In the study, the micro fuel cells were designed and fabricated in-house through a deep UV lithography technique, and a metal-organic chemical vapor deposition technique was used as microstructure material for fuel cell flow field plates. The conductive and insulating flow field plates include interdigitated, serpentine, parallel, and mesh. The experiments with several operation condition include of different cell operating temperatures, different reactant flow rates, and different operating times. This study of various operating parameters shows the physical phenomenon in the current density distribution in fuel cell reaction area and water accumulation in flow channel, and results are represented by VI curve and PI curve. The relationship of physics phenomenon between fuel cell¡¦s power production and water production rates from the current density distribution and water accumulation, can be found through visualization.

In this study, deposition of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxide films on Gd-doped ceria (CGO) substrates by an electrostatic assisted ultrasonic spray pyrolysis (EAUSP) method was demonstrated for the first time. The electrostatic field employed for directing the aerosol stream towards the substrate was shown to be indispensable for film deposition. The XRD result indicates that a single phase of cubic perovskite was obtained in the calcined films. SEM examination reveals that the electric field strength had a profound effect on film porosity with weaker field resulting in higher porosity. The results of impedance measurement on LSCF//CGO//LSCF cells indicate that the area specific resistance (ASR) values of current LSCF films and their activation energies are comparable to that obtained by conventional sample preparation routes. In view of the simplicity, efficiency and economy of film deposition and the sound electrochemical characteristics of the obtained films manifested in current work, it is concluded that EAUSP method is a promising method for preparation of SOFC electrode films. Besides the EAUSP method, electrostatic spray deposition (ESD) method was also employed to deposit LSCF films. The growth mechanism of LSCF films deposited on silicon wafer was studied by examining a series of films obtained with increasing deposition durations. The results show that the film formation mechanism in the initial stage depends on the deposition temperature, and films with a unique porous structure were obtained when a deposition temperature lower than the boiling point of the precursor solution was used. Deposition parameters were also varied systematically to deposit LSCF cathode films on CGO substrates to obtain symmetrical cells. The microstructure and morphology of obtained films were investigated by X-ray diffraction and SEM, and the area specific resistances of the symmetrical cells measured by electrochemical impedance spectroscopy (EIS). The minimum interfacial ASR value associated with the LSCF cathodes was 0.25 ohm¡Ecm2 at700 ¢XC. NiO-SDC (Sm0.2Ce0.8O1.9)/SDC/LSCF (La0.6Sr0.4Co0.2Fe0.8O3-£_) cells with either single layer or double layer cathode were also fabricated and tested. The single layer LSCF cathode was made by stencil printing while the double layer one was prepared by depositing a thin porous layer on the SDC electrolyte by ESD before stencil printing LSCF. The maximum power density increased from 1.04 to 1.18 Wcm-2 at 700¢XC when the LSCF inter-layer was introduced. The results showed that the ASRs of the cells reduced to half with the addition of the LSCF inter-layer.

In this paper a proton exchange membrane fuel cell (called PEMFC) stack was developed to power or charge 3C products without any voltage transformer. PEMFC stacks made with traditional bipolar plates to generate a high voltage are usually by accumulating multiple single fuel cells together. The design with traditional heavy and large bipolar plates is inconvenient for 3C products to generate a high voltage in a finite volume. To solve this problem, a heterogeneous carbon fiber bunch unipolar plate is adopted to replace traditional bipolar plates, and a special membrane electrode assembly (called MEA) with multiple sets of banded electrodes is used to replace a traditional MEA that is made with only a set electrodes. With this new design, the fuel cell voltage can easily increase in a layer. The designed stack can provide multiple voltages and currents by proper series and/or parallel connections. The variable voltage 16-cell fuel cell is composed of 4-layer 4-banded type MEAs and 5 heterogeneous carbon fiber bunch bipolar plates. The 16-cell stack is divided into 4 sets. Each set of 4 series connection cell is arranged in a line in 4 different layers. The 4-cell sets can connect by series/parallel on the two ends of the stack. The total volume of the 16-cell stack is 385cm3 and its weight is 365g. The new design can power or charge certain 3C products directly. If 2 sets of 4-cell fuel cells are connected in series, the stack can provide 2A at 3.6V. With the above 2 sets of 2*4-cell connected in parallel, the stack can provide 3.5A at 3.6V. If the 4 sets of 4-cell are all connected in series, the stack can provide 1.8 A at 7.2V. These voltages and currents derived from these stacks can power or charge a mobile phone, a photo camera and a video camera directly. If a higher voltage or current are needed, two or more 16-cell stacks can be connected in series XI or parallel. Then notebooks or any other 3C products in which higher power are needed can be driven.

