Highly Durable and Active PtCo Alloy/Graphitized Carbon Black Cathode Catalysts by Controlled Deposition of Stabilized Pt Skin Layers

Abstract

We have developed a new, facile solution-phase method for the controlled deposition of targeted numbers (1 to 4) of Pt-skin layers on PtCo alloy nanoparticle surfaces that have already been heat-treated in the presence of H 2 to form an initial single Pt layer, both processes being carried out with the particles on a carbon support. In particular, we report the deposition of an additional single atomic layer of Pt to produce a stabilized dual-layer Pt-skin formed on Co-enriched nanocore surfaces, which were dispersed on graphitized carbon black (Pt 2AL -PtCo HT(2) /GCB). This catalyst was found to provide high mass-activity, three times greater than that of a commercial Pt/CB catalyst for the oxygen reduction reaction and distinctively high durability during standard potential-cycling test-protocols designed to simulate load changes and start/stop cycles in 0.1 M HClO 4 solution at 65 • C. This method is one of the simplest, most convenient possible approaches to obtain both high activity and high durability for cathode catalysts used in polymer electrolyte fuel cells. It has been shown that the electrochemical oxygen reduction reaction (ORR) on Pt catalysts is enhanced by alloying with non-precious metals such as Fe, Co, and Ni, dependent on their composition. [1][2][3][4] It has also been clearly demonstrated with X-ray photoelectron spectroscopy (XPS), in situ electrochemical quartz-crystal microbalance (EQCM) and in situ electrochemical scanning-tunnelling-microscope (EC-STM) measurements at sputtered Pt alloy electrodes that nonprecious metal elements were leached out in acidic solution, followed by the spontaneous formation of an atomically flat "Pt-skin layer" on the alloy surface.2-6 The specific activity for the ORR on the Pt-skin layer has been enhanced by an electronic structure modified by that of the underlying alloy.5,7 For polymer electrolyte fuel cells (PEFCs), nano-sized platinum and platinum-based alloy (Pt-M) particles dispersed on high surface area carbon black (CB) support materials (Pt/CB, Pt-M/CB) have been used as highefficiency electrocatalysts. The properties of these nanoparticle cathode catalysts depend on their size and/or composition, shape, and distribution. However, the ORR activity and the durability of these nanoparticles are still insufficient for widespread utilization of PEFCs in applications such as power generation units for fuel cell vehicles (FCVs) and residential co-generation systems. In such practical PEFCs (operated at 60-90• C), two typical degradation modes of the cathode catalysts have been recognized: (1) Pt dissolutionredeposition caused by oxidation/reduction reactions at Pt particle surfaces during frequent load-cycling, and (2) corrosion of the carbon support from positive potential excursions during start/stop or fuel starvation, resulting in losses of Pt active surface area due to detachment of Pt nanoparticles from the support and subsequent agglomeration.The optimum particle size or shape of Pt or Pt-M alloy nanoparticles still has not been conclusively de...

Oxygen Reduction Activity and Durability of Ordered and Disordered Pt3Co Alloy Nanoparticle Catalysts at Practical Temperatures of Polymer Electrolyte Fuel Cells

“…One of the promising methods for improving the durability, while maintaining high MA k , is the formation of stabilized Pt-skin layers, which we developed recently. 53,54 The formation of stabilized Pt skin layers on the controlled Pt-alloy crystal structure is now in progress in our laboratory.…”

In Situ FTIR Analysis of CO-Tolerance of a Pt-Fe Alloy with Stabilized Pt Skin Layers as a Hydrogen Anode Catalyst for Polymer Electrolyte Fuel Cells

“…% metal loading) was prepared by the use of a high surface area (specific surface area = 780 m 2 ·g −1 ) carbon black support (Denka Co. Ltd., Tokyo, Japan) in the same manner as that described previously [11,20]. A commercial catalyst c-Pt 2 Ru 3 /C (TEC61E54, 50 wt.…”

Research and Development of Highly Active and Durable Electrocatalysts Based on Multilateral Analyses of Fuel Cell Reactions

“…In contrast, the degradation of MA k for Pt xAL -PtCo/GCB was suppressed considerably: the N 1/2 values were 14 and 128 times longer, respectively, than those for PtCo/GCB and c-Pt/CB. It is noteworthy that the j k value on Pt xAL -PtCo/GCB was maintained at a constant value to 30,000 cycles, suggesting that the stabilized Pt skin layers protected the PtCo core perfectly from the dissolution of Co. 52 A very high durability of Pt xAL -PtCo/GCB versus high potentials (simulating startup/shutdown-cycles of FCVs) is clearly seen in Fig. 3(b).…”

“…Very recently, we have successfully prepared monodisperse PtCo alloy NPs, having approximately two atomic layers of stabilized Pt skin, dispersed uniformly on graphitized carbon black (GCB) or high-surface-area carbon black (Pt xAL -PtCo/ GCB or Pt xAL -PtCo/C, 1 < x¯2 by more recent analysis). 52,53 Figure 3 shows the variations in MA k at 0.85 V on Pt xAL -PtCo/GCB as a function of log N in potential changes at 65°C, simulating loadcycles and startup/shutdown-cycles of FCVs. The initial MA k for both Pt xAL -PtCo/GCB and PtCo/GCB was a factor of ca.…”

“…[37][38][39][40][41][42] Based on these results, new practical catalysts have been synthesized. [43][44][45][46][47][48][49][50][51][52][53][54][55] New catalyst layers or membraneelectrode-assemblies (MEAs) have been developed in order to make the electrocatalysts work effectively under widely varying operating conditions of relative humidity (RH) and temperature. [56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] Changes in the distribution of the specific resistance in the PEM and oxygen partial pressure at the CL/PEM interface during operation, [73][74][75] as well as the degradation mechanism of the MEAs, 59,63,64,76,77 were also analyzed to feedback information to the syntheses of PEMs and catalysts.…”