Electrochemical Surface Restructuring of Phosphorus-Doped Carbon@MoP Electrocatalysts for Hydrogen Evolution

An electrochemical-induced surface restructuring strategy is developed to design phosphorus-doped carbon@MoP electrocatalysts which exhibits excellent activity for the hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes. The activation process and the fundamental mechanism of the prominent synergistic interaction between the phosphorus-doped carbon and MoP are elucidated. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00737-w.


S1.2.1 Synthesis of p-xylylenediphosphonic acid (H4xdp) [S1]:
The ligand was synthesized by reacting alpha, alpha'-Dibromo-p-xylene with triethyl phosphite and followed by refluxing the obtained oil with conc. hydrochloric acid according to the literature method. Block colorless crystals were obtained from the water solution by slow evaporation.

S1.2.2 Synthesis of [(MoO2)2(xdp)(H2O)2]· 2H2O [S1]
Mo-MOF precursor was prepared according to previous work [S1]. In a typical procedure, Na2MoO4· 2H2O (0.240 g, 1.0 mmol) was stirred together with p-xylylenediphosphonic acid (H4xdp) (0.140 g, 0.5 mmol) in 16ml deionised water. The pH of the solution was adjusted to pH 1 by dropwise addition of conc. hydrochloric acid. The acidified solution was then placed in a 25 cm 3 Ace pressure tube and heated at 120 °C for 15 h. The resultant white crystalline material was thoroughly washed with deionised water several times and dried at 80 °C for 12 h under vacuum.

S1.2.3 Preparation of MoP@PC Nanowires
In a typical procedure, 500 mg Mo-MOF precursor was placed in a porcelain boat. Then, the boat was heated at 900 °C under a constant flow of N2 at 30 mL min −1 for 120 min with the warming rate of 20 °C min -1 . The final black powder was collected when the temperature dropped to room temperature under N2.

S1.3 Electrochemical Activation
The in-situ electrochemical activation was carried out in 0.5 M H2SO4 under a N2 atmosphere to avoid possible oxidation caused by O2 in air. This was conducted by using the three-electrode system of CHI 760E electrochemical workstation (CH Instruments, Inc., Shanghai). MoP@PC was used as the working electrode, carbon rod was used as the counter electrode, Ag/AgCl (saturated KCl-filled) was used as the reference electrode. The electrochemical activation was performed by cycle voltammetry (CV) from -0.2 to 0.2 V vs RHE in 0.5M H2SO4, portion of the activation is shown in Fig. S7.

S1.4 Characterization
The crystal structure of sample was characterized by powder X-ray diffraction (XRD) (PANalytical Inc.) using Cu Kα irradiation operating at 45 KV and 40 mA with a fixed slit. Morphology of sample was observed by a JEOL JSM-7500F (Japan) Field Emission Scanning Electron Microscopy (FESEM). TEM (HRTEM) images were measured using a JEOL JEM2100F (Japan)Transmission Electron Microscope for investigating the information on lattice and fringe. Nitrogen sorption isotherms were measured at 77 K using an Autosorb volumetric gas sorption analyzer (Quantachrome, USA). TGA was conducted on a thermal analyzer (Mettler Toledo TGA/SDTA85, Canada) from room temperature to 1000 °C in N2 atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Thermo ESCALAB 250 (USA) spectrometer using an Al Kα (1486.6 eV) photon source. Raman spectrum was recorded using JY HR800 under ambient conditions. The X-ray absorption near edge structure (XANES) measurement was performed at Singapore Synchrotron Light Source, facility for catalysis research (XAFCA) beamline.
Electrochemical measurements were performed at room temperature, catalyst ink was typically made by dispersing 20 mg of catalyst in 2 mL ethanol. After adding 0.5 mL of 0.05 wt% of Nafion solution (Gashub, Singapore) and ultrasonication, an aliquot of 5 µL was pipetted onto the glassy carbon electrode (0.0706 cm 2 ) to reach the catalyst loading of 0.56 mg cm −2 . In a three-electrode configuration, Polarization curves were collected by CHI 760E electrochemical workstation at room temperature. Carbon rod as the counter electrode, Ag/AgCl and saturated calomel electrode (SCE) were used as the reference electrodes in acid and alkaline electrolyte, respectively. All the potentials shown were recorded with respect to the reversible hydrogen electrode (RHE) without IR correction. Current density was normalized to the geometrical area of the working electrode. Polarization data are collected at the scan rate of 5 mV s −1 on a rotation disk electrode under 2000 rpm. EISs were carried out in a potentiostatic mode in the frequency range of 10 6 to 1 Hz with the amplitude of 5 mV.

S1.5 Electrochemically Active Surface Area
The electrochemically capacitance measurements were conducted by cyclic voltammograms from 0.10 to 0.30 V with various scan rates (10,20,30,40,50,60,70,80,90, 100 mV s -1 ) as shown in Fig. S5. The capacitive currents were measured in a potential where no faradic processes were observed. According to the previous report [2] , the specific capacitance, a flat standard with 1 cm 2 of real surface area, is approximately 40 μF cm -2 . Thus, the electrochemical active surface area can be calculated by following Eq. (S1):

S1.7 DFT Calculations
All calculations were performed using Vienna Ab-initio Simulation Package (VASP) of MedeA software, the generalized gradient approximation (GGA) of Perdew−Becke−Ernzerhof (PBE) is used for the exchange-correlation functional [S3-S5] The MoP@C240 model was built by encapsulating a MoP cluster with a graphitic carbon cage C240, which performed well in previous study [S6, S7]. In the construction of model MoP@C239P1, and C239P1, P atom was introduced by substituting C atom in the carbon cage. All structures were fully relaxed to the ground state and spin-polarization was considered in all calculations. The convergence of energy and forces were set to 1 × 10 −4 eV and 0.01 eV Å -1 , respectively. An energy cutoff of 400 eV and a Gamma k-point sampling were found to get convergent geometry. For HER, the free energies of the intermediates were obtained by ΔG(H*) = ΔE(H*) + ΔZPE − TΔS, where ΔE(H*), ΔZPE and ΔS is the binding energy, zero-point energy change and entropy change of adsorption H, respectively. The ΔZPE and ΔS were obtained according to the method reported by Norskov [S8, S9].