NSRRC Activity Report 2023

018 NSRRC ACTIVITY REPORT 2023 Fig. 4 : (A) In situ XAS analysis for revealing the active sites and catalytic mechanism of the representative dendritic solid-solution PdPtRhIrRu HEA nanocrystals during HER. In situ XANES spectra conducted at different potentials corresponding to the (a) Pd K-edge, (b) Pt L 3 -edge, (c) Rh K-edge, (d) Ir L 3 - edge, (e) and Ru K-edge. (B) In situ EXAFS spectra conducted at different potentials at the (a) Pd K-edge, (b) Pt L 3 -edge, (c) Rh K-edge, (d) Ir L 3 - edge, (e) and Ru K-edge. [Reproduced from Ref. 3] and phase-separated PdPtRhIrRu nanocrystals (3.00, 2.68, and 2.63 mA/cm 2 , respectively) were much larger than that of commercial Pt/C (1.34 mA/cm 2 ). In situ XANES measurements with applied potentials were carried out to reveal the actual active sites. The XANES spectra at the Pd K-edge, Pt L 3 -edge, Rh K-edge, Ir L 3 -edge, and Ru K-edge of the representative dendritic PdPtRhIrRu HEA nanocrystals under an open circuit potential (OCP) and different applied potentials (0.05 and − 0.05 to − 0.15 V RHE ) for HER in the acidic 0.5 M H 2 SO 4 electrolyte are shown in Figs. 4(A) and 4(B) . It was observed that the absorption edge of Ru shifts to a lower energy when the applied potential was increased. The shift could be attributed to an increase in electron density and adsorbed hydrogen ions on the Ru sites during HER ( Fig. 4(A) ). These observations suggest that Ru sites are the direct active sites for HER, while the other elements contribute to modifying the electronic structure and increasing the entropy of PdPtRhIrRu HEA nanocrystals. This was confirmed by DFT calculations of the hydrogen adsorption free energy ∆ G H which showed that the Ru sites near the step site causes defects around the Ru atoms, which could largely weaken hydrogen adsorption on Ru and consequently promote HER activity. On the other hand, in situ XAS experiments for HOR in the H 2 - saturated 0.1 M KOH solution showed that the Pd sites play an important role in HOR, as evidenced by the dramatic shift in the main Pd K-edge to a lower energy as the applied potential was gradually changed from 0.05 to 0.9 V RHE under an OCP. Durability tests were carried out compared to commercial Pt/C and phase-separated PdPtRhIrRu nanocrystals, and it was found that the dendritic solid–solution PdPtRhIrRu nanocrystals showed greatly enhanced activity and durability toward HER and HOR. In conclusion, by analyzing quantitative experimental results obtained from reduction kinetics and entropy of mixing experiments of a method involving dropwise addition of a five-metal (quinary) precursor, the researchers demonstrated a novel and reliable method to synthesize HEA nanocrystals in a controllable and predictable manner. The results reported here are expected to lead to a paradigm shift in the design of HEA nanocrystals. (Reported by Ashish Chainani) This report features the work of Tung-Han Yang and his collaborators published in Sci. Adv. 9 , eadf9931 (2023).