Size and composition distribution dynamics of alloy nanoparticle electrocatalysts probed by anomalous small angle X-ray scattering (ASAXS)

Abstract

Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.

Molecular dynamics simulations of ternary PtxPdyAuz fuel cell nanocatalyst growth

“…A possible way consists in combining noble metals (Pt, Pd, Au ...) [5][6][7][8][9][10][11] and non-noble metals for preparing binary and ternary nanocatalysts. While non-noble metals are interesting from a cost reduction point of view, they may lead to reduced stability compared to pure platinum due to their dissolution capability [12][13][14][15][16][17]. Pt x Pd 1-x .…”

In-Operando Anomalous Small-Angle X-Ray Scattering Investigation of Pt3Co Catalyst Degradation in Aqueous and Fuel Cell Environments

“…14,[18][19][20][21][22] A comprehensive discussion of the ASAXS method and its use in characterizing carbon-supported metal catalysts can be found in several publications and textbooks. [23][24][25][26][27][28][29] One advantage this technique has is its ability to obtain element specific PSDs.…”

“…From changes in the Pt and Cu PSDs obtained through ASAXS, Yu et al were able to show the in-situ formation of a Pt enriched shell surrounding a PtCu core through electrochemical Cu dissolution of a carbon supported Pt 25 Cu 75 electrocatalyst and Tuaev et al showed the same type of evolution for a Pt 25 Ni 75 electrocatalyst. 21,22 The ASAXS technique was used in this study to determine Pt PSDs of nanoparticle Pt 3 Co electrocatalysts in-situ and in-operando for two accelerated stress test protocols and ex-situ Pt and Co PSDs of the electrocatalyst before and after cycling. A catalyst with a Pt:Co atomic ratio of 3:1 was chosen for this study, as this ratio has been shown to have both the highest ORR activity and activity stability.…”

Trends in anomalous small-angle X-ray scattering in grazing incidence for supported nanoalloyed and core-shell metallic nanoparticles

“…F part (q, R) and F co (q, R c ) are the form factor for a spherical scattering object of radius R for the whole particle and of a radius R c for the core, respectively. Only a limited number of papers are dedicated to investigations of core-shell particles by SAXS technique [23][24][25], and even less by ASAXS technique [19,26,27]. Figure 3 shows data extracted from recent works of Haug and coworkers [27].…”

“…contributing significantly to the total scattering signal. This is the case for metal catalyst particles in porous carbon support [9,[17][18][19][20]. As the carbon matrix is inhomogeneous, the system should be described using a three-phase model [21].…”