The polyanionic hydrogel evaporator with Donnan effect achieves high-efficiency solar water-electricity cogeneration

Freshwater scarcity and energy shortage are critical bottlenecks for global sustainable development. Solar-driven interfacial evaporation enables simultaneous freshwater harvesting and power generation. Yet performance degradation caused by salt accumulation severely limits the long-term serviceability of conventional evaporators. Herein, a polyanionic hydrogel evaporator (PSAP hydrogel) with a prominent Donnan effect is developed for efficient and stable solar water-electricity cogeneration. The PSAP hydrogel is fabricated from polyvinyl alcohol (PVA), sodium alginate (SA), 2-acrylamido-2-methylpropane sulfonic acid (AMPS) incorporated with phytic acid-modified carbon nanotubes (PA@CNTs). Abundant hydrophilic groups (-OH, -COOH, -SO₃H, -PO₄, and -NH-) endow the hydrogel network with numerous fixed negative charges, as validated by a zeta potential of -21.9 mV. The integrated PA@CNTs confer the hydrogel with a visible-light absorption efficiency exceeding 92%. Benefiting from the regulated water molecular state within the network, the equivalent water evaporation enthalpy is reduced to 1428.21 kJ kg^-1, contributing to a high evaporation rate of 2.09 kg m^-2 h^-1 under one-sun irradiation. Enabled by the Donnan effect, the negatively charged network selectively traps cations (e.g., Na⁺) and repels anions (e.g., Cl^-) during solar-driven water transport. Such asymmetric ion migration induces continuous charge separation and generates a stable Donnan potential during evaporation. The optimized device yields an open-circuit voltage of 202.5 mV and a maximum power density of 2.22 mW m^-2 under one-sun illumination. This work provides a feasible strategy for the design of multifunctional hydrogel evaporators toward high-efficiency solar desalination and concurrent clean power generation.