Materials
Ni chloride hexahydrate (NiCl2·6H2O, guaranteed reagent (GR), ≥ 98 % purity) was purchased from Alfa Aesar, boric acid (H3BO3, GR, ≥ 99 % purity), Cobalt acetate tetrahydrate (Co(Ac)2·4H2O, analytical reagent (AR), ≥ 99.5% purity) and Ni nitrate hexahydrate (Ni(NO3)2·6H2O, GR, ≥ 99 % purity) were obtained from Aladdin Reagent Co. Ltd. Kaolin, thiourea (CH4N2S, AR, ≥ 90% purity), thioacetamide (TAA, C2H5NS, AR, ≥ 90% purity) were purchased from Sinopharm Chemical Reagent Co., Ltd, and cyanamide (CN2H2 AR, ≥ 95 % purity) was purchased from Macklin. Potassium hydroxide (KOH, AR, ≥ 85 % purity), sodium hydroxide (NaOH, AR, ≥ 96 % purity), lithium hydroxide (LiOH, AR, ≥ 99 % purity) were obtained from Sinopharm Chemical Reagent Co., Ltd, and cesium hydroxide (CsOH, AR, ≥ 99 % purity) was purchased from Macklin, which were all directly used without pre-purification. Ni foam (Kunshan Shengshi Jingxin New Material Co., Ltd) was cut into 1 × 2 × 0.15 cm3 pieces. Ni foam was washed three times with acetone and deionized water alternatively, then etched in diluted HCl solution (6.0 M) for 15 min, and finally washed thoroughly with deionized water and ethanol under ultrasonication for 15 min, respectively. ITO (indium tin oxide) glass with 185-nm thick ITO film and resistance of 6-8 Ω (Shenzhen South China Xiangcheng Technology Co., Ltd) was cut into 1 × 2 × 0.11 cm3 pieces. Deionized water was used as solvent throughout the whole experimental process.
Preparation of ITO/Ni@Ni(OH)2 electrodes
The ITO glass was used as the anode (working electrode), the platinum tablet was used as the cathode (counter electrode) and the Ag/AgCl/3.5 M KCl electrode was used as the reference electrode. In brief, 0.5 mmol NiCl2·6H2O and 5 mmol H3BO3 were dissolved into 50 mL deionized water, which was used as electrodeposition solution to obtain Ni particles on ITO substrates. 0.5 mmol Ni(NO3)2·6H2O was then dissolved in 50 mL deionized water, which was used for the subsequent electrodeposition of Ni(OH)2 shell on Ni particles. To prevent the influence of dissolved oxygen in the solution, the electrolyte solutions were purged with ultra-high purity nitrogen for 30 min before electrodeposition. The Ni particles were electrodeposited on ITO substrate at a constant potential of -1.2 V vs. Ag/AgCl and the subsequent electrodeposition of Ni(OH)2 shell on Ni particles was performed at a constant potential of -1.0 V vs. Ag/AgCl. The program automatically ends when the quantity of electric charge for deposition reaches 10 mC cm-2, and the whole process was controlled by an Autolab PGSTAT302N. After the electrodeposition of Ni@Ni(OH)2 core-shell nanoparticles on ITO, the ITO anode was thoroughly cleaned with deionized water.
Preparation of NF/NiSX/C3N4 electrodes
This preparation method refers to ref. 11. The pre-cleaned NF (1.0 × 2.0 cm) was placed in a tube furnace beside a crucible containing 4.0 g thiourea (CH4N2S). Thiourea powder downstream of a N2 flow at 10 mL min-1. The temperature of tube furnace was raised from room temperature to 450 ℃ and maintained for 2 h, The heating rate is 1.4 ℃ min-1, then cooled to room temperature to obtain the NiSX/C3N4. The samples were cleaned with deionized water and then dried at room temperature for 12 h.
Preparation of NF/Ni3S2 electrodes
This preparation method refers to ref. 44. A piece of pre-cleaned NF was wrapped by Teflon tape with exposure area of ~1 cm2, and then immersed into a hydrothermal kettle containing a 40 mL aqueous solution of 640 mg thioacetamide (TAA, C2H5NS). Then, the hydrothermal kettle was sealed for hydrothermal reaction at 180 ℃ for 6 h to obtain the NF/Ni3S2. After the hydrothermal kettle was cooled down to room temperature, the samples were cleaned with deionized water and ethanol and then dried at room temperature for 12 h.
Preparation of NF/Ni(OH)2 electrodes
0.5 mmol Ni(NO3)2·6H2O was dissolved in 50 mL deionized water, which was used for the subsequent electrodeposition of Ni(OH)2 on pre-cleaned NF. The electrodeposition of Ni(OH)2 on NF was performed at a constant potential of -1.0 V vs. Ag/AgCl. The program automatically ends when the quantity of electric charge for deposition reaches 100 mC cm-2 to obtain the NF/Ni(OH)2.
