A theoretical model and numerical simulation of a direct methanol fuel cell (DMFC) is developed to simulate the reaction mechanisms and the cell voltage under several different designing parameters and operational conditions. The results of a numerical simulation include the distributions of the proton current density, the concentration of methanol, the electrochemical reaction rates, the overpotential losses, and the pressures within proton exchange membrane layer, catalyst layer and diffusion layer. In addition, the influence of aforementioned operational conditions on methanol crossover in a direct methanol fuel cell is also investigated. Finally, the results of the model are compared to the results from the experimental work. The results show that increasing of temperature, pressure and anode catalyst loading can enhance the performance of a direct methanol fuel cell, and the concentration of methanol plays an important role in its performance. The optimal concentration of methanol for a direct methanol fuel cell is about 2M. Methanol crossover can be suppressed by decreasing methanol concentration and increasing thickness of polymer electrolyte membrane (PEM). However, under operating condition of high current density, thick PEM and low methanol concentration will cause large concentration overpotential and ohmic losses, respectively.

The direct methanol fuel cell (DMFC) performance has been improved via two approaches. The first approach reduces methanol crossover in the membrane electrode assemblies (MEAs) by incorporating a methanol barrier layer onto an anode electrode of the MEA. The second approach increases the triple phase boundaries via the modified morphology of catalyst layers in the MEA. Methanol barrier layers containing a composite layer of Nafion/mordenite (MOR), Nafion/zeolite Y (ZY), Nafion/montmorillonite (MMT) or Nafion/titanate (TN) were distributed onto the anode of an MEA. The performance of these MEAs were tested in a single cell DMFC for temperatures between 30-80 °C and methanol concentrations of 1 M-4 M and compared with a standard MEA to identify changes in power output. At 2 M methanol concentration and 80 °C, the MEAs featuring with Nafion/0.50 wt% MMT and Nafion/0.50 wt% TN delivered higher power densities, 19.76% and 26.60%, respectively, than that of standard MEA. The catalyst morphology has been adjusted by the dilution of catalyst ink to prevent an agglomeration of catalyst particles, resulting in the increased triple phase boundaries which are the phases for electrochemical reactions and for the transportation of electron and proton products. The new-standard MEA presented the best improvement in power density of 81.15% over the conventional counterpart at 80 °C and 2 M methanol concentration. This modified procedure was further utilised for MEAs fabrication. Further investigation has been carried out by the selected Nafion/MMT layer. The MMT loading of 0.25 wt%-1.00 wt% were incorporated onto the barrier layer where the Nafion/0.25 wt% MMT layer illustrated the best performance. This MEA attributed the highest power density of 69.14 mW cm⁻² which is 2.76% higher than 67.23 mW cm⁻² of the new-standard MEA at 80 °C and 2 M methanol concentration. The best improvement in power density, 27.09%, was obtained at low temperature and low methanol concentration of 30 °C and 1 M. The power density was 25.30 mW cm⁻² when compare to 19.91 mW cm⁻² of the new-standard MEA. These results suggest that the methanol barrier layer and the modified morphology of catalyst layer accomplish the aim of improving DMFC performance.

The experimental test and analysis of the direct methanol fuel cell (DMFC), which is made with a newly developed heterogeneous composite carbon fiber unipolar/bipolar plate, is performed in our lab. The work from the making of carbon fiber unipolar plate to the design of single-cell DMFC is also included in this study. The experimental work of various control parameters, such as methanol concentration, operating temperature, and the air flow rate, is also conducted in this thesis. The formation of carbon dioxide in anode is inspected during experiment. From a series of experimental test, we have understood the characteristics of DMFC better. The experimental result and experience can also provide the references of the application and development of DMFC in the future. According to our experiment, we find that the assembling of the new-type unipolar/bipolar plate doesn¡¦t need to use the large compressing force to reduce the contact resistance like those of the traditional unipolar/bipolar plates. The structure of the DMFC stack made with the new carbon fiber unipolar/bipolar plate is simple and weight light. However, the experimental results still show that the factors that affect the performance of the DMFC fuel cell are similar to those with the conventional unipolar/bipolar plates. For example, increasing the reactive temperature of fuel, proper methanol concentration, and proper content of catalyst all can effectively improve the power density of a DMFC. The structure of the methanol mixture directly stored in the flow channel of the anode is simple. However, the design exists the problems of the crossover of methanol, the stripping of the anode electrode, and the removal of the carbon dioxide. Special attention is needed to overcome and improve those problems in making DMFC stacks. Or the performance of the cell will decline after long period operation.

