Surface reconstruction of cobalt phosphide nanosheets by electrochemical activation for enhanced hydrogen evolution in alkaline solution† †Electronic supplementary information (ESI) available: Experimental details, XRD, SEM, TEM, XPS, and electrochemical measurements. See DOI: 10.1039/c8sc04589e

The surface reconstruction of cobalt phosphide nanosheets is investigated by an in situ electrochemical strategy for enhanced hydrogen evolution.

Scientific ESCALAB 250 spectrometer with AI Kα radiation as the excitation source. Binding energies for the high resolution spectra were calibrated by setting C 1s to 284.6 eV. Fourier transformed infrared (FT-IR) spectra were recorded with a Nicolet IS10 FT-IR spectrometer. The concentration of elements was measured on inductively couple plasma optical emission spectroscopy (ICP-OES, Agilent Technologies).
Raman spectrum were collected using a Renishaw in-Via Raman microprobe equipped with a Leica DM 2500 M microscope system. Raman spectrum were excited by a 523 nm diode-pumped laser at a resolution of 4 cm -1 in the range between 100-4000 cm -1 .
Electrochemical measurements: For the preparation of working electrodes, 8.0 mg of CoP powders and 30 μL of Nafion (5 wt%) were dispersed in 1 mL of ethanol and water (1:1 in volume). The mixture was ultrasonicated for at least 30 minutes to generate a homogeneous ink, then 8 μL of ink was dropped onto the glassy carbon electrode with a diameter of 5 mm. The amount of CoP catalyst deposited on the glassy carbon electrode was controlled as 0.4 mg cm -2 . All electrochemical measurement was performed at room temperature by using a CHI 760E electrochemical workstation (CH Instrument, Shanghai, China) with a standard three-electrode cell. An Ag/AgCl in 3M KCl solution acted as the reference electrode, and a graphite rod (6 mm in diameter) was used as counter electrode. An aqueous solution of 1.0 M KOH (pH = 14) was used as the electrolyte. All potentials were referenced to a reversible hydrogen electrode (RHE) with 90% iR correction: E vs RHE = E vs.Ag/AgCl + 0.0591*pH + E Ag/AgCl θ (0.209) -iR, where the R was referred to the average ohmic resistance obtained from EIS measurement. 1 The in-situ surface reconstruction was carried out by chronpotentiometry under a constant current density of 20 mA cm -2 for 10 hours in 1.0 M KOH. Electrochemical tests of CoP-A were conducted after the V-t curves becoming stable. The scan rate of cyclic voltammetry (CV) is 0.05 V s -1 , while that is 0.005 V s -1 for linear sweep voltammetry (LSV).
The Tafel plots were obtained by fitting the polarization curves as overpotential η versus log current density (log[j]). Electrochemical impedance spectroscopy (EIS) was performed from 100 KHZ to 0.01 HZ at -150 mV (vs RHE) with an amplitude of 5 mV. Cyclic voltammetry (CV) was conducted to measure the electrochemical double layer capacitance (EDLC) at non-Faradaic potential ranging from 0.15 to 0.35 V (vs. RHE) with various scan rates (5, 10, 15, 20, 25, 30, 35, 40 mV s -1 ), which can estimate the effective electrochemical surface area. The long-stability of catalysts was also tested by chronoamperometry, which was carried out at constant overpotential of 120 mV for 25 hours. A flow of N 2 was maintained during the experiment.

The evaluation of main electrochemical parameters ( b, j o , C dl , ESCA, TOF). 2
Tafel slope (b) and exchange current density (j 0 ): Tafel plots can be obtained by fitting the polarization curves as overpotential versus log current density: η= a + blog [j], η is overpotential, j is current density and b is Tafel slope, which obtained by fitting in the linear region of Tafel curve and used to indicate the characteristics of the hydrogen evolution reaction kinetics. Exchange current density (j 0 ) is the intersection of the Tafel curve in the extension in linear region, which also reveals the kinetics of hydrogen evolution reaction and the intrinsic activity of electrocatalysts. Electrochemical double layer capacitance (C dl ) and        Ms [1] : Mass loading of catalyst (mg cm -2 ); η on [2] : Onset overpotential (mV); η 10 [3] : Overpotential of current density at 10 mA cm -2 (mV); b [4] : Tafel slope derived from LSV (mV dec -1 ); Cd [5] : Double layer capacity at non-faradic region (mF cm -2 ); S 0 [6] : Electrochemical surface area (cm 2 ); J 0 [7] : Exchange current density (mA cm -2 ); TOF [8] : Turnover frequency (s -1 ).
Note: NA means no available.