Y. Kamihara
Keio Univ.,
Japan
Keywords: superconductivity, oxygen evolution reaction, metal-air-battery, oxygen deficiency, YBCO
Summary:
Oxygen evolution reaction (OER) is an essential part of the electrolysis of water in the fields of renewable energy, and rechargeable metal–air batteries. However, since the large overpotential of OER causes an energy loss in electrolysis of water, an appropriate OER electrocatalyst is required. The OER requires an electrode as the catalyst to enhance its reaction efficiency. Practical, existing OER catalysts are metal oxides of noble metals like Iridium (Ir) and Ruthenium (Ru). The practical OER catalysts are expensive, and their introduction into large-scale industrial facilities and infrastructure requires high costs, which presents a barrier to smart society. The search for alternative materials that are both cheaper and possess high catalytic activity is required to remove the barrier. One approach for an inexpensive OER catalyst is research on transition metal layered oxides with oxygen vacancies. Indeed, Hirai et al., in [S. Hirai, et al., J. Mater. Chem. A 6, 15102-15109 (2018). Oxygen vacancy-originated highly active electrocatalysts for the oxygen evolution reaction], reported that an layered perovskite-related compound with oxygen vacancies Sr2VFeAsO3-delta (delta >= 0.5) exhibits the lower OER overpotential and the larger reaction rate than those of conventional OER catalyst. Thus, layered perovskite-related compounds possess functionality as electrochemical catalysts. A representative layered perovskite-related compound is the orthorhombic YBa2Cu3O7-delta (YBCO), known as a high-critical-temperature superconductor. As the delta increases in YBCO, superconducting phase disappears, it becomes an insulator, and its crystal structure becomes tetragonal. The functionality of tetragonal YBCO that does not exhibit a superconducting phase is still unknown. In this report, the function of tetragonal YBa2Cu3O7-delta (YBCO) was investigated as a new OER catalyst, which enhances the activity because of the reaction mechanism caused by oxygen deficiency. YBCO samples were synthesized via a sol-gel reaction and calcined at 760, 820 and 920 deg. C. Crystal structures were determined by powder X-ray Diffraction (XRD). The overpotentials of YBCO as an OER catalyst were measured by Rotating Ring Disk Electrodes (RRDE). OER activity of YBCO is superior to a bare glassy carbon and carbon black with regards to OER overpotential and Tafel slope. Moreover, the crystal structure of YBCO may play a role in the enhancement of OER activity. Tetragonal YBCO exhibits better OER catalyst activity than that of orthorhombic YBCO. [N. Takehara, R. Kuramochi, Y. Kamihara et al. Mater. Sci. Tech. Jpn., accepted for publication, (in Japanese)]