Cu-based Electrocatalyst For Coupling CO₂ Reduction With CH₃OH Oxidation Towards Efficient Formic Acid Production

With the rapid development of social industrialization,the massive consumption of coal,oil,natural gas and other non-renewable energy has caused a sharp rise in the concentration of carbon dioxide（CO₂）in the atmosphere,leading to global climate problems.Using renewable electricity to convert CO₂ into high-value chemicals provides a potential way to reduce the concentration of atmospheric CO₂.However,this technology still faces many challenges:（i）CO₂ has higher reaction energy barrier,and the limited mass transfer efficiency on electrode surface lead to the low current density during CO₂ reduction reaction;（ii）the competitive hydrogen evolution reaction in aqueous electrolyte system and the diversity of CO₂ reduction products increase the difficulty for highly selective electrocatalytic CO₂ reduction to specific target products.In addition,in the electrocatalytic full electrolysis system of CO₂reduction,the anodic water oxidation and oxygen evolution reaction has high reaction potential（1.23V vs.RHE）and slow reaction kinetics,increasing the energy consumption and restricting the reaction rate of the entire electrochemical process.As the product,oxygen is low value-added,which greatly reduces the energy utilization efficiency of the system.To solve the above problems,we developed a bifunctional copper-based catalyst.In the full electrolytic cell,this catalyst can couple the cathode carbon dioxide reduction reaction with the anode methanol oxidation reaction to high value-added formic acid,significantly improving the energy utilization efficiency and showing excellent electrocatalytic stability.The main research contents of this paper are as follows:1.Copper nanowires with rough surface（E-Cu NWs）were grown on copper foils by simple electrochemical oxidation and reduction.The structural characterizations of E-Cu NWs show that the E-Cu NWs has numerous grain boundaries with Cu⁺sites in the surface.The results of electrochemical tests and in-situ Raman spectroscopy showed that these fully exposed Cu⁺sites were stable in the electrocatalytic process,they were conducive to the stability of the key intermediate*OCHO in the reduction of CO₂ to formic acid,thus improving the performance of the electrocatalytic CO₂reduction.At–0.97 V vs.RHE,the Faraday efficiency of formic acid is up to 90.2%,and the partial current density is 35.0 m A cm^-2 which is much higher than that of H-Cu NWs(Faraday efficiency of formic acid is only 36.8%,and the partial current density is 14.5 m A cm^-2),mostly,this result is better than the copper-based catalysts reported so far.2.The performance and mechanism of catalyst in electrocatalytic methanol oxidation reaction were also investigated.In a wide potential range of 1.32～1.82 V vs.RHE,the Faraday efficiency of methanol oxidation to formic acid is near to 100%,and the current density can reach more than 200 m A cm^-2.The overpotential required to produce a current density of 10 m A cm^-2 was 274 m V lower than the oxygen evolution reaction.The reaction path of methanol oxidation to formic acid was revealed by a series of in-situ Raman spectra at different potentials or at different times.3.In a full electrolytic cell in which CO₂ reduction was used as the cathode and methanol oxidation as the anode（CO₂RR-MOR）to achieve the simultaneous and highly selective preparation of formic acid,E-Cu NWs was used as the cathode and anode catalysts,respectively.Compared with the conventional CO₂ reduction-oxygen evolution reaction electrolysis system（CO₂RR-OER）,the overpotential is significantly reduced by 760 m V to achieve a current density of 10 m A cm^-2.The yield of formic acid in CO₂RR-MOR system reaches 1151.9μmol h^-1 cm^-2 at 2.2 V cell voltage,which is 2.66 times of that in CO₂RR-OER system.At the same time,the energy consumption of 1 kg formic acid produced by CO₂RR-MOR system is 1.872k Wh,far lower than that of 2.912 k Wh produced by CO₂RR-OER system.In addition,on account of the electrochemical in situ reproducibility of the bifunctional E-Cu NWs,the CO₂RR-MOR system can operate stably for more than 250 hours at a current density of 40 m A cm^-2 by using the cathodic and anodic exchange strategy.