The effects of actively pulsing reactant flow rates into solid polymer electrolyte fuel cells were investigated in this thesis. First, work was conducted to determine the magnitude of voltage response to pulsed reactant flow on a direct hydrogen proton exchange membrane (PEM) cell. The effects of pulsed reactant flow into a direct methanol fuel cell (DMFC) were then considered. The PEM work showed substantially greater response to pulsed air flow than to pulsed fuel flow. It was found that several parameters affect the magnitude of cell response to active flow control (AFC). Increasing current load, increasing the magnitude of flow oscillation, decreasing the frequency of oscillation, and decreasing the average level of excess reactant supplied were found to maximize both the level of voltage oscillations and the decrease in cell power from steady state performance. Greater response to pulsed oxidant flow is believed to have been observed due to effects brought about by changes in membrane humidity. In contrast, pulsed fuel flow showed the greatest response in the study of DMFC technology. In this case, time averaged cell voltage was found to increase as the time averaged fuel flow rate was reduced. The increase in average cell power is the result of a reduction in methanol crossover; sustainable increases of up to 6% in power output were measured. The parameters found to effect the increase in cell power observed include the frequency of oscillation and the time-averaged NOSfuel. Pulsed air flow on the DMFC did not show any such rise in voltage, supporting the hypothesis that a reduction in methanol crossover is the phenomenon which brings about enhanced 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).

<p>In polymer electrolyte fuel cells (PEFC) chemical energy, in for example hydrogen, is converted by an electrochemical process into electrical energy. The PEFC has a working temperature generally below 100 °C. Under these conditions water management and transport of oxygen to the cathode are the parameters limiting the performance of the PEFC.</p><p>The purpose of this thesis was to better understand the complex processes in different parts of the PEFC. The rate-limiting processes in the cathode were studied using pure oxygen while varying oxygen pressure and humidity. Mass-transport limitations in the gas diffusion layer using oxygen diluted in nitrogen or helium was also studied. A large capacitive loop was seen at 1-10 Hz with 5-20 % oxygen. When nitrogen was changed to helium, which has a higher binary diffusion coefficient, the loop decreased and shifted to a higher frequency.</p><p>Steady-state and electrochemical impedance spectroscopy (EIS) models have been developed that accounts for water transport in the membrane and the influence of water on the anode. Due to water drag, the membrane resistance changes with current density. This gives rise to a low frequency loop in the complex plane plot. The loop appeared at a frequency of around 0.1 Hz and varied with <em>D</em>/<em>L_m</em>², where <em>D</em> is the water diffusion coefficient and <em>L_m</em> is the membrane thickness. The EIS model for the hydrogen electrode gave three to four semicircles in the complex plane plot when taking the influence of water concentration on the anode conductivity and kinetics into account. The high-frequency semicircle is attributed to the Volmer reaction, the medium-frequency semicircle to the pseudocapacitance resulting from the adsorbed hydrogen, and the low-frequency semicircles to variations in electrode performance with water concentration. These low-frequency semicircles appear in a frequency range overlapping with the low-frequency semicircles from the water transport in the membrane. The effects of current density and membrane thickness were studied experimentally. An expected shift in frequency, when varying the membrane thickness was seen. This shift confirms the theory that the low-frequency loop is connected to the water transport in the membrane.</p>

<p>This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells.</p><p>A model is formulated to describe an experimental cell setup consisting of a Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario.</p><p>Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.</p>

While there is a substantial amount of information regarding the electrochemical behavior of fuel cells and there components little to no information is available regarding the mechanical properties of fuel cell materials in stack setups. This set of experiments was set up to test mechanical properties of gas diffusion layer and bipolar plate materials in a one cell setup. Samples were clamped to specified pressures and deformation properties were observed and measured. Measurements were taken of impingements of the gas diffusion layers into the gas flow channels. A limit for compression of cell configurations was found to be approximately 300psi. Upon reaching the compression limit bipolar plates collapse and materials between plates show signs of breakage. Under compression diffusion media showed impingement into the gas flow channels as well as substantial compression of the three layer stack.

