Investigation of Perovskite/Spinel Oxides−Based Oxygen Electrocatalysts for Electrochemical Energy Storage and Conversion

• Author / Creator

  Zhang,Yaqian

• Energy storage and conversion devices, such as fuel cells, metal-air batteries (MABs) and water electrolyzers, provide significant approaches for renewable energy storage, which address the challenge of intermittent generation from renewable energy system. However, the practical application of fuel cells and/or MABs is often hampered by the sluggish kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). The development of efficient, stable and low-cost oxygen electrocatalysts is critical to realize the practically viable electrochemical devices, and to advance those sustainable technologies. So far, noble-metals-based electrocatalysts, such as Pt, Ir, Ru, have been exclusively used as electrochemical catalysts in oxygen reaction and hydrogen reaction. Although the precious metal-based-electrocatalyst demonstrated remarkable catalytic activities, their high cost and limited availability have impeded their practical application. For transition metal oxides to become more competitive oxygen reduction reaction (ORR) catalysts, substantial progress is required to advance their enhanced catalytic activity and durability. I demonstrated a novel method to fabricate high-performance perovskite electrocatalyst via combining nano architecture designs, internal structures engineering, and Ag nanoparticles (NPs) in situ exsolution. The synthesized Ag‑(PrBa)0.95Mn2O5 (Ag-PBMO5) catalyst exhibits favorable ORR catalytic activity in terms of overpotential (Eonset ~ 0.92 V vs RHE and E1/2 ~ 0.81 V vs RHE), and outperforms the Pt/C with respect to durability. Several characterization techniques, including transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) etc., were applied alongside density functional theory calculations to understand the possible active sites and the synergistic coupling effects that contributed to the high ORR performance. The introduction of a secondary electrocatalyst (Ag NPs) through in situ exsolution leads to significant ligand effect and facilitates electron transfer and ion migration within the oxygen reduction reaction.Inspired by the optimal performance achieved on perovskite oxides with multicomponent and sufficient active centers through in-situ exsolution, further effort was made towards this direction in developing cobalt phosphide - PrBa0.5Sr0.5Co1.5Fe0.50O5+δ (CoP-PBSCF). The CoP-PBSCF was successfully prepared via an in-situ exsolution and post phosphatization process. The integration of CoP and perovskite oxides endowed synergistically active sites, which subsequently contributed to extended functionalities and better activities. The CoP-PBSCF demonstrates a significantly enhanced trifunctional electrocatalytic activity towards ORR/OER and hydrogen evolution reaction (HER). The as-synthesized multifunctional electrocatalysts have been successfully applied for the application of both Zn-air batteries and overall water splitting.Besides non-stoichiometric perovskite oxides, other alternative oxygen electrocatalysts could be spinel oxides. Ultrafine sub-10 nm MnFe2O4 crystals were grown on the ultrathin NiCo2O4 nanosheets, leading to a highly effective surface area and a strong cooperative effect. The distinct architecture and complex composition afford an excellent bifunctional oxygen electrocatalytic activity in alkaline condition. The practical rechargeable Zn-air battery with the resulting hybrid (MnFe2O4/NiCo2O4) electrocatalyst demonstrates a high round-trip efficiency (a low discharge-charge voltage gap of 0.81 V at a reversible current density of 10 mA cm-2) and an outstanding durability, which outperforms the commercial bifunctional Pt/Ru/C electrocatalyst. This work holds the promise to open a new possibility in designing novel transition metal based bifunctional catalysts as the alternatives to the noble metals for the application in energy related devices.

• Subjects / Keywords

  • in situ exsolution
  • oxygen evolution reaction
  • oxygen reduction reaction
  • oxygen electrocatalyst
  • perovskite oxides

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3862BT5G

• License

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