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1 Metallic Iron-Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media Xia Long 1, Guixia Li 2, Zilong Wang 1, HouYu Zhu 2, Teng Zhang 1, Shuang Xiao 1, Wenyue Guo 2, Shihe Yang 1 * 1. Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 2. College of Science, China University of Petroleum, Qingdao, PR China Supporting Information 1. Experimental details 1.1 Chemicals: all chemical reagents were used as received without further purification. 1.2 Synthesis of FeNi based layered hydroxides: FeNi LDHs were synthesized by one step hydrothermal method with different feed ratios..145 ml of 1 M ferrous nitrate (Fe(NO 3 ) 3 ) aqueous solution, 1.45 ml of 1 M nickel chloride (NiCl 2 ) aqueous solution, were mixed in the beaker with 7.8 ml DI water. Then 5.6 ml of.5 M urea aqueous solution ad 2 ml of.1 M trisodium citrate (TSC) were added into the beaker in sequence with magnetic stirring. The mixed solution was transferred to a 1 ml Teflon lined stainless steel autoclave for hydrothermal reaction at 15 for 24 h. The FeNi-CO 3 LDH was collected by centrifuge, washed with water three times and then dried. 1.3 Synthesis of FeNi based layered sulfides: For formation of FeNi sulfide nanosheets, 8 mg of FeNi LDH precursor was dispersed into 4 ml of ethanol, followed by the addition of.1125 g of thioacetamide (TAA). Thereafter, the mixture was transferred into a Teflon-lined stainless steel autoclave, which was subsequently heated at 12 ºС for 6 h. after centrifugation and washed with ethanol for S1

2 several times, the FeNi-S ultrathin nanosheets were obtained. The reactions could be described by the following equations: S1,S2 CH 3 CSNH 2 + C 2 H 5 OH CH 2 (NH 2 )C(OC 2 H 5 )-SH CH 3 (NH 2 )C(OC 2 H 5 )-SH + C 2 H 5 OH CH 3 (NH 2 )C(OC 2 H 5 ) 2 + H 2 S H 2 S + FeNi-OH FeNi-S + CH 3 COOH + H 2 O In order to further improve the crystallinity, the final products were annealed under N 2 atmosphere at 35 ºС for 2 h. 1.4 Sample preparation for electrochemical characterizations: 5 mg of catalyst was dispersed in 1 ml of water-ethanol solution (V water /V ethanol =48/5), and then 2 ul of 5 wt% Nafion was added. The mixed solution was sonicated for 3 min. Then 1 ul of catalyst ink (containing 5 ug of catalyst) was loaded into a glass carbon electrode with diameter of 5 mm (the loading of catalyst was.254 mg/cm 2 ). 1.5 Electrochemical characterizations: Electrochemical studies were carried out in a standard three electrode system controlled by a CHI 76D electrochemistry workstation. Catalysts loaded on glass carbon electrode (5 mm in diameter) was used as the working electrode, coiled platinum wire and graphite rod as the counter electrodes respectively (the electrochemical performance was recorded by using Pt as the counter electrode if not indicated) and Ag/AgCl electrode as the reference electrode. The reference was calibrated against and converted to revisable hydrogen electrode (RHE). Linear sweep voltammetry (LSV) was carried out at 5 mv/s for the polarization curves and.1 mv/s for Tafel plots. The catalysts was cycled about 5 times of cyclic voltammetry (CV) until a stable CV curve was developed before measuring polarization curves of catalysts. All LSV polarization curves have not been ir-corrected if not indicated. Chrompotentiometry (CP) was carried out under a constant current density of 5 ma/cm 2, 1 ma/cm 2, and 2 ma/cm Structure and morphology characterization: Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX) samples were prepared by drop-dry the aqueous suspensions onto silicon wafer, and was characterized by a JSM-67F and by a 639 respectively. Transmission electron microscopy (TEM) samples were prepared by drop dry the catalysts suspensions onto copper grids and was measured by 21 TEM. Atomic force microscopy (AFM) samples were prepared by drop dry the catalysts suspensions onto silicon wafer and was characterized by Veeco diinnova with Si tip. S2

