M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

4 6 H I G H L I G H T S 2 0 2 3 I

X-ray diffraction images platinum nanoparticles at work in an electrochemical cell

Bragg coherent diffraction imaging has been used at beamline ID01 to measure the distribution of strain within a single platinum nanocatalyst under electrochemical control. The results should aid in designing more efficient nanocatalysts for applications such as fuel cells and electrolysers.

Controlling the degree of lattice strain is commonly used to optimise the chemisorption energies of adsorbates onto metallic nanocatalysts, and thus to accelerate the rate of (electro)catalytic reactions, in particular those involved in decarbonised electrochemical energy storage and conversion systems such as fuel cells and electrolysers.

First unveiled by Hammer and Nørskov using density functional theory (DFT) calculations [1,2], the so-called d-band theory found practical application in energy storage and conversion systems, and the production of high-value products. One important prediction of this theory is that catalysts binding oxygenated species slightly weaker than platinum Pt(111) would enhance the rate of the oxygen reduction reaction (ORR), of prime importance for fuel cells. Later, it was predicted that locally concave sites are more active in the ORR than terrace sites, while locally convex or buried sites contribute poorly to the reaction rate [3]. However, strain is experimentally measured on billions of nanocatalysts, and experimentalists assume that it is evenly distributed on each of them. So far, it has not been possible to map the distribution of strain in a single metal nanoparticle nor its dynamics while being operated.

In this work, Bragg coherent diffraction imaging (BCDI) was used at beamline ID01 to image the distribution of strain within a ca. 200-nm single Pt nanocatalyst in situ (under electrochemical control). BCDI was used to reconstruct the electron density and the displacement field at different electrode potentials. Figure 29 shows that positive strain (tension) accumulates on the top and bottom facets, while edges, corners and side facets experience negative strain (compression) as the electrode potential increases. Strain later dynamically propagates from the surface to the bulk of the Pt nanoparticle as the potential increases.

DFT calculations on different slab models corresponding to particular features of the nanoparticles (Figure 30) were used to confirm the experimental findings and to provide insight into the atomistic origin of the strain dynamics. The results indicate that bisulfate ions adsorb preferentially onto the corner and edge atoms, inducing local compression of the lattice strain. Furthermore, these were independent of the nanoparticle size, suggesting that the distribution of strain is morphology-related

Fig. 29: Revealing the influence of the electrode potential on the surface

strain (ε002).