Browsing by Subject "Oxygen evolution"
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Item Design of nanocomposites for electrocatalysis and energy storage : metal/metal oxide nanoparticles on carbon supports(2012-08) Slanac, Daniel Adam; Johnston, Keith P., 1955-; Stevenson, Keith J.; Mullins, Charles C.; Korgel, Brian A.; Ferreira, Paulo J.Controlling catalyst morphology and composition are required to make meaningful structure-activity/stability relationships for the design of future catalysts. Herein, we have employed strategies of presynthesis and infusion or electroless deposition to achieve exquisite control over catalyst composite morphology. The oxygen reduction (ORR) and the oxygen evolution reactions (OER) were chosen as model systems, as their slow kinetics is a major limiting factor preventing the commercialization of fuel cells and rechargeable metal air batteries. In acid, bimetallic (Pt-Cu, Pd-Pt) and monometallic (Pt) catalysts were presynthesized in the presence of capping ligands. Well alloyed Pt-Cu nanoparticles (3-5 nm) adsorbed on graphitic mesoporous carbon (GMC) displayed an ORR activity >4x that of commercial Pt. For both presynthesized Pt and Pt-Cu nanocrystals on GMC, no activity loss was also observed during degradation cycling due to strong metal-support interactions and the oxidation resistance of graphitic carbon. Similar strong metal-support interactions were achieved on non-graphitic carbon for Pd3Pt2 (<4 nm) nanoparticles due to disorder in the metal surface This led to enhanced mass activity 1.8x versus pure Pt, as well as improved stability. For basic electrolytes, we developed an electroless co-deposition scheme to deposit Ag (3 nm) next to MnOx nanodomains on carbon. We achieved a mass activity for Ag-MnOx/VC, 3x beyond the linear combination of pure component activities due to ensemble effects, where Ag and MnOx domains catalyze different ORR steps, and ligand effects from the unique electronic interaction at the Ag-MnOx interface. Activity synergy was also shown for Ag-Pd alloys (~5 nm), achieving up to 5x activity on a Pd basis, resulting from the unique alloy surface of single Pd atoms surrounded by Ag. Lastly, we combined arrested growth of amorphous nanoparticles with thin film freezing to create a high surface area, pure phase perovskite aggregate of nanoparticles after calcination. Sintering was mitigated during the high temperature calcination required to form the perovskite crystals. The high surface areas and phase purity led to OER mass activities ~2.5x higher than the benchmark IrO2 catalyst.Item Investigation of lithium cobalt oxide under oxygen evolution reaction conditions(2015-05) Colligan, Nora Jean; Manthiram, Arumugam; Hwang, GyeongMetal-air batteries are drawing much attention as the active material O₂ could be directly used from air. The fundamental electrochemical reactions occurring in metal-air batteries are the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which require the use of efficient catalysts to lower the overpotential and improve the efficiency. Many less expensive oxide catalysts are gaining much attention, but the mechanisms involved are still not well understood. The electrocatalytic OER performance of various forms of lithium cobalt oxide has been studied to systematically establish the surface level catalytic mechanisms. The low-temperature lithiated spinel form of LiCoO₂ (designated as LT-LiCoO₂) exhibits lower overpotentials than the high-temperature layered form of LiCoO₂ (designated as HT-LiCoO₂), but this is shown to be a result of the increased surface area afforded by the lower-temperature synthesis conditions. Raman spectroscopy, along with the presence of an irreversible peak during the first cycle of the OER, demonstrates that the mechanism for OER is the same for both the forms of LiCoO₂. At the surface level, lithium is removed during the first cycle of the OER, forming Co₃O₄ on the surface, which is likely the active site during the OER. This work highlights the importance of determining the nature of the catalyst surface when investigating the electrocatalytic properties of bulk materials.Item Synthesis and systematic study of Co₃O₄-based catalysts for oxygen reduction and oxygen evolution reactions(2015-05) Liu, Siyang, M.S.E.; Manthiram, Arumugam; Yu, GuihuaCo₃O₄-based composite materials are good electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in alkaline solutions. Here, this thesis first investigated the individual functionality of Co₃O₄ and the N-doped carbon nanoweb (CNW) in ORR and OER. The Co₃O₄/CNW bifunctional catalysts were synthesized by an in situ growth of Co precursors onto CNW followed by a controlled heat treatment. Rotating disk electrode measurements were utilized to provide insight into the specific functions of Co₃O₄ and CNW in the composite material during catalysis. It was found that Co₃O₄ alone exhibited poor ORR catalytic activity. However, in the presence of CNW, Co₃O₄ assisted the selective four-electron oxygen reduction over the two-electron pathway. Co₃O₄ acted as the primary catalytic site for OER and CNW improved the electronic conduction between Co₃O₄ and the current collector. CNW underwent serious degradation at the high potential of the OER, but its stability improved greatly upon the deposition of Co₃O₄. Two possible mechanisms for the improved catalytic stability are discussed. The findings demonstrate the specific functions of Co₃O₄ and CNW in catalyzing the OER and ORR and further establish an understanding of the synergy of the composite in electrocatalysis. Based on the critical functionality of Co₃O₄ in stabilizing carbon materials in the OER potential region, it is of interest to investigate novel synthesis methods to prepare nano-sized Co₃O₄ that can provide more active sites for catalytic reactions and thus, improve the OER kinetics. Here, in situ electrochemical generation of 2-dimensional Co₃O₄ (2D-Co₃O₄) nanoplates were achieved by scanning CoO[subscript x]/Co precursors in 1 M KOH solution. X-ray diffraction characterization suggested that CoO[subscript x]/Co precursors were oxidized to Co₃O₄ before the onset potential of OER. Scanning electron microscopy showed that oxidation from CoO[subscript x]/Co to 2D-Co₃O₄ was associated with the formation of hexagonal nanoplates. The 2D-Co₃O₄ exhibited excellent OER catalytic activity and stability probably due to the effective mass transfer through the 2D structure.