A nano-scale understanding of the degradation mechanisms responsible for the performance loss of high surface area (HSA) catalysts implemented in polymer electrolyte membrane fuel cells (PEMFC) is essential for the development of improved catalysts. Here we present a systematic study of the degradation mechanisms of a HSA carbon supported Pt (Pt/C) catalyst. By monitoring the electrochemical surface area (ECSA) loss under accelerated stress test (AST) protocols with identical location transmission electron microscopy (IL-TEM), it is shown that different degradation mechanisms are responsible of the performance loss of Pt/C depending on the applied AST protocol. Three different AST protocols have been applied, i.e. i) load cycles between 0.6-1.0 V RHE , ii) start/stop cycles between 1.0-1.5 V RHE , and iii) a treatment consisting of a mix of both conditions (cycling between 0.4-1.4 V RHE ). During load cycles the main degradation mechanisms are nanoparticle (NP) migration as well as Pt dissolution and re-deposition. However, after applying start/stop cycles only NP detachment from the carbon support is detect, whereas the third AST protocol induces all degradation modes concomitantly.Low temperature proton exchange membrane fuel cells (PEMFCs) are considered as future clean and efficient power sources to be implemented in portable, stationary, and automotive applications. Despite recent advances, for such applications further improvements of electrocatalysts with respect to their activity and stability are needed. 1 In order to improve catalyst stability, a key factor for development is a detailed understanding of the degradation mechanisms responsible for the performance loss; in particular an understanding how the degradation mechanisms dependent on the operation conditions. However work studying the degradation of high surface area (HSA) carbon supported fuel cell catalysts is still mainly descriptive. In general such degradation studies can be divided into in-situ and ex-situ investigations. In in-situ investigations the whole membrane electrode assembly (MEA) is tested, 2-4 while in ex-situ measurements electrochemical half cells are employed; 5-7 thus studying only the performance of the catalyst itself as influence of other factors such as the electrode structure are minimized. Both methods usually consist of applying an accelerated stress test (AST) and monitoring the change in electrochemical active surface area (ECSA) with time. The ECSA loss is a direct measure for stability, however provides no mechanistic insight.There is a consensus in literature that three main degradation mechanisms are responsible for an ECSA loss: i) Platinum dissolution, with and without re-deposition (the latter case is dubbed electrochemical Ostwald ripening) ii) particle migration and subsequent coalescence, and iii) detachment of the Pt nanoparticles (NPs) from the support due to carbon corrosion. But only few studies are able to distinguish mechanistic details of degradation on the nano-scale. Especially particle migration ...

A colloidal synthesis approach is presented that allows systematic studies of the properties of supported proton exchange membrane fuel cell (PEMFC) catalysts. The applied synthesis route is based on the preparation of monodisperse nanoparticles in the absence of strong binding organic stabilizing agents. No temperature post-treatment of the catalyst is required rendering the synthesis route ideally suitable for comparative studies. We report work concerning a series of catalysts based on the same colloidal Pt nanoparticle (NP) suspension, but with different high surface area (HSA) carbon supports. It is shown that for the prepared catalysts the carbon support has no catalytic co-function, but carbon pre-treatment leads to enhanced sticking of the Pt NPs on the support. An unwanted side effect, however, is NP agglomeration during synthesis. By contrast, enhanced NP sticking without agglomeration can be accomplished by the addition of an ionomer to the NP suspension. The catalytic activity of the prepared catalysts for the oxygen reduction reaction is comparable to industrial catalysts and no influence of the particle size is found in the range of 2-5 nm.

In this work, Pt nanoparticles prepared by a colloidal method are supported on high surface area carbons. The electrocatalysts synthesized by this method have well-separated, size-controlled nanoparticles with tunable interparticle distance, and thus enable the examination of the particle proximity effect on the oxygen reduction reaction (ORR). The particle proximity effect proposes that the activity of fuel cell catalysts depends on the distance between the catalyst particles and is here for the first time demonstrated for high surface area catalysts; i.e. catalysts which can be used in fuel cells. Based on rotating disk electrode (RDE) experiments, we show that the kinetic current density of ORR depends on the distance between the neighboring nanoparticles, i.e. the ORR activity increases with decreasing interparticle distance.

For the first time, shape-controlled Pt 3 Sn, PtSn, and PtSn 2 intermetallic nanocrystals were synthesized in octadecene (ODE) by a versatile hot-injection method with 1,2-hexadecanediol (HDD) as the reducing agent. Transmission electron microscopy (TEM) measurements reveal that the metal composition has an influence on the particle morphology: with the increase in the Sn content, the Pt/Sn nanoparticles obtained by the hot-injection synthesis show flower-like, irregular faceted, cubic/tetrahedral, hexagonal, and spherical/nanowire structures. A facile phase-transfer preparative procedure for the synthesis of Pt/Sn core/shell nanoparticles was also developed, in which ligand-free Pt nanoparticles were used as precursors. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements confirm a Pt-core/Snshell structure. The surface characteristic of the Pt/Sn core/shell nanoparticles was also investigated by IR spectroscopy of CO adsorption experiments (i.e., with a highly surface sensitive technique). These experiments reveal a few Pt atoms to be left on the surface as adsorption sites for CO. However, the intensity of the corresponding infrared (IR) bands is almost negligible. Furthermore, Pt/Sn random-alloy nanoparticles with different metal compositions and particle sizes were synthesized in this work by heating-up methods. Energy dispersive X-ray (EDX) and XRD analyses show different alloying extent of Sn with Pt.

The influence of organic ligands on the geometric and electronic surface properties of colloidally prepared Pt nanoparticles (Pt NPs) was investigated by means of diffuse reflectance infrared spectroscopy (DRIFTS) and using CO as probe molecule. Dodecylamine (DDA) and dodecylthiol (DDT) were used as surface functionalizing ligands that exhibit the same hydrocarbon tail but different functional groups. While for DDA an electronic donor effect as well as a geometric effect was found, the effect of DDT was mainly geometric in nature. For high DDA and DDT coverages, on-top sites were blocked for CO adsorption to a higher extent than bridge and threefold hollow sites. Whereas DDA is only weakly adsorbed and displaced by strongly binding adsorbates (e.g., CO), DDT is strongly attached to the particle surface forming an evenly distributed capping layer. With decreasing thiol coverage, blocking of bridge and threefold hollow sites became more pronounced than on-top site blocking. The influence of both ligands on the selectivity of the hydrogenation of crotonaldehyde catalyzed by Pt NPs was investigated. For amino-functionalized NPs the catalytic properties did not differ from that of "unprotected" Pt NPs. In contrast, an increased selectivity can be found for thiol-functionalized particles, which, according to the IR-spectroscopic investigations, was attributed to a geometric modification of the surface by the ligand.