Unified Framework for Osmotic Energy Conversion

Osmotic energy systems employ charged nanoporous membranes and nanochannels to convert salinity gradients into electrical power, offering a promising route to harvest one of Earth’s most abundant yet underexploited renewable resources—the oceans—and providing reliable, carbon-free electricity. However, to date, no known self-consistent theoretical model can accurately predict the electrical response of realistic charge-selective systems. Thus, empirical and phenomenological models are used to increase the performance while attributing the increase to various mechanisms, which are mostly incorrect and incomplete. This leads to a costly empirical trial-and-error optimization process, which typically yields only incremental improvements. Here, we provide a self-consistent theoretical model for the current-voltage ( I − V ) response, verified by nonapproximated numerical simulations, that establishes a detailed framework for osmotic energy conversion in realistic systems comprising a charge-selective membrane and the often-neglected larger bulk reservoirs. Our model delineates the effects of the surface charges, the bulk concentration ratio, and the often-neglected effects of the bulk reservoirs. The model provides analytical expressions for all the major transport characteristics at zero current ( I = 0 ), including the Ohmic resistance, R Ohmic , the voltage at zero current, V I = 0 , and the transport number, τ , which is a metric for the selectivity of the system. The I = 0 insights are carried over to the results at the energy harvesting limit of zero voltage ( V = 0 ), allowing a straightforward analysis. Two of our key results are unexpected. First, the concentration at which the harvestable electrical power is maximal is not the concentration at which the harvestable electrical current is maximal. Second, overlooking the effects of the bulk reservoirs leads to overestimations of the harvestable power by several orders of magnitude. Our proposed framework lays the theoretical foundation for a new generation of sustainable nanofluidic energy systems relevant to addressing global energy and water challenges.