Degradation of the All-Vanadium Redox Flow Battery in the Presence of Metal Impurities

Abstract

Vanadium redox flow batteries (VRFBs) are a promising technology to advance grid scale energy storage and renewable energy generation integration. However, the cost of the electrolyte is a major challenge for implementation of VRFBs [1, 2]. Electrolyte quality and purity have a significant impact on the cell performance and cost. The presence of impurities even at low concentrations in the vanadium electrolyte solution can cause the instability of the electrolyte and influence cell performance, energy density, operating temperature range, electrochemical kinetics, and production/operation costs [3-6]. However, there is no universal standard for the electrolyte specifications in the market, and a high purity electrolyte is favored by researchers and technology developers to avoid potential detrimental impacts of impurities on the system performance. There is thus a need for improved understanding of the impact of electrolyte impurities on the degradation of VRFBs is vital for commercialization of VRFBs [7]. This study aims to evaluate the effect of iron, aluminum, and manganese ions (Mn2+, Fe2+ and Al3+) on the VRFB performance and the degradation of materials used in the VRFB cell.The battery performance was evaluated using a ‘zero-gap’ flow cell with an electrode area of 5 cm2. An electrolytic solution containing 1.6 M VOSO4 solution in 3 M H2SO4 was circulated through the cell. Thermally treated carbon papers were used as the cathode and anode electrodes. For charge-discharge experiments, constant current density (in the range 10 to 80 mA cm−2) was applied with 1.65 and 0.8 V as upper and lower voltage limits. The effects of each impurity were studied at 0.1 M concentrations through charge-discharge experiments. Material characterization analysis (SEM-EDS, XRD, Raman spectroscopy, and UV-Vis) were conducted before and after cycling to provide a better understanding of the effects of the impurities on the electrode, membrane, and electrolyte degradation. Based on the results obtained from these experiments, the effects of each impurities in the electrolyte can be ascertained providing an important reference for electrolyte manufacturing and regeneration.Figure 1 compares the morphologies of the carbon paper electrodes used in the positive and negative sides of the VRFB, before and after 200 cycles of operation, captured by SEM (Figure 1a, 1b). Carbon papers have a smooth fiber surface with small flakes scattered on the surface. The surface of the fresh carbon paper was smooth as shown in Fig. 1a. In the absence of impurities, the electrode morphology appears to be almost unchanged by after 200 cycles, although the surface of the carbon fibers may be slightly rougher. However, a different structure was observed are cycling with an electrolyte containing Al3+ impurity ions. A solid phase has blocked the electrode pores and precipitated on the active surface area of the electrode. This precipitate adhered to the carbon paper electrode surface and changed the surface structure, hindering interfacial contact between the electrolyte and electrode [8] resulting in performance degradation of VRFB. The observations of electrode morphology changes are consistent with voltammetric analysis and the observed degradation of battery performance.