Study Of Advanced Air-Cathodes For Magnesium-Air Batteries

Among all kinds of battery technologies,metal-air batteries?MABs?possess theoretical values of high specific energy densities.The energy densities of such metals can be comparable to that of gasoline.The extremely high specific energy densities of these MABs make them the most attractive battery technologies for sustainable energy storage & conversion applications.Despite their early beginning,none of the MABs at present is at a stage for large-scale industrial deployment.Among different types of MABs,primary aqueous MABs might be a short-term solution and are closer to replacing other conventional batteries for many highpower demanding applications if their practical energy/power densities and lifetime can be further enhanced in the future.Among different MABs,Magnesium-air?Mg-air?batteries are expected to have the second highest specific energy density and the second largest cell voltage.However,this battery technology currently is still at the research stage,thus has not fulfilled its full potential due to several challenges associated with the anode,cathode and the electrolyte.In Mg-air batteries,air-cathodes have been identified to be the major challenge towards the maturity of this battery and even other MABs due to the highly sluggish oxygen reduction reaction?ORR?at the air-cathodes.At the air-cathode,this ORR reaction mainly takes place at the triple-phase boundary where the solid electrode is simultaneously interfaced with liquid electrolyte and gaseous O₂.Therefore,developing active catalysts to expedite ORR kinetics and designing proper electrode architecture to enlarge the three-phase interface are critical and would greatly benefit the battery performance.Most MABs including Mg-air batteries have used an alkaline aqueous electrolyte.However,in cells containing alkaline aqueous electrolyte,concerns associated with both anode and cathode have to be addressed such as the passivating Mg?OH?₂ formed on the anode,and the precipitation of carbonate in the cathode caused by carbon dioxide adsorption from the air,which will form the interfacial film on the anode and block the electro-catalyst pores in the cathode catalyst layer,and eventually terminate the discharge process due to the high interfacial impedances.This thesis aims at overcoming these limitations by studying cost-effective nonnoble metal Co TMPP catalysts and associated air-cathodes under non-aggressive,safe and environmentally friendly neutral aqueous electrolyte.The synthesis method and effects of various carbon supports and heat-treatment temperatures on catalysts' physical properties and catalytic ORR activities are deeply studied.The ORR kinetic parameters and reaction mechanism of Co-based catalysts are proposed,which would enhance the understanding of Co-based catalysts for ORR in neutral electrolyte.The obtained results and conclusions would significantly contribute to the proper selection of carbon materials for non-noble metal catalyst complexes.A novel air-cathode based on the Co-based catalyst is designed,developed and fabricated for MABs,particularly in a neutral electrolyte.The advantages of the design method of forming the air-cathode decrease the internal electronic resistance and gas flow restriction of the system,and therefore increase air permeability as well as water transportation to the reaction sites.By using such an integrated structure of air-cathodes,the cost-effective air-cathodes in terms of materials and manufacture compared to the commercial ones and the overall fabrication procedure are achieved,and the method can be easily transferred into a continuous industrial manufacture process.For the characterization of the Co-based catalysts associated air-cathodes,a novel electrochemical half-cell is designed and fabricated for diagnosing,evaluating and analyzing their electrochemical performance of the aircathodes at operating conditions that are as close as possible to that of an actual MAB while offering a more controlled laboratory environment and economic way for down-selecting and optimizing the air-cathodes.