Advancement in clean energy to cut pollution and global warming is one of the most significant of our duties as scientists. For this target, interdisciplinary research is pivotal, where chemistry is at the core. We pursue electrochemistry to accomplish green-energy technology, thus aiming to understand electrochemical reactions to resolve scientific and engineering challenges. We have particularly focused on gaining fundamental insights regarding the role and reaction mechanism of electrocatalysts and applying this knowledge to electrochemical energy systems. We have achieved this research target with advanced nanomaterial and in-situ observation of electrochemical reactions at surfaces and interfaces.