Supporting Information Hierarchical Mesoporous Zinc-Nickel-Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High Performance Electrodes for Supercapacitors Chun Wu a, Junjie Cai a, Qiaobao Zhang a, Xiang Zhou a, Ying Zhu a, Pei Kang Shen b, *, Kaili Zhang a, * a Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong b Department of Physics and Engineering, Sun Yat-sen University, Guangzhou, 510275, China * Corresponding authors, E-mail address: kaizhang@cityu.edu.hk and stsspk@mail.sysu.edu.cn Figure S1, Figure S2, Figure S3, Figure S4, Figure S5, Figure S6, Figure S7, Figure S8, Figure S9 Table S1 References S-1
Figure S1 Schematic illustration of the as-fabricated ZNCO//AC asymmetric supercapacitor in 6 M KOH electrolyte S-2
(a) (b) 5 µm 3 µm (c) (d) 5 µm 3 µm (e) (f) 5 µm 3 µm Figure S2 Low and high-magnification SEM images of various ZNCO nanostructures on different substrates: (a) and (b) Si wafer, (c) and (d) Cu foam, (e) and (f) carbon cloth S-3
(a) (b) 50 µm 10 µm (c) (d) 50 µm 10 µm (e) (f) 50 µm 10 µm Figure S3 Low and high-magnification SEM images of ZNCO nanostructures prepared from different salts with the same ratios between Co, Ni and Zn at 130 o C for 5 h. (a) and (b) chlorates, (c) and (d) acetates, (e) and (f) sulfates. S-4
s s Figure S4 (a) Co 2p, (b) Ni 2p, (c) Zn 2p and (d) O 1s XPS spectra of ZNCO electrode material XPS analyses provide a careful investigation of the compositions of the ZNCO electrode materials. Obviously, the Co 2p spectrum in Fig. S4(a) presents two prominent peaks at 795.2 ev for Co 2p 3/2 and 780.0 ev for Co 2p 1/2, indicating the presence of Co 3+ oxidation state. 1 While a very intense, characteristic satellite at ~787.0 ev can be observed, the approximate position of which is indicated by the arrow of Fig. S4(a), demonstrating the existence of Co 2+. 2 Moreover, the Ni 2p spectrum in Fig. S4(b) shows two obvious peaks at 854.9 and 872.9 ev, which is the Ni 2p 3/2 and 2p 1/2 binding energies, respectively. 2 The Ni 2p spectrum also can be fitted by considering two spin-orbit doublets and two shake-up satellites, in which the nickel 2p 3/2 spectra can be assigned to two distinct peaks, indicating the presence of S-5
both Ni 2+ and Ni 3+. 3,4 And the peak with binding energy value at 1020.3 in Fig. S4(c) is ascribed to Zn 2p 3/2, indicating the Zn 2+ oxidation state. 5,6 Furthermore, the high-resolution spectrum for the O 1s region (Fig. S4(d)) shows four oxygen contributions. The fitting peak of O 1 at 529.0 ev and peak of O 2 at 529.5 ev are attributed to the metal-oxygen bonds. 7 The well-resolved O 3 component corresponds to a high number of defect sites with low oxygen coordination in the material with small particle size. 8 Furthermore, the component O 4 at 531.4 ev can be attributed to hydroxide oxygen in Co-O-OH, which is agreement with the peak at 780.5 ev of Co 2p 3/2 binding energy. 9 From above results, the binding energy values at 780.1 and 780.5 ev of Co and 855.3 ev of Ni can be assigned to CoOOH and NiOOH, 10,11 indicating that the energy storage mechanism of the ZNCO electrode originated mainly from Faradic redox reactions assigned to the M-O/M-O-OH, where M represents both Ni and Co ions. 12 Based on the Faradic redox reactions, the theoretical specific capacitance can be calculated as follows: The required amount of power used for electrolysis is of 1 mol active material is 1 F which is equal to 96485 C. For ZNCO electrode material, its molar mass is 485.6 g mol -1 according to the EDX measurement (ZnNi 1.6 Co 3.5 O 7.5 ), however, the element taking part in the Faradic redox reactions are Co and Ni (Zn would not to be covered), molar mass is 404.2 g mol -1, so theoretical faradaic capacitance is calculated as follows: C = Q U = 96485 (1.6 + 3.5)/404.2 U Where U is the voltage window, Q is electrical energy per 1 gram. When U is 0.5V in charge/discharge measurement, the theoretical faradaic capacitance is about 2434.8 F S-6
