Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride

Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady-state active sites on ENRR catalysts that follow the Mars-van Krevelen mechanism via quantitative isotope-exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active-site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate-limiting step in the ENRR is likely the initial N equivalent to N bond activation via the addition of a proton and an electron to the adsorbed N-2 on the N vacancies to form N2H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.