Core-shell catalysts mass activity

Abstract In this chapter, we review recent works of dealloyed Pt core-shell catalysts, which are synthesized by selective removal of transition metals from a transition-metal-rich Pt alloys (e.g., PtMs). The resulted dealloyed Pt catalysts represent very active materials for the oxygen reduction reaction (ORR) catalysis in terms of noble-metal-mass-normalized activity as well as their intrinsic area-specific activity. The mechanistic origin of the catalytic activity enhancement and the stability of dealloyed Pt catalysts are also discussed. 

Figure 19.8 illustrates the effect of hold time at each current density point in MEA tests using different nanostructured thin film (NSTF) MEAs, showing increased MEA mass activity at 900 mV using 5 s holds (matching RDE values more closely) compared with lower MEA mass activity using 1,050 s holds at each point in the polarization curve [13] similar variations in activity between the MEA and RDE techniques may also be expected for core-shell catalyst materials. 

As discussed previously, the DoE has set a target of catalyst activity of four times fhaf of pure Pt/carbon. This can be expressed as activity per mass of Ft or acfivify per cost equivalent. Three general approaches have been investigated to achieve this target Pt alloys, Pt core-shells, and non-Pt catalysts... 

Particle size is an important parameter that strOTigly influences the activity of catalyst nanoparticles. There are extensive studies on the particle size effect of Pt particles on the ORR catalysis, showing that the area-specific activity increased as the particle size increases due to a lower portion of low-coordinated Pt atoms on the edges and comers [13, 74—77]. Taking the surface area into account, this leads to a maximum mass activity at particle size of around 3-5 imi. Similar trends were also reported in PtsM alloy catalysts [13, 78, 79]. Eor dealloyed nanoparticles, situation becomes more complex as the morphologies and core-shell stractures were found to be dependent on the particle size, being an additional contribution to the overall size effect. 

It is still tmclear that which kind of core-shell structure contributes the most to the macroscopic overall catalytic activities of the dealloyed catalyst particle ensemble. Researchers at General Motors found that the single-core-shell nanoparticles provided most of the activity for dealloyed PtCus catalyst [3], while Snyder et al. fotmd that nanoporous dealloyed PtNis particles exhibited higher mass activity and specific activity compared to the dealloyed solid nanoparticles PtNi3 with smaller sizes [80]. Further studies are needed to clarify this issue. 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

To enhance the mass activity of Pt, the modification of the catalytic platinum surface by the addition of a second or multiple metals has been developed to remarkably reduce Pt loading while enhancing the catalytic performance. For CO and methanol oxidation reactions, while PtRu is the practical electro-catalyst, PtMo (core-shell) and other ternary catalysts show very high activity in the oxidation of CO and/or methanol. Well-defined Pt3Ni surfaces and Pt3(CoNi) are two good examples to show how judicious design can be beneficial to dramatically enhanced activity in catalyzing ORRs. 

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