(550g) Understanding V2+/V3+ Reaction on Metal Electrocatalysts for Vanadium Redox Flow Batteries

The world’s energy demand, which is expected to grow by 50% by 2050,¹ is currently largely met by burning fossil fuels that emit carbon dioxide (~35 billion metric tons of CO₂ released in 2020).¹ The large increase in CO₂ in the atmosphere has led to a rise in the Earth’s temperature by ~0.40 °C in the past two decades,² causing harmful climate change and a huge loss to biodiversity. This rising energy demand can be met sustainably by using energy from clean renewables like solar and wind, which is expected to supply ~56% of the global electricity demand by 2050.³ However, the intermittent nature of solar and wind necessitates the development of low-cost energy storage technologies such as redox flow batteries (RFBs) for ensuring grid reliability. RFBs store energy in flowing electrolytes rather than in an electrode and are highly promising to store renewable electricity at a large scale because of high power densities, long electrode lifetimes, and their ability to decouple energy and power, enabling high scalability.

All-vanadium redox flow batteries (VRFBs) are the most developed RFBs, but they suffer from high costs partly due to the slow reaction rate at the negative electrode, preventing them from reaching the Department of Energy’s capital cost target.⁴ VRFBs store energy in different oxidation states of vanadium (VO₂⁺/VO²⁺//V²⁺/V³⁺) dissolved in acidic electrolytes. V²⁺/V³⁺ reaction contributes ~80% of the total overvoltages in VRFBs, lowering their energy efficiency.⁵ Using active electrocatalysts to eliminate the kinetic overvoltage of V²⁺/V³⁺ reaction alone will increase the energy storage efficiency from 77 to ~86%, lowering the capital costs.⁶

Certain metal electrocatalysts like Bi,⁷ Cu,⁸ Sn,⁹ and Sb supported on carbon have shown improved V²⁺/V³⁺ kinetics,¹⁰ but the cause of enhancement is unknown, preventing rational electrocatalyst design. Further, it is uncertain whether the improved kinetics is due to the metal or a change in functional groups (or increase in surface area) on carbon as a result of metal deposition. Our prior work has shown that V²⁺/V³⁺ reaction proceeds through an adsorbed vanadium intermediate on glassy carbon, and by controlling the energetics of the intermediate, the V²⁺/V³⁺ charge transfer can be enhanced.^6,11 All these tested metal electrocatalysts adsorb hydrogen weakly, so we hypothesize that weakly adsorbed hydrogen facilitates vanadium intermediate formation by not competing for active sites. If this is true, the hydrogen adsorption (ΔG_H) and vanadium intermediate adsorption (ΔG_I) free energy would control the reaction rate, and there will be an ideal ΔG_H and ΔG_I resulting in the highest catalytic activity, based on the Sabatier principle of catalysis. The relation between the electrocatalyst’s ΔG_H, ΔG_I, and V²⁺/V³⁺ reaction has not been previously explored.

In this work, we explore the hypothesis that weakly adsorbed hydrogen and vanadium intermediate leads to higher V²⁺/V³⁺ activity by conducting kinetic measurements on metal electrocatalysts (unsupported on carbon) with varied ΔG_H and ΔG_I. We extract exchange current densities (i_o), apparent frequency factors, and apparent activation energies (E_a) from experimental steady state current and electrochemical impedance spectroscopy measurements conducted at various rotation rates, using a rotating disk electrode setup. The measurements at various rotation rates allows us to deconvolute the kinetic and mass transfer contributions, and hence obtain an accurate estimation of the kinetic parameters. We test metals with varying ΔG_I evaluated by density functional theory and that adsorb hydrogen strongly (W), weakly but experimentally untested (Au, Ag), and weakly but previously shown to improve V²⁺/V³⁺ performance (Bi, Cu) (Figure 1a). The electrochemical active surface area (ECSA) of these electrocatalysts are measured through either underpotential deposition (Cu, Pb), or N₂O titration method depending on the metal electrocatalyst. The ECSA normalized rates allows fair comparison between activities of various electrocatalysts, which we use to infer mechanistic insights of V²⁺/V³⁺ reaction on metal electrocatalysts.

We show that the metal electrocatalysts are ~100 times more active compared to glassy carbon for V²⁺/V³⁺ reaction (Figure 1b), and their activity trend is correlated with the adsorption energy of vanadium intermediate. The i_o and E_a on these metal electrocatalysts varies with State of Charge (SoC = [V²⁺]/([V²⁺] + [V³⁺])) similar to glassy carbon, confirming the presence of an adsorbed intermediate. The trend in the apparent frequency factors allow us to deduce that the adsorbed hydrogen on the metal electrocatalyst acts as a poison and prevents the formation of the active intermediate lowering the reaction kinetics. Further the high activity on metal electrocatalysts confirm that the improved performance of V²⁺/V³⁺ is primarily due to metal itself, and not due to the change in carbon functionalization or increase in surface area. These findings can be utilized to develop more efficient electrocatalysts like metal alloys with ideal ΔG_H and ΔG_I, thereby eliminating the V²⁺/V³⁺ kinetic overvoltage and allowing us to take a step forward in creating low cost VRFBs for a sustainable future. Additionally, the methodology used in this study to evaluate kinetic parameters and compare various metal electrocatalysts accurately can be extended to other redox couples employed in flow batteries like Cr²⁺/Cr³⁺, Fe²⁺/Fe³⁺, Cu²⁺/Cu⁺, etc.

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