Engineering proton generation and transport within alginate-based hydrogel photocatalytic platform for efficient hydrogen production.

While contemporary research on photocatalytic water splitting predominantly focuses on catalyst designs, critical environmental impacts, specifically water status, and ionic transportation, remain largely unnoticed. Upon the establishment of four hydrogen bonds, the intramolecular covalent OH bonds of strongly hydrogen-bonded (SHB) waters are significantly weakened, as supported by Raman spectroscopy analysis. The weakened OH bonds, and thus lowered energy barriers for molecular dissociation, potentially make SHB waters serve as dynamic proton sources. Within the studied alginate-incorporated double-network hydrogels, the density of hydrogen bonds for evolving SHB waters is influenced by the available contacts between hydrogel segments and contained waters. However, structural engineering intended to densify the hydrogel and thus increase SHB waters unavoidably introduces steric hindrance to proton transport via hydronium ions. Unveiled as the rate-determining factor for hydrogen production within hydrogels, the efficient supply of protons to catalytically active sites should be improved alongside the enhanced abundance of proton-generating SHB waters. Therefore, a dilemma arises regarding the hydrogel densification. Surprisingly, the spread of local surface plasmon resonance-induced electric fields of dispersed plasmonic nanoparticles has been newly unveiled to evolve "proton-transportation highways" within hydrogels. As restricted proton transportation is successfully decoupled from efficient proton generation via hydrogel densification, a remarkable hydrogen evolution rate of 604 μmol g-1 h-1 has been achieved upon the sacrificial-agent-assisted splitting of artificial seawaters confined within hydrogel platforms with dispersed catalysts, superior to that via the splitting of non-confined seawaters.