Single-Crystalline Mesoporous Palladium and Palladium-Copper Nanocubes for Highly Efficient Electrochemical CO 2 Reduction

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

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

Phase-Controlled 1T Transition-Metal Dichalcogenide-Based Multidimensional Hybrid Nanostructures

Open AccessCCS ChemistryRESEARCH ARTICLE14 Jul 2022Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks Xiaomin Ren†, Chunzhi Li†, Wanchao Kang†, He Li, Na Ta, Sheng Ye, Linyan Hu, Xiuli Wang, Can Li and Qihua Yang Xiaomin Ren† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 , Chunzhi Li† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 , Wanchao Kang† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000 , He Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Na Ta State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Sheng Ye State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Linyan Hu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Xiuli Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Can Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 and Qihua Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 https://doi.org/10.31635/ccschem.021.202101090 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Enhancing the charge separation efficiency is highly effective strategy in improving the photocatalytic activity of covalent organic frameworks (COFs) which have the problems of low conductivity and difficult dissociation of excitons. In this work, we report the sevenfold increase in apparent quantum efficiency resulting from the use of a near-single layer COF (SLCOF) in photocatalytic hydrogen evolution compared with bulk COF. Detected by transient absorption spectroscopy characterization, 100% of photogenerated long-lived electrons in the near-SLCOF can be extracted and participate in the photocatalytic process. However, the electron extraction efficiency declined to only about 11% when the COFs were increased to eight layers, implying the difficulty of charge migration among COFs interlayers. The near-SLCOF was prepared by deposition of self-exfoliated COFs colloids on SiO2, driven by their strong affinity. This work not only sheds light on the significant influence of COF layer thickness on the charge separation efficiency but also provides a new route to prepare and stabilize COF layers for practical applications. Download figure Download PowerPoint Introduction Photocatalytic water splitting to produce hydrogen is a desirable approach to sustainably store intermittent solar energy. Two-dimensional (2D) covalent organic frameworks (COFs) have emerged as a novel type of photocatalyst due to their unique optoelectronic properties and π-conjugated skeleton which can be designed at the molecular level.1–10 Visible-light-responsive 2D COFs for example, diacetylene functionalized COFs,11 azine COF,12 sp2-carbon-linked triazine-cored COFs,13–15 thiazolo thiazole-linked COFs,16 hydrazone-based COFs,17 and sulfone-containing COFs18 have been synthesized and used for visible-light-driven H2 production. However, most COFs show mediocre activity in photocatalytic hydrogen evolution (PHE) compared with traditional inorganic semiconductors, which is mainly related to the difficulty in dissociation of excitons and the rapid recombination of photogenerated electrons and holes during the photocatalytic process.19 Several strategies have been developed to improve the charge separation efficiency of COFs, for example, incorporating donor–acceptor (D–A) moieties20,21 or halogen atoms in COFs,22,23 constructing novel π-conjugated building blocks,24 and generating junctions with other semiconductors.25 In addition to the above strategies, reducing the particle size of COFs to nanometer scale is a more facile method to improve the charge separation efficiency due to the possibility of the charge carrier recombination being reduced in the short diffusion distance.26 Therefore, the single-layer COFs (SLCOFs) offering the minimum diffusion distance for charge carriers should be the perfect candidate for photocatalysis. Although SLCOFs can be successfully prepared on