Probing the Influence of Anions on Charge Transfer in Redox Couples for Flow Battery Applications
Authors: Agarwal Harsh , Florian Jacob , Goldsmith Bryan R. , Singh Nirala 
Published: July 2021

Integrating intermittent renewables like solar and wind to meet rising energy demand has necessitated the development of low-cost energy storage technologies like flow batteries for ensuring grid reliability. Aqueous redox flow batteries (RFBs) are a promising technology for storing energy on a large scale and releasing it based on demand. RFBs store energy in different oxidation states of species dissolved in electrolytes, which undergo charge transfer by accepting or donating electrons on the electrodes’ surface to store and release electricity. Several aqueous RFBs based on 3d transition metal ion redox couples have been demonstrated. Most of these RFBs employ acidic electrolytes and use porous carbon felts as electrodes due to their high surface area.1 The anions in these electrolytes affect the structure of the reactive species in solution by altering the metal ion coordination sphere. Additionally, these anions influence the charge transfer process either by getting adsorbed on the electrode surface and changing the number of available sites for the reaction, or changing the energy of the adsorbed intermediate. However, there is a lack of systematic study that identifies these structures and simultaneously looks at the adsorbed intermediates to understand the reaction mechanism in the presence of anions, hindering the design of novel electrolytes and electrocatalysts to control the charge transfer.

In this work, we study the influence of anions on the V2+/V3+ redox couple by identifying the structures of V2+ and V3+ complexes and conducting kinetic measurements on a controlled glassy carbon surface in various acidic electrolytes (HClO4, H2SO4, HCl, HBr, and HI) to get mechanistic insights of V2+/V3+ charge transfer. We choose V2+/V3+ because of its enormous potential as negative electrode in aqueous vanadium-based RFBs and the similarity in the observed kinetic enhancement in the presence of chloride anions, as also seen for Cr2+/Cr3+, Fe2+/Fe3+, and Cu+/Cu2+ redox couples,2 suggesting the possible similarity in the reaction mechanism. Using a combination of extended X-ray absorption fine structure, UV-Vis spectroscopy, and density functional theory calculations, we show that V2+ exists as [V(H2O)6]2+ in all electrolytes, while V3+ exists as a mixture of [V(H2O)6]3+ and [V(H2O)5SO4]+ in H2SO4, and [V(H2O)6]3+, [V(H2O)5X]2+, and [V(H2O)4X2]+ (X = Cl, Br, and I) in HCl, HBr, and HI. We evaluate the exchange current densities (io
) and apparent activation energies (Ea
) in these electrolytes using a rotating disk electrode setup. We show that the io
follows the order H2SO4 < HCl < HClO4 < HBr < HI, while the Ea
follows the order of H2SO4 > HClO4 > HCl > HBr > HI. This difference in orders of io
and Ea
for HCl and HClO4 occurs because io
has a contribution from both apparent frequency factor and Ea
, highlighting the need to consider both apparent frequency factor and Ea
independently to infer mechanistic insights.

We show that anions influence the V2+/V3+ kinetics by changing the energetics of the adsorbed intermediate. This is identified by the decreasing Ea
with increasing adsorption energy of the *[anion-V3+] intermediate. Because the adsorption energy of the intermediate follows the same order as anion polarizability,3 it also explains the use of anion polarizability to explain the kinetic behavior of these redox couples in previous studies. The apparent frequency factor follows the order opposite to the coverage of anions on glassy carbon, indicating the coverage is not responsible for the observed trends in V2+/V3+ activity in these acidic electrolytes. We compare the trends in adsorption energy of similarly forming intermediates during charge transfer for these other kinetically similar behaving redox couples (Cr2+/Cr3+, Fe2+/Fe3+, and Cu+/Cu2+) to rationalize trends in their inner-sphere charge transfer behavior. Further, we highlight how our findings show that electrocatalyst and electrolyte engineering to tune the intermediate’s energy is a viable strategy to control the charge transfer by comparing V2+/V3+ kinetic measurements on various metal electrocatalysts.

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