Laser-induced graphene nanostructures for electrochemical energy storage

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Σαμαρτζής, Νικόλαος

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The surging increase in the prevalence of low-energy demanding wearable/portable devices, alongside with the imperativeness for sustainable power output from intermittent renewable energy sources, and the transcendence towards electric automotive technologies, unavoidably established more rigorous standards for electrochemical energy storage (EES) devices. Potential approaches towards satisfying these elevated standards involve either the improvement of the EES devices’ components (electrodes, electrolytes, separators), or the inauguration of new manufacturing protocols that, in relation to current technologies are eco-friendlier, more cost efficient, faster, safer, and more industrially relevant. Carbon-based materials have always been very popular as EES electrode materials, owing to their abundance and low cost, high electronic conductivity, lightweight, and remarkable chemical stability. In addition, carbon can be nanostructured via numerous methods, which allow the tuning of its porosity and surface area, as well as its functionalization with heteroatoms, with the potential to endow carbon with new fascinating properties. The most common approaches towards synthesizing porous carbon involve thermal or chemical methods, which however often necessitate the feedstock of harmful reagents and release hazardous byproducts or require the use of inert atmosphere. Addressing these limitations, laser-assisted graphitization is an enticing approach for synthesizing porous graphene-like structures in ambient conditions, eliminating the requirement for supplementary chemicals. Most importantly, lasers are compatible with additive-manufacturing processes, allowing the in-situ formation of graphene on a desired substrate, and they are inherently endowed with high spatial resolution, which allows the fabrication of miniaturized and patterned graphene-like electrodes. This dissertation focuses on the laser-assisted synthesis of graphene-like structures for use as electrode materials in supercapacitors. The aim is not only to synthesize the desired material but to directly fabricate functional electrodes. This thesis commences with two introductory chapters discussing the fundamentals of supercapacitors (Chapter 1) and graphene-based EES electrode materials (Chapter 2). Followingly, a brief description of the physicochemical characterization techniques, employed here, 10 is presented in Chapter 3. The results and discussion of the experiments performed in the current dissertation are summarized in Chapters 4-7. Chapter 4 focuses on the synthesis of high-quality graphene-like structures originating from the laser irradiation of biomass-derived (grape molasses) and laboratory synthesized (phenol-based resin) carbon precursors. The synthesis is optimized on the basis of laser fluence, and the graphene material is produced in free powder form. After an extensive physicochemical characterization of the synthesized graphene-based structures, utilizing a broad range of techniques such as, Raman spectroscopy, SEM, TEM, XPS, XRD, TGA, N2 physisorption, and sheet resistance measurements, the graphene materials with optimal properties were evaluated as electrodes in symmetric supercapacitors. The electrochemical characterization was conducted using various electroanalytical techniques, such as CV, GCD, and EIS. Given that the graphene structures were obtained in free powder form, the respective electrodes were fabricated after casting an appropriate graphene-containing slurry onto the current collectors. Chapter 5 stands out as a pivotal section in this dissertation, building upon the research outlined in Chapter 4. It unveils a groundbreaking laser-based method (International patent application No.: PCT/GR2021/000029) that enables the simultaneous synthesis and transfer of graphene onto a chosen substrate. The Laser-assisted Explosive Synthesis and Transfer (LEST) method allows the direct fabrication of functional graphene electrodes, avoiding the use of slurries and the several steps of wet-chemistry approaches. The graphene film synthesis by the LEST method was optimized (on the basis of laser fluence) using commercially available polyimide polymer. It was also utilized to demonstrate the proof-of-concept for graphene production starting from the carbon precursors studied in Chapter 4. Apart from the preparation of pure graphene electrodes, the LEST process can be also used in the preparation of graphene-based nanohybrids. For instance, the formation of graphene/SiOx nanohybrids was optimized using a commercially available polyimide tape precursor (a polyimide foil coated with a silicone adhesive on its one side). In addition, the formation of graphene/MnO nanohybrids was also demonstrated using proper precursor for the metal oxide nanoparticles. All the above-mentioned graphene electrodes were successfully implemented in symmetric supercapacitors operating with a liquid aqueous electrolyte. An asymmetric supercapacitor configuration was 11 fabricated for the graphene/MnO nanohybrid. It was shown that the addition of pseudocapacitive MnO nanoparticles leads to a significant enhancement of the device capacitance in comparison to the symmetric configuration. Lastly, a modified and improved version of the LEST process was demonstrated, which allows the deposition of graphene on flexible heat-sensitive substrates. The modified version relies on the modification of the irradiation geometry, and in particular, the change of the laser beam inclination angle. Studying the effect of this change in relation to the variation of the distance between the carbon precursor and the acceptor substrate, revealed interesting new results with eminent technological potential for soft, flexible substrates. The modified LEST method was successfully implemented in the fabrication of an interdigitated and micro-flexible capacitors. Chapter 6 demonstrates the laser-based simultaneous synthesis and transfer of fluorine-doped graphene like structures in ambient conditions, avoiding the use of any harmful reagent. The main aim was to optimize the process and characterize with physicochemical methods the respective F-doped graphene films. These films are produced after applying the LEST method for a three-layered laminate composed of fluorinated ethylene propylene // polyimide // fluorinated ethylene propylene structure, (thickness; PI: 25μm and FEP: 2.5μm). For this particular structure, it was found that fluorine doping can reach up to 3.3%. Interestingly, F-doping does not compromise the conductivity of the graphene films, as fluorine participates in semi-ionic C-F bonds (bonded to sp2 carbon), instead of covalent bonds which would increase the sp3 carbon content. In addition, the introduction of fluorine was shown to increase the specific surface area and hydrophobicity of the deposited graphene films. Chapter 7 can be conceptually divided into two parts. The first part deals with the LEST process applied to a Si/C precursor, aiming to the fabrication of Li-ion battery (LIB) anode. As it is demonstrated, the SiC-precursor can be decomposed by the laser beam into graphene-like structures decorated with nanocrystalline Si and SiOx, emerging as gases after the decomposition, which can re-deposit onto the graphene structures. The process took place directly onto Cu foil, i.e. the anode current collector. The second part of this chapter deals with the pure graphene and the F-doped graphene films prepared by the LEST method, which are used as cathode materials of Zn-ion hybrid supercapacitors. Switching from a pure capacitive EES device configuration (anode:cathode; capacitive:capacitive) to a hybrid EES device 12 configuration (anode:cathode; battery-type:capacitive) boosts the resulting energy density by one order of magnitude without compromising the power density. The F-doped graphene cathode was found to be superior to the undoped graphene cathode. This is mostly due to the increase of the specific surface area caused by the presence and structure morphology change due to fluorine. Further, there are indications that the fluorine functional groups provide additional Faradaic charge storage due to the interaction with the Zn2+ ions of the electrolyte.



Γραφένιο, Υπερπυκνωτές, Νανοδομές, Σύνθεση γραφενίου με laser, Ηλεκτροχημική αποθήκευση ενέργειας, Νανοϋβρίδια γραφενίου, Φθοριωμένο γραφένιο