Preparation of NF/Co3O4 electrodes
This preparation method refers to ref. 45. Co(Ac)2·4H2O (3.6 mL, 0.2 M) and CN2H2 (0.6 mL) were added to deionized water (48 mL). A piece of pre-cleaned NF was immersed into a hydrothermal kettle containing the mixture solution and hydrothermal reacted at 180 °C for 4 h to obtain the NF/Co3O4. After the reaction, the samples were washed with deionized water and ethanol and then dried at room temperature.
Characterizations
The micro morphology of the electrode and kaolin were measured by field-emission scanning electron microscope (FE-SEM, ULTRA-55, Zeiss, Germany). The composition and nature of Ni oxides were characterized by X-ray photoelectron spectroscopy (XPS, K-Alph, Thermo Scientific, American). The water structure of electrolytes and the chemical compositions of electrodes were characterized by laser confocal Raman spectrometer (LabRAM HR800, Horiba, France), a He-Ne laser with 532 nm excitation wavelength and a 50×microscope objective with a numerical aperture of 0.55 were used in all Raman measurements, a Si wafer was used to calibrate Raman frequency before each experiment. The surface charge density of kaolin was measured by zeta electric potential analyzer (Zetasizer Nano ZS90, Malvern, Britain). The particle size of kaolin was measured by Laser particle sizer (Mastersizer 2000, Malvern, Britain), and the element contents in electrolyte were identified using inductive coupled plasma emission spectrometer (ICP, OPTIMA7000DVI, Perkin Elmer, American). The surface groups of kaolin were recorded by Fourier Transformed Infrared (FTIR) spectra (Vector-22 spectrometer, Bruker, Germany).
Electrochemical tests
All electrochemical experiments were carried out with an Autolab Electrochemical workstation and a glass three-electrode cell, using platinum electrode as the counter electrode, Ag/AgCl electrode as the reference electrode, and the aforementioned ITO/Ni@Ni(OH)2 as working electrode. Linear sweep voltammetry (LSV) measurements of ITO/Ni@Ni(OH)2 electrodes were performed with 1.02-2.02 VRHE at a scan rate of 50 mV s-1, and LSV measurements of NF based electrodes were performed at a scan rate of 5 mV s-1. Electrochemical impedance spectra (EIS) were measured within frequency range of 105-0.1 Hz, and current density–time (j-t) was performed at a constant potential of 2.52 VRHE (unless otherwise noted). The overpotential (η) was calculated according to: η = ERHE -1.23 V. All the potentials were adjusted relative to reversible hydrogen electrode (RHE) according to: ERHE= EAg/AgCl + 0.059 × pH +0.1967 V, unless stated otherwise, all potentials were referenced to the RHE.
To evaluate the effect of adding kaolin into electrolyte on electrocatalytic performance, the ITO/Ni@Ni(OH)2 electrode was firstly aged with 30 cycles of CVs within 0-1.0 VRHE in the KOH electrolyte until the OER performance was stable, and then the electrode was aged with 30 cycles of CVs within 0-1.0 VRHE in the KOH electrolyte with kaolin to evaluate the effect of kaolin addition. In addition, all the test was performed under stirring.
Computational Details
All the AIMD simulations were performed with CP2K/Quickstep code46,47 within a hybrid Gaussian/Plane-Wave (GPW) scheme48. Electronic cores were represented by Geodecker-Teter-Hutter (GTH) pseudopotentials49-51. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional52 was applied together with dispersion corrections according to the DFT-D3 method53,54. DZVP-MOLOPT-SR-GTH basis sets55 were used along with plane waves expanded to a 500 Rydberg energy cutoff. The deuterium mass is substituted for all protons to reduce the time step size and to limit nuclear quantum effects in our AIMD simulations. The simulations were sampled by the canonical (NVT) ensemble employing Nosé-Hoover thermostats56,57 with a time step of 1.0 fs at a finite temperature of 300 K.
A periodic p (2 × 2) kaolin (010) surface (H32O72Al16Si16) was solvated by a roughly 22.6 Å thick water film containing 111 explicit H2O molecules. The parameters of the resulting simulation cell were 14.77 Å × 10.30 Å × 33.50 Å and 90.00˚ × 90.00˚ × 105.04˚. Another water box containing 111 H2O molecules was employed as a comparison. With the molar mass of water to be 18 g mol-1, the density of the water region in the two models is calculated to be about 1 g cm-3. The above two neutral models were initially equilibrated for 5 ps. A hydrogen atom was deleted in the last configuration of each trajectory to yield simulation cells containing a hydroxide anion. The anion containing simulation cells did not include a counterion and the systems therefore presented a net charge neutralized by a uniformly charged background58. The hydroxide was fixed in the middle of the water box by freezing distance between the surface site and the hydrogen oxygen O∗. Its hydrogen coordination was restrained during an equilibration run of 10 ps. The last 5 ps of each trajectory was taken to analysis the radial distribution functions g(r) and coordination numbers. The averaged electrostatic potential was calculated from the structure of last frame in the trajectory. The adsorption energy of one water molecule on the kaolin (010) surface and 3-coordinated hydroxide was calculated respectively as: