Speaker
Description
Ion-exchange membranes play a central role in micro- and nanofluidic technologies for desalination, osmotic energy conversion, and electrochemical separation. Their performance is commonly characterized by macroscopic quantities such as selectivity and open-circuit voltage (OCV), often assumed to be intrinsic material properties. However, experimental observations increasingly challenge this assumption, especially under flow conditions where concentration polarization develops. In particular, the strong and systematic dependence of the OCV on hydrodynamic conditions remains insufficiently understood. Clarifying the physical mechanisms underlying this behavior is essential for accurate membrane characterization and for the rational design membrane-based devices.
We investigate the origin of flow-dependent OCV variations using a milli-fluidic cell in which two electrolyte streams of different KCl concentrations are separated by a cation-exchange membrane. The cell geometry allows precise control of flow velocity, membrane dimensions, and concentration ratios over several orders of magnitude. Open-circuit potentials are measured under steady-state conditions, and segmented electrodes are used to probe spatial variations of electrochemical activity along the membrane. Post-mortem electrode analysis and direct current measurements provide evidence of internal current circulation despite the absence of net external current. To rationalize these observations, we develop a two-dimensional transport model based on coupled Nernst–Planck equations under electroneutrality, accounting for advection, diffusion, and electromigration in the electrolytes, as well as selective transport in the membrane. Donnan equilibrium is imposed at membrane interfaces, and electrode kinetics are incorporated through Nernst boundary conditions and finite interfacial resistance. The model self-consistently determines the spatial distribution of concentrations, local current density, and membrane potential under the global open-circuit constraint.
Experiments reveal that the OCV is not solely determined by the imposed concentration ratio and membrane material, but strongly depends on flow velocity, membrane length, and electrolyte concentration. At low velocities, the OCV can decrease by more than 30% compared to its high-velocity plateau. Electrode segmentation and surface analysis demonstrate the existence of steady, closed loops of ionic current circulating within the cell, driven by spatially varying concentration polarization along the flow direction. The model quantitatively reproduces the measured OCV curves across a wide range of conditions using a limited set of physically meaningful parameters. It shows that finite membrane selectivity allows counter-propagating ionic fluxes, leading to a reversal of the dominant charge carrier along the membrane and to local changes in effective selectivity, including sign inversion. As a result, selectivity emerges as a spatially varying property rather than a uniform membrane characteristic. These internal current loops can persist when a net external current is allowed and translate directly into reduced extractable power in closed-circuit operation.
This work demonstrates that open-circuit conditions in ion-exchange membrane systems can host substantial internal ionic circulation, fundamentally altering the interpretation of OCV and selectivity measurements. These findings have direct implications for membrane characterization, reverse electrodialysis performance, and electrode design, highlighting the benefits of electrode segmentation to suppress parasitic current loops. The results open new perspectives for controlling spatially resolved ionic transport in micro- and nanofluidic systems, and motivate future studies on loop dynamics and their exploitation in advanced electrochemical devices.