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.


Estimation of electrochemical active surface area (ECSA) and Turnover frequency calculations (TOF)
Since the effective surface area of the catalyst is linearly proportional to the double layer capacitance (C dl ), we measured the capacitive currents of the single crystal electrode in the potential range of 0.19 ~ 0.29 V vs. RHE with various scan rates (80, 100, 110, 120, 130, 140 mV/s, etc), where no faradic processes are happened. The specific capacitance is determined by plotting the capacitive currents as a function of scan rate. The specific capacitance can be converted into an electrochemical active surface area (ECSA) using the specific capacitance value for a flat standard with 1 cm 2 of real surface area.
According to the XPS and calculations, we assumed that the Pt is the active center and all the exposed Pt atoms in the (001) plane are possible active sites.
The total number of hydrogens turn overs was calculated from the current density according to: [2] # 2 = ( 2 ) (

Associated content
Supporting Information includes crystallographic information files of PtSn 4 , measured at 100 K and 300 K (CIF).

Computation details
Density functional theory (DFT) method as implemented in the Vienna ab initio Simulation Package (VASP) was used for the electronic-structure calculations. The exchange-correlation was considered in the revised Perdew-Burke-Ernzerhof (rPBE) parameterized generalized gradient approximation (GGA) and spin-orbital coupling (SOC) was included. After H adsorption, the H atoms are fully relaxed until the force less than 10 meV / Ȧ. The energy cutoff is set as 300 eV. The k-points grid is 6 x 6 x 1.

Water dissociation kinetics calculation
The most energy favorable pattern for water molecule and OH + H pairs adsorbing on slab are obtained by geometric optimization. To search a minimal energy path for water decomposition on the slab, a 2 × 2 × 1 supercell for both Pt (111) and PtSn 4 (010) is adopted, and the complete LST/QST method embedded in CASTEP as implemented in Materials Studio is employed for searching the transition states. The convergence criterion of force is set to be 0.01 eV/Å.

Catalytic activity of PtSn 4 and Pt
The Gibbs free energy ΔG H* of H adsorption on the basal surface of each material are calculated to describe the trends of hydrogen evolution reaction with the following equation: Here, ΔE ZPE and ΔS H are the difference in zero points energy and entropy between the adsorbed species and the gas phase molecule, respectively. For the adsorbed species, they can be determined from the vibrational frequencies of the adsorbed species using normal mode analysis with DFT calculation. [4] While for the gas phase molecule, the zero point energy of H 2 and the TS at room temperature (300K) can be looked up from standard molecular tables. [5] A good catalyst for hydrogen evolution is characterized with a ΔG H value close to 0.