Bimetallic Nickel Cobalt Sulfide as Efficient Electrocatalyst for Zn–Air Battery and Water Splitting

Highlights Bimetallic nickel cobalt sulfide (Ni,Co)S2 nanosheet arrays were demonstrated as a multifunctional catalyst for OER, HER, and ORR. First principle calculations were performed to probe the rate-limiting step, which involves the formation of *OOH from HO− on the (Ni,Co)S2 surface. A water-splitting system was designed with the (Ni,Co)S2 serving as both cathode and anode, and a Zn–air battery cathode electrocatalyst. Electronic supplementary material The online version of this article (10.1007/s40820-018-0232-2) contains supplementary material, which is available to authorized users.


S1 Materials Characterizations
The crystal structure of each sample was studied by using X-ray diffraction for phase analysis (XRD, X' Pert PRO PHILIPS with Cu Kα radiation). X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra) was conducted to study the elementary composition and the bonding characteristics in each sample. The morphology and highresolution images were characterized using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, TecnaiTM G2 F30, FEI, USA). Raman spectra were acquired using a Jobin-Yvon LabRam HR80 spectrometer (Horiba Jobin Yvon, Inc.) with 532 nm line of Torus 50 mW diode-pumped solid-state laser under backscattering geometry. Electrochemical measurements were performed in a standard three-electrode electrochemical cell using an electrochemical workstation (CHI660e).

S4 HER Measurements
In the standard three-electrode electrochemical cell of electrochemical workstation (CHI660e), graphite electrode, Ag/AgCl, and glassy carbon electrode were used as the counter, reference, and working electrode. All the data were recorded at a sweep rate of 5 mV s -1 after applying a number of cyclic voltammetric scannings until they were stable. Current density was normalized to the geometrical area of the working electrode. The electrochemical measurements were iR-corrected until otherwise specified. The potential of Ag/AgCl is related to RHE by the equation of E(RHE(V))= E(Ag/AgCl) + 0.197 V+ 0.059*pH.

S5 OER Measurements
For OER characterization, the typical process is similar to that of HER, except that the test was conducted in an alkaline electrolyte 0.1 M KOH.

S6 ORR Measurements
The catalyst ink was pipetted onto the disk electrode or ring disk electrode to obtain a catalyst loading of 0.2 mg cm -2 , which was used to test for ORR. Electrochemical experiments were carried out in O2-saturated 0.1 M KOH electrolyte for ORR. The potential range is cyclically scanned between 0.2 and 1.0 V vs. RHE with a scan rate of 2 mV s -1 . The CV and LSV were obtained at the ambient temperature after purging with O2 or N2 gas for 30 min. The potential cycling was repeated until stable voltammogram curves were obtained. RDE measurements were made at rotating rates varying from 400 to 2400 rpm, at a scan rate of 2 mV s -1 .
Kinetic parameters were obtained on the basis of the following Koutecky-Levich (K-S4/S14 L) equation: 1/j=1/jk+1/(Bɷ 1/2 ) B, the slope of K-L plot, can be obtained from the following: where j is the measured current density, jk is the kinetic current density, ɷ is the rotation speed (the constant of 0.2 is used when the rotation speed is expressed in rpm), n is the electron transfer number, F is the Faraday constant (96485 C mol -1 ), , v is the kinetic viscosity of the electrolyte (0.01 cm 2 s -1 ).

S7 Zn-air Batteries
Home-made electrochemical cells of rechargeable Zn-air battery are constructed in the present work. The active material, made as detailed in the experimental procedure, was coated on carbon paper substrate as the air cathode. A polished Zn plate was employed as the anode, and a 6 M KOH + 0.2 M Zn(Ac)2 aqueous solution was utilized as the electrolyte. Battery tests were performed at room temperature using a LAND CT2001A instrument. In the cycling test, one cycle typically consists of one discharging step (2 mA cm -2 for 5 min) followed by one charging step of the same current density and duration time.

S8 Calculation of Effective Active Surface Area (ECSA)
The double layer capacitance (Cdl) is obtained by cyclic voltammetry at different scan rates (in the range of 20~180 mV s -1 ) to be linearly proportional to effective active surface area (ECSA). The potential is in the range from 0.2 to 0 V vs. Ag/AgCl, the Cdl is estimated by plotting the difference between anodic and cathodic currents (ja-jc) at 0.1 V vs. Ag/AgCl against various scan rates, where the slope is double Cdl. The specific capacitance Cdl 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. The specific capacitance for a flat surface is generally found to be in the range of 20-60 μF cm -2 [S2]. In the following calculations of ECSA we assume 40 μF cm -2 , ECSA = 40

S9 Turnover Frequency (TOF) in OER Calculation [S3]
The TOF values were calculated by assuming that every metal atom is involved in the catalysis: Here, j (mA cm -2 ) is the measured current density, S is the surface area of the electrode, the number 4 means 4 electrons per mol of O2, F is the Faraday's constant (96,485 C mol −1 ) and n is the moles of coated metal atom on the electrode. S8/S14

S10 Turnover Frequency (TOF) in HER Calculation
Voltammetric charges (Q) is calculated by the following equation [S4]: Where F is Faraday constant (96,480 C mol -1 ), n is the number of active sites. The factor 2 suggests that the formation of one hydrogen molecule needs two electrons in HER. In the experiment, the voltammetry curve is obtained by CV measurements with phosphate buffer (pH = 7) at a scan rate of 50 mV s -1 . When the number of voltammetric (Q) is obtained after deduction of the blank value.
The turnover frequency (TOF) can be calculated with the following equation:

TOF = I/Q
Where I(A) is the current of the polarization curve, we obtained it from the LSV measurements. Fig. S21 a The Cdl obtained via cyclic voltammetry at different scan rates. b The TOF of (Ni,Co)S2, NiS2 and CoS2. c EIS of (Ni,Co)S2, NiS2 and CoS2, the insert is analogue circuit diagram. d The i-t curve of (Ni,Co)S2 at -0.2V vs.RHE for 8×10 4 s