single-crystal surfaces and solid–vapor–liquid interfaces,27–30 such materials are unsuitable for application in photocatalysis due to the difficulty in preventing the stacking of freestanding SLCOFs and the scale-up synthesis. Recently, partitioning the interlayer space of COFs and the acid-aided exfoliation method have been used for the synthesis of monolayer COFs, but the yields of SLCOFs by these approaches have not been high. The facile synthesis and stabilization of SLCOFs still remains a challenge.31,32 Herein, we report the preparation of a near-SLCOF by self-exfoliating of COF colloids in the presence of SiO2 nanospheres and other supports which have strong affinity for COF colloids. COFs with near-single to multiple layers were successfully deposited on SiO2 nanospheres under fine control, providing an ideal model to study the relationship between layer thickness and charge separation efficiency. It was found that almost all the photogenerated long-lived electrons in the near-SLCOF could be used for H2 production, and this value decreased sharply with the increase of COF , which elucidated the remarkable improvement in charge separation efficiency by decreasing the diffusion distance. Experimental Methods Synthesis of TP-TTA colloids The synthesis of 1,3,5-triformylphloroglucinol (TP)-4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA) COF colloids was similar to the method in the literature with a slight modification.33 In a typical process, 29.4 mg (0.14 mmol) of TP was dissolved in 0.5 mL of dimethyl sulfoxide (DMSO). The solution was added dropwise to a flask containing 58 mL of 0.05 M hexadecyl trimethyl ammonium bromide (CTAB) aqueous solution. After ultrasonication, 1.8 mL of 0.05 M sodium dodecyl sulfate (SDS) aqueous solution was added to form solution A. Separately, 49.6 mg (0.14 mmol) of TTA was dissolved in 1 mL of DMSO. The solution was added dropwise to a flask containing 58 mL of 0.05 M CTAB aqueous solution. After ultrasonication, 1.8 mL of 0.05 M SDS solution was added to form solution B. Finally, the solutions A and B were mixed, and 5.8 mL of acetic acid was added to the resultant solution. After reacting at 30 °C for 48 h, a completely transparent orange colloidal solution was formed. Synthesis of TP-TTA/SiO2-x In a typical process, the pH of the TP-TTA colloids solution was adjusted to 7 with NaOH (1 M), followed by the addition of the desired amounts of SiO2 colloid solution, which was diluted to 15 mg/mL with water. After stirring at room temperature for 24 h, the mixture was heated at 110 °C to evaporate the solvent to afford solid products. After thoroughly washing the mixture with ethanol five times and degassing at 120 °C for 12 h under vacuum, TP-TTA/SiO2-x was obtained. TP-TTA/MOx (MOx = TiO2, WO3, Nb2O5, ZrO2) and TP-TTA/CdS were synthesized in a similar procedure to TP-TTA/SiO2-8 with the exception that CdS/MOx supports were used instead of SiO2 colloid solution (for details, see Supporting Information). PHE A flask charged with 50 mg of photocatalyst and 30 mL of 0.1 M ascorbic acid water solution was degassed by three freeze–pump–thaw cycles. An aqueous solution of H2PtCl6 (3 wt % of COF content) was injected into the flask under inert gas. The reaction mixture was illuminated with a 300 W Xenon lamp (PLS-SXE300/300 UV, Perfect Light, China) with a cutoff filter of 420 nm. The temperature of the reaction solution was maintained at 25 °C. Gas samples were taken with a gas-tight syringe (Hamilton 1700) and run on an Agilent 6890 gas chromatograph with a thermal conductivity detector (TCD) referencing against standard gas with a known concentration of hydrogen. Hydrogen dissolved in the reaction mixture was not measured, and the pressure increase generated by the evolved hydrogen was neglected in the calculations. Results and Discussion The TP-TTA colloids confined in CTAB/SDS micelle were prepared according to the method in the literature33 using TP and TTA as monomers. The size of TP-TTA colloids is ∼28 nm as measured by the dynamic light scattering (DLS) method ( Supporting Information Figure S1 and for synthesis details, see Supporting Information). TP-TTA/SiO2-x (x denotes the layer number of TP-TTA) samples with different COF layers were prepared by dispersing commercial SiO2 nanospheres (particle size ∼26 nm) in TP-TTA colloid solution (Scheme 1). The layer number of TP-TTA on SiO2 was facilely controlled by varying the mass ratio of TP-TTA colloids and SiO2 nanospheres in the initial mixture. Scheme 1 | The illustration of preparation of TP-TTA/SiO2-x (x denotes the layer number of TP-TTA) by self-exfoliating of TP-TTA colloids. Download figure Download PowerPoint The TP-TTA content of TP-TTA/SiO2-1, TP-TTA/SiO2-5, and TP-TTA/SiO2-8 was, respectively, 1.0, 4.1, and 7.1 wt % determined by 1H NMR analysis of digested TP-TTA/SiO2-x (for details, see Supporting Information and Supporting Information Figure S2). The Fourier transform infrared (FT-IR) spectra of TP-TTA/SiO2-x clearly showed the vibrations assigned to C=C, C=O and the aromatic ring respectively at 1578, 1625, and 1598 cm−1, together with the vibrations from triazine ring at 1370 and 1510 cm−1, implying the existence of TP-TTA with β-ketoenamine linkage34–36 (Figure 1a and Supporting Information Figures S3a, S3b, S3e, and S3f). No obvious absorption peaks attributed to CTAB and SDS were observed in the FT-IR spectrum of TP-TTA/SiO2-8, indicating no or a lesser amount of residue. The 13C cross-polarization total suppression of sidebands (CP-TOSS) NMR spectrum of TP-TTA/SiO2-8 exhibited characteristic chemical shifts at 183 and 106 ppm representing –C=O of the keto form and –C=C of the aryl ring. The chemical shifts at ∼170 and ∼131 ppm were assigned to the C atoms of triazine units and the C atoms directly connected to the triazine units37 (Figure 1b). The signals in the range of 14–35 ppm assigned to alkane carbons of CTAB and SDS appeared in the NMR spectrum of TP-TTA/SiO2-8. Thermogravimetric analysis (TGA) showed ∼6.9±1 wt % of organic content in TP-TTA/SiO2-8 ( Supporting Information Figure S4), in agreement with the content of TP-TTA determined by 1H NMR, showing the low amount of surfactant in the sample. The relatively stronger signals of the surfactant were mainly due to the cross-polarization experiment. For the 13C atom with a 1H atom directly connected to it, stronger coupling between 1H and 13C enhanced the intensity of the signal because the magnetization transfer from 1H to 13C by simultaneously applying matching radiofrequency fields to both spins, according to the Hartmann–Hahn condition. The combination of FT-IR spectra and 13C CP-TOSS NMR characterization confirmed the existence of TP-TTA on SiO2. Figure 1 | (a) FT-IR and (b) 13C CP-TOSS NMR spectra of TP-TTA/SiO2-8, (c) UV–vis spectra of TP-TTA/SiO2 samples dispersed in water (A1 and A8 refer to the absorbance at 450 nm). Download figure Download PowerPoint The uniformly dispersed nanospheres with smooth surfaces identical to the parent SiO2 were observed in the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of TP-TTA/SiO2-x samples, and the absence of other polymer phases implied that TP-TTA was mostly deposited on the surface of SiO2 (Figure 2a and Supporting Information Figures S5a–S5d). The scanning TEM (STEM) images and elemental mappings of TP-TTA/SiO2-x clearly showed a uniform distribution of C elements on the surface, suggesting that TP-TTA evenly wrapped on SiO2 nanoparticles (NPs) (Figures 2b–2d). The energy-dispersive system (EDS) mapping of nitrogen species was collected simultaneously. Due to the intrinsic low N content in the as-prepared sample, the signal-to-noise ratio of N elemental mapping was too low to generate an image with high quality. Although we had tried to obtain a better result by increasing both the dose rate and the integration time, the serious beam damage and the carbon contamination during lengthy collection times significantly degraded the spatial resolution of EDS mapping. The thickness of TP-TTA on the SiO2 increased from TP-TTA/SiO2-1 to TP-TTA/SiO2-8 which was evidenced by the increased intensity of the carbon signal. The TP-TTA layer thickness of TP-TTA/SiO2-8 was approximately 3.0 nm measured from the STEM elemental mapping. It is difficult to get the accurate thickness of