In-situ raman spectroscopy reveals the reconstructions of NiMoO4 for neutral water oxidation: Insights from activation processes
image: In-situ Raman spectroscopy provides direct spectroscopic evidence for the activation behavior of NiMoO4 in different electrolyte environments, clarifying the crucial role of alkaline activation in enhancing the OER performance of NiMoO4 under neutral conditions. This finding offers important guidance for the development of efficient self-reconstructing catalysts for neutral water electrolysis.
Credit: Nano research, Tsinghua University Press
Green hydrogen production is essential and highly desirable for achieving a sustainable and carbon-neutral future. Electrochemical water electrolysis is immensely promising because of its high-purity hydrogen output and high potential for large-scale applications. At present, although electrochemical water splitting in alkaline or acidic media demonstrates excellent performance, the corrosion of equipment and catalysts severely restricts its extensive industrial-scale usage. Alternatively, water electrolysis in neutral media has attracted increasing attention due to its lower corrosiveness in mild electrolytes.
The sluggish kinetics of the oxygen evolution reaction (OER) have always been a critical barrier that needs to be overcome, especially in neutral circumstances with ultra-low concentrations of hydroxide and protons. Therefore, designing efficient electrocatalysts with high activity in neutral media remains an urgent challenge. A team of researchers led by Hongru Hao and Bo Wei from Harbin Institute of Technology recently published an interesting study in Nano Research, which reveals the reconstruction mechanism of NiMoO4 during neutral water oxidation using in-situ Raman spectroscopy
The work provided important insights for the development of efficient catalysts for neutral water electrolysis. They systematically investigated the dynamic structural evolution of NiMoO4 precatalyst during electrochemical activation in different electrolyte. “Albeit surface reconstruction of precatalysts for alkaline oxygen evolution reaction is extensively studied, the dynamic structure evolution and true active phase in neutral media remain largely unexplored. Here, we carried out in-situ mechanistic comparison of alkaline vs. neutral activations on neutral-pH OER activity, taking NiMoO4 as a model precatalyst,” said Prof. Bo Wei.
The results reveal that, when electrochemically activated in harsh alkaline conditions, NiMoO4 undergoes rapid and complete reconstruction with the formation of a considerable NiOOH active phase. On the other hand, direct activation in neutral media only results in weak reconstruction with a thin NiOOH layer on NiMoO4 surface. Accordingly, the electrode oxidized in a basic electrolyte yields a superior OER performance compared to that directly treated in mild pH condition.
Density functional theory calculations further revealed the reaction mechanism. They constructed four models: NiMoO4, NiOOH-NiMoO4, NiOOH-MoO42-, and NiOOH. The calculation results show that the rate-determining step of NiOOH (transformation from *O to *OOH) has a Gibbs free energy change of 1.95 eV, significantly lower than that of NiMoO4 (2.33 eV) and NiOOH-NiMoO4 (2.17 eV). This indicates that the NiOOH active catalyst derived from alkaline electrooxidation can effectively lower the energy barrier of the OER reaction and enhance the catalytic reaction rate.
The OER onset potential in 1 M PBS solution of NiMoO4-KOH was 1.719 V, which was significantly lower than that of NiMoO4-PBS (1.914 V). At a current density of 10 mA·cm-2, the overpotential of NiMoO4-KOH is measured to be 548.3 mV, which is notably smaller than that of NiMoO4-PBS with a value of 789.1 mV. In addition, the NiMoO4-KOH catalyst demonstrated excellent stability with stable operation for 100 hours at 50 mA·cm-2.
This study deepens the understanding of the neutral water oxidation reaction, providing insightful guidelines for the development of efficient electrocatalysts for neutral water electrolysis with self-reconstruction. The “alkaline pre-activation/neutral-operation” strategy is proven efficient for neutral water electrolysis, which is also universally applicable for practical synthesis. This study uncovers crucial insights in catalyst design, highlighting that the deep reconstruction of precatalysts in alkaline conditions is critical for high performance neutral water oxidation.
Other contributors include Jian Zhou, Hongcheng Zhao and Zhe Lv from the School of Physics, Heilongjiang Provincial Key Laboratory of Advanced Quantum Functional Materials and Sensor Components, Harbin Institute of Technology in Harbin; and Jiahui Wang from the Department of Applied Physics, Nanjing University of Science and Technology in Nanjing, and Lingling Xu from the Key Laboratory for Photonic and Electronic Bandgap Materials Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University in Harbin.
This work was supported by the Natural Science Foundation of China (22279025, 21773048, 52472195) and the Fundamental Research Funds for the Central Universities (2023FRFK06005).
About the Authors
Dr. Bo Wei is a professor in the School of Physics, Harbin Institute of Technology (HIT). He received his Ph.D. from Harbin Institute of Technology in 2008. In 2013, he worked at Curtin University and then worked as a Humboldt Research Fellowship in Institute of Physical Chemistry at RWTH Aachen University, Germany (2014–2015). His research focuses on solid oxide fuel cells and water splitting, etc.
https://homepage.hit.edu.cn/weibo
About Nano Research
Nano Research is a peer-reviewed, open access, international and interdisciplinary research journal, sponsored by Tsinghua University and the Chinese Chemical Society, published by Tsinghua University Press on the platform SciOpen. It publishes original high-quality research and significant review articles on all aspects of nanoscience and nanotechnology, ranging from basic aspects of the science of nanoscale materials to practical applications of such materials. After 18 years of development, it has become one of the most influential academic journals in the nano field. Nano Research has published more than 1,000 papers every year from 2022, with its cumulative count surpassing 7,000 articles. In 2024 InCites Journal Citation Reports, its 2024 IF is 9.0 (8.7, 5 years), and it continues to be the Q1 area among the four subject classifications. Nano Research Award, established by Nano Research together with TUP and Springer Nature in 2013, and Nano Research Young Innovators (NR45) Awards, established by Nano Research in 2018, have become international academic awards with global influence.