A hierarchical porous aerogel nanocomposite of graphene/NiCo₂S₄ as an active electrode material for supercapacitors

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1 375.0 W kg and a power density of 26.2 kW kg at 3.7 kWh kg , which suggests this device has great potential applications in energy storage. ly stud becaus s), eas ost re the supercapacitor [10,12]. Recently, many researchers have developed various composites of carbonaceous materials, MOHSs, or/and conducting polymers. cal/insulating ma- an extremely low . In addition, they cal property, large re, aerogels can be supercapacitors. ace areas and rich ercapacitors with pores and relatively low electrical conductivity. This issue can be overcome by combining 3D aerogels with high capacitance mate- rials such as MOHS and conducting polymers. Herein, 3D RGO/NiCo2S4 aerogels were prepared by a one-step hydrothermal approach. The as-prepared aerogels were character- ized by scanning electron microscopy (SEM), transmitted electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron * Corresponding author. E-mail address: hoanv@ntu.edu.vn (N.V. Hoa). Contents lists availab Journal of Science: Advanc .e l Journal of Science: Advanced Materials and Devices 6 (2021) 569e577Peer review under responsibility of Vietnam National University, Hanoi.of EDLC. However, MOHSs and conducting polymers usually have low electric conductivity that results in low specific capacitance of EDLC [11,12,15]. However, the capacitance of graphene aerogels is unsatisfactory at large current densities because of their micro-(MOHSs), carbon-based materials, and conducting polymers [5e9]. Carbonaceous materials store energy through pure electrostatic charge accumulation at electrodeeelectrolyte interfaces through electrical double-layer capacitance (EDLC). In contrast, metal oxides and conductive polymers store energy through rapid and reversible redox or Faradaic charge reactions on the electrode's surface, which is known as a pseudocapacitive electrode. Generally, pseudocapa- citive electrodes can provide higher specific capacitance than that catalyst supports, sorption materials, and electri terials due to their highly porous structure with density and a high specific surface area [13,14] have high electrical conductivity, good mechani surface area, and high chemical stability. Therefo applied as an active electrode