Ammonia is synthesized through the energy intensive and centralized Haber-Bosch process. This process requires high temperature, 573~773 K, and pressure, 200~300 bar, leading to about 1%~1.5% of total energy consumption worldwide. Electrochemical ammonia synthesis at mild operating conditions could reduce the energy intensity and enable small scale, distributed NH3 production. However, the catalytic process is limited by very poor catalyst Faradaic efficiency. We use density functional theory methods to examine the elementary electrocatalytic processes involved in N2 reduction on iron-based oxides, hematite and magnetite. First-principles ab initio thermodynamics approach are applied to determine the surface termination with respect to both applied potential (RHE scale) and Miller index. Under N2 electroreduction conditions, the surfaces are partially reduced and are terminated by coordinatively unsaturated metal sites or hydroxyl/hydrogen groups, which are crucial for providing active sites for N2 reduction. Using the proton-shuttling model developed by our group combined with thermodynamic criteria, we find that Ni doping can strongly facilitate the N2 reduction kinetics by reducing the limiting potential for the target reaction. The Ni dopant position and concentration impact the hydrogen evolution kinetics substantially different, thus affecting the selectivity. We further study the Fe dopant effect in Ni-based oxide phase, namely, layered β-Ni(OH)2 and NiO. The performance of the catalyst is better when Fe is in higher percentage compared to Ni.

Towards efficient N2 electroreduction to ammonia through DFT investigation of mixed iron/nickel oxides

2019 Spring National Meeting

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