3 1.7 Other characterizations: X-ray diffraction (XRD) sample were prepared by drop dry catalysts aqueous solution onto glass substrate to form a thin film and then measured by PW183 (Philips); X-ray photoelectron spectroscopy (XPS) samples were prepared by centrifuging the catalyst suspension at 13 rpm for 3 min and washed wished with alcohol. The dried samples were measured by XPS on PHI 56. Energy dispersive X-ray spectroscopy (EDS) sample was prepared by drop dry catalysts suspension on Si wafer and then measure by JSM 63 (JEOL). Brunar-Emmett-Teller (BET) surface areas and N 2 absorbing-desorbing isotherms were recorded on a Coulter SA 31 surface area analyzer. The resistivity was measured by compressing the catalyst powder into a compressed thin film and then the four-point probes method was applied to measure the resistivity of the catalyst films. The experiments were carried out on HL 55PC (Bio-Rad) with the resistivity/hall measurement system under room temperature and 77 K (liquid N 2 ) respectively. 1.8 Computational methods and details: All calculations were performed using DFT with the program package of DMol3 in Materials Studio of Accelrys Inc. The exchange-correlation energy was calculated within GGA-PW91 approximation. The double-numeric quality basis set with polarization functions (DNP) was used in the calculations. The density functional semicore pseudopotential (DSPP) method was employed for the metal atoms. In NiS (11) (Ni:6; S:6) and FeNiS (11) (Fe: 1; Ni: 5; S: 6) slab models, two layers with six atoms per lay were adopted. A fermi smearing of.5 Hartree and a real-space cutoff of 4. Å were used to improve the computational performance. All calculations were performed with spin-polarization. The energy per adatom was calculated from the differential adsorption energy, E H, and was derived as E H = [ + ], where n is the number of hydrogen atoms adsorbed on the surface and E(surf + nh) is the total energy of the surface with n atoms adsorbed. The negative sign of E H corresponds to the energy gain of the system because of H atom adsorption. S3

4 Figure S1. SEM images of FeNi LDH. Figure S2. EDX mapping of FeNi sulfide nanosheets showing the existence of Ni, Fe and S. (A) SEM image, (B) Ni mapping, (C), Fe mapping, and (D) S mapping. S4

5 B Height (nm) ~ 2 nm Position (nm) D 2. Height (nm) ~1.7 nm position (µm) Figure S3. (A) AFM height image and (B) corresponding height vs position curve (white line in A) of a FeNi sulfide nanosheet. (C) AFM height image and (D) corresponding height vs position curve (white line in C) of a FeNi LDH. A (1) (12) (11) (11) α-ins B Intensity (a.u.) (11) (3) (211) (21) (3) (6) (12) (131) β-ins LDH Precursor Intensity(a.u) α-ins β-ins LDH precursor θ (deg.) Binding Energy(eV) Figure S4. XRD pattern (A) and XPS spectra (B) of LDH precursor (black curves), β-ins (red curves) and α-ins (blue curves). S5

6 A INS-Ni fitting 2P3/2 B INS-Fe fitting 2P3/2 Intensity (a.u.) 2P1/2 Intensity (a.u.) 2P1/ Binding Energy (ev) Binding Energy (ev) Figure S5. XPS spectra of β-ins nanosheets, (A) Ni, and (B) Fe region fitting of β-ins nanosheets. Figure S6. SEM images of β-nis nanosheets (A, B) and Ni(OH) 2 nanosheet precursor(c, D). S6

7 A 35 3 INS: m 2 /g B 4 35 NiFe LDH: m 2 /g Vads cc/g(stp) Vads cc/g(stp) Ps/Po Ps/Po C Vads cc/g(stp) NiS: 8.44 m 2 /g Ps/Po D % Average Diameter (nm) Figure S7. N 2 adsorbing-desorbing isotherm curve of INS nanosheets (A), FeNi LDH nanosheet precursor (B), and NiS nanosheets (C), (D) BJH pore size distribution curve of INS ultrathin nanosheets. J (ma/cm 2 ) bare GC β-nis α-nis INS α-ins Pt E (V vs RHE) Figure S8. Electrochemical performances of all catalysts including α-nis, β-ins, α-nis, α-nis, bare GC and Pt on HER in acidic solution. S7

