Supporting Information Heterostructured arrays of Ni x P/S/Se nanosheets on Co x P/S/Se nanowires for efficient hydrogen evolution Weiqiang Tang, 1 Jianying Wang, 1 Lixia Guo, 1 Xue Teng, 1 Thomas J. Meyer, 2 and Zuofeng Chen*,1 1 Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China 2 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States *Corresponding author: zfchen@tongji.edu.cn S-1
EXPERIMENTAL Chemicals. Co(NO 3 ) 2 6H 2 O, NiSO 4 6H 2 O, Na 2 S H 2 O, NaH 2 PO 2, NaBH 4, K 2 S 2 O 8, NH 4 F, NH 3 H 2 O, urea and Se powder were purchased from Sigma-Aldrich Co. Carbon cloth (CC, thickness ~ 0.5 mm) was obtained from Shanxi Lizhiyuan Material of Battery Co. Ltd (China). 20 wt% Pt/C was purchased from Alfa Aesar Chemical Reagent Co. Sulfuric acid (98%) were purchased from Energy Chemical Reagent Co. High-purity air and argon (99.999%) gases were purchased from Shanghai Gases Co. All the chemical reagents were used as received without further purification. All electrolyte solutions were prepared by ultrapure water (18 MΩ cm) unless stated otherwise. Apparatus. Scanning electron microscope (SEM) images, energy dispersive X-ray (EDX) elemental analysis data and EDX elemental mapping images were obtained at Hitachi S-4800. The time for EDX mapping images is 10 min. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were obtained using JEM-2100, JEOL. Powder X-ray diffraction (XRD) was measured by Bruker Foucs D8 via ceramic monochromatized Cu Kα radiation of 1.54178 A, operating at 40 kv and 40 ma. The scanning rate was 5 per min in 2θ. X-ray photoelectron spectroscopy (XPS) for elemental analysis was conducted on a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer. The carbon 1s peak (284.6 ev) was used for internal calibration. The amounts of Ni and Co in the hybrid material were determined by the inductively coupled plasma optical emission spectroscopy (ICP-OES) using the Perkin Elmer S-2
ICP-OES Optima 8300. Electrochemical measurements were performed on a CHI 660E electrochemical workstation (Chenhua Corp., Shanghai, China). The three-electrode system consisted of a working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE, ~0.244 V vs. NHE). All experiments were performed at 22 ± 2 C. Procedure. Synthesis of Co 2 (OH) 2 CO 3 nanowires on carbon cloth. Co 2 (OH) 2 CO 3 nanowires array on carbon cloth was prepared employing hydrothermal reaction. In a typical procedure, 2 mmol of Co(NO 3 ) 2 6H 2 O, 4 mmol of NH 4 F and 10 mmol of urea were dissolved in 35 ml distilled water and stirred to form a clear pink solution. A piece of carbon cloth (CC) was cleaned by sequential sonication in acetone, ethanol and water several times to remove the surface impurities. The above solution and CC were then transferred to a 50 ml Teflon-lined autoclave, which was sealed and maintained at 110 C for 5 h. After cooling down to room temperature, the resulting CC was rinsed several times with distilled water and ethanol, followed by drying 2 h at 60 C to obtain Co 2 (OH) 2 CO 3 NW-CC. Synthesis of heterostructured Ni(OH) 2 nanosheets@co 2 (OH) 2 CO 3 nanowires on carbon cloth. Typically,the as-prepared Co 2 (OH) 2 CO 3 nanowires array was used as the scaffold for the growth of Ni(OH) 2 nanosheets in a simple chemical bath. To prepare the solution for chemical bath deposition (CBD), 50 mmol of NiSO 4 6H 2 O and 10 mmol of K 2 S 2 O 8 were dissolved in 90 ml deionized water; 10 ml of aqueous ammonia was then added into the beaker. After immersing into the CBD solution for 10 min at room temperature, the resulting electrode was taken off and rinsed with S-3
