Efficient Catalysts for VO2 +/VO2+ Redox Couples in Vanadium Redox Flow Batteries

Abstract

The Vanadium Redox Flow Battery (VRFB) is an efficient energy storage technology for large scale stationary applications. One of the main challenges of VRFB with conventional non-activated graphite felt electrodes (GFE) is slow kinetics of redox reactions V2+/V3+ on the negative electrode and VO2 + /VO2+ reaction on positive electrodes. The activation of GFE with catalytic coatings may enhance the rate of redox reactions due to the decrease of the charge transfer resistance and increase of wettability and specific surface area.In this work, we report on activation of positive GFEs with supported/unsupported metal and metal oxide coatings (metal loading 0.5-1mg/cm2). The electrochemical study was conducted by 1) cyclic voltammetry (CV) measurements at 1 mV/s, 20oC in 0.2M VOSO4/2M H2SO4, cycling tests of commercial VRFBs (9 cm2) with positive activated GFEs and negative thermally treated GFE at 500oC in air (GFE500) at i=80 mA/cm2, 20oC in a commercial vanadium electrolyte 1.6M [V]/2M H2SO4, and 2) electrochemcial impedance spectroscopy (EIS) (solution (Rs) and charge trasfer resistance(Rct)).Analysis of the peak current ratio (Jpa/Jpc= 1.43-1.55) of CVs measured on supported catalysts (samples 2,3, Table 1) of positive activated GFE demonstrates that they have the similar reversibility for VO2 + /VO2+ redox reactions compared to baseline elecrodes GFE500 (Jpa/Jpc= 1.36, sample 1). However, these catalysts have better electrochemical activity for redox reactions VO2 + /VO2+ than GFE500 as their peaks potential separation ΔEp of 0.15-0.17V are significantly less than that on GFE500 (ΔEp=0.345V). It is important to note, they have significantly lower charge transfer resistance of 0.46-0.57 Ohm.cm2 than that of baseline GFE500In-situ cycling tests of VRFBs with a positive activated GFE by 40% (50%Ir40%Ru10%Se)/5Sb-SnO2 (rGO) and negative GFE500 (sample 2, Table 1) demonstrate the higher energy efficiency (EE) of 86.4 than that of VRFBs with baseline electrodes GFE500 (EE-85.4%) (Table 1). Table 1. Electrochemical characterization of activated positive graphite felt electrodes in half cells and VRFB # Catalyst Jpa/Jpc ΔE (V) Rct (Ohm.cm2) EE*,% CE*,% VE*,% 1 GFE500 1.36 0.345 5 85.4 96.4 87.5 2 [40% (50%Ir40%Ru10%Se/5%Sb-SnO2)]/GFE500 1.43 0.172 0.46 86.4 95.9 90.2 3 [75% (Ta0.6Nb0.4TiO2)/C]/GFE500 1.55 0.15 0.63 86.2 95.6 89.9 *EE- Energy efficiency, CE- Coulombic efficiency, VE - Voltage efficiency Fig.1 Electrochemical performance of VRFB (energy,Coulombic, voltage efficiency, and charge/discharge capacity) with positive activated GFE500 (40% (70%Ir20%Ru10%Se)/C (rGO) and negative GFE500 at i=80mA/cm2, T=20oC , 25 ml/min in the commercial vanadium electrolyte (1.6M [V]/2M H2SO4) Figure 1