Superlative Activity and Durability of Stabilized Pt-Skin Pt-M (M=Fe, Co, Ni) Alloy Cathode Catalysts

Abstract

Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is one of the most important issues for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs). To obtain high mass activity (MA), Pt alloy nanoparticles dispersed on high-surface-area carbon support have been prepared. However, the durability of conventional PtM alloys (M = Fe, Co, Ni) have been insufficient due to an appreciable dealloying of M component at high temperatures (> 60 °C).1, 2 Recently, to increase both the MA and durability, we have succeeded in preparing new catalyst with two atomic layer of Pt skin (stabilized Pt skin layer) formed on PtCo-core alloys nanoparticles supported on graphitized carbon black (GCB; 150 m2 g-1) 2, 3 and high surface area carbon black (C; 800 m2 g-1).4 The preparation is briefly described below.3, 4 The PtCo alloy (1:1 atomic ratio) nanoparticles with uniform size and composition were highly dispersed on GCB or C was performed by the nanocapsule method.5 The PtCo/GCB or PtCo/C was heat-treated in H2 atmosphere in order to form nearly one atomic layer (Pt1AL) of Pt skin on the surface of the PtCo nanoparticles (denoted as Pt1AL–PtCo) by the segregation of Pt atoms from interior of the alloy core. Then, one more atomic layer of Pt-skin was formed on Pt1AL–PtCo (denoted as Pt2AL–PtCo) via controlled Pt deposition from a Pt-complex-containing aqueous solution with H2 bubbling. Finally, the Pt2AL–PtCo/GCB or Pt2AL–PtCo/C catalysts thus obtained were heat-treated in H2 at 200 oC.The kinetically-controlled mass activity (MA k) and specific activity (j k) for the ORR at Nafion-coated Pt2AL–PtCo/GCB and Pt2AL–PtCo/C electrodes were examined in O2-saturated 0.1 M HClO4 solution at 65 oC by multi-channel flow double electrode cell.1 The accelerated durability test (ADT) was examined by a standard potential step protocol (E = 0.6 V ↔ 1.0 V vs. RHE, holding 3 s at each E) simulated the load-change for the fuel cell vehicle (FCV)6 in N2-purged 0.1 M HClO4 solution at 65 oC.Figure 1 shows the plots of MA k and j k values at Nafion-coated Pt2AL–PtCo/GCB and Pt2AL–PtCo/C electrodes as a function of the potential step cycles (N). The initial values of j k at Pt2AL–PtCo/GCB (d TEM = 2.9 ± 0.2 nm) and Pt2AL–PtCo/C (d TEM = 3.3 ± 0.5 nm) were identical with that of PtCo/GCB, which was 2.5 times larger than that of a commercial standard catalyst c-Pt/C. The value of MA k at c-Pt/C decreased severely to 10% of the initial value after N = 30,000, whereas the j k was nearly unchanged. The MA k and j k values at PtCo/GCB were drastically decreased with increase N and were equal to that for c-Pt/CB after N = 30,000 due to the dealloying of Co. In contrast, the value of j k at the Pt2AL–PtCo/GCB electrode was maintained constant value during the ADT, and the retention of MA k at N = 30,000 was as high as 68%. A similar trend was seen for the Pt2AL–PtCo/C, at least, up to N= 10,000. Thus, the dealloying of Co was almost completely suppressed by the stabilized Pt skin layer on the surface regardless of GCB and C support.We have also succeeded in preparing the Pt2AL–PtM alloy (M = Fe and Ni) dispersed on C support by the same method as described above. The average particle sizes d TEM of the Pt2AL–PtFe/C and Pt2AL–PtNi/C were 2.9 ± 0.4 nm and 3.2 ± 0.4 nm, respectively. The Pt2AL–PtFe/C and Pt2AL–PtNi/C also exhibited high MA k. We will present the effect of alloy components on the ORR activity and durability.This work was supported by the funds for the “SPer-FC” project from the NEDO of Japan.