Supporting Information for

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1 Electronic Supplementary Material (ESI) for Chemical Communications. This journal is The Royal Society of Chemistry 2014 Supporting Information for Metal Nanoparticles Directed NiCo 2 O 4 Nanostructure Growth on Carbon Nanofibers with High Capacitance Long Chen, Jiahua Zhu * Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA Corresponding author: jzhu1@uakron.edu Phone: (330) S1. Experimental S1.1 Materials Polyacrylonitrile (PAN, M w 120,000), nickel(ii) nitrate hexahydrate, cobalt(ii) nitrate hexahydrate, N,N-dimethylformamide (DMF) and urea were purchased from Sigma Aldrich. Ethanol was supplied from Decon Labs, Inc. All materials were used without further purification. S1.2 Polyacrylonitrile fiber Polyacrylonitrile fibers were electrospun from 12 wt% PAN/DMF solution. The applying voltage, feed rate of polymer solution and needle tip to collector distance was set at 15.0 kv, 7.0 μl/min and 15.0 cm, respectively. The fabricated PAN fibers were dried at 50 o C for 8 hours to remove the residual solvent and then oxidized at 250 o C for 2 hours in air atmosphere. S-1

2 S1.3 Metal nanoparticle coated carbon nanofibers (MNPs/CNFs) The MNPs/CNFs were prepared by dip coating and subsequent carbonization processes. Firstly, the oxidized PAN fibers were immersed in 0.5 M Co(NO 3 ) 2 and Ni(NO 3 ) 2 aqueous solutions for 12 hours. Then, the fibers were separated from nitrate solutions and dried at 80 o C overnight. Secondly, the Co(NO 3 ) 2 and Ni(NO 3 ) 2 doped fibers were carbonized at 800 o C for 2 hours in N 2 atmosphere with a heating rate of 5 o C/min and the products were named as Co/CNFs and Ni/CNFs. As a control, pure carbon nanofibers (CNFs) were obtained by annealing the oxidized PAN fibers following the same heating profile. S1.4 Preparation of NiCo 2 O 4 /MNPs/CNFs Wet chemical synthesis method was used to synthesize NiCo 2 O 4 nanostructures. Typically, 1.0 ml 1.0 M Ni(NO 3 ) 2, 2.0 ml 1M Co(NO 3 ) 2 and 12.0 mmol urea were added to a mixture of ethanol (20.0 ml) and DI water (20.0 ml) to form a pink solution. Pure CNFs and MNPs/CNFs were immersed in the pink solution (10 ml) and reacted in a sealed reactor at 85 o C for 24 hours. Afterwards, the products were washed by DI water in ultrasonication bath to remove the loosely attached products and dried at 60 o C. The products were annealed in air at 250 o C for 2 hours to obtain crystallized NiCo 2 O 4. Three products were obtained from this process, NiCo 2 O 4 /CNFs, NiCo 2 O 4 /Co/CNFs and NiCo 2 O 4 /Ni/CNFs. S1.5 Characterization The morphology of PAN fiber, CNFs, MNPs/CNFs and NiCo 2 O 4 /MNPs/CNFs were characterized by scanning electron microscopy (SEM, JEOL-7401) and transmission electron microscopy (Tecnai T12T/STEM). Samples for TEM observation were prepared by drying a drop of sample powder ethanol suspension on carbon-coated copper TEM grids. The loading of metal nanoparticles and NiCo 2 O 4 on the CNFs were determined by S-2

3 thermogravimetric analysis (TGA, TA instrument Q500). TGA studies on the samples were conducted in air atmosphere from 20 o C to 800 o C with a ramp rate of 10 o C/min. The powder X-ray diffraction analysis of the samples was carried out with a Bruker AXS D8 Discover diffractometer with GADDS (General Area Detector Diffraction System) operating with a Cu-K α radiation source filtered with a graphite monochromator (l = Å). The Raman analysis was characterized with a Horiba LabRam HR Micro Raman Spectrometer within the range of 400~3000 cm -1. The electrochemical testing of the NiCo 2 O 4 /(MNPs)/CNFs nanocomposites were evaluated with the two-electrode method on a VersaSTAT 4 electrochemical workstation (Princeton Applied Research). The working electrodes were prepared by mixing the NiCo 2 O 4 /(MNPs)/CNFs nanocomposite (80 wt%), carbon black (10 wt%), polyvinylidene fluoride (10 wt%). A drop of N,N-dimethylformamide was added into the mixture and grinded into paste. Then the paste was dropped onto the pre-cleaned Ni foam ( mm) and then dried at 90 o C for 12 hours in a vacuum oven to ensure the binding between active material and Ni foam current collector. All electrodes were soaked in 1.0 M KOH electrolyte overnight before electrochemical tests. Two active electrodes are separated by an insulating porous separator (whatman filter paper) and sandwiched between a coin-cell current collector (CR2430). Finally, the assembled cell was loaded into a cell holder. The cyclic voltammograms (CV) were recorded at different scanning rate 2, 5, 10, 20, and 50 mv/s in the potential range of V, chargedischarge tested were conducted at current density range of A/g. Electrochemical impedance spectroscopy (EIS) tests were performed using a sinusoidal signal with mean voltage of 0 V and amplitude of 10 mv over a frequency range of 1,000,000 to 0.01 Hz. S-3

4 S2 Results and discussion S2.1 Microstructure Fig. S1. SEM of (a) electrospun polyacrylonitrile fibers, (b) NiCo2O4/CNFs, (c) NiCo2O4/Co/CNFs and (d) NiCo2O4/Ni/CNFs. Fig. S1(a) shows the microstructure of as-spun polyacrylonitrile fibers with uniform fiber diameter of about 870 nm. After carbonization, the fiber morphology is still remained but the size reduced to 470 nm due to the carbonization induced shrinking effect. The subsequent coating of NiCo2O4 on CNFs was not successfully, only small amount of rod shaped NiCo2O4 was observed, Fig. S1(b). Complete coatings were observed by introducing Co and Ni nanoparticles on CNFs, Fig. S1(c&d). S-4

