Effect of IrO2 crystallinity on electrocatalytic behavior of IrO2-Ta2O5/MWCNT composite as anodes in chlor-alkali membrane cell

IrO2-Ta2O5 multi-wall carbon nanotube (MWCNT) composite coatings were synthesized on Ti electrodes at different calcination temperatures from 350 to 550 degrees C used as anodes in membrane cell for brine electrolysis. The physicochemical properties and electrochemical performance of the coatings were investigated by simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC-TGA), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) analysis. Results indicate that the degree of IrO2 crystallinity significantly affects the coating properties. XRD pattern of the coating prepared at 350 degrees C has shown no reflection peaks indicating that the IrO2 and Ta2O5 were amorphous. The DSC-TGA showed two major exothermic peaks at 475 and 575 degrees C attributed to the crystallization of IrO2 and oxidation of Ta2O5 from chloride precursor solution, respectively. XPS data reveals the presence of both Ir valance states, which confirms the reversible redox transition of Ir (III)/Ir (IV). The Raman spectra of the coatings demonstrated that the MWCNT gradually loses its tube structure at the calcination temperatures of 450 and 550 degrees C, and they transform into a graphite-like structure by crystallization of IrO2. However, in the coating without IrO2, the modification of the MWCNT structure was not observed at the calcination temperature of 550 degrees C. The performance of calcined anodes for brine electrolysis was studied using a membrane cell, which showed that the output current density reduces with increasing calcination temperature. The results of the EIS analysis at oxygen evolution potential showed that the charge transfer resistance of IrO2-Ta2O5-MWCNT composite increases from 1.1 to 18.2 (Omega.cm(2)) due to gradual IrO2 crystallization, which illustrates a sharp reduction in the electrochemical OER activity.