A joint research group led by Associate Professor Hiro Minamimoto and Professor Minoru Mizuhata of the Graduate School of Engineering at Kobe University and Associate Professor Masaaki Yoshida of the Graduate School of Sciences and Technology for Innovation at Yamaguchi University announced on September 11 that they have developed a new nickel-based catalyst that enhances the efficiency of the oxygen evolution reaction (OER) that creates oxygen in water electrolysis. They confirmed high reaction activity even under strong alkaline conditions and high durability against temporary reverse currents that cause degradation. The team also clarified how changes in atomic arrangement and electronic states around the catalyst lead to high activity and durability. This achievement is expected to contribute to cost reduction of green hydrogen. The results were published in the American Chemical Society journal ACS Electrochemistry on August 12.
Alkaline water electrolysis (AWE) is attracting attention as a method for producing green hydrogen from renewable energy-derived electricity as it provides an advantage in terms of cost. AWE is a technology that decomposes water in an alkaline electrolyte to obtain hydrogen and oxygen, consisting of the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. Non-noble metal catalysts responsible for the OER, which determines the overall reaction rate, are required to achieve both high activity and long-term stability.
In conventional synthesis methods, such as high-temperature heat treatment and hydrothermal methods, high-energy synthesis conditions can cause surface inhomogeneity and reduced reproducibility, necessitating highly reproducible synthesis processes. Additionally, reverse currents occur during startup and shutdown, which causes electrode degradation, and countermeasures have been sought.
Therefore, in this study, the team directly fabricated fluorine-doped α-Ni(OH)2 catalyst films using the liquid phase deposition (LPD) method, which allows synthesis at low temperature and low load.
As a result, they confirmed high reaction activity even under strong alkaline conditions. They also revealed that precise control of the pH of the reaction solution enables fine adjustment of the electronic state and surface state without impairing crystallinity.
In tests simulating repeated startup and shutdown operations using a two-electrode cell that mimics actual operating environments, the catalyst showed high durability against temporary reverse currents that cause degradation. Furthermore, the researchers discovered that slight differences in the pH of the precursor solution systematically affect electrode activity.
To clarify the origin of the high activity, they conducted operando X-ray absorption spectroscopy analysis, observing the reaction in real time, and were able to visualize how NiOOH formation during operation and local fluorine coordination similar to NiF2 affect conductivity and activity. Based on these findings, they proposed a "reaction field design guideline" with pH, fluorine introduction amount, and film formation conditions as key factors.
This work is expected to contribute to the design of highly efficient alkaline water electrolysis electrodes that do not depend on rare metals. In the future, the team plans to proceed with additional evaluations such as Faradaic efficiency, which measures how much of the electricity consumed is used for the target reaction.
Minamimoto commented: "Our research group has long explored inorganic material synthesis methods in aqueous solutions, and in recent years we have been advancing the preparation of electrode materials as an application of this. While the possibility of fluorine incorporation into the material during the synthesis process was anticipated, the detailed mechanism and functional effects were not clear. By introducing the unique measurement method of co-author Associate Professor Masaaki Yoshida, we were able to demonstrate for the first time how fluorine is incorporated and what role it plays during electrode use. This achievement provides a new perspective on material design, and we plan to develop it toward process development and innovative material design with practical application in mind."
Journal Information
Publication: ACS Electrochemistry
Title: Integrating Electrochemical Analyses and Operando X-ray Spectroscopy of Fluorine-Doped α-Ni(OH)2 for Alkaline Water Electrolysis
DOI: 10.1021/acselectrochem.5c00136
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