Electrochemical electrodes based on graphene materials for sensing and energy applications
Date of Issue2016-03-15
School of Chemical and Biomedical Engineering
Due to the unique properties, graphene has changed the landscapes of many areas (e.g. sensing and energy storage and conversion). The large defect-free surface area, unparrelled charge carrier mobility and wide electrochemical potential window make graphene a promising conductive platform for efficient electrochemical reactions. In this PhD thesis, dimensional-tailored graphene-based electrochemical electrodes were designed and developed, aiming to explore the potential of graphene materials in electrochemistry and their applications in novel sensor and energy devices. We attempt to contribute new understanding and concepts on the design of graphene-based electrochemical electrodes mainly in terms of synthesis of multi-dimensional graphene structures, functionalization of these distinct graphene structures (e.g. compositing with metal oxides and heteroatom doping), and study of their physiochemical and electrochemical properties. At last, the performance of multi-dimensional functionalized graphene materials for sensing and energy storage and conversion were evaluated. In chapter 3, a novel graphene/cobalt oxide hybrid electrode was fabricated by transferring chemical vapor deposition (CVD)-grown two dimensional (2D) graphene strip onto a glass micropipette, subsequent electrochemical deposition of nanostructured crystalline cobalt hydroxide, and final thermal annealing process. Our results showed that uniform flower-like crystal structure of Co3O4 was formed on the pipette tip. In electrochemical tests, the hybrid electrode exhibited apparent redox peaks with high stability. Such needle-like electrode was employed for non-enzymatic glucose detection, which exhibited high sensitivity in a wide linear range of 50 to 300 μM. Using a bare graphene needle counter electrode and a graphene/Co3O4 hybrid working electrode, I glucose can be probed in a micro-droplet with a lower detection limit < 10μM. In comparison with the conventional planar electrodes or sensors, such needle like electrode enables sensitive detection in a small volume or three-dimensionally addressable local- detection with high spatial resolution. Deliberate invasion of heteroatoms (e.g. B, N, P, O and S) into hexagonal carbon lattices of graphene is another effective strategy to confer it new or improved electrochemical capabilities. In chapter 4, a green and facile synthesis approach for heteroatom-doped graphene/bacteria composite was demonstrated using chemically- exfoliated graphene oxide as the precursor and E.coli as the reducing agent, spacer and doping source. The yielded porous carbon composite exhibited superior performance in oxygen reduction reaction (ORR) and lithium ion battery (LIB). Enriched and well- exposed active sites and synergistic effects of multi-dopants resulted in the carbonaceous graphene/bacteria composite an onset ORR potential of ~ -0.09 V versus saturated Ag/AgCl and a superior four-electron behavior along with remarkable durability and methanol tolerance. LIB measurements showed the hybrid a high initial discharge capacity of 1048 mA h g -1 and reversible capacity of 587 mA h g-1 at current density of 0.5 C. The gradually increased discharging capacity of rGO/E.coli electrode after an initial decrease (reaching a value of 501.5 mAh g-1 at the 380th cycle) indicated its excellent stability. The hybrid anode also exhibited a high Coulombic efficiency of 97~ 99% (from the 5th cycle onwards) along with excellent high rate capability. To achieve the full potential applications of graphene, three-dimensional (3D) graphene free-standing foam was constructed in chapter 5, where nickel foam (1.0 mm thick) and ethanol were employed as the chemical vapor deposition substrate and II precursor, respectively. The 3D architecture simultaneously accesses to the porous architecture and excellent intrinsic properties of 2D gaphene. 3D grahpene/manganese dioxide (MnO2) composite was further prepared via a simiple hydrothermal method. The morphology of the MnO2 nanostructures was found to be readily controlled by reaction conditions. Serving as a supercapacitor electrode, the free-standing 3D hybrid gave a significant specific capacitance (560 F/g at the current density of 0.2 A/g) and remarkable cycling stability (98% retention after 1000 cycles). Graphene quantum dots (GQDs) are another kind of graphene derivatives but possess distinct structural, electronic and optical properties from graphene sheets due to strong quantum confinement and edge effects. In the last contribution, we reported a new electrochemical strategy for facile synthesis of high quality GQDs from monolithic CVD- grown 3D graphene. Room temperature ionic liquid was used as the electrolyte, which gave high ionic conductivity and wide electrochemical potential window. Applying a constant voltage of 5 V for 100 s was found to give high yield of GQDs while achieving uniform etching of graphene foam and preserving its 3D structure. The synthesized GQDs were of uniform distribution in lateral diameter (~3 nm) and thickness (mostly single-layered), and exhibited strong blue photoluminescence (quantum yield, 10%). Ionic liquid functionalization endowed GQDs the ability for sensitive and specific optical detection of Fe3+ ions (theoretical detection limit ≈ 7.22 μM).