx, Mn-N2O2Cx, and Zn-N2O2Cx is CO, and that of V-N2O2Cx is CH4, while the other six catalysts would produce HCOOH. Moreover, the best pathway for CH4 formation is also unveiled, that is, * + CO2 → C*OOH → C*O + H2O → C*HO → C*HOH → C*H + H2O → C*H2 → C*H3 → * + CH4. In short, TM-N2O2Cx catalysts exhibit high catalytic activity for electrocatalytic CO2RR, which provides useful information for experimental scientists to synthesize these promising CO2RR catalysts.

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Direct CO2 conversion using renewable energy source (e.g. solar energy) to value-added chemicals has attracted much attention for alternative fuel production. An efficient and durable electrocatalyst is essential to achieve practical level of CO2 conversion. In the recent years, nanostructuring of metal catalysts (e.g. Au, Ag, Cu) has been reported for enhancing CO2 reduction reaction (CO2RR) activity in aqueous solutions. In particular, nanostructured Au formed on a thick Au foil by electrochemical treatment has been intensively focused due to the superior properties regarding low overpotential and enhanced stability.[1] However, the utilization of thick Au foils can increase the cost and is also unavailable for photoelectrochemical (PEC) CO2RR which is the simplest approach for solar CO2 conversion. Accordingly, we recently demonstrated that mild electrochemical treatment enabling the formation of nanoporous Au thin film on Si photocathode for PEC CO2RR.[2] Although it shows exceptional CO2RR activity with only 20 nm-thick nanoporous Au layer, stability issue still remains to be solved for practical CO2 conversion. Here, we propose a novel electrochemical synthesis strategy to form Au nanostructured thin film for further improved CO2RR activity. The electrochemical synthesis is simply performed with 160 nm-thick Au thin film on Si substrate by anodization of Au thin films to grow Au oxide at constant potential (2.5 V (vs. RHE) for 40 min) followed by electro-reduction of the grown Au oxide at constant current density (-0.5 mA cm-2 for 10 min) in 0.2 M KHCO3 solution. Nanoporous Au nanostructures with height of ~180 nm were formed as a result of the electrochemical treatment. The CO2RR activities (overpotential, durability) for the nanostructured Au is significantly improved as compared to bare Au thin film. For example, our CO selectivity is to 66% and 98% at -0.35V and -0.39V, respectively, (bare Au: 0 and 20%, respectively). Nanostructured Au also shows better stability at -0.49V (~2 hours for over 90% selectivity) as compared to bare Au (20% selectivity at an hour). In order to further improve the stability of our nanostructured Au during CO2RR, we introduce additional electrochemical oxidation and reduction cycles. Interestingly, declined CO2RR activity (CO selectivity: ~70%) is revived to have 97% CO selectivity after an additional cycle. Furthermore, after the 4th electrochemical cycles, our Au nanostructures are stable over 6 hours while producing over 90% CO selectivity. We performed a systematic morphological and microstructural investigation of Au nanostructures during the repeated electrochemical treatment cycles. In particular, the height of Au nanostructured increases to about 390 nm and roughness factor is doubled after the 4th treatment. TEM investigations confirmed that remarkably increased number of grain boundaries (GBs) on the 4th treated Au samples. In addition, most GBs disappear after 15 hours of CO2RR. These results indicate that extended stability by repeating cycle of electrochemical synthesis is attributed to increased GBs on a roughened Au nanostructure. Finally, we applied our Au nanostructures to Si photocathodes for PEC CO2RR with Si photocathodes by forming Au micro-mesh. Micro-mesh Au on Si enables electrochemical treatments selectively on Au surface without deteriorating the underlying Si. Our Si photocathode with nanostructured Au co-catalyst remarkably achieves ~82% CO selectivity at -0.07 V which is more positive than CO2/CO redox potential under 1 sun illumination with extended stability. Therefore, our electrochemical synthesis would provide an efficient strategy to improve and recycle nanostructured Au activity for photo- and electrochemical CO2reduction. [1] Yihong Chen, Christina W. Li, and Matthew W. Kanan, J. Am. Chem. Soc. 134, 19969 (2012) [2] Jun Tae Song, Hyewon Ryoo, Minhyung Cho, Jaehoon Kim, Jin-Gyu Kim, Sung-Yoon Chung, and Jihun Oh, Adv. Energy Mater. 7, 1601103 (2017)

Influence of tetraethylammonium cation on electrochemical CO2 reduction over Cu, Ag, Ni, and Fe surfaces

