Miniature Fuel Cell with Monolithically Fabricated Si Electrodes -Fabrication of Pd-Pt Catalyst By H-UPD-Slrr-

Abstract

Miniature fuel cells have attracted attention as an ultimate portable power source, and we have developed monolithically fabricated Si electrodes for the miniature fuel cells[1]. Recent prototypes showed satisfying power density about 500mW/cm2 at 313K though the reaction area was only 1 mm2. Besides, the prototype cells used porous Pt as a catalyst layer, and the amount of Pt was quite large. 3.6mg/cm2of Pt was used and the amount of Pt must be reduced. Then, to reduce the Pt amount, slight deposition of Pt on porous Pd was attempted using UPD-SLRR (Underpotential Deposition, and Surface Limited Redox Replacement), and obtained Pd-Pt catalyst showed quite promising results[2]. Furthermore, preliminary results suggested that the Pd-Pt catalyst had high CO tolerance. We are also interested in the use of biomass derived hydrogen for our miniature fuel cells and the high CO tolerance is strongly needed. Therefore, further study of the Pd-Pt catalyst obtained using the UPD-SLRR technique. However, Cu UPD was used for SLRR and the reproducibility was not good enough, because bath change was needed between UPD and SLRR processes, and Cu oxidation was not negligible. Recently, Nutariya et al. reported that UPD-SLRR reaction is available using hydrogen UPD[3]. If this process could be applied to our porous Pd as shown in figure 1, no bath change is needed and repetitive deposition of Pt or other noble metal would be realized, and optimization of the Pd-Pt catalyst would be possible. In this study, Pt deposition on the porous Pd was attempted by H-UPD-SLRR process, and the power generation by the obtained fuel cells was demonstrated. Experimental The Si substrate used in this study was N-type (Resistivity 0.004-0.006 Ohm-cm), (100) oriented, double side mirror polished and 100 um thick wafer. The porous Pd was formed on a Si substrate using anodize porous Si. H-UPD-SLRR was performed on the porous Pd in a solution of 1mM H2PtCl6 and 0.1 M HClO4. MSE was used as a reference electrode. As a counter electrode, Pt wire was used. The electrolyte solution was deaerated for 30 minutes with N2 gas and the whole vessel for the H-UPD-SLRR was filled with N2 during the experiment. H-UPD process was performed at -600mV vs. MSE for 30 seconds. Subsequently, SLRR process was performed by opening the circuit until the electrode potential reached -280mV vs. MSE. Using the slightly Pt deposited porous Pd layer was used as catalyst layers, prototype fuel cells were built. Power generation tests were performed by the cells whose anode and cathode catalysts were Pd-Pt and Pt, respectively. Prototype cells were set into the aluminum casing at 313K in a temperature controllable chamber. 10 sccm pure hydrogen or hydrogen containing 100ppm CO were supplied to the anode. 5sccm oxygen was supplied to the cathode. Power generation was performed for more than 1 hour, and tolerance of the catalyst to the CO poisoning was observed. Results Figure 2 shows cross sectional SEM image of the porous Pd-Pt catalyst layer. Cross sectional EDS analysis of the porous Pd-Pt catalyst layer showed slight Pt deposition, and it seemed that expected reaction was realized(figure 3). The amount of Pt was estimated to be about 20 μg/cm2from the charge of underpotential H deposition. Figure 4 shows preliminary result of power generation test of Pd-Pt catalyst. Red line shows the polarization curve with pure hydrogen supply, and blue line shows polarization curve with hydrogen containing 100 ppm CO. Though the power density was dropped with 100 ppm CO, the power density kept almost constant more than 1h, though conventional catalyst shows quick power drop with 100ppm CO. With these results, CO tolerance was reconfirmed with Pd-Pt catalyst obtained by novel H-UPD-SLRR process. With the H-UPD-SLRR process, amount of Pt deposition can be finely controlled, and further optimization of the catalyst will be performed.