Anomalous Behavior of Anion Exchange Membrane during Operation of a Vanadium Redox Flow Battery

Abstract

Vanadium redox flow batteries (VRFBs) are becoming an integral component of renewable energy solutions, microgrids, and backup storage systems. The reduction in cell resistance due to advancements in the electrode and overall component design allows flow cells to operate at higher current/power densities. During discharging of an experimental flow cell at higher current densities, under certain conditions, an unexpected voltage drop and subsequent recovery were observed. Similar effects of voltage drop and recovery have been observed in NiCd and primary lithium–thionyl chloride (LTC) batteries but have not yet been reported for the VRFB in peer-reviewed publications. This study aims to understand the causes behind the voltage drop effect in a VRFB under high power density operation.

The Influence of Free Acid in Vanadium Redox‐Flow Battery Electrolyte on “Power Drop” Effect and Thermally Induced Degradation

“…3) Determination of the cell resistance from EIS to compare with initial cell resistance value. 4) Galvanostatic discharge at relatively high current density to enable the appearance of “power drop‐off effect.” [ 3 ] 5) Investigation of catholytes, i.e., V(V)‐ n at high SoC at room and elevated temperature.…”

“…One of a reason for it may be the loss of vanadium species as vanadium pentoxide precipitates on the anion exchange membrane, which is supposed to happen during the electrolysis to high SoC. [3,11] During the electrolysis of V(IV) solutions, sulfate anions are expected to move through membrane from anolyte to catholyte according to the commonly suggested mass balance equations (Equation (4)-(5)). [19] ðanolyteÞ…”

“…[ 3 ] This effect, i.e., reversible decrease in a discharge voltage, is more pronounced after a longer exposure of cell to a high SoC, at higher discharge current densities or at low temperatures. [ 3 ] The appearance of “power drop” is suggested to be caused by reversible adsorption or temporary precipitation of vanadium(V) species onto membrane, which is a pH‐dependent process. [ 3,4 ] In this work, we apply a fully charging of a VRFB with anion exchange membrane as a stressor to ascertain if there is any correlation between the initial electrolyte composition and a tendency of the VRFB to degradation.…”

“…[ 3 ] The appearance of “power drop” is suggested to be caused by reversible adsorption or temporary precipitation of vanadium(V) species onto membrane, which is a pH‐dependent process. [ 3,4 ] In this work, we apply a fully charging of a VRFB with anion exchange membrane as a stressor to ascertain if there is any correlation between the initial electrolyte composition and a tendency of the VRFB to degradation. The initial electrolyte is often composed of a 50%:50% mol mixture of vanadium(III) and vanadium(IV) species in sulfuric acid and is denoted further as V 3.5+ electrolyte.…”

“…In case of constant current discharge of a VRFB, which starts from a higher SoC (over 80%), the “power drop” effect has been recently and for the first time reported for a cell with anion exchanger membrane and vanadium electrolyte based on sulfuric acid matrix. [ 3 ] This effect, i.e., reversible decrease in a discharge voltage, is more pronounced after a longer exposure of cell to a high SoC, at higher discharge current densities or at low temperatures. [ 3 ] The appearance of “power drop” is suggested to be caused by reversible adsorption or temporary precipitation of vanadium(V) species onto membrane, which is a pH‐dependent process.…”