Xiaoming
Sun
,
Hao
Lu
,
Peng
Liu
,
Thomas E.
Rufford
,
Rohit R.
Gaddam
,
Xin
Fan
and
X. S.
Zhao
*
The School of Chemical Engineering, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia. E-mail: george.zhao@uq.edu.au
First published on 18th December 2017
We report a vacuum-thermal strategy for the preparation of a composite electrode material consisting of reduced graphene oxide and nickel oxide nanoparticles, which displays interesting electrocapacitive properties. Graphene oxide thermally expands in a vacuum and simultaneously nickel(II) acetylacetonate decomposes to form NiO nanoparticles between graphene layers. This method not only allows the uniform dispersion of NiO nanoparticles between graphene layers but also enables simultaneous reduction of graphene oxide. The structural and electrochemical advantages of both reduced graphene oxide and nanoscale NiO particles are maintained. The reduced graphene oxide–NiO composite exhibits a specific capacitance of 880 F g−1 at a current density of 1 A g−1 in 6 M KOH and a 93.1% retention of initial capacitance after 5000 cycles at 5 A g−1.
Graphene has been studied widely as a promising electrode material for SCs.5–8 Many studies have investigated methods to expand and/or exfoliate graphite to prepare graphene materials, including mechanical expansion methods,9 chemical methods,10,11 and thermal methods.12,13 Among them, methods involving thermal expansion at high temperatures14 and sometimes in a vacuum15 are a facile approach.
In addition, metal oxides with a high pseudocapacitance such as nickel oxide (NiO),16–18 manganese oxide,19–21 and iron oxide22,23 have been combined with graphene to prepare composite electrodes for SCs. Such composite electrode materials are reported to benefit from excellent kinetics of charge transfer, structural stability during charge/discharge processes, and good electrical conductivity.
We have previously reported the capacitive improvement of graphene by introducing metal oxides,24 conducting polymers,25 carbon nanotubes,26 and carbon nanospheres.27 Here, we demonstrate a facile approach to produce a composite electrode of expanded graphene oxide (EGO) decorated with NiO nanoparticles (this composite is labelled EGO–NiO) as an advanced electrode material for SCs. The preparation is schematically illustrated in Fig. 1. Nickel(II) acetylacetonate [Ni(acac)2] was selected as the nickel precursor, which can diffuse between graphene sheets of graphene oxide (GO) flakes.28–30 Upon heating up to 550 °C, Ni(acac)2 decomposes to form NiO nanoparticles31,32 on the GO sheet surface. The interlayer space of the GO sheets was simultaneously expanded. Meanwhile, a large number of oxygen-containing groups on GO were thermally reduced.11 The deposited NiO nanoparticles prevent restacking of the reduced GO (RGO), providing sufficient space for electrolyte ions to move while the NiO nanoparticles contribute significantly to pseudocapacitance.
We compared the physical and chemical properties, and electrochemical performance of the EGO–NiO composite to those of three samples produced by other methods. (a) Thermally treated GO flakes (T-GO) were prepared from GO by the same two-step procedure at 220 °C and 550 °C under vacuum as described for EGO–NiO. (b) A GO–nickel oxide composite, GO–solNiO, was prepared from a water soluble nickel precursor, Ni(NO3)2, by the same procedures as those used for EGO–NiO. And (c) a GO–nickel oxide composite, phGO–NiO, was prepared by physically mixing GO with Ni(acac)2 at a mass ratio of 1:1, and this mixture was also heated at 550 °C for 3 h in a vacuum.
Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests were performed on an Autolab PGSTAT 3020N. The CV scans were collected in a potential window of 0–0.5 V and at scan rates of 5–50 mV s−1. The GCD tests were performed at current densities from 1 to 20 A g−1 in the potential window of 0–0.5 V. Electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential with an amplitude of 10 mV in the frequency range of 10−2 to 105 Hz.
The specific capacitance of a single electrode was calculated from GCD profiles using eqn (1):
(1) |
Fig. 2 (a) XRD patterns of GO, GO/Ni(acac)2, and EGO–NiO, and (b) small-angle XRD pattern of EGO–NiO; JCPDS 47-1049 is the index pattern of NiO. |
We used AFM to examine the EGO–NiO composite more closely, and observed NiO nanoparticles decorated on the flakes of RGO (Fig. S2a†). The height of an RGO flake was estimated, from a cross-sectional contour of the flake, to be approximately 0.9 nm (Fig. S2b†), which is consistent with other reports for a single layer of graphene.35,36 The thickness of the NiO nanoparticles on the RGO surface was estimated, from the AFM images, to be around 4.2 nm and this result is consistent with the EGO–NiO interlayer estimated from the small-angle XRD pattern (Fig. 2b). Thus, the AFM results together with the XRD patterns of EGO–NiO suggest that the significantly expanded interlayer spacing is attributed to the presence of NiO nanoparticles between the graphene sheets decomposed from Ni(acac)2.
