Ion-Exchanging Graphenic Nanochannels for Macroscopic Osmotic Energy Harvesting

The Gibbs free energy difference between seawater and river water can be tapped by selective ion transport across charged nanochannels, referred to as reverse electrodialysis (RED). However, existing single pore and micro/nanofluidic RED systems have shown poor prospects for scalability and practical implementation. Herein, we present a macroscopic RED system, utilizing a cation-selective membrane or an anion-selective membrane. The membranes comprise reduced graphene oxide (rGO) nanosheets decorated uniformly with TiO2 nanoparticles. The nanosheets are covalently functionalized with polystyrene (PS) and subsequently linked to sulfonate or quaternary amine functional groups to obtain cation and anion selectivity, respectively. The membranes show excellent ion transport properties along with high power densities demonstrated under artificial salinity gradients. The cation-exchange membrane (CEM) delivered a power density of 448.7 mW m-2 under a 500-fold concentration gradient, while the anion-exchange membrane (AEM) produced a substantial power output of 177.8 mW m-2 under a similar gradient. The efficiencies ranged from 10.6% to 42.3% for CEM and from 9.7% to 46.1% in the case of AEM. Testing under varying pH conditions revealed higher power output under acidic conditions and substantial power output across the entire pH range, rendering them practically viable for sustainable energy harvesting in acidic and alkaline wastewaters.