Kazunori Nishio

A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3. This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

Thermodynamic guiding principles in selective synthesis of strontium iridate Ruddlesden-Popper epitaxial films

We demonstrate the selective fabrication of Ruddlesden-Popper (RP) type SrIrO3, Sr3Ir2O7, and Sr2IrO4 epitaxial thin films from a single SrIrO3 target using pulsed laser deposition (PLD). We identified that the growth conditions stabilizing each phase directly map onto the phase diagram expected from thermodynamic equilibria. This approach allows precise cation stoichiometry control as evidenced by the stabilization of single phase Sr3Ir2O7 for the first time, overcoming the close thermodynamic stability between neighboring RP phases. Despite the non-equilibrium nature of PLD, these results highlight the importance of thermodynamic guiding principles to strategically synthesize the targeted phase in complex oxide thin films.

Synthesis and electronic properties of Fe2TiO5 epitaxial thin films

We investigate the growth phase diagram of pseudobrookite Fe2TiO5 epitaxial thin films on LaAlO3 (001) substrates using pulsed laser deposition. Control of the oxygen partial pressure and temperature during deposition enabled selective stabilization of (100)- and (230)-oriented films. In this regime, we find an optical gap of 2.1 eV and room temperature resistivity in the range of 20–80 Ω cm, which are significantly lower than α-Fe2O3, making Fe2TiO5 potentially an ideal inexpensive visible-light harvesting semiconductor. These results provide a basis to incorporate Fe2TiO5 in oxide heterostructures for photocatalytic and photoelectrochemical applications.