Katsushi Fujii (Ph.D., Science) received his B.Eng. degree in 1983 and his Ph.D. degree in 1994, both from Osaka University. He joined Mitsubishi Chemical Corporation in 1983. From 2004 to 2017, he held several academic positions at research institutes and universities, including roles as Researcher, Associate Professor, and Professor. Since 2017, he has been a Research Scientist at the Center for Advanced Photonics, RIKEN. His research until 2011 focused on GaAs compound semiconductor bulk growth and the epitaxial growth of AlGaAs, AlGaInP, and AlGaInN by MOVPE, as well as the fabrication and characterization of light‑emitting devices (LDs and LEDs). In 2004, he began research on photoelectrochemical water splitting using nitride compound semiconductors. In 2011, he initiated research on system‑level approaches to renewable energy storage. His current research interests include energy storage systems, solar‑to‑chemical energy conversion (solar‑to‑X), electrochemical hydrogen evolution from water, electrochemical CO₂ reduction, electrochemistry, and semiconductor science.