Development and characterisation of anode materials for polysulphide-air redox flow battery

Abstract

The aim of this thesis is to develop a highly active negative electrode for the polysulphide-air RFB. The investigation started from a fundamental understanding of the aqueous polysulphide electrolyte, including the speciation and redox reactions. A numerical model was shown to be a valuable method to approximate the equilibrium concentrations of sulphur species within aqueous polysulphide solutions. S32- and S42- are shown to be the predominant polysulphide species in the 1 mol kg-1 Na2S2/1 mol kg-1 NaOH and the 0.02 mol kg-1 Na2S2/1 mol kg-1 NaOH electrolytes. Possible aqueous Sx2-/HS- redox reactions were summarized from previous studies, whose potentials were calculated using the numerical model derived ionic concentrations. Electrochemical measurements were mostly performed in a three-electrode cell setup. CV measurements of glassy carbon and nickel disk electrodes showed multiple peaks during the anodic and cathodic polarizations of a dilute polysulphide electrolyte, corresponding to the formation of an initial sulphur or NiSx layer and the subsequent electrochemical formation and dissolution of polysulphide and sulphur species on the initial sulphur or NiSx layer. Passivation of both disk electrodes at high anodic potentials was observed. Polarization tests of the two disk electrodes in a concentrated polysulphide electrolyte suggested nickel as a better catalyst than glassy carbon towards aqueous Sx2-/HS- redox reactions. A number of NiSx/carbon paper electrodes were prepared, using electrodeposition, impregnation and electroless deposition methods, and characterized for their electrocatalytic performance in a concentrated polysulphide electrolyte. Those composite electrodes exhibited much better catalytic activities than the catalyst-free carbon paper electrodes, however lower than a sulphidised acid-treated Ni foam which was demonstrated in a 5 cm2 single-cell polysulphide-air RFB. A maximum power density of ~4.8 mW cm-2 was achieved. The performance of the polysulphide-air RFB was largely limited by the polysulphide-side half-cell, due to the large charge transfer resistance associated with the negative electrode, as indicated by the EIS measurements.