Supporting Information Engineering NiS/Ni 2 P Heterostructures for Efficient Electrocatalytic Water Splitting Xin Xiao, Dekang Huang, Yongqing Fu, Ming Wen, Xingxing Jiang, Xiaowei Lv, Man Li, Lin Gao, Shuangshuang Liu, Mingkui Wang, Chuan Zhao, # Yan Shen*, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 4374, P. R. China College of Science, Huazhong Agricultural University, Wuhan 437, P. R. China Faculty of Engineering and Environment, Northumbria University, Newcastle Upon Tyne, NE18ST, U.K. School of Chemistry Science and Engineering, Tongji University, Shanghai 292, P. R. China # School of Chemistry, The University of New South Wales, Sydney, NSW 252, Australia * Corresponding author: E-mail address: cica_sheny@mail.hust.edu.cn. S-1
Turn-over frequency analysis We employed the method proposed by G. C. Dismukes et al to estimate the turn-over frequency (TOF). 1 We used the following formula to calculate the TOF: TOF = #total hydrogen turn overs / cm2 of geometric area #active sites / cm 2 of geometric area The total number of hydrogen turn overs was calculated from the current density according to: # H 2 = (j ma 1 C s 1 1 mol e cm2) ( ) ( ) 1 ma 96485.3 C (1 mol H 2 2 mol e ) (6.22 123 H 2 molecules ) = 3.12 1 mol H 2 1 15 H 2/s ma cm2 per cm 2 The active sites per real surface area is calculated from the following formula: The hexagonal unit cell has a molar volume of NiS: 9.76 g / mol Vm = Fw/ρ = = 16.52 cm 3 5.5 g / cm 3 mol NiS Each formula unit contains 2 atoms of Ni and S. The average surface occupancy thus becomes: 2 atoms/formula unit No. of active sites (NiS) = ( 16.52 cm 3 / mol NiS 6.221 1 23 mol 1 ) cm 2 The hexagonal unit cell has a molar volume of Ni2P: Vm = Fw/ρ = 148.37 g / mol 7.351 g / cm 3 = 2.184 cm 3 mol Ni 2P 2/3 = 1.75 1 15 surface atoms / Each formula unit contains 3 atoms of Ni and P. The average surface occupancy thus becomes: 3 atoms/formula unit No. of active sites (Ni2P) =( 2.184 cm 3 / mol Ni 2P 6.221 1 23 mol 1 ) 2/3 = 2. 1 15 surface atoms / cm 2 Since the exact hydrogen binding site and the exact cell parameters of NiS/Ni2P/CC is not known, we estimate the number of active sites as the number of surface sites (including both Ni, S and P atoms as possible active sites) from the average value of NiS and Ni2P. No. of active sites (NiS/Ni2P) = 1.875 1 15 surface atoms / cm 2 Finally, plot of current density can be converted into a TOF plot according to: TOF = (3.12 115 H2/s cm 2 per ma cm 2) j #active sites A ECSA S-2
Intensity (a.u.) O Ni S P Ni C Ni 3 6 9 Binding energy (KeV) Figure S1. The EDX spectrum of the NiS/Ni2P/CC..294 nm NiS (11) 69.3.277 nm NiS (3) 5 nm (c) (d).281 nm Ni 2 P (11) 56.2.253 nm Ni 2 P (2) 5 nm Figure S2. and (c) HRTEM images of NiS/Ni2P/CC; and (d) The twodimensional lattices (both lattice distances and the angle between two crystallographic directions) of NiS and Ni2P in different area of NiS/Ni2P/CC, respectively. S-3
Ni-K (c) S-K (d) P-K (e) O-K (f) Figure S3. STEM EDX mapping of NiS/Ni2P/CC (a-e); (f) Selected area electron diffraction pattern for NiS/Ni2P/CC. Intensity (a.u.) Intensity (a.u.) 2 3 4 5 6 7 8 2 Theta (degree) 2 3 4 5 6 7 8 2Theta (degree) Figure S4. XRD patterns of samples of NiS/CC and Ni2P/CC. -5-1 -15 NiS/CC Ni 2 P/CC NiS/Ni 2 P/CC 3 2 1 NiS/CC Ni 2 P/CC NiS/Ni 2 P/CC -2 -.6 -.4 -.2. 1.2 1.5 1.8 2.1 Figure S5. and Non-iR corrected polarization curves of various catalysts for HER and OER, respectively. S-4
6 j (ma mg -1 ) -2-4 NiS/CC Ni 2 P/CC NiS/Ni 2 P/CC j (ma mg -1 ) 3 NiS/CC Ni 2 P/CC NiS/Ni 2 P/CC -6 -.4 -.3 -.2 -.1. -3 1. 1.2 1.4 1.6 Figure S6. The mass activity per unit area of the catalysts for HER and OER. 3 2 1 1. 1.2 1.4 1.6 1.8 Figure S7. The LSV polarization curve of Ni(OH)2/CC for OER. 45 3 15 45 3 15-15 1. 1.2 1.4 1.6 45 3 15 (c) -15 1. 1.2 1.4 1.6-15 1. 1.2 1.4 1.6 Figure S8. The CV curves of NiS/CC, Ni2P/CC, and (c) NiS/Ni2P/CC in 1 M KOH solution at a scanning rate of 5 mv s -1. S-5