8 A B -5 J (ma/cm 2 ) no ir 7% ir 8% ir 9% ir 1% ir J (ma/cm 2 ) Pt wire counter electrode graphite rod counter electrode E ( V vs RHE) E (V vs RHE) Figure S9. (A). Polarization curves of α-ins nanosheets under different ir compensation levels. (B). Polarization curves of α-ins nanosheets on HER by using graphite rod (red curve) and Pt wire (black curve) as the counter electrode, respectively. From the LSV curve, the overpotential at the current density of 1 ma/cm 2 for a-nis is recorded to be 155 mv, substantially smaller than that of b-nis (22 mv), further confirming the beneficial metallic character of the HER catalysts. Moreover, polarization curves of a-ins nanosheets under different ir compensation levels were recorded to circumvent the influence of solution resistivity (Figure S9A). Clearly, the overpotential needed to achieve a certain current density was further decreased in our favor with increasing ir compensation level α-ins: m 2 /g Vads cc/g(stp) Ps/Po Figure S1. N 2 adsorbing-desorbing isotherm curve of α-ins nanosheets. S8

9 A 4 3 B 5 4 J (ma/cm 2 ) J (ma/cm 2 ) E (V vs RHE) Scan rate (mv/s) Figure S11. Electrochemical surface area (ECSA) tests of α-ins towards HER in the acidic electrolytes of.5 M H 2 SO 4. (A) CV curves of α-ins with various scan rates, (B) charging current density differences plotted against scan rates. The linear slope, equivalent to twice of the double-layer capacitance C dl, was used to represent the ECSA. log (j/a) β-nis 48 mv/dec α-nis 4 mv/dec Pt 33 mv/dec E (V vs RHE) Figure S12. Calculated exchange current density of α-ms (black), β-ms (red) and Pt (blue) by applying extrapolation method to the corresponding Tafel plots. We measured the exchange current density, which tells how vigorously the forward and reverse reactions occur during dynamic equilibrium. [1c] Due to difficulty of determining the active site S9

10 and the site density of the powder catalyst, the geometrical current density instead of absolute exchange current density was calculated here to make a qualitative comparison of the electrochemical catalytic performances of these catalysts. From the Tafel plot, the log(j/j ) of the β-ins ultrathin nanosheets catalyzed HER is when extrapolated to the overpotential of (Figure S12). According to the Tafel equation, the geometrical exchange current density is calculated to be 14 µa/cm 2. For metallic α-ins nanosheets, the geometrical exchange current density is 2 µa/cm 2, which is significantly higher than that of β-ins. Though this value is lower than the that of Pt (.78 ma/cm 2, consistent with the literature value), it is among the largest reported values for non-noble metals based HER catalysts without any carbon material on the GC electrode (Table S2). Figure S13. (A) Schematic reaction pathway of HER on β-ins ultrathin nanosheets in acid environment. (B) The kinetic energy barrier profiles of HER on β-ins and β-nis nanosheets. The yellow, blue and red spheres in A 1 -A 3 represent S, Ni, and Fe atoms, respectively. The HER pathways on β-ins and β-nis catalysts were also investigated (Figure S13). The energy barrier of the rate-determining step and the energy release for H 2 formation on β-nis are.44 ev and 2.24 ev, respectively, and the corresponding values on β-nis are.46 ev and ev, respectively, confirming the higher catalytic performance of β-ins than of β-nis. Moreover, the energy barriers on both β-ins and β-nis are much higher than that on α-ins, suggesting a better catalytic performance of α-ins on HER, which is in accord with the experimental results. S1

11 Table S1. Chemical composition and atomic concentrations of β-ins nanosheets measured by XPS. Peak Position BE (ev) Atomic Mass Atomic Conc% Ni 2p Fe 2p O 1s C 1s S 2p Table S2. Comparison of HER performance in.5 M H 2 SO 4 for FeNi-S ultrathin nanosheets with other HER catalysts. Ref. catalysts η (mv) Tafel slope (mv/dec) J exchange (ma/cm 2 ) j (ma/cm 2 ) E j (mv) S3 NiMnN x /C MoN/C S4 Ni 2 P nanoparticle S5 MoS 2 /RGO S6 Mo 2 C/CNT S7 Mo 2 C MoS S8 β-mo 2 C γ-moc S9 MoC x nanooactahedrons S1 WS 2 nanoflakes S11 WS 2 /Graphene S12 exfoliated WS S13 exfoliated WS S14 MoS 2 /Au S15 FeP S16 CoP (on Ti) S17 CoP/CNT S18 CoP/CC S19 NiMoN x /C S2 Co-NRCNTs S21 CoSe 2 NP/CP S22 CoSe nanobelts S23 NiS nanoframes This work INS nanosheets α-ins nanosheets S11

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