The over-exploitation of fossil fuels and the consequent increase of atmospheric CO2 concentration have aroused people's attention to energy and environmental issues.(1) In terms of reducing CO2 content, the electrocatalytic CO2 reduction reaction (CO2RR) can effectively solidify CO2 and convert it into high value-added chemicals. There are many CO2RR reduction products, including CO, HCOOH, C2H5OH, CH4,C2H4, etc.(2) However, in the CO2RR process, the hydrogen evolution reaction (HER) in the water phase has a lower reduction potential, making the CO2RR products less selective, which has become the most important problem restricting its practical application. Therefore, improving the selectivity and activity of CO2RR products has important research significance and practical application value.In this work, a series of Cu2O/Ag nanocrystals (NCs) with different crystal faces were prepared by hydrothermal method. Specifically, cubic Cu2O (c-Cu2O) enclosed with {100} facets, rhombic dodecahedral Cu2O (d-Cu2O) enclosed with {110} facets, and octahedral Cu2O (o-Cu2O) enclosed with {111} facets, are mixed with Ag nanospheres to form c-Cu2O/Ag, d-Cu2O/Ag, and o-Cu2O/Ag bimetallic tandem catalysts. The catalysts can improve the high activity and selectivity of CO2RR to produce C2H4 product by adjusting different crystal faces. We can find from Figure 1 that o-Cu2O/Ag tandem catalyst exhibits an impressive Faradaic efficiency (66.8%) and partial current density (17.8 mA cm-2) for C2H4 product at -1.2 VRHE. This result provides a new strategy for improving the selectivity of CO2RR to produce C2+ products by adjusting the crystal facet engineering. Figure 1. (a) SEM images of o-Cu2O/Ag, (b) LSV curves of three Cu2O/Ag catalysts in Ar and CO2-saturated 0.5 M KHCO3 solutions, (c) Partial current density of C2H4 formation on the three Cu2O/Ag catalysts at -1.2 V, (d) Faradaic efficiencies of C2H4 on the three Cu2O/Ag NCs. Reference [1] P. De Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo and E. H. Sargent, Science, 364 (2019).[2]Y. Zheng, A. Vasileff, X. Zhou, Y. Jiao, M. Jaroniec and S. Z. Qiao, J Am Chem Soc, 141, 7646 (2019). Figure 1

Harnessing in-situ oxidation of nanostructured mxene to modulate the electronic structure for improved selectivity for electrochemical carbon dioxide reduction

Ultrathin ZnIn2S4 nanosheet arrays activated by nitrogen-doped carbon for electrocatalytic CO2 reduction reaction toward ethanol

The performance of the electrocatalytic CO2 reduction reaction (CO2RR) is highly dependent on the microenvironment around the cathode. Despite efforts to optimize the microenvironment by modifying nanostructured catalysts or microporous gas diffusion electrodes, their inherent disorder presents a significant challenge to understanding how interfacial structure arrangement within the electrode governs the microenvironment for CO2RR. This knowledge gap limits fundamental understanding of CO2RR while also hindering efforts to enhance CO2RR selectivity and activity. In this work, we investigate this knowledge gap using a tunable system featuring superhydrophobic hierarchical Cu nanowire arrays with microgrooves (NAMs). Adjusting the NAM structure tunes multiple synergistic effects in the microenvironment, which include stabilization of the microwetting state, confinement of CO*, improvement to local CO2 concentration, and modulation of the local pH. Notably, using mass transport modeling, we quantify the role of the gas-liquid-solid interface in boosting local CO2 concentrations within several microns of the interface itself. Leveraging these effects, we elucidate how CO* and H* competitively occupy active sites, influencing reaction pathways toward multicarbon products based on tuning the microenvironment. Consequently, we provide new insights into why the optimized configuration significantly increased CO2RR activity by 690% (as normalized by electrochemical active surface area), C2+ product selectivity by 72%, and Faradaic efficiency by 36%, compared to CO2RR with hydrophobic Cu foil. Based on these insights, our findings unlock new opportunities to engineer the CO2RR microenvironment through the rational organization of hierarchical interface materials in gas diffusion electrodes toward improved CO2RR selectivity and activity.

Photo-/electrocatalytic approaches to CO2 conversion on Cu2O-based catalysts

The electrocatalytic CO2 reduction reaction (CO2RR) to fuels driven by electrocatalysts is a viable strategy for efficient utilization of emitted CO2. CO2RR involves multiple‐steps, including adsorption, activation, hydrogenation, etc. At present, copper‐tin alloy catalysts have shown the capability to reduce CO2 to formic acid or formate. However, their poor adsorption and activation capacities for CO2 molecules, as well as the sluggish kinetics in *H supply restrict the proton‐coupled electron transfer processes in the electrocatalytic CO2RR to produce formic acid. In order to solve the above problems, the ultra‐small SnO2/Cu6Sn5/CuO nanocatalysts with superscalar phase boundaries are fabricated by laser sputtering. The introduction of SnO2 enhances the adsorption and activation of CO2, while CuO promotes H2O decomposition and provides abundant *H intermediates, resulting in tandem catalytic sites on the SnO2/Cu6Sn5/CuO composite catalysts and thus leading to excellent CO2RR activity and high selectivity to formic acid. The Faradic efficiency of formic acid (FEHCOOH) at the SnO2/Cu6Sn5/CuO electrode reaches 90.13% along with a high current density of 25.2 mA cm−2 at −0.95 V versus reversible hydrogen electrode. The role of the multiphase boundaries constructed by introduction of oxides is confirmed by in situ infrared spectroscopy and kinetic isotope effects experiments, which is consistent with the design concept.