Investigation of Bipolar Electrochemically Exfoliated Graphene for Supercapacitor Applications

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

Bipolar anodic electrochemical exfoliation of graphite powders

The electrochemical exfoliation of graphite has attracted considerable attention as a method for large-scale, rapid production of graphene and graphene oxide (GO). As exfoliation typically requires direct electrical contact, and is limited by the shape and/or size of the starting graphite, treatment of small graphite particles and powders, the typical form available commercially, is extremely difficult. In this study, GO nanosheets were successfully prepared from small graphite particles and powders by a bipolar electrochemical process. Graphite samples were placed between two platinum feeder electrodes, and a constant current was applied between the feeder electrodes using dilute sulfuric acid as the electrolyte. Optical microscopy, atomic force microscopy, X-ray diffractometry, Raman spectroscopy, and X-ray photoelectron spectroscopy were employed to examine the samples obtained after electrolysis. The results obtained from these analyses confirmed that anodic electrochemical exfoliation occurs in the graphite samples, and the exfoliated samples are basically highly crystalline GO nanosheets with a low degree of oxidation (C/O = 3.6–5.3). This simple electrochemical method is extremely useful for preparing large amounts of graphene and GO from small particles of graphite

Recent advances in graphene-based materials for fuel cell applications

The unique chemical and physical properties of graphene and its derivatives (graphene oxide, heteroatom-doped graphene, and functionalized graphene) have stimulated tremendous efforts and made significant progress in fuel cell applications. This review focuses on the latest advances in the use of graphene-based materials in electrodes, electrolytes, and bipolar plates for fuel cells. The understanding of structure-activity relationships of metal-free heteroatom-doped graphene and graphene-supported catalysts was highlighted. The performances and advantages of graphene-based materials in membranes and bipolar plates were summarized. We also outlined the challenges and perspectives in using graphene-based materials for fuel cell applications

Novel application of electrochemical bipolar exfoliated graphene for highly sensitive disposable label-free cancer biomarker aptasensors

Label-free aptasensors can be a promising point-of-care biosensor for detecting various cancer diseases due to their selectivity, sensitivity, and lower cost of production and operation. In this study, a highly sensitive aptasensor based on gold-covered polyethylene terephthalate electrodes (PET/Au) decorated with bipolar exfoliated graphene is proposed as a possible contender for disposable label-free aptasensor applications. Bipolar electrochemical exfoliation enables simultaneous exfoliation, reduction, and deposition of graphene nanosheets on prospective electrodes. Our comparative study confirms that the bipolar exfoliated graphene deposited on the negative feeding electrode (i.e., reduced graphene oxide) possesses better electrochemical properties for aptasensing. The optimized aptasensor based on bipolar exfoliated graphene deposited on PET/Au electrodes exhibits a highly sensitive response of 4.07 μA log c−1(unit ofc, pM) which is linear in the range of 0.0007-20 nM, and has a low limit of detection of 0.65 pM (S/N = 3). The aptasensor establishes highly selective performance with a stability of 91.2% after 6 days. This study demonstrates that bipolar electrochemistry is a simple yet efficient technique that could provide high-quality graphene for biosensing applications. Considering its simplicity and efficiency, the BPE technique promises the development of feasible and affordable lab-on-chip and point-of-care cancer diagnosis technologies

2D semiconductor nanomaterials and heterostructures : controlled synthesis and functional applications

Two-dimensional (2D) semiconductors beyond graphene represent the thinnest stable known nanomaterials. Rapid growth of their family and applications during the last decade of the twenty-first century have brought unprecedented opportunities to the advanced nano- and opto-electronic technologies. In this article, we review the latest progress in findings on the developed 2D nanomaterials. Advanced synthesis techniques of these 2D nanomaterials and heterostructures were summarized and their novel applications were discussed. The fabrication techniques include the state-of-the-art developments of the vapor-phase-based deposition methods and novel van der Waals (vdW) exfoliation approaches for fabrication both amorphous and crystalline 2D nanomaterials with a particular focus on the chemical vapor deposition (CVD), atomic layer deposition (ALD) of 2D semiconductors and their heterostructures as well as on vdW exfoliation of 2D surface oxide films of liquid metals

