Electrochemical impedance spectroscopy for the study of catalysis and electrode degradation in vanadium redox-flow batteries

Abstract

Vanadium redox-flow batteries (VRFBs) are emerging as promising systems for large-scale energy storage. Yet, certain key aspects require for substantial improvements in order to achieve broad commercial success of this technology. In that respect, further development of electrode materials in terms of increased and durable electrocatalytic activity is of essence, especially in the negative half cell. Myriads of approaches for surface modification of the predominantly employed macroporous carbons have thus been proposed so far. However, upon thorough review of pre-existing literature, it became evident that meaningfully benchmarking and quantifying the effects of those procedures with regard to resulting performance enhancement is anything but a straightforward task. The emphasis of this thesis was therefore put on establishing methodical approaches for reliable and unambiguous quantification of electrode kinetics by properly utilizing electrochemical techniques, predominantly electrochemical impedance spectroscopy (EIS), and exploiting their full capabilities. In an exemplary study on bismuth-modified carbon felt electrodes it was demonstrated how applying proper normalization to ex-situ impedance data enables thorough characterization and standardized comparison of electrocatalytic effects induced by incorporation of metal (oxide) particles or any other type of supposedly activating treatment. Intrinsic catalytic activity of Bi for the V(II)/V(III) redox reaction has been verified and stability of modified electrodes was assessed for the first time. By eliminating the often misleading impact of electrode wetting, reproducibility of obtained results was greatly enhanced compared to other approaches commonly pursued in the open literature. Subsequently, the innovative concept of distribution of relaxation times (DRT) analysis was introduced to the field of in-situ examination of VRFB cells. Loss processes during operation of the battery have been unraveled before proving the feasibility of DRT-based monitoring of electrode ageing. Gained insights underline the importance of keeping future research focused on the negative half cell electrode, at least in terms of electrocatalysis and degradation. Further investigations explored how synergistic use of additional techniques may complement the experimental capabilities of EIS to determine electrode characteristics in the most comprehensive way. This involved unequivocal separation of respective current contributions from V(III) -reduction and parasitic hydrogen evolution reaction (HER) at planar model electrodes during rotating ring-disc electrode (RRDE) and alternating current cyclic voltammetry (ACCV) measurements as well as visualization of bismuth dissolution and redeposition in an operating VRFB by utilization of X-ray-based imaging procedures.