Abstract: 

Electrochemical interfaces govern the performance of technologies ranging from energy conversion devices to chemical sensors, yet fundamental knowledge gaps persist in our understanding of these critical boundaries. In ion-exchange membranes (IEMs), the relationship between non-equilibrium ion transport and thermodynamically quasi-equilibrated interfaces remains misunderstood. We demonstrate that IEMs, which are inherently non-equilibrium systems at nonzero temperature, generate additional free energy in the form of intra-membrane liquid-junction potentials that can amplify cell potentials beyond traditional Nernstian predictions. Through the development of a steady-state diffusion model based on discretized Fick's laws and the Nernst-Planck treatment of electrodiffusion, transient open-circuit potential measurements are used to extract intra-membrane ion diffusion coefficients. This analysis reveals that steady-state interfacial Donnan potentials are often far smaller than the equilibrated Donnan potentials predicted by Donnan Theory due to concentration polarization in boundary layers and coupled ion crossover. Additionally, we demonstrate that net power generation from devices utilizing IEMs can be achieved when electron flux in the external circuit exceeds ionic leakage flux across the membrane, allowing for the conversion of chemical potential to electricity.