Energy storage for intermittent renewable energy has become increasingly more important. The all vanadium redox flow battery (VRFB) is one of the few options to store energy electrochemically. One of the remaining challenges is the low power density (< 0.1 W cm^-2) of the VRFB, caused by sluggish kinetics of the redox reactions. To alleviate this drawback, many studies tried to heterogeneously catalyse the redox reactions. However, up to now, there is no consensus in the literature which of the two half-cell reactions, the V²⁺/V³⁺- or the VO²⁺/VO₂⁺-reaction, features the higher rate constant [1]. In the literature, rate constants of the two half-cell reactions spread over four orders of magnitude [2]. We will show that this uncertainty in the literature is due to two factors:

• An apparent catalytic effect appears when porous electrode materials are investigated with linear or cyclic voltammetry. The porosity reduces the separation between anodic and cathodic peak suggesting enhanced kinetics when indeed only an interplay of enlarged electrochemical surface area and impeded diffusion within the porous electrode is present;
• The Faradaic current I_F is proportional to both wetted surface area A_wet and the rate constant k₀: I_F ∝ A_wet k₀. As both parameters vary with electrode composition, it is often difficult to distinguish catalytic from surface-area effects.

This complex situation is unravelled by using a newly developed procedure based on electrochemical impedance spectroscopy. Combining these results and x-ray photoelectron spectroscopy we showed that surface functional groups such as hydroxyl, carbonyl and carboxyl increase the wetted surface area A_wet, increase the rate constant k₀ for the V²⁺/V³⁺-redox reaction but decrease the rate constant k₀ for the VO²⁺/VO₂⁺-redox reaction.
In addition, we investigated homogeneous catalysis of the VO²⁺/VO₂⁺ redox reaction. We show that transferring the system from 1 M sulfuric acid to 1 M phosphoric acid increases the electron transfer constant k₀ up to 67 times [3]. This observation was confirmed by chronoamperometry, electrochemical impedance spectroscopy as well as qualitatively by cyclic voltammetry and symmetric flow cell tests. We discuss the complex reaction mechanism which is a combination of electron transfer and chemical reaction. In a laboratory scale flow cell that employed dilute vanadium electrolyte, we were able to show that the over-voltages can be significantly lower in phosphoric acid as compared to sulfuric acid.

[1] Fink, H.; Friedl, J.; Stimming, U. Composition of the Electrode Determines Which Half-Cell’s Rate Constant Is Higher in a Vanadium Flow Battery. J. Phys. Chem. C 2016, 120 (29), 15893–15901.

[2] Friedl, J.; Stimming, U. Determining Electron Transfer Kinetics at Porous Electrodes. Electrochim. Acta 2017, 227, 235–245.

[3] Holland-Cunz, M.; Friedl, J.; Stimming, U. Anion effects on the redox kinetics of positive electrolyte of the all-vanadium redox flow battery. Journal of Electroanalytical Chemistry 2017, 10.1016/j.jelechem.2017.10.061.