the TP-TTA layer for TP-TTA/SiO2-1 and TP-TTA/SiO2-5 due to the weak signals of C elemental mapping. The thickness of the TP-TTA layer was calculated to be 0.46 and 1.81 nm, respectively, for TP-TTA/SiO2-1 and TP-TTA/SiO2-5 based on the density formula (for details, see Supporting Information). With the distance between adjacent layers of TP-TTA COFs approximately 0.37 nm by Vienna Ab-initio Simulation Package (VASP) (as shown in Supporting Information Table S1), the TP-TTA layer numbers for TP-TTA/SiO2-1, TP-TTA/SiO2-5, and TP-TTA/SiO2-8 were ∼1, ∼5, and ∼8, respectively. Figure 2 | (a) TEM image of TP-TTA/SiO2-8. STEM images and elemental mapping of (b) TP-TTA/SiO2-1, (c) TP-TTA/SiO2-5, (d) TP-TTA/SiO2-8, (e) TP-TTA/TiO2, and (f) TP-TTA/CdS. HRTEM images of (g) TP-TTA/TiO2 and (h) TP-TTA/CdS. Download figure Download PowerPoint The specific surface area of TP-TTA/SiO2-x was measured by Ar sorption at 87 K ( Supporting Information Table S2 and Figure S6) and calculated with the Brunauer–Emmett–Teller (BET) method. The specific surface area of TP-TTA/SiO2-1 was almost identical to the parent SiO2. The TP-TTA/SiO2-5 and TP-TTA/SiO2-8 showed higher BET surface area than SiO2. No micropore assigned to TP-TTA COF at 0.6 and 0.9 nm could be observed in the pore size distribution curve of TP-TTA/SiO2-1, implying a monolayer structure. TP-TTA/SiO2-5 and TP-TTA/SiO2-8 afforded the micropore from TP-TTA COF, further confirming the presence of multilayer TP-TTA ( Supporting Information Figure S6). The powder X-ray diffraction (PXRD) patterns of TP-TTA/SiO2-8 and TP-TTA displayed intense peaks at 5.7°, 9.2°, and 14.9°, respectively, attributed to the 100, 110, and 210 reflections, implying that the multilayer TP-TTA on SiO2 has a crystalline structure ( Supporting Information Figure S7). Two kinds of stacking arrangements (eclipsed AA and staggered AB) were applied to generate the hexagonal unit cells for TP-TTA COF. After geometrical optimization of the models, the experimental PXRD patterns of the TP-TTA COFs and TP-TTA/SiO2-8 were more likely to match the simulated diffraction patterns of the AA-stacking models, in good agreement with reports in the literature.38 The deposition of TP-TTA with varied thickness on SiO2 suggested that the exfoliation of TP-TTA colloids occurred during the deposition process. CdS, TiO2, WO3, Nb2O5, and ZrO2 were also used as supports for the deposition of TP-TTA. The high-resolution TEM (HRTEM) images showed a uniform coating of TP-TTA layers on the surface of CdS/TiO2 with a clear boundary between TP-TTA layers and TiO2/CdS, which was further confirmed by the STEM images and elemental mapping (Figures 2e–2h). The thickness of TP-TTA layers was 3.6 and 2.6 nm, respectively, for TP-TTA/TiO2 and TP-TTA/CdS, indicating the exfoliating deposition behavior of COF layers on the above solid supports. No polymer phase could be observed in the SEM image of TP-TTA/TiO2 and TP-TTA/CdS ( Supporting Information Figures S5e–S5h). The appearance of the characteristic vibrations of TP-TTA in the FT-IR spectra of TP-TTA/CdS and TP-TTA/TiO2 confirmed the formation of hybrid materials ( Supporting Information Figures S3c and S3d). Unfortunately, the SEM images and the corresponding EDS mapping results showed the coexistence of COF aggregates and metal oxide NPs when using WO3, Nb2O5, and ZrO2 as supports, implying that TP-TTA colloids can hardly be exfoliated on the surface of WO3, Nb2O5, and ZrO2 ( Supporting Information Figure S8). The interaction between adjacent layers of COFs involves the noncovalent van der Waals force.39–41 The size of TP-TTA COF colloids is ∼28 nm due to the surrounding compact surfactant layer preventing further growth and flocculation.33 Density functional theory (DFT) calculation showed that the exfoliation energy of TP-TTA COF (AA stacking) is directly related to the layer thickness ( Supporting Information Figure S9). The more layers, the more difficult it is to exfoliate. Consequently, the TP-TTA colloids with much smaller layer number than bulk TP-TTA COFs had weak interlayer strength which facilitated the stripping of TP-TTA layers. Furthermore, TP-TTA colloids synthesized