Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition

Highlights Three-phase heterojunction can adjust the ∆G of H/O-intermediates to boost catalytic activity. At ± 1000 mA cm−2, Ni/MoO2@CN exhibits low hydrogen/oxygen evolution reaction overpotentials (267/420 mV). Ni/MoO2@CN used as bifunctional electrodes can work at 1000 mA cm−2 for 330 h in 6.0 M KOH + 60 °C condition. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00744-x.


S1.1 Computational Methods
The Vienna Ab initio Software Package (VASP 5.3.5) code was used to obtain all the density functional theory (DFT) calculations under the projected augmented wave (PAW) approach and Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation [S1 -S4]. The cutoff energy was set at 450 eV for the plane-wave basis set. The Monkhorst-Pack (MP) grids were employed to optimize the Brillouin zone of the surface unit cell, and the k-point mesh density is 2π×0.04 Å −1 [S5]. In order to complete the geometric optimization, the force and electronic self-consistent iteration were converged to 0.01 eV Å -1 and 10 −5 eV, respectively. For reducing the underestimation of the electronic band gap and the excessive tendency to delocalize the electron density, the electronic structure of catalysts was obtained by the PBE+U method. Herein, the Hubbard parameter of Ni and Mo were set to U−J=3 and 5 eV, respectively. To avoid interactions between periodic images, the vacuum layer was set to 15 Å.

S1.2 Preparation of Ni/MoO2@CN Nano-needle
All reagents were produced by Aladdin Reagent Co., Ltd with no further purification. The 1.0 M HCl was used to remove the oxide on the surface of NF (1.0×2.0 cm 2 ) under the ultrasound condition, and following washed with ultra-pure water and ethanol for about 30 min. Then, the NF was immersed in a mixed solution [20 mL ethylene glycol, 5 mL ultra-pure water, 54 mg (0.186 mmol) Ni(NO3)2· 6H2O and 210 mg (0.170 mmol) (NH4)6Mo7O24· 4H2O], which put into a 50 mL steel autoclave and maintained for 12 h at 140 °C . Subsequently, when the temperature cooled down to 25 °C , the NF was cleaned by C2H5OH and ultra-pure water, and vacuum dried at 80 °C for 12 h. Finally, it heated at different temperatures (350, 450, and 500 °C ) for 2 h under the reducing atmosphere [5% H2+95% Ar, named as (Ni-MoO2)@CN nano-needle]. The mass loading of Ni/MoO2@CN nano-needle is 15.2 mg cm −2 by ultrasonication method to remove the materials from NF. The samples with Ni/Mo molar ratios of 1:5 and 1:9 were prepared by the same method. Besides, MoO2@CN and Ni@CN were obtained by the same method without Ni and Mo source, respectively; Ni/MoO2 was obtained in pure water solution with Mo and Ni source.

S1.3 Characterization
The SU8220 scanning electron microscopy (SEM, HITACHI, Japan) was employed to study the surface morphology of the samples. The G2 80-300 Titan ETEM (FEI Co., USA) worked at 300 kV to obtain the energy dispersive X-ray (EDX) spectroscopy and high-resolution transmission electron microscopy (HRTEM) images. The D8 Advance X-ray diffraction (XRD) with λ=0.15406 nm CuKα radiation (SmartLab, Rigaku Co., Japan) to research the crystal structure of catalysts. The state of elements for catalyst was obtained by the ESCALab 250Xi X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific, USA) with an Al X-ray source worked at 150 W. The Horiba Jobin Yvon Inc., France, λ(He/Ne)=532 nm Raman spectrometer obtained the Raman spectroscopy.

S1.4 Electrochemical Measurements
Traditional three-electrode cell (include: all samples, reversible hydrogen electrode and graphite bar were used as work, reference and counter electrode, respectively) were used to evaluate linear sweep voltammetry (LSV), electrochemical impedance spectra (EIS) and chronopotentiometry (CP) for all catalysts, and obtained by electrochemical workstation (Germany) under 1.0 M KOH+30 °C solution containing saturated N2. EIS was evaluated at −0.2 and 1.5 V for HER and OER with the range from 100,000 to 0.1 Hz and the amplitude is 5 mV. The iR correction potential (Ecorr) was obtained by the following equation: (1) Ecorr=Emea-iRs, the actually measured potential and solution resistance were the Emea and Rs. The WS performance was tested by the two-electrode cell at the same environment. The Tafel plots were originated from LSV curves by the formula: [(2) η=blog|j|+a], the current density, intercept and Tafel slope are j, a and b, respectively.
Furthermore, the cathode/anode noble metal ink contained 40 wt% IrO2/C and 20 wt% Pt/C (purchased from Aladdin without further purification), which dispersed in a mixed solution [5.0 wt% Nafion (40.0 μL) and ethanol (0.96 mL)]. Subsequently, it was spread on the 0.5 cm 2 NF (named as IrO2/C and Pt/C).                     We used the active surface redox sites method to study the TOFs of Ni/MoO2@CN, Ni/MoO2, and Ni@CN for OER, by calculating the redox surface sites of Ni 2+ /Ni 3+ without the capacitive current [S7, S11-S15]. As shown in Fig. S44, the Ni/MoO2@CN, Ni/MoO2 and Ni@CN are tested in 1.0 M KOH solution and the region is 1.0 to 1.8 V vs. RHE. The total number of active atoms is equal to the calculated charge of the peak Qs divided by the charge of an electron (1.6×10 −19 C), and the formula is Ns=Qs/Qe, which is from the one-electron reaction of Ni 2+ /Ni 3+ .