Design and implementation of multi-walled carbon nanotube electrodes for electrochemical sensors and thermocells

Carbon nanotubes (CNTs) have been widely used in electrochemical sensors and hermocells due to their excellent electrochemical, mechanical, chemical, electrical, and thermal properties. Both of electrochemical sensors and thermocells rely on redox reactions happening at the interfaces of electrodes and electrolytes to realize their functions. However, achieving high-performance CNT-based electrochemical sensors and thermocells still faces challenges, notably in achieving high mass transfer coefficient, while keeping time and material consumption during fabrication processes at the minimum. To address these challenges in CNT-based electrochemical sensors, this thesis first enhances mass transfer coefficient at vertically aligned multi-walled carbon nanotube nanoelectrode arrays (VCNs), which are fabricated by encapsulating multi-walled carbon nanotube (MWCNT) arrays in epoxy and exposing tips of MWCNTs. This thesis investigates the effect of array diameter and forced convection on mass transfer coefficient at VCNs. Compared with macro-sized VCNs in stationary conditions, reducing array diameter to 50 µm achieves a 3-fold enhancement of mass transfer coefficient, while introducing forced convection at a flow rate of 2000 µL/min achieves a 20-fold enhancement. Theoretically, mass transfer coefficient can be further increased by reducing array diameter, increasing flow rate, and reducing channel dimension. Based on the study of VCNs, this thesis proposes a different configuration of CNT-based electrodes, namely, multi-walled carbon nanotube band electrodes (CBEs). CBEs are prepared by encapsulating MWCNT sheets in epoxy and exposing tips of MWCNTs. The band electrodes are designed with micro-sized width (≈ 200 nm) and macro-sized length (≥ 3 mm) to respectively achieve high mass transfer coefficient and high response current. Compared with macro-size VCNs and individual CNT electrodes, CBEs respectively achieve a 32-fold enhancement on the mass transfer coefficient and a hundred-fold enhancement on the response current. This thesis further proposes a third kind of CNT-based electrodes, namely, multiwalled carbon nanotube sheet electrodes (CSEs), which are fabricated by pulling MWCNT sheets from spinnable MWCNT arrays and adhering them on insulating polyimide-glass substrates. Compared with other electrode fabrication processes, fabrication of CSEs requires the minimum time (few minutes) and material (no surfactant or binder material). This thesis systematically investigates the dependence of the electrochemical responses of CSEs on the length of MWCNT leading wires, the layer number of MWCNT sheets, air plasma treatment, and forced convection. By controlling these factors, air plasma treated single layer CSEs can achieve high standard heterogenous rate constant and high mass transfer coefficient. To address these challenges, as well as achieving high electrical conductivity, in CNT-based thermocells, this thesis fabricates MWCNT sheets on stainless steel sheets and meshes to respectively form CSSEs and CSMEs, and explores their application in thermocells. By investigating the dependence of output power on the layer number of MWCNT sheets, air plasma treatment, and substrate type, it is found that the mass transfer resistance is the limiting factor and reduces with increasing MWCNTs surface coverage of CSSEs. The small inter-distance between neighboring MWCNTs hinders diffusion of electrolytes into the MWCNT sheets, CSSEs therefore behave as planar electrodes rather than porous electrodes. This PhD thesis presents detailed studies on designing and implementing VCNs, CBEs, and CSEs for electrochemical sensors, and CSSEs and CSMEs for thermocells. These electrodes successfully achieve high mass transfer coefficient while keeping time and material consumption during fabrication processes at the minimum. The excellent performances of these electrodes in electrochemical sensors and thermocells demonstrate the successful design of these electrodes, and provide guidelines for designing next generation of CNT-based electrodes.Doctor of Philosophy (MAE