Chemical insights into the roles of nanowire cores on the growth and supercapacitor performances of Ni-Co-O/Ni(OH)2 core/shell electrodes

Nanostructured core/shell electrodes have been experimentally demonstrated promising for high-performance electrochemical energy storage devices. However, chemical insights into the significant roles of nanowire cores on the growth of shells and their supercapacitor behaviors still remain as a research shortfall. In this work, by substituting 1/3 cobalt in the Co3O4 nanowire core with nickel, a 61% enhancement of the specific mass-loading of the Ni(OH)2 shell, a tremendous 93% increase of the volumetric capacitance and a superior cyclability were achieved in a novel NiCo2O4/Ni(OH)2 core/shell electrode in contrast to a Co3O4/Ni(OH)2 one. A comparative study suggested that not only the growth of Ni(OH)2 shells but also the contribution of cores were attributed to the overall performances. Importantly, their chemical origins were revealed through a theoretical simulation of the core/shell interfacial energy changes. Besides, asymmetric supercapacitor devices and applications were also explored. The scientific clues and practical potentials obtained in this work are helpful for the design and analysis of alternative core/shell electrode materials.

in NF and the loading of active material can be dramatically increased as the available surface area is greatly enlarged for the deposition of active materials. Various materials, such as carbon-based nanomaterials [29][30][31][32][33][34] , TiN 35 , TiO 2 36 , FeO x 37 , NiS x 38 , Ni 39 , (Ni,Co)O x /(OH) x 39 -41 , have been employed as cores for Ni(OH) 2 shell growth. The Ni, Co oxides based cores are of great interests, because they not just serve as an agent to increase the surface area but also contribute to the total capacitance owing to their own high electrochemical activities 40,[42][43][44][45][46][47][48][49] . Tang et al. reported a Co 3 O 4 nanowires/Ni(OH) 2 core/shell hybrid on nickel foam electrode and achieved a specific capacitance around 15 Fcm −2 at a current of 5 mAcm −2 41 . NiCo 2 O 4 nanowires have shown better supercapacitor performance than Co 3 O 4 . However, NiCo 2 O 4 nanowires/Ni(OH) 2 core/shell formation and its supercapacitor performance are unclear. It is interesting to find out whether the supercapacitor performance can be increased if the nanowire core is changed to NiCo 2 O 4 as the substrate for the growth of the same Ni(OH) 2 shell. It is more interesting to find whether a partial substitution of Co by Ni in the core can significantly affect the growth and performance of Ni(OH) 2 shell. A comparative study on the Ni(OH) 2 shell growing on Co 3 O 4 and NiCo 2 O 4 nanowire core materials and their performances in supercapactor devices are worth to be conducted.
In this work, we carried out a comparative study on supercapacitor performances for the core/shell structures of Ni(OH) 2 grown on two different nanowires: NiCo 2 O 4 and Co 3 O 4 . It was found that NiCo 2 O 4 /Ni(OH) 2 core/shell structure interestingly showed a great improvement in supercapacitor performance comparing with the Co 3 O 4 /Ni(OH) 2 core/shell electrode. The volumetric and gravimetric capacitances increased 93% and 56%, respectively. And the capacitance retention also enhanced to 96.5% for the NiCo 2 O 4 /Ni(OH) 2 electrode compared with the Co 3 O 4 /Ni(OH) 2 (74.4%) after 1000 cycles. Both the enhancement of the specific mass-loading of the Ni(OH) 2 shell and the more electrochemically active NiCo 2 O 4 core contributed to its superior performances. Through theoretical simulations, the chemical adsorption energy between the NiCo 2 O 4 core and Ni(OH) 2 shell was found to be smaller than that of the Co 3 O 4 /Ni(OH) 2 structure, which revealed the mechanisms behind the influences of such a compositional change in the nanowire core material on the core/shell's properties. In addition, asymmetric supercapacitor devices were fabricated to demonstrate their great potentials for practical applications. The experimental evidences and scientific understandings achieved in this work are of great values for the design and interpretation of other core/shell systems.

