Performance and cycle life of carbon- and conductive-based air electrodes for rechargeable Zn-air battery applications

The development of high-performance, cyclically stable bifunctional air electrodes are critical to the commercial deployment of rechargeable Zn-air batteries. The carbon material predominantly used as support material in the air electrodes due to its higher surface area and good electrical conductivity suffers from corrosion at high oxygen evolution overpotentials. This study addresses the carbon corrosion issues and suggests alternate materials to replace the carbon as support in the air electrode.;In this study, Sm0.5Sr0.5CoO3-delta with good electrochemical performance and cyclic lifetime was identified as an alternative catalyst material to the commonly used La0.4Ca 0.6CoO3 catalyst for the carbon-based bifunctional electrodes. Also, a comprehensive study on the effects of catalyst morphology, testing conditions on the cycle life as well as the relevant degradation mechanism for the carbon-based electrode was conducted in this dissertation. The cyclic life of the carbon-based electrodes was strongly dependent on the carbon support material, while the degradation mechanisms were entirely controlled by the catalyst particle size/morphology. Some testing conditions like resting time and electrolyte concentration did not change the cyclic life or degradation mechanism of the carbon-based electrode. The current density used for cyclic testing was found to dictate the degradation mechanism leading to the electrode failure.;An alternate way to circumvent the carbon corrosion is to replace the carbon support with a suitable electrically-conductive ceramic material. In this dissertation, LaNi0.9Mn0.1O3, LaNi 0.8Co0.2O3, and NiCo2O4 were synthesized and evaluated as prospective support materials due to their good electrical conductivity and their ability to act as the catalyst needed for the bifunctional electrode. The carbon-free electrodes had remarkably higher catalytic activity for oxygen evolution reaction (OER) when compared to the carbon-based electrode. However, the LaNi0.8Co0.2O 3-based electrode was unstable during OER after a short time period, while the LaNi0.8Co0.2O3 and NiCo2O 4-based electrodes were stable. The LaNi0.8Co 0.2O3-based electrodes had reasonable ORR activity but stability during oxygen reduction reaction (ORR) was limited due to the flooding of the electrode caused by the extremely hydrophilic nature of the perovskite electrode. On the contrary, the NiCo2O4-based electrode showed reasonable stability for both ORR and OER even during aggressive cyclic lifetime testing at a higher current density of 50 mA.cm-2 and could be a potential support material to replace carbon in bifunctional air electrodes for rechargeable Zn-air battery applications.