Interfacial Water State Engineering for Photocatalytic Hydrogen Evolution: From Immersed to Nonimmersed Systems

Traditional immersed photocatalysis is constrained by the "water layer dilemma," where bulk water causes parasitic optical losses and impedes mass transfer. To overcome this, interfacial water state engineering via nonimmersed architectures offers a transformative paradigm shift by decoupling the reaction interface from the bulk liquid. Establishing a novel framework centered on water supply modalities and interfacial states, this review provides a fresh perspective on nonimmersed systems, tracing the transition from externally supplied to autonomous self-supplied architectures. We dissect how engineering the physical form and phase of water reconfigures the local microenvironment to maximize photon utilization and accelerate gas evolution kinetics. Nevertheless, scaling these systems for widespread application remains challenged by key hurdles in water-film thickness regulation and heat-mass coupling stability. Addressing these challenges through mechanistic understanding of interfacial water dynamics is essential to unlocking the full potential of nonimmersed architectures, which offer a transformative pathway for geographically versatile distributed hydrogen production and global carbon neutrality.