Results and Discussion
The core/shell electrode preparation involves a hydrothermal deposition of Ni-Co-O nanowire cores on nickel foam (NF) and a chemical bath deposition of Ni(OH) 2 shells on the nanowires as illustrated in Fig. S1  NF/NiCo 2 O 4 /Ni(OH) 2 electrodes turn to be light green as shown in the insets of Fig. 1a' Fig. 2a EDS results for Co 3 O 4 and NiCo 2 O 4 nanowires are obtained using the transmission electron microscope equipped with EDS facility as shown in Fig. 3. To distinguish Ni and Co, the profiles in the energy region between 6.5 keV and 8 keV are magnified in the inset. For the Co 3 O 4 nanowire, only Co K α1 (6.931 keV) and Co K β1 (7.649 keV) peaks are detected. For the NiCo 2 O 4 nanowire, additional Ni K α1 (7.480 keV) and Ni K β1 (8.267 keV) peaks appear. The EDS spectra data analysis reveals a Ni/Co ratio close to the formulated ratio for NiCo 2 O 4 , but (I)

Electrodes
Loading of core (mg) Loading of shell (mg) Loading of core/shell (mg) SA-C (m 2 ) ΔE a (eV)    this ratio cannot be accurate due to the overlapping of Ni K α1 and Co K β1 peaks. The Cu and C signals come from the TEM grid. Individual core and core/shell wires are inspected by using scanning transmission electron microscopy (STEM) and elemental mapping. The STEM and elemental mapping images in    52,53 . In addition, the CV curve of a bare NF presents a negligible contribution compared with the nanowire core coated electrodes in Fig. 6a. The enclosed area of the CV loop for NF/NiCo 2 O 4 is also larger than NF/Co 3 O 4 , indicating a larger capacitance value of NF/NiCo 2 O 4 than NF/Co 3 O 4 . This is consistent with the longer discharging time in the discharge curve of NF/NiCo 2 O 4 than NF/Co 3 O 4 in Fig. 6b. The calculated gravimetric, volumetric and areal specific capacitances of core and core/shell electrodes at a current density of 2.5 mA/cm 2 are presented in Table 1(II). The current density dependent discharge and specific capacitance curves for those core and core/ shell electrodes are also presented in Figs S2 and S3. It should be pointed out that in this work the specific capacitance (SC) values were calculated from the discharge measurements, following Equation (1),   2 are increased about 56% and 93%, which are much larger than the relative loading increase of Ni(OH) 2 (36%). Therefore, it can be concluded that the increased loading of Ni(OH) 2 shell results in a crucial capacitance enhancement, but the capacitances of the core/shell electrodes are not fully contributed by the Ni(OH) 2 shell but also counted on the cores' contribution. Additional evidences are provided by the CV curves of the NF/Co 3 O 4 /Ni(OH) 2 and NF/NiCo 2 O 4 /Ni(OH) 2 electrodes in Fig. 6c. After the growth of Ni(OH) 2 shell, the enclosed areas of CV loops for both core/shell electrodes are significantly extended in contrast to their core electrodes (Fig. 6a). Similar phenomena were also observed in other Ni/Co oxides or hydroxides based core/shell electrodes 12,39,[53][54][55] . Comparing with NF/Co 3 O 4 /Ni(OH) 2 , there are traceable characteristics of left-shifted electrochemical reaction peaks in NF/NiCo 2 O 4 /Ni(OH) 2 , which could be resulted from the similar features in the CV loops of their cores in Fig. 6a. In Fig. 6d, the cycling performances of these two core/shell electrodes are examined at a charge-discharge current of 50 mA/cm 2 . After 1000 cycles, the residual capacitance of NF/NiCo 2 O 4 /Ni(OH) 2 is 96.5%, which is much higher than that of NF/Co 3 O 4 /Ni(OH) 2 (74.4%).
