Unraveling the viscosity impact on volumetric transfer in redox flow batteries

Flow batteries are being increasingly deployed in grid-scale energy storage applications. However, long-term operation of flow batteries still suffers from a different extent of capacity decay. While the effects of ion diffusion and side reactions on capacity degradation have been identified and further minimized by improvement in materials, the mechanism of volumetric transfer and its influence in capacity still receive insufficient attentions that impedes further capacity optimizations for flow batteries. In order to gain an in-depth understanding of volumetric transfer mechanism in flow batteries, six different types of flow batteries are adopted in this study and further classified in accordance with electrolyte viscosities for investigations. Experimental results show that a net volumetric transfer in a conventional flow battery highly depends on viscosity values of the two half-cell electrolytes and is virtually towards the half-cell possessing a smaller electrolyte viscosity. For flow batteries with a mixed electrolyte in both half-cells, moreover, cycling tests further demonstrate a zero net transfer under similar viscosity measurements of both half-cell electrolytes. Unraveling the viscosity impact on volumetric transfer is greatly beneficial to facilitate deeper understandings of transport phenomena in flow batteries, which can contribute to realize long-term flow battery operation with a superior capacity retention.