In Situ XAS Studies of PEM and HT-PEM Fuel Cell Electrocatalysts
Open AccessIn situ and in operando X-ray absorption spectroscopy (XAS) was used to examine the poisoning effects and roles of various adsorbates on platinum electrocatalysts. XAS is ideal for providing both structural and electronic information on electrocatalysts under in operando conditions; the extended X-ray adsorption fine structure (EXAFS) technique provides the structural information, and the X-ray absorption near edge spectra (XANES) providing adsorbate information. In this work the ∆µ-XANES analysis technique was used for the first time to provide information on Pt skin thicknesses and relative coverage of specific di-oxygen (e.g. OOH, HOOH) adsorbates during fuel cell operation. The Δμ XANES data at the Co or Ni K-edge are reported for de-alloyed PtCox and PtNix catalysts (6 different catalysts at different stages of life). These de-alloyed catalysts are 6-8 times more reactive than pure Pt and meet the 2017 DOE beginning of life target Pt mass activity (0.44 A mgPt-1). Ex situ high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) and in situ X-ray absorption spectroscopy (XAS) demonstrate that these NPs have a semi-ordered Pt1-1.5M core and a Pt skin. The results clearly reveal that the catalytic enhancements result primarily from compressive-lattice strain effects. The Δμ XANES results reveal a dual volcano-like behavior in the activity with increasing Ni/Pt ratio. The first volcano results from build-up of OOH with decrease in OH, and the second from a decrease in OOH with an increase in HOOH following the well-known Sabatier principle for catalysts. We also develop a new technique to determine at what potential (Vpen) the outer Pt skin on the nanoparticle is penetrated by O in situ. The data indicate that cycling produces a "characteristic" Pt skin robustness (porosity or thickness) so that if the Pt skin begins "thin" it grows to a "characteristic" thickness and if it begins very thick it thins to the same "characteristic" thickness. This work is believed to be the first in situ XAS study to shed light on the nature of the Pt skin on a core-shell particle, its thickness and/or porosity, and how it changes under real operating conditions in a fuel cell. High temperature polymer electrolyte fuel cells (HT-PEMFCs) using polybenzimidazole (PBI) membranes were also studied as a function of phosphoric acid loading, potential and temperature. For the first time, extensive in-operando XAS investigations were carried out on Pt/C fuel cell catalysts (cathode and anode) at different temperatures and H3PO4 concentrations at varying fuel cell voltages. Under in-operando conditions, significant H3PO4 anion coverage of the Pt nanoparticles is observed, but the high temperature allows both H (anode) and O (at the cathode) to displace the hydrogen phosphate anions, so that the poisoning effects are not highly detrimental to fuel cell operation. We also conclude that the effects of CO poisoning on the HT-PEMFCs operating at 170°C is much smaller than in normal PEMFCs because the adsorbed phosphate anions and higher temperature weaken the Pt-CO bond.
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Caldwell_gwu_0075A_12168.pdf | 2018-01-16 | Open Access |
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