Catalytic Activity of Nitrides and Oxides for Electrochemical Ammonia Synthesis: Computational Guides and Experimental Insights
Authors: Abghoui Younes , Skulason Egill 
Published: February 2020

By mimicking the naturally occurring enzymatic process, ammonia can be produced electrochemically from nitrogen and water at ambient conditions where protons are solvated in the electrolyte by water splitting and electrons are driven to the surface of catalyst via electricity supplied by renewable sources of energy. In this manner, the ammonia synthesis becomes a zero-carbon emitting process and more decentralized compared to the Haber-Bosch. Here we present the result of comprehensive density functional theory (DFT) calculations on the surface of transition metal nitrides and transition metal oxides as two different cathode materials for catalyzing nitrogen conversion to ammonia electrochemically in aqueous solutions and at ambient conditions. The aim was to find candidates that were stable and active toward ammonia formation while simultaneously suppressing the competing reaction of hydrogen evolution. Among the nitrides, the most promising candidates are found to be VN, ZrN, NbN, and CrN1-4, which are predicted to catalyze ammonia formation at overpotentials of 0.50 to 1.40 V versus the standard hydrogen electrode (SHE). However, the most promising oxide candidate is found to be the (110) facets of rutile NbO2, which is anticipated to produce ammonia at relatively low overpotentials of 0.57 V versus SHE5. IrO2 was actually predicted to be the most active oxide catalyst for synthesis of ammonia with only an overpotential of 0.36 V, but this catalyst might contribute a portion of the supplied electrical potential to formation of hydrogen gas too, and thus yields lower rate of ammonia formation. Following these computational guidance, experimentalists investigated the catalytic activity of VN nanoparticles and measured 6% faradaic efficiency (FE)6 towards ammonia synthesis but with significant suppression of hydrogen evolution. Nonetheless, engineering the catalyst surface (as elaborated by the DFT results) is a key factor for enhancement of performance and efficiency on these materials that should be the scope of future works. Also Huang et al.
7got inspiration from our theoretical predictions on oxides and thus tested NbO2 experimentally where they reported 32% FE towards ammonia formation at ambient conditions and in aqueous electrolyte. This is currentlya world record in the field. Both these theoretical and experimental investigations have therefore opened up new avenues for the possibility of improved activity and selectivity of cathode material toward electrochemical ammonia synthesis.
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