Polyelectrolyte Gradient Hydrogels for Efficient Solar Evaporation

Freshwater scarcity is an escalating global challenge, and solar evaporation has emerged as a promising and sustainable desalination strategy. Although Janus‐structured solar evaporators offer improved directional water transport compared to bulk‐structured counterparts, their fabrication is complex, and the top polyelectrolyte layer is often relatively thick. Sustaining a high and homogeneous osmotic pressure within this layer remains challenging, thereby impeding continuous water delivery to the evaporative surface and constraining overall efficiency. In this study, a facile and controllable method to fabricate polyelectrolyte gradient hydrogels via an electric‐field‐driven grafting method is presented. The resulting surface‐localized polyelectrolyte gradient generates an exponentially increasing osmotic pressure, significantly enhancing directional water transport. Quaternized poly(4‐vinylpyridine) is employed as the functional polyelectrolyte owing to its strong hydrogen‐bond disrupting capability, which effectively lowers the energy required for evaporation. The optimized gradient evaporator, fabricated within just 10 min, exhibited an impressive osmotic pressure difference of 125 kPa and evaporation rate of 4.5 kg m−2 h−1. Field experiments demonstrated excellent stability under 7‐day operation, achieving a high salt rejection ratio of 99.99% and a freshwater production rate of 14 kg m−2 day−1.