at 30 °C have lower degrees of polymerization than COFs synthesized by the traditional solvothermal method, which further impaired the interaction strength among TP-TTA layers. When the interaction strength between the solid supports and TP-TTA COF colloids is stronger than the π−π staking (AA) strength between the TP-TTA layers, the exfoliation of TP-TTA COF colloids may occur. This is reasonable considering that the 2D imine-linked COF powders can be exfoliated in the presence of acid by temporarily weakening their interlayer stacking through electrostatic repulsion.42 To decrease the surface tension, the TP-TTA single layer tends to deposit on solid supports driven by the H-bonds or other interactions. In the presence of higher amounts of TP-TTA colloids in the synthesis mixture, multilayer TP-TTA was formed on the supports, possibly by the restacking of the single layer due to π–π interactions driven by the high temperature used for the solvent evaporation. Another important parameter for the exfoliating TP-TTA colloids on supports is the pH value of the colloid solution. By mixing SiO2 and TP-TTA colloids (adjusted to pH of 7), the positively charged TP-TTA colloids (zeta potential value of 60 mV, Supporting Information Figure S10) interacted with negatively charged SiO2 nanospheres (isoelectric point of 1.5–3.5) through electrostatic interactions to destroy the colloids. Thus, the TP-TTA was released from the micelles and delaminated into a single-layer, driven by the strong interaction of SiO2 and TP-TTA colloids. The control experiment was performed by mixing SiO2 and TP-TTA colloids with pH adjusted to 1. The TEM image of the resultant material showed the coexistence of SiO2 nanospheres and irregularly shaped TP-TTA ( Supporting Information Figure S5i). At pH of 1, the surface of SiO2 is positively charged, which does not favor the contact with the positively charged TP-TTA colloids. The control experiment signifies the importance of surface electrostatic interactions in successful deposition of TP-TTA layers on SiO2. In comparison with previously reported methods,43 the self-exfoliation of COF colloids could precisely control the layer thickness, and the SiO2 support could prevent the stacking of COF layers during the practical application process. More importantly, this method is easy for the scale-up synthesis. The color changed gradually from light yellow to yellow brown when TP-TTA content increased ( Supporting Information Figure S11). The UV–vis spectra of TP-TTA/SiO2-x water suspension gradually showed red shifts of the absorption edge from 427 to 440 nm with increased layers (Figure 1c). This can be attributed to an increased conjugation length with layer thickness and/or the J-type aggregation with the chromophores between adjacent layers.44,45 The UV–vis reflectance of solid TP-TTA/SiO2-x showed a similar tendency ( Supporting Information Figure S11). Calculated by Tauc plots,46,47 the optical band gaps of TP-TTA/SiO2-x varied from 2.28 to 2.41 eV, showing a slight increase of band gap with the decrease of the TP-TTA layer ( Supporting Information Figure S11). Mott–Schottky tests were performed to determine the band minimum of the materials ( Supporting Information Figure The for TP-TTA COF and TP-TTA/SiO2-x was hydrogen which is than the potential ( Supporting Information Figure The above results the for PHE by TP-TTA/SiO2-x and TP-TTA COFs under The PHE activity of TP-TTA/SiO2-x and corresponding bulk TP-TTA COFs ( Supporting Information Figures and was in a H2 evolution reaction under light 420 nm) with ascorbic acid as a and NPs from H2PtCl6 as a (for details, see Supporting Information). showed no H2 evolution occurred light and with a that the PHE reaction only under light and in the presence of a shown in Figure the of H2 increased with the TP-TTA COF and TP-TTA/SiO2 The H2 evolution rate was relatively during the 0.5 h, possibly due to the of which is a for TP-TTA/SiO2-1 H2 in h, much higher than the content in the showing that the H2 is not from the of the The experiment confirmed that the H2 from water ( Supporting Information Figure The H2 evolution rate was calculated 