Abstract
A nickel hydroxide (Ni(OH)2)-modified reduced graphene oxide (rGO) foam electrode for supercapacitor application is presented in this study. The electrode was made without binder through a one step process. In order to compare the supercapacitor performance of as synthesized graphene foam electrode, rGO and rGO:carbon black (CB) standard electrodes with PVDF-HFP binder were also synthesized . All electrodes were then characterized structurally (XPS, EDS, RAMAN, XRD, and FT-IR) and morphologically (SEM). Ni-containing electrodes were found in the α-Ni(OH)2:rGO structure. NirGO3 has 800 Fg-1 capacitance, while rGO:CB has 900. Thus NirGO3 has the best capacitive performance among binder-free electrodes, comparable to rGO:CB.
Keywords: foam electrode, supercapacitor, self-templated, nickel hydroxide
  1. IntroductionSupercapacitors (SCs), as environmentally friendly and renewable energy storage technology, have recently become popular in the scientific community. When compared to conventional batteries, their relatively long lifetime and higher power density makes them good candidates for electrical application. A typical supercapacitor contains positive and negative electrodes separated by an ion-conducting electrolyte [1]. As a critical component of SCs, the electrodes are prepared by the slurry casting method that needs adding some high conducting material and/or a binder (generally a polymer). The function of conductive (and porous) additives is to improve the interaction at the current collector and the electrode material interface to reduce the series resistance of the device. However, additives and insulating binders have some disadvantages, such as aggregation or clogging pores of active material resulting in loss of volume and relatively lower volumetric and gravimetric capacitance of electrodes [2, 3]. In SC applications, carbonaceous materials, conductive polymers, and metal oxides/hydroxides have generally been used as electrode materials. Particularly, graphene-based composites have commonly been used in those applications due to their large surface area (theoretical surface area up to 2630 m2 g−1), high electronic conductivity, mechanical flexibility and excellent thermal stability [4-7]. However, π-π interactions and weak van der Waals forces lead graphene nanosheets to aggregate to be stacked back into graphite, resulting in decreased accessible surface area and a limited electrolyte ion diffusion rate. To overcome this problem, some other carbon-based materials (carbon black (CB), carbon nanotube (CNT), etc.) or metal oxide/hydroxides nanoparticles (Ni(OH)2, Fe2O3, MnO2, etc.) have been inserted into the electrode structure [8-11]. Although high capacitance values ​​(maximum theoretical capacitance: 2082 F g−1) can be achieved with electrodes prepared, a decrease in conductivity may occur due to the introduction of impurities into the structure and the increase in the distances between the lamellas [8, 9]. From this perspective, binder-free, lightweight graphene foams (GF) with porous structure and high surface area have recently emerged as an essential development [12-14] . 3D GF is a hierarchical material with a porous structure exhibiting suitable properties such as a highly conductive path, extensive surface area, improved charge-carrying mobility, and lower resistance [15]. Electrons can be transferred efficiently in the system with low resistance because of the ideal interconnected network and 3D macrostructure. Moreover, it is possible to obtain more functional structures for various applications by making GF composites with different nanoparticles or semiconductor metal oxides/hydroxides[16-22]. In the literature, the most viable and efficient methods proposed for the preparation of 3D GF and GF-based hybrid structures are hydrothermal, chemical vapor deposition, template, freeze drying, and electrochemical preparation. [23-26]. For instant, it was reported that an Au and rGO based 3D composite material was coated on a carbon electrode surface by electrochemical deposition [27]. This method formed a three-dimensional structure with an interconnected network having relatively larger surface area and higher conductivity. It was also determined that the layer distances of the rGO nanolayer were increased by the presence of Au nanoparticles, providing