Hierarchical carbon nanotube and graphene oxide networks for multifunctional applications
Creators: name: De Marco, Martina
Publisher: name: Imperial College London
Year: 2016

description: Assembling carbon nanomaterials (CNs) into networks and macrostructures is a potentially effective approach for the development of a wide array of technologies, including energy storage and production devices. CNs, such as carbon nanotubes (CNTs) and graphene (G), are characterised by impressive mechanical and electrical properties, however, these features are related to the high quality, individualised single carbon species.1, 2 Producing two/three-dimensional CN architectures presents several hurdles, mainly concerning the need to disassemble the pristine CN aggregates, the damages inflicted on the carbon framework during processing, and the consequent lack of mechanical strength and/or reduced electrical conductivity of the final material. Suitable methods for preparing CN (macro)structures retaining the extraordinary properties of the fundamental CN units, have yet to be fully developed. This Thesis addresses these issues by suggesting two different methodologies for the synthesis of CN networks, which are tailored to specific applications of the final structures. A novel cross-linking strategy of single-walled carbon nanotubes (SWCNTs) is developed, yielding highly connected, high surface area (&gt; 750 m2 g-1) and electrically conductive (&gt; 15 S m-1) cryogels. The cryogels are demonstrated to be effective electrodes within fully working electrochemical devices. In contrast to cross-linking strategies already explored in literature, the SWCNTs are individualised at high concentrations (up to 0.25 M), and cross-linked with p-diiodobenzene without shortening or damaging the carbon framework via a “reductive chemistry” route.3 Careful control of the absolute charge concentration in the system is found to be crucial for maximising the extent of debundling and grafting, with a suggested optimum at 15 mM. Optimised synthesis parameters in turn determine the accessible surface area and the conductive properties of the final freeze-dried cryogels. Multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) hybrid networks are produced by a simple mixing approach, and used as supports for the CO2 adsorbents Layered Double Hydroxides (LDHs).4 Due to a strong synergistic interaction between the two CNs, hybrid GO/MWCNT systems significantly outperform the pure CNs as a support for LDHs, providing improved CO2 adsorption capacity and dramatically enhanced multicycle thermal stability (up to 96% of gas capacity retention after 20 cycles of adsorption-desorption). Detailed materials characterisation at several stages of the multicycle CO2 adsorption process, links the improved performance to the microstructure, showing that the hybrid GO/MWCNT substrate provides a superior surface area and LDH dispersion from the start, and resists sintering more effectively than either pure GO or pure MWCNTs. A systematic investigation of the relative proportions of the three-phase mixture (MWCNT/GO/LDH) consistently identifies a GO/MWCNT 1:1 ratio as the optimum for both surface area and sorbent performance, particularly when added in small amount (10 - 20%) to LDHs.