Publication date: 21st July 2025
Palladium hydrides (PdHx) are a model system for studying phase transitions and hydrogen (H) absorption in materials. Well-studied in the gas phase, they are also relevant in electrochemistry, particularly electrocatalysis, where the H:Pd ratio can be controlled though electrochemical potential. PdHx hydrides exhibit a slightly expanded lattice at low H content (x < 0.05), known as the α-phase, which transforms into a lattice-expanded β-phase at higher H content. While wide-angle X-ray scattering can be used to monitor the in situ absorption of H into commercial Pd nanoparticles (NPs) 3.6 nm in size [1] as well as related phenomena such as H trapping, crucial aspects of the mechanism and kinetics of PdHx formation remain elusive. Specifically, it is unclear whether the α- and β-phases coexist, and if the Pd NPs undergo isotropic H insertion, following a core-shell model, or if preferential H absorption pathways exist, as suggested by a spherical cap model. Furthermore, the small size of the facets makes it difficult to determine the distribution of strain fields across single NP.
In this study, employing Bragg Coherent X-ray Diffraction Imaging (BCDI) and focusing on the 111 Bragg reflection, we obtained information on the morphology, projected strain, displacement fields, and d-spacing of single 300 nm Pd NPs at various electrode potentials relevant to H adsorption, H absorption, and H2 evolution [2]. We examined changes in lattice constants for both α and β phases and reconstructed individual Pd NPs in each individual phase. The reconstructions revealed a continuous increase in the Pd lattice parameter, indicating an isotropic expansion of the NP. Additionally, we observed heterogeneous strain in the reconstructed Pd NPs, with tensile strain accumulating on the {111} and {100} facets, while the lattice in the edges and corners of the atoms appeared compressed. Finally, we will show how BCDI can be used to gain insights into H absorption/desorption mechanism and kinetics.
Figure 1. Diagram showing the in situ cell used to reconstruct a single nanoparticle in 3D under potential control from diffraction images.
This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement HERMES No. 952184 and grant agreement CARINE No. 818823.
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