Improving the Selectivity of Commercial Membranes in Vanadium Redox Flow Batteries through the Direct Deposition of an Additional Selective Layer

Abstract

The evolution of both energy demand and energy supply requires large energy storage systems to face the challenges related to the not-programmable nature of renewable energy source. Vanadium Redox Flow Batteries (VRFB) is a promising solution for these challenges due to its decoupled energy and power, high flexibility, fast response time and long cycle life [1]. However, some technological issues limit the competitiveness of the technology by increasing the cost of the system. One of these issues is vanadium cross-over, which is the undesired transport of vanadium ions through the not-ideally selective membrane, causing battery self-discharge, capacity loss and electrolyte imbalance [1]. Relatively thick membranes (>50 µm) are usually employed in the commercial applications to mitigate cross-over losses, leading to increased voltage ohmic losses and high system costs. Indeed, the membrane is one of the main cost items of the technology as it may contribute up to 50% of total stack costs [2]. Therefore, the development of a low-cost and selective separator will improve the technology. As demonstrated by the authors [3], an effective strategy to reduce system costs and cross-over losses is to improve the selectivity of NafionTM membranes with an additional selective layer, allowing to reduce the thickness, thus the costs and the ohmic losses. The additional selective layer, referred as “the barrier”, proposed by the authors is a layer of controlled morphological properties directly deposited on the membrane that introduces a tortuous path for the ion transport, successfully mitigating vanadium fluxes through the separator [3]. In previous works the barrier was deposited by Reactive Spray Deposition Technology (RSDT) [3], but in this work the barrier was manufactured via Ultrasonic Spray Coating (USC), a commercial and easily scalable technique. Exploiting the USC for the manufacturing of the barrier accelerates the scale-up of the barrier to a scale closer to the commercial applications (100 cm2). Moreover, the barrier is composed by only commercially available materials (NafionTM ionomer, Vulcan® XC-72R and silica nanoparticles) and deposited on commercial membranes, thus facilitating a future industrial production.In this work, further developments of the barrier with respect to previous works [4] are discussed. In particular, the deposition process is further investigated by evaluating the influence of the heat-plate temperature and the ink flow rate on the evaporation process of the solvent and its impact on the performance of the barrier. Regarding barrier composition, different commercial polymers (AquivionTM and NafionTM ) were employed both as ionomer in the barrier layer and as supporting membrane to analyse the influence of different equivalent weight and the coupling of different materials on the selectivity of the barrier. Moreover, the barrier was also deposited on thin membranes (<25 µm) to enable their use in commercial applications, reducing system costs and ohmic losses.The barrier layers were characterized combining morphological and electrochemical characterization. SEM analysis was conducted to relate the morphology of the layer with the ink composition and the deposition parameters. Instead, charge-discharge cycles with voltage cut-offs were performed to evaluate the influence of the different barrier layers on the capacity loss and the efficiency of the battery. A commercial electrolyte was used for the cycling test to assess the ability of the barriers in operating conditions closer to the ones of real applications. In these operating conditions the capacity loss due to cross-over was reduced by nearly 20%, as reported in Figure 1, while the net vanadium transport by 30% with respect the bare membrane, as resulted from ICP-OES analysis of electrolytes at the end of the test. The barrier effectiveness is further improved adopting different electrolyte composition.Finally, the combination of ink and deposition parameters that ensured the highest selectivity were used to deposit a barrier with an active area of 100 cm2. The barrier was able to mitigate cross-over losses and reduced the capacity decay due to cross-over, confirming its effectiveness also at higher cell area.Figure 1 – Comparison of the discharged capacity with NafionTM N212 and the barrier during charge-discharge cycles with cut-off voltages at 100 mA cm-2. Cycles at different current densities were performed at beginning and end of test to evaluate battery performance in different conditions.