Highly-stable gradient poly(anion-π) hydrogels with superior salt-tolerant property for giant osmotic energy harvesting

Harvesting ocean salinity energy has attracted large attentions based on different kinds of asymmetric ionic membranes. Gradient polyelectrolyte hydrogels demonstrated superior power generation capacity due to their abundant charged groups and rectification behaviors. However, they generally suffer from the electrostatic screening and dramatic swelling, leading to unsatisfactory energy conversion performance. Here, an anti-swelling, salt-tolerant gradient polyanion hydrogel is reported for high-efficiency osmotic energy conversion. The gradient polyanion hydrogel is prepared by ultraviolet (UV) induced copolymerization of anionic monomer and aromatic monomer. Benefiting from the mutual electrostatic interaction and π-π conjugation interaction, the gradient polyanion hydrogel presents the outstanding anti-electrostatic screening and anti-swelling performance. As a result, they can maintain excellent cation selectivity and ionic diode effect for providing an ultrahigh power density of 16.4 W/m2 when mixing artificial seawater and river water. This new salt-tolerant gradient polyelectrolyte hydrogel provides a promising way to recover energy from seawater and wastewater.