Proton-Conducting Solid Oxides for Hydrogen Energy System

The energy system utilizing hydrogen as a secondary energy is a potent option for the realization of decarbonized societies. Large-scale energy storage is essential to use renewable energies, which are inherently unstable and do not match demand from human activities. Water electrolysis provides a method for large-scale energy storage by converting electrical energy into hydrogen energy. While various hydrogen utilization technologies exist, because hydrogen serves as a fuel both thermally and electrochemically, fuel cells efficiently convert hydrogen into electricity. Therefore, water electrolysis and fuel cells are important technologies contributing to carbon neutrality.
The application of solid oxide cells (SOC), utilizing proton-conductive metal oxides as electrolytes, holds promise for solid oxide fuel cells and steam electrolysis. This presentation discusses the applicability of these materials to hydrogen energy systems.
Proton conduction in metal oxides was discovered by Iwahara in the early 1980s, with its mechanism explained by the hydration of oxygen vacancies. Introducing cations with lower valences than the original ones in the lattice of the host crystals, most typically perovskite-type oxides, creates oxygen vacancies under electrically neutral conditions. At relatively low temperatures, around 600°C or lower, where hydration proceeds under humid conditions and hence protons (hydrogen ions) appear in the lattice as conducting species.
The primary characteristic of proton-conductive oxides is their lower operating temperatures compared to conventional oxide ion-conducting solid electrolytes typified by stabilized zirconia. The temperature dependence of proton conduction is smaller than that of oxide ions. Therefore, high conductivity is maintained up to relatively low temperatures. The second feature is the potential for higher fuel utilization when used as fuel cells. When applied to steam electrolysis, it offers the advantage of obtaining hydrogen without steam. These characteristics are attributed to the conductive species being protons, making them promising for more efficient fuel cells and steam electrolysis.
However, there are still several unresolved challenges. One of them is electronic leakage. Proton-conducting oxides generally exhibit hole conduction in oxidative atmospheres. This leads to a decrease in Faradaic efficiency in hydrogen production via steam electrolysis and results in additional fuel consumption in the case of fuel cells. The authors are addressing the control of transport phenomena as an approach to suppress electronic leakage in proton-conductive cells. For instance, it was found that electronic conduction in the electrolyte can be blocked by gadolinium-doped ceria (GDC). Effective suppression of such electronic leakage is based on electrode reactions, particularly at the positive electrode, namely the air/steam electrode. Despite the electrolyte being proton-conducting, in many cases mixed conductors of oxide ions and electrons are used for the electrodes. The functionality of GDC as an electron-blocking layer and its facilitation of electrode reactions stem from the properties of this electrode material and its reaction mechanisms. Understanding the roles of each material in a series of electrode reactions allows for comprehension of this phenomenon.

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Biography

Dr. Hiroshige Matsumoto is a Professor, Associate Director, and Lead Principal Investigator at the Advanced Energy Conversion Systems Thrust, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University. He holds a Ph.D. in Engineering, Applied Chemistry from The University of Tokyo, where he conducted a molecular dynamics study on the relationship between local structure and chemical properties of glasses. After obtaining his Ph.D., he worked as a research associate at Nagoya University and Tohoku University before moving to Kyushu University, where he currently serves. Dr. Matsumoto's research focuses on solid state electrochemistry in relation to environment and energy, with significant contributions in the following areas: materials study on ion conducting ceramics, water/steam electrolysis using proton conducting inorganic materials, fuel cells using proton conducting inorganic materials, and nanoionics on solid state ionics. His research and contributions to the field of solid state electrochemistry have positioned him as a leading expert, particularly in the field of proton-conducting metal oxides for the development of environmentally friendly and efficient energy conversion systems.