Fuel cells are direct energy conversion devices to generate electricity. They have the lowest emission level of all forms of electricity generation. Fuel cells require no combustion of the fuel. The thermal energy gained from fuel cells may be utilized in micro turbines (gas turbines). In this work, first and second law analyses are performed on a hybrid system consisting of a solid oxide fuel cell (SOFC) combined with a micro turbine to be able to find an optimum point of pressure and corresponding mass ratio to gain maximum work output. Also another system with same equipments only without a gas turbine is investigated to see the effects of gas turbine. The analyses are performed utilizing a code written in MATLAB for each of the equipments. Fuel used is biomass with a certain concentration. To be able to use biomass in a fuel cell-micro turbine hybrid cycle, it is gasified and converted into a certain calorific value gas, with the use of gasifiers. In this study fluidized bed gasifier is utilized since it has the advantage of good mixing and high heat transfer leading to a uniform bed condition. Desulphuration and gas filter units will be implemented in order to clean the producer gas before being used in hybrid system. For a certain percentage of the fuel that may pass through the fuel cell without being used, a combustor is utilized. Optimum point mass and pressure ratios for system are MR = 0.6411 and Pr = 8. Gas turbine supplies more power and higher efficiency to the system. There are different choices for fuel selection in hybrid systems. The reason why biomass is examined among these is that it decreases the depletion of energy carriers and reduces the environmental impact.

It is desirable to increase the operation temperature of proton exchange membrane fuel cells above 100oC due to fast electrode kinetics, high tolerance to fuel impurities and simple thermal and water management. In this study / the objective is to develop a high temperature proton exchange membrane fuel cell. Phosphoric acid doped polybenzimidazole membrane was chosen as the electrolyte material. Polybenzimidazole was synthesized with different molecular weights (18700-118500) by changing the synthesis conditions such as reaction time (18-24h) and temperature (185-200oC). The formation of polybenzimidazole was confirmed by FTIR, H-NMR and elemental analysis. The synthesized polymers were used to prepare homogeneous membranes which have good mechanical strength and high thermal stability. Phosphoric acid doped membranes were used to prepare membrane electrode assemblies. Dry hydrogen and oxygen gases were fed to the anode and cathode sides of the cell respectively, at a flow rate of 0.1 slpm for fuel cell tests. It was achieved to operate the single cell up to 160oC. The observed maximum power output was increased considerably from 0.015 W/cm2 to 0.061 W/cm2 at 150oC when the binder of the catalyst was changed from polybenzimidazole to polybenzimidazole and polyvinylidene fluoride mixture. The power outputs of 0.032 W/cm2 and 0.063 W/cm2 were obtained when the fuel cell operating temperatures changed as 125oC and 160oC respectively. The single cell test presents 0.035 W/cm2 and 0.070 W/cm2 with membrane thicknesses of 100 &micro / m and 70 &micro / m respectively. So it can be concluded that thinner membranes give better performances at higher temperatures.

Proton exchange membrane (PEM) fuel cell technology is promissing alternative solution to today&rsquo / s energy concerns providing clean environment and efficient system. Decreasing platinum (Pt) content of fuel cell is one of the main goals to reduce high costs of fuel cell technology in the way of commercialization. In this target, porous carbons provide an alternative solution as a support material for fuel cell electrocatalysts. It is also essential to increase surface area of carbon support material to have well dispersion of the Pt nanoparticles. The aim of this thesis is to synthesize mesoporous carbon supports named as hollow core mesoporous shell (HCMS) carbon and prepare their corresponding electrocatalysts with platinum impregnation method. HCMS carbon supports were synthesized by using two different carbon sources. As a first approach, phenol/paraformaldehyde couples were used and carbon source exhibited 1053 m 2 /g BET surface area and 1.046 nm BJH adsorption pore diameter. Second approach was to use divinylbenzene (DVB) as a carbon source with an initiator named as azo bis isobuytronitrile (AIBN) differing synthesis criteria. It is observed that using AIBN/DVB, pore sizes increased up to 3.44 nm. Platinum impregnation was conducted by microwave irradiation method using hydrogen hexachloroplatinate (IV) hydrate as a platinum precursor. The first achievement was to increase platinum loading up to 44 wt % on commercial Vulcan XC 72 by using ethylene glycol as a reducing agent. Using different reducing agents such as hydrazine, sodium borohydrate with a combination of ethylene glycol, platinum loading reached up to 34 wt % on HCMS carbon support. Accordingly, 34 wt %, 32 wt % and 28 wt % Pt/HCMS carbon supported electrodes preparation was achieved. The sizes of the platinum nanoparticles were calculated by XRD analysis as 4 nm, 4.2 nm and 4.5 nm for 28 wt %, 32 wt % and 34 wt % Pt/HCMS carbon supported electrodes respectively. Characterizations of catalysts were performed by ex situ (N 2 adsorption, TGA, SEM, TEM and Cyclic Voltammetry) and in situ (PEMFC tests) analysis.

