Synthesis of graphene-based hybrid nanomaterials for energy storage applications
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Date
2025-01-27
Authors
Μινωτάκης, Κωνσταντίνος
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Abstract
The rapid growth of global energy demand driven mainly by technological advancements, increased industrialization and fossil fuel unsustainability has posed a significant challenge for the scientific community as well as for the rest of the world, demanding an innovative solution. Humanity’s shift towards electric-based technologies including wearable electronics or electric vehicles has significantly heightened the demand for more advanced electrochemical energy storage systems. This need is further amplified by the limited utilization of renewable energy sources which challenges the development of sustainable energy solutions. To address these demands, scientific efforts have been focused on supercapacitor technology. Supercapacitors, with their remarkable electrochemical properties such as high-power density, exceptional cycle life and fast charge/discharge capabilities, have emerged both as a supportive component to widely used technological applications such as batteries and a potential standalone energy storage solution.
Various carbon-based nanostructures have been widely used as electrode materials in supercapacitor devices, not only for being cost-effective and eco-friendly but also due to their exceptional properties such as high electronic conductivity, large specific surface area, high porosity, and excellent chemical stability, making them very versatile materials for electrochemical applications. However, even while possessing such great properties, carbon-based materials alone cannot effectively bridge the gap between the high-energy and high-power density needs in certain energy storage systems. This issue can be effectively approached by combining these carbon-based materials with established pseudocapacitive or battery-type type materials which can offer high energy density. This certain combination of two different electrochemical energy storage mechanisms can cover both grounds of carbon-based material’s lack of energy density and pseudocapacitive/battery-type material’s, lack of conductivity or limited cycle life. To satisfy the criteria above, scientific research has turned its focus on using metal oxides/hydroxides as pseudocapacitive or battery-type materials in combination with carbon-based electrodes for hybrid supercapacitor technology.
This study focuses on the fabrication of supercapacitor devices which use graphene-like structures as electrodes, synthesized via the laser-assisted method. The laser-assisted graphene synthesis is an environmentally friendly and cost-effective approach for synthesizing porous graphene-like structures under ambient conditions, while also enabling the in-situ synthesis of graphene on a desired surface or substrate. This method also enables the successful addition and transfer of metal oxides/hydroxide compounds onto the graphene-like structure. During the 1st and 2nd chapters, this thesis focuses on the theoretical background of the basic principles of capacitor and supercapacitor technology as well as on the significance and properties of carbon and graphene-like materials. The 3rd chapter introduces and briefly describes the physicochemical and electrochemical characterization techniques that were used during the experimental section, providing an insight on their usefulness and basic principles. The 4th chapter shortly discusses the preparation of the laser-assisted graphene electrode samples as well as the assembly of the supercapacitor cells.
The 5th chapter focuses on the synthesized graphene electrodes decorated with various Manganese Oxide (MnO) mass-loadings. The MnO-containing electrodes were tested in an asymmetric two-electrode device configuration while some of them were tested on a three-electrode device configuration. Both systems were operated in an alkaline electrolyte environment (6M KOH). The asymmetric two-electrode devices were electrochemically tested, demonstrating that the incorporation of MnO nanoparticles onto the electrode structure, outperformed electrochemically the reference symmetric device in every case. This indicates that the electrochemical enhancement of the graphene-like electrodes was successful while also revealing that the enhancement diminishes after a certain mass-loading threshold, highlighting the importance of optimizing the MnO loading for maximum performance. The electrode with the moderate MnO mass-loading outperformed every other device, exhibiting an enhanced areal capacitance (14.3mF/〖cm〗^2), good rate capability (70% at 5 mA/〖cm〗^2) and excellent cycling stability (99% after 10,000 cycles). The electrochemical testing of the three-electrode device revealed significantly higher capacitance values for the MnO-containing electrodes. This suggests that the energy storage mechanism becomes imbalanced when transitioning to a two-electrode configuration, leading to electrochemical performance degradation. When the best performing MnO electrode was tested in a symmetric configuration it revealed no significant electrochemical enhancement indicating that the pseudocapacitive contribution from Mn-based nanoparticles is predominantly revealed when used as positive electrodes.
On a similar approach, the 6th chapter focuses on the electrochemical evaluation of the synthesized graphene electrodes incorporated with Ni-based nanoparticles in with various mass-loadings which exhibit battery-like behavior. Physicochemical measurements revealed the excellent quality of graphene while the electrochemical testing performed on a three-electrode device under alkaline electrolyte environment (6M KOH), demonstrated the successful incorporation of 〖Ni(OH)〗_2 compounds on the laser-assisted graphene’s surface. In every case, all Ni containing electrodes outperformed the pure graphene electrodes in terms of capacity and rate capability, while the electrode with the highest Ni mass-loading outperformed the other electrodes exhibiting an excellent areal capacity (5.5 μAh/〖cm〗^2) and a rather satisfactory rate capability (62% at 10 mA/〖cm〗^2). An intriguing behavior emerged during the cycling stability testing of the Ni-containing electrodes, characterized by a critical increase of the capacity, followed by a sudden drop. This behavior is likely attributed to specific electrochemical and physicochemical processes that occurred withing the aqueous electrolytic environment.
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Keywords
Graphene, Supercapacitors, Laser-assisted graphene, Graphene nanohybrids