Rational engineering of multifunctional hydrogel-based photocatalysts for solar energy conversion and environmental remediation.

Hydrogel-based photocatalytic systems have emerged as a transformative platform at the interface of soft matter chemistry, catalysis, and sustainable energy conversion. Unlike conventional particulate photocatalysts, embedding semiconductor materials within three-dimensional, water-rich polymer networks creates a highly hydrated and tunable microenvironment that regulates mass transport, light-matter interactions, and interfacial charge dynamics. This review critically examines the rational design principles of multifunctional photocatalytic hydrogels, emphasizing how gelation chemistry, network topology, and interfacial engineering synergistically govern catalytic performance. Recent advances in defect engineering, heterojunction construction, functionalization and hybridization with conductive and plasmonic components are discussed as key strategies to tailor band structures and suppress charge recombination within confined hydrogel architectures. The hydrogel matrix plays a key role in coupling adsorption with photocatalysis, stabilizing reactive intermediates, and enabling spatially confined redox pathways, thereby enhancing efficiencies in photocatalytic hydrogen evolution, hydrogen peroxide production, ammonia synthesis, carbon dioxide reduction, organic pollutant degradation, biomass conversion, and advanced oxidation processes, including (self-)Fenton and peroxymonosulfate and persulfate activation for environmental remediation. Beyond material design, this review addresses challenges related to photon transport, interfacial charge migration, reactor configuration, and scalability, linking microscale engineering with macroscale performance. A comparative assessment against conventional photocatalysts highlights their strengths, limitations, and translational potential. Finally, future directions are outlined through a structured strengths, weaknesses, opportunities, and threats (SWOT) analysis, guiding the development of next-generation hydrogel photocatalytic systems for sustainable solar-to-chemical energy conversion and environmental remediation.