Optimization Of Electrocatalytic Nitrogen Reduction Performance By Electrolyte Design

Ammonia（NH₃）is an important chemical with high energy density widely used in food production and the manufacture of chemical products.However,the industrial application of the new electrocatalytic nitrogen reduction（NRR）process is limited due to its low NH₃ yield and Faradaic efficiency（FE）.This is mainly due to the difficulty in activating N₂ and the severe competitive hydrogen evolution reaction（HER）in the aqueous electrolyte,which results in insufficient NRR activity and selectivity of the catalytic system,making the development of an electrocatalytic system with high NRR activity and selectivity extremely challenging.Therefore,catalytic design and HER inhibition are the key to the development of the NRR ammonia synthesis process.Due to its unique layered structure and excellent conductivity,WSe₂ has been widely used in various studies of electrocatalytic reactions.However,its low proton adsorption energy and difficulty in exposing active sites result in insufficient NRR catalytic activity.Therefore,it is necessary to regulate its electronic structure to enhance its NRR catalytic performance.Moreover,in the novel water-in-salt electrolyte（WISE）,the high salt/water mass ratio creates a unique solvation structure that strongly constrains the active water molecules within the solvation shell of the metal cations,resulting in a reduced water activity and effectively suppressing water dissociation.Herein,the NRR performance of vacancy-rich WSe_2-x in WISE was theoretically investigated using density functional theory（DFT）calculations and molecular dynamics（MD）simulations to predict its catalytic activity in this study.Based on this,WSe_2-xwith abundant Se vacancies was synthesized and characterized to verify the theoretical predictions of its NRR catalytic performance in WISE through experimental testing.Combining the theoretical and experimental results,the underlying mechanism of how WISE enhances the NRR catalytic performance of WSe_2-x was revealed.The specific contents include:（1）Theoretical simulation study on the electrocatalytic nitrogen reduction performance of WSe_2-x in WISETheoretical studies of the NRR catalytic performance of WSe_2-x were conducted through DFT calculations and MD simulations.It was found that the formation of Se vacancies causes charge redistribution on the surface of WSe_2-x,inducing neighboring W atoms to exhibit Lewis acidity.Additionally,the conductivity and surface electron emission capability of WSe_2-x were improved,enhancing its N₂ adsorption and activation ability,reducing the thermodynamic energy barrier of the NRR process,and enabling WSe_2-x to exhibit Satisfying NRR catalytic performance.Moreover,the study of the NRR catalytic performance of WSe_2-x in WISE showed that compared to dilute electrolyte（DE）,the solvation structure in WISE is dominated by[Li（H₂O）₄]⁺,forming a rigid solvation shell centered around Li⁺that binds active water molecules,reducing proton transfer and thereby inhibiting competitive HER.Furthermore,when the Li Cl O₄ concentration reached 12 m(mol kg^-1),the local concentration of N₂ near WSe_2-x was highest,enabling WSe_2-x to exhibit optimal NRR catalytic performance.（2）Preparation of vacancy-rich WSe_2-x and its electrocatalytic nitrogen reduction performanceVacancy-rich WSe_2-x was successfully prepared by liquid-phase exfoliation and thermal treatment,and its structure and morphology were characterized.The research results indicate that WSe_2-x has a layered structure with rich Se-vacancies.The NRR catalytic performance of WSe_2-x was tested in DE（0.5 m Li Cl O₄）and WISE（12 m Li Cl O₄）,and the results showed that WSe_2-x exhibited excellent NRR catalytic performance in the same electrolyte environment.It was mainly attributed to the promotion of the electrical conductivity and electrochemically active surface area（ECSA）of WSe_2-x by the Se vacancies.In WISE,the FE and NH₃ yield of WSe_2-x reached 62.5%and 181.3μg h^-1 mg^-1,respectively,which were significantly improved compared to those in DE and consistent with the theoretical research results.Based on the experimental and theoretical results,it can be concluded that WISE can effectively enhance the NRR catalytic performance of WSe_2-x,primarily due to its suppression of the HER in aqueous electrolytes and the increase of the local concentration of N₂ near WSe_2-x.