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4.3 Summary and Final Remarks Throughout the work presented and discussed in this chapter, it was shown how seve- ral of the physicochemical parameters relevant for RFB electrolytes can be determined. The reported values are summarised in Table 4.1, divided into the four groups of investi- gated structures (ferri-/ferrocyanide, hydroxybenzoquinones, carboxybenzoquinones, and anthraquinones) to facilitate comparison between them. Diffusion coefficients were determined on both stationary and rotating electrodes and were generally in the range of 1.15 × 10−6 cm2 s−1 to 7.30 × 10−6 cm2 s−1. In the cases where diffusion coefficients were determined with both methods, the Randles-Ševčík analysis resulted in lower estimates than the Levich analysis. This was attributed to the Randles- Ševčík equation only being fully applicable for reversible systems, which none of the investigated systems were. It is also observed that the anthraquinones exhibit the lowest diffusion coefficients among the investigated compounds, as expected from their bulkier structure. Regardless, the diffusion coefficient of the investigated quinones are in the same area or higher than those of V3+ and V2O5 in acidic media ((1.41±0.12)×10−6 and 1.4 × 10−6 cm2 s−1, respectively [57, 58]). The investigated quinones should thus result in reasonably low mass transfer overpotentials when used in RFBs. The standard rate constants determined through RDE voltammetry show a wider span of values, ranging from 6.25 × 10−4 cm s−1 to 3.43 × 10−2 cm s−1. The nature of functional groups and supporting electrolyte thus appears to have a greater influence on the kinetics than on the diffusion. The anthraquinones in most cases exhibit more facile electrode kinetics than both the ferri-/ferrocyanide species and hydroxybenzoquinones. The transfer coefficients also span widely, from 0.098 to 0.558, although the lowest values are more likely caused by the RDE method not producing reliable estimates for redox couples with facile electrode kinetics. As a comparison to the vanadium chemistry, standard rate constants for V3+ and V2O5 in acidic media of 1.7 × 10−5 cm s−1 and 7.5 × 10−4 cm s−1 were previously reported [57, 58]. All of the investigated compounds, with the exception of DHBQ, thus exhibit more facile electrode kinetics than the vanadium couples used in VRFBs. The investigated quinones and the ferri-/ferrocyanide are therefore favourable to be used in RFB electrolytes in terms of reducing the activation overpotentials caused by sluggish electrode kinetics. Bulk electrolysis proved useful for an initial evaluation of the chemical stability of ferri- /ferrocyanide in alkaline media. It was also used to show how the capacity of DHTA and DHPA was lost to presumably redox inactive degradation products at pH 14. It would be interesting to continue the study of DHTA and DHPA in the future, both in strong and milder alkaline media, to investigate if they are chemically stable in milder alkaline conditions. It would furthermore be beneficial to investigate the degradation products with NMR and chromatography methods, and determine their alkaline solubility if shown to be stable at milder conditions. Although these ex situ methods provide useful preliminary knowledge about electrochemi- cal behaviour and chemical stability, they do come with two major shortcomings: Firstly, the electrochemistry is studied on electrode surfaces markedly different from the ones used in flow cells. The planar disk WEs used in standard three-electrode configurations are non-ideal representations of the porous carbon electrodes used in flow cells, so the determined electrode kinetic parameters could be significantly different in situ. The diffu- 4.3. Summary and Final Remarks 69PDF Image | Organic Redox Flow Batteries 2023
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