Abstract
The direct methanol fuel cell (DMFC) is an attractive and promising power generator, which generates electricity by an electrochemical redox reaction of methanol and oxygen, enabling a wide range of applications from small sensors, portable electronic devices, to automobiles. However, the slow methanol electro-oxidation and severe methanol crossover undermine the DMFC performance. On the other hand, a high loading of noble metal electrocatalysts make it too expensive for commercialization. In this thesis, two solutions are proposed to tackle these issues: an efficient anode with a low loading of noble metal electrocatalysts to enhance the methanol electro-oxidation; a proton exchange membrane coated with a methanol blocking layer to reduce the methanol crossover.Firstly, the study demonstrated the feasibility of a high-performance membrane-electrode-assembly (MEA), with low electrocatalyst loading on carbon nanotubes (CNTs), which were grown directly on carbon cloth as an anode. The direct growth of CNTs was realized by microwave plasma-enhanced chemical vapor deposition using CH4/H2/N2 as precursors. The cyclic voltammetry and electrochemical impedance measurements with 1 mM Fe(CN)63-/4- redox reaction reveal a fast electron transport and a low resistance on the direct grown CNT. The electrocatalysts, platinum and ruthenium, were coated on CNTs by sputtering technique to form the Pt-Ru/CNTs-CC anode (Pt-Ru/CNTs-CC). The MEA, the sandwiched structure which comprises 0.4 mg cm-2 Pt-Ru/CNTs-CC as the anode, 3.0 mg cm-2 Pt black as the cathode and Nafion 117 membrane at the center, performs very well in a direct methanol fuel cell (DMFC) test. The micro-structural MEA analysis shows that the thin electrocatalyst layer is uniform, with good interfacial continuity between membrane and the gas diffusion layer.Secondly, protonated polyaniline (PANI), a stable and electrically conducting polymer, was directly polymerized on a Nafion 117 membrane (N117), forming a composite membrane, to act as a methanol blocking layer (PANI/N117), whose was evaluated to reduce the methanol crossover in the DMFC. A PANI layer coated on the N117 has a thickness of 100 nm, with an electrical conductivity of about 13.24 S cm-1. The methanol permeability of the PANI/N117 is 41% less than that of the N117 at room temperature, suggesting that the PANI/N117 can effectively reduce the methanol crossover in the DMFC. The MEAs using the conventional N117 (N117-based MEA) and the new developed PANI/N117 (PANI/N117-based MEA) were compared to the feeding of 1, 2, 4, 6 and 8 M methanol at 60 oC. The output power of the N117-based MEA is reduced at higher methanol concentration, which is due to the methanol crossover of the N117. However, the PANI/N117-based MEA exhibits higher output power at higher methanol concentration. The maximum power density of the PANI/N117-based MEA is 70 mW cm-2 at 6 M methanol solution. This value is double that of the N117-based MEA under identical conditions. This work also suggests that the methanol-crossover rate of the PANI/N117-based MEA is about 60% lower than that of the N117-based MEA from 1 M to 6 M methanol solutions. The PANI/N117-based MEA performs well at elevated methanol concentration, suggesting the potential for long-term operation of small-scale DMFCs.