Hierarchical CoNiSe2 nano-architecture as a highperformance electrocatalyst for water splitting

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1 Nano Res. Electronic Supplementary Material Hierarchical CoNiSe2 nano-architecture as a highperformance electrocatalyst for water splitting Tao Chen and Yiwei Tan ( ) State Key Laboratory of Materials-Oriented Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing , China Supporting information to DOI /s Figure S1 (a) SEM micrograph of the pristine Ni foam substrate. (b) A representative side-view SEM image of the hierarchical CoNiSe2 nano-architecture grown on nickel foam (denoted as CoNiSe2/Ni). Figure S2 EDS spectrum of the CoNiSe2 nanorods. Address correspondence to ytan@njtech.edu.cn

2 Nano Res. Figure S3 (a) SEM image of the product grown on Ni foam, prepared in the absence of 2-methoxy-5-nitroaniline. (b) HRTEM image of the as-synthesized CoNiSe2/poly(2-methoxy-5-nitroaniline) core/sheath nanorods grown on Ni foam, which are obtained without washing. Figure S4 (a) (d) SEM images taken at different magnifications, (e) HRTEM image, and (f) XRD diffractogram of the NiSe nanosheets supported on Ni foam. For comparison, the intensities and positions for pure NiSe reference are given according to the JCPDS database (the red lines at the bottom of panel f). The XRD reflections marked by asterisks in panel f are indexed to the Ni foam substrate.

3 Nano Res. Figure S5 XPS spectra collected from the NiSe nanosheets scratched from the surface of the NiSe/Ni electrode. (a) XPS survey spectrum, (b) Ni 2p, and (c) Se 3d detail spectra. The low-magnification SEM images in Figure S4a c show Ni foam is uniformly covered with the ultrathin NiSe nanosheets that are interconnected with one another. Intriguingly, many interconnected NiSe nanosheets orientated in many different directions are integrated into rose-like superstructures. High-magnification SEM image in Figure S4d exhibits that the thickness of individual NiSe nanosheets is smaller than 10 nm. The HRTEM image of the nanosheets scratched from Ni foam substrate reveals the single crystal feature as well as the lattice fringe spacings of 2.74 and 2.06 Å corresponding to the interplanar spacing of (101) and (102) planes from hexagonal NiSe phase, respectively (Figure S4e). Excluding the strong peaks labelled by asterisks from the Ni foam, the XRD pattern in Figure S4f shows that the crystallographic structure of the as-prepared NiSe nanosheets is identified as a hexagonal phase with lattice parameters a = Å and c = Å (JCPDS No , space group P6 3 /mmc (194),). Figure S5 demonstrates the XPS analyses of the NiSe nanosheets scratched from the Ni foam substrate, clearly confirming the presence of Ni and Se signals together with O and adventitious C species in the survey spectrum (Figure S5a). The peak fitting of Ni 2p reveals that similar to the CoNiSe 2 nanorods, the surface Ni species of the NiSe nanosheets are Ni 2+ ions (Figure S5b). Se 3d core level XPS spectrum shows that there are a doublet from Se 3d 1/2 and Se 3d 3/2 and a weak peak at 59.9 ev related to Se oxides (Figure S5c). Nano Research

4 Nano Res. Figure S6 CV curves after 1 and 3000 potential cycles for the OER on the CoNiSe2/Ni electrode. All curves are recorded at a potential scan rate of 2 mv s 1. No visible shift in overpotential is observed for driving 100 ma cm 2. A 95% retention of current density is achieved at the highest potential (1.595 VRHE) after 3000 potential cycles. Figure S7 XRD diffractogram of the CoNiSe2/Ni electrode that is subjected to continuous electrolysis (OER) operation over 20 h. The peaks marked by asterisks stem from the Ni foam substrate. For comparison, the intensities and positions for pure CoNiSe2 reference are given according to the JCPDS database (JCPDF No ) (the orange lines at the bottom). Figure S8 (a) Low- and (b) high-magnification SEM images of the CoNiSe2/Ni electrocatalyst, which are taken after chronopotentiometric measurements at a current density of 100 ma cm 2 for the OER over a period of 20 h.

