The Fabrication of Conductive and Corrosion-Resistant Oxide Thin Films for Fuel Cells and Hydrogen Generation Devices

In recent years, fuel cell-based power generation technologies have been the subject of study as part of efforts towards a decarbonized society. In Polymer Electrolyte Fuel Cells (PEFCs), both the positive and negative electrodes are in a corrosive environment of high temperature and high humidity, and current must be drawn from both electrodes. Consequently, the separator must have both high corrosion resistance and electrical conductivity. Furthermore, the use of costly titanium and carbon mixtures as separators is currently prevalent. For this reason, our laboratory has reported on cost reduction by coating inexpensive stainless-steel separators with SnO₂and In₂O₃, which have both corrosion resistance and high electrical conductivity ^[1][2]. Furthermore, hydrogen is expected to become a next-generation energy source that does not emit carbon dioxide when used, and water electrolysis, which can generate hydrogen in combination with zero-emission power sources such as renewable energy, is being studied for a sustainable society ^[3][4]. Among these, Proton Exchange Membrane Water Electrolysis (PEMWE) has attracted attention due to the high purity of the hydrogen gas that can be extracted and the ability to operate at high current densities. Furthermore, PEMWE exposes the separator to higher potentials (≥ 2 V vs. RHE) than fuel cells (0.7 V), necessitating the development of higher performance corrosion resistance and electrical conductivity. In this study, experiments were conducted to demonstrate the possibility of applying Tin Oxide (SnO₂) and Indium Oxide (In₂O₃), which have been the subject of previous research, not only to fuel cell separator coatings but also to separators for water electrolysis.<br/>Low-resistance SnO₂ and In₂O₃ films were formed on Ti and Stainless Steel (SUS304) substrates, respectively, using Mist Chemical Vapor deposition (CVD) method. The contact resistance was measured for the vertical component of the sample using the four-terminal method. For the corrosion resistance test, a constant potential polarization test was conducted in a sulfuric acid solution (H₂SO₄, pH 3) to simulate the internal environment. The test duration was 72 hours, with a potential of 2 V vs. RHE applied. The contact resistance of the SnO₂ and ITO films on Ti substrates was 7.6 mΩcm² and 6.4 mΩcm², respectively. These values are below the US Department of Energy (DOE) technical target contact resistance for fuel cells (10 mΩcm²), indicating that the resistance of the deposited oxide materials is well below the target value for separator applications. The resistance values of SnO₂ and ITO thin films formed on stainless steel for separator applications were 11.5 mΩcm² and 8.1 mΩcm², respectively. The contact resistance of uncoated stainless steel alone was measured to be 50.1 mΩcm², indicating the usefulness of the conductive oxide film as a coating material. The elevated contact resistance observed on the SUS substrate in comparison to the Ti substrate is postulated to be attributable to the formation of a thermal oxide film at the interface between the SUS and the oxide, resulting from the heating effects of the mist CVD method during film formation. Consequently, it is anticipated that a further reduction in resistance can be achieved through the optimization of the substrate pre-treatment. Therefore, further lowering of resistance can be expected by carrying out substrate pre-treatment. Furthermore, the test solution of SnO₂ on Ti substrate after a constant potential polarization test was analyzed by ICP-AES, and it was confirmed that neither the Sn contained in the film nor the Ti component of the substrate eluted, indicating good corrosion resistance.<br/><br/>[1] K. Kaneko et al., Jap. J. Appl. Phys., 57, 117103(1-6) (2018).<br/>[2] T. Hattori et al., J. Soc. Mater. Sci. Jpn., 73, 356-363 (2024).<br/>[3] C. Lu et al., Adv. Energy Mater, 11, 2002926(1-10) (2021)<br/>[4] J. Hemauer et al., Int. J. Hydrog. Energy, 48, 25619-25634 (2023)