In recent years, palladium (Pd) has been extensively studied for a possible alternative for Pt that has been most commonly used as a catalyst in fuel cells. However, Pd shows lower activity than Pt towards the cathodic oxygen reduction reaction (ORR) and also exhibits poor tolerance toward carbon monoxide (CO) poisoning occurring in the anode process. To improve its performance, alloying Pd with other transition metals has been suggested as one of promising solutions as the Pd-based alloys have been found to boost the ORR activity and yield significant improvement in the CO tolerance. However, a detailed understanding of the alloying effects is still lacking, despite its importance in designing and developing new and more cost effective fuel cell catalysts. This is in large part due to the difficulty of direct characterization. Alternatively, computational approaches based on quantum mechanics have emerged as a powerful and flexible means to unravel the complex alloying effects in multimetallic catalysts; such first principles-based computational studies have provided many invaluable insights into the mechanisms of catalytic reactions occurring on the alloy surfaces. Using first-principles density-functional theory calculations, we have examined the surface reactivity of Pd-based bimetallic catalysts with the aim of better understanding the alloying effects in association with atomic arrangement, facet, local strain, ligand interaction, and effective atomic coordination number at the surface. More specifically, this thesis work has focused on examining the following topics: Role of Pd ensembles in selective H₂O₂ formation on AuPd alloys; Effect of local strain and low-coordination number at the surface on the performance of Pd monomer in selective H₂O₂ formation; Different facet effects on the activity of Pd ensembles towards ORR; Structure of ternary Pd-Ir-Co alloys and its reactivity towards ORR; Pd ensembles effects on CO oxidation on CO-precovered Pd ensembles; Role of ligand and ensembles in determining CO chemisorptions on AuPd and AuPt. Our first principles-based theoretical investigation of bimetallic alloys offers some insights into the rational design and development of alloyed catalysts. / text

Fuel cells are promising energy conversion devices, but they have not been widely adopted because of their very high cost. The most expensive component of a fuel cell is the membrane electrode assembly, a polymer film coated with catalyst material. The catalyst layer is fabricated by depositing and drying a liquid ink on the membrane. The membrane can rapidly absorb water from the ink, causing swelling strain sufficient for wrinkling, which can cause defects in the finished product. These challenges limit most catalyst layer fabrication to individual preparation by hand. We seek to understand the swelling phenomenon in a way that enables the control of defects during mass production. Membrane swelling is a transient, three-dimensional, coupled mass transfer, heat transfer and solid mechanics problem. Existing models describe the membrane in fuel cell operating conditions, making use of simplifying assumptions that are not valid for predicting manufacturing defects. We present a new model incorporating effects that are missing from other models. We present simulation results for scenarios relevant to the control of defects. Simple spatial variations in water content can cause curl and wrinkling; we establish criteria for the formation of these defects by simulating the membrane's response when subjected to the full pre-swollen coating and drying process. We investigate the sensitivity of wrinkling to nonuniformity in the coating and to processing conditions in the coating line. We propose a rationale for controlling wrinkling caused by these effects and for diagnosing coating defects using the membrane's wrinkling response. We show how the membrane behaves differently depending on whether the coating is applied to one side or to both sides simultaneously. We have designed and constructed a machine to pre-swell the membrane, apply a coating and then dry the coating under controlled tension, speed, temperature and humidity. We present the design and discuss how the machine may be used, together with the membrane model, to predict and control defects in catalyst-coated membranes. / text

