Enhanced Photocatalytic Hydrogen Evolution in Hybrid Hydrogels Coated with an Electrospun PVDF Fibrous Layer with an Unconnected Porous Structure via Multiple Scattering and Water Retention

Efficient photocatalytic hydrogen evolution in hybrid hydrogels containing photocatalysts remains a challenge due to the rapid loss of water and poor light harvesting. To overcome this drawback, an electrospun poly(vinylidene fluoride) (PVDF) fibrous layer with an unconnected porous structure is coated on hybrid hydrogels to improve the photocatalytic hydrogen evolution. The hybrid hydrogels are prepared by the thermally initiated polymerization of the monomers poly(ethylene glycol) methyl ether methacrylate (OEGM₃₀₀) and di(ethylene glycol) methyl ether methacrylate (MEO₂MA) with g-C₃N₄/Pt nanosheets. After that, an electrospun PVDF fibrous layer with a porous structure is constructed on the hydrogel surface by first electrospinning a solution containing PVDF and poly(ethylene oxide) (PEO) and then removing PEO. To optimize photocatalytic performance, the weight fraction of PEO is increased from 0.2 to 0.5 and finally to 0.8. Simultaneously, the porous structure in the nanofibers switches from an unconnected to a fully connected state. Due to the existence of unconnected pores in the PVDF fibrous layer, the evaporated water molecules from hydrogels can be captured and returned to the liquid state. They act as tiny mirrors to reflect the light scattered from the hydrogel surface. Thus, light harvesting is enhanced. The hydrogen evolution rate prominently increases to 3491 μmol h^–1 g^–1, which is almost double that without a fibrous layer covering. In addition, evaporation also slows down due to cycles of evaporation and condensation of water molecules. The weight loss of hybrid hydrogels coated with an electrospun PVDF_0.5 fibrous layer with an unconnected porous structure is only 43.5% to that without any PVDF fibrous layer. It significantly prolongs the lifetime. For this reason, the present hybrid hydrogel system is well suited for photocatalytic hydrogen evolution in deserts and prairies, which are rich in solar energy but lack water.