Designing NiO Nanoparticles as Electrocatalysts for Nitrate Reduction into Ammonia

The introduction of the Haber—Bosch (HB) process in the early twentieth century enabled the large-scale production of NH₃, swiftly becoming one of the most crucial chemical products worldwide due to its extensive application in agriculture as a fertilizer. Moreover, NH₃ has recently garnered significant interest as a potential renewable energy storage system, given its capacity to serve as a source of hydrogen [1].
However, the Haber—Bosch process, reliant on atmospheric nitrogen and fossil fuel, requires H₂ for NH₃ production, as well as high process temperature and pressure, contributing to approximately 1.6% of the annual global CO₂ emissions [2]. A promising alternative lies in the electrochemical nitrogen reduction reaction (E-NRR) to synthesize NH₃ under ambient conditions. However, to date, this process has a limited yield production and a low selectivity due to the high stability of the N2 molecule and the presence of parasitic reactions, primarily leading to water (solvent) conversion into hydrogen [4].
A more recent focus has emerged towards the reduction of NO₃^‒, as it can be more easily converted into NH₃ with a significant Faradaic efficiency (FE) and high yield. Moreover, owing to the prevalent use of nitrogen-based fertilizers, this process possesses a significant real-case application towards wastewater treatment, where high NO₃^‒ levels have often been detected [3].
This study presents the utilization of a nanostructured NiO electrocatalyst, prepared by precipitation in an aqueous medium and calcinated at 600 °C, for the reduction of NO₃^‒ into NH₃, achieving an average FE of 36% and a production rate ranging from 28 to 107 μg h cm^‒2, depending on the initial NO₃^‒ concentration. The experiments were conducted in an H-type cell, utilizing three different concentrations of KNO₃ (NO₃^‒ source), i.e. 0.1, 0.05, and 0.008 M. A second investigated experimental parameter was the concentrations of the supporting electrolyte (i.e., K₂SO₄), which was used at 0.4, 0.45, and 0.492 M. The tests were conducted under an applied potential (E) of ‒1.4 V vs. Ag/AgCl for a duration of 2 h.
In this contribution, we will show the main outcomes derived from this newly explored electrocatalyst, highlighting the main structure--performance correlations.

References:
[1] H. Shen, C. Choi, J. Masa, X. Li, J. Qiu, Y. Jung and Z. Sun, Chem, 2021, 7, 1708–1754.
[2] P. Zhang, W. Xiong and M. Zhou, Nano Materials Science, 2020, 2, 353–359.
[3] Q. Liu, Q. Liu, L. Xie, Y. Ji, T. Li, B. Zhang, N. Li, B. Tang, Y. Liu, S. Gao, Y. Luo, L. Yu, Q. Kong and X. Sun, ACS Appl Mater Interfaces, 2022, 14, 17312–17318.
[4] B. Yang, W. Ding, H. Zhang and S. Zhang, Energy Environ Sci, 2021, 14, 672–687.

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 948769, project title: SuN2rise).