In polymer electrolyte fuel cells (PEFC) chemical energy, in for example hydrogen, is converted by an electrochemical process into electrical energy. The PEFC has a working temperature generally below 100 °C. Under these conditions water management and transport of oxygen to the cathode are the parameters limiting the performance of the PEFC. The purpose of this thesis was to better understand the complex processes in different parts of the PEFC. The rate-limiting processes in the cathode were studied using pure oxygen while varying oxygen pressure and humidity. Mass-transport limitations in the gas diffusion layer using oxygen diluted in nitrogen or helium was also studied. A large capacitive loop was seen at 1-10 Hz with 5-20 % oxygen. When nitrogen was changed to helium, which has a higher binary diffusion coefficient, the loop decreased and shifted to a higher frequency. Steady-state and electrochemical impedance spectroscopy (EIS) models have been developed that accounts for water transport in the membrane and the influence of water on the anode. Due to water drag, the membrane resistance changes with current density. This gives rise to a low frequency loop in the complex plane plot. The loop appeared at a frequency of around 0.1 Hz and varied with D/Lm2, where D is the water diffusion coefficient and Lm is the membrane thickness. The EIS model for the hydrogen electrode gave three to four semicircles in the complex plane plot when taking the influence of water concentration on the anode conductivity and kinetics into account. The high-frequency semicircle is attributed to the Volmer reaction, the medium-frequency semicircle to the pseudocapacitance resulting from the adsorbed hydrogen, and the low-frequency semicircles to variations in electrode performance with water concentration. These low-frequency semicircles appear in a frequency range overlapping with the low-frequency semicircles from the water transport in the membrane. The effects of current density and membrane thickness were studied experimentally. An expected shift in frequency, when varying the membrane thickness was seen. This shift confirms the theory that the low-frequency loop is connected to the water transport in the membrane. / <p>QC 20121011</p>

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24644号 / 工博第5150号 / 新制||工||1983(附属図書館) / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 河瀬 元明, 准教授 中川 浩行, 教授 外輪 健一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells. A model is formulated to describe an experimental cell setup consisting of a LixNi0.8Co0.15Al0.05O2 composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged LixNi0.8Co0.15Al0.05O2 composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario. Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23287号 / 人博第1002号 / 新制||人||236(附属図書館) / 2020||人博||1002(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 田部 勢津久, 准教授 戸﨑 充男 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

<p>Proton exchange membrane fuel cells and lithium polymer batteries are important as future power sources in electronic devices, vehicles and stationary applications. The development of these power sources involves finding and characterising materials that are well suited r the application.</p><p>The materials investigated in this thesis are the perfluorosulphonic ionomer Nafion^TM(DuPont) and metal oxides incorporated into the membrane form of this material. The ionomer is used as polymer electrolyte in proton exchange membrane fuel cells (PEMFC) and the metal oxides are used as cathode materials in lithium polymer batters (LPB).</p><p>Crystallinity in cast Nafion films can be introduced by ion beam exposure or aging. Spectroscopic investigations of the crystallinity of the ionomer indicate that the crystalline regions contain less water than amorphous regions and this could in part explain the drying out of the polymer electrolyte membrane in a PEMFC.</p><p>Spectroscopic results on the equilibrated water uptake and the state of water in thin cast ionomer films indicate that there is a full proton transfer from the sulphonic acid group in the ionomer when there is one water molecule per sulphonate group.</p><p>The LPB cathode materials, lithium manganese oxide and lithium cobalt oxide, were incorporated <i>in situ</i> in Nafion membranes. Other manganese oxides and cobalt oxides were incorporated <i>in situ</i> inside the membrane. Ion-exchange experiments from HcoO₂to LiCoO₂within the membrane were also successful.</p><p>Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray diffraction were used for the characterisation of the incorporated species and the Nafion film/membrane.</p>