Quantifying Decay Rates of Electrochemically Active Species Using Microelectrode Voltammetry

Abstract

The availability, abundance, and energy density of fossil resources have driven progress over the last century leading to dramatic improvements in quality of life worldwide. However, to avert catastrophic climate change, there is an increasingly urgent need to reduce carbon emissions without stifling economic growth. Deep decarbonization necessitates mass deployment of renewable technologies, such as wind and solar photovoltaic, to displace existing carbon-intensive electricity generation. Though increasingly inexpensive, the inherent difficulties in converting the intermittent output of renewable installations into reliable, dispatchable power sources represents an important constraint and motivates research into storage technologies to buffer the intermittency without significantly increasing the cost of electricity.There are several large-scale energy storage technologies available but all suffer some drawbacks. Pumped hydro storage, currently responsible for 99% of the energy storage market in the U.S., has high energy efficiency and is inexpensive with long cycle and calendar life; however, there have been few new installations in the U.S. due largely to the difficulty of permitting new sites, financing large projects, and meeting geographic requirements1,2. Redox flow batteries (RFBs) are an electrochemical technology that holds promise for long duration energy storage due to their location independence, lower capital cost, decoupled energy and power specifications, and inherent safety. The current state of the art RFBs employ vanadium as the active species which is relatively expensive. As system level costs decline, the cost of RFBs will approach materials cost and so finding less expensive materials if critical to reducing RFB price. Some redox active species used to store electricity in RFBs undergo decomposition due to residing outside the thermodynamic solvent stability window, optical sensitivity, or air sensitivity. These decay pathways consume active materials and reduce the lifetime of the RFB. Determining the kinetic parameters of this decomposition and quantifying the impact on system lifetime is the task we address here.Here, we demonstrate the utility of microelectrodes for quantifying rapid decay in electrochemical systems containing soluble redox active components. By holding a sufficiently reducing or oxidizing potential on a microelectrode, as compared to the equilibrium potential of the redox couple of interest, limiting currents can be continuously monitored without significantly altering the state-of-charge of the bulk solution. From this current, a concentration can be extracted and its time-dependent evolution can be fit to a general rate equation. As a model reaction, we evaluate the decay of permanganate, which is under consideration as a positive electrolyte material in alkaline aqueous RFBs3. Preliminary studies have shown permanganate auto reduces to manganate and oxygen, introducing a potential flammability hazard and decreasing the lifetime of RFBs4–6. A better understanding of the factors that influence this decay process will inform mitigation strategies. We find that the decay of permanganate is strongly dependent on supporting electrolyte pH. The decay order was found to approximate a second order with the rate constant near 40 M-1 min-1 in the least stable supporting salt concentrations and approaches zero in the most stable conditions. These numerically fit parameters are in close agreement with observed decomposition times and agree with parameters calculated using UV-vis spectroscopy. The talk will include development of the microelectrode technique, benchmarking with model compounds and UV-vis spectroscopy, and, finally, demonstration on a rapidly decaying model species, permanganate. We anticipate that the technique developed here can be applied to other systems experiencing similarly rapid decay that frustrates conventional methodologies.