- 242 views
- 267 downloads
Gas Diffusion Layers Impregnated with Metal Oxide Decorated and Nitrogen-Doped Carbon Nanotube Catalysts for Electrically Rechargeable Zn-Air Batteries
-
- Author / Creator
- Aasen, Drew
-
Secondary zinc-air batteries (ZABs) have garnered interest in recent years as a promising technology for energy storage due to their minimal safety concerns, low cost, and a high theoretical energy density. However, many issues still need to be resolved for commercialization of ZABs. Many of these issues are associated with the air electrode, such as the slow kinetics of the oxygen reduction and oxygen evolution reactions (ORR and OER, respectively) and poor cycle life. The former has been addressed previously through the use of noble metal catalysts such as Pt, Ru, and Ir, as well as their oxides. However, these catalysts are expensive and suffer from performance degradation during battery cycling. As the battery is cycled, electrolyte will flood the electrode and pass through the catalyst layer, resulting in performance losses. Therefore, development of low-cost catalysts and a simple electrode preparation technique to help mitigate the effects of flooding is desired. This work focusses on the development of impregnated air electrodes using transition metal (Mn, Co, Fe, and Ni) oxide decorated, nitrogen-doped carbon nanotube (N-CNT) catalysts to improve the performance and cycling efficiency of ZABs. The effect of the impregnation technique as a form of electrode preparation was investigated through cross sectional scanning electron microscopy (SEM) and electrochemical tests such as galvanostatic charge and discharge rate tests (battery rate tests), as well as linear sweep voltammetry (LSV). The N-CNT supported catalysts were characterized using SEM, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the catalysts was evaluated using cyclic voltammetry (CV), LSV, electrical impedance spectroscopy (EIS), and battery rate tests. The best performing samples were then selected for use in an electrically rechargeable ZAB.The first study involved the impregnation and synthesis of air electrodes using Mn3O4 decorated N-CNT catalysts for ORR. The catalysts were prepared using a simple precipitation method. The air electrode was simultaneously impregnated with the catalysts during synthesis through soaking and vacuum filtration. The impregnated electrode showed superior performance to electrodes prepared by conventional spray coating. Furthermore, the Mn3O4/N-CNT impregnated electrodes had superior ORR performance to other Mn3O4 catalysts from the literature, as well as similar ORR performance to commercially available Pt-Ru/C catalysts. The electrode was coupled with electrodeposited Co-Fe on Ni foam and was cycled in a tri-electrode cell. The tri-electrode cycling performance was comparable to that of Pt-Ru/C under the same tri-electrode cycling conditions. The second study involved the preparation of bimetallic (Co,Fe)3O4 decorated N-CNTs as a bifunctional and highly stable catalyst for ZABs. The catalyst and electrode preparation were achieved again by simultaneous precipitation and impregnation. Characterization through TEM and XPS indicated mixed valence for both Co and Fe in a spinel oxide phase. Electrochemical testing of the (Co,Fe)3O4/N-CNT impregnated electrodes showed comparable ORR activity and superior OER activity to Pt-Ru/C at 20 mA cm-2. Bifunctional cycling of (Co,Fe)3O4/N-CNT at a current density of 10 mA cm-2 exhibited exceptional stability with a discharge/charge efficiency of 58.5% after 200 cycles (100 h), which compared favorably with Pt-Ru/C under the same conditions (55.3% after 200 cycles). The third study investigated the catalytic performance of transition metal based bimetallic and trimetallic oxides on N-CNTs for ZABs. Using the developed synthesis and impregnation technique, 6 bimetallic oxides (Mn-Co, Co-Fe, Mn-Fe, Ni-Co, Ni-Fe, Ni-Mn) and 3 trimetallic oxides (Ni-Co-Fe, Ni-Mn-Fe, Mn-Co-Fe) were synthesized on N-CNTs. Ni-Mn oxide on N-CNTs (NiMnOx/N-CNT) had the best ORR performance of the bimetallic oxides, while Ni-Fe oxide on N-CNTs (NiFeOx/N-CNT) and (Co,Fe)3O4/N-CNT had the best OER performance and bifunctional performance, respectively. The trimetallic oxide systems were selected based on the battery rate test performance and discharge/charge efficiencies of the bimetallic oxide catalysts. Electrochemical testing of the trimetallic oxides on N-CNTs showed improved activity towards OER when compared with the bimetallic oxides. Furthermore, the trimetallic oxides had similar ORR performance to Pt-Ru/C and superior OER performance in battery rate tests. Bifunctional cycling of the trimetallic oxide catalysts showed good cycling stability and superior efficiencies to Pt-Ru/C after 200 cycles (100 h) at a current density of 10 mA cm-2. Ni-Co-Fe oxide on N-CNTs (NCFO/N-CNTs) had the best cycling performance of the trimetallic oxide catalysts and the best OER activity of all 10 catalysts tested in the study. Bifunctional cycling of NCFO/N-CNT at a current density of 20 mA cm-2 demonstrated better cycling efficiency than Pt-Ru/C after 100 cycles (53.2% vs 41.3%, respectively).
-
- Subjects / Keywords
-
- Graduation date
- Spring 2020
-
- Type of Item
- Thesis
-
- Degree
- Master of Science
-
- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.