Decoupling Heat Loss and Mass Transfer Limitation in Solar Reactor for Fractional Water Production

Integrating photocatalysis with interfacial solar steam generation enables efficient solar‐driven water treatment for pure water production and large‐scale water use. Although 1D‐water‐transfer‐pathway structures from conventional solar desalination provide useful insights for thermal localization, applying them directly to photothermal‐photocatalytic systems limits performance due to poor mass transfer. Herein, an integrated solar‐thermal reactor with optimized design is prepared to overcome the trade‐off between heat loss and product inhibition. The structural design synergistically combines thermal insulation, short mass transfer distance, and efficient channels to achieve localized heating and improved mass transfer. With the structural optimization, steam and H 2 O 2 production rates of 2.10 kg m −2 h −1 and 11.95 mmol m −2 h −1 , respectively, are achieved under 1.0‐sun irradiation. Over 14 days of outdoor testing, the prototype achieves 13.34 L m −2 of steam and 600 L m −2 of oligotrophic water disinfection per day. This work demonstrates an optimized structure, offering guidance for balanced thermal and mass transfer in next‐generation photothermal‐photocatalytic reactors.