Synthesis and properties of nanocrystalline manganese-cobalt oxide coatings prepared by anodic electrodeposition

Anodic electrodeposition is a versatile and cost-effective technique to prepare nanocrystalline oxide coatings with distinct morphologies, cation distributions, oxidation states and defect chemistry. The aim of this thesis work is to develop Mn-Co oxide coatings by anodic electrodeposition for energy storage and conversion applications, such as electrode materials for electrochemical supercapacitors and protective coatings for ferritic stainless steel interconnects in solid oxide fuel cells (SOFCs).;The rock salt-to-spinel structural transformation occurred in the oxide nanocrystals annealed at a temperature of 500°C in air. The structural transformation involves the migration of Mn/Co cations from the octahedral interstices to the tetrahedral interstices, which is accompanied by the reduction of Mn/Co cations and oxygen evolution. The cation ordering process leads to unique magnetic properties. The Mn-Co oxide coatings were reduced into metallic Co and Mn3O4 phases when annealed in forming gas (5%H 2-95%N2). The forming gas pretreated Mn-Co oxide coatings not only acted as a protective barrier to reduce Cr outward diffusion, but also improved the electrical performance of the AISI 430 substrates.;The electrochemical performance of MnO2 nanocrystals with different crystal structures was evaluated. Improved frequency response was observed in cycled antifluorite MnO2, while no improvement in the impedance response was observed in the rock salt MnO2 and hexagonal epsilon-MnO 2 electrodes during charge-discharge cycles. A dissolution-redeposition mechanism is proposed to account for different impedance response of MnO 2 electrodes with different morphologies and crystal structures in terms of morphological and structural evolution.;The electrochemical process for the anodic electrodeposition of nanocrystalline coatings in Mn-Co-O system was investigated to understand the relationship between process parameters and the morphology/chemistry/crystal structure of the Mn-Co oxide coatings. The chemistry, morphology and crystal structure of the Mn-Co-O coatings can be manipulated by adjusting deposition parameters, including solution temperature, metal ion concentration, current density and complexing agent type. Cobalt-rich and crack-free Mn-Co-O coatings were successfully prepared as a protective coating for SOFC interconnects. MnO2 nanocrystals with three types of crystal structures were demonstrated as electrochemical supercapacitor electrodes: epsilon-MnO2 prepared from complex-free solutions, defective rock salt MnO2 from EDTA-containing solutions and defective antifluorite MnO2 from citrate-containing solutions.