Developing a reliable, simple, cost-efficient and eco-friendly method for scale-up production of high-quality graphene-based materials is essential for the broad applications of graphene. Up to now, various manufacturing methods have been employed for synthesizing high quality graphene, however aggregation and restacking has been a major issue and the majority of commercially available graphene products are actually graphite microplates. In this study, bipolar electrochemistry techniques have been used to exfoliate and deposit graphene nanosheets in a single-step process to enable high performance device application.
In the first part of this study, bipolar electrochemistry concept is utilized to design a single-step and controllable process for simultaneously exfoliating a graphite source and depositing both graphene oxide (GO) and reduced graphene oxide (rGO) layers on conductive substrates. The electrochemical performance of the fabricated graphene-based materials as the electrode for supercapacitors has been investigated. Areal capacitance of 1.932 mF cm-2 for the rGO, and 0.404 mF cm-2 for GO at a scan rate of 2 mV s-1 were achieved. Moreover, a cut-off frequency of 1820 Hz was obtained, which is a promising characteristic for AC filtering applications.
Although the physicochemical characteristics of produced graphene have been evaluated in the first part, the exfoliation and deposition mechanisms were still unclear. In the second part of this dissertation, a novel modified BPE system with an electrically connected graphite-platinum couple acting as the bipolar electrode has been designed in order to decouple and investigate the contribution of anodic/cathodic exfoliation and deposition of graphene in the BPE process. Electron microscopy and infrared spectroscopy results indicate that both anodic and cathodic exfoliation of graphene could take place regardless of the type of polarization; however, the morphology and deposition rate highly depend on the polarization. Furthermore, the graphene fabricated by anodic exfoliation was found to show higher levels of oxidation compared to the graphene produced by cathodic exfoliation.
In the last part of this study, for the first time, a vertically aligned graphene layer was deposited on a micro-sized interdigitated gold current collector by a modified bipolar electrochemistry method. Both time domain and frequency domain electrochemical performance of on-chip micro-supercapacitors (MSCs) were evaluated. An areal capacity of 640.9 μF cm-2 at a scan rate of 2 mV s-1 and 239.31 μF cm-2 at discharge current density of 25 μA cm-2 was delivered with an excellent cyclability. Most importantly, the MSC exhibited a very fast response (cut-off frequency of 3486 Hz) and very close to ideal performance (phase angle reached -83.2°) at low frequencies.
For the first time, this dissertation reported the modified BPE method as a novel approach for three in one exfoliation, deposition and reduction of high-quality graphene with vertically aligned and porous structure. The unique design of the BPE cell enabled the author to study the BPE mechanisms and measure the bipolar current for the first time. The method could successfully be employed to fabricate fast response microsupercapacitors based on vertically aligned graphene nanosheets
