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 incorrect mechanisms. This leads to a costly empirical trial-and-error optimization process, which typically yields only incremental improvements. Here, we provide the first self-consistent theoretical model, verified by non-approximated numerical simulations, that establishes a detailed framework for osmotic energy conversion in realistic systems. Our model delineates the effects of each parameter in the system, including 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 , and the voltage at zero current, V _I=0 . The I = 0 insights are carried over to the results at the energy harvesting limit of zero voltage (V = 0), allowing for the first time 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 pressing global challenges.