The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) >> Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) >> Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.

Among the most challenging issues in technologies for electrochemical energy conversion are the insufficient activity of the catalysts for the oxygen reduction reaction, catalyst degradation and carbon-support corrosion. In an effort to address these barriers, we aimed towards carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored physical properties. We present here a new class of metallic materials with tunable near-surface composition, morphology and structure that have led to greatly improved affinity for the electrochemical reduction of oxygen. The level of activity for the oxygen reduction reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycrystalline Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.

PtM alloys (M = Co, Ni, Fe, etc.) have been extensively studied for their use in fuel cells, both in well-defined extended surfaces, [1] as well as in nanoparticles. [2] After the report about exceptional activity of Pt 3 Ni(111)-skin surface [1a] for the oxygen reduction reaction (ORR) a lot of efforts have been made to mimic this catalytic behavior at the nanoscale. It has been shown that a Pt 3 Ni(111) crystal annealed in ultrahigh vacuum (UHV) shows an oscillating segregation profile, with the outermost layer consisting of pure platinum while the second layer is enriched in nickel compared to the bulk composition. [1a, 3] Such a surface we termed Pt skin, and owing to the presence of the non-noble metal in the subsurface layer it has altered electronic properties compared to the monometallic Pt single crystal with the same orientation. Accordingly, altered electronic properties induce a change in adsorption behavior, specifically a shift of surface-oxide formation to higher potentials. [1a,b] This adsorption behavior is believed to be the origin of the high activity for the ORR. On the opposite side of the potential scale, the adsorption of hydrogenated species, denoted as underpotentially adsorbed hydrogen (H upd ), is also largely affected on Pt-skin surfaces. [4] Despite numerous efforts dedicated to synthesize nanocatalysts with Pt-skin-type surfaces, [2c, 5] it still remains a challenge to claim their existence at the nanoscale. To systematically resolve this issue, we attempt to provide fundamental insight into the adsorption properties of well-defined Pt-skin surfaces under relevant electrochemical conditions and to transfer that knowledge to corresponding nanocatalysts.For that reason, we first examine the formation and composition of Pt-skin surfaces by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM) in UHV, and second we study the composition of the surfaces in an electrochemical environment to establish their adsorption properties. We demonstrate by cyclic voltammetry that the surface coverage of H upd on Pt skin is about half of that found on Pt(111), whereas the surface coverage of a saturated monolayer of carbon monoxide is similar for both surfaces. This is an important finding, which provides a link towards accurate determination of the electrochemically active surface area of nanoscale catalysts. The developed methodology provides additional evidence for the existence of Pt-skin surfaces on Pt-bimetallic nanocatalysts and can substantially diminish errors in the evaluation of the real surface area and catalytic activity.A thorough examination of the Pt-skin surfaces was performed in view of their importance in electrocatalysis as well as in response to recent questions and doubts in the Figure 1. Surface characterization of Pt 3 M(111) surfaces in UHV: STM images of A) sputtered and B) annealed Pt 3 Ni(111) surfaces. The brightness in colors is a measure of the depth profile, with each color change marking a single atomic step. Low-energy ion scattering spectra ...

LETTERfactor. 19 A hexagonal unit cell of a = b = 2.775 Å and c = 6.797 Å, where c-axis) Pt [111], was used in our measurements and analysis for convenience.' ASSOCIATED CONTENT b S Supporting Information. Details on the specular rod data and fitting as well as the resonance scattering data and fitting. This material is available free of charge via the Internet at http://pubs.acs.org.

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In this study, we report a methodology which enables the determination of the degradation mechanisms responsible for catalyst deterioration under different accelerated stress protocols (ASPs) by combining measurements of the electrochemical surface area (ECSA) and Pt content (by X-ray fluorescence). The validation of this method was assessed on high surface area unsupported Pt nanoparticles (Pt-NPs), Pt nanoparticles supported on TaC (Pt/TaC) and Pt nanoparticles supported on Vulcan carbon (Pt/Vulcan). In the load cycle protocol, the degradation of Pt-NPs and Pt/Vulcan follows associative processes (e.g. agglomeration) in the first 2000 cycles, however, in successive cycles the degradation goes through dissociative processes such as Pt dissolution, as is evident from a similar decay of ECSA and Pt content. In contrast, the degradation mechanism for Pt nanoparticles dispersed on TaC occurs continuously through the dissociative processes (e.g. Pt dissolution or particle detachment), with similar decay rates of both Pt content and ECSA. In the start-up/shut-down protocol, high surface area Pt-NPs follow associative processes (e.g. Ostwald ripening) in the first 4000 cycles, after which the degradation continues through dissociative processes. On the other hand, dissociative mechanisms always govern the degradation of Pt/TaC under start-up/shut-down protocol conditions. Finally, we report that Pt nanoparticles supported on TaC exhibit the highest catalytic activity and long term durability of the three nanoparticle systems tested. This makes Pt/TaC a potentially valuable catalyst system for application in polymer electrolyte fuel cell cathodes.

Polymer electrolyte membrane fuel cells (PEMFCs) are under intense research and development for transportation applications. It has been shown that a highly active, lower cost, oxygen reduction reaction (ORR) catalyst can be made by replacing a portion of the costly Pt catalyst with a transition metal, in this case Ni [1]. Further gains can be achieved through dealloying the PtNi alloy catalyst to create a Ptrich skin or shell, although spontaneous dealloying during fuel cell operation poses a significant durability issue [2]. While usually deployed in nanoparticle form, highly active Pt 3 Ni 7 nanostructured thin films (NSTF) have also been demonstrated, and the substantial increase in both the specific activity and specific surface area has been attributed to a complex interplay between composition, grain size, lattice strain, and the catalyst nanoparticle morphology, e.g., Pt-skin, -shell or -skeleton structures [3].In this work, dealloying and annealing pre-treatments have been performed on Pt 3 Ni 7 NSTF aimed at improving the specific activity and area, as well as operational durability. The effect of these pretreatments on catalyst morphology at the nanoscale has been characterized by quantitative scanning transmission electron microscopy (STEM) techniques including energy dispersive X-ray spectroscopy, selected area electron diffraction, and electron tomography. These results are correlated with X-ray diffraction, X-ray fluorescence, and cyclic voltammetry measurements to understand structure-activity relationships, which direct further process development for enhanced performance. These measurements were performed following five major stages of the catalyst preparation and performance testing: growth, dealloying, annealing, conditioning, and voltage cycling. High-angle annular dark-field (HAADF)-STEM images of the PtNi NSTF catalyst at each of these stages is shown in Figure 1.The dissolution of Ni from the as-deposited Pt 3 Ni 7 catalyst via dealloying and annealing treatments resulted in a Pt-rich surface layer. Fuel cell conditioning, an activation process performed by cycling the fuel cell on and off, induced the largest change in the NSTF catalyst whiskers, which are transformed into a nanoporous Pt 7 Ni 3 alloy. The voltage-cycled electrode was further studied by electron tomography to reveal an open-pore network (Figure 2). The available surface area (electrochemically active surface area) was quantified and compared with an annealed whisker to show that the pore network and loss of the whisker interior may provide additional catalytically active sites in the cycled NSTF electrodes [4].References:[1] V.R.