Temperature and potential-dependent structural changes in a Pt cathode electrocatalyst viewed by in situ XAFS (original) (raw)
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Pt–Co cathode electrocatalyst behaviour viewed by in situ XAFS fuel cell measurements
Journal of Power Sources, 2008
The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan XC-72 catalyst using X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XAS results have been obtained ex situ and in situ using a specially optimized for XAS measurement fuel cell (down to 6 keV). The results are compared with those obtained for pure Pt catalyst on the same carbon support under the same working conditions.
In situ X-ray absorption spectroscopy and X-ray diffraction of fuel cell electrocatalysts
Journal of Power Sources, 2001
The utility of in situ X-ray absorption spectroscopy (XAS) in determining structural parameters, through analysis of the extended X-ray absorption ®ne structure (EXAFS), and electronic perturbations, through a white line analysis of the X-ray absorption near edge structure (XANES), is demonstrated for Pt/C, PtRu/C and PtMo/C fuel cell electrodes. The results provide veri®cation that the enhancement of CO tolerance of the alloy catalysts occurs via an intrinsic mechanism for the PtRu alloy, whilst a promotion mechanism is in operation for the PtMo alloy. Preliminary results of an in situ powder X-ray diffraction (XRD) method which utilises synchrotron radiation (SR) and a curved image plate detector are also presented, using Pd/C as an example. The lattice expansion upon formation of the b-hydride is clearly observed. #
Frontiers in Energy, 2017
A method is described to determine the internal structure of electrocatalyst nanoparticles by in situ X-ray absorption spectroscopy (XAS). The nondestructive spectroscopic technique typically utilizing synchrotron radiation as the source measures changes in the X-ray absorption coefficient as a function of energy. The bulk technique has found its use for materials characterization in all scientific areas, including nanomaterials. The analysis of the internal structure of nanoparticles reveals interatomic distances and coordination numbers for each element, and their values and mutual relations indicate whether the elements form a homogeneous or heterogeneous mixture. The core-shell heterogeneous structure in which certain elements are predominantly located in the core, and others form the encapsulating shell is of particular importance in catalysis and electrocatalysis because it may reduce the amount of precious metals in nanoparticles by replacing the atoms in the core of nanoparticles with more abundant and cheaper alternatives. The examples of nanoparticle structures designed in the laboratory and the approach to model efficient catalysts through systematic analysis of XAS data in electrochemical systems consisting of two and three metals are also demonstrated.
Journal of Non-Crystalline Solids, 2014
In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations of the changes in the local geometric and electronic structure of Pt nanoparticles used as a cathode catalyst in proton exchange membrane fuel cell (PEMFC), working under controlled potential cycling conditions. The body of the results obtained suggests that in the first stage of PEMFC operation, small particle dissolution was a dominant process. Subsequent 100 h of work led to the progressive agglomeration of nanoparticles followed by a pronounced growth of the mean nanoparticle size. At the same time, high-quality XAFS spectra analysis demonstrated that negligible changes in structural local ordering and a slight increase in Pt 5d-electron density occurred during the whole FC operation period under consideration.
Actualizing In Situ X-ray Absorption Spectroscopy Characterization of PEMFC-Cycled Pt-Electrodes
Journal of The Electrochemical Society, 2018
The family of the PtM (M represents transition metals such as Co, Ni, Pd, etc.) alloys is the most promising cathode electrocatalysts for proton exchange membrane fuel cells (PEMFCs) owing to their superior oxygen reduction reaction (ORR) activity to pure Pt. However, the activity gain fades with long-term PEMFC operation, and the degradation mechanism is not yet fully understood. To truly understand the degradation mechanism of the carbon supported PtM nanoparticles (PtM/C) in the cathode of a membrane electrode assembly (MEA) upon long-term PEMFC operation, it is essential to characterize the PEMFC-cycled electrode under working conditions. Herein, we showed that operando X-ray absorption spectroscopy (XAS) characterization of PtM/C electrocatalysts cycled in a PEMFC has inherent difficulties since Pt and especially M dissolve during PEMFC operation and migrate into the membrane; the bulk XAS spectrum is an average of the signals from the electrode and the membrane. Alternatively, we developed a method that allows for in situ XAS characterization on PEMFC-cycled PtM/C electrocatalysts. We justified the method by showing that the dissolved species in the membrane were separated from the PtM/C electrocatalyst in the cathode, and the in situ XAS signals arose exclusively from the electrocatalyst.