1 h reaction to the influence of the The H2 evolution rate decreased from to when increasing the layer numbers from 1 to (Figure TP-TTA/SiO2 samples were more than TP-TTA COF with the H2 evolution rate of the samples had much lower content of TP-TTA. Therefore, it is reasonable to that the more precisely the thickness, of TP-TTA significantly photocatalytic activity by the diffusion distance of charge The H2 evolution rate of TP-TTA/SiO2-1 was as high as this remarkable the for reported ( Supporting Information Table Figure | (a) PHE as a of reaction with 50 mg TP-TTA/SiO2-x and TP-TTA COFs under nm, in the presence of 30 mL 0.1 M ascorbic acid and wt % (b) The of layer numbers of TP-TTA with PHE rate and (c) of H2 (d) of H2 evolution Download figure Download PowerPoint The apparent quantum efficiency of TP-TTA/SiO2-1 was measured using a 300 W Xenon lamp with a and nm filter (Figure for details, see Supporting Information). The of at different was in good agreement with optical absorption The at 440 nm for TP-TTA/SiO2-1 which is times higher than TP-TTA COF with of With increased layer thickness, the decreased (Figure In a PHE no activity was observed h using TP-TTA/SiO2-1 as the model photocatalyst (Figure The TP-TTA/SiO2-1 had similar and optical absorption properties as the the high of TP-TTA/SiO2-1 during the photocatalysis ( Supporting Information Figure Figure | absorption of (a) TP-TTA/SiO2-1 and (b) TP-TTA/SiO2-8 in water in 0.1 M ascorbic acid and in 0.1 M ascorbic acid with at 450 nm, at nm). (c) for TP-TTA/SiO2-1 and TP-TTA/SiO2-8 in the presence of ascorbic (d) The electron extraction efficiency of TP-TTA/SiO2-1 and TP-TTA/SiO2-8. Download figure Download PowerPoint The of TP-TTA/SiO2 samples was almost the suggesting that the high PHE activity of TP-TTA/SiO2-1 was not due to the different for ( Supporting Information Figure to the surface of COFs has a influence on the PHE The water contact of TP-TTA/SiO2-x was measured, and the results showed that TP-TTA/SiO2 samples and TP-TTA COFs had a similar water contact of ( Supporting Information Figure implying that the surface is not for the in PHE spectroscopy and of TP-TTA/SiO2-x samples were performed and compared with TP-TTA shown in Supporting Information Figure the of TP-TTA/SiO2-x increased with the increase of the layer number of suggesting that TP-TTA could improve charge The of TP-TTA/SiO2 samples also increased with the decrease of the number of TP-TTA layers, indicating that the transmission distance was for the separation of photogenerated charge carriers ( Supporting Information Figure a the PHE rate of TP-TTA/SiO2-1 was enhanced to TP-TTA COF. To further the charge separation efficiency of TP-TTA/SiO2 samples, transient absorption spectra of the water of TP-TTA/SiO2-1 and TP-TTA/SiO2-8 were In the presence of the the signal at nm was observed for both samples, and the was increased at the scale in the presence of the (Figures and Supporting Information Figure confirming that in such long-lived signal in is from electrons and the The initial could be used to the number of electrons due to the between the electron numbers and the intensity of the signal. The initial increased from for TP-TTA/SiO2-1 to for TP-TTA/SiO2-8 in ascorbic The ratio = of the initial of TP-TTA/SiO2-8 and TP-TTA/SiO2-1 was in good with the ratio = of their absorbance at 450 nm under the intensity (Figure 1c). This good agreement that all the light by TP-TTA/SiO2-x generate long-lived electrons in the presence of ascorbic their similar dissociation TP-TTA/SiO2-1 and TP-TTA/SiO2-8 have of electrons the when the to of the initial of and respectively (Figure The behavior of electron extraction from TP-TTA/SiO2 samples to was by the was (Figure and Supporting Information Figure In the presence of the signal reduced for TP-TTA/SiO2 samples, indicating the migration of electrons to to participate in the For TP-TTA/SiO2-1, the signal to a weak absorption signal by the of indicating that the corresponding to the weak absorption participate in the The electron

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

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