With the fast development of vehicles and electronics, advanced energy storage devices with high energy density and large power density are in great demand. Because of the particular physical and chemical properties of graphene, in this thesis, we focus on taking advantage of graphene to improve the energy storage ability of devices especially supercapacitors (high power density) and Li-S battery (large energy density). Briefly, we have separately investigated graphene as conductive substrate/template to shape nanoparticle size and morphology, as free-standing materials for flexible electrode and also as advanced separator to achieve ion sieving.

In Chapter 1, after giving an introduction at the first beginning, recent representative published work and research trend in related research area have been summarized. In Chapter 2, to fully exploit the capacity of pseudocapacitive materials, we have innovatively come up with the idea to use graphene as template to shape nanoparticle morphology and enhance their contact with graphene substrate. As a proof of concept, we have prepared the droplet-shape Ni3S 2 on graphene and investigated the "contact effect" between graphene and Ni3S2 on the final performance in supercapacitors. Our result shows that droplet-shape Ni3S2 nanoparticles tightly attached onto graphene contribute to higher specific capacitance and better rate capability, compared to the simple physical mixture of round-shape Ni3S2 nanoparticles with graphene. Furthermore, Ni 3S2 nanoplatelets, another structure that ensues its good contact with graphene, has been obtained via a one-step H2 reduction method, together with excellent performance for supercapacitors in Chapter 3. Meanwhile, to explore the potential application of graphene in wearable electronics, 3-D graphene synthesized by CVD method was incorporated with ordered porous carbon (OPC) as a free-standing flexible electrode, and with this structure, high performance and good mechanical robustness are achieved in Chapter 4.

Additionally, with the use of graphene, we work on Li-S battery to overcome its poor capacity and short cycle life that have hindered its practical use. In order to alleviate "shuttle effect" and maximize sulfur utilization, functional molecule reduced and modified graphene has been integrated with sulfur of ultrafine particle size via a solution based method. As a result, improved capacity and cycling durability are realized as discussed in Chapter 5. In Chapter 6, in addition to cathode design, we propose a continuous monolayer CVD graphene-coated PP membrane to replace commercialized PP as the separator for Li-S battery. Ascribe to its superior 2-D structure and mechanical robustness, graphene can help mitigate the shutting process by selectively blocking the translocation of lithium polysulfides, thus to improve sulfur utilization and enhance cycle life. To further study the detailed mechanism of this "blocking effect", molecular dynamic simulation was carried out to model Li-S battery system. Our simulation results show that, the selectivity of nanopore is dependent not only on pore size but also the nanopore structure. Specifically, at specific pore size range, graphene can selectively block lithium polysulfides but maintain Li+ transport, and the selectivity can be further enhanced by modifying the nanopore with negatively charged functional groups. The simulation result is in agreement with our hypothesis and also provides us a direction to further optimize the nanopore size on graphene to improve the performance of Li-S battery.