Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX
pubs.acs.org/JPCC
Low-Potential Pyridinium Anolyte for Aqueous Redox Flow Batteries Christo S. Sevov,†,§ Koen H. Hendriks,†,§ and Melanie S. Sanford*,†,§ †
Joint Center for Energy Storage Research, 9700 South Cass Avenue, Argonne, Illinois 60439, United States Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
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S Supporting Information *
ABSTRACT: Aqueous redox flow batteries (RFBs) can serve as inexpensive grid-scale energy storage devices. A key challenge for developing these systems is identifying storage materials that undergo reversible redox events at potentials near the voltaic limits of aqueous media. This work details the development of a benzoylpyridinium-based anolyte for this application. A combination of electrochemical and spectroscopic studies guided the selection of a supporting electrolyte to mitigate anolyte-catalyzed proton reduction at the low potentials. These insights were used to achieve stable one-electron cycling with KOH as the support in both a H cell and in a laboratory-scale flow cell. In the latter experiment, cycling versus iron ferrocyanide afforded an aqueous RFB with an open-circuit voltage exceeding 1 V.
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INTRODUCTION The integration of renewable energy sources into the electrical grid requires the support of large-scale energy storage devices.1,2 Such devices serve multiple roles, including alleviating the mismatch between the supply and demand for electrical power and also providing load leveling while intermittent renewable energies are harnessed. Redox flow batteries (RFBs) represent an attractive emerging technology for this application.3−6 RFBs consist of solutions of solvated redox-active molecules that are charged as they flow over inert electrodes. As such, these systems can be inexpensively scaled by simply adding more anodic solution (anolyte) or cathodic solution (catholyte) to external reservoirs. Key to the deployment of high-capacity RFBs is the design of the electroactive solutions. Ideally, these solutions should be composed of inexpensive solvents, supporting salts, and redoxactive compounds while still maintaining a high open-circuit voltage (OCV).7,8 In this regard, aqueous RFBs offer the advantage that they employ water as the solvent in conjunction with acids, bases, or inexpensive salts as the supporting electrolytes. In current commercial RFBs, aqueous electrolytes are paired with vanadium salts as the energy storage materials.9 However, the redox-active component of these systems, vanadium, remains expensive and in low abundance relative to what is necessary for global energy storage demands.10 Redox-active organic and organometallic molecules have attracted attention as inexpensive alternatives to aqueous vanadium salts. Despite extensive efforts in this area, the most common anolytes (viologen,11−15 quinones16−18) and catholytes [iron coordination complexes,11,12,17 2,2,6,6-tetramethylpiperidinyl-N-oxide (TEMPO)13−15,19,20] for aqueous RFBs afford relatively low OCVs (
