Boosted Energy Extraction from the CapMix Process by Grafting with Titratable Polymers

Abstract Salinity gradient energy offers a sustainable route to convert ionic chemical potential differences into usable power. Capacitive mixing enables this conversion without membranes, but suffers from limited ion regulation at electrode interfaces. Here we show that, by grafting electrode surfaces with titrating polymers, the performance can be substantially improved. Using Grand Canonical Monte Carlo simulations with exact image-charge Ewald summations, we demonstrate how the coupled effects of ion adsorption and charge regulation in response to an external potential can be harnessed. Grafted electrodes are shown to deliver substantially more energy relative to bare surfaces, driven by charge regulation effects that exploit the pH difference that typically exists between rivers and the ocean. While the effect is in principle maximized at high grafting densities and moderate chain lengths, the performance is fairly robust to variations in these parameters, within reasonable bounds. Complementary classical polymer Density Functional Theory calculations confirm these trends, validating the mechanistic framework. This work also establishes a practical approach to harvest electrical energy during wastewater neutralization, where acidic (or alkaline) effluents serve as complementary reservoirs, and offers a promising strategy to couple environmental remediation with renewable energy recovery.