Obstacles of solar-powered photocatalytic water splitting for hydrogen production: A perspective from energy flow and mass flow

Solar-driven photocatalytic water splitting for hydrogen production has emerged as one of the foremost scientific issues. However, the efficient, low-cost, and large-scale operable system is still missing. Photocatalytic water splitting has multiple processes that involve light absorption, charge excitation and transfer, mass transfer, and chemical reaction, making it extremely complicated and challenging. Herein, we review the whole-process photocatalytic water splitting from two kinds of “flow”: energy flow and mass flow. The energy flow represents transmission and conversion of solar energy through the concentrator, reactor, reaction solution, and photocatalyst. The mass flow refers to transfer of reactants and products in the gas-liquid-solid multiphase environment. For the first time, we clearly describe the energy flow and mass flow in photocatalytic water splitting from multiple spatiotemporal scales, and point out that the low efficiencies of photocatalytic water splitting are attributed to obstacles in energy flow and mass flow, as well as non-coupling and mismatching of energy flow and mass flow. The mechanistic insights learned from this perspective indicate that, in addition to the material optimization, the scientific and engineered design of sunlight collection, interfacial reaction, and mass transfer is of great significance.