This book presents a new design strategy for high proton-conducting oxides with high chemical stability—donor doping into oxides with intrinsic oxygen vacancies—aimed at bridging the “Norby gap” (200–500 °C). It reframes the classic limitation of the conventional acceptor-doped perovskites by exploiting electrostatic donor–proton repulsion to suppress proton trapping by dopants and lower activation energies. The topic is treated through an integrated materials-design narrative spanning defect chemistry, synthesis, structure determination, transport measurements, and atomistic simulations. A central result is the discovery of cubic perovskites, exhibiting bulk proton conductivity of 0.01 S cm⁻¹ at 235 °C and high chemical stability under oxidizing, reducing, and CO₂-containing atmospheres. The book further establishes how donor oxidation state affects oxygen deficiency, hydration, and conductivity trends. Readers will find a transferable guideline: start from an intrinsically oxygen-deficient framework, then use donor doping to stabilize a cubic perovskite, yielding fast proton transport. Customers can expect a practical roadmap for designing chemically stable electrolytes for protonic ceramic fuel cells (PCFCs) and electrolysis cells (PCECs) operating at the intermediate temperatures. This topic is important to publish now because intermediate-temperature PCFCs and PCECs are rapidly progressing, yet a general, mechanism-based strategy to overcome the conductivity–stability bottleneck remains urgently needed.

Kei Saito is an Assistant Professor at Institute of Science Tokyo, working in the Yashima Group. He received his B.Sc. and M.Sc. in Science from Tokyo Institute of Technology and his Ph.D. in Science from Institute of Science Tokyo in 2025. His research focuses on solid-state ionics, particularly ceramic proton conductors with intrinsic oxygen vacancies. By combining materials synthesis, neutron diffraction, and computational approaches, he studies proton transport mechanisms and develops new proton-conducting oxides for energy conversion technologies such as protonic ceramic fuel cells and electrolysis cells.