The development of a control-oriented dynamic relative humidity model for a polymer electrolyte membrane (PEM) fuel cell stack is presented. This model is integrated with a first law based thermal model, which tracks energy flow within four defined control volumes in the fuel cell; the cathode channel, anode channel, coolant channel, and fuel cell stack body. Energy and mass conservation equations are developed for each control volume. On top of mass conservation, electro-drag and osmosis models were also implemented within the model to account for the major modes of vapor transfer through the membrane between the anode and cathode. Requisite alterations to the thermal model as well as mass flow rate calculations are also discussed. Initially, the model utilized a single lumped control volume for the calculation of all values each channel (anode and cathode). This lumped value method is computationally inexpensive, and makes the model optimal for control design. However, investigation of the mass-based Biot number showed the need for greater granularity along the length of the channels to properly capture the relative humidity dynamics. In order to improve the resolution of the model, while still minimizing the computation expense, the model was subdivided into a series of lumped value models. The cathode channel was the point of focus as it is the major concern from a controls perspective. This method captures the proper trends found in far more complex CFD models, while still maintaining a quick calculation time. Different levels are subdivision (3 and 6 submodels) are investigated, and the differences discussed. Particularly, temperature range, relative humidity range, the effect on the modeled voltage, and calculation time are compared. This control-oriented model is low order and based on lumped parameters, which makes the computational expense low. Formulation of this model enables the development of control algorithms to achieve optimal thermal and water management. / text

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19090号 / エネ博第314号 / 新制||エネ||64 / 32041 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 佐川 尚, 教授 野平 俊之 / 学位規則第4条第1項該当

Fuel cells are one of most environmental friendly energy sources; they have many advantages and may be used in many applications from portable electronic devices to automotive components. Proton exchange membrane (PEM) fuel cells are one of most reliable fuel cells and have advantage such as rapid-startup and ease of operation. This dissertation focuses on PEM fuel cell stack optimization based on operation experimental research and numerical modeling study. This dissertation presents three major research activities and the obtained results by the Ph.D candidate. A novel stack architecture design is introduced in order to decrease mal-distribution and non-uniform output performance between individual cells in order to improve the stack performance. Novel stack architecture includes a novel external bifurcation flow distribution delivery system. One major issue of uniform distribution of reactants inside individual fuel cells and between fuel cells in a fuel cell stack is solved by the novel stack architecture design. A novel method for uniform flow distribution was proposed, in which multiple levels of flow channel bifurcations were considered to uniformly distribute a flow into 2ⁿ flow channels at the final stage, after n levels of bifurcation. Some detailed parameters such as the flow channel length and width at each level of bifurcation as well as the curvature of the turning area of flow channels were particularly investigated. Computational fluid dynamics (CFD) based analysis and experimental tests were conducted to study the effect of the flow channel bifurcation structure and dimensions on the flow distribution uniformity. Optimization design and factors influential to the flow distribution uniformity were also delineated through the study. The flow field with the novel flow distribution was then considered to be used in a cooling plate for large fuel cell stacks and a possible method for cooling electronic devices. Details of the heat transfer performance, particularly the temperature distributions, on the heating surface as well as the pressure losses in the operation were obtained. In the second part of the research, experimental testing, analytical modeling, and CFD methods were employed for the study and optimization of flow fields and flow channel geometry in order to improve fuel cell performance. Based on the experimental results, a serpentine flow field is chosen for CFD and modeling analysis. Serpentine flow channel optimization is based on the parametrical study of many combinations of total channel width and rib ratio. Modeling analysis and in-house made computational code was used to optimize the dimensions of flow channels and channel walls. It is recommended that cell channel design should use a small total channel width and rib ratio. Proton exchange membrane fuel cells were fabricated based on the optimization results. Experimental tests were conducted and the results coincided with the numerical analysis; therefore, small total width and rib ratio design could significantly improve the fuel cell performance. Three dimensional (3D) CFD simulations for various PEM fuel cells were conducted to investigate information such as water and reactants distribution. The direct simulation results of current density distribution proclaim how the channel design influences the performance. The final section of research is stack bipolar plate flow field optimization. Optimized channel geometries are applied to the serpentine channel design for the stack. This serpentine channel design evolved to parallel-serpentine channel and symmetric serpentine channel design. Experimental tests of the stacks using the above flow fields are compared to one another and the results recommend use of the novel symmetric serpentine flow channel for stack bipolar design to achieve best performance.

