Development of Electrocatalysts for Oxygen Electrodes in Alkaline Electrochemical Systems

Abstract

In recent years as a larger proportion of our energy needs are being met by renewable energy sources, research and development in energy storage is becoming more significant. Oxygen electrodes, found in electrical energy storage applications such as fuel cells, water electrolysers and metal-air secondary batteries, face the demand for improved performance. In view of this, the research in this thesis focuses on the synthesis and development of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts to overcome the slow kinetics of the oxygen electrochemical reactions in alkaline media, followed by the investigation of their combined performance in a tri-electrode zinc-air secondary cell. The ORR performance of various transition metal oxides and carbonaceous materials was initially compared against benchmark catalyst Pt/C using rotating disc electrode measurements. Amorphous MnOx combined with Vulcan XC-72R was found to demonstrate high ORR activity and good stability over the range of cathodic current densities tested. The influence of the synthesis parameters of amorphous MnOx on its ORR activity was subsequently investigated and it was found that optimal amorphous MnOx catalyst can be synthesised with a molar ratio of MnO4-/ Mn2+ of 2.67, by adding KMnO4 to Mn(CH3COO)2 in a basic solution of pH 12 at 295 K. Similarly, the OER performance of transition metal oxides and hydroxides coated on metal mesh was compared and electrodeposited Ni-Fe hydroxide was reported to display high activity and durability when held at anodic potentials. Based on this, various compositions of Ni-based binary and Ni-Fe based ternary metal hydroxides were screened with a unique microelectrode set-up at high current densities up to 1 A cm-2. Ni-Fe-Co hydroxide showed most improved OER performance. The effect of electrodeposition parameters on the electrocatalytic performance of Ni-Fe-Co hydroxide were examined and used to further optimise the catalyst. Ni-Fe-Co hydroxide cathodically deposited at 300 mA cm-2 for 240 s at 22 ºC, pH 3.9 was found to demonstrate best OER performance, giving an overpotential of 235 mV at 0.1 A cm-2. The electrodes with optimised catalysts were tested in an in-house built zinc-air cycling set-up, demonstrating energy efficiencies of 58-61% up to 40 h at 20 mA cm-2 in 4 M NaOH + 0.3 M ZnO at 333 K.