Fuel cells are the tools which convert chemical energy into electricity directly by the effective utilization of hydrogen and oxygen (or air). One of the most important barriers for the fuel cell commercialization is the durability of the fuel cell components in the long term operations. In this study, the durability of the PEM fuel cell electrocatalysts were investigated via cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments in order to determine the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) which corresponds to the half cell reactions in the fuel cell. PEM fuel cell electrodes mainly composed of carbon supported Pt catalysts. In long term operations due to Pt dissolution and carbon corrosion some properties of the electrocatalysts can be changed. Performance losses in catalysts mainly depend on / i) decrease in the total metal surface area (SA) and the electrochemically active surface area (ESA) due to the increase in the particle size ii) decrease in the tafel slope potential in ORR and iii) increase in carbon corrosion. In this study, these properties were examined via accelerated degradation tests performed in CV and RDE. The catalysts having different Pt loadings, synthesized with different ink compositions, pH values and microwave durations were investigated. The commercial catalysts having Pt loadings of 20, 50 and 70 (wt %) were tried and best results were obtained for Pt/V (50 wt %) catalyst. Different carbon to Nafion&reg / ratios of 4, 8, 12 in the ink composition were tried. C/N ratio of 8 gave the best result in Pt dissolution and carbon corrosion degradation tests. The catalysts prepared at different pH values of 1.4, 6.25 and 10 were tried and the catalyst prepared at pH of 10 was less degraded in Pt dissolution test and the catalyst prepared at pH of 6.25 showed better resistance to carbon corrosion. Catalysts prepared under different microwave durations of 50, 60 and 120 s were tried and the catalyst prepared at 60 s gave the best performances.

A mathematical model for a proton exchange membrane fuel cell is the focus of this thesis. Modeling and simulations are carried out with an aim to understand the influence of operational and geometrical parameters on the inner reaction and performance of a proton exchange membrane fuel cell, and discuss the distributions of physical phenomena in membrane and catalyst layer. Than, the results of modeling are compared and analyzed with the experiments, and discuss the reasons of influences of the performance of PEMFC. The results show that activation overpotential is the major reason of influence of the performance at low current density (less than ), and diffusion and ohmic overpotential are substantially increased at high current density (great than ). The membrane of higher membrane conductivity and more thin, increasing pressure of cathode gas and use oxygen can enhance the performance of a PEMFC. The performance almost no influence for the catalyst layer over 0.3£gm. The catalyst layer thin and uniform can decrease coating of this layer. The results of modeling and experiments show that experiments have contact resistance between materials, and the performance slightly lower than performance of modeling, and the differences that at high current density great than low current density.

In this dissertation, new high frequency link power conversion systems for the fuel cell power conditioning are proposed to improve the performance and optimize the cost, size, and weight of the power conversion systems. The first study proposes a new soft switching technique for the phase-shift controlled bi-directional dc-dc converter. The described dc-dc converter employs a low profile high frequency transformer and two active full-bridge converters for bidirectional power flow capability. The proposed new soft switching technique guarantees soft switching over wide range from no load to full load without any additional circuit components. The load range for proposed soft switching technique is analyzed by mathematical approach with equivalent circuits and verified by experiments. The second study describes a boost converter cascaded high frequency link direct dc-ac converter suitable for fuel cell power sources. A new multi-loop control for a boost converter to reduce the low frequency input current harmonics drawn from the fuel cell is proposed, and a new PWM technique for the cycloconverter at the secondary to reject the low order harmonics in the output voltages is presented. The performance of the proposed scheme is verified by the various simulations and experiments, and their trade-offs are described in detail using mathematical evaluation approach. The third study proposes a current-fed high frequency link direct dc-ac converter suitable for residential fuel cell power systems. The high frequency full-bridge inverter at the primary generates sinusoidally PWM modulated current pulses with zero current switching (ZCS), and the cycloconverter at the secondary which consists of only two bidirectional switches and output filter capacitors produces sinusoidally modulated 60Hz split single phase output voltage waveforms with near zero current switching. The active harmonic filter connected to the input terminal compensates the low order input current harmonics drawn from the fuel cell without long-term energy storage devices such as batteries and super capacitors.

