Spinel‐type <scp> Ni <sub>2</sub> GeO <sub>4</sub> </scp> electrocatalyst for electrochemical ammonia synthesis via nitrogen reduction reaction under ambient conditions
Dohun Kim, Subramani Surendran, Yoongu Lim, Hyeonuk Choi, Jaehyoung Lim, Joon‐Young Kim, Mi‐Kyung Han, Uk Sim
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.