Study Of The Electrocatalytic Reduction Of CO₂ By Mn-based And Cu-based Transition Metal Catalyst

Carbon dioxide electrocatalytic reduction(CO2ER)technology can convert stable CO2 molecules into easy-to-store and transport organic compounds like CO,formic acid,methane,methanol,ethanol,ethylene,acetylene,and n-propanol using renewable energy sources like solar and wind.It is both a future-oriented pollution reduction measure and a viable energy storage option.However,low selectivity of C1 products,poor catalytic process stability,and difficulty in forming C2+products with high added value continue to limit the technology’s use.This paper is dedicated to the development of manganese(Mn)and copper(Cu)based-transition metal catalysts with abundant reserves and low costs,to achieve high selectivity of the C1 product(CO),catalytic system stability,and effective conversion of C1 products to high value-added C2+products.It serves as a useful guide for creating a carbon dioxide electrocatalytic reduction system that is both efficient and long-term.The following are the key research findings and conclusions of this paper:Although transition metal Mn-based materials have been shown to have catalytic activity for electroreduction conversion of CO2 to CO,inadequate catalytic stability is a performance barrier.To boost the catalytic activity of CO production and considerably improve the stability of Mn-based catalysts,we created a nitrogen-doped graphene aerogel to support MnO crystals rich in oxygen defects.The catalyst shows excellent CO2ER performance.The highest Faradaic efficiency(FE)for CO is 86%.The catalyst exhibits a negligible overpotential of 160 mV.And after 10 hours of operation,the performance hasn’t deteriorated considerably.The main active site is MnO,and N doping dominated by pyridine nitrogen efficiently inhibits the hydrogen evolution reaction of the competitive reaction,according to electrochemical characterization.DFT results reveal that MnO crystals with oxygen defects and N doping on the support can significantly reduce the free energy barrier of*CO2-intermediate and promote the progress of CO2ER.Further improving CO selectivity is difficult due to the difficulties of CO2 molecule activation on the surface of Mn.Cu-based catalysts were developed,which belong to the 3d transition metal family but are easier to activate reactant CO2 than Mnbased materials,were prepared.To increase the adhesion between the metal site and the support,we described a new in situ exsolution of Cu NPs from the interstitial region of a nickel-based hydroxide framework.The hydrothermal approach is used to achieve interstitial doping of copper ions,and the Cu ions are successfully exsolution into Cu nanoparticles by electric activation at ambient temperature.XAFS,DFT simulation,and various characterization methods are used to explore the doping mechanism,clarify the exsolution mechanism,and prove the general applicability of this approach.Compared with the traditional in-situ exsolution with perovskites by substitution mechanism,this method overcomes several limitations.And the particle size of nanoparticles can be regulated by controlling the reduction potential of exsolution.The Cu-based electrocatalyst with a particle size of～4 nm prepared by this method has excellent CO selectivity as high as 95.6%and shows high stability over at least 40 h.Compared with Mn-based catalysts,the selectivity and stability of CO are significantly improved.The catalyst also shows the potential to form multi-carbon products with a high pH.Compared with the electrolyte system of 1 M and 5 M KOH,the CO2ER performance is the best at the concentration of 3 M,the total FE of C2+products up to 67%,of which the FE of ethanol is 32%and that of acetate is 26%.Given the low selectivity of Cu-based catalysts in the electrocatalytic conversion of CO2 to the specific C2+product(ethanol),a series of Ag-modified Cu-based catalysts were prepared based on the design idea of tandem catalysts.Introducing Ag,a dominant metal that can convert CO2 into CO,improves the coverage of*CO on the Cu-based active sites,promotes the occurrence of C-C coupling,and synchronously optimizes the adsorption of specific intermediates on the active sites.The performance of CO2 electrocatalytic reduction catalysts with various Cu/Ag ratios and single metal control groups was explored.CuOAg0.4 has the best catalytic performance.The H2 selectivity of the CuOAg0.4 catalyst is the lowest,and FE is 11%.The FE of ethanol is as high as 61%,the maximum partial current density of ethanol is 76 mA cm-2,and the selectivity of C2+product is nearly 80%.Compared with the CuO control group,CuOAg0.4 the enhancement factor for ethanol is 11 times.The results show that the alloying of Ag and CuO can effectively inhibit the hydrogen evolution reaction and improve the activity of C2+products dominated by ethanol.Based on this,the gas diffusion layer in the working electrode is modified by superimposing a PTFE hydrophobic layer,which can improve the stability under high current density by～9 times,inhibit the hydrogen evolution reaction and contribute to the formation of higher-order products.In addition,taking single metal CuO and bimetallic CuOAg0.8 as examples,the regulation of the anion effect on the catalytic system was investigated.The addition of halogen ions(Cland Br-)and bicarbonate ions contributes to the formation of C2+products.Compared with the control group,in CuO catalysts,the peak faraday efficiency of C2+product in chloride ion addition system was increased from 33%to 68%(mainly ethanol).For the CuOAg0.8 catalyst,halogen ions contribute to the reaction path of ethylene in the low reduction potential range and increase the selectivity of alcohols in the high potential range.After adding 0.5 M Cl-ion,C2+faraday efficiency increased from 60%to 71%.Rather than alcohols,HCO3-contributes to the production of ethylene.It can release more CO2 by attaching to metal sites,resulting in a nearly 2-fold increase in current density.The presence of sulfate ions reduces the current density of the system,which could be attributed to the formation of an insoluble sulfate crystal on the catalyst’s surface.The sulfate ion can also aid in the production of formate.The addition of the sulfate ion increases the FE of formate to 70%.This conclusion serves as a useful guide for furthering the Cu-based catalyst electrolysis system’s optimization.