The production of solar fuels from H2O, sunlight, and CO2 is a promising approach to achieve a sustainable society. Plasmonic-metal nanostructures that can harvest sunlight and generate non-equilibrium hot carriers capable of catalyzing chemical reactions offer an appealing material platform for solar fuel generators. Compared with conventional photoelectrochemical devices composed of semiconductors, in which light absorbers and catalysts are usually decoupled, plasmonic photocatalysts offer unique size-dependent optical properties combined with catalytic surfaces that can influence chemical reactivity. Especially interesting for CO2 reduction where the complex multi-step chemical pathway influences product selectivity, it has been proposed that plasmon excitation may reduce activation barriers, or change the population of adsorbed molecules. Despite much promise, the main limitation of such systems is the insufficient light-matter interaction and difficulties associated with charge transfer, leading to low energy efficiency. To realize practical fuel production, the project aims to engender significant electromagnetic coupling between optical modes for enhancing plasmon-induced photoelectrochemical reactions. The hybrid mode can enhance light absorption and alter the hot-carrier separation and transfer dynamics to boost up the overall efficiency. The strong near-field enhancement may also help with chemical binding to assist coupling for higher-value products. For bridging the knowledge from nanoscale catalysts to practical photoelectrochemical reactions, in-situ TEM and in-situ SERS will be used to understand the real-time evolution of the catalytic surface (structural and chemical changes) during operation under illumination and/or electrical bias with high spatial resolution.