Development of a Selective Layer Via Ultrasonic Spray Coating for Vanadium Redox Flow Batteries: Analysis of Ink Composition and Component Scale-up

Abstract

Vanadium Redox Flow Battery (VRFB) is a promising technology for stationary energy storage because of its long cycle life and independent energy to power ratio. However, the competitiveness of the technology is hindered by some technological issues, such as vanadium cross-over. Vanadium cross-over is the undesired transport of vanadium ions trough the ion-exchange membrane, causing capacity loss and electrolyte imbalance. Usually, to mitigate cross-over fluxes between the two half-cells, relatively thick membranes (>50 µm) are employed in the commercial applications, meaning increased ohmic losses, reduced energy efficiency and higher system capital cost: indeed, the membrane can contribute up to 50% the cost of the stack [1]. The state-of-the-art membranes for VRFB are made of NafionTM because of its high conductivity, but its not-ideal selectivity towards vanadium ions prevents the use of thin membranes. Alternative cation exchange membranes like SPEEK or SPI are promising because of their reduced permeability, but the low conductivity limits system power density. Instead, anion exchange membranes are still limited by the poor chemical stability and low conductivity [2].The authors in a recent work [3] successfully demonstrated the possibility of mitigating vanadium cross-over though an innovative additional selective layer directly deposited on the membrane. The selective layer, termed as barrier, is a porous component whose pores size tortuous path, thickness and composition are designed to improve ion/proton selectivity. As reported in Cecchetti et al., the barrier layer, manufactured through Reactive Spray Deposition Technology (RSDT) significantly reduced battery self-discharge without hindering the efficiency of the battery [3].In this work, the possibility of manufacturing the barrier via Ultrasonic Spray Coating (USC) is investigated. USC is a commercial and easily scalable technique that exploits the ultrasonic vibration of the nozzle tip to uniformly atomize and spray an ink on a substrate [4]. In particular, the influence of the process parameters and ink composition on the selectivity of the barrier layer and efficiency of the battery were investigated through both morphological and electrochemical characterization in batteries of 4 cm2 and 25 cm2 active area. The deposited barrier layers were directly deposited on NafionTM 212 to form thin layers of 10 µm thickness (Figure 1). The barriers were composed of commercial silica and Vulcan® XC-72R nanoparticles, that introduce a tortuous path for the vanadium ions through the separators, and NafionTM ionomer, acting as a binder and ensuring proton conductivity.After identifying the combination of process parameters and ink composition that ensures the lowest self-discharge of the battery, the scale-up of the barrier at commercial scale was investigated by directly deposition the barrier on NafionTM 212 to obtain an active area of 100 cm2. The 100 cm2 barrier was tested in a VRFB in cycles of charge-discharge cycles with cut-off voltages of 1.65 V and 1 V showing a capacity decay rate of 0.11 % cycle-1 at 50 mA cm-2: four time lower than the reference value (0.44 % cycle-1) reported by Rodby et al. for VRFB in the same range of current density [5].