Investigating the V(IV)/V(V) electrode reaction in a vanadium redox flow battery – A distribution of relaxation times analysis

•Design of 3D printed flow cell to precisely control the measurement conditions.•EIS data are analyzed using the DRT tool to study the VO2+/VO2+ redox reaction.•Electrochemical processes are identified according to their typical time constant.•Novel method to determine the SoC of the battery electrochemically. Due to the worldwide increasing energy demand and the urgency to act due to climate change, new energy storage technologies are required to balance the intermittent power supply of renewable energy sources. While the vanadium redox flow battery (VRFB) must still overcome lifetime and efficiency challenges, the technology is a promising candidate for large-scale energy storage. Thus, conducting experiments in a setup that closely mimics the operating conditions is vital for gaining new insights into the reactions and transport processes in a VRFB. We developed a novel 3D printed flow cell to study the individual half cell reactions of a VRFB under precisely controlled operating conditions. Using electrochemical impedance spectroscopy combined with the distribution of relaxation times analysis, we could identify the processes occurring in the half cell with the V(IV)/V(V) redox reaction by varying experimental parameters. We assigned peaks in different frequency ranges to the electrochemical reaction, the transport processes through the porous electrode structure, and the ion transport. This information is essential in the search for optimized operating conditions to improve the VRFB efficiency. The presented method also allows electrochemical measurements to determine the electrolyte's state of charge in the half cell. Thus, no additional instruments are required – only a potentiostat with a frequency response analyzer module. This feature was verified by selecting random electrolyte compositions. [Display omitted]