A Proposed Mechanism for the Cerium Electron Transfer for Use in RFB Applications

Redox flow batteries (RFBs) are a promising technology for large scale energy storage, including grid-scale storage of renewable energy, due to their longer lifecycles and easier scalability than other, more developed battery technologies, e.g., lithium-ion. 1 Despite their promising nature, RFBs are currently too expensive for market deployment. For instance, the U.S. Department of Energy has reported a target capital cost for new energy storage technologies of 150 $/kWh, 2 yet recent reports of the all-vanadium RFB indicate the current capital cost is at least 555 $/kWh. 1 The large expense of RFBs is in part driven by inefficiencies in mass transport, ohmic, and kinetics, resulting in large losses in voltage at high current densities. One way to improve the efficiency of RFBs is to explore new chemistries that increase the voltage window of the battery, effectively reducing the significance of any incurred voltage losses. The cerium redox reaction between Ce 3+ and Ce 4+ is being considered for use at the positive electrode in an aqueous RFB, due to its high positive redox potential that ranges between 1.28 V vs SHE in HCl and 1.74 V vs SHE in HClO 4 . 3 Despite being thermodynamically promising for RFB applications, the kinetics of the Ce 3+ /Ce 4+ electron transfer have been reported to be a limiting performance factor in studies of cerium RFB systems. 4 The cerium charge transfer mechanism, which is currently not known, must be elucidated to improve the kinetics. Identifying whether the Ce 3+ /Ce 4+ electron transfer occurs via an inner- or outer-sphere mechanism would allow for advances in the kinetics because it would indicate whether the electrode (inner-sphere) or the electrolyte (outer-sphere) controls the rate of the electron transfer. Previous Ce 3+ /Ce 4+ kinetic studies 4 have demonstrated that the kinetic behavior varies significantly depending on the conditions used, e.g., electrode material and geometry, acid concentration, and total cerium, Ce 3+ and Ce 4+ concentration. The wide range of kinetic performance parameters reported in the literature makes it difficult to discern whether the electrode or electrolyte is controlling the rate of the reaction. In this study, to first identify the role of the electrode we analyze the Ce 3+ /Ce 4+ electron transfer kinetics in sulfuric acid on three different electrode materials, platinum (Pt), glassy carbon (GC), and boron-doped diamond (BDD). We utilize a rotating disk electrode (RDE) configurati