NH₃ has received much attention as one of CO₂-free hydrogen carriers. Haber-Bosch method via nitrogen reduction reaction (N₂RR) has been used to produce NH₃ at the high temperature and pressure conditions. However, it suffers from the high emission of CO₂ and high energy expense. The electrochemical N₂RR at mild temperature and pressure conditions has been proposed to be the alternative to energy-intensive Haber-Bosch process. One of bottlenecks in developing the electrochemical ammonia synthesis process is the poor activity of catalysts. Thus, it is imperative to develop the highly efficient electrochemical N₂RR catalyst. In this study, by using quantum mechanics-based first-principles density functional theory (DFT) calculation, we designed single and double X atom catalysts supported on MXene. First, two different N₂RR mechanisms were examined on the model MXene-based catalyst, which leads to the identification of a descriptor for measuring the catalytic activity toward NH₃ production via electrochemical N₂RR. Next, using a descriptor, we computationally screened and selected the novel catalyst through the composition engineering for enhancing NH₃ production. Here, the tuning factor in controlling N₂RR catalysis is the different types of MXene [M₂CO₂ (M= Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr)] and of X [X=Fe, Ru, Os] atoms. We found the Os single atom supported on V₂CO₂(Os/V₂CO₂) as a promising candidate, which exhibited the improved NH₃ productivity [whose onset potential (U= −0.22V) is close to the equilibrium NH₃ production potential (U= 0.057V)] compared to the conventional Ru catalyst. Furthermore, our DFT calculation showed the slightly lower onset potential (U= −0.27V) for hydrogen evolution reaction (HER) on the Os/V₂CO₂case than the N₂RR case (U= −0.22V), suggesting the higher NH₃ selectivity than H₂ production. In addition, the electronic structure and charge transfer analysis were investigated to understand the origin of enhanced activity of MXene based catalysts toward NH₃ production. Our theoretical study provides the fundamental mechanism of electrochemical N₂ reduction on MXene-based catalyst and gives the insight for the next generation catalysts for hydrogen storage applications.

HoChang Song

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