High entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries

dc.contributor.authorMongwe, Agnes Monosi
dc.contributor.supervisorOzoemena, Kenneth Ikechukwu
dc.contributor.supervisorHaruna, Aderemi B.
dc.date.accessioned2025-07-01T11:43:19Z
dc.date.issued2024-08
dc.descriptionA dissertation submitted in fulfilment of the requirements for the Master of Science degree, to the faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2024.
dc.description.abstractThe development of effective and stable rechargeable zinc-air batteries (RZABs) using noble-metal free bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has been a key challenge to its practical applications. This MSc research work strategically investigated some synthetic methods aimed at tuning the physicochemistry and electrochemical properties of two electrocatalysts (i.e., noble-metal free high entropy spinel oxide (HESOx) ((CoCuFeMnNi)3O4) and spinel Fe2CoO4 for rechargeable zinc-air batteries). For HESOx, a simple and reproducible Pechini method was used to synthesize a homogeneous nanosized electrocatalyst HESOx-550. The HESOx-550 was thereafter supported on onion-like carbon (OLC) in (1) an acidic environment to produce HESOx-550/OLCAT (where AT stands for acid-treated) and (2) a nonacidic environment to produce HESOx-550/OLC. The effects of the different synthesis environments on these three samples were thoroughly investigated using different analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA) and Nitrogen Gas Adsorption analysis. The Nitrogen Gas Adsorption analysis results show that the HESOx-550/OLCAT has the largest surface area and more volume. The electron paramagnetic resonance (EPR) and O1s XPS data consistently proved that HESOx-550/OLCAT has improved oxygen vacancies which are essential in improving conductivity and offering abundant reaction sites. The HESOx-550/OLCAT shows the best bifunctional ORR and OER electrocatalytic performance with a bifunctionality index (ΔE) of 0.70 V in 1 M KOH. In addition, the RZAB air electrode with HESOx 550/OLCAT exhibits high areal capacity (60 mAh cm-2) and areal energy density (73.2 mWh cm-2) with a long-term cycle stability over 112 h in 6.0 M KOH and 0.2 M zinc acetate. The HESOx-550/OLCAT RZAB shows better electrochemical performance than 10wt.% Pt/C- IrO2 when cycled over 315 h under 27% depth of discharge condition. For Fe2CoO4, iron cobalt-based electrocatalysts on Vulcan carbon support were synthesized using a simple reduction method to produce two composites (FeCo-Fe2CoO4/CAnnealed), and (FeCo Fe2CoO4/CMicrowave). The physicochemical analytical methods such as XRD, XPS, Raman, TGA and Nitrogen Gas Adsorption analysis were used to investigate the samples. The electrochemical analysis showed that the FeCo-Fe2CoO4/CAnn had a very low “bifunctionality index” (ΔE) of 0.76 V and the FeCo-Fe2CoO4/CAnn air cathode RZAB demonstrated good stability for over 50 h under harsh DOD conditions (35.2%). The assembled RZABs have areal energy densities of 48.4 mWhcm-2 and 60.5 mWhcm-2 which are higher than the minimum recommended areal energy density of 35 mWhcm-2 (and better than most electrocatalysts reported in the literature). This study has significant contributions to the progress of practical applications of RZABs.
dc.description.sponsorshipSasol-NRF
dc.description.submitterMMM2025
dc.facultyFaculty of Science
dc.identifier0000-8617-0569
dc.identifier.citationMongwe, Agnes Monosi. (2024). High entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries. [Master's dissertation, University of the Witwatersrand, Johannesburg]. WIReDSpace. https://hdl.handle.net/10539/45305
dc.identifier.urihttps://hdl.handle.net/10539/45305
dc.language.isoen
dc.publisherUniversity of the Witwatersrand, Johannesburg
dc.rights©2024 University of the Witwatersrand, Johannesburg. All rights reserved. The copyright in this work vests in the University of the Witwatersrand, Johannesburg. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of University of the Witwatersrand, Johannesburg.
dc.rights.holderUniversity of the Witwatersrand, Johannesburg
dc.schoolSchool of Chemistry
dc.subjectOxygen reduction reaction (ORR)
dc.subjectOxygen evolution reaction (OER)
dc.subjectRechargeable zinc-air batteries (RZAB)
dc.subjectDepth of discharge (DOD)
dc.subjectAreal energy density
dc.subjectHigh entropy spinel oxide (HESO)
dc.subjectBifunctional electrocatalysts
dc.subjectUCTD
dc.subject.primarysdgSDG-9: Industry, innovation and infrastructure
dc.subject.secondarysdgSDG-13: Climate action
dc.titleHigh entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries
dc.typeDissertation

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