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

Abstract

The stability of reported organic materials for redox flow batteries (RFB) continues to improve. Consequently, the relevance of analytical techniques to assess degradation rates also grows. To contribute to the development of in operando thermal stability assessment techniques, we evaluated the commonly-reported heating setups using the ferrocene-based FPMAm-co-METAC polymer (PFc) in a Zn-based hybrid RFB with a size-exclusion membrane. In the first stage, the conditions for RFB cycling were selected and evaluated. The amperometric SOC measurement technique revealed oxygen intolerance of the PFc. While no polymer cross-over was detected, cross-over of its hydrolysis products occurred and facilitated the capacity fade. Adjustment of membrane pore size and electrolyte composition helped to mitigate the hydrolyzed products' cross-over. In the second stage, different heating setups for the thermal stability evaluation of PFc were compared. Eventually, a thermostatic setup established the desired temperature most accurately and homogeneously, while the popular oil/sand bath setup exhibited a deviation of 22 °C down from the expected 60 °C. The PFc stability was further evaluated from ambient conditions (28 °C) to 60 °C. At temperatures above 50 °C a facilitated capacity fade was observed. The volumetrically unbalanced, compositionally symmetric flow cell cycling has unraveled that the degradation was caused by catholyte self-reduction and following half-cell imbalances. A mechanism involving the ferrocene complex decomposition is proposed as the origin of the catholyte self-reduction. Finally, the properties and thermal stability of the PFc material as well as the reliability of the studied heating setups are discussed.