Kinetics of the Bromine Reaction at Carbon-Polymer Composite Electrodes in the Presence of Complexing Agents

The zinc-bromine hybrid redox flow battery (RFB) is one of the few battery systems that have seen implementation for the medium to large scale energy storage.  One of the issues identified with this flow battery is that of bromine crossover from the positive electrode to the zinc electrode, leading to loss of current efficiency.  One of the ways in which this problem can be overcome is through the use of organic-based complexing agents which tie up the bromine as a thin surface layer and an immiscible liquid phase which is pumped from the electrode surface into an external reservoir.  In this paper, we examine in detail the kinetics and chemistry of formation of these complexed bromine ion pairs and establish the concentrations of the various complexing agents required to achieve full complexation of the electrogenerated bromine species.  The initial phases of the study used small chain (2,3 and 4-carbon) symmetrical tetraalkylammonium bromide salts to investigate the bromide oxidation and the stages of film formation on a variety of carbon-composite electrodes commonly employed in RFB systems.  These data were then compared to the classical responses obtained on Pt and glassy carbon electrodes.  What is clearly evident from the study is that the electrochemical reversibly of the Br^-/Br₂ reaction is attenuated on the carbon-composite surfaces in the presence of the complexing agents and leads to a second oxidation process at more positive potentials (Figure 1).   The nature of this second oxidation and of the surface species formed is investigated using in-situ Raman spectroscopy and its electrochemical characteristics examined using electrochemical impedance spectroscopy and potentiodynamic polarisation techniques.  Substantial changes in the electrochemical responses are recorded when film formation occurs and this event also led to a large increase in the measured impedance, requiring the use of an additional low frequency time constant element in the Randles equivalent circuit employed to fit the impedance spectrum acquired.  The magnitudes and trends observed in the impedance parameters at the different concentration and complexing species employed will be discussed in the presentation.  Since in an operational Zn-Br₂ RFB system during charge, the oxidation reaction will be occurring on composite carbon surface covered with the Br_x^- complex, how this film impacts on the kinetics of the charge and discharge reaction will impact on the voltage efficiency.  The understanding of the behaviour of these simpler complexing agents will thus enable the electrochemistry of more complex species, e.g. 1-methyl-1-ethylpyrrolidinium bromide, to be better understood and so lead to improvements in the efficiency of the process occurring at the positive electrode of the zinc-bromine hybrid RFB.