The Cell-in-Series Method: A Technique for Accelerated Electrode Degradation in Redox Flow Batteries

Abstract

We demonstrate a novel method to accelerate electrode degradation in redox flow batteries and apply this method to the all-vanadium chemistry. Electrode performance degradation occurred seven times faster than in a typical cycling experiment, enabling rapid evaluation of materials. This method also enables the steady-state study of electrodes. In this manner, it is possible to delineate whether specific operating conditions induce performance degradation; we found that both aggressively charging and discharging result in performance loss. Post-mortem x-ray photoelectron spectroscopy of the degraded electrodes was used to resolve the effects of state of charge (SoC) and current on the electrode surface chemistry. For the electrode material tested in this work, we found evidence that a loss of oxygen content on the negative electrode cannot explain decreased cell performance. Furthermore, the effects of decreased electrode and membrane performance on capacity fade in a typical cycling battery were decoupled from crossover; electrode and membrane performance decay were responsible for a 22% fade in capacity, while crossover caused a 12% fade. This paper is part of the JES Focus Issue on Redox Flow Batteries-Reversible Fuel Cells.The vanadium redox flow battery (VRFB) is a potential gridscale energy storage technology that has attracted significant research interest.1-5 The cell architecture, electrodes, and membranes have been considerably optimized, resulting in increased efficiency and power density of VRFBs.3,5-10 Other research has focused on improving energy density by enhancing vanadium solubility through the use of additives in the electrolyte solution; these compounds may also enhance electrode activity. [11][12][13] As these improvements continue, the long-term durability of VRFBs becomes an issue with strong implications for commercial viability. Therefore, methods to study component durability are of growing interest.Unlike traditional secondary batteries, VRFBs do not suffer from appreciable unrecoverable capacity fade. They promise to have long lifetimes due to the ability to rebalance the electrolyte, 14-16 which negates capacity fade and may allow them to last for thousands of cycles and for several years. While the vanadium electrolyte is stable, 17 the durability of some components in the VRFB has not been studied in detail. Changes in the electrode can result in increased activation/ charge transfer resistance and decreased mass transport; changes in the membrane may result in increased ohmic resistance in the cell. These phenomena contribute to a decrease in the capacity that can be accessed at high currents as well as to reduced energy efficiency. Herein, we present a methodology and initial findings for studying performance degradation in VRFBs. The energy efficiency loss in a typical VRFB cycling experiment is deconvoluted into contributions from decreased electrode performance and increased ohmic resistance within the membrane. The capacity fade mechanism, often attributed to crossover alone...

Performance of Different Carbon Electrode Materials: Insights into Stability and Degradation under Real Vanadium Redox Flow Battery Operating Conditions

“…The O/C ratio after acid soaking increased significantly for both materials, in line with findings by previous authors. 26 After long term cycling the oxygen content of both cycled materials were compared to the acid-soaked samples as a baseline. A decrease in O/C ratio of approximately 33% was observed for both carbon materials from the negative half-cell.…”

“…36 Additionally the vanadium ions themselves have also been suggested to induce changes in the oxygen content of carbon electrodes. 26 The type of oxygen groups is shown in Figures 4c and 4d. C-OR, C=O and COOH groups were present on the surface of heat treated and acid soaked carbon paper and carbon felt with C-OR groups being the most abundant (see Supplementary Information, Figures S2, S3 and S4).…”

Parasitic Hydrogen Evolution at Different Carbon Fiber Electrodes in Vanadium Redox Flow Batteries

“…21,24 In the case of operation at high states of charge (SoC), parasitic hydrogen evolution exacerbates the well-known electrolyte imbalance, which is caused by solvent crossover and emerges during long-term cycling. 25,26 This effect leads to an incremental capacity loss during subsequent charge-discharge cycles and an increase in cell resistance. Additionally, gas bubbles trapped in the porous electrode reduce the utilization of the electrode surface area.…”

A review of all‐vanadium redox flow battery durability: Degradation mechanisms and mitigation strategies

“…Recently, Mench's group developed a cell‐in series (CIS) technique for an accelerated electrode degradation test. In this method, two cells were connected in series with one cell undergoing discharge and the other cell undergoing charge.…”

“…Polarization curves before and after 54 cycles at 200 mA cm −2 (reproduced and modified with permission from ECS) [Colour figure can be viewed at wileyonlinelibrary.com]…”