Graphene Hydrogel Materials for Next Generation Energy Storage DevicesPublic Deposited
Electrochemical energy storage devices have become increasingly relevant to the operation and sustainability of the modern world, as proliferation of mobile electronics, renewable electrical energy generators, electrical vehicles, and various high-tech bio-medical sensing device continues. The widespread need of easier to produce, better performing, and multifunctional energy storage materials in these applications is becoming increasingly urgent. It is particularly demonstrated in energy storage devices such Li-ion batteries and supercapacitors, where the intense research in the last two decades have yet to satisfy the consumer and industrial demand for better material design. One candidate material has been met with intense interests in the recent years. Graphene, and its chemical derivatives, graphene oxide and reduced graphene oxide, offer potential scalable material solutions for higher performing battery and supercapacitor materials. Due to its unique chemical structure, graphene has excellent chemical stability and high electrochemical performance. And its chemical derivative, graphene oxide, offers scalable and cost-effective processing methods due to its hydrophilic nature and ability to be easily manipulated via simple solution-based chemistry. Nonetheless, majority of contemporary studies have not fully utilized the potential benefits of graphene, due to the limitation of the two-dimensional electrode design where graphene layers are often closely packed into dense films and coating, and thereby reducing its active surface sites in electrochemical applications resulting in decreases the performance of the constructed devices.', 'In this dissertation, the development of a novel class of graphene based materials, graphene hydrogel, is discussed in detail. The free-standing, three-dimensional, and highly porous structure of the graphene hydrogel is composed of loosely stacked and cross-linked graphene sheets, resulting in higher specific surface area and increase in electrochemical active sites when compared to the two-dimensional graphene films. Implementation of the graphene hydrogel materials were successfully demonstrated in both Li-ion battery and supercapacitor devices in this dissertation. ', 'For battery application, a porous 3D graphene hydrogel composite embedded with Si nanoparticles coated with an ultrathin SiOx layer is successfully synthesized using a solution-based self-assembly process. The excellent electrochemical performance can be attributed to the porous, open cell 3D structure of graphene hydrogel, which provides a large internal space and flexible, electrically conductive graphene matrix that can accommodate volumetric changes of Si nanoparticles. Its highly porous 3D structure of high specific surface area allows rapid diffusion of Li-ions and easy penetration of electrolyte, resulting in high specific capacity even at high current density. ', 'A polyurethane supported graphene hydrogel composite was fabricated and used as flexible electrode material for supercapacitors. Nano and micro-particles of electrochemically active materials, graphite and MnO2, were mixed with graphene oxide solution and then treated with a mechanical spray dry method. The resulting powder consists of the said active materials wrapped with graphene oxide sheets. The hydrophilic nature of the graphene oxide wrapping allows for even suspension of hydrophobic active materials in aqueous suspension. Graphene hydrogel can then be synthesized through hydrothermal reduction of the aqueous suspension of such composite particles, with even distribution of active hydrogel throughout the hydrogel microstructure. Through mechanical and electrochemical testing, the hydrogel electrode material demonstrated its capability as highly flexible supercapacitor electrode. The resulting understanding in the fabrication and electrochemical properties of such composite system paved the foundation for the designs of future multifunctional electrode materials capable of withstanding high degree of elastic deformation. ', 'Both the electrochemical and mechanical properties of this three-dimensional electrode material were studied in detail. The formation of stress induced surface defects on graphene coating were reported in this dissertation. These stress induced defects exposed edge planes of graphene layers to electrolyte solution and resulted in significantly increase in the electrochemical activity of the material. Such phenomenon was not previously reported in literature and offered new insights on the effects of mechanical deformation have on interaction between electrolyte and flexible graphene based electrode. The effects of intercalated monovalent and divalent cations have on the mechanical properties of the graphene hydrogel were also observed. Graphene hydrogel samples were treated with solutions of monovalent and divalent salts. The graphene hydrogel samples were able to retain trace amount of monovalent and divalent ions, resulting noticeable increases to the elastic modulus of the graphene hydrogel, particularly in samples treated with divalent cations Mg2+ and Ca2+. The results correlate with previous studies conducted on two-dimensional graphene structure.