It is found that the NF/NiCo 2 O 4 /Ni(OH) 2 has much better capacitive performances than NF/Co 3 O 4 /Ni(OH) 2 . The more electrochemically active nature of the NiCo 2 O 4 nanowire than Co 3 O 4 plays a role, however the much enhanced mass-loading of Ni(OH) 2 shell on the NiCo 2 O 4 core than Co 3 O 4 contribute more to their capacitances. In order to find out the reasons for the loading increase of the Ni(OH) 2 shells on the NiCo 2 O 4 core, the microstructures of the cores, especially the surface area, need to be examined, because the cores instead of bare nickel foams now serve as the effective substrates for the Ni(OH) 2 growth. The analysis of the surfaces and pores of the Co 3 O 4 and NiCo 2 O 4 cores is performed through N 2 adsorption-desorption isotherm together with TEM images as shown in Fig. 7. The mesoporous nature of the nanowire cores are revealed by a type-IV adsorption-desorption isotherm for both Co 3 O 4 and NiCo 2 O 4 in Fig. 7a 56 . The hysteresis loop for the Co 3 O 4 can be further classified to type H3, which indicates slit-like pores. The hysteresis loop for the NiCo 2 O 4 belongs to type H4, indicating slit-like pores with larger pore sizes 57 . In addition, the hysteresis loop of the Co 3 O 4 core initiates at a larger relative pressure than NiCo 2 O 4 , suggesting a relative smaller pore size in the Co 3 O 4 core 57 . In Fig. 7b, the Barret-Joyner-Halenda (BJH) pore size distribution curves give a pore size of 7.  Generally, growth rate (r) follows a kinetic law written as 58 , where R and C are constants, and Δ E a is the chemical affinity, i.e. the system energy change before and after growth. In this case, Δ E a is defined as the adsorption energy and expressed as: Here, E ad is the total energy per cluster of Ni(OH) 2 unit on respective nanowire core system, E c is the total energy of clean systems and E r is the total energy of per cluster of Ni(OH) 2 . To reveal this variation of chemical affinity between the cores and Ni(OH) 2 shell, the adsorption energy of Ni(OH) 2 unit on the core surfaces are calculated using density functional theory (DFT) with the generalized gradient approximation of Perdew and Zunger and Ion cores modeled with projector augmented wave (PAW) potentials as implemented in the VASP 59-61 . All calculations were performed with an energy cutoff of 500 eV, which had been tested for total energy convergence. For unit-cell calculations, a dense Monkhost-Pack grid of 8 × 8 × 8 k-ponits sampling was used and reduced to 2 × 2 × 1 k-ponits samples on nanowire or surface calculations. The convergence for energy was chosen as 10 −5 eV between two ionic steps, and the maximum force allowed on each atom is 0.02 eVÅ −1 . The cluster of Ni(OH) 2 sourced from Ni(OH) 2 crystal with a Trigonal P-3M1 structure was put in a cubic box with the lattice constant of 15 Å, the relative ground state energy was used to as reference state energy. Here, Co 3 O 4 nanowire is considered as the prototype, which has 36 Co and 48 O atoms in its primitive unit cell. The ideal nanowire is cut initially from optimized bulk Co 3 O 4 crystal and, subsequently, all atoms are fully optimized. Upon relaxation, the structure of ideal clean Co 3 O 4 nanowire reconstructed and which was adopted for adsorption calculations shown in Fig. 8a. Using Ni to replace tetrahedron Co in Co 3 O 4 nanowire forms NiCo 2 O 4 nanowire containing 24 Co, 12 Ni and 48 O atoms as shown in Fig. 8b.
After simulation, theoretical adsorption energy (Δ E a ) values of − 3.10 eV for Ni(OH) 2 on NiCo 2 O 4 and − 2.17 eV for Co 3 O 4 are calculated. The negative values reveal decreased total energies for both cases after deposition. But the lower Δ E a of the NiCo 2 O 4 /Ni(OH) 2 core/shell indicates a more favorable growth of Ni(OH) 2 on the NiCo 2 O 4 than Co 3 O 4 , which is well supported by the experimental observation of a much higher mass-loading of Ni(OH) 2 on the NiCo 2 O 4 than Co 3 O 4 (see Table 1(I)). As a result, the capacitive performances of the core/ shell electrodes are going to be affected by their respective cores and subsequent Ni(OH) 2 shells. Additionally, the active material's parting from electrodes during cycling is one of the major concerns for the capacitance loss in literature. In our case, the calculated smaller adsorption energy between Ni(OH) 2 and NiCo 2 O 4 than Co 3 O 4 also indicates a stronger connection between Ni(OH) 2 and NiCo 2 O 4 , which is responsible to its better cycling ability as shown in Fig. 6d.
