title: Heterostructured Electrocatalysts for Oxygen Electrode in Rechargeable Zinc-Air Batteries
Creators: name: Dong, Mengyang
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Zinc-Air Batteries
subject: Oxygen Electrode
subject: Heterostructured Electrocatalysts

description: Nowadays, under the background of environmental pollution and energy crisis, with the continuous development of various forms of new energy, energy conversion and storage devices are essential to the utilization of renewable energy. Among them, clean battery technology is developing rapidly. Compared with traditional batteries including lithium batteries, Zn-air batteries have unique advantages and face significant development opportunities due to their high theoretical energy density, safety and environmental protection. However, as a secondary battery, the rechargeable Zn-air battery is charged and discharged at the air cathode. The high overpotential and slow kinetic process of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which occur respectively, seriously affect the actual efficiency of the battery, which has become an essential obstacle for the commercial use of Zn-air batteries. In order to explore the excellent theoretical performance of Zn-air batteries, it is an essential mean to use electrocatalysts with ORR/OER dual catalytic function to improve the cathode reaction efficiency. Based on this, this thesis reviews the structure, reaction mechanism, structure and performance characteristics of various oxygen reaction electrocatalysts of Zn-air batteries, and analyzes the material factors affecting the performance of catalysts. Utilizing strategies such as porous carbon, heteroatom doping, transition metal oxides, single-atom modulation, and defect control, a series of composite materials with high ORR/OER dual catalytic activity were prepared. The structure, electrochemical properties and catalytic performance of the materials in Zn-air batteries were studied. In addition, this thesis also investigates the application of catalysts in miniature wearable solid-state Zn-air batteries combined with various new battery configuration designs. In this dissertation, a series of transition metal carbon-based bifunctional catalysts with excellent performance were prepared based on various building elements. However, the analysis and research in this thesis revealled the fine materials science contents such as the electronic state distribution of the adsorption and bonding of heteroatom-doped carbon to oxygen reaction intermediates, and the regulation pathway of adsorption sites caused by defects. Understanding these contents is very important for explaining the structure-activity relationship of catalytic materials. Studying a bifunctional catalyst for Zn-air batteries that can work stably and maintain a terrific constant charge-discharge potential gap, withstanding high current density, will eventually lead to a revolution in the battery industry. However, most of the reported electrocatalysts in high-rate Zn-air batteries to date do not have enough durability, suffering from unstable nanostructures, poor electrical conductivity, low active sites, and high overpotentials. In view of this, it is the ultimate proposition of the road to Zn-air batteries applications to pursuit ultrastable and cheap catalysts that can alleviate particle aggregation, have abundant active sites and low resistance. In addition, compared with liquid Zn-air batteries, the performance of all-solid-state Zn-air batteries in various reports, is often poor, which originates from the insufficient water retention and conductivity of solid-state electrolytes, improper catalyst loading method and battery configuration. Various factors such as design, especially the research and development of new solid-state electrolytes, are of great significance for the development of wearable rechargeable Zn-air batteries with both flexible mechanical properties and charge-discharge efficiency. In order to solve the above problems and these drawbacks of Zn-air batteries, different characterization techniques are used to determine the similarity or commonality of composite electrocatalysts with high efficiency and activity. The main research contents are as follows: (1) Spinel-type metal oxides, as a group of the transition metal oxides, are considered as one of the most promising bifunctional oxygen electrocatalysts due to its unique electronic structure, mixed metalic valence centres with redox behaviours, abundance and environmental friendliness. In the first work, a facile one-step hydrothermal method is reported