고분자 전해질 연료전지 양극용 고활성 PtNi/C 전극의 제조와 평가

Abstract

We prepare carbon-supported PtM catalysts which Pt is selectively adsorbed on metal surface by using isoelectric point (IEP) difference method. If the pH of slurry is adjusted between IEPs of metal oxide and carbon, surface charge of metal oxides becomes positive and that of carbon becomes negative. And then we introduce platinum anionic precursor such as chloroplatinic acid (H2PtCl6) to the slurry. Because the surface charge of carbon is negative, the introduced platinum anionic precursor should be adsorbed on metal oxide surface not on carbon surface. Among the tested metal oxides, for nickel oxide and copper oxide Pt is preferentially adsorbed on metal oxide surface. In the case of the other metal oxides such as Fe, Ru, Co, Cr and Mn, Pt was not preferentially adsorbed on metal oxide surface. For nickel oxide and copper oxide we monitor the amount of Pt that is adsorbed on metal oxide and on carbon as time at various conditions such as pH of solutions, metal contents, or initial Pt concentration. In the case of nickel oxide, the difference of Pt amount adsorbed on nickel oxide and carbon is decreasing after about 20 h of adsorption time. As initial Pt concentration is decreasing in solution, the ratio of Pt amount is a little increasing. But Pt contents in PtNi are also decreasing as decreasing Pt concentration. As nickel contents is decreasing, the ratios of Pt amount adsorbed on nickel is strangely increasing. The condition that shows the highest ratio of Pt amount adsorbed on nickel is low Pt concentration and low nickel contents. For copper oxide as initial Pt concentration is decreasing, Pt contents are also decreasing but the ratios of Pt amount on copper don’t have any tendency dislike on nickel. However the ratios of Pt amount are increasing as copper contents are decreasing like nickel.We carry out the oxygen reduction activity for PtNi/C and PtCu/C electrocatalysts prepared by IEP difference method. For PtNi/C Pt and Ni contents are 8.7 wt% of Pt and 16.7 wt% of Ni and for PtCu/C Pt and Ni contentrs are 6.8 wt. % Pt and 14.5 wt. % Cu. From the results of the XRD, XPS and XAFS for PtNi200 and PtNi400 it is apparent that the adsorbed platinum exists as both small particles and platinum metal that is not alloyed or strongly interacting with nickel. In the case of PtNi600, however, it was found that the Pt is perfectly alloyed with Ni at 600°C and the size of the PtNi particles is 1.9 nm. Moreover PtNi600 exhibits 39% improved mass activity than 10% Pt/C catalyst but specific activity is not much improved because of its small particle size. In the case of PtCu/C electrocatalysts, it does not show more improved the oxygen reduction activity than commercial Pt/C catalysts.In order to evaluate catalytic activity in detail we prepare PtNi/C electrocatalysts with very low content of Pt by the same method. PtNi[Ni] catalysts has 1.76 wt.% Pt and 2.09wt.% of Ni and about 74% of Pt is preferentially adsorbed on nickel surface. PtNi[C] catalysts are prepared by the method that Pt is selectively adsorbed on carbon surface by using a platinum cationic precursor. From AAs results for PtNi[C] catalyst all of Pt is selectively adsorbed on carbon. Among prepared catalysts, PtNi[Ni]-600 (reduced at 600°C under H2 atmosphere) shows the highest oxygen reduction activity in half cell. PtNi[Ni]-800 also exhibit the high oxygen reduction activity as PtNi[Ni]-600. However in the case of PtNi[C] series catalysts, all of them do not show the high oxygen reduction activity. From the results of XAFS, the value of white line area for PtNi[Ni]-600 and PtNi[Ni]-800 that show high oxygen reduction activity is around 4.6. And their coordination number is 4.0 (Pt-Pt), 6.0 (Pt-Ni) and 3.8 (Pt-Pt), 6.3 (Pt-Ni), respectively. The ratio of Pt-Ni to Pt-Pt coordination number is 1.5 and 1.66, respectively. The catalysts that show the high oxygen reduction activity have specific geometric and electronic properties. After 100h under OCV mode, the performance of single cell do not decrease and even a little increase for PtNi[Ni]-600 catalyst. From TEM, SEM and Ni dissolution test, we can confirm that such increase for PtNi[Ni]-600 is due to Ni dissolution.