(398i) Bridging Mechanism of V2+/V3+ Reaction on Glassy Carbon in Acidic Electrolytes for Vanadium Redox Flow Batteries

With the world’s energy demand expected to increase immensely in coming decades, along with the adverse effects of CO₂ emissions on environment by burning of fossil fuels, there is a need to shift to renewable energy sources like solar and wind. These technologies are expected to generate ~50% of total world’s electricity by 2050, almost double to the current levels. However, despite the enormous potential of renewable resources in solving the world’s electricity demands, they suffer from intermittency necessitating storing the excess electricity and releasing based on demand. Although pumped hydroelectric currently provides the majority of electricity storage, its geographical limitations prevent its further scaling, motivating searches for alternative electricity storage technologies.

Redox Flow Batteries (RFBs) have shown tremendous capability to store and release renewable electricity at a multimegawatt-hours scale with long electrode lifetimes. RFBs have a unique ability to decouple energy and power which provides them high scalability and flexibility for grid-energy storage applications. Among aqueous RFBs, all-vanadium RFBs (VRFBs: VO₂⁺/VO²⁺//V²⁺/V³⁺) are among the most developed RFBs and the closest to commercial implementation. However, VRFBs suffer from a high kinetic overvoltage of the slower V²⁺/V³⁺ redox couple (V²⁺ ⇄ V³⁺ + e^−, V vs SHE) that lowers the overall battery efficiency.¹ Despite substantial work on VRFBs, there is a lack of fundamental understanding of V²⁺/V³⁺ redox couple chemistry, particularly the role of electrolyte and the associated charge transfer. VRFBs with carbon electrodes employing hydrochloric acid (HCl) or mixed acid electrolytes (HCl/H₂SO₄)¹ have higher current densities compared with the traditionally used sulfuric acid (H₂SO₄) electrolyte, but it is unclear if the enhancements are due to kinetics, mass transport, or conductivity.

Recently, we have reported V²⁺/V³⁺ kinetic measurements on glassy carbon in HCl, H₂SO₄, and mixed HCl/H₂SO₄ electrolytes for VRFBs. We extract exchange current densities (i_o) via steady-state current measurements (i_o,Tafel) and impedance spectroscopy (i_o,Rct) and apparent activation energies (E_a). Our results from both independent techniques to measure i_o show that the V²⁺/V³⁺ reaction kinetics in HCl are ~2.5 times faster than in H₂SO₄ or HCl/H₂SO₄. By comparing values of i_o,Rct to i_o,Tafel, we also show that V²⁺/V³⁺ reaction is an overall two electron transfer reaction in HCl (with one electron in rate determining step, RDS), as opposed to an overall one electron transfer reaction in H₂SO₄ and HCl/H₂SO₄. We propose that the enhanced V²⁺/V³⁺ redox kinetics in HCl are caused by the charge transfer proceeding through more polarizable chloride (*Cl) bridge, instead of through surface-bound hydroxyl (*OH) groups in H₂SO₄ and HCl/H₂SO₄.² We further hypothesize that the anion’s bridging strength, which is a measure of deformability in the presence of electric field,³ will govern the V²⁺/V³⁺ reaction kinetics, with stronger bridging strength corresponding to higher activity.

In this talk, we will discuss our V²⁺/V³⁺ kinetic measurements on glassy carbon in hydrobromic (HBr) and hydriodic (HI) acid to test the hypothesis of bridging strength as the sole criteria governing V²⁺/V³⁺ reaction kinetics. The bridging strength of anions follows the order, I > Br > Cl > OH,³ so based on our hypothesis we expect the V²⁺/V³⁺ reaction rates to follow the same trend. The V²⁺/V³⁺ exchange current densities in HBr are ~1.5 times higher than in HCl, with 5 to 15 kJ mol^-1 lower E_a (Figure 1). The i_o increases and the E_adecreases with increase in State of Charge (SoC) in HBr, similar to the behavior observed in HCl and H₂SO₄. We determine that the reaction in HBr is an overall two electron transfer reaction, with one electron involved in RDS, similar to HCl. Further, the V²⁺/V³⁺ reaction is HI has kinetics faster than HBr, with a maximum rate at ~50% SoC. The i_o,Rct in HI indicates the reaction mechanism changes to an overall one electron transfer. However, we notice that the rates in HI are affected immensely by the presence of small amount of sulfate anions. This points to the bridging strength not being a sole descriptor for the reaction kinetics. We attribute this observed trend in rates of V²⁺/ V³⁺ reaction to be dependent on the stability of the vanadium intermediate formed on the surface of glassy carbon, computed using density functional theory.

REFERENCES:

1. Choi, C. et al. Renew. Sustain. Energy Rev. 69, 263–274 (2017).
2. Agarwal, H. et al. ACS Energy Lett. 4, 2368–2377 (2019).
3. Anbar, M. et al. J. Phys. Chem. 69, 973–977 (1965).