Tokyo Institute of Technology discovered a novel oxide with the highest recorded proton conductivity in the intermediate-to-low temperature range

Graduate Student Kei Saito and Professor Masatomo Yashima of the School of Science at the Tokyo Institute of Technology announced the discovery of a new material that exhibits the world's highest proton conductivity ever recorded in the medium-to-low temperature range. The material design strategy employed for this new material was completely different from conventional methods. Through crystal structure analyses and theoretical calculations, they elucidated the factors that contribute to the high proton conductivity of this material. Their methodology, which resulted in the discovery of the new material BaSc_0.8Mo_0.2O_2.8 with a remarkably high proton conductivity in the medium-to-low temperature range, diverges from the conventional strategies, in which 3D irregular essential oxygen vacancies are donor-doped into perovskites.

They observed that the high proton conductivity was due to factors such as a low activation energy, which resulted from suppressed proton trapping. This breakthrough is expected to facilitate the development of high-performance proton-conducting fuel cells that can operate at low temperatures. The findings were published in the international academic journal Nature Communications on November 17, 2023.

Proton conductors are materials that demonstrate proton (H⁺) conduction, and they are anticipated to be applied as clean energy materials in electrochemical devices such as protonic ceramic fuel cells (PCFCs), hydrogen pumps, and hydrogen sensors. To achieve a high proton conductivity, oxygen vacancies are introduced into the crystal structure through acceptor doping, i.e., doping of cations, whose valence states are lower than those of the cations present in the host compound. However, in the medium-to-low temperature range, a gap known as the "Norby gap" exists, in which materials with high proton conductivities are absent. This gap is observed because of the proton trapping phenomenon, in which oxygen vacancies capture protons.

In this study, the researchers focused on the cubic perovskite BaScO_2.5, which served as the parent material with irregular essential oxygen vacancies. The cubic perovskite BaSc_0.8Mo_0.2O_2.8 was synthesized through a solid-state reaction. Electrical conductivity measurements showed that the ratio of the electrical conductivity measured in H₂O airflow to that detected in D₂O airflow was approximately equal to the theoretical value of 1.41, which was derived based on the classical theory of isotope effect. The material showed high chemical and electrical stability. The bulk conductivity was also the highest in the world. The factors contributing to the observed high bulk proton conductivity were extracted through Rietveld and thermogravimetric analyses. The proton concentrations derived from the structural and thermogravimetric analyses were in good agreement with each other, indicating that water was incorporated into the bulk.

The diffusion coefficient of this new material was found to be higher than that of other proton conductors; this high diffusion coefficient was one of the factors that contributed to its high bulk proton conductivity. Additionally, the diffusion coefficient increased as the selenium concentration increased.

The activation energy of the new material was confirmed to be lower than that of other acceptor-doped proton conductors. This low activation energy indicates that proton trapping is suppressed in this new material and was one of the factors that contributed to the observed high proton conductivity at low temperatures. The practical application of these materials in fuel cells is expected to eliminate the need for expensive platinum and heat-resistant materials. These materials can significantly reduce the manufacturing costs of fuel cells.

Yashima stated: "By exploring new materials with a high oxygen vacancy concentration, we have discovered a groundbreaking new material that combines high proton conductivity and high stability. However, advocating for these material design guidelines in a top-tier journal requires various experiments, analyses, and calculations, supported by strong evidence, such as 1) sample synthesis, 2) various electrochemical measurements, 3) precise crystal structure analyses, and 4) first-principles calculations. This study was completed through the dedicated efforts of Mr. Saito. With the use of this groundbreaking new material featuring a high proton conductivity and high stability, we anticipate the development of highly efficient fuel cells, which can solve energy and environmental issues."