for the synthesis of a high-performance bifunctional oxygen electrocatalyst, cobalt-doped Mn3O4 nanocrystals supported on graphene nanosheets (Co-Mn3O4/G). Compared to pristine Mn3O4, this Co-Mn3O4/G exhibits greatly enhanced electrocatalytic activity, delivering a half-wave potential of 0.866 V for the ORR and a low overpotential of 275 mV at 10 mA cm−2 for the OER. The Zn-air battery built with Co-Mn3O4/G shows a reduced charge–discharge voltage of 0.91 V at 10 mA cm−2, a peak power density of 115.24 mW cm−2 and excellent stability without any degradation after 945 cycles (315 h), outperforming the state-of-the-art Pt/C–Ir/C catalyst-based device. This work offers an efficient strategy to synthesize spinel-type complex oxide materials in high-performance bifunctional oxygen electrocatalyst areas. (2) In order to make the Zn-air batteries work well at a high current, structural optimization is imperative. In the second work, a rapid seeding synthesis strategy is reported for the fabrication of impregnated Co3O4-based carbon ultra-thin nanosheets (Co3O4/C-NS) architecture induced by CoMOF as a bifunctional electrocatalyst. The impregnated ultra-thin nanosheets network would provide prolific pathways for efficient mass transfer, which allows the inner active sites to be accessible to electrolyte and oxygen. Additionally, the MOF-derived carbon matrix would suppress the aggregation of Co3O4 nanoparticles and increase the stability of the catalyst during the high-density charge/discharge cycling. Our Co3O4/C-NS exhibits uniform morphology, high specific area, low internal resistance, and superior ORR and OER activity to the benchmark Pt/C and Ir/C, respectively. Furthermore, the Zn-air batteries fabricated with the assynthesized electrocatalyst afford remarkably stable charge/discharge at a high current density of 25 mA cm-2, surpassing most of the previously reported catalysts. The material engineering approach highlighted herein exemplifies a facile yet effective avenue towards stable, efficient and robust non-noble metal-based electrocatalysts. (3) Single-atom catalysts (SACs) have attracted great interest in the field of catalysis, mainly because SACs not only possess the advantages of homogeneous and heterogeneous catalysts, but also possess some unique properties. In the third work, NiCo-LDH with electrocatalytic Ni and Co were grown on Ni, Co-codoped, hierarchically ordered macroporous carbon (NiCo-LDH@NiCo-SAs/OPC) derived from pyrolysis of ZIFs via a facile method. The strong coupling between NiCo-LDH and NiCo- SAs/OPC not only sharply facilitates the electron transfer but also result in high chemical stability against the corrosion during charging and discharging processes. Additionally, interconnected hierarchically porous structures were involved in NiCo-SAs/OPC via introducing removable templates, which would serve as channels to accelerate mass transport (O2 and electrolytes) during electrochemical steps. The obtained hierarchically porous NiCo-LDH@NiCo-SAs/OPC possesses abundant atomically dispersed Ni-Nx, Co-Nx sites and continuous species/charge transport channels, and exhibits good bifunctional ORR/OER electrocatalytic performance, which is superior to the corresponding noble metals Pt/C and RuO2 catalysts. More importantly, the rechargeable Zn metal-air batteries assembled with NiCo-LDH@NiCo-SAs/OPC also exhibited good charge-discharge performance and long-term stability. (4) Alloy-based electrocatalysts have been studied as bifunctional catalysts for ORR/OER for a long-time. In the fourth work, NiCo bi-alloy particles are used to embed onto the carbonized MF framework (NiCo@CMF), which has shown excellent performance, providing a new idea for designing other non-precious metal ORR/OER bifunctional electrocatalysts. More importantly, NiCo@CMF electrode can be processed into various shapes, furthermore, the assembled Zn-air battery shows pretty good flexibility during application as well as an appreciable charge-discharge voltage gap. While maintaining high-efficiency battery performance, the battery exhibits excellent bending mechanical properties. These works provide the power supply for nextgeneration smart wearable devices. The unique machinable NiCo@CMF electrode will have many potential applications, providing more possibilities for the design of wearabletype Zn-air batteries, and the cost-effectiveness of the NiCo@CMF electrode allows it to be fabricated on a large scale, providing a more economically viable avenue to the Zn-air batteries technology. This strategy can even be extended to other wearable devices for wider promotion.