a biocompatible interface. In another study, the GF-TiO2composite was obtained by a procedure called one-step-hydrothermal method at 180 °C in an autoclave for wastewater treatment. Wang and co-workers reported that the hybrid nanosheet they prepared exhibited much superior performance for removing methylene blue (MB) and chromium (VI) than individual pure graphene foam or pure TiO2 sheets [28]. Similar structures containing Ni, NiO, and Ni(OH)2 have been obtained to be used in supercapacitor applications. Huang et al. reported a graphene/NiO-based film through various preparation processes. The prepared graphene/NiO film achieved a specific 540 F g−1 at 2 A g−1 and kept 80% of initial capacitance for 2000 CDC [29]. On the other hand, Pore et al. also proposed another method to obtain a graphene/NiO foam. In that procedure, the electrode material was prepared with a hydrothermal method by annealing at 400oC. A specific capacitance of 727.1 F g−1 at 1mA cm−2 was achieved with a stability over 80% after about 9000 cycles [3][26]. In addition, an electrode based on Ni(OH)2 for supercapacitor application was reported by Viswanathan and co-workers[8]. As well known, Ni(OH)2 has good solubility in acids such as H2SO4. A chemical reduction method was achieved to obtain insoluble rGO/Ni(OH)2 structure showing a specific capacitance value over 130 F g−1 in 1M H2SO4 electrolyte. This current study developed a self-templated foam based on Ni and rGO, and its structural analyses was carried out. The prepared foams in different weight ratios of Ni and rGO were directly grown on the current collector. These electrodes were electrochemically characterized as supercapacitor electrodes in aqueous electrolytes and compared with standard rGO and rGO:CB reference electrodes.
  2. Experimental Section
  3. Preparation of electrode material and fabrication of PSC
The Hummer‘s method was modified to synthesize GO efficiently and a chemical reduction method was applied to obtain rGO [30]. In order to determine effects of both foam structure and Nickel derivative, three types of electrode composition were prepared. The standard electrode (rGO) was obtained by mixing rGO and binder PVDF at a ratio of 95:5 (w/w) in acetone. The standard secondary electrode (rGO:CB) was obtained by mixing rGO, CB and binder PVDF in the ratio of 80:15:5 (w/w) in acetone. Both reference electrodes were coated on current collector (1*1 cm2) by drop casting method. Initially, rGO powder (10 mg) and nickel nitrate hexahydrate in the various weight ratio of 1:3, 1:6 and 1:12 was added into 2.5 mL ethylene glycol, and then the mixtures were sonicated for 3 h (presented weight ratios of starting materials are optimized values). Subsequently, the resulting dispersions were cast onto a 250 °C pre-heated graphite sheet (the active material is 1.0-1.3 mg in 1*1 cm2 surface area). Adding reactant dispersion onto the hot graphite sheet causes NOx gas’s evolution, and binder-free NirGO foam forms on the current collector. Consequently, these electrodes were used as working electrode and characterised electrochemically in the electrochemical cell system to determine their capacitive performances.
Structural Characterization of Electrode Materials Methods
X-Ray Diffraction (XRD, Rigaku Ultima IV), depth profiling Raman spectroscopy (XploRA Raman Microscope), Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS50 FTIR), energy dispersive spectroscopy (EDS, JEOL JSM-7100-F), and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha), methods were used for the structural characterization of rGO and NirGO composites. Moreover, scanning electron microscopy (SEM, JEOL JSM-7100-F) analyses were carried out to take micrographs of all electrodes for morphological analyses.
XRD patterns were recorded to analyse phase compositions of NirGO3 and rGO with Cu Kα radiation (λ = 1.54 Å), while step size and scan rate are 0.02° and 2°/min, respectively, in a 2θ range from 3° to 95. Furthermore, Raman spectra of rGO and NirGO3 were used to determine the intensity and position changes in the D and G bands of rGO and the Raman shift peaks of Ni(OH)2 in the 400-2500 cm−1. Furthermore, the phase of Ni(OH)2 and its interaction with rGO through the functional groups was also clarified by carrying out FT-IR analyses in the range from 400 to 4000 cm−1.
