Guan, Bowen; (2023) In situ modification of reduced graphene oxide for energy storage applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The use of renewable energy requires energy storage technologies, for instance to improve the efficiency and reliability of power grids and the application in electric vehicles, both require an effective and susceptible energy storage device. Efficient electrochemical energy storage systems have therefore received intensive research effort over the past few decades, with a focus on batteries and supercapacitors due to their rapid and consistent energy output, adaptability under different operation conditions and low carbon footprint compared to conventional fossil fuels. Since the emergence of graphene in 2004,1 it has gained the interest of many in the field of energy storage due to its high electric conductivity, mechanical and chemical stability, high specific surface area among other desirable properties.2, 3 However, the direct synthesis of graphene such as physical exfoliation and chemical vapor deposition (CVD) are extremely inefficient and costly. 4 An alternative method to produce graphene in large quantity is via the reduction of graphene oxide (GO), which is a synthesis technique that has been adopted by many researchers to obtain high quality graphene.5 The goal of this study is to design an electrode material for supercapacitors (SCs) and potassium ion batteries (PIBs) that can fulfil a wide range of applications, from electric vehicles to power grids using one single material as precursor for the electrodes of SCs and PIBs, graphene oxide (GO). Hence, simplifying the entire manufacturing process and improving the economies of scale. Some of the key criteria that dictates the overall performance of SCs and PIBs include capacitance, energy/power density, life cycle, equivalent series resistance (ESR) among others, the study has been conducted in pursuance of improving the above key aspects of SCs and PIBs by performing chemical (chemical activation and transitional metal doping) and physical modifications (morphological and structural modifications during the reduction proces) on GO during the reduction process. To achieve a facile and lowcost fabrication of electrodes, all research works were based on the different alterations performed using GO as the single precursor that has undergone different reduction conditions to obtain the resultant reduced graphene oxide (rGO), which is the resultant material used for the electrodes of energy storage devices. All the research works were based on reductions methods with simple and cost-effective methods and any materials used for chemical modifications were chosen based on their cost effectiveness and environmental friendliness, such as transitional metal oxides. The thesis is comprised of an introduction and research methodology of SCs and PIBs followed by three major research works listed as follows: (1&2) A general introduction about SCs and PIBs, describing their working principles, the different types of materials used as electrodes and electrolytes, characterization techniques and methodology of this study. (3) It was demonstrated in this work, a facile synthesis method of multivalence manganese oxide (MnOx) on activated reduced graphene oxide (A-rGO) via a simple one pot hydrothermal method for supercapacitor (SC) electrodes. Potassium permanganate (KMnO4) was used as both the precursor for the growth of MnOx as well as acting as an activation material for the rGO planes. The as synthesized MnOx/A-rGO was coated onto a nickel foam and undergone a three-electrode system electrochemical analysis using 1M KOH as the electrolyte. The best performing sample have shown a specific capacitance of 473.9 A g-1 at 1A g-1, with a capacitance retention of 99% after 4000 cycles at 5A g-1 outperforming most of the manganese-graphene composite materials for SC applications. In the broader context, the MnOx-ArGO was synthesized using a facile one-pot method that can be produced at large scale in an economic feasible way and has showed optimal energy storage and cycling performances due to the enhanced porosity and pseudocapacity, enabling it to be used as a potential material for supercapacitor electrodes. (4) Potassium-ion batteries (PIB) are viewed as one of the potential replacements for lithium-ion batteries (LIB) due to their affordability and availability. Similar to LIBs, carbon is among the most researched anode materials for PIB applications. Despite this, the vast majority of carbon materials have poor rate performance and cyclability due to the considerable volume expansion of the carbon layers, which causes irreversible structural breakdown. By thermally annealing crumpled graphene oxide (CGO) and graphene oxide (GO) to form reduced crumpled graphene oxide-graphene oxide (rCGO-rGO) mixtures, we have produced a 3D/2D composite carbon material with remarkable potassium ion storage ability. The best performing 75rCGO-25rGO has shown a specific capacitance of 489 mAh g-1, 340 mAh g-1, 299 mAh g-1, 251 mAh g-1 , 222 mAh g-1, 195 mAh g-1, 154 mAh g-1 and 108 mAh g-1 at 0.05 A g-1, 0.1 A g-1, 0.2 A g-1, 0.5 A g-1, 1 A g-1, 2 A g-1, 5 A g-1 and 10 A g-1 respectively, with an excellent stability of 116.7 mAh g-1 after cycling at extremely high current densities of 5 A g-1 for 2000 cycles. Various electrochemical analysis and physical characterizations were conducted on the rCGO-rGO composite material to study its ion storage behaviour and the synergistic effects between the two materials with distinct morphologies. To summarize , a crumpled reduced graphene oxide-reduced graphene oxide composite was synthesized via an aerolisation crumpling process of graphene oxide followed by the reduction with thermal annealing. The obtained 3D-2D mixed structure has provided a higher mechanical stability and enhanced ion storage capability due to the strong 3D crumpled structure and increased surface area to accommodate more ions. It’s capacitance and life cycle has surpassed many of the carbon-based materials used for KIB anodes, making it a promising material for KIB electrodes. (5) Hydrothermal reduction and thermal exfoliation of graphene oxide material are two of the most widely used technique to obtain reduced graphene oxide. The reduced graphene oxide produced from these two techniques have completely different physical and chemical properties. Hereby, we have investigated the different charge mechanisms of the reduced graphene oxide synthesized using the thermal exfoliation technique and hydrothermal technique, we have also synthesized a composite material of crumpled graphene oxide and graphene oxide that improved the rate and cycle performance of the materials at high current densities. Hydrothermally reduced graphene oxide (HGO) has displayed the best rate performance, with a capacity of 461.9 mAh g-1 at 0.05 A g1 , whereas the thermally exfoliated 75TECGO-TEGO exhibited the best cyclability with a capacity of 241.8 mAh g-1 after cycling at 0.1 A g-1 for 200 cycles. In this research, two of the most commonly used GO reduction methods were investigated as alternative ways to reduce the 3D crumpled rGO and 2D rGO composite in the previous chapter, namely hydrothermal reduction and thermal exfoliation. The reduced product resulted in chemical and physical differences that largely affected the charge storage behaviour and structural integrity of the carbon material, providing a better understanding of the impact of different reduction methods of GO. In summary, the research study developed distinct solutions of rGO based electrodes for SCs (MnOx activated rGO) and PIBs (3D and 2D carbon composite), achieving superior performance characteristics via chemical and physical modifications on the GO and finally studied the impact of different reduction methods (hydrothermal reduction and thermal exfoliation) of GO for PIB applications.

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