Pt/C catalyst is now widely used for electrode of polymer electrolyte membrane fuel cell(PEMFC). Nano-sized Pt particles impregnated on conductive carbon supports provide larger catalytic surfaces and highly porous structures. However, its use is limited when carbons meet water vapor at high voltages in unitized regenerative fuel cell and reformed hydrogen fuel containing CO(Carbon monoxide) gas is used. Carbon corrodes and deteriorates performance of hydrogen generation from water vapor. Pt is poisoned by CO gas and lose its function as a catalyst of hydrogen oxidation.
Therefore, in this study, new support materials of Pt catalyst have been investigated to overcome the weakness of carbon support. Titanium sub-oxides (Magneli phases) and $La^{3+}$ substituted cerate have been suggested as a functional support.
In chapter 1, firstly single cell operating conditions and cell constituents have been optimized for the following test. Cell temperature, pressure, gas flow rate and temperature of humidifier was set up as 80 ℃, 2atm, 100sccm : 200sccm $(H_{2} : O_{2})$ and 90 ℃ : 85 ℃ $(H_{2} : O_{2})$. Nafion 115, 1035 membrane, Nafion ionomer and teflon coated carbon paper was selected for the cell constituents. Membrane/Electrode assembly was fabricated by hot pressing at 130 ℃, 90atm, for 1 minutes. Composition of Nafion ionomer and Pt loadings in electrode was important for the improvement of cell performance which was correlated with the area of tri-phase active surfaces.
The cell performance is known to be affected not only by composition of electrode but also by its structure. The structure of catalyst can be controlled by fabrication process and improved by cell activation procedure. Although many kinds of coating method have been applied to the fabrication process of catalytic layer such as hand brushing, screen printing, rolling and spraying, screen printing becomes one of the most popular methods due to its convenience and adaptability. However, there are two things to be focused on in the screen printing. The first is swelling problem of membrane. In the conventional screen printing, Nafion membrane must be temporally changed to $Na^+$ form to avoid swelling trouble during the hot press decal transferring. $Na^+$ formed Nafion membrane is stable at the temperature range from 150 ℃ to 160 ℃ during the hot pressing. Also in the previous direct screen printing, the slurry was also applied to membrane in $Na^+$ form or tetra-butyl ammonium form to stabilize the catalytic layer. Anyhow both processes are still subject to being complicated yet. The second is organic solvent like glycerin which is essential for the coating property but assumed to deteriorate the activity in electrode. Usually organic solvent used for the screen printing is required to be highly viscous, so it can not be easily removed due to its elevating boiling temperature. Therefore, improved screen printing method which is rather simple without additive stabilization of catalytic layer steps but suppress the swelling trouble has been presented. And cell activation and its performance has been investigated by focusing on the glycerin.
Two kinds of simplified screen printing methods have been suggested which are with or without gasket unified. The unified method revealed better performance especially at high current area due to blocking the gas leak during the operation which induce mass transport polarization. Anyhow both methods were competent to the previous methods in spite of eliminating the membrane treatment process to $Na^+$ form. These methods provide simplified and faster fabrication chances.
I-V and C-V characteristics indicated that glycerin tends to degrade activity of catalysts obstructing gas flow and ionic conduction and retards activation time linearly with increase of glycerin. However, minimum addition was necessary for the better coating of catalytic layer. Futhermore Pt loading can be controlled by varying glycerin addition. Optimized condition of glycerin for cell performance was 1:1 about 5% Nafion solution. From the TG and viscosity experiments, it could be conjectured that glycerin gradually escapes from the electrode during the cell activation rather than simply evaporates.
Pt/C catalysts were made from two kinds of chemical method. Conventional sulfito complex route and direct reduction route were taken. Nano-sized Pt particles under 2nm could be impregnated on carbon support from direct reduction route, while some Pt clusters were impregnated from sulfito complex route. Speed of stirring precursors and dropping reduction agent were key processes for the impregnation of nano-sized Pt particles. Cell performances were measured using the Pt/C catalysts made from two kinds of catalysts and commercial one. Compared with commercial Pt/C(E-tek), cell performance was better due to larger surface area, although Pt composition in Pt/C made from direct reduction route was lower.