distilled water, followed by drying at 80 C for 2 h to obtain Ni(OH) 2 NS@Co 2 (OH) 2 CO 3 NW-CC. Synthesis of heterostructured Ni 2 P nanosheets@cop nanowires on carbon cloth. To prepare Ni 2 P NS@CoP NW-CC, the as-prepared Ni(OH) 2 NS@Co 2 (OH) 2 CO 3 NW-CC and NaH 2 PO 2 were put at two separate positions in a porcelain boat with NaH 2 PO 2 at the upstream side of the furnace. The samples were firstly heated at 300 C for 120 min with a heating rate of 2 C min 1 under Ar atmosphere, and then naturally cooled to ambient temperature under Ar atmosphere. For the sake of comparison, individual CoP NW-CC and Ni 2 P NS-CC were also prepared by the same method. Synthesis of heterostructured NiCo chalcogenides. For the synthesis of NaHSe solution, 0.065 mg NaBH 4 was dissolved into 2 ml deionized water. Then 0.059 mg Se powder was added into the NaBH 4 solution. After gently stirred for several minutes, clear NaHSe solution was obtained. In a typical procedure, one piece of as-prepared Ni(OH) 2 NS@Co 2 (OH) 2 CO 3 NW-CC, 30 ml argon statured deionized water and 2 ml fresh prepared NaHSe solution were filled into a Teflon lined stainless steel autoclave. The autoclave was heated at 160 C for 10 h, and then cooled to room temperature naturally. After washing and drying, the Ni 0.85 Se NS@Co 0.85 Se NW-CC electrode was obtained. The synthesis procedure of Ni 3 S 4 NS@Co 3 S 4 NW-CC is similar to that of Ni 0.85 Se NS@Co 0.85 Se NW-CC except that 2 ml NaHSe solution with Na 2 S solution (0.1 M) and the autoclave was heated at 90 C for 9 h. Tafel plot. The current-potential data of the Ni 2 P NS@CoP NW-CC electrode were S-4
obtained by linear sweep voltammetry (LSV) at a very slow scan rate (0.1 mv/s). The Tafel slope was obtained from the LSV plot using a linear fit applied to points in the Tafel region. The solution resistance measured prior to the data collection (using ir test function) was used to correct the Tafel plot for ir drop. Calculation of ECSA. The electrochemically active surface areas (ECSAs) are evaluated by measuring their double layer charging capacitance in 0.5 M H 2 SO 4 solution. Briefly, a potential range where no apparent Faradaic process occurred was determined by using the cyclic voltammetry (CV). The charging current (i c ) in this potential range was then measured from CVs at different scan rates. The relation between i c, the scan rate (ν), and the double layer charging capacitance (C DL ) was governed by eq 1. The ECSA, which is directly proportional to C DL, can be evaluated from the slope of the plot of i c vs. ν. i c = νc DL (1) S-5
Fig. S1. SEM images of Ni(OH) 2 NS-CC of different magnifications. (A), low magnification; (B) high magnification. Fig. S2. XRD pattern of Ni(OH) 2 NS-CC and the bare carbon cloth. S-6
Fig. S3. Co 2p 3/2 XPS (A) and Ni 2p 3/2 XPS (B) of Ni(OH) 2 NS@Co 2 (OH) 2 CO 3 NW-CC. In the left spectrum, the two peaks at binding energies of 781.3 and 785.8 ev can be assigned to Co 2p 3/2 XPS of Co 2 (OH) 2 CO 3 and its satellite peak, respectively. In the right spectrum, the two peaks at binding energies of 855.6 and 861.1 ev can be assigned to Ni 2p 3/2 XPS of Ni(OH) 2 and its satellite peak, respectively. Fig. S4. SEM-EDX spectrum of Ni 2 P NS@CoP NW-CC. S-7
Fig. S5. The ICP-OES fitting curves of (A) Ni and (B) Co. The catalyst material derived from 10 min CBD reaction show the best electrocatalytic performance. We determined the atomic ratio of Ni and Co by ICP-OES measurement, and the result reveals an atomic ratio of 1: 1.1 for Ni: Co in Ni 2 P NS@ CoP NW-CC. Fig. S6. Survey XPS of Ni 2 P NS@CoP NW-CC. S-8
Fig. S7. (A,B) SEM images and (C) TEM image of CoP NW-CC. (D,E) SEM images and (F) TEM image of Ni2P NS-CC. Fig. S8. XRD patterns of (A) CoP NW-CC and (B) Ni2P NS-CC. S-9
