Untapped Potential: The Need and Pathways to High-Voltage Aqueous Redox Flow Batteries

Abstract

With the increasing desire to decarbonize, decentralize, and fortify the power grid, we must develop cost-effective electrical-energy storage (EES) technologies that can support a diverse set of large-scale applications1. Redox flow batteries (RFBs) are a potential energy storage solution that is increasingly competitive, on a capital cost basis, for EES applications that require longer discharge durations due to their unique decoupling of energy and power2. Both non-aqueous RFBs (NAqRFBs) and aqueous RFBs (AqRFBs) are possible, but AqRFBs are currently the dominant class of RFB chemistries due to their significantly lower solvent cost, inherently higher electrolyte conductivity, and relative safety (i.e., reduced flammability). The major advantage of NAqRFBs is they are not limited by the electrochemical stability window (ESW) of water, allowing for higher cell voltages. Most techno-economic studies, to date, have considered the maximum OCV of AqRFBs to be ≤1.5 V due to the common misconception that the ESW of water is limited to ~1.5 V. However, through careful consideration of electrolyte composition, cell design, and operating practices, AqRFBs with potentials >1.5 V can exist with minimal and/or manageable water splitting. Development of such systems would increase the attractiveness of this class of chemistries and is a promising pathway to low-cost RFB systems, especially since most NAqRFBs have not been able to stably achieve >3 V (generally, the voltage needed to overcome other techno-economic barriers of NAqRFBs to make them competitive).We have performed basic techno-economic analyses to demonstrate the impact of higher cell voltages (up to 2.5 V) for AqRFB capital cost , and also consider the secondary benefits that can arise from the flexibility provided by higher power and energy densities. A major benefit seen at cell voltages ≈ 2 V or higher is significantly lower projected capital costs. There are multiple challenges to realizing AqRFBs with such high potentials, and we will discuss some potential pathways to enabling realistic systems. This includes the mitigation and remediation of hydrogen and oxygen evolution, as well as stability issues with other RFB components. We will also point out the need for fundamental research on a variety of topics, and new experimental protocols, that can assist the community in developing AqRFBs with higher OCVs.We hope this perspective will inspire the field to reexamine the perceived upper limits of AqRFB chemistries and to explore options for achieving high-voltage systems. References Intergovernmental Panel on Climate Change. Global Warming of 1.5 C. https://www.ipcc.ch/sr15/ (2018).Darling, R. M., Gallagher, K. G., Kowalski, J. A., Seungbum, H. & Brushett, F. R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Energy Environ. Sci. 7, 3459–3477 (2014).