Dirac Nodal Arc Semimetal PtSn4 : An Ideal Platform for Understanding Surface Properties and Catalysis for Hydrogen Evolution

Abstract

Conductivity, carrier mobility, and a suitable Gibbs free energy are important criteria that determine the performance of catalysts for a hydrogen evolution reaction (HER). However, it is a challenge to combine these factors into a single compound. Herein, we discover a superior electrocatalyst for a HER in the recently identified Dirac nodal arc semimetal PtSn₄ . The determined turnover frequency (TOF) for each active site of PtSn₄ is 1.54 H₂ s^-1 at 100 mV. This sets a benchmark for HER catalysis on Pt-based noble metals and earth-abundant metal catalysts. We make use of the robust surface states of PtSn₄ as their electrons can be transferred to the adsorbed hydrogen atoms in the catalytic process more efficiently. In addition, PtSn₄ displays excellent chemical and electrochemical stabilities after long-term exposure in air and long-time HER stability tests.

Keywords: Dirac semimetal; PtSn4; electrocatalysis; hydrogen evolution reaction; surface states.

Figure 1

Motivations for searching for advanced electrocatalysts beyond pure Pt: a) The Pt mass ratios in binary compounds that include Pt. b) HER volcano plot for PtSn₄ and selected metal catalysts. The Gibbs free energy of PtSn₄ is very close to that of pure Pt. c) Band structure of Pt. The metallic topological surface states are shown in yellow. The bulk states (upper and lower parts) and surface states (middle part) are separated by the red line. The band structure of PtSn₄ with non‐closed Dirac node arc is also shown. d) The room temperature mobility and conductivity of PtSn₄ with the selected advanced electrocatalysts.

Figure 2

Electrochemical performance of PtSn₄ single‐crystal catalyst. a) HER polarization curves for the Cu wire, Pt foil, 20 % Pt/C, and PtSn₄ single crystal. b) Corresponding Tafel plots for the Pt foil, 20 % Pt/C, and PtSn₄ single crystal in 1 m KOH. c) Overpotential of the PtSn₄ single‐crystal catalyst at 10 mA cm⁻² and some recently reported results for HER electrocatalysts. d) TOF values for the PtSn₄ single‐crystal catalyst and other recently reported results for HER electrocatalysts. e) Current–time (i–t) chronoamperometric response of the PtSn₄ electrocatalyst for increasing current densities from 20–300 mA cm⁻², at an overpotential of 46 mV. The inset shows the changes in the current density during stirring of the electrolyte with a magnetic bar.

Figure 3

Structure information of bulk single‐crystal PtSn₄. a) Crystal structure of the PtSn₄ derived from the single‐crystal XRD at 300 K. The exposed Pt atomic layer was constructed as shown by the upper left panel. b) A typical SEM image of the obtained PtSn₄ single crystal. c) HRTEM image of the PtSn₄ sample prepared using the focused ion beam technique (FIB). Inset: the selected area diffraction (SAED) pattern recorded along the [010] crystal orientation. d) Temperature‐dependent resistivity along the [010] direction. An ultralow ρ of 42 μΩ cm at room temperature was observed. e) Calculated surface Fermi surface based on the semi‐infinite model. The XPS results for the f) Pt 4f and g) Sn 3d.

Figure 4

Bonding information in the crystal and the electron transfer mechanism. a) Contour plots of the total charge distribution of PtSn₄ in the (010) plane. The electronic charges are almost all distributed in the vicinity of the Pt and Sn atoms. b) The 3D‐electron localization function mapping of the (010) surface shows that there is a weak bonding between the in‐plane Sn atoms. c) The 3D‐electron localization function (ELF), with an iso‐surface value of 0.72. d) Atomically resolved STM topography of the PtSn₄ (010) surface. Inset: the corresponding fast Fourier transform (FFT). The line profile measured along the pink line in (d) indicates a lattice parameter of 0.451 nm; the bias voltage was V _b=100 mV with a tunnel current of I _set=200 pA. e) The Gibbs free energy diagram of the PtSn₄ (010) surface with an exposed Sn layer, Pt layer, Pt metal (111) face, and Pt nanocluster (55 atoms) calculated at the equilibrium potential of different models, and the water dissociation energy berries on the Pt (111) and PtSn₄ (010) surfaces. f) Band structure of the PtSn₄ (010) surface with an exposed Pt layer, but without H adsorption. The robust surface states are along the direction Χ → Μ in the Brillouin zone, located just below the Fermi energy. g) Evolution of the Pt derived surfaces states after hydrogen adsorption when exposing the Pt layer. The surface states donate electrons to the adsorbed H 1s orbital, and shift above the Fermi level.