Power generation mechanism by ion transport in electrically conductive papers

• A coupled electrokinetic–electrochemical model with equivalent circuit model is developed for transpiration-driven power generators. • Ion diffusion near the wet–dry interface generates voltage even without redox reactions. • Maximum output power is achieved when the conductivity of dry region is higher than that of wet region, and their conductivity difference becomes largest. • Conductivity non-uniformity of the CNT composite paper must be suppressed below 5% to ensure power variations within 15%. This study presents a coupled numerical and experimental investigation into the performance of transpiration-driven electrokinetic power generators (TEPG) employing carbon nanotube (CNT)/pulp composite papers. A one-dimensional, time-dependent model was developed based on Nernst–Planck equation for ion transport and charge conservation equations to elucidate the underlying mechanism of electrical power generation. To specifically evaluate the contribution of Faradaic redox processes, electrode kinetics were incorporated via the Butler–Volmer equation, while an equivalent circuit model was implemented to account for the simultaneous electronic and ionic conduction pathways within the composite paper. Results showed that the electromotive force is generated due to ion diffusion near the wet-dry interface, even without Faradaic electrode reactions. Experiments conducted with deionized water and 1 M hydrochloric acid validated the theoretical framework, demonstrating that zinc oxidation at the anode significantly boosts the open-circuit voltage through a synergistic interaction between TEPG electrokinetics and redox reactions. The CNT content in the paper composite influenced the energy generation performance due to variations in swelling-induced resistivity. Optimal power density is achieved when the conductivity of the wet region remains lower than that of the dry region, establishing a favorable potential gradient. Furthermore, a stochastic analysis of material non-uniformity—originating from fiber flocculation and random in-plane orientation—highlights that the magnitude of local conductivity fluctuations significantly impacts output stability. To maintain power variations within 15%, the local conductivity non-uniformity must be suppressed below 5%. Significantly, these findings provide essential design rules for the optimization of scalable hydrovoltaic systems, emphasizing the need for precise structural control during material fabrication.