New Insights into the Surfactant-Assisted Liquid-Phase Exfoliation of Bi2S3 for Electrocatalytic Applications

During water electrolysis, adding an electrocatalyst for the hydrogen evolution reaction (HER) is necessary to reduce the activation barrier and thus enhance the reaction rate. Metal chalcogenide-based 2D nanomaterials have been studied as an alternative to noble metal electrocatalysts because of their interesting electrocatalytic properties and low costs of production. However, the difficulty in improving the catalytic efficiency and industrializing the synthetic methods have become a problem in the potential application of these species in electrocatalysis. Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for lab- and industrial-scale synthesis of 2D-material colloidal inks. In this work, we present, to the best of our knowledge, for the first time a systematic study on the surfactant-assisted LPE of bulk Bi2S3 crystalline powder to produce nanosheets (NSs). Different dispersing agents and LPE conditions have been tested in order to obtain colloidal low-dimensional Bi2S3 NSs in H2O at optimized concentrations. Eventually, colloidally stable layered nano-sized Bi2S3 suspensions can be produced with yields of up to ~12.5%. The thus obtained low-dimensional Bi2S3 is proven to be more active for HER than the bulk starting material, showing an overpotential of only 235 mV and an optimized Tafel slope of 125 mV/dec. Our results provide a facile top-down method to produce nano-sized Bi2S3 through a green approach and demonstrate that this material can have a good potential as electrocatalyst for HER

Electrochemical Exfoliation of Graphene and its Characterisation

A green approach is reported for the production of few layered graphenes (FLGs) via electrochemical route utilising the benefits of anodic exfoliation process, wherein electrochemical intercalation of nitrate ions into pyrolytic graphite resulted in electrochemical exfoliation of nitrate ions-intercalated graphite electrode. The role of applied potential in intercalation and concentrations of nitric acid are well defining factors in controlling the number of layers in FLGs. The success of this approach was confirmed by FTIR, wherein smaller particles of intercalated graphite led to broader peaks due to increased interaction with light wave. The SEM images showed several layers of graphene stacked together and slightly twisted at edges. An increased exfoliation in intercalated graphite was revealed by XRD patterns. Desirable conductive properties of the FLGs synthesised makes it a viable option for utility as conductive ink

Memristive Non-Volatile Memory Based on Graphene Materials

Resistive random access memory (RRAM), which is considered as one of the most promising next-generation non-volatile memory (NVM) devices and a representative of memristor technologies, demonstrated great potential in acting as an artificial synapse in the industry of neuromorphic systems and artificial intelligence (AI), due its advantages such as fast operation speed, low power consumption, and high device density. Graphene and related materials (GRMs), especially graphene oxide (GO), acting as active materials for RRAM devices, are considered as a promising alternative to other materials including metal oxides and perovskite materials. Herein, an overview of GRM-based RRAM devices is provided, with discussion about the properties of GRMs, main operation mechanisms for resistive switching (RS) behavior, figure of merit (FoM) summary, and prospect extension of GRM-based RRAM devices. With excellent physical and chemical advantages like intrinsic Young’s modulus (1.0 TPa), good tensile strength (130 GPa), excellent carrier mobility (2.0 × 105 cm2∙V−1∙s−1), and high thermal (5000 Wm−1∙K−1) and superior electrical conductivity (1.0 × 106 S∙m−1), GRMs can act as electrodes and resistive switching media in RRAM devices. In addition, the GRM-based interface between electrode and dielectric can have an effect on atomic diffusion limitation in dielectric and surface effect suppression. Immense amounts of concrete research indicate that GRMs might play a significant role in promoting the large-scale commercialization possibility of RRAM devices

Graphene - Its role in energy storage devices

This presentation provides an overview of graphene, methods of preparation of graphene, important applications and its role in energy storage devices, namely, fuel cells, supercapacitors, lithium batteries and hydrogen storage