| Summary: | 碩士 === 東海大學 === 化學工程學系 === 93 === One of the factors caused the decrease in the efficiency of fuel cells was due to the slow kinetics in oxygen reduction reaction (ORR) on cathode. In this thesis, the core-shell bimetallic Co rich core-Pt rich shell /C electrocatalysts were prepared by two stages method in order to increase in the electroactivity of ORR and decrease in the loading of Pt. First of the stages was prepared Co/C by the thermal decomposition of Co organometallic compound (precursor), and then Pt was reduced and deposited on Co/C in the second step. The grain size and crystallinity, particle size, and compositions of the preparing electrocatalysts were analyzed by XRD, TEM, and AAS, respectively. The electroactive areas and properties of home-made Pt/C and Co rich core-Pt rich shell /C electrocatalysts was studied with cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in 0.5 M HClO4 aqueous solution.
The lower degree of alloying of Co rich core-Pt rich shell /C without thermal annealing process was obtained to be in the range of 7.15 ~ 13.10%. The maximum ekectroactive area of the home-made electrocatalysts was found to be 117.69 m2 g-1 for Co rich core-Pt rich shell (1:1)/C. Electrocatalysts without annealing showed the higher mass and specific activities compared with Pt/C. The maximum specific and mass activities were obtained to be 18.69 μAcm-2 and 10.52 A g-1 for Co rich core-Pt rich shell (1:2)/C.
The Co atoms were diffused from inside of the core-shell electrocatalysts to the surface and resulted in the increase in the degree of alloying. The specific activity of core-shell electrocatalysts annealed in N2 environment increased with the increase in the degree of alloying. The electroactive areas of the home-made core-shell electrocatalysts annealed in air and then reduced in H2 atmosphere were greater than the home-made and commercial Pt/C electrocatlysts due to the formation of the porous structure. On the other hand, the electroactivity for ORR on the core-shell electrocatalysts annealed in air and then reduced in H2 was less than that without annealing process.
Identical two Tafel slopes for ORR were found on the home-made electrocatalysts; i.e. -54 ~ 69 mV dec-1 for a lower overpotential and -106 ~ 126 mV dec-1 for a higher overpotential. The ecperimental results indicated that the similar ORR mechanisms were obtained for the home-made electrocatalysts. The specific activity of home-made core-shell electrocatalysts was 2.5 ~ 4.1 folds compared with the home-made Pt/C.
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