Extremely Low Electrical Contact Resistance at the Interface of Carbon-Fiber-Based Gas Diffusion Layer and Anatase TiO₂ Thin Films

With the rapid development of polymer electrolyte membrane fuel cells (PEMFC), there is a growing need to reduce the contact resistance between a gas diffusion layer (GDL) and stainless-steel separators within stacked cells. However, stainless-steel separators coated with metal oxides or nitrides exhibit a high contact resistance exceeding 10 mΩ cm², whereas gold-coated stainless steel exhibits 1–5 mΩ cm². Therefore, the development of alternative coating materials is critical. Here, we demonstrate that the contact resistance is reduced by an order of magnitude to 1.4 mΩ cm² using anatase Nb-doped TiO₂ for the coating of separators. First, we fabricate electrically conductive TiO₂ with a resistivity of 6.2 × 10^–3 Ω cm on glass substrates using mist chemical vapor deposition. Next, we minimized the contact resistance using Bayesian optimization and robots. Bayesian optimization helps find optimal conditions, and automated experiments with robots help to prepare and evaluate more samples. The contact resistance is the lowest value reported for metal oxides and nitrides and is comparable to that of gold. Furthermore, the coated anatase Nb-doped TiO₂ exhibits high corrosion resistance under acidic conditions. These results highlight anatase Nb-doped TiO₂ as a promising coating material for high-power-density PEMFC separators.