Modification Of SPEEK Membrane By Importing Positive Charge To Regulate CO₂ Hydrogenation Cathode Potential

Nowadays,increasing CO₂ emissions lead to a series of environmental problems,which has affected the lives of people everywhere.It is urgent to capture and utilize CO₂.CO₂hydrogenation is one of the effective ways to transform and utilize them.The electrochemical hydrogen pump unit?EHPR?has become the focus of research because of its normal temperature and pressure operation and low mass transfer resistance.However,when EHPR is used for CO₂ electrochemical reduction,it is found that the low CO₂ reduction current efficiency due to insufficient cathode potential.The import of liquid buffer layer between the cathode and the membrane can solve some problems,but meanwhile bring new limitations,such as increasing mass transfer resistance and unsuccessfully suppressing H₂ evolution for a long time.In this paper,based on the mechanism of the liquid buffer layer to adjust the cathodic potential to suppress the hydrogen evolution reaction,a new method for controlling the cathodic potential and achieve high-efficiency hydrogenation of CO₂ in the negatively charged proton exchange membrane is proposed.The surface of the SPEEK membrane was modified by ion exchange method and layer by layer self-assembly?LbL?.The modified membrane can effectively regulate the cathode potential and achieve high-efficiency hydrogenation of CO₂.The modified membrane achieves a high cathodic potential of 1.8V?vs RHE?at a little current density?<40 mA/cm²?,which is higher than the cathodic potential of the buffer layer at50 mA/cm²?1.2V vs RHE?,while the cathode potential at 90 mA/cm² without positive treatment of SPEEK is lower than 0.6V vs RHE,the hydrogenation efficiency is extremely low?<5%?and unsuccessfully used for CO₂ hydrogenation.The positively-modified membrane instead of the liquid buffer layer can effectively reduce the mass transfer resistance of the EHPR device,wherein the modified membrane has an ohmic resistance of 0.2?and a buffer layer of 3.0?.The positive charge of the modified membrane can be stably existed and the H₂ evolution is stably inhibited for a long time.After 10 hours of reaction,the H₂ evolution current efficiency of the two modified membranes is stably controlled at about 10%and 15%;while the H⁺in the buffer layer EHPR buffer continuously accumulates,and up to 50%after 10h.The modified membrane has more excellent hydrogenation performance than the buffer layer.The modified membrane has a current density of 25.2 mA/cm²,the formation efficiency of HCOOH can reach87.0%,and the reaction conversion is 15%and the reaction rate is 209 nmol.cm^-2.s^-1,while those for the buffer layer are 13.6 mA/cm²,78.6%and 70.9 nmol.cm^-2.s^-1.Furthermore,a theoretical electric double layer model was established for the ion-exchanged modified SPEEK,revealing the mechanism of the positively-modified membrane to regulate the cathode potential.By analyzing the charge efficiency and cathode potential,the effect of factors on the surface modification of SPEEK membranes such as the sulfonation degree,immertion time,cation type,etc.of the base membrane were explored.The above factors directly affect the amount of positive charge,thereby affecting the cathode potential and current density,thereby affecting the hydrogenation efficiency.Therefore,optimizing the above factors can achieve voltage and current matching and improve hydrogenation efficiency.Through theoretical analysis and experimental verification,it is found that the increase of sulfonation degree of SPEEK base film can adjust the cathode potential more effectively.The hydrogenation product needs the balance of voltage sites and current sites.Among them,DS57%SPEEK modified membrane has the best hydrogenation performance;with the increase of immersion time of SPEEK,higher cathode potential can be adjusted and hydrogenation reaction can be promoted;different ions have different binding ability to SO₃^-,and Mg²⁺can form higher cathode potential as divalent ion and promote hydrogenation reaction.