Exploring Various Catalysts for Nitrogen Reduction to Ammonia

Abstract

Electrochemical nitrogen reduction reaction (eNRR), one of the most studied problems in catalysis, has the potential to replace the century-old Haber-Bosch process for ammonia synthesis. However, a low-cost, highly selective and efficient catalyst for eNRR is yet to be discovered. Metal based catalysts have been at the heart of numerous studies for eNRR, however there has been a shift towards metal-free catalysts due to their environmentally benign nature and low cost. In this study, we analyze 2D boron-based electrocatalysts for nitrogen reduction and explore the effect of carbon substitution on their activity. Using first-principles calculations, we demonstrate that pristine and carbon (C) substituted 𝛼 borophene catalyze eNRR at exceptionally low limiting potentials of -0.33 V and -0.25 V, respectively. These 𝛼 borophene based catalysts also show high selectivity towards eNRR over the competing hydrogen evolution reaction (HER). Our results show that C-substituted 𝛼 borophene exhibits outstanding catalytic activity towards eNRR via the distal pathway with a small activation barrier of 0.56 eV, almost half the value reported for Ru (0001). Finally, we identify two novel descriptors, adsorption energy of *NNH and charge transfer to *N2, which can be used in high-throughput screening of catalysts for eNRR. These findings demonstrate the strong potential of metal-free borophene based catalysts for efficient electrochemical reduction of N2, enabling low-cost and sustainable NH3 synthesis.We revisit the nitrogenase FeM cofactor (M=Fe/Mo/V) based catalysts for eNRR to decipher the key mechanism at play. Further, catalysts based on transition metal nitrides (TMNs) and transition metal dichalcogenides (TMDCs) are also being explored for the study and compared with the state-of-the-art materials available. The dominance of one NRR mechanism like the Mars Van Krevelen (MvK) mechanism on TMNs over others like the dissociative NRR is also examined. These findings will provide a more comprehensive and elaborate view of the ammonia synthesis for N2 reduction reaction space.