3 3 j ( ma cm -2 ) -3 5.3.4.5 (c) 1 mv s -1 7 mv s -1 3 mv s -1 9 mv s -1 5 mv s -1 11 mv s -1-3 1 mv s -1 7 mv s -1 3 mv s -1 9 mv s -1 5 mv s -1 11 mv s -1.3.4.5-5 1 mv s -1 7 mv s -1 3 mv s -1 9 mv s -1 5 mv s -1 11 mv s -1.3.4.5 Figure S9. Typical CV curves of electrode NiS/CC, Ni2P/CC, and (c) NiS/Ni2P/CC in 1 M KOH solution with different scanning rates. 5 μm 5 μm Figure S1. SEM images of NiS/Ni2P/CC after HER and OER stability tests. S-6
Ni 2p P 2p Intensity (a.u.) After HER After OER Intensity (a.u.) After HER After OER Intensity (a.u.) (c) 88 87 86 85 Binding energy (a.u.) S 2p After HER After OER 138 135 132 129 Binding energy (ev) 165 162 159 Bindingenergy (ev) Figure S11. High-resolution Ni 2p, P 2p, and S 2p (c) XPS spectra of NiS/Ni2P/CC after HER and OER stability tests. Figure S12. and TEM images of NiS/Ni2P/CC after HER and OER stability tests, respectively. S-7
Table S1. Elements analysis of NiS/Ni2P/CC before and after stability tests. Elements Wt% [a] Wt% [b] Wt% [c] Ni 57.92 55.33 4.17 S 12.38 8.75 2.96 P 11.56 7.37 4.85 O 18.13 28.56 52.3 Note: [a] represents the content of elements of NiS/Ni2P/CC before stability test; [b] and [c] represent the content of elements after the HER and OER stability tests, respectively. Table S2. Comparison of electrochemical performance for NiS/Ni2P/CC with other non-noble metal bifunctional electrocatalysts for water splitting. catalyst η HER / mv [a] η OER / mv [a] E / V [b] Ref NiS/Ni 2P 111 255 1.67 This work Ni 3S 2 29 29 1.7 2 Ni 9S 8 26 33 >1.8 3 CoP 112 29-4 CP/CTs/Co-S 24 32 1.74 5 Ni 3S 2 26 41-6 NiS 165 3 1.64 7 Ni 5P 4 19 27 1.68 8 Ni-P 12 36 1.68 9 NiCo 2S 4 25 35 1.63 1 CoP/rGO 13 37 1.7 11 Note: [a] the HER or OER overpotential at 2 ma cm -2 in 1 M KOH solution; [b] the overall water splitting overpotential at 1 ma cm -2 in 1 M KOH solution. S-8
REFERENCES (1) Laursen, A.; Patraju, K.; Whitaker, M.; M. Retuerto, Sarkar, T.; Yao, N.; Ramanujachary, K.; Greenblatt, M.; Dismukes, G. Nanocrystalline Ni5P4: a Hydrogen Evolution Electrocatalyst of Exceptional Efficiency in Both Alkaline and Acidic Media. Energy Environ. Sci. 215, 8, 127-134. (2) Feng, L.; Yu, G.; Wu, Y.; Li, G.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High- Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 215, 137, 1423-1426. (3) Chen, G.; Ma, T.; Liu, Z.; Li, N.; Su, Y.; Davey, K.; Qiao, S. Efficient and Stable Bifunctional Electrocatalysts Ni/NixMy (M= P, S) for Overall Water Splitting. Adv. Funct. Mater. 216, 26, 3314-3323. (4) Liu, Y.; Hu, W.; Han, G.; Dong, B.; Li, X.; Shang, X.; Chai, Y.; Liu, Y.; Liu, C. Novel CoP Hollow Prisms as Bifunctional Electrocatalysts for Hydrogen Evolution Reaction in Acid Media and Overall Water-Splitting in Basic Media. Electrochimica Acta 216, 22, 98-16. (5) Wang, J.; Zhong, H.; Wang, Z.; Meng, F.; Zhang, X. Integrated Three Dimensional Carbon Paper/Carbon Tubes/Cobalt-Sulfide Sheets as an Efficient Electrode for Overall Water Splitting. ACS Nano 216, 1, 2342-2348. (6) Zhu, T.; Zhu, L.; Wang, J.; Ho, G. In Situ Chemical Etching of Tunable 3D Ni3S2 Superstructures for Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A 216, 4, 13916-13922. S-9
(7) Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 216, 52, 1486-1489. (8) Ledendecker, M.; Calderón, S.; Papp, C.; Steinrück, H.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and Their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem. Int. Ed. 215, 127, 12538-12542. (9) Liu, Q.; Gu, S.; Li, C. Electrodeposition of Nickel-Phosphorus Nanoparticles Film as a Janus Electrocatalyst for Electro-splitting of Water. J. Power Sources 215, 299, 342-346. (1) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions, Adv. Funct. Mater. 216, 26, 4661-4672. (11) Jiao, L.; Zhou, Y.; Jiang, H. Metal-Organic Framework-Based CoP/Reduced Graphene Oxide: High-Performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 216, 7, 169-1695. S-1