Design and Development of an Innovative Barrier Layer in Vanadium Redox Flow Batteries
Authors: Zago Matteo , Cecchetti Marco , Ebaugh Thomas Allen , Yu Haoran , Bonville Leonard J. , Gambaro Chiara , Meda Laura , Maric Radenka , Casalegno Andrea 
Published: May 2020

Vanadium Redox Flow Battery (VRFB) is an electrochemical energy storage technology that exploits red-ox reactions of vanadium ions dissolved in acidic solutions to convert electrical energy into and from chemical energy. VRFB is an interesting and promising technology because of its high efficiency, independent energy to power ratio, high flexibility and long cycle life [1]. However, VRFB commercialization is still hindered by some technological issues, among which is the capacity loss induced by the undesired transport of vanadium ions across the ion-exchange membrane. Depending on the nominal operating condition, the choice of suitable membrane for VRFB application results from the trade-off between low vanadium permeability and high proton conductivity.
Nafion® is widely used in VRFB: it presents high conductivity and good mechanical and chemical stability, but it is not ideally selective towards vanadium ions. This implies the adoption of thicker membrane to limit capacity loss, leading to increased ohmic loss and system costs [2]. Alternative cation exchange membranes (CEMs) are SPEEK or SPI that are promising because of their reduced permeability, but the low conductivity and stability hinder their competitiveness with respect to Nafion®. Anion exchange membranes (AEMs) are interesting alternatives, but the poor chemical stability and low conductivity limit their application in VRFB. Amphoteric ion exchange membranes (AIEM) combine the functional groups of both CEM and AEM, obtaining both low permeability and good conductivity. However, the complex preparation results in high costs [3].
In this work, an additional selective layer, described in the patent WO 2019/197917, was deposited on a commercial cation exchange membrane. The selective layer, termed as barrier, is a porous component in which pores size, tortuous path, thickness and the composition are designed to improve layer selectivity towards vanadium ions. The barrier was manufactured with reactive spray deposition technology (RSDT), which is a flame-based synthesis process unique to Dr. Radenka Maric’s research group. Fabrication of the barrier layer by RSDT is a one-step process in which carbon-rich particles are synthesized in the flame and a mixture of commercial carbon black particles and ionomer is sprayed directly onto the membrane. In this work different barrier morphologies have been designed, manufactured and tested.
The barrier layer was tested in a 25 cm2 cell during cycles at constant capacity in both charge and discharge [4]. After each charge and discharge step the cell was kept in OCV for 90 seconds in order to acquire cell OCV and get an insight into battery state of charge (SoC). Figure 1 reports the evolution of SoC after each step of charge: it is worth noting that the VRFB with barrier presents a significant reduction of SoC decay (i.e., capacity loss) compared to a reference VRFB employing Nafion® 115. Moreover, the VRFB with barrier exhibits a slightly higher efficiency and a good stability in time.
In addition to electrochemical testing, the structure of the barrier layer was characterized using TEM, SEM and nitrogen adsorption. TEM and nitrogen adsorption provide information on pore structure and pore-size distribution, which can be related to the performance of the barrier layer during cell testing, while SEM shows that the coating is uniform in thickness and consistent from sample to sample.

References:

[1] A.Z. Weber et al., Journal of Applied Electrochemistry 41 (2011) 1137-1164.
[2] B. Jiang et al., Journal of Membrane Science 510 (2016) 18-26.
[3] Y. Shi et al., Applied Energy 238 (2019) 202-224.
[4] M. Cecchetti et al., Journal of Power Sources 400 (2018) 218-224.

Figure 1