Theoretical Approach toward Optimum Anion-Doping on MXene Catalysts for Hydrogen Evolution Reaction: an Ab Initio Thermodynamics Study.

Abstract

Developing highly active catalysts for hydrogen evolution reaction based on earth-abundant materials is challenging. Nitrogen doping has recently been reported to improve catalytic properties by modifying the electrochemical properties of titanium carbide MXene. However, systematic doping engineering, such as optimization of doping concentration, doping site, and thermodynamic phase stabilization have not been systematically controlled, which retards the reliable production of high-activity MXene catalysts. In this study, the optimum doping concentration of nitrogen and doping process conditions on O-functionalized Ti2C MXene for hydrogen evolution reaction were investigated using density functional theory with thermodynamics. To confirm the optimum nitrogen concentration, the catalytic properties are examined considering the Gibbs free energy of hydrogen adsorption and conductivity for 2.2-11.0 at % nitrogen concentration. It was confirmed that 8.8 at % nitrogen-doped Ti2CO2 had optimum catalytic properties under standard conditions. Moreover, when the doping concentration was higher, the decrease in the adsorption energies of hydrogen and the transition in the energy dispersion of the conduction band led to deterioration of the catalytic properties. Through theoretical results, the feasible process conditions for optimum nitrogen concentration while maintaining the structure of MXene are presented using a thermodynamics model taking into account chemical reactions with various nitrogen sources. This study provides further understanding of the nitrogen-doping mechanism of Ti2CO2 for hydrogen evolution reactions.