g -1, which is lower than that of the charge/discharge measurement, the reason may be that the obtained capacitance is the contribution of faradaic capacitance from redox reaction at interfaces and EDLC from high surface area of the porous electrode material. S-7
Figure S5 Capacitance retention and coulombic efficiency of the ZNCO electrode at a current density of 10 A g -1 S-8
Figure S6 Electrochemical performance of the ZNCO electrode: (a) and (c) CV curves of the ZNCO-3 and ZNCO-7 at different scan rates, (b) and (d) charge/discharge curves of the ZNCO-3 and ZNCO-7 under different current densities, (e) and (f) Comparisons of CV and charge/discharge curves among ZNCO-3, ZNCO and ZNCO-7 (With the temperature during the hydrothermal process were 3, 5 and 7 h, the samples were marked with ZNCO-3, ZNCO and ZNCO-7, respectively) S-9
(a) (b) 5 µm 5 µm Figure S7 SEM images of (a) ZNCO-3 and (b) ZNCO-7 S-10
Figure S8 (a) CV curves of AC electrode under different scan rates, (b) charge/discharge curves of AC electrode at different current densities, (c) specific capacitance of AC electrode at different current densities and (d) the Nyquist plots (the inset shows the expanded high-frequency region of the plot) of the AC electrode As presented in Fig. S8(a), a good double layer capacitive behavior in aqueous system on AC electrode is obtained. The charge/discharge curves of the AC electrode (Fig. S8(b)) are almost symmetrical isosceles lines, demonstrating that the AC material possesses stable electrochemical properties. The electrode delivers a specific capacitance of 247.9, 231.1, 219.3, 210.1 and 203.5 F g -1 at the current density of 1, 2, 3, 4 and 5 A g -1, respectively. The ac impedance spectrum depicted in Fig. S8(d) indicates a good capacitive behavior of the AC electrode. S-11
Figure S9 Capacitance retention and coulombic efficiency at more than 3000 charge/discharge cycle of the ZNCO//AC ASC device at a current density of 3 A g -1 S-12
Table S1 Comparison of specific capacitance of ZNCO electrode with different metal oxide electrodes reported in literature Electrode material Current Specific Ref. density capacitance ZnCo 2 O 4 /MnO 2 nanocones 1 A g -1 2339 F g -1 13 NiCo 2 O 4 /MnO 2 nanoflakes 1 A g -1 935 F g -1 14 ZnCo 2 O 4 nanorods/nickel foam 1 A g -1 1400 F g -1 15 NiCo 2 O 4 @NiO nanoarrays 1 A g -1 2105 F g -1 16 CoMn 2 O 4 nanowire 1 A g -1 2108 F g -1 17 Mn-Ni-Co nanowires 1 A g -1 638 F g -1 18 Mn-Ni-Co composites 1 A g -1 1260 F g -1 19 ZNCO nanowires 1 A g -1 2481.8 F g -1 This work References (1) Zhong J. H.; Wang A. L.; Li G. R.; Wang J. W.; Ou Y. N.; Tong Y. X. Co 3 O 4 /Ni(OH) 2 Composite Mesoporous Nanosheet Networks as a Promising Electrode for Supercapacitor Applications. J. Mater. Chem. 2012, 22, 5656-5665. (2) Kim J. G.; Pugmire D. L.; Battaglia D.; Langell M. A. Analysis of the NiCo 2 O 4 Spinel Surface with Auger and X-ray Photoelectron Spectroscopy. Appl. Surf. Sci. 2000, 165, 70-84. S-13
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