Oxygen Reduction Reaction Activity and Durability of Platinum/Cobalt Alloy Nanoparticle Catalysts in Alkaline Media

Abstract

Introduction Anion exchange membrane fuel cells have recently attracted considerable attention as promising energy conversion devices because of possible use of non-precious metals as electrocatalysts such as Ni and Co. In order to achieve high fuel cell performance, it is essential to develop electrocatalysts that have high oxygen reduction reaction (ORR) activity and durability in alkaline media. Previously, our group reported that PtCo alloy nanoparticles with controlled particle size, alloy composition, and metal loading could be prepared by nanocapsule method.1 The PtCo/C thus obtained showed high ORR activity and reasonable durability in acidic media compared to those of the commercial Pt/C and Pt/C catalysts.2 In contrast, ORR properties of Pt alloy catalysts in alkaline media have not been investigated as much. In this study, we report ORR activity, and durability of our Pt/Co catalysts prepared by nanocapsule method in alkaline media, in particular, the effect of alloy composition on these properties. The results will be compared with those of the commercial Pt/C and PtCo/C catalysts in details. Experimental Platinum/cobalt alloy nanoparticle catalysts were prepared by nanocapsule method,1 and supported on acetylene black (AB) that had 800 m2 g-1 specific surface area. The prepared catalysts (PtCo/AB) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray fluorescence (XRF). The loaded amounts of the metals on carbon support were quantified by thermogravimetric analyses (TGA) in air from room temperature to 600 °C. Electrochemical measurements were performed with a typical rotating ring disk electrode (RRDE) equipment with a gas-tight Pyrex glass cell in 0.1 M KOH. A ring-shaped platinum wire and a reversible hydrogen electrode (RHE) were used as the counter electrode and the reference electrode, respectively. All electrode potentials are stated relative to the RHE. The ORR catalytic activity was measured at 25 °C under air saturated conditions. The durability test of PtCo/AB was performed in 0.1 M KOH saturated with nitrogen at 40 °C. The potential was stepped between 0.6 V and 1.0 V, with a holding period of 3 sec at each potential (6 sec for one cycle). After a given number of potential step cycles, cyclic voltammetry and RRDE measurement were performed as mentioned above to examine the changes in ECA and the ORR activities. Results & discussion XRD patterns of PtCo/AB showed characteristic peaks assignable to platinum/cobalt alloys and carbon support. TEM images revealed that the obtained catalysts contained platinum/cobalt alloy nanoparticles well-distributed on AB support. The average particle sizes were ca. 3 nm. TG measurement suggested that metal loading amounts were ca. 20 wt% nearly comparable to the target values. Five catalysts with different compositions (Pt100-xCox: x = 31, 39, 42, 51, 73 atom%) were obtained. Cyclic voltammograms of PtCo/AB catalysts indicate characteristic peaks of oxidation/reduction of platinum and cobalt and hydrogen adsorption/desorption. The onset potential for ORR of PtCo/AB catalysts were higher than that of the commercial Pt/C catalyst. It was found that the PtCo/AB (x = 34 and 42) was the most active for ORR. The highest mass activity (MA) at 0.85 V was ca. 767 A g-1 Ptat the optimum composition. The optimum composition of Co was higher than that (ca. 25 atom%) in acidic media. The yield of hydrogen peroxide via two electron transfer mechanism was similar to that of the commercial Pt/C catalyst. In the durability test, however, MA of PtCo/AB catalysts decreased with the potential step cycle. After 30000 cycles, the MA of PtCo/AB catalysts was comparable to that of the commercial Pt/C catalyst, implying the dealloying during the durability test. Details of the differences between in alkaline media and in acidic media will be discussed. Acknowledgement This work was partly supported by Japanese Science Technology Agency (JST), CREST.