Spinel‐type <scp> Ni ₂ GeO ₄ </scp> electrocatalyst for electrochemical ammonia synthesis via nitrogen reduction reaction under ambient conditions

Abstract

The electrochemical nitrogen reduction reaction (ENRR) is considered an environmentally-friendly process for ammonia production under ambient conditions, compared to the conventional Haber-Bosch method. However, the electrochemical synthesis of ammonia suffers from poor selectivity and low efficiency owing to constrained catalytic activity and the competitive hydrogen evolution reaction (HER). Spinel-type solid materials have received considerable interest owing to their various applications in catalysis, energy storage, and optical devices. Particularly, mixed transition-metal oxides with a formula of A2BO4 exhibit improved catalytic activity owing to the presence of multiple balance sites for cations (A2+/3+ and B2+/3+/4+) in their structure. In this study, Ni2GeO4 nanoparticles synthesized via a facile hydrothermal method were investigated as a novel electrocatalyst for the ENRR. X-ray photoelectron spectroscopy confirmed the presence of octahedrally-coordinated Ni2+ and tetrahedrally-coordinated Ge4+ in the prepared spinel-type Ni2GeO4 material. Notably, the Ni2GeO4 electrocatalyst achieved an excellent Faradaic efficiency of 3.57% and an ammonia yield rate of 3.06 μg h−1 cm−2 at −0.1 V vs RHE, owing to the enhanced charge transfer and chemical activity caused by charge polarization between the polyhedral units. The results of this primary investigation can inspire the development of Ni2GeO4-based electrocatalysts using innovative approaches to realize superior ENRR activity.