Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case

Abstract

Design of active and stable Pt-based nanoscale electrocatalysts for the oxygen reduction reaction (ORR) will be the key to improving the efficiency of fuel cells that are needed to deliver reliable, affordable and environmentally friendly energy. Here, by exploring the ORR on Pt single crystals, cubo-octahedral (polyhedral) Pt NPs with different sizes (ranging from 2 to 7 nm), and 7-8 nm Pt NPs with different shapes (cubo-octahedral vs. cube vs. octahedral), we presented surface science approach capable of rationalizing, and ultimately understanding, fundamental relationships between stability of Pt NPs and activity of the ORR in acidic media. By exploring the potential induced dissolution/re-deposition of Pt between 0.05 and 1.3 V, we found that concomitant variations in morphology of Pt(111) and Pt(100) lead to narrowing differences in activity between Pt single crystal surfaces. We also found that regardless of an initial size or shape, NPs are metastable and easily evolve to thermodynamically equilibrated shape and size with very similar activity for the ORR. We concluded that while initially clearly observed, the particle size and shape effects diminish as the particles age to the point that it may appear that the ORR depends neither on the particle size nor particle shape. IntroductionThe last two decades have witnessed remarkable progress in our ability to chemically synthesize metal nanoparticles (NPs) ranging in size from 1 to 10 nm with unique electrocatalytic properties. 1-4 Because the electronic properties of metal NPs in this size range are not unique, 5 (electro)catalyst research with such materials has focused on the variations in the reaction rate or selectivity with characteristic dimensions of metallic catalysts. While Boronin and co-workers 6 pioneered this approach to understand the "crystalline size effect" in heterogeneous catalysis, Kinoshita 7 and others 8,9 used this tactic to understand the "particle size effect" in electrocatalysis. There is no simple ideal structure that will model all the aspects of NP catalysts, particularly in the configuration that are used in electrolytic cells. However, if one considers the equilibrium shape of a face-centered cubic (fcc) metal NP such as a cubooctahedron, consisting of (111) and (100) facets bounded by edge atom rows that are like the topmost rows in the (110) surface, single-crystal surfaces may serve as a reasonable model for

Atomic-scale restructuring of hollow PtNi/C electrocatalysts during accelerated stress tests

“…S1 for particle size distributions). Clearly, the single PEMFC testing was the most damaging: regardless of the initial morphology and chemical composition of the fresh electrocatalyst, solid spherical nanoparticles with size comprised between 5 and 8 nm particles were found after 5000 cycles in solid electrolyte and T = 80 • C. This result supports the conclusions of Li et al [43] that preferentially shaped nanoparticles return to their equilibrium shape after polarization at electrode potential E > 0.90 V vs. RHE.…”

“…It is unclear yet whether the presence of adislands is inherent to bulk or nanometre-sized Pt surfaces or results from the mechanism of formation of the hollow nanoparticles [41]. In the frame of the first hypothesis, note that Strmcnik et al [42] and Li et al [43] have reported adislands of size ca. 5 × 5 nm 2 on freshly-prepared Pt(1 1 1) and Pt(1 0 0) single crystals, respectively.…”

A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells

“…They explained the maximal mass activity at 2.2 nm based on density functional theory calculations performed on fully relaxed NPs. 97 Li et al 98 found that the ORR activity of well dispersed and uniform Pt NPs is indeed dependant on their size, and the effect is decreased in the range 2-7 nm.…”

Probing the nanoscale structure of the catalytically active overlayer on Pt alloys with rare earths

“…[16] Earlier studies on Pt and Pt alloys suggest that the active site is located on the closely packed (111) surface. [17][18][19][20] According to a model based on density functional theory (DFT) calculations, the catalytic activity for oxygen reduction shows a Sabatier-volcano dependence on the binding to the key reaction intermediate, HO * (where HO * denotes a hydroxyl group adsorbend on an active site). [21] The optimal catalyst should exhibit moderate binding to HO * , neither too strong nor too weak, with a binding to HO * around ~0.1 eV weaker than Pt(111).…”