Thermodynamics analysis of charged hydrogels for osmotic power applications

Osmosis driven by the difference in salt concentration between saltwater and freshwater can be used to produce electricity. But the fouling and low efficiency have hindered this renewable energy technology from reaching its potential. To overcome these challenges, researchers have looked to charged hydrogels, which can transform a salinity gradient into energy through cycles of swelling and deswelling.

To gain a theoretical understanding of the materials’ thermodynamics, Zhang et al. studied two types of charged hydrogels under pressure: polyelectrolytes and polyampholytes, each exhibiting different swelling behaviors in response to salinity.

The researchers designed a model for studying the swelling/deswelling cycle of hydrogels under pressure. They calculated the total released free energy and the maximum energy efficiency in one cycle, demonstrating that energy efficiency is highly dependent on the charge and the elasticity and their interplay.

According to their calculations, polyelectrolytes outdid polyampholytes in every way. They found that the higher energy efficiency of the polyelectrolytes is typically achieved at lower salinity levels. At high salinity levels, the hydrogels behave as if they’re overloaded, since the number of charges on the polymers is not sufficient to match the ions in the saline solution.

Experiments performed on hydrogels of each type in a proof-of-concept device confirmed the theoretical findings. The thermodynamic efficiency of the researchers’ polyelectrolyte design exceeded 10%, far exceeding the 2% mark found in previous charged hydrogel literature.

“Our study suggests the potential of polyelectrolyte hydrogels for energy extraction from low- to moderate-salinity sources and provides a framework for their development,” author Sui Zhang said.

Source: “Thermodynamic analysis and material design to enhance chemo-mechanical coupling in hydrogels for energy harvesting from salinity gradients,” by Sui Zhang, Shaoting Lin, Xuanhe Zhao, and Rohit Karnik, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0013357 .