In situ study of oxidation states of platinum nanoparticles on a polymer electrolyte fuel cell electrode by near ambient pressure hard X-ray photoelectron spectroscopy

Abstract

We performed in situ hard X-ray photoelectron spectroscopy (HAXPES) measurements of the electronic states of platinum nanoparticles on the cathode electrocatalyst of a polymer electrolyte fuel cell (PEFC) using a near ambient pressure (NAP) HAXPES instrument having an 8 keV excitation source. We successfully observed in situ NAP-HAXPES spectra of the Pt/C cathode catalysts of PEFCs under working conditions involving water, not only for the Pt 3d states with large photoionization cross-sections in the hard X-ray regime but also for the Pt 4f states and the valence band with small photoionization cross-sections. Thus, this setup allowed in situ observation of a variety of hard PEFC systems under operating conditions. The Pt 4f spectra of the Pt/C electrocatalysts in PEFCs clearly showed peaks originating from oxidized Pt(ii) at 1.4 V, which unambiguously shows that Pt(iv) species do not exist on the Pt nanoparticles even at such large positive voltages. The water oxidation reaction might take place at that potential (the standard potential of 1.23 V versus a standard hydrogen electrode) but such a reaction should not lead to a buildup of detectable Pt(iv) species. The voltage-dependent NAP-HAXPES Pt 3d spectra revealed different behaviors with increasing voltage (0.6 → 1.0 V) compared with decreasing voltage (1.0 → 0.6 V), showing a clear hysteresis. Moreover, quantitative peak-fitting analysis showed that the fraction of non-metallic Pt species matched the ratio of the surface to total Pt atoms in the nanoparticles, which suggests that Pt oxidation only takes place at the surface of the Pt nanoparticles on the PEFC cathode, and the inner Pt atoms do not participate in the reaction. In the valence band spectra, the density of electronic states near the Fermi edge reduces with decreasing particle size, indicating an increase in the electrocatalytic activity. Additionally, a change in the valence band structure due to the oxidation of platinum atoms was also observed at large positive voltages. The developed apparatus is a valuable in situ tool for the investigation of the electronic states of PEFC electrocatalysts under working conditions.

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

“…Despite recent advances, nearly all in situ studies of energyrelated materials have been performed using bulk sensitive spectroscopies, such as hard X-ray XAS and XRD. The recent coupling of operational systems with surface sensitive techniques, such as X-ray photoelectron spectroscopy (XPS), provides significant opportunity for shedding new light on phenomena occurring at the solid/liquid electrified interface (38,39,40,41,42,43,44,45,46,47,48). In this work, we study, under operando conditions, the chemical and structural evolution of different conformal CoO x catalyst layers formed by plasma-enhanced atomic layer deposition (PE-ALD) on p + -Si substrates (37) as a function of the applied electrochemical potential.…”

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…[372,373] BL36XU has also been a trailblazer in combining ambient pressure environments with operando setups for fuel cells, which will be discussed in detail in Section 3.3.3. [372,374,375] Today, a number of AP-HAXPES beamlines is operational, including P22 (PETRA III), X07MB (SLS), and 9.3.1 (ALS). [57,58,376] In addition, beamline 8A at the Pohang Accelerator Laboratory, which is set up as a soft X-ray AP-HAXPES endstation, uses an off-line Cr Kα source to perform AP-HAXPES.…”

X-ray-Based Spectroscopic Techniques for Characterization of Polymer Nanocomposite Materials at a Molecular Level

“…The interaction between gas molecules and polymeric materials is also possible: for example, the interaction of water to hydrophilic polymer chains in superabsorbent hydrogel such as a soft contact lens can be probed effectively [146]. In the nanocomposite systems, a lower number of studies using NAP-XPS can be found in the literature regarding the interaction of relevant molecules with inorganic fillers on or near the surface of the nanocomposite, e.g., the chemical state of Pt in Pt/C electrodes under humid condition [148]. Probing the interaction at organic/inorganic interfaces was recently attempted: Pletincx et al investigated the mechanism of the interactions among three components of water vapor, poly(methyl methacrylate) (PMMA) chain, and aluminum oxide surface [149].…”