Elucidating the Effects of Temperature on Nonaqueous Redox Flow Cell Cycling Performance

Abstract

The impact of cell temperature is a relatively underexplored area within the burgeoning field of nonaqueous redox flow batteries (NAqRFBs). Here, we investigate the effect of elevated temperature on the performance of nonaqueous redox electrolytes and associated flow cells. Using a model compound, N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT), in a propylene-carbonate-based electrolyte, we experimentally measure the temperature dependence of relevant physicochemical properties (i.e., electrolyte conductivity, viscosity, diffusivity) and electrochemical characteristics (i.e., chemical and electrochemical reversibility) across a temperature range of 30 °C to 70 °C. We then perform flow cell studies, finding that while ohmic and mass transport resistances decrease significantly with increases in temperature for the MEEPT/MEEPT+· redox couple, accessible electrolyte capacity gradually reduces at temperatures > 50 °C. Ex-situ, post-test characterization using microelectrode voltammetry suggests that this capacity fade is due to instability of the MEEPT radical cation. Finally, using MEEPT as a posolyte and a model viologen negolyte (bis(2-(2-methoxyethoxy)ethyl)viologen), we assemble a full cell and perform polarization analyses, observing a 2× increase in the peak power density when the operating temperature is increased from 30 °C to 70 °C. Broadly, this work highlights opportunities for systematic engineering of nonaqueous electrolytes and flow cells for higher power operation at elevated temperatures.

Evaluation of in situ thermal stability assessment for flow batteries and deeper investigation of the ferrocene co-polymer

“…Additionally, in the study by Quinn et al , the thermal stability of N -(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT) material was assessed in a non-aqueous electrolyte, applying a decently accurate heating setup (Table S1†) and performing in the 30 to 70 °C temperature range with 10 °C increments. 24 Unfortunately, capacity fade values were not stated for the performed symmetric cycling and, thus, we have extracted the values from the figure five in the corresponding publication by converting the figure into data points using the WebPlotDigitizer program. 56 The obtained charge capacity from time dependency is depicted in Fig.…”

“…Due to the scarcity of the thermal stability evaluations of organic cathode materials, the obtained capacity fade rates of the newly emerged MEEPT study are valuable for providing an idea about the current state of the reported results. Nevertheless, we emphasize that the stated values represent a rough estimation, obtained by digitizing the graph from the work of Quinn et al 24 Hence, firstly, the capacity fades calculated from the original values may differ. Secondly, even if the digitalization of the figure was accurate, the statements on the fade rate behavior, which are based on such short battery tests, are of low reliability.…”

“…• Positioning of the probe outside from the RFB electrolyte and preliminary calibration for the proper correspondence of the probe readouts to the RFB reached electrolyte temperature as in the work of Quinn et al 24 may be a good alternative for an accurate heating setup;…”

“…21,22 In particular for cathodic organic materials, there have been only a few species with moderate to high capacity fade rate presented. 18,23,24 Consequently, it is of great interest to assess the thermal stability of the currently developed stable RFB organic materials to indicate the promising species, and to better understand the mechanisms of the molecules' decomposition.…”

Leveraging Temperature-Dependent (Electro)Chemical Kinetics for High-Throughput Flow Battery Characterization

“…Symmetric cells with only one heated reservoir were used to cycle aqueous TEMPO-based RAOMS, however no statistical difference in temporal capacity fade rates at room temperature vs 40°C was observed. 45,46 Quinn & Ripley et al 47 investigated the effect of elevated temperature on the physicochemical and electrochemical properties, and RFB performance, of nonaqueous RAOMs up to 70°C. Many of the temperature effects explored in Ref.…”