Highly Energy-Efficient Vanadium Redox Flow Batteries with Thermo-Chemically Activated Graphite Felt Electrodes

Abstract

The Vanadium Redox Flow Battery (VRFB) is a promising energy storage technology for large scale stationary applications that require a long cycle life of over 10 years. The specific feature of VRFB is the slower kinetics of redox reactions (V2+/V3+) on the negative electrode (NE) as compared to the kinetics of the redox reaction (VO2 + /VO2+) on the positive electrode (PE) and poor kinetic reversibility.. The commonly used electrodes for VRFBs are graphite felt electrodes (GFE). One of the main challenges facing these GFEs is their activation that enhances a rate of redox reactions due to the increase of GFE wettability and surface area. Another challenge is capability to maintain GFE activation over cycling.In this work, we report on thermal and thermo-chemical activation of GFEs which leads to the formation of functional groups (carboxyl C=O, phenolic C-O, and adsorbed O or H2O for the enhancement of the electrode wettability and active surface area. The physicochemical characterization of the activated GFEs was performed using X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The electrochemical study was conducted by cyclic voltammetry (CV) measurements in 0.2M VOSO4/2M H2SO4 at 1 mV/s and cycling tests (50 cycles, three tests for every GFE) in a commercial 9cm2 VRFB ( Standard Energy Corp.) using a charge/discharge current density of 80 mA/cm2 at 20oC and a commercial vanadium electrolyte 1.6M [V]/2M H2SO4. The increase of temperature of GFE heat treatment of GFEs from 400 to 500oC resulted in the growth of the energy efficiency (EE) of VRFBs with such electrodes from 81.3 to 87.2%, respectively, due to the increase of adsorbed oxygen or water from 1.6 to 8.72% and C-O (C-OH) groups from 0.49 to 2.09%, respectively and wettability (contact angle (CA) decreases from 78.7 to 65.1o). Analysis of CVs of the redox reaction VO2 + /VO2+ shows the decrease of the peak of potential separation ΔEp (Epa-Epc) from 0.357 to 0.345 V and the peak of current ratio (Jpa/Jpc) from 1.406 to 1.26, indicating lower polarization due to the increase of the electrode surface area, acceleration and reversibility of these redox reaction, respectively (Table 1). The proposed additional treatment of the heat treated GFE at 400oC (GFE400) in strong oxidants H2O2, H2SO4 and HNO3 for the decomposition of C=O and the enhancement of the content C-O groups and GFE wettability didnt result in the performance improvement (VRFB with GFE400+10%H2O2 - energy efficiency of 78%).The thermal treatment of GFE at 500oC demonstrated improvement of VRFB cell performance, namely growth of energy efficiency to 87.2% and VE to 90.1%, % due to the increase of wettability and active surface area of GFEs. Figure 1