On the electronic structure and hydrogen evolution reaction activity of platinum group metal-based high-entropy-alloy nanoparticles

Abstract

We report the synthesis of high-entropy-alloy (HEA) nanoparticles (NPs) consisting of five platinum group metals (Ru, Rh, Pd, Ir and Pt) through a facile one-pot polyol process. We investigated the electronic structure of HEA NPs using hard X-ray photoelectron spectroscopy, which is the first direct observation of the electronic structure of HEA NPs. Significantly, the HEA NPs possessed a broad valence band spectrum without any obvious peaks. This implies that the HEA NPs have random atomic configurations leading to a variety of local electronic structures. We examined the hydrogen evolution reaction (HER) and observed a remarkably high HER activity on HEA NPs. At an overpotential of 25 mV, the turnover frequencies of HEA NPs were 9.5 and 7.8 times higher than those of a commercial Pt catalyst in 0.05 M H₂SO₄ and 1.0 M KOH electrolytes, respectively. Moreover, the HEA NPs showed almost no loss during a cycling test and were much more stable than the commercial Pt catalyst. Our findings on HEA NPs may provide a new paradigm for the design of catalysts.

Fig. 1. (a) HAADF-STEM image of IrPdPtRhRu HEA NPs and its corresponding EDX maps showing (b) Ru-L, (c) Rh-L, (d) Pd-L, (c) Ir-L, and (f) Pt-L.

Fig. 2. Core-level XPS spectra showing the existence of five elements of IrPdPtRhRu HEA NPs in the regions of (a) 277–350 eV and (b) 55–88 eV. The dashed lines assign the peaks. (c) Metallic compositions obtained by XRF and XPS.

Fig. 3. (a) Comparison of the experimental VB spectrum of IrPdPtRhRu HEA NPs and the linear combination of VB spectra of the monometal NPs. The difference of the spectra is shown as cyan regions. (b) Comparison of the VB spectra of IrPdPtRhRu HEA NPs and Pt NPs.

Fig. 4. (a and d) Polarization curves, (b and e) geometric current densities at an overpotential of 50 mV, and (c and f) TOF of IrPdPtRhRu HEA NPs and other monometallic catalysts in (a–c) 0.05 M H₂SO₄ solution and (d–f) 1.0 M KOH solution.

Fig. 5. Comparison of the polarization curves between the initial cycle (red) and after 3000 cycles (black). (a) IrPdPtRhRu HEA NPs and (b) commercial Pt/C catalyst. The dashed arrow demonstrates the loss of overpotential at a j_geo of 10 mV cm⁻².

Fig. 6. TOF value at −0.05 V_RHE as a function of the experimental d-band centre of the tested catalysts in (a) 0.05 M H₂SO₄ and (b) 1.0 M KOH solutions. The d-band centre is relative to the Fermi level. The light blue regions show the trend of the activity following d-band centre theory.