The XRD patterns of GO–solNiO and phGO–NiO are included in Fig. S3.† In Fig. S3a,† the diffraction peaks of the (111) and (200) planes of crystalline NiO are again observed in GO–solNiO and phGO–NiO. However, we did not observe any characteristic GO peak around 2θ = 12.6° in Fig. S3a,† nor any peak that could be attributed to the expansion of GO in the small angle XRD patterns (Fig. S3b†) of GO–solNiO and phGO–NiO. These XRD results suggest that the NiO was formed as a separate phase instead of between graphene layers in GO–solNiO and phGO–NiO, and highlight that precursor selection and pre-anchoring into graphene layers is critical to achieving the EGO–NiO structure.
The porous structure of sample EGO–NiO was characterized by nitrogen sorption analysis. As shown in Fig. S4,† sample EGO–NiO exhibits a type IV isotherm, which indicates the existence of abundant mesopores. The BET surface area is 128 m2 g−1.
The SEM images of EGO–NiO in Fig. 3a and b show that the EGO–NiO retained the flake structure of GO, with dispersed NiO particles and layers of reduced GO visible in the images. The nanosized NiO particles and the RGO layers are also observed in the TEM image shown in Fig. 3c. The distribution of NiO particle sizes calculated from TEM images (Fig. 3d) shows that the average size of NiO particles was 7.8 nm, and we interpret this size as the width of nanoparticles parallel to the RGO sheet planes. The nanocrystal lattice of NiO was captured in the HRTEM image (Fig. 3e), which shows a well-defined crystalline lattice spacing of 0.21 nm, consistent with the (200) crystallographic plane of NiO. Fig. 3f and g show the carbon and nickel elemental maps, obtained from TEM-EDS respectively, which confirm the well-dispersed state of NiO nanoparticles in EGO–NiO.
The SEM images of GO–solNiO and phGO–NiO in Fig. S5a and c† show densely stacked GO flakes several microns thick, which is consistent with the XRD results which indicate that no obvious GO expansion occurs when the soluble nickel precursor is used or upon physical mixing of Ni(acac)2 with GO. Considering the obvious peaks of crystalline NiO observed for sample GO–solNiO, along with the nanoparticles observed in the SEM image shown in Fig. S5b,† it can be concluded that the NiO nanoparticles in sample GO–solNiO existed on the surface of GO flakes instead of being interspersed between graphene layers. We also observe in Fig. S5d† that NiO particles were formed on the external GO surfaces of phGO–NiO. Hence, we postulate that the key step in the preparation of EGO–NiO is the diffusion of Ni(acac)2 in a basic GO suspension, which facilitates π–π interactions between the aromatic acetylacetonate (acac) ligands and the graphitic layers of the sp2 carbon network. This concept has been demonstrated for other metal-acetylacetonate compounds with GO and graphene materials.28–30 However, for phGO–NiO, during vacuum heating, due to the lack of expansion of GO, the narrow interlayer space causes an obstruction, preventing Ni(acac)2 molecules from diffusing in. Consequently, Ni(acac)2 adsorbs on the external surfaces of GO and at high temperature sublimes to deposit NiO only on the external GO surfaces.
The nickel content of these three composite materials was analysed by EDS from SEM (Fig. S6†). Interestingly, the nickel content of EGO–NiO and GO–solNiO is 2.85 and 1.40 at% respectively, which are lower than that of phGO–NiO (8.8 at% Ni). This may possibly be due to the leaching of unbound Ni ions during the washing process.
The XPS survey spectra in Fig. 4a confirm the presence of carbon, oxygen, nickel species on the surface of EGO–NiO, and only carbon and oxygen species on GO. The C 1s XPS spectrum of GO (Fig. 4b) was deconvoluted into four components: CC at a binding energy of 283.7 eV, C–C at 284.6 eV, a dominant C–O peak associated with epoxy and alkoxy groups at 286.6 eV, and a CO peak associated with carbonyl groups at 288.2 eV.37,38 In contrast, the dominant peak in the C 1s XPS spectrum of EGO–NiO (Fig. 4c) is the C–C peak, with a low intensity C–O peak and no observable CO peak. This change observed for EGO–NiO indicates the reduction of oxygen species on the GO and a partial restoration of the conjugated sp2 graphene network after the vacuum-thermal treatment. The Ni 2p XPS spectrum of EGO–NiO in Fig. 4d exhibits two main peaks at binding energies of 871.5 eV assigned to Ni 2p1/2 and 853.7 eV assigned to Ni 2p3/2. We attribute the satellite peaks of Ni 2p1/2 at a binding energy of 880.8 eV and Ni 2p3/2 at 861.8 eV in Fig. 4d to Ni2+ in NiO.39–42 These XPS results confirm the reduction of GO and the formation of NiO after the vacuum heating process to prepare EGO–NiO.
