Vanadium cross-over through the not-ideal selective membrane is one of the main technological issues that hinder the competitiveness of Vanadium Redox Flow Battery (VRFB). Commercial applications usually employ relatively thick membranes (>50 µm) to mitigate battery capacity loss and electrolyte imbalance due to cross over, resulting in high ohmic losses and high capital cost: indeed, the membrane can contribute up to 50% of stack cost. To overcome this problem, the authors presented in a previous work an innovative selective layer directly deposited on the membrane via Reactive Spray Deposition Technology (RSDT), named as “barrier”. The barrier is a porous component whose composition, pore size, tortuous path and thickness are designed to improve the selectivity. The developed barrier was able to significantly reduce the self-discharge of the battery without affecting battery efficiency. Differently from membrane treatments proposed in the literature, the novelty of this approach consists in the direct deposition on already commercially available membranes, allowing a easily scalable manufacturing process.In this work Ultrasonic Spray Coating (USC), a commercial and scalable technique, is investigated as manufacturing technique for the barrier. USC exploits ultrasonic vibrations in the nozzle to uniformly atomize and spray an ink directly on a substrate. This work focused on how spray process parameters and ink composition influence the selectivity of the barrier and the efficiency of the battery. In particular, among the process parameters, ink flow rate, shaping gas flow rate, atomizing power and heat-plate temperature were analysed. As regards the composition of the ink, it was studied in terms of amount and type of solvent, amount and type of ionomer and amount and type of nanoparticles, as well as the supporting membrane. The investigation was conducted through both morphological and electrochemical characterizations with a lab-scale flow battery in segmented cell configuration, permitting local measurement of current distribution. Moreover, a physical based modelling analysis supported the investigation.

At the moment, the most performing barrier is deposited on Nafion^TM 212 with a thickness of 10 µm. The composition consists in a combination of commercial silica and Vulcan^® XC-72R nanoparticles with Nafion^TM ionomer as binder.

This layer has been also deposited with an active area of 100 cm2 to investigate the scale-up at larger scale. During testing the 100 cm² barrier showed a capacity decay rate of 0.11 % cycle^-1 at 50 mA cm^-2: significantly lower than the reference value for a VRFB (0.44 % cycle^-1) in the same range of current density reported by Rodby et al.