Fig. S9. LSV curves of the Ni 2 P NS@CoP NW-CC and bare CC at a scan rate of 2 mv s 1 in 1 M KOH; Insert shows the Tafel plots of Ni 2 P NS@CoP NW-CC. As can be seen, the Ni 2 P NS@CoP NW-CC electrode is also highly active toward the HER in the alkaline solution with a low onset overpotential of 25 mv and a small Tafel slope of 43.2 mv dec 1. S-10
Table S1. Comparison of the HER activity of our hybrid electrode with other none-noble-metal HER electrocatalysts in acidic solution. Electrocatalyst j η b Electrolyte Ref. (ma cm 2 ) (mv) (mv dec 1 ) solution Ni 2 P NS@CoP NW-CC 10 55 48 0.5 M H 2 SO 4 this work Ni 2 P/CoP HNS/CC 10 85 40 0.5 M H 2 SO 4 [1] MoP NP 10 125 54 0.5 M H 2 SO 4 [2] Ni 2 P hollow NP 10 116 46 0.5 M H 2 SO 4 [3] CoP/Ti 10 90 43 0.5 M H 2 SO 4 [4] NiP 2 NS/CC 10 75 51 0.5 M H 2 SO 4 [5] FeP NWs/Fe foil 10 96 39 0.5 M H 2 SO 4 [6] FeP 2 NWs/Fe foil 10 61 37 0.5 M H 2 SO 4 [6] MoP 2 NS/CC 10 58 63.6 0.5 M H 2 SO 4 [7] CoP/rGO 10 105 50 0.5 M H 2 SO 4 [8] CoSe 2 NW/CC 10 130 32 0.5 M H 2 SO 4 [9] Ni 3 S 2 NS 10 170 / 0.5 M H 2 SO 4 [10] CoP/WS 2 10 150 64.6 0.5 M H 2 SO 4 [11] Al-CoP/CC 10 63 43 0.5 M H 2 SO 4 [12] Mo 2 C porous NP 10 144 47.5 0.5 M H 2 SO 4 [13] Supplementary references (1) Wang, A.-L.; Lin, J.; Xu, H.; Tong, Y.-X.; Li, G.-R. Ni 2 P-CoP hybrid nanosheet arrays supported on carbon cloth as an efficient flexible cathode for hydrogen evolution. J. Mater. Chem. A 2016, 4, 16992-16999. (2) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Closely interconnected network of S-11
molybdenum phosphide nanoparticles: a highly efficient electrocatalyst for generating hydrogen from water. Adv. Mater. 2014, 26, 5702-5707. (3) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270. (4) Pu, Z.; Liu, Q.; Jiang, P.; Asiri, A. M.; Obaid, A. Y.; Sun, X. CoP nanosheet arrays supported on a Ti plate: an efficient cathode for electrochemical hydrogen evolution. Chem. Mater. 2014, 26, 4326-4329. (5) Jiang, P.; Liu, Q.; Sun, X. NiP 2 nanosheet arrays supported on carbon cloth: an efficient 3D hydrogen evolution cathode in both acidic and alkaline solutions. Nanoscale 2014, 6, 13440-13445. (6) Son, C. Y.; Kwak, I. H.; Lim, Y. R.; Park, J. FeP and FeP 2 nanowires for efficient electrocatalytic hydrogen evolution reaction. Chem. Commun. 2016, 52, 2819-2822. (7) Zhu, W.; Tang, C.; Liu, D.; Wang, J.; Asiri, A. M.; Sun, X. A self-standing nanoporous MoP 2 nanosheet array: an advanced ph-universal catalytic electrode for the hydrogen evolution reaction. J. Mater. Chem. A 2016, 4, 7169-7173. (8) Jiao, L.; Zhou, Y.-X.; Jiang, H.-L. Metal-organic framework-based CoP/reduced graphene oxide: high-performance bifunctional electrocatalyst for overall water splitting. Chem. Sci. 2016, 7, 1690-1695. (9) Liu, Q.; Shi, J.; Hu, J.; Asiri, A. M.; Luo, Y.; Sun, X. CoSe 2 nanowires array as a 3D electrode for highly efficient electrochemical hydrogen evolution. ACS Appl. Mater. Interfaces 2015, 7, 3877-3881. S-12
(10)Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-index faceted Ni 3 S 2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 2015, 137, 14023-14026. (11) Jin, J.; Zhu, Y.; Liu, Y.; Li, Y.; Peng, W.; Zhang, G.; Zhang, F.; Fan, X. CoP nanoparticles combined with WS 2 nanosheets as efficient electrocatalytic hydrogen evolution reaction catalyst. Int. J. Hydrog. Energy 2017, 42, 3947-3954. (12) Zhang, R.; Tang, C.; Kong, R.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. Al-Doped CoP nanoarray: a durable water-splitting electrocatalyst with superhigh activity. Nanoscale 2017, 9, 4793-4800. (13) Ji, L.; Wang, J.; Guo, L.; Chen, Z. In situ O 2 -emission assisted synthesis of molybdenum carbide nanomaterials as an efficient electrocatalyst for hydrogen production in both acidic and alkaline media. J. Mater. Chem. A 2017, 5, 5178-5186. S-13