MIcroband Electrode Platforms for Identifying Homogeneous and Heterogeneous Factors Limiting Redoxmer Performance for Redox Flow Batteries

Chemical and electrochemical limitations of redox-active species (<i>redoxmers</i>) that cause long-term performance losses in redox flow battery devices may not be immediately evident during conventional electroanalytical experimentation. In both aqueous and nonaqueous solutions, effects of uncompensated resistance and non-idealities in the redoxmer response can lead to difficulties in identifying mechanistically complicating factors. Here, we posit that in contrast to ~cm² electrodes typically used for the initial screening of redoxmers, ultramicroelectrodes – electrodes with at least one critical dimension in the micron or sub-micron range – can be used directly or integrated into microfabricated devices to reveal such limitations. Complicating factors can present in the form of heterogenous, i.e. related to reactivity at the electrodes, or homogeneous, i.e. related to the behavior of the charged molecules in solution. In this talk, we will present our approaches using microfabricated band electrodes (MBEs) and interdigitated array electrodes (IDAs) to tackle these analytical needs for both aqueous and non-aqueous systems. Emphasis will be made on approaches designed to inform on processes at timescales relevant to the flow battery experiment.<br/><br/>In a first example we will discuss an automated generation-collection experiment with IDAs is used to determine the homogeneous rates of decomposition of redoxmers in highly systematic experiments. Taking advantage of several simplifications afforded by this experiment enables the swift estimation of the impact of interfering species, redoxmer concentration, and other electrolyte conditions on the chemical stability of these systems. In a second example, we will show how MBEs are used to obtain heterogenous kinetics of electron transfer with great versatility, avoiding complications associated with the use of high scan rates in typical cyclic voltammetric experiments at macroelectrodes. We will discuss prospects for integrating these methodologies into systematic experiments similar to those conducted using scanning electrochemical microscopy and associated techniques to promptly identify mechanistic limitations in redoxmer systems.