PNNL

Abstract

Redox flow batteries (RFBs) have been investigated over the past several decades as a promising energy storage system for grid applications due to their unique features that include the separation of energy and power output, high safety, and long cycle life. It is therefore vital but still in severe deficiency to understand the reliability of RFBs, and the mechanisms that cause degradation with time. One of the primary challenges involves the unseparated contributions from individual electrodes due to the lack of a stable reference electrode (RE) especially for long-term cycle testing in a scaled cell. Herein, we first develop a long-term stable reference electrode for a scaled all-vanadium redox flow battery (VRFB). The newly developed reference electrode, based on a dynamic hydrogen electrode (DHE) with novel design of Pt electrodes, demonstrated its high accuracy and long-term stability that enables in-situ monitoring of individual electrode potentials up to 500 cycles. By introducing the RE approach to decouple the cathode and anode, the reliability and degradation mechanism of a scaled VRFB were investigated. It was found that the cell degradation occurred significantly in the initial 100 cycles in terms of performance fading (capacity and efficiencies), owing to the increase in cell polarization induced by electrolyte (vanadium ion) crossover. By further measurements of the charge-discharge voltage profile, OCV, and overpotential of full cell and individual electrodes (vs. DHE), it was revealed that the cathode reaction played a much more significant role than the anode reaction in limiting the capacity particularly in the discharge process. However, the anode has a larger contribution to the overall cell degradation upon long-term cycling. The polarization curve measurement disclosed the details on the ohmic loss and mass transport loss contributed by the cathode and anode. This exploratory work will be beneficial for the insight into the reliability/degradation mechanism of a VRFB and future design and development of a stable reference electrode in the practical application of a scaled ESS.