In Situ Determination of the Potential Distribution within a Copper Foam Electrode in a Zinc-Air/Silver Hybrid Flow Battery

This work describes a novel methodology for measuring the potential distribution within the porous copper foam electrode of a zinc-air/silver hybrid (ZASH) flow battery by using local potential probes. The suitability of dynamic hydrogen electrodes (DHEs) and a quasi-reference electrode as probes is evaluated, with the latter chosen in view of stability. Liquid and solid-phase potentials are recorded at varying applied current densities over multiple charge-discharge cycles. Various zinc structures are found within specific overpotential ranges, with moss-like structures appearing between 7.8 mV and 13.2 mV and the desired boulder structures in the range of 22 mV to 100 mV. Regardless of the current density, the highest liquid-phase potentials are always measured in the outermost region of the porous foam near to the separator. In practice, this means that increasing the thickness of the copper foam over about 5 mm does not provide significant performance benefits. Conversely, solid-phase potentials across the copper foam remain nearly uniform, resulting in negligible effects on local overpotential. The presented technique provides unique insights into the behavior of porous electrodes in electrochemical energy conversion technologies, facilitating the determination of the optimal properties for maximum efficiency, such as electrode thickness. Porous electrodes enhance mass transfer to a greater electrode surface area and improve fluid flow distribution of the electrolyte. Their use in flow batteries has therefore become increasingly important in recent years. The application of quasi-reference electrodes to a copper foam in a zinc-air/silver hybrid flow battery reveals a non-uniform utilization across the foam thickness with the highest activity near the counter electrode. image