5 Nano Res. Figure S9 XPS spectra of the CoNiSe2 nanorods after the OER or HER in the (a) Co 2p, (b) Ni 2p, and (b) Se 3d core level regions. The samples were subjected to galvanostatic operations at a current density of 100 ma cm 2 for the OER or HER over a period of 20 h. Figure S10 XPS spectra of the NiSe nanosheets after the OER or HER in the (a) Ni 2p and (b) Se 3d core level regions. The samples were subjected to galvanostatic operations at a current density of 100 ma cm 2 for the OER or HER over a period of 20 h. Nano Research

6 Nano Res. Figure S11 CV curves after 1 and 3000 potential cycles for the HER on the CoNiSe 2 /Ni electrode. All curves are recorded at a potential scan rate of 2 mv s 1. A 94% retention of current density is achieved at the most negative potential ( V RHE ) after 3000 potential cycles. Figure S12 The crystallographic structures of the hexagonal (a) CoNiSe 2 with lattice constants a = Å and c = Å and (b) NiSe with lattice constants a = Å and c = Å. For simplicity, we assume that Co and Ni ions are orderly distributed in the CoNiSe 2 lattice, as shown in panel (a). The electrochemical active surface area (A ECSA ): Turnover frequencies (TOF): Total H 2 turnovers = H 2 s 1 cm 2 /ma cm 2, which is obtained according to the report by Kibsgaard et al.. 1 It is well known that it is difficult to precisely determine the nature and number of active sites for HER and OER. 1 Furthermore, in fact, all surface species usually demonstrate synergistic effects on improving the electrocatalytic activity and therefore take part in the catalytic process together. 1,2 Therefore, the number of active sites is assumed to be the total number of surface sites (N ss ), i.e., the constituent Se and transition metal ions. CoNiSe 2 and NiSe contain 12 metallic ions and 12 selenium ions in their separate unit cell and have a unit

7 Nano Res. cell volume of and Å 3, respectively. Therefore, the surface sites per real surface area for CoNiSe 2 (N SS(CoNiSe2) ) and NiSe (N SS(NiSe) ) are:, thus The TOF values are calculated to be and H 2 s 1 for the CoNiSe 2 and NiSe catalysts at V versus RHE, respectively. Table S1 Comparison of the electrocatalytic activity of various chalcogenides for the OER sample catalyst loading (mg cm 2 ) overpotential (η OER ) (mv) Ref cited in the text CoNiSe 2 nanorods (100 ma cm 2 ) this work CoSe 2 nanobelts (10 ma cm 2 ) 15 CoSe 2 nanosheets (79.5 ma cm 2 ) 16 CoSe 2 /CeO 2 nanobelts (10 ma cm 2 ) %Ag-CoSe 2 nanobelts (22.36 ma cm 2 ) 21 NiSe nanowires on Ni foam (100 ma cm 2 ) 25 NiSe nanowires on Ni foam not mentioned 400 (35 ma cm 2 ) 26 Ni 3 S 2 nanorods on Ni foam (10 ma cm 2 ) 28 Ni 3 S 2 nanosheets on Ni foam (10 ma cm 2 ) 29 NiCo 2 S 4 nanowires (100 ma cm 2 ) 32 NiCo 2 S 4 nanowires not mentioned 260 (10 ma cm 2 ) 33 Ni x Fe 1 x Se 2 DO hybrid nanoplates (262 ma cm 2 ) 34 (Ni 0.75 Fe 0.25 )Se 2 nanosheets (100 ma cm 2 ) 35 (Ni, Co) 0.85 Se nanotubes ~5 300 (36.4 ma cm 2 ) 36 Co 0.13 Ni 0.87 Se 2 nanoparticles (100 ma cm 2 ) 38 Table S2 Comparison of the electrocatalytic activity of various chalcogenides for the HER sample catalyst loading (mg cm 2 ) overpotential (η HER ) (mv) Ref cited in the text CoNiSe 2 nanorods (10 ma cm 2 ) 171 (100 ma cm 2 ) this work CoS film 1.35 (μmol cm 2 ) 397 (50 ma cm 2 ) 12 CoS 2 nanowires 1.7 ± (10 ma cm 2 ) 13 CoSe 2 nanoparticles (10 ma cm 2 ) 181 (100 ma cm 2 ) 14 CoSe 2 nanosheets not mentioned 152 (10 ma cm 2 ) Nano Research

