(Invited) High Efficiency, Ultras-table Solar Hydrogen Production Utilizing Industry Standard Materials

Solar water splitting and green hydrogen fuel production has been extensively studied but is confronted with fundamental challenges of simultaneously achieving both high efficiency and long-term stability. The commonly utilized approaches include photovoltaic-electrolysis (PV-E), photoelectrochemical, and photocatalytic processes, and numerous materials have been studied. In this presentation, I will report on our recent developments of gallium nitride (GaN) based photoelectrochemical and photocatalytic water splitting studies. GaN is one of the most produced semiconductors. By alloying with indium, the energy bandgap of InGaN can be continuously varied across nearly the entire solar spectrum. Recent studies have further shown that the conduction and valence band edge positions of InGaN can staddle water redox potentials for indium compositions up to 40-50%, corresponding to an energy bandgap ~1.7 eV. This is a unique property not commonly seen in other materials. Significantly, by growing GaN nanostructures under N-rich conditions, the surfaces can be transformed to be N-rich, which can protect against photocorrosion and oxidation during harsh photocatalysis reaction conditions. Studies have further shown that, during the initial solar water splitting reaction, the N-rich surfaces are transformed to be gallium oxynitride, which not only further protects the surfaces of the light absorber but exhibits significantly enhanced photocatalytic and photoelectrochemical performance with continued reaction, which is in direct contrast to the rapid performance degradation of other known light absorbers. We have demonstrated double-junction InGaN/Si photoelectrode that can exhibit solar-to-hydrogen efficiency of 10% with stability over 100 hrs under practical two-electrode measurement conditions. Moreover, we have developed unique photocatalytic process wherein high efficiency solar hydrogen can be produced utilizing tap water, or seawater, without any wire connection, or electricity input. The demonstration of large-scale solar water splitting systems and the performance will be discussed and reported. Moreover, the use of GaN-based photocatalytic nanostructures for converting carbon dioxide and methane to liquid fuels under sunlight will be presented.