Delamination of Reinforced Membranes in Vanadium Redox Battery Cells

Abstract

Redox-flow batteries (RFBs) possess compelling attributes for stationary energy storage. In particular, the decoupling of energy and power in RFBs enables the cost effective use of redox active materials with inherently low energy density1. This decoupling is especially advantageous for long-duration applications, i.e., discharge times ≥ 5hours at rated power2. All-vanadium redox battery (VRB) cells typically consist of a proton-conducting membrane between two porous carbon-fiber-based electrodes. The VII/VIII and VIV/VV redox couples, dissolved in strong sulfuric acid, flow through the negative and positive electrodes, respectively. The primary reactions during charge and discharge are the redox reactions of these two vanadium couples. Protons in an ion-exchange membrane (IEM) typically carry charge between the two electrodes. Movement of reactant species from one electrode to the other through the membrane results in inefficiency and reversible capacity losses and is undesirable3. Therefore, for a given membrane material, there is an optimal membrane thickness to achieve the desired performance with respect to both voltage efficiency (i.e., membrane ohmic loss) and coulombic efficiency (i.e., membrane crossover)4. State-of-the-art VRB cells employ relatively thin IEMs to enable higher ionic conductance (e.g., ≤ 25 μm thick3), which enables high power densities and reduces VRB stack costs2, 5. Perfluorinated sulfonic-acid (PFSA) membranes are typically used in VRBs due to their outstanding stability in the highly oxidizing positive electrolyte (containing VV)6. Thin PFSA membranes are often reinforced with a porous matrix to improve the mechanical robustness and reduce dimensional changes when exposed to varying levels of humidity. In addition to reducing swelling, some reinforced membranes (e.g., GORE-SELECT) can significantly decrease V permeability (33% to 47%) compared to non-reinforced membranes with the same equivalent weight (EW), while the corresponding increase in area-specific resistance (ASR) with reinforcement is smaller (13% to 18%)7. Therefore, reinforced membranes can potentially enable improved VRB cell performance, as well as being easier to handle during stack assembly. Unfortunately, reinforced membranes can undergo delamination in VRB cells. An example of this is shown in Fig. 1, which is a picture of a reinforced membrane from 3M Corporation that was removed from a stack after cycling for more than 2,000 hours. Vionx Energy’s cells use interdigitated flow fields (IDFF), which enable high performance with low pressure drops8, 9. The clear (darker) areas in Fig. 1, where the membrane remains relatively intact, coincide with the ribs of the IDFF. The white areas are where the membrane has separated into layers. These areas coincide with the IDFF channels. There is residual electrolyte between the delaminated layers. This talk shall describe the mechanisms that we hypothesize to be the causes of this delamination of reinforced membranes in VRB cells. We shall also describe accelerated test protocols that can be used to assess the likelihood for a membrane to delaminate in a RFB cell and discuss various mitigation strategies, including reinforced-membrane architectures that are less prone to delamination in RFB cells.