Electrocatalysts play an important role in the transition towards a sustainable society. Nanomaterials are used as catalysts for a range of electrochemical reactions, and the activity and selectivity of nanocatalysts depend on many factors.1 While extensive efforts have been devoted in optimizing electrode performance with high activity and selectivity, the poor stability of nanomaterials becomes a critical issue. The dynamic nature of nanoparticles under electrochemical conditions results in structural variations that determine the selectivity and activity on the single nanoparticle level. Therefore, thorough understanding of electrocatalysts requires both bulk electrode and single nanoparticle analysis.

TOC. a. Schematic drawing of in situ electrochemical cell for X-ray diffraction studies. b, c. Measured diffraction intensity of Cu nanoparticle in absence (b) and presence (c) of CO2 saturated 0.1 M KHCO3. d. Top isosurface view (top) of the reconstructed modulus. Side isosurface view (bottom) of the reconstructed modulus. The missing electron density corresponding to a part with a different crystallographic orientation (twin) is clearly visible. The tick spacing corresponds to 50 nm. e, f. 2D cross-sections of displacement field of Cu nanoparticle in absence (b) and presence (c) of CO2 saturated 0.1 M KHCO3.

In this work, we demonstrate a versatile in situ X-ray diffraction-based methodology which enables the structure and phase analysis of bulk electrodes and lattice strain analysis of single nanoparticles. A novel in situ electrochemical cell configured in back-illumination geometry has been fabricated to perform X-ray diffraction (XRD) and Bragg Coherent Diffraction Imaging (BCDI) with unfocused and focused beam respectively. In situ XRD has been collected during stepped cyclic voltammetry CO₂ saturated 0.1 M KHCO₃ to trace the phase evolution of Cu electrode, while ex situ BCDI has been performed on single Cu nanoparticle in absence and presence of CO₂ saturated electrolyte. BCDI results demonstrate that that displacement appears mostly at the twin boundary and edges even without the presence of electrolyte. After introducing electrolyte, the displacement field at the twin boundary and the surface change significantly, which can be caused by adsorbed species or surface oxidation. With the strain and displacement field information, we observed the influence of electrolyte on the surface and boundary structure, which can help to identify the active sites and holds great promise for establishing the structure-performance relationship.