This dissertation focuses on the preparation of new Nafion®/ ex-situ silica nanocomposites membranes and the impact of particle size of spherical silica particles on the nanocomposites' properties. To achieve acceptable power production, fuel cell polymer membranes are required with good proton conductivity, water retention, thermal and mechanical stability. However, to avoid poisoning of fuel cell electrocatalysts with CO or other fuel contaminants, they must be operated at temperatures (>100 °C). At these temperatures, fuel cell membranes dehydrate resulting in dramatic decreases in proton conductivity or complete failure as membranes crack due to volumetric stress from water loss. Even if fuel cell is kept in a humidified chamber, increasing temperature will eventually shut the cell down as Nafion®'s bicontinuous structure "dissolves" into a single poorly conducting phase at temperatures above the polymer's Tg.This research provides systematic studies of effects of silica particle size on properties of silica-Nafion® nanocomposites. Results of this study include new insights into requirements for reproducible particle syntheses, practical methods for avoiding silica particle floatation during Nafion® nanocomposite membranes preparation, and a summary of the influence of particle size and functionalization on Nafion® membrane properties. Stober particle syntheses showed high sensitive to ammonia concentration and we discovered that literature procedures' variability is likely due to researchers failure to actually measure ammonia concentration in their aqueous base (which can be 50% or more off). Homogeneous nanocomposite membranes, as determined by AFM and SEM, were successfully prepared using more viscous dispersions. It was observed that nanocomposites membranes with small particles (<50 nm) showed significant increases in proton conductivity at temperatures above 80 °C. Surface modification of the silica particles improved the proton conductivity at 80 °C. Enhancement on proton conductivity was more pronounced with small modified particles at temperatures < 80 °C but unmodified particles were better than modified particles at temperatures >80 °C. Small, unmodified particles led to enhanced thermal stability of the Nafion® ionic domain, however, surface modification did not result in any thermal stability enhancement. Contrary to the expected, mechanical properties of the Nafion® were degraded by adding the silica particles, especially with smaller particles (<50nm).

Research was performed on the development of relationships between the microstructure of nickel and yttria-stabilized zirconia (YSZ) coatings and the processing parameters used for their deposition by atmospheric plasma spraying (APS). Research was also performed on the development of relationships between the microstructure of plasma sprayed Ni-YSZ coatings and the electrochemical performance of metal-supported solid oxide fuel cells (SOFCs) incorporating these coatings as anodes. Three APS processes were used to deposit Ni-YSZ coatings: dry-powder plasma spraying (DPPS), suspension plasma spraying (SPS), and solution precursor plasma spraying (SPPS). These processes differ in the form of the feedstock injected into the plasma. The composition of the Ni-YSZ coatings deposited with each spray process could be controlled through adjustment of the plasma gas composition and stand-off distance, as well as adjustment of feedstock properties including agglomerate size fraction for DPPS, NiO particle size and suspension feed rate in SPS, and the enthalpy of decomposition of the precursors used in SPPS. The porosity of the Ni-YSZ coatings could be controlled through the addition of a sacrificial pore forming material to each feedstock, with coating porosities up to approximately 35% being achieved for each coating type. Metal-supported SOFCs were fabricated to each have anodes deposited with a different plasma spray process, where all anodes had nominally identical composition. The microstructures obtained for each anode type were distinctly different. SPPS led to the most uniform mixing of the smallest Ni and YSZ particles. These anodes most resembled typical structures from anodes fabricated using conventional methods. It was found that the polarization resistance, Rp, associated with the high frequency (&gt; 1 kHz) range of the impedance spectrum correlated to the three phase boundary length (TPBL) density of each anode, with lower Rp values corresponding to higher TPBL densities. It was also found that the Knudsen diffusion coefficient and effective ordinary diffusion coefficient of the porous anodes correlated with the Rp associated with the low frequency (&lt; 1 kHz) range of the impedance spectrum. Therefore, the impedance spectrum can be used to compare microstructural differences among plasma sprayed Ni-YSZ anodes.