Abstract
The long-term maintenance of stable redox-flow-battery performance requires a strategy to manage dynamic volume imbalances between electrolyte reservoirs. Even with ion-selective membranes as separators, bulk liquid exchange can occur within the reactor because of pressure differences across its positive and negative chambers. Disproportionation chemistries, wherein both electrolyte reservoirs have identical composition in the discharged state, tolerate species crossover reversibly. These make possible the use of low-cost porous separators, but such materials are more hydraulically permeable. Here we propose an approach to mitigate reservoir imbalance, using the nonaqueous vanadium acetylacetonate disproportionation chemistry as a test platform. We demonstrate a benchtop-scale control scheme, which relies on a camera that monitors reservoir volumes linked to a microcontroller that regulates the differential flowrate between flow streams. The balancing apparatus introduces no extra wetted components and enables real-time measurements of intra-cycle imbalance due to electroosmosis. All software and firmware is made available for community use.
Highlights
Several methods have been proposed for managing both differences in volumes and differences between average oxidation states between electrolyte reservoirs in redox flow batteries (RFBs)
The driving forces for bulk-electrolyte volume imbalance include mechanical pressure, osmotic pressure, and species-varying electroosmotic drag, as well as state-of-charge-dependent liquid properties such as density and viscosity, whose changes during cycling influence the previously mentioned phenomena. Any of these driving forces can result in bulk volume changes, either within a single cycle or cumulatively across several charge/discharge cycles. The latter cumulative effect can be negated through an appropriate volume balancing scheme
All experiments were conducted in an argon-filled glovebox with
Summary
Several methods have been proposed for managing both differences in volumes and differences between average oxidation states between electrolyte reservoirs in redox flow batteries (RFBs). The driving forces for bulk-electrolyte volume imbalance include mechanical pressure, osmotic pressure, and species-varying electroosmotic drag, as well as state-of-charge-dependent liquid properties such as density and viscosity, whose changes during cycling influence the previously mentioned phenomena. Any of these driving forces can result in bulk volume changes, either within a single cycle or cumulatively across several charge/discharge cycles. The latter cumulative effect can be negated through an appropriate volume balancing scheme. Volume balancing eliminates a troubling (albeit reversible) source of capacity fade during cycling experiments, which helps to isolate factors that control other possible failure mechanisms
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