SEM analyses of rGO, rGO-CB and NirGO3 electrode materials were performed to examine their morphology. The 250X and 50kX magnitude micrographs were recorded with secondary electrons detector (SE). In addition, EDS analyses were used to determine the composite electrodes’ chemical composition (amount of C, Ni and O). XPS analysis was carried out for a detailed characterization of the composition. In the complete spectrum of the XPS survey, elements of Ni, C, and O were detected clearly. Moreover, their chemical states were confirmed by the deconvolution of Cs1, Os1, and Ni 2p.
Electrochemical Characterization of Prepared Supercapacitors
As it was mentioned above, CV, EIS and CDC techniques were carried out for determining the capacitive performance of SC cells by using a CH Instrument CHI 660B electrochemical workstation. Three NirGO electrodes with different weight ratios and two rGO and rGO: CB reference electrodes were performed in the three-electrode system. In the electrochemical cell, an aqueous solution of 1M H2SO4 was used as the electrolyte, while platinum (Pt) wire and Ag/AgCl were used counter as and reference electrodes, respectively. As stated in section 2.1., rGO:Binder was used as the standard electrode structure. In addition, a second standard electrode with a higher surface area was prepared by adding CB to the rGO:Binder mixture.
It was aimed to compare the capacitive performances of NirGO-based foamed electrodes and the standard electrodes according to their CV analysis results. All voltammograms were recorded between -0.4 V and +0.6 V at the potential window.
The specific capacitance (\(C_{\text{sp}})\) of NirGO1, NirGO3 and NirGO6-based electrodes were calculated by using the equation below [31].
\begin{equation} C_{\text{sp}}=\frac{i}{\text{ms}}\nonumber \\ \end{equation}
In the equation, \(i,\) \(m,\) and \(s\) represent the average current, the average mass of electrode materials (binder and additive), and the scan rate (\(\Delta V/\Delta t\)), respectively.
With EIS analysis, it is possible to determine the resistance across the entire device, electrode (related to its bulk resistance), and electrode-electrolyte interface (related to the charge transfer). All spectra were obtained with the measurements taken in the 10 MHz-100 kHz. The bulk (Rb) and the charge transfer (Rct) resistance values were calculated from these spectra.
The galvanostatic CDC curves of all electrodes were recorded between -0.4V and +0.6 V potential range under 1 mA current. From these graphs, the specific capacitances (\(\complement\), F g-1) ​​were calculated for each electrode using the formula below.
\begin{equation} \complement=\frac{I*t}{m*V}\nonumber \\ \end{equation}
In the equation, I , the discharge current (A), is multiplied by∆t , the discharge time (s). This value is then divided by the product of ∆V, the potential operating window (V ), andm, the mass of the electrode material (g ). Besides, CDC plots were also used to calculate the energy density (E , Wh kg−1) and the power density (P , W kg−1) by using the following equations:
\(E=\frac{1}{2}C_{\text{sp}}V^{2}\)
\begin{equation} P=\frac{E}{\text{Δt}}\nonumber \\ \end{equation}
In the first equation, the calculated specific capacitance,Csp (F g−1) is multiplied by the potential operating window, V (V), while in the second equation, the calculated Energy Density, E , is divided by the discharging time, Δt (s) represent.
Results and discussion
Structural Characterization of Electrode material
Raman spectroscopy, X-ray diffraction, FT-IR, XPS and EDX analyses were used to elucidate the structure of the composite materials. Due to the best overall capacitive performance achieved by NirGO3, some structural and morphological analyses were reported and evaluated for that active material.
The Raman spectra exhibited in Fig. 1(a) belong to rGO and NirGO3 electrode materials. In the spectra of rGO and NirGO3, the D band corresponding to the defects and the disorders, while the G band corresponding to the vibration of carbon atoms (sp2-bonded) in the hexagonal lattice were observed at 1320 and 1596 cm−1, respectively [32]. Furthermore, peaks at around 545 and 781 cm−1, which correspond to the Ni—OH bond, were also observed in the NirGO3 spectrum. These results indicated that the composite contains oxide/hydroxide forms of Ni rather than Ni(NO3)2 [33].
