Layered Janus Gel for Efficient Portable Solar H₂ Production From Atmospheric Moisture

Photothermal catalytic atmospheric water splitting enables off-grid in situ H² 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 SrTiO₃:Al photocatalytic top layer, and a strategically engineered polyvinylidene fluoride interlayer, designed for clean and efficient in situ H₂ production. The uniquely designed interlayer synergistically facilitates efficient water desorption from the hygroscopic matrix, creates a liquid-exclusion interface minimizing H₂ diffusion resistance, and serves as a protective barrier to inhibit hygroscopic component oxidation. Benefiting from this design, this Janus layered system achieves sustained H₂ 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 H₂ 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 H₂ supply systems.