Thermodynamic analysis and material design to enhance chemo-mechanical coupling in hydrogels for energy harvesting from salinity gradients

Coupling between solution salinity and the mechanics of charged hydrogels presents an opportunity to harvest osmotic energy in a clean and sustainable way. By applying mechanical pressure to retard the swelling or deswelling of hydrogels in saline solutions, the free energy of mixing is converted into mechanical work. This study developed a theoretical framework and experimentally investigated the potential of hydrogels for energy production from salinity gradients. Mathematical modeling revealed the effect of parameters including the charge and elastic modulus of hydrogels, applied pressure, and the solution salinity on energy conversion using different thermodynamic cycles. With proper material design and process control, the thermodynamic efficiency of an ideal process was predicted to exceed 5% with 10 mM and 600 mM NaCl solutions. Experiments with poly (styrene sulfonate) hydrogels verified the theoretically predicted trends and demonstrated more than 10% thermodynamic efficiency for moderate-salinity sources, due to the unique swelling-strengthened mechanical properties of the gels. The study suggests the potential of polyelectrolyte hydrogels in the extraction of energy from low- to moderate-salinity sources and provides a framework for their design.