Study of the atomic structure and morphology of the Pt 3 Co nanocatalyst
Journal of Physics: Conference Series, 2009
It has been shown that Pt3Co nanoparticles used as a catalyst for cathode of Proton Exchange Membrane Fuel Cells (PEMFC) enhance oxygen reduction reaction (ORR) activity even by a factor of two compared to pure Pt nanoparticles. The local structure and chemical disorder of a commercially available Pt3Co nanocatalyst supported on high surface area carbon were investigated. High-quality XAFS spectra were collected at the ELETTRA synchrotron XAFS 11.1 beamline. XAFS spectra analysis have been performed accounting for the reduction of the coordination number and degeneracy of three-body configurations, resulting from transmission electron microscopy (TEM) and x-ray diffraction (XRD) extracted mean particles diameter, size distribution and expected surface atom contributions. The presence of a Co-Co first neighbour EXAFS signal is shown to be related to the degree of the alloy's chemical disorder. This is a good starting point for analyzing the atomic structure of Pt3Co nanocrystalline system and their changes as a function of alloy preparation or working conditions when they operate as a catalyst in PEMFC.
The Journal of Physical Chemistry C, 2007
We establish relationships between the atomic structure, composition, electrocatalytic activity, and electrochemical corrosion stability of carbon-supported Pt-Co alloy nanoparticles in electrode catalyst layers. These Pt-Co catalysts have received much attention for use as cathode layers in polymer electrolyte membrane fuel cells (PEMFCs) because of their favorable oxygen-reduction-reaction (ORR) activity and suspected corrosion stability. We reported an enhancement of activity of low-temperature Pt 50 Co 50 of 3 times that of pure carbon supported Pt catalysts. The use of synchrotron X-ray diffraction has enabled structural characterization of the alloy nanoparticles both before and, importantly, after electrocatalysis under fuel cell like conditions. From this, a detailed picture of the relative activity and stability of Pt-Co alloy phases as a function of synthesis conditions has emerged. We have investigated the structure, composition, chemical ordering, and concentration of Pt-Co alloy phases in (i) a dry, freshly synthesized nanoparticle catalyst, (ii) the catalytic electrode layer in a proton-conducting polymer electrolyte before electrocatalytic activity, and (iii) the same electrode layer after electrocatalytic activity. We find that Pt 50 Co 50 catalysts annealed at 600°C consist of multiple phases: a chemically ordered face-centered tetragonal (fct) and two chemically disordered face-centered cubic (fcc) phases with differing stoichiometries. The Co-rich fcc phase suffers from corrosive Co loss during the preparation of conducting polymer electrode layers and, more significantly, during the ORR electrocatalysis. Most importantly, these fcc phases exhibit high catalytic activities for ORR (about 3× compared to a pure Pt electrocatalyst). Pt 50 Co 50 catalysts annealed at 950°C consist mainly of the fct Pt 50 Co 50 phase. This phase shows favorable stability to corrosion in the conducting polymer electrode and during electrocatalysis, as the relative intensities of fcc(111)/fct(101) peak ratio remained consistently around 0.5 before and after preparation of conducting polymer electrode layers and before and after electrochemical measurements; however, it exhibits a lower catalytic ORR activity compared to the low-temperature fcc alloy phases (about 2.5× compared to a pure Pt electrocatalyst). Our results demonstrate the complexity in these multiphase materials with respect to catalyst activity and degradation. By understanding of the relationships between crystallographic phase, chemical ordering, composition, and the resulting electrochemical activity and corrosion stability of fuel cell catalysts within polymer-electrolyte/catalyst composites, we can move toward the rational design of active and durable catalyst materials for PEMFC electrodes.
2013
The project was launched on September 15, 2009. Over the grant period, we have accomplished all the research objectives proposed in the proposal. Specifically, we have developed and performed Monte Carlo (MC) simulations predicting the surface composition of a series of Pt binary and ternary alloy catalysts, carried out first-principles transition state calculations of the ORR on Pt and Pt alloy surfaces, developed and performed kinetic Monte Carlo (KMC) simulations predicting the activity for the ORR on Pt surfaces, and derived the relation between surface composition and catalytic activity for the ORR on Pt alloys. As a result of our study, ten research articles supported by this grant have been published in peer-reviewed journals.