Electrocatalytic Reduction Of CO₂ By Cationic Quaternary Ammonium Salt

Quaternary ammonium salts are useful catalysts for CO₂ electrochemical reduction reaction.However,there are some problems along with the quaternary ammonium salts utilization,such as high overpotential and low reduction current.In this research,the property of quaternary ammonium cation is modified to increase the current and reduce the overpotential of CO₂ electrochemical reduction.The property of quaternary ammonium cation is modified by two methods.Detailed information is shown as following:In first method,alkoxy is used to replace ethyl in Tetraethylammonium bromide.This method is to reduce the overpotential,i.e.positively shift the reduction onset potential,and prevent the current decrease of CO₂ electrochemical reaction.Cyclic voltammetry is used to detect the electrochemical property of the modified quaternary ammonium salts.The results indicate that the electrochemical window of[N_22,1O1,1O1]Br?[N_22,2O1,2O1]Br?[N_222,,1O1]Br are narrow,so that they would be reduced before the reduction of CO₂.The electrochemical window of[N_222,2O1]Br is relatively wider than others.It cannot be reduced before the reduction of CO₂.Comparing to[N₂₂₂₂],the CO₂ electrochemical reduction current is not decreased.However,the shift of CO₂ reduction onset potential is not significant.For the second method,double quaternary ammonium salt has a unique chemical structure.Comparing with the traditional single quaternary ammonium salt,quaternary ammonium salt has two cationic centers,meaning the double quaternary ammonium salts have positive charge of two.Consequently,the two cationic centers can compare with two CO₂^·-ions.Thus,the activation energy of CO₂ gaining electrons is reduced,and the CO₂ reduction onset potential is shifted.Dibromo alkyl and trimethylamine are used to synthesize di-quaternary ammonium salt,and the electrochemical test is carried out.Results show that as the chain length increases,the CO₂ reduction onset potential shifts positively first and then shifts negatively.When the hexyl is used,the onset potential is the most positive,i.e.-1.02 V(vs Ag/AgI/I^-1).The Tafel method,EIS and DFT calculations are used for explanation.The Tafel and EIS results show that when[N₁₁₁6N₁₁₁]?PF₆?₂ is used the catalyst,it has the smallest charge transfer resistance and the largest exchange current density.CO₂ is easier to be reduced.DFT simulations suggest that the central nitrogen atom of double quaternary ammonium salt is electrically negative.Thus,the carbon directly connected to the nitrogen is negatively charged,and the hydrogen?a-H?connected to such carbon is positively charged.As a result,the electrostatic attraction between the oxygen of CO₂^·-and a-H leads to the formation of ion pairs.The stronger the electrostatic attraction is,the easier the ion pair can be formed,and the CO₂ reduction onset potential shifts further to positive direction.With the increase of carbon chain length of the double quaternary ammonium salt,the electrostatic attraction of the ?-H and the oxygen atoms in CO₂^·-increases and then decreases.The electrostatic attraction between the ionic pairs of the a-H in[N₁₁₁₅N₁₁₁]⁺ and CO₂^·-,[N₁₁₁6N₁₁₁]⁺ and CO₂^·-is the strongest.