Extracting Salinity Gradient Energy via Antifouling Poly(acrylic acid-co-acrylamide) Hydrogels in Natural Water

Salinity gradient energy (SGE) is an increasingly important form of renewable energy occurring in nature when river water streams flow into the sea. Charged polymeric gels have been recently proposed to convert SGE into mechanical energy by utilizing their volumetric response to solution salinity difference. However, most of these materials have drawbacks such as mechanical weakness, manufacturing challenges, and poor durability caused by various kinds of fouling, hampering the new promise of SGE harvest. This study develops a facile, yet versatile radiation cross-linking method to fabricate robust and antifouling poly(acrylic acid-co-acrylamide) hydrogels that can overcome the above-mentioned shortcomings. These cost-effective hydrogels exhibit the superior capacity for the external load due to the hydrogen bonds between the carboxyl and amide groups, and desirable antifouling effect, i.e., high resistance to multivalent cations, bovine serum albumin, and inorganic particles. The hydrogel-based osmotic engine obtains a power density of 1.72 mW/g and energy efficiency (EE) of 2.84%, exceeding the values achieved by existing hydrogels under model 3.5% NaCl–0.035% NaCl cycling solution. Moreover, in a subsequent test to extract SGE in the natural matrix of seawater and river water mixing, we showed for the first time the copolymer hydrogels, unlike common single-charged hydrogels that fail to swell by severe absorption of Ca2+ and Mg2+ ions, enable the multiply cycling and achieve a power density of 1.12 mW/g and EE of 1.17% under optimal ionic density. Therefore, the facileness and versatility of the present radiation method make P(AA-co-AAm) hydrogels suitable for large-scale manufacturing and potential incorporation into SGE harnessing.