Exhibiting high proton conductivity at 300°C

A research group led by Professor Yoshihiro Yamazaki of the Platform of Inter-/Transdisciplinary Energy Research, Kyushu University, has successfully developed an electrolyte^*1 material for solid oxide fuel cells^*2 that function at 300°C. Solid oxide fuel cells are attracting attention as next-generation fuel cells because they use hydrogen as fuel to generate electricity and emit no carbon dioxide during power generation.

The research group achieved, for the first time worldwide, a proton conductivity of 0.01 S/cm or higher at 300°C by substituting a high concentration of scandium into some metallic portions of the electrolyte materials barium stannate and barium titanate. Proton conductivity is a measure of the mobility of protons (hydrogen ions) through a material, and when this value is high, a fuel cell’s power generation becomes more efficient. In addition, this electrolyte material maintained structural stability even in high-CO₂ environments and showed high durability.

*1 Electrolyte
An ion-conducting substance that enables ion transport inside a fuel cell

*2 Solid oxide fuel cell
A fuel cell that produces electricity by oxidizing fuels such as hydrogen. It generates power with high efficiency.

Cost hinders widespread adoption of solid oxide fuel cells

Solid oxide fuel cells function at high temperatures

Solid oxide fuel cells are characterized by their use of hydrogen as fuel and their lack of CO₂ emissions during power generation. Despite attracting attention as a technology suitable for reducing CO₂ emissions, commercialized versions currently operate at high temperatures between 700 and 800°C and therefore require heat-resistant materials. However, because heat-resistant materials are expensive, the high material costs have hindered widespread adoption of solid oxide fuel cells. If the viable temperature for efficient power generation can be lowered to around 300°C, it would not only reduce costs by using cheaper materials, but it would also shorten startup times and simplify thermal management. As a result, solid oxide fuel cells could see much wider adoption than at present, including in household power generation systems and vehicular applications.

The dilemma of proton concentration and proton traps

To enable solid oxide fuel cells to function at low temperatures, electrolyte materials that exhibit high proton conductivity at low temperatures are essential. Therefore, the research group focused on proton-conducting oxides that exhibit high proton conductivity at temperatures ranging from 400 to 600°C.

To dramatically increase proton conductivity, it is necessary to increase both “proton concentration” and “proton mobility.” In proton-conducting oxides, the proton concentration increases in proportion to the concentration of a substitutional dopant. A substitutional dopant refers to an element that can substitute for other atoms. However, increasing the concentration of a dopant is known to induce the phenomenon called “proton trapping.” This is a phenomenon in which proton migration is restricted by the substituted element, with proton mobility decreasing significantly in the low-temperature range around 300°C. This problem had remained unresolved since the discovery of proton-conducting oxides in 1981.

Successful development of a new electrolyte material with octahedral connectivity as the key

Two types of electrolyte materials showing high performance

In this study, the proton conductivity of two proton-conducting oxides, barium stannate and barium titanate, was evaluated for materials with high-concentration scandium (Sc) substitution. As a result, a proton conductivity of 0.01 S/cm or higher, which is the level required for polycrystalline electrolyte materials in solid oxide fuel cells, was achieved for the first time worldwide at the intermediate temperature of 300°C (Fig. 1). In addition, it is crucial for electrolytes to maintain performance in environments that contain CO₂ during power generation. These materials demonstrated high durability, with no degradation in function, even under high CO₂ concentrations.

Reasons for Enhanced Proton Conductivity

The reasons why these electrolyte materials were able to achieve high proton conductivity are twofold.

The first is that barium stannate and barium titanate have a “softer” lattice than conventional zirconium (Zr)-containing materials, allowing the incorporation of a larger amount of scandium. This enabled Sc substitution at high concentrations of 70% and 80%.

The second reason is that proton mobility did not decrease even when a high concentration of Sc was added, meaning that proton trapping was avoided. To clarify this point, the research group conducted simulations of proton migration. Normally, protons reside adjacent to oxygen and hop between oxygen atoms in the octahedral structure (Fig. 2, left). When scandium octahedra are not connected, isolated octahedra trap protons and hinder their rapid movement instead. However, high-concentration Sc substitution causes the octahedra to connect, enabling protons to migrate rapidly along these linked pathways (Fig. 2, right).

These results demonstrate that high-concentration scandium substitution is effective not only for raising the concentration of protons but also for suppressing the drop in mobility.

A major step toward the realization of a hydrogen energy society

Two types of electrolyte materials exhibited high proton conductivity, even at 300°C, and demonstrated high CO₂ tolerance. Building upon the findings from this study is expected to enable further development of electrolyte materials that function at even lower temperatures. These results will significantly accelerate the practical implementation and broadening application of hydrogen-fueled fuel cells.