Structures and Free Energies of Cerium Ions in Acidic Electrolytes and Structures of Electrolyte-Electrode Interfaces to Inform Kinetic Performance in Redox Flow Battery Applications
Authors: Buchanan Cailin , Ko Eunbyeol , Goldsmith Bryan R. , Singh Nirala 
Published: December 2020

Despite the interest in renewable energy as a substitute for fossil fuels, renewables make up less than 20%1 of total electrical energy in the United States in part because their intermittency prevents them from being a dominant fraction without storage.2 The redox flow battery (RFB) is a promising energy storage technology because of its high-power density and long lifetime,2 which can be used to address intermittency by decoupling customer demand and renewable electricity generation. Currently, however, RFBs incur substantial energy losses during operation at high current densities and are therefore too costly for market deployment. A promising chemistry for the positive electrode of a RFB which would improve efficiency by increasing the total voltage is the cerium redox couple, which has a potential up to 0.74 V greater than the positive vanadium electrode.3,4 The Ce3+/Ce4+ electron transfer redox potential changes depending on the electrolyte,3 which we hypothesize is due to the anion complexation thermodynamics. It is not currently clear whether only one or both of the Ce ions are complexed by anions in solution. Knowledge of the structures and free energies of cerium ions in acidic electrolytes would enable control over the Ce3+/Ce4+ redox potential for RFB applications. Additionally, understanding whether the Ce3+/Ce4+ redox reaction involves a change in the inner sphere is important for discerning the charge transfer mechanism and kinetics.
In addition to a lack of information on the structure of Ce3+ and Ce4+ anions in acids, previous work in zinc-cerium5,6 and hydrogen-cerium7 batteries show that the cerium electrode kinetics are a limiting factor on battery performance. Our preliminary kinetic studies of the redox couple in sulfuric acid indicate that the kinetics are influenced more by the electrolyte than by the electrode material, which is consistent with an outer-sphere electron transfer mechanism. If the Ce3+/Ce4+ electron transfer does proceed via an apparent outer-sphere mechanism, the reorganization energy,8 which is dependent on the electrolyte, will control the rate of reaction. To determine whether the reaction follows an outer-sphere mechanism, we must isolate the behavior of the cerium ions at the electrode interface. In this study, we therefore explore the structure of the electrode-electrolyte interface as well as study the structure of cerium anion complexes.
To probe the electrode-electrolyte interface, we use in-situ Extended X-ray Absorption Fine Structure (EXAFS) of the platinum (Pt) L3-edge of a Pt electrode during Ce3+ oxidation in two of the most relevant electrolytes for battery applications, H2SO4 and CH3SO3H. At the high positive potentials applied, the Pt is oxidized, and therefore the relevant electrode is a Pt-oxide. To study the ionic structure in different aqueous acidic electrolytes (i.e., HCl, H2SO4, H3NSO3, CH3SO3H, HNO3, CF3SO3H, and HClO4) and extract free energies of Ce3+ and Ce4+ complexes, we use experimental UV-Vis spectroscopy and EXAFS, and Density Functional Theory (DFT) calculations. Based on the combination of spectroscopy, DFT calculations, and the direction of the redox potential shift from non-complexing media, we hypothesize that Ce4+ is complexed by anions in electrolyte, while Ce3+ is hydrated by water. We find from the UV-Vis spectra that the dominant Ce3+ structure is the same in all acids studied, and that the dominant Ce4+ complex changes with electrolyte. EXAFS of Ce3+ allows us to determine a coordination number of nine water molecules. Our DFT calculations enable us to extract bond lengths of the first coordination shell of the cerium ions. The DFT-predicted free energies of Ce4+ anion complexation agree with free energies we calculated from redox potentials reported in literature, supporting our hypothesis that the shift in redox potential can be attributed to the anion complexation of Ce4+. Additionally, this agreement shows that we can predict standard redox potentials for cerium in simple aqueous electrolytes. Ultimately, these characterization findings clarify the fundamental interactions of cerium ions with their electrolyte environment and a relevant electrode as well as the Ce3+/Ce4+ thermodynamics and will guide kinetics studies of the Ce3+/Ce4+ redox couple for use in redox flow battery applications.

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