The NF/NiCo 2 O 4 /Ni(OH) 2 electrode is applied as the anode in an asymmetric supercapacitor cell that is assembled with a NF/reduced graphene oxide (RGO) as the cathode and a 6 M KOH aqueous solution as the electrolyte (see the inset of Fig. 9a). A practical application case of this supercapacitor cell as the power supply for a mini fan is manifested in Fig. 9a.
The CV curves in Fig. 9b show great electrochemical activities of the both cell over a large potential range from 0 to 1.7 V in the aqueous electrolyte at a scan rate of 5 mV/s. But the cell consisting of NF/NiCo 2 O 4 / Ni(OH) 2 electrode presents an extra pair of peaks around 0.6 V and 13.5 V, which could be attributed to the more active NiCo 2 O 4 nanowire core as revealed in the half cell test (Fig. 6). Galvanostatic discharge curves shown in Fig. 9c give rise to areal specific capacitance values of 6.5 Fcm −2 and 7.5 Fcm −2 at a discharge current density of 2.5 mAcm −2 , respectively. Additional current density dependent discharge curves are presented in Fig. S4. In the Ragone plot in Fig. 9d 2 and NiCo 2 O 4 /Ni(OH) 2 core/shell structures were successfully prepared on nickel form substrate through hydrothermal and chemical bath depositions. These core/shell electrodes were applied in a full asymmetry supercapacitor device with reduced graphene oxide as the cathode and KOH aqueous solution as the electrolyte. The great application potentials were demonstrated by the practical case and capacitive characterizations. More interestingly, comparative studies of the NiCo 2 O 4 /Ni(OH) 2 and Co 3 O 4 /Ni(OH) 2 core/shell electrodes revealed their distinct capacitive behaviors and different loading ability of Ni(OH) 2 shell. And the causes were further investigated through theoretical simulation and surface analysis of the core/shell interfaces. It was found that the adsorption energy of Ni(OH) 2 on NiCo 2 O 4 is smaller than Co 3 O 4 , which resulted in more Ni(OH) 2 shell loading and better cycling stability of NiCo 2 O 4 /Ni(OH) 2 electrode. In this work, in addition to an exploration of the NiCo 2 O 4 nanowire/Ni(OH) 2 core/shell based electrode for supercapacitor applications, a comprehensive understanding of the core materials and their impacts on the core/shell structures and finally the performances of the whole cell was established. It is of great practical values to analyze various core/shell structures and develop better electrode candidates for supercapacitors.

Methods
Nanowires synthesis. The spinel NiCo 2 O 4 nanowires were synthesized by a facile hydrothermal method. NF/Co 3 O 4 , i.e. NF/NiCo 2 O 4 /Ni(OH) 2 or NF/Co 3 O 4 /Ni(OH) 2 electrode was subjected to a high speed rotation rinsing at 500 rpm for 3 minutes and then dried at 120 °C in air.

Cathode electrode fabrication.
A mixture paste of 90 wt% reduced graphene oxide (Graphene Supermarket) and 10 wt% PTFE was spread onto a 2 cm × 2 cm Ni foam to form the RGO-based cathode. Then the electrode was dried at 70 °C in air for 2 hours, pressed at 8 MPa, and then kept at 120 °C in air for 12 hours.
Material and device characterizations. The morphology and microstructure of the synthesized core/shell electrodes were characterized by scanning electron microscopy (SEM, Zeiss) and transmission electron microscopy (TEM, JEOL 2010) with energy dispersive X-ray (EDS) analyzer. The structure was measured by X-ray diffraction (XRD) (BRUKER D8 ADVANCE), selected area electron diffraction (SAED) and high resolution TEM. The surface area and porosity of nanowire cores were evaluated by Brunauer-Emmett-Teller (BET, Micromeritics ASAP 2020) N 2 adsorption-desorption measurements. Electrochemical measurements of single electrodes in a half cell were carried out in a three-electrode arrangement with the prepared electrode as working electrode, a platinum plate as counter electrode and a saturated calomel electrode (SCE) as reference electrode in 6 M KOH aqueous electrolyte. Cyclic voltammetry (CV) and galvanostatic charge/discharge test of respective single electrodes and full cells were evaluated by Solartron Electrochemical System SI 1287.