A Proposed Mechanism for the Cerium Electron Transfer for Use in RFB Applications
Authors: Buchanan Cailin , Herrera Dylan , Goldsmith Bryan R. , Singh Nirala 
Published: July 2021

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 Ce3+ and Ce4+ 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 HClO4.3 Despite being thermodynamically promising for RFB applications, the kinetics of the Ce3+/Ce4+ 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 Ce3+/Ce4+ 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 Ce3+/Ce4+ kinetic studies4 have demonstrated that the kinetic behavior varies significantly depending on the conditions used, e.g., electrode material and geometry, acid concentration, and total cerium, Ce3+ and Ce4+ 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 Ce3+/Ce4+ 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) configuration for each of these electrode materials to control for mass transfer effects, and we study the effect of Ce3+ and Ce4+ concentration on kinetic performance, quantified by exchange current density. The purpose of these experiments is two-fold; first, to determine whether there are significant differences in kinetic performance as a function of electrode material, which would suggest an inner-sphere electron transfer, and second, to identify a rate law for the Ce3+/Ce4+ electron transfer. This rate law can be derived using mechanistic theories such as Marcus Theory and fitted to the experimental data to identify key performance parameters such as reorganization energy (dependent on electrolyte). These efforts will help to identify the charge transfer mechanism, which as noted earlier, is critical to furthering the use of the cerium redox couple in RFB applications. We find that the exchange current densities as a function of Ce4+ concentration for each RDE material are within a factor of five of each other, which could suggest the electron transfer occurs via an outer-sphere mechanism. However, our Ce K-edge EXAFS analysis of the Ce ions in sulfuric acid, as well as our previous Ce L3-edge EXAFS and computational study5 on the Ce ion structures, demonstrates that an inner-sphere structural change occurs during the electron transfer, as Ce3+ prefers the [Ce(H2O)9]3+ structure, while Ce4+ is complexed by at least one anion. This indicates that the electron transfer cannot occur in a simple single outer-sphere step. Thus, to reconcile these seemingly contradictory kinetic and structural data, we propose a mechanism which is consistent with our kinetic data in which a quasi-equilibrated ligand exchange reaction occurs in addition to the rate-determining electron transfer step. Identification of a likely Ce3+/Ce4+ charge transfer mechanism will help to advance the eventual goal of using the cerium redox couple in RFB systems by helping to demonstrate which aspects are critical to performance, e.g., electrode or electrolyte effects.

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