Vol.5 No.1 2012

Research paper : Durable polymer electrolyte fuel cells (PEFC) for residential co-generation application (K. Tanimoto et al.)−60−Synthesiology - English edition Vol.5 No.1 (2012) term operations. Also, the behavior of platinum in the alloy catalyst may be related to the durability. 4.2 Degradation observation of the real cellTo observe the fine structure of the real cell to which the aging test was done at micro level, the structural observation of the electrode catalysts was done using the transmission electron microscope (TEM). In the TEM observation, the first issue was to fabricate a sample that could be observed under TEM from the real cell subject to the aging test. The cell with which the generation test was done in the simulated aging condition was disassembled, and the thin EM sample slice of the membrane-electrode assembly (MEA) was made using an ultra microtome. Using this method, the structures of the electrode catalyst and the electrolyte membrane could be preserved and observed. Observations were done for the samples subject to the aging tests under various conditions including lack of fuel, potential flux cycle, and high potential.[2]-[4] From the measurement of the particle size distribution under TEM observation, it was found that the size of the electrode catalyst particle increased, the composition of the Pt-Ru fine particle changed[2] as the Ru eluted from the anode Pt-Ru/C catalyst as seen by energy dispersive X-ray spectroscopy (EDS), and the phenomenon occurred where the metal particles of the electrode catalyst precipitated into the electrolyte membrane depending on the test condition, as shown in Fig. 5. For the particles that precipitated into the electrolyte membrane, it was found that the particle size distribution and the spatial distribution and precipitated particles changed by the influence of the type of gas supplied to the anode and cathode and by the thickness of the electrolyte membrane.[4] Figures 5a and 5b are the TEM images of the surrounding area of the cathode when the cathode potential was retained at 1.0 V, nitrogen gas was supplied, and the electrolyte membranes with thickness 50 m (Fig. 5a) and 175 m (Fig. 5b) were used. The lower part of the photo is the Pt/C catalyst layer. In the case of the thin electrolyte membrane, many particles were precipitated near the Pt/C catalysts layer. When air was supplied to the cathode (Fig. 5c and 5d), the precipitated particles were distributed to areas further from the catalyst layer compared to when nitrogen was supplied. It was thought that the concentration inside the membrane of the hydrogen that permeated the membrane from the anode affected the precipitation distribution of the platinum particle in the membrane. In the EM observation, as shown in Fig. 5, the structural assessment of the electrode catalyst fine particles from m to several nm is possible. The effectiveness of the fine structural analysis in practical materials such as PEFC was demonstrated. As the advancement of the recent EM technology is dramatic, the observation of a single atom of Pt is possible, and we think detailed data on the electrode catalyst structure can be obtained. With the improvement of the spatial resolution of EM, it is becoming possible to study the details of the electron state of carbon with the increased sensitivity of electron energy loss spectroscopy (EELS). Detailed information can be obtained for the carbon degradation of the cathode catalyst.4.3 Clarification of the degradation mechanism in a model cellWhile it was known through experience that the start-stop operation of PEFC enhanced degradation, in 2005, an American researcher proposed the theory that the degradation occurred due to the “reverse current decay mechanism.”[5] This theory states that when the fuel supply Fig. 4 Two issues on which weight must be placed for stack degradationTwo issues important in the degradation mechanism related to the decrease of cell performance are extracted based on the generation data of the companies participating in the project. Decreased gas diffusion due to wetnessNormally, the water produced by the reaction of oxygen and hydrogen is emitted from the cell along with the reaction gas through the gas diffusion layer (such as carbon cloth) that has been treated with water repellent. The water accumulates due to the decreased water repellency of the gas diffusion layer in the generation over time, and the necessary oxygen cannot be readily supplied to the catalyst layer.Decrease in CO poisoning resistanceIn long-time operation for several years, the blocking of the active site of carbon monoxide and the ruthenium of Pt-Ru alloy, the oxidation of ruthenium, and the separation of platinum and ruthenium occur, and these cause the decrease of carbon monoxide resistance.When the hydrogen produced from hydrocarbon fuel is used, about 1 ppm of carbon monoxide is mixed.The carbon monoxide bonds with the active site of platinum to render it inactive.Currently, the Pt-Ru alloy catalyst is used to add resistance to carbon monoxide, but degradation also occurs with this alloy.Anode catalyst layerPlatinum fine particle(5-2 nm)Carbon particle (1-0.1µm)Carbon monoxideHydrogenSeparatorPlatinum fine particleOxygenGas diffusion layerCatalyst layerH+WaterCarbon paperCarbon clothCarbon blackPolymer electrolyte membraneGas flow channelGas flow channelSchematic of PEFC structure cross section


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