NSRRC Activity Report 2023

Physics and Materials Science 017 The researchers analyzed the reduction kinetics of the metal precursors and entropy of mixing as shown in Figs. 2(a)−2(c) . This showed that the number of each metal precursor reached a steady state within 80 min in Fig. 2(a) and the instantaneous percentages of five elements as a function of reaction time converged, as shown in Fig. 2(b) . Most importantly, the calculated instantaneous entropy of mixing ∆ S mix > 1.5 R after ~10 min ( Fig. 2(c) ) corresponds to a high-entropy region which resulted in dendritic solid-solution HEA nanocrystals. Figures 2(d) and 2(e) show the TEM and HAADF–STEM images of the dendritic solid–solution PdPtRhIrRu nanocrystals, revealing a three-dimensional dendritic morphology consisting of several branches. HAADF–STEM images shown in Figs. 2(f)−2(i) indicate that the dendritic nanocrystals contained many step atoms ( Fig. 2(g) ), local disordered atoms ( Fig. 2(h) ), and vacancy defects ( Fig. 2(i) ), corresponding to an abundance of low-coordination defect sites on the surface. The Fast-Fourier transform pattern in the inset of Fig. 2(f) shows at least two sets of face-centered cubic (FCC) diffraction features, suggesting a polycrystalline sample. In addition, the FCC structure of the dendritic nanocrystals was confirmed by the XRD pattern ( Fig. 2(j) ). The EDS maps shown in Figs. 2(k) and 2(l) revealed the distributions of Pd, Pt, Rh, Ir, and Ru elements in each dendrite nanocrystal, and the atomic percentages of Pd, Pt, Ir, Rh, and Ru were 22.8, 21.9, 20.2, 18.4, and 16.7%, respectively. In addition, as shown in Figs. 2(m) and 2(n) , XPS results suggested that the dendritic nanocrystals were in the metallic state as seen by their clean spin-orbit split, single-peak asymmetric core level spectra. After characterizing the crystal structure, morphology and metallic nature of the dendritic nanocrystals, the researchers then carried out ex situ XANES and EXAFS measurements at the TPS 44A Quick-scanning X-ray Absorption Spectroscopy beamline to acquire element specific electronic structure information on the dendritic solid–solution PdPtRhIrRu nanocrystals. The XANES spectra ( Figs. 3(a)−3(e) ) showed that the absorption edges of all elements in the HEA nanocrystals were very similar to their corresponding metal foils and confirmed that the elements were in the metallic state, consistent with XPS analysis. Next, the coordination environments of the mixed elements in the HEA nanocrystals were investigated using their EXAFS spectra, as shown in Fig. 3(f) . The radial distances of the 4d elements in the HEA nanocrystals became longer than those of the metallic foil counterparts. In contrast, the radial distances of the 5d elements in the HEA nanocrystals became shorter compared to the metallic foils. These results confirmed the electronic structure and coordination of the solid–solution PdPtRhIrRu metallic nanocrystals. The researchers then evaluated the nanocrystals’ catalytic activities towards HER in an acidic 0.5 M H 2 SO 4 electrolyte. The linear sweep voltammogram (LSV) polarization curves of the four catalysts showed that the dendritic solid–solution PdPtRhIrRu nanocrystals exhibited the smallest overpotential of 30 mV (versus reversible hydrogen electrode, V RHE ) at a current density of 10 mA/cm 2 . The typical solid–solution PdPtRhIrRu HEA nanocrystals and the dendritic PdPtRhIrRu HEA nanocrystals showed geometric current densities of 21.2 and 26.8 mA/cm 2 at a potential of − 0.05 V RHE , which were 3.78 and 4.79 times higher than that of the phase-separated PdPtRhIrRu nanocrystals, respectively. In addition, the catalytic activities for HOR under alkaline conditions were examined by LSV polarization curves in N 2 - and H 2 -saturated 0.1 M KOH solutions. An anodic current over 0 V RHE was obtained upon H 2 saturation of the electrolyte, suggesting H 2 oxidation. The geometric current densities of the solid–solution PdPtRhIrRu nanocrystals without dendritic morphology, with dendritic morphology, Fig. 3 : The coordination structure and electronic structure of dendritic solid– solution PdPtRhIrRu HEA nanocrystals characterized by ex situ XAS analysis. The corresponding metallic foils and oxides were also compared. XANES spectra for the (a) Pd K-edge, (b) Pt L 3 -edge, (c) Rh K-edge, (d) Ir L 3 -edge, and (e) Ru K-edge. (f) EXAFS spectra for all five elements in the dendritic solid–solution PdPtRhIrRu HEA nanocrystals. [Reproduced from Ref. 3]