Layered Janus Gel for Efficient Portable Solar H ₂ Production From Atmospheric Moisture

Photothermal catalytic atmospheric water splitting enables off‐grid in situ H 2 production but remains constrained by low energy conversion efficiency, sacrificial agent dependence, and sorbent degradation. Herein, we report a Janus‐structured gel with asymmetric wettability between its top and bottom surfaces, consisting of a LiCl‐modified nanofibrillated cellulose hygroscopic bottom layer, a SrTiO 3 :Al photocatalytic top layer, and a strategically engineered polyvinylidene fluoride interlayer, designed for clean and efficient in situ H 2 production. The uniquely designed interlayer synergistically facilitates efficient water desorption from the hygroscopic matrix, creates a liquid‐exclusion interface minimizing H 2 diffusion resistance, and serves as a protective barrier to inhibit hygroscopic component oxidation. Benefiting from this design, this Janus layered system achieves sustained H 2 production from air across diverse humidity levels. At 90% relative humidity, it delivers a hygroscopic capacity of 4.21 g H2O g sorbent −1 , a H 2 production rate of 78.50 mmol h −1 m −2 , and a solar‐to‐hydrogen efficiency of 0.35%. These findings demonstrate the promise of the Janus‐structured gel for portable, off‐grid, and green H 2 supply systems.