Fig. 1(b) shows the XRD graph of reference rGO gave a compatible pattern with the literature [31, 34]. The broad peak of (002) graphitic crystal planes in rGO was observed between 20-30°, while the peak belonging to the (100) plane, π-π stacking of graphene, appeared at 44.2° as evidence for the formation of rGO. Ni(OH)2 has two phases, α and β, in the layered hexagonal structure. Although the structure occurred in the β-phase form has high crystallinity and more stability, the large surface area of ​​the α-phase shows better capacitive performance due to its low crystallinity. In the pattern of NirGO, four characteristic peaks appeared at 10.95°, 33.76°, 45.21° and 59.08° belonging to (003), (016), (018), and (110) planes, respectively, belong to the α-phase of Ni(OH)2. However, the peak which must be observed at 20.59° originating from the (006) plane of Ni(OH)2 could not be detected because the graphitic crystal peak in the same region covers it. Furthermore, the disappearance of the sharp peak of the (100) graphene plane and the observed broad peaks of α-Ni(OH)2 indicate that the obtained composite has a highly amorphous character [33, 35]. Increasing the amorphous form in the structure provides higher capacitance with Ni-based composite electrodes. It has also been previously reported that the α-crystal phase gives better capacitive performance [36]. Therefore, it can be stated that the prepared material is suitable for supercapacitor applications in terms of its crystal structure.
Fig. 1(c) illustrates the FT-IR spectra of rGO and NirGO3. rGO presents several oxygen functionalities remaining in rGO [37]. In addition, both electrode materials show very similarly shaped FT-IR spectra, as they have almost the same functional groups except for the Ni—O—H structure. Due to the hydrogen bond-free nature of β-Ni(OH)2, gives a sharp vibration peak at about 3645 cm-─1 corresponding to stretching vibration of O—H. However, for the FT-IR spectrum of α-Ni(OH)2, this peak is either unobservable or rarely detectable as a shoulder due to the high rate of hydrogen bond signal [38, 39]. In the spectrum of NirGO3, there is a relatively broader O—H band, between 3100 cm− 1 and 3650 cm−1, attributed to stretching vibration belong to hydroxyl groups of α-Ni(OH)2 with hydrogen-bonded and H2O intercalated in between rGO layers[26]. In addition, a noticeable redshift was observed for FT-IR peaks of NirGO3, particularly in the strong vibrations of the C—O, C=O and —OH bond, compared to that of rGO. On the other hand, in the rGO‘s spectrum, the peaks at 1557 (C—C stretch) and 1627 cm−1 (C=C stretch) are fused in the FT-IR spectrum NirGO, forming a broad peak appeared around 1614 cm−1 due to the interaction between rGO and Ni(OH)2. The peaks at about 1384, 1081, and 1042 cm−1 belong to C—C, C—O, and C—O—C bonds, respectively[26, 35]. These results also support the Raman analyses.
From the SEM images of rGO, rGO-CB and NirGO3 electrode materials, it was determined that the porous structure of NirGO3 was more evident than that of rGO (Fig. 2(a)-(f)). Although it could not be determined numerically, the pore size distribution of NirGO3 material was more uniform than that of other materials. As a result, it can be proposed that the foam (or sponge) structured morphology is obtained, and the surface area is increased while preparing the electrode material composed from NirGO3. On the other hand, looking at the 50kX image of the rGO electrode material modified with CB, it could be noticed that the morphology was similar to that of rGO. In 250X extended images, however, it was clear that the CB containing sample was more porous than pure rGO. This is because CB is deposited between the rGO sheets, reducing π-π stacking. In the lower (NirGO1) and higher (NirGO6) concentrations of Ni(NO3)2, organic and inorganic phases were separated, and the formation of an ideal foam structure did not occur (Fig. S1(a-d)). In conclusion, SEM analyses show that NirGO3 material has a morphologically significant advantage in capacitor applications.