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coherence) of such device-relevant catalysts with enhanced surface-to-volume ratio are permanently modified by the electrochemical environment of their application systems (Figure 116a and Figure 117a for Pd and Pt, respectively). The ability of Pd to absorb hydrogen and to form Pd hydrides is already well-documented. However, monitoring the structure of Pd nanoparticles during cyclic voltammetry allowed the researchers to unveil the electrochemically driven Pd hydride phase transition, which, until now, had mostly been investigated in the gas phase. The high-quality X-ray diffraction patterns
measured under operando conditions provided the first detection of theoretically predicted supersaturated and undersaturated metastable states involved in a core-shell mechanism of the phase transition (Figure 116b).
In addition, monitoring the structure of Pt/C during cyclic voltammetry allowed the establishment of a near-linear correlation between the expansion of the bulk lattice parameter of Pt nanoparticles and their oxide surface coverage (Figure 117b). Since the latter is known to be a descriptor of catalyst activity toward numerous reactions
Fig. 116: Monitoring hydrogen absorption in Pd.
a) 3D representation and corresponding 2D projection
of wide-angle X-ray scattering (WAXS) pattern intensities
(1 in 20) plotted as a function of time and the momentum
transfer Q recorded during a Pd cyclic voltammetry
experiment in a N2-purged 0.1 M NaOH electrolyte at room
temperature. The evolution of the electrochemical potential
applied, and electric current measured, are also represented
on the same time scale. b) PdHx α ↔ b phase transition
lattice constant hysteresis at the steady state (square
symbols interpolated by black lines) measured after 60 s hold at various potentials,
superimposed with the data from cyclic voltammetry at
5 mV s-1 (circle symbols). The time resolution is 333 ms.
Fig. 117: Monitoring adsorption on Pt. a) Cyclic voltammograms (solid line) and first derivative of the lattice constant (dashed line) of Pt/C in N2-purged 0.1 M HClO4 recorded at room temperature with a potential sweep rate of 5 mV s-1. b) Linear regression (dashed line) between lattice constant variation and adsorbate-related electrical charge.