W/Mo Doped Bismuth-based Electrocatalyst For CO₂ Electroreduction To Formic Acid

The massive utilization of fossil fuels has not only led to a yearly increase in the concentration of carbon dioxide（CO₂）in the atmosphere,causing environmental problems such as the greenhouse effect,but has also contributed to the current energy shortage.Therefore,the conversion and utilization of CO₂ has attracted attention worldwide.Since electricity is widely available and renewable,electrocatalytic CO₂reduction reaction（e CO₂RR）can convert CO₂ into high value-added chemical products and clean fuels,which is a promising strategy to address the CO₂ problem in the future.Formic acid is an important C₁ product of e CO₂RR.The economic and technical analysis shows that e CO₂RR to formic acid is highly economically feasible.However,there are many problems with the current e CO₂RR technology:（1）high reaction overpotential and high energy consumption;（2）the limited mass transfer of CO₂ and the severe side reactions（hydrogen evolution reaction,HER）in aqueous solutions lead to low current density and slow reaction rates;（3）product complexity and low selectivity of target products.Therefore,the design and preparation of efficient catalysts are particularly critical to improve the product selectivity and catalytic activity of e CO₂RR to formic acid.In this thesis,a series of works were conducted with the goal of developing W/Mo doped bismuth-based catalytic materials for efficient e CO₂RR to formic acid,and the relationships between W/Mo doping and other factors and catalytic performance were investigated.The main contents are as follows:（1）The precursors bismuth tungstate nanosheets and bismuth molybdate nanosheets were synthesized by a hydrothermal method.The precursors were then formed into catalysts re-Bi₂WO₆ NSs and re-Bi₂Mo O₆ NSs by in-situ electrochemical reduction.In addition,a series of characterizations show that the precursors and catalysts had the same elemental composition and similar nanosheets structures.The role of the W/Mo doping is investigated for catalytic activity and reaction mechanism for e CO₂RR.Electrochemical performance tests showed that the re-Bi₂WO₆ NSs exhibited a maximum Faraday efficiency 85.8%and a partial current density of–8.5 m A cm^–2 for the formic acid at–0.9 V,which was superior to that of the re-Bi₂Mo O₆ NSs and Bulk Bi.W doping and the nanosheets structure were the main reasons for the high catalytic activity of the re-Bi₂WO₆ NSs catalyst.Besides,the transformation of the precursor and the real active site of the catalyst was investigated by in-situ Raman,and the results reveal that the true active site of the catalyst was Bi⁰.（2）Based on the above study,the catalytic performance of e CO₂RR for formic acid was further improved by active sites reconstitution and surface roughening strategies.re-Bi WO NPs and re-Bi Mo O NPs catalysts with roughened surfaces were designed and formed form the precursors by in-situ electrochemical reduction.The re-Bi WO NPs electrocatalyst exhibits excellent electrocatalytic activity for the generation of HCOOH with a high faradaic efficiency(FE_HCOOH)of over 90%at the applied potential from–0.8 to–1.9 V（vs.RHE）with the corresponding j_HCOOH ranging from–13.9to–116.78 m A cm^–2.Moreover,the maximum FE_HCOOH reaches 94.7%at–1.0 V with j_HCOOH of–24.96 m A cm^–2,and the catalytic performance can remain stable after 32 h of continuous electrolysis,showing excellent potential for industrial applications.The experimental results further illustrate that the role of W/Mo doping in bismuth-based electrocatalysts and the real active sites of Bi⁰ for the bismuth-based electrocatalysts in e CO₂RR in this work.