In chapter 2, firstly titanium sub-oxides (Magneli phases) and $La^{3+}$ substituted cerate were synthesized. Magneli phase are a sub-stoichiometric composition of titanium oxides as the general formula $Ti_{n}O_{2n-1}$ $(10 ≥ n ≥ 4)$ which have higher electrical conductivity but more resistive in corrosion. They are identifiable compounds and not simply doped titania or casual mixture of $TiO_{x}$ (x<2). They were produced from high temperature over 1050$^\circ C$, reducing titania in a hydrogen atmosphere. $La^{3+}$ substituted cerate are nonstoichiometric oxide with a lot of oxygen vacancies maintaing its fluorite structure. Due to a number of defects, oxygen in the surface or bulk can be evolved and stored more easily.
Using the prepared metal oxide supports, Pt/Magneli phase, Pt/Mn-Magneli phase, $Pt/CeO_{2}$, $Pt/La_{x}Ce_{1-x}O_{2-x/2}$ , $(Pt/C, La_{0.3}Ce_{0.7}O_{1.85})$ and Pt/(C,$La_{0.3}Ce_{0.7}O_{1.85}$) catalysts have been fabricated. The size of Pt particles impregnated on metal oxide supports was small enough to make surface area of Pt over 60 ㎡/g$. This value was about twice larger than that of Pt black, 31 ㎡/g$. Pt/Magneli phase, Pt/Mn-Magneli phase are applied to the electrode of URFC(Unitized Regenerative Fuel Cell) for hydrogen regeneration without separate fuel supplier and $Pt/CeO_{2}$, $Pt/La_{x}Ce_{1-x}O_{2-x/2}$, $(Pt/C, La_{0.3}Ce_{0.7}O_{1.85})$ and $Pt/(C,La_{0.3}Ce_{0.7}O_{1.85})$ are applied to the electrode of PEMFC for enhancing the resistance of CO poisoning.
The conductivity of Magneli phase and Mn-doped Magneli phase are measured. The conductivity of Mn-doped Magneli phase was higher than undoped Magneli phase and increased with doping level to 3%. For the Magneli phase, the conductivity of `Ebonex` composition which composed of $Ti_{4}0_{7}$, $Ti_{5}0_{9}$, $Ti_{6}0_{11}$, $Ti_{7}0_{13}$, $Ti_{8}0_{15}$ was the highest.
The hydrogen regeneration performance of Pt/Maneli phase was higher than that of Pt black due to large Pt surfaces, despite Pt loading of Pt/Magneli phase was an half of Pt black. However, the enhancement was not so big because pore volume in Pt/Magneli phase was very low. The composition of Nafion ionomer was optimized differently for Pt black and Pt/Magneli phase. In Fuel cell mode, the cell performance of each catalysts was increased as the follow sequence; Pt/Mn(3%)-$Ti_{4}O_{7}$ < Pt black < Pt/Ebonex < Pt/Mn(3%)-Ebonex. In electrolyzer mode, the sequence was Pt/C < Pt black < Pt/Ebonex < Pt/Mn(3%)-$Ti_{4}O_{7}$ < Pt/Mn(3%)-Ebonex. Cell performance could be improved when supports of Pt were applied and Manganese was doped in Magneli phase.
On the other hand, $La^{3+}$ can be substituted in $Ce^{4+}$ site until 30% composition. Over 40% substitution, $La_{2}O_{3}$ phase precipitated and pyrochloric cation ordering between $La^{3+}$ and $Ce^{4+}$ showed up. Catalytic activity about CO preferential oxidation was measured for $Pt/CeO_{2}$, $Pt/La_{x}Ce_{1-x}O_{2-x/2}$ (0.1
