Research On The Application Of Zero-Gap Membrane Electrode Assembly Reactor In Electrocatalytic CO₂RR

Electrochemical carbon dioxide reduction reaction（e CO₂RR）is a mild and controlled CO₂treatment method that can convert greenhouse gas CO₂into renewable fuels and high value-added compounds.On the one hand,CO₂RR can not only reduce the concentration of CO₂in the atmosphere,but also alleviate the energy crisis to a certain extent,which has attracted more and more attention.The effect of electrocatalytic CO₂RR is closely related to catalyst,electrolyte membrane and reactor.In recent years,researches on catalysts have gradually matured,and catalysts with high selectivity and high catalytic activity emerge in an endless stream.In order to make CO₂RR reach the industrial standard as soon as possible,people begin to focus on the reactor design.In this paper,a self-made zero-gap membrane electrode assembly（MEA）reactor was used to investigate the effect of a zero-gap MEA reactor on the production of gas product ethylene and liquid product formic acid by CO₂RR using Cu₂O catalyst and Sn-doped Bi dendrite catalyst with tip effect.Meanwhile,a small-area（4cm²）zero-gap MEA reactor was paralleled to form a reactor.And the large area（25cm²）zero-gap MEA reactor is designed to explore the problems existing in the process of industrialization of zero-gap MEA reactor.The main research results of this paper are as follows:（1）The cubic Cu₂O catalyst containing（111）crystal surface was prepared by a mild chemical precipitation coupled with chemical reduction protection method.The electrocatalytic performance of CO₂RR to produce ethylene was investigated in a traditional H-type reactor and zero-gap MEA reaction.The best faraday efficiency of ethylene production is 11.31%and the maximum current density is 77 m A cm^-2,which indicates that the exposed（111）crystal surface of Cu₂O catalyst has good electrocatalytic performance of ethylene production from CO₂RR.Compared with the traditional H-type reactor,the optimal faraday efficiency of ethylene production in zero-gap MEA reactor was increased by 13.94%,reaching 25.25%,due to the increased mass transfer effect of flowing CO₂gas and anode electrolyte.At the same time,due to the gathering action of the bipolar plate and the use of gas diffusion electrode（GDE）,when the cell voltage is 3.2V,the total current density of the reaction system can reach 250 m A cm^-2,which is much higher than that of the traditional H-type reactor,and basically meets the current density requirements required for industrialization(>200 m A cm^-2).（2）The catalyst of Sn-doped Bi dendrites with tip effect was successfully prepared and the catalyst was successfully prepared by ultrasonic in copper foam substrate.The catalyst was used in zero-gap MEA reactor with different ion exchange membranes.When proton exchange membrane（PEM）is used,because the working principle of PEM collects a large amount of H⁺at the cathode,the competitive reaction of CO₂RR hydrogen evolution reaction（HER）is easier to occur.The faraday efficiency of hydrogen is around 70%,while the best faraday efficiency of formic acid is only 10.69%.The overall faraday efficiency is slightly less than 100%due to the difficulty of cathode side product collection.When bipolar membranes（BPM）are used,due to the occurrence of HER,a large amount of OH^-after hydrolysis will temporarily appear near CO₂molecules due to the consumption of H⁺from a microscopic point of view.As the proton transfer process of BPM is more difficult than that of PEM,the accumulation of H⁺at the cathode side is less than that of PEM.Therefore,H⁺cannot neutralize the OH^-generated by the reaction in time,resulting in the reaction of CO₂and OH^-to form KHCO₃,resulting in carbon loss,and the overall faraday efficiency is only about 50%.Although BPM has a low current density,the optimal faraday efficiency of formic acid can reach 26.84%,and formic acid production is detected in both the cathode and anode,confirming the existence of"cross over".When anion exchange membrane（AEM）is used,the current density of the system can reach 200 m A cm^-2,and the best faraday efficiency of formic acid is 86.31%,which proves that the zero-gap MEA reactor is feasible to produce liquid products,formic acid products are all detected at the anode,and the"cross-over"is stronger than BPM.However,at high current density,formic acid of anode will be oxidized,resulting in the decrease of faraday efficiency.（3）The parallel operation of small area reactor and large area reactor were investigated,and the problems existed in the process of industrialization of zero-gap MEA reactor were investigated.Among them,when we used a small-area zero-gap MEA reactor for parallel test,it was found that the overall current density decreased by about half compared with a single MEA reactor,which was caused by the decrease of mass transfer effect after parallel operation and the increase of system contact resistance.In addition,a zero-gap MEA reactor with an effective area of 25 cm²was successfully designed in this work,which showed good performance.In particular,the current density of large-area MEA reactor is even higher than that of small-area MEA reactors when the cell voltage is lower than 2.9V.When the cell voltage is higher than 2.9V,we found the"area effect"of the large-area zero-gap MEA reactor,that is,the appearance of Cu₂O catalytic material at the inlet and outlet changes greatly due to the full progress of the catalytic reaction and the strong mass transfer process,which leads to the degradation of the electrochemical performance of the catalyst.