Real-Time Reservoir Balancing and Leak-Free Nonaqueous Cell Design for Flow Batteries

Abstract

Benchtop-scale testing of redox flow batteries presents numerous operational challenges related to electrolyte imbalance, leakage, and chemical compatibility. Electrolyte imbalance creates a capacity-limiting reservoir and necessitates manual or automatic transfer of electrolyte between reservoirs to rebalance. Leakage of electrolyte in the cell, often at tubing connections or interfaces between the graphite plates, gaskets, and membrane, also reduces capacity and experimental repeatability. The testing of increasingly common nonaqueous electrolytes requires not only strict wetted material compatibility across a range of solvents but also greater resistance to leaks – the lower surface tensions of many organic solvents (vs. water) makes them more prone to leakage. The above issues are more easily dealt with in large-scale, permanent installations that do not have the benchtop-scale requirements of manual assembly/disassembly, modularity, and configurational simplicity. Additionally, benchtop flow batteries often use peristaltic pumps, a type of positive displacement pump which create large pressure and flowrate pulsations compared to the continuous output of centrifugal pumps commonly used in large scale systems. The use of individual peristaltic pumps for each side of the flow battery can create an electrolyte imbalance when an asymmetric pressure drop occurs across the cell, causing a Darcy flow of electrolyte through the membrane separator. Other aspects of the flow battery can exacerbate this imbalance, such as using porous separators which are more permissive to electrolyte crossover, or more viscous electrolytes which create higher pressure differentials in the hydraulic circuit at a given flowrate. A new pump control strategy for flow batteries is designed and implemented which controls the speed and flowrate of the pumps in real time in response to the reservoir levels, as shown in Figure 1. This control scheme maintains balanced electrolyte reservoirs by minimizing the average differential pressure across the membrane. Minimal pressure drops are achieved by effectively matching the pressures in each half-cell of the battery by adjusting each pump’s speed and flowrate independently, so that both pumps are operating at the same pressure on their respective fluid mechanical system curves. Additionally, a more leak-resistant cell gasket design shown in Figure 2 is developed, while maintaining chemical compatibility for nonaqueous systems. These two developments are employed in a long-term cycling experiment using a nonaqueous disproportionation electrolyte. Figure 1