In Situ Synthesis of Vertical Standing Nanosized NiO Encapsulated in Graphene as Electrodes for High‐Performance Supercapacitors

Abstract NiO is a promising electrode material for supercapacitors. Herein, the novel vertically standing nanosized NiO encapsulated in graphene layers (G@NiO) are rationally designed and synthesized as nanosheet arrays. This unique vertical standing structure of G@NiO nanosheet arrays can enlarge the accessible surface area with electrolytes, and has the benefits of short ion diffusion path and good charge transport. Further, an interconnected graphene conductive network acts as binder to encapsulate the nanosized NiO particles as core–shell structure, which can promote the charge transport and maintain the structural stability. Consequently, the optimized G@NiO hybrid electrodes exhibit a remarkably enhanced specific capacity up to 1073 C g−1 and excellent cycling stability. This study provides a facial strategy to design and construct high‐performance metal oxides for energy storage.

After that, g-Ni foam was used as the conductive substrates for synthesizing NiO nanosheet arrays. Briefly, Ni(NO 3 ) 2 ·6H 2 O (1 mmol) and urea (5 mmol) were added to 40 ml of deionized water to obtain a mixed solution. Then, g-Ni foam were transferred to the 50 ml reaction vessel and maintained at 120 o C for 6 h. The fabricated precursors were washed deionized water and ethanol for several times. The synthesized precursors were annealed at Ar atmosphere in 350 o C for 1 h to obtain NiO. [2,3] The mass loading of NiO was calculated by the weight difference before and after hydrothermal and annealing processes. The mass loading of NiO is about 1.60 mg cm -2 .

Synthesis of vertical standing nanosized NiO encapsulated in graphene (G@NiO).
Plasma enhanced chemical vapor deposition (PECVD) enables growth of various carbon nanometerials on kinds of substrate without catalysts under the low temperature. In this case, we conducted PECVD process to encapsulate NiO by graphene. Firstly, the obtained NiO was heated to 350 o C at the Ar atmosphere. In the 2 next step, a mixture of CH 4 and Ar with a gas flow rate ratio of 5sccm/95sccm was introduced to the chamber. During this process, the chamber pressure was kept at 500 Pa and the plasma source was turned on at a power of 200W. And the plasma condition was maintained for 0.5min, 1min, 2min and 3min before cooling down the room temperature (briefly named as G@NiO-0.5, G@NiO-1, G@NiO-2 and G@NiO-3). For comparison, the pristine NiO samples were also treated under the plasma source of H 2 and Ar (10sccm/95sccm) for 1min (named as NiO-Ni-1). The mass loading of G@NiO samples was calculated by subtracting the weight before hydrothermal process from the weight after PECVD process. The mass loading of were also carried out.
Electrochemical tests. The electrochemical tests were conducted in the three-electrode configuration by CHI 760E and PARSTAT 4000A, using 2 M KOH as electrolyte. While the obtained sample was directly used as working electrode, Pt foil and Hg/HgO electrode were used as the counter and reference electrodes, respectively.
Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 0.1 to 10 5 Hz with an amplitude of 5 mV. The specific capacitance (C s ) and specific capacity (C m ) of obtained samples was calculated by the following equations: (1) where C, I, ∆t, m, and ∆V mean the specific capacitance, the current, the discharging time, the mass loading of the active materials and the potential window, respectively. [3] In order to widen the potential window, the symmetric supercapacitor (ASC) device was assembled using G@NiO-1 as positive electrode and nitrogen-doped graphene hydrogels (NGH) as negative electrode. The synthesis process of NGH was according to previous research [4]. The energy density (E, Wh kg -1 ) and power density (P, W kg -1 ) of assembled ASC devices were calculated by the following equations [5] : where C, V and ∆t mean the specific capacitance, potential window and discharging time in the ASC device. The energy density is calculated based the mass of active materials (total active material loadings of the G@NiO and NGH).

Mass loading calculations.
The mass loading of graphene on the g-Ni foam was calculated by the weight difference before and after CVD process, using a high-precision balance (Denver Instruments, sensitivity: 0.01 mg). And ten samples with a size of 2 × 5 cm 2 were weighed. The graphene loading on g-Ni foam was calculated to be about 0.08mg cm -2 . In order to calculate the graphene layers on NiO nanosheets by PECVD process, the obtained G@NiO products were dissolved in 3M HCl for 24 h. After that, the obtain products were washed by water and ethanol for several times, and dried at 80 o C overnight. By weighing these products, the graphene layers on NiO can be estimated to about 0.03 mg cm -2 for G@NiO-0.5, 0.08 mg cm -2 for G@NiO-1, 0.13 mg cm -2 for G@NiO-2 and 0.17 mg cm -2 for G@NiO-3, respectively. And the mass loading of graphene layers on NiO was all subtracted the 4 mass loading of graphene on g-Ni substrate by CVD.
As for the ratio of NiO/Ni, we calculated the mass loading of NiO by annealing the  Figure S1. Raman spectrum of g-Ni foam As shown in Figure S1, the Raman spectrum of g-Ni foam shows three major bands at about 1350 (D band), 1580 (G band) and 2700 cm -1 (2D band). [6] The D band is induced by the disordered carbon atoms, while G band represents the sp 2 -hybridized graphitic carbon atoms. [7] The weak D band and high G band in g-Ni foam suggests that the graphene on Ni-foam shows the high degree of graphitization. [6,7]  Ni. [8,9] As shown in Figure S4b, O 1s can be deconvoluted into two peaks at the binding energies of 529.5 eV and 531.2 eV, which can be indexed to O 2from NiO and O 2form OH -. [10,11] It can be found that the intensity of Ni was increased with the longer plasma exposition time, while the intensity for O 2from NiO was gradually decreased. In order to further investigate the effect of graphene layers on the electrochemical performance, we also prepare the porous graphene (PG) by dissolving G@NiO-1 sample in 3M HCl for 24 h (see Figure S8). As shown in Figure S8a, the PG shows the porous structures (markd with red cycles) after dissolving process. It suggests that the interconnected graphene do not fully encapsulate the nanoparticles. In addition, we also prepared the electrodes by mixing the PG (80% wt.%), acetylene black (10 wt.%) with polyvinylidene fluoride (10 wt.%) binder in N-methyl pyrrolidinone solvent. The mixed slurry were coated onto g-Ni foam and dried at 80 o C overnight.
The electrochemical tests demonstrates that PG shows the limited specific capacity (only about 125 C g -1 at 1 A g -1 ), which is much lower than that of G@NiO.