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

Abstract IrO 2 –Ta 2 O 5 multi-wall carbon nanotube (MWCNT) composite coatings were synthesized on Ti electrodes at different calcination temperatures from 350 to 550 °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 IrO 2 crystallinity significantly affects the coating properties. XRD pattern of the coating prepared at 350 °C has shown no reflection peaks indicating that the IrO 2 and Ta 2 O 5 were amorphous. The DSC-TGA showed two major exothermic peaks at 475 and 575 °C attributed to the crystallization of IrO 2 and oxidation of Ta 2 O 5 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 °C, and they transform into a graphite-like structure by crystallization of IrO 2 . However, in the coating without IrO 2 , the modification of the MWCNT structure was not observed at the calcination temperature of 550 °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 IrO 2 –Ta 2 O 5 -MWCNT composite increases from 1.1 to 18.2 (ꭥ.cm 2 ) due to gradual IrO 2 crystallization, which illustrates a sharp reduction in the electrochemical OER activity.