8 Nano Res. (Continued) sample catalyst loading (mg cm 2 ) overpotential (η HER ) (mv) Ref cited in the text CoSe 2 nanoneedles (20 ma cm 2 ) 19 CoSe 2 nanotubes (10 ma cm 2 ) 20 Co(S 0.73 Se 0.27 ) 2 nanowires (10 ma cm 2 ) 157 (100 ma cm 2 ) 22 CoSP nanowires not mentioned 48 (10 ma cm 2 ) 23 NiS 2 nanosheets (10 ma cm 2 ) 24 NiSe nanowires on Ni foam (10 ma cm 2 ) 25 porous NiSe 2 /Ni foam not mentioned 143 (10 ma cm 2 ) 27 Ni 3 S 2 nanosheets on Ni foam (10 ma cm 2 ) 29 Fe Ni S (10 ma cm 2 ) 30 Ni 0.33 Co 0.67 S 2 nanowires (10 ma cm 2 ) 118 (100 ma cm 2 ) 31 NiCo 2 S 4 nanowires (50 ma cm 2 ) 305 (100 ma cm 2 ) 32 NiCo 2 S 4 nanowires not mentioned 210 (10 ma cm 2 ) 33 CoSe 2 /NiSe 2 nanowires (10 ma cm 2 ) 37 Co 0.13 Ni 0.87 Se 2 nanoparticles (10 ma cm 2 ) 38 Table S3 Comparison of a two-electrode alkaline electrolyzer using 3d-TM chalcogenide electrocatalysts for overall water electrolysis sample support catalyst loading (mg cm 2 ) cell voltage (V) Ref cited in the text CoNiSe 2 CoNiSe 2 Ni foam (10 ma cm 2 ) this work NiSe NiSe Ni foam (10 ma cm 2 ) 25 Ni 3 S 2 Ni 3 S 2 Ni foam (~13 ma cm 2 ) 29 NiCo 2 S 4 NiCo 2 S 4 carbon cloth (10 ma cm 2 ) 32 NiCo 2 S 4 NiCo 2 S 4 Ni foam not mentioned 1.63 (10 ma cm 2 ) 33 Co 0.13 Ni 0.87 Se 2 Co 0.13 Ni 0.87 Se 2 Ti foil (10 ma cm 2 ) 38 Ni 5 P 4 Ni 5 P 4 Ni foil ~1.7 (10 ma cm 2 ) CoP-MNA CoP-MNA Ni foam (10 ma cm 2 ) NiCoP NiCoP Ni foam (10 ma cm 2 ) Ni 1.5 Fe 0.5 P Ni 1.5 Fe 0.5 P carbon fiber (10 ma cm 2 ) Angew. Chem. Int. Ed. 2015, 54, Adv. Funct. Mater. 2015, 25, 7337 Nano Lett. 2016, 16, Nano Energy 2017, 34, 472. References [S1] Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen F.; Jaramillo, T. F. Energy Environ. Sci. 2015, 8, [S2] Tan, Y.; Wang, H.; Liu, P.; Cheng, C.; Zhu, F.; Hirata, A.; Chen, M. Adv. Mater. 2016, 28,