Magister Scientiae - MSc / Fuel cells convert chemical energy from a fuel into electricity through chemical reaction with oxygen. This possesses some challenges like slow oxygen reduction reaction (ORR), overpotential, and methanol fuel cross over in a direct methanol fuel cell (DMFC). These challenges cause inefficiency and use of higher amounts of the expensive platinum catalyst.Several binary catalysts with better ORR activity have been reported. In this study we investigate the best catalyst with better ORR and MOR performances and lower over-potentials for PEMFC and DMFC applications by comparing the in-house catalysts (10%Pt/C, 20%Pt/C,30%Pt15%Ru/C, 40%Pt20%Ru/C, 30%PtCo/C, 20%Pt20%Cu/C and 20%PtSn/C) with the commercial platinum based catalysts (10%Pt/C, 20%Pt/C, 20%Pt10%Ru/C, 20%PtCo/C,20%PtCu/C and 20%PtSn/C) using the cyclic voltammetry and the rotating disk electrode to determine their oxygen reduction reaction and methanol tolerance. HRTEM and XRD techniques were used to determine their particle size, arrangement and the atomic composition. It was observed that the 20%Pt/C in-house catalyst gave the best ORR activity and higher methanol oxidation current peaks compared to others catalysts followed by 20%Pt10%Ru/C commercial catalyst. The 20%PtCo/C commercial, 30%PtCo/C in-house and 20%PtSn/C in-house catalysts were found to be the most methanol tolerant catalysts making them the best catalysts for ORR in DMFC. It was observed that the ORR activity of 20%PtCo/C commercial and 30%PtCo/C inhouse catalysts were enhanced when heat treated at 350 0C. From XRD and HRTEM studies, the particle sizes were between 2.72nm to 5.02nm with little agglomeration but after the heat treatment, the particles were nicely dispersed on the carbon support.

This thesis focuses on developing an alternative system for membrane electrode assembly (MEA) formation to use with a direct methanol fuel cell (DMFC). The approach involves incorporating inorganic fillers with an industry standard Nafion polymer as part of a methanol resistant composite barrier layer at the anode/membrane interface of MEA featuring Nafion 117 membranes. This procedure is used to reduce the fuel cell losses related to the crossover of un-oxidised methanol through the membrane and prevent its subsequent reaction at the cathode. The inorganic filler used within this study was mordenite that has Si/Al ratio of 5 and by incorporating this into the barrier layer a superior DMFC performance has been achieved in comparison to a standard MEA featuring a Nafion 117 membrane. The voltage, current density and power density used as a measure of DMFC performance under a range of methanol molarities (1M-4M) and cell temperatures (40°C-70°C) have been taken for both the novel and standard MEA. Linear sweep voltammetry (LSV) and AC impedance spectroscopy (ACIS) were used to give some insight into what was occurring within the MEA with regards to methanol crossover current and the proton conductivity within the DMFC. To obtain the best possible DMFC performance a range of mordenite loadings from 0wt%1.0wt% were utilised and an optimum loading of 0.5wt% was reached. MEA which featured mordenite that had undergone ion exchange into a protonated form (from the sodium form) and had a silane functional group (glycidoxypropyltrimethoxysilane) grafted onto the surface, gave DMFC performances that were as much as 50% better than the standard. The highest power density obtained with this MEA was 43.6mW/cm-2 compared to the 35mW/cm-2 obtained using the standard. Values obtained for the methanol crossover current and proton conductivity under working DMFC operating conditions showed that this novel MEA had as much as 16% lower methanol permeability compared to the standard combined with comparable proton conductivity when using a 1M methanol feed. The durability of a novel MEA featuring the 0.5wt% functionalised H-mordenite composite barrier layer was tested in the DMFC and compared to a standard MEA at a constant current of 50mA/cm-2 over 100 hours. The cell potential fell by 0.1mV/h in comparison to a 0.23mV/h loss observed with the standard. The work reported within this study aims to show that by incorporating a thin Nafion/mordenite composite layer at the anode/membrane interface within an MEA will result in improvements in DMFC performance. The development of this technology has led to the application for a patent due to the potential for the commercial development of DMFC using this novel approach.