In this study a three-dimensional mathematical model is developed to simulate the flow field and mass transfer in a PEM fuel cell. In the model, the effects of the different flow channel structures in heterogeneous composite carbon fiber bipolar plates and traditional hard surface bipolar plates on the performance are studied. The results show that, the cell performance with the heterogeneous composite carbon fiber bipolar plates have better performance than that with the traditional hard surface bipolar plates, whether in the parallel flow channel structures or the serpentine flow channel structures. The reason is that, the heterogeneous composite carbon fiber ribs are porous material, so it allows the reactants and products transport uniformly even in the rib zone. This greatly improved the mass transfer and the gases distribution in the fuel cell. With the traditional bipolar plates, the reactants can only enter the reaction zone from the side of carbon cloth under ribs, so that the performance in this area under rib is relatively poor. In the simulation of the flow channel structures, we detect that, due to the single inlet serpentine flow channel have stronger convective effects that forced reactants to flow through the whole reaction zones, so it has better performance at high current density than in the singles inlet parallel flow channel. In addition, the results also show that, higher fuel stoichiometric number and operated pressure and properly humidified at anode will all improve the performance of the fuel cell.

Membrane electrode assembly (MEA) is the foundation of the single cell as well as the core of the fuel cell when generating electricity. Its work efficiency is the key factor for single cell performance. This study aims to understand the variation between the conventional method and the decal method during the MEA process. By observing the microstructure morphology of electrode and the performance of single cell, as well as analyzing internal resistance and its stabilization, the advantages and disadvantages of MEA in the two methods is analyzed. The decal condition is 135¢XC, 15 kg/cm , 2.5 min at a high temperature (50¢XC 3M methanol), in air-breathing under atmosphere system. The maximum power density is approximately 22.5 mW/cm which is very close to the result of conventional method. The decal method is better than the conventional method particularly in regards to the high current density performance. It shows that there is an efficient influence of the decal method on the methanol mass transfer and it also improves its polarization and enlarges the current. If the single cell is operated in the high temperature, the fuel mass transfer can be advanced in the decal method and its performance can be raised. However, in the manufacturing process, more time has to be spent when producing the MEA. This experiment can be used as a reference on the single cell operation environment and manufacturing time for future studies.

Abstract The conventional hydrophilic electrode used to spray the catalyst on the level-off carbon layer and the utilization of catalysts can only be reacted between the gas and the catalyst; however, the internal catalyst of the proton exchange membrane cannot be reacted. In order to increase the reaction of the catalyst, the hydrophobic pillared micro structures (HPMS) are made on the carbon layer, so that the gas can reach the catalyst in the internal membrane so that a reaction on large surface between the gas and the catalyst can be achieved. It is easier to build the gas channel in the internal HPMS than the structures of the carbon layer. As a result, more gas can be sent to the internal catalyst thus enlarging the reaction zone and more reactions between the gas and the catalyst is then achieved. The carbon powder is sprayed in the conventional HPMS in the deposition process. The HPMS are formed after the gravity process while the powder is passing the metal netmask and the manufacturing time is long. The experimental design uses electroforming to make the micro porous structure so that the hydrophobic carbon layer can be stamped thereby forming the HPMS. It has been proven that the time for the manufacturing process can be shortened if the micro structured metal template is applied. The micro structured metal template is used to stamp the small and large HPMS on the side electrode of the cathode, the stamping HPMS pressure was 500kg/cm2. With the same catalyst quantity the surface of the small HPMS was raised 63% and its performance was up to 55%; the surface of the large HPMS was raised 30% and its performance was up to 30%. The catalyst quantity of the cathode was reduced from 0.5mg/cm2 to 0.25 mg/cm2 and its performance remains the same. The experiment¡¦s results indicate that the reaction of the catalyst was only on the surface between the gas and the catalyst. Either small or large HPMS or after reducing the catalyst quantity can all raise the performance of the fuel cell as well as economize the catalyst. And by two kind of different size dimension microstructure metal template manufacture small or large HPMS, the electrode power density all may achieve 720mW/cm2 and 595mW/cm2.

The design and method of cell assembly plays an important role in the performance of PEM fuel cell. The cell assembly will affect the contact behavior between the bipolar plates, flow-channel plates, gas diffusion layers (GDLs) and membrane electrode assembly (MEA). From the past studies, it was noted that the flow-channel plates in the cell will be deformed while the cell was assembled by locking with bolts. This phenomenon may lead to leakage of fuels, high contact resistance and malfunctioning of the cells. The main aim of this research is to study the variation of the deformation mode of the flow-channel plat in a micro-PEM fuel cell assembly subjected to different bolts locking sequences. The commercial FEM package, ANSYS, was adopted to model the three-dimensional single micro-PEMFC FEM model and the numerical simulation analyses were performed. The effect of the bolts locking sequence on the deformations of flow-channel plate in the micro-PEMFC was presented. A most properly bolts locking sequence was proposed also.