Water engineering via surfactant coacervates enables efficient and robust solar hydrogen evolution.

Water is essential in photosynthesis, acting both as a reactant and a medium regulating charge transport and reaction kinetics. While catalyst design has received significant attention, the impact of water structure on reactivity remains almost unexplored. Here, we report that water engineering via surfactant coacervates can significantly improve the performance of artificial photosynthesis for hydrogen production. By creating dynamic aqueous coacervate phases within a disrupted hydrogen-bond network, we create environments that yield more active, disordered water molecules. This tuned water state yields a stable hydrogen evolution rate of 918 mmol·g-1·h-1 and an apparent quantum efficiency of 69% at 420 nm in an aqueous methanol solution, demonstrating competitive catalytic performance. Spectroscopic and calorimetric characterizations, as well as computational simulations, indicate that surfactant coacervates increase the amount of intermediate water while weakening hydrogen bonding, thereby reducing the apparent activation energy, promoting charge transfer, and facilitating efficient catalysis. This study presents an approach to regulate water structure to enhance catalytic reactivity for solar hydrogen production, with broad potential for applications in water-involved catalytic systems.