5 Fig. S2. TEM microstructure of NiCo 2 O 4 /Co/CNFs after 5 min sonication. The NiCo 2 O 4 nanosheet structure is easily to be peeled off by short time sonication. Fig. S2(a) shows that part of the nanosheet has been peeled from the surface and bare area was observed. The peeled nanosheet can be clearly observed in Fig. S2(b). Fig. S3. Raman spectra of CNFs, Co/CNFs and Ni/CNFs. Raman spectra of CNFs, Co/CNFs and Ni/CNFs are characterized in Fig. S3. It is apparent that no obvious peak could be observed on pure CNFs, indicating its amorphous feature. S-5

6 It is within expectation since the stabilized polyacrylonitrile is annealed at 800 o C that is much lower than the graphitization temperature of o C. However, the decorated Co and Ni nanoparticles will catalyze the graphitization process at relatively lower temperature, e.g. 800 o C. This phenomenon has been reported in our previous publications. 1 The appearance of strong D band and G band in Co/CNFs and Ni/CNFs confirms the catalytic graphitization process during annealing. In Co/CNFs, the typical peaks at 477, 516 and 682 cm -1 correspond to the E g, F 2g1, A 1g modes of Co 3 O 4 nanoparticles. 2 In Ni/CNFs, one strong peak at about 508 cm 1 is observed, which could be attributed to the first-order longitudinal optical (LO) phonon modes of NiO. 3 S2.2 Composition analysis Table S1 summarized the yield of cobalt nickel carbonate hydroxide hydrate after reaction on each substrate. It is worth mentioning that the total amount of cobalt nickel carbonate hydroxide hydrate increases after adding the three substrates in the reaction media. The yield value of the three samples cannot be compared since similar weight of CNFs, Co/CNFs and Ni/CNFs are used (the density and surface area are different among these samples). Table S1. Yield of cobalt nickel carbonate hydroxide hydrate on different substrate. On fiber (g) In solution (g) Total (g) Yield (%) Control / / CNFs % Co/CNFs % Ni/CNFs % S-6

7 Fig. S4. TGA curves of pure CNFs, nanoparticle decorated CNFs (Co/CNFs. Ni/CNFs) and NiCo 2 O 4 coated nanocomposite fibers (NiCo 2 O 4 /CNFs, NiCo 2 O 4 /Co/CNFs. NiCo 2 O 4 /Ni/CNFs). Table S2. Composition analysis of nanocomposites by TGA. Samples Weight percentage CNFs Metal NPs NiCo 2 O 4 at 800 o C (%) (%) (%) (%) CNFs 1.3 / / / Co/CNFs 24.5 / / / Ni/CNFs 16.3 / / / NiCo 2 O 4 /CNFs NiCo 2 O 4 /Co/CNFs NiCo 2 O 4 /Ni/CNFs S-7

8 S2.3 Electrochemical property Fig. S5. CV and galvanostatic charge/discharge curves of NiCo 2 O 4 /CNFs (a&d), NiCo 2 O 4 /Co/CNFs (b&e) and NiCo 2 O 4 /Ni/CNFs (c&f). Fig. S6. Proposed equivalent circuit model for EIS results. S-8

9 Table S3. Summary of EIS modelling results for three nanocomposites. Samples R s, Ω C DL, F R ct, Ω W C F, F NiCo 2 O 4 /CNFs E NiCo 2 O 4 /Co/CNFs E NiCo 2 O 4 /Ni/CNFs E Fig. S7. Cycling stability of NiCo 2 O 4 /CNFs, NiCo 2 O 4 /Co/CNFs. NiCo 2 O 4 /Ni/CNFs measured using charge/discharge over cycles. All the samples show retention increase at lower cycle numbers due to the electrode oxidation/activation, Fig. S7. It is interesting to observe that the NiCo 2 O 4 /Co/CNFs exhibit the highest retention, which is above 100% after cycles. Slight degradation is observed in NiCo 2 O 4 /CNFs and NiCo 2 O 4 /Ni/CNFs, which still remains >90% retention after cycles. All these results indicate that these electrode materials are excellent candidates in terms of longterm cycling stability. S-9

10 Fig S8. XRD pattern of the fibrous composites after coating with cobalt nickel carbonate hydroxide hydrate. (before 250 o C calcination) The XRD pattern of the materials before calcination has been provided to confirm the crystalline structure of the intermediate, cobalt nickel carbonate hydroxide hydrate, Fig. S8. The strong peak at 2θ=11.5 o is observed in all three samples, which corresponds to the typical (003) crystal plane of cobalt nickel carbonate hydroxide hydrate (PDF# ). The relatively weak peak at 2θ=24.3 o is attributed to the (006) plane of cobalt nickel carbonate hydroxide hydrate. Reference 1 J. Zhu, M. Chen, N. Yerra, N. Haldolaarachchige, S. Pallavkar, Z. Luo, T. C. Ho, J. Hopper, D. P. Young, S. Wei and Z. Guo, Nanoscale, 2013, 5, (a) T. Yu, Y. W. Zhu, X. J. Xu, Z. X. Shen, P. Chen, C. T. Lim, J. T. L. Thong and C. H. Sow, Adv. Mater., 2005, 17, 1595; (b) V. G. Hadjiev, M. N. Iliev and I. V. Vergilov, J. Phys. C: Solid State Phys., 1988, 21, L A. C. Ferrari and J. Robertson, Phys. Rev. B, 2000, 61, S-10