Breaking scaling relations in nitric oxide reduction by surface functionalization of MXenes

Abstract

The electrocatalytic NO reduction reaction (NORR) towards NH₃ has attracted increasing attention as a promising strategy to cope with both NH₃ synthesis and NO removal challenges. However, the catalytic efficiency is seriously impeded by the intrinsic linear scaling relations between adsorption energies of NORR intermediates. Herein, we propose a design principle to break the linear scaling relationships of key intermediates in the NORR for promoted reaction thermodynamics via modifying the surface functionalization of MXenes. Our calculation results reveal that surface functionalization can remarkably reduce the limiting potentials of the NORR on MXenes from −1.09 V (bare Ti₂C) to −0.08 V (Ti₂CS₂) and −0.14 V (Ti₂CO₂) by decreasing the free energy difference of the rate-determining step (*NH₂ + H⁺ + e⁻ = *NH₃). Then, we highlighted that modifying surface terminations on MXenes can selectively tune the adsorption strength of *NH₂ and *NH₃, thus breaking the linear scaling relations between their adsorption energies. Further analysis on the charge difference and density of states illuminate that the modulation mechanism originates from the surface termination induced tunable electric properties of MXenes. Furthermore, Ti₂CS₂ and Ti₂CO₂ MXenes possess superior selectivity towards NH₃ by inhibiting the competing production of H₂. Our work not only provides a deeper insight into the essential effect of surface functionalization on the catalytic activities of MXenes, but also proposes general design principles for high-performance NORR electrocatalysts.