An XAS experimental approach to study low Pt content electrocatalysts operating in PEM fuel cells

Abstract

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

Effect of heat treatment on the activity and stability of carbon supported PtMo alloy electrocatalysts for hydrogen oxidation in proton exchange membrane fuel cells

“…A greater increase in the magnitude of white line with the increase in the applied potential is observed for the catalyst of small particle size (1 nm) in comparison to large particle sized (3.7 nm) catalyst. This increase in white line intensity with increasing applied potential for the catalysts with two different particle sizes is associated with a modification of average Pt oxidation state and has been discussed in detail by various authors [31,36,37]. Mukerjee et al [9] observed a small increase in the Pt white line magnitude for PtMo/C when compared with Pt/C, indicating an increase in the number of Pt 5d band vacancies in PtMo/C due to alloying.…”

Synthesis and X-ray Characterization of Cobalt Phosphide (Co2P) Nanorods for the Oxygen Reduction Reaction

“…These techniques have been robust for not only distinguishing bimetallic core-shell vs. alloy architectures but also to understandthe formation of metal-oxide shells that emerge from cleaning pre-treatments in preparation for functional testing. [45][46][47][48] Such careful study of the average and local crystallographic structures of our NRs provides valuable insights into the structure-property relations in shape-dependent electrocatalysis. Besides the unique 1-D structure, Co 2 P NRs supported on carbon exhibit intriguing catalytic performance for catalyzing ORR in an alkaline medium.…”

Balance of Nanostructure and Bimetallic Interactions in Pt Model Fuel Cell Catalysts: In Situ XAS and DFT Study

“…By contrast, high-energy x-rays used in absorption spectroscopy at the Pt L edges can readily penetrate liquid electrolytes. [10][11][12][13][14][15][16][17][18][19][20][21] Recently, we demonstrated that high energy resolution fluorescence detection xray absorption spectroscopy (HERFD XAS) at the Pt L 3 edge, applied to a well-defined Pt monolayer on a Rh(111) substrate, is a surface sensitive probe of Pt-O surface interactions that can unambiguously differentiate between the chemisorption of oxygen-containing species and surface oxide formation. 22 Moreover, the nature of Pt oxides and their formation mechanism at high potentials was found to be strongly influenced by the underlying metal substrate.…”