Fig. 4 XPS survey spectra of GO and EGO–NiO (a), C 1s XPS spectrum of GO (b), C 1s XPS spectrum of EGO–NiO (c), and Ni 2p XPS spectrum of EGO–NiO (d). |
Fig. 5 summarises the electrocapacitive performance of EGO–NiO electrodes in GCD and CV tests performed in 6 mol L−1 KOH. The GCD profiles measured at a current density of 1 A g−1 in Fig. 5a show that the EGO–NiO electrode delivered a much longer discharge time than the GO–solNiO and phGO–NiO electrodes, and it follows that the EGO–NiO specific capacitance of 880 F g−1 calculated from the GCD discharge profile was significantly greater than the capacitances of GO–solNiO (182 F g−1) and phGO–NiO (165 F g−1). The capacitance of EGO–NiO was also much greater than the charge storage capacity of a GO electrode (Fig. S7†). Similar to literature reports, Fig. 5a indicates the loss of coulombic efficiency at low current density, which is possibly due to the slow electrochemical reaction at nanostructured metal oxide electrodes at lower current density.43 Furthermore, Fig. 5b shows that the electrochemical performance of EGO–NiO remained excellent at fast charge–discharge rates with a specific capacitance of 428 F g−1 at current densities up to 20 A g−1.
Fig. 5c demonstrates the stability of the EGO–NiO electrode over 5000 consecutive GCD cycles at a current density of 5 A g−1. After 5000 charge–discharge cycles, the EGO–NiO electrode retains 93.1% of its initial capacitance. Moreover, a capacitance retention of 80.1% can be achieved after 5000 cycles at a high current density of 20 A g−1 (Fig. S8†). The relatively quicker capacitance fade and shorter cycle life at higher current density may possibly be due to the severe structural change of electrode materials under fast charging–discharging.
Based on these results and the characterisation of the electrode materials, we attribute the outstanding performance (high capacitance and electrochemical stability against the electrolyte) of the EGO–NiO electrode to several factors, including the sufficient surface area accessible to electrolyte ions, removal of oxygen-containing groups thereby improving the conductivity, and the pseudocapacitance that NiO particles contribute to.44
The CV curves of EGO–NiO (Fig. 5d) exhibit redox peak-pairs with an anodic peak at around 0.48 V in the forward scan and a cathodic peak at around 0.35 V in the backward scan. These peaks correspond to NiO electrochemical reactions associated with the redox couple Ni2+/Ni3+:45,46
NiO + OH− ↔ NiOOH + e− |
It is noted that two obvious cathodic peaks were observed with the increase of scan rates. They were caused by the transformation of NiOOH to α-Ni(OH)2 in the alkaline solution during the charging process and the instability of α-Ni(OH)2, which results in the formation of β-Ni(OH)2.47–49 Accordingly, the two oxidation reactions of Ni(OH)2 give two varieties of oxyhydroxide, β and γ NiOOH, which can explain the two reduction peaks observed during the backward scan in Fig. 5d.50
The Nyquist plot (ESI Fig. S7d†) produced from EIS measurements with the EGO–NiO electrode features a low x-axis intercept in the high frequency region indicating a low equivalent series resistance (ESR), and a negligible semicircle at medium frequencies, much smaller than that of the GO electrode (Fig. S7†). The diameter of the semicircle represents the charge transfer resistance (Rct). Furthermore, in the low-frequency region, the curve of EGO–NiO is close to a vertical straight line and this shape demonstrates that the resistance to the diffusion of ions in the electrode is small.51–53 These EIS results, that indicate fast electron and ion transport in the composite electrode, are consistent with the excellent capacitance retention of the EGO–NiO electrode at high current densities (Fig. 5b).
Fig. 6 compares the rate capability of EGO–NiO with those of some NiO–graphene composite electrode materials reported in the literature,54–58 and the same reference data are tabulated in Table S1.† The EGO–NiO electrode prepared in this work showed higher or comparable specific capacitance, rate capability, and cycling stability in comparison with other electrode materials. It can be seen that EGO–NiO has an excellent rate capability at high current densities up to 20 A g−1. In the EGO–NiO composite, the removal of oxygen-containing groups improves the electrical conductivity of the RGO network. RGO plays the role of a conductive support for the deposition of NiO nanoparticles. The good interfacial contact between NiO and RGO promotes the conductivity of the electrode, thus ensuring a good rate capability.
Fig. 6 A comparison of the rate capability of the electrode material prepared in this work with those reported in the literature.54–58 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00420f |
This journal is © The Royal Society of Chemistry 2018 |