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TY  - THES
AU  - Wang, Zhiyuan
TI  - Oxygen reduction reaction and oxygen evolution reaction mechanisms investigation of the non-noble bifunctional electrocatalysts in alkaline electrolyte
PB  - RWTH Aachen University
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2018-223399
SP  - 1 Online-Ressource (xv, 139 Seiten) : Illustrationen
PY  - 2018
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, RWTH Aachen University, 2018
AB  - Metal-air batteries have attracted plenty of attentions because of their high theoretical energy densities. They are considered to be the next generation of energy storage devices for the electric vehicles. However, right now there still some challenges restrict the practical application of rechargeable metal-air batteries, the main challenges is the intrinsic sluggish reaction kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during discharge and charge, resulting in a limited practical energy densities. Because extra energy required overcoming the high-activation barriers, which limit the energy conversion in an electrochemical process, the extent of the barrier is defined by the overpotential. The overpotentials from both ORR and OER significantly lead to the low power output and poor round-trip efficiency of rechargeable metal-air batteries. Therefore, development of efficient bifunctional and cost effective electrocatalysts for ORR and OER is the critical issue for rechargeable metal-air batteries. Transition metal oxides and transition metal oxide-carbon hybrids are found to be the promising candidate as bifunctional electrocatalysts among plent of catalysts. The electrocatalytic activity of the catalyst generally depends on the electronic structure properties of the catalyst, which in turn is impacted by the defect structure of the material. In this thesis, a series of MnOx/C composites and perovskite structured (La0.65Sr0.3)xFeO3-δ catalysts with different A-site stoichiometry are designed and prepared at different temperatures, their properties are investigated by several characterization methods. The influence of the preparation temperature on the properties of the MnOx/C composites, and the influences of the preparation temperatures and A-site stoichiometry on the properties of the perovskite structured (La0.65Sr0.3)xFeO3-δ catalysts are studied in detail. The MnOx/C 120°C composite performs a better ORR and OER activity than the other two composites, because of its smaller particle size, larger electrochemical active surface area (ECSA), more content of reactive active sites and more efficient electron transfer pathway. The ORR and OER mechanisms on the surfaces of the MnOx/C composites are investigated by rotating ring disk electrode (RRDE) technique. Most of the O2 is reduced through a direct 4-electron pathway catalytic by the MnOx/C composites; only a small part of O2 is reduced through an indirect 2×2-electron pathway with the generation of intermediate H2O2. The OER catalyzed by the MnOx/C composites are through direct 4-electron pathway. For the LSF catalysts with the same A-site stoichiometry, the higher ORR and OER activity are obtained at lower calcination temperature; because the content of reaction active site and the content of oxygen vacancy are decreasing with the increasing of the calcination temperature. For the LSF catalysts with same calcination temperature, the best ORR activity is obtained at moderate content of oxygen vacancy. While the best OER activity is obtained at highest content of oxygen vacancy. Their ORR and OER mechanisms are also investigated by RRDE technique. All of the LSF catalysts possess two ORR potential ranges. The first ORR range is in the low potential values, and the O2 is reduced through the indirect 2×2-electron pathway. In the second ORR range with high potential values, the O2 is reduced through the direct 4-electron pathway. The selection of the two ORR mechanisms on the surface of the transition metal oxide is mainly decided by the electron transport efficiency and the O2 adsorption model on the active site. The OER catalytic by LSF catalysts are through 4-electron pathway. This direct 4-electron pathway on the surface of the transition metal oxide is occurred under the synergistic effect of two-reaction active sites of M3+. The electrocatalytic activities of the LSF catalysts towards ORR and OER are affected seriously by the conductivities, adding some highly conductive carbon into the LSF catalysts can improve their ORR and OER activities, because the electron transport during the ORR and OER is important for the catalytic activity of the material. In this work, the influence factors of the transition metal oxide and transition metal oxide/carbon hybrid towards ORR and OER are found, and their mechanisms of ORR and OER in alkaline media are studied.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2018-223399
UR  - https://publications.rwth-aachen.de/record/722099
ER  -