title: Electrochemical synthesis of 2D materials and their applications in energy storage
Creators: name: Zhang, Yubai
Publisher: name: Griffith University
Year: 2021
Subjects: subject: electrochemical exfoliation
subject: graphite
subject: sodium ion battery anode
subject: electrochemical graphene oxide
subject: vanadium oxide
subject: aqueous zinc ion battery cathode
subject: electrochemical production
subject: molybdenum disulfide

description: 2D materials have inspired the intrigue of researchers and industries for its potential to improve the performance of existing materials in energy storage field. However, wide application of 2D material such as graphene and transition metal dichalcogenides in batteries is not implemented since the tremendous challenges and issues, the quality, quantity, and cost concerns impede its commercialization. Electrochemical approach performs as a controllable and scalable method for exfoliating, expanding, and functionalizing the pristine bulk materials on-demand. Sodium ion batteries, a promising candidate for lithium ion batteries, and aqueous zinc ion batteries, a safe energy storage system have received considerable attention in recent decades. The research herein focuses on the electrochemical exfoliation of graphite for its application in sodium ion battery anode, adopting the electrochemical graphene oxide (EGO) as functional agent combining with vanadium oxide for aqueous zinc ion battery cathode, and electrochemical production of molybdenum disulfide in a packed bed reactor. The PhD thesis generally is composed of three parts. In the first part, graphite is exfoliated and oxidized in a packed bed reactor. The effects of boron doping and oxidation on the graphene-based material were studied for high performance sodium ion battery anode respectively in Chapter 2 and Chapter 3. The electrochemical route from natural graphite to graphene oxide is investigated in terms of concentration of acid electrolyte (sulfuric acid). It was found that 12 M sulfuric acid reacted graphene oxide could deliver higher capacity of sodium ion battery than other concentrations. Boron doped graphene was synthesized by a twostep reaction, electrochemical fabrication of the tetraborate anions intercalated graphite oxide followed by reduction by annealing at 900 °C for 3 h under Ar gas. It was found that the boron doped graphene containing 0.21 at. % of boron was highly defective delivers a good capacity of 129.59 mAh g-1 at the current density of 100 mA g-1 and a long-term cyclic stability under current density of 500 mA g-1 retaining 100.20 mA g-1 after 800 cycles. The battery performance of boron doped graphene is better than that without boron doping. To further improve the sodium ion battery anode performance, mildly reduced graphite oxide with layered structure was synthesized by a simple electrochemical oxidation of expanded graphite followed by mildly heating reduction as reported in Chapter 3. The irrigated pipe in the expanded graphite packed bed assists with diffusion of electrolyte. A fast thermal reduction at 150 °C for 20 min on the electrochemical graphite oxide achieves a controlled deoxygenation and maintaining of the large interlayer gap of the product for high sodium storage capacity. The thermally processed electrochemically produced graphite oxide could deliver a high reversible capacity of 268 mAh g-1 at a current density of 100 mA g-1, and 163 mAh g-1 at a high current density of 500 mA g-1 and a good capacity retaining capability (in average 0.0198% loss per cycle) over 2000 cycles. In the second part, the EGO was integrated with vanadium oxide as cathode material for aqueous zinc ion battery. A simple spray dry method is applied to generate electrode materials, which is catering to industrial production. The aqueous mixture for spray drying is formed by quenching the molten V2O5. The products received after spray drying is vanadium oxide hydrate of amorphous structure. The zinc ion storage performance is investigated in terms of content of graphene oxide in the composite. The fabricated amorphous V2O5-EGO composite xerogel with 2D heterostructure possesses high zinc ion storage capability, high rate performance and stable cycling stability due to the functionality of graphene embedded in the composite material. In the third part, inspired by the common intercalation electrochemistry of graphite and transition metal dichalcogenide, exfoliation for 2D MoS2 from its bulk crystal powder is investigated by using the packed bed set up. Organic solvent is found to be a critical factor in the electrochemical activation and the mechanical exfoliation process. The MoS2 bulk crystal can be exfoliated to few-layer nanosheets with stable solution dispersibility. This finding further broadens the horizon of electrochemical production of transition metal dichalcogenides through a scalable approach of electrochemical reaction in packed bed. To sum up, this PhD thesis represents a huge step forward for EGO applications in sodium ion battery anode and aqueous zinc ion battery cathode. In addition, it develops a scalable production of vanadium oxide/graphene material by the spray dry method. The utility of the packed bed electrochemical reactor is extended to transition metal dichalcogenide MoS2. This work will be a valuable guidance for adoption of graphene, vanadium oxide, and MoS2 in the market of energy storage materials.