Perovskite oxide electrode materials for energy conversion and storage
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The development of renewable energy conversion and storage technologies has become a leading focus of the fields of Materials Chemistry and Electrochemistry, with the ultimate goal of converting energy generated by renewable sources such as wind and solar into chemical fuels that can be stored and then utilized with clean energy conversion technologies. With energy densities primarily being a factor of fuel weight, metal-air batteries and fuel cell technologies are attractive candidates, having energy densities near gasoline. In addition, storage solutions that are both fast charging/discharging and have extended lifetimes are needed to implement renewable generation technology on a wide scale. In this vain, perovskite oxide catalysts were chosen as a model system to investigate both their applications as pseudocapacitor electrodes for storing energy and as air electrodes for both the reduction of oxygen and water electrolysis during the generation of energy. For energy storage, LaMnO [subscript 3±δ] was shown to be an active pseudocapacitor electrode, taking advantage of oxygen vacancies are charge storage sites that could intercalate oxygen anions from the electrolyte. For energy generation, a number of systems were studied. Firstly, the connection between the element in the active site and the chemical functionalities of the carbon support were identified using a number of perovskite systems, LaBO3 (B = Mn, Co, Ni, Ni₀.₇₅5Fe₀.₂₅), and high surface area carbons that were either unfunctionalized (reduced graphene oxide) or nitrogen-doped. Out of this work came the hypothesis of lattice-oxygen being an intermediate reactant during the OER and that the chemical functionalities of the carbon support were crucial to the ORR. This work was extended further by examining the system La₁₋ [subscript x] Sr [subscript x] CoO [subscript 3-δ] (0 ≤ x ≤ 1). Through a combination of rigorous characterization of the materials coupled with computation modeling, the connection was made between the covalency of the Co-O bond and access to an OER pathway utilizing lattice oxygen. On the ORR side, the connection between support interactions with the catalyst was extended to demonstrate that the ORR followed a 2-site series pathway with O₂ being reduced to HO₂⁻ on the carbon support and HO₂⁻ being further reduced to OH⁻ on the perovskite surface.