SYNTHESIS OF MIXED METAL OXIDE BASED THIN FILMS A SUITABLE ALTERNATIVE FOR ELECTRO-OXIDATION OF METHANOL IN DIRECT METHANOL FUEL CELL

Abstract

The present work focused on the fabrication of cost friendly electro-catalysts for the oxidation of methanol with enhanced efficiency. NiO, ZrO2, Y2O3, NiO-ZrO2/FTO and NiOY2O3/ FTO thin films were deposited on FTO glass substrate via simple deposition method. Different characterization techniques were used to characterize the prepared thin films. SEM confirmed the uniformity and porosity while EDS confirmed the elemental composition of the prepared thin films. Other techniques like XRD, Raman spectroscopy and FTIR revealed the crystallinity phases and successful formation of mixed metal oxide thin films without any additional impurities. Electrocatalytic behaviour of prepared thin films towards methanol oxidation was studied using potentiostat based on three electrode system, in 0.5M NaOH electrolyte solution at scan rate of 100 mV/s, using Pt wire as counter electrode, Ag/AgCl as reference electrode and prepared metal oxide composite thin film as working electrode, providing potential range of - 0.5 to 2.0V. Different electrochemical techniques like cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA) studies were performed to evaluate the efficiency of prepared metal oxide thin films for methanol oxidation. NiO-Y2O3/FTO thin film showed current density value of 7 mA/cm2 vs. 0.65 V with 0.8 M methanol solution at scan rate of 100mV/s. NiO-ZrO2/FTO thin film showed current density value of 10 mA/cm2 vs. 0.65 V with 0.6 M methanol solution at scan rate of 100mV/s. Comparatively, pure NiO,Y2O3 and ZrO2observed no prominent oxidation peaks which is attributed to the synergistic effect between metal oxides in mixed metal oxide thin films (NiO-Y2O3/FTO and NiO-ZrO2/FTO) . Furthermore, it was found that with increase in methanol concentration and scan rate, current densities of the prepared thin films increases, which inferred that oxidation of methanol is diffusion-controlled process and increase in scan rate increases electron transfer process which ultimately enhances the peak current density. Concentration studies revealed that thin films with 0.1: 0.1 (NiO: Y2O3/ZrO2) precursor concentration showed best results with oxidation peak at the lowest oxidation potential and lowest onset potential. Current-time plot was obtained to check the stability of prepared thin films towards methanol oxidation. NiOZrO2/ FTO and NiO-Y2O3/FTO thin films observed current decay of 10% and 6.8% of initial current density values. This decay may be due to the reduction of methanol concentration near electrode surface with time or due to accumulation of carbonaceous reaction intermediates like CO on electrode surface. Electrochemical impedance studies were conducted to further evaluate the electrocatalytic performance of prepared films. Nyquist plot of NiO-ZrO2/FTO showed decreased Rct value of 34.14Ω compared to pure NiO and ZrO2 with Rct values of 88.42Ω and 2779Ω, respectively. Nyquist plot of NiO-Y2O3/FTO showed decreased Rct value of 22.8Ω compared to pure NiO and Y2O3 with Rct values of 88.4Ω and 3189Ω, respectively. These mixed metal oxide thin films showed enhanced catalytic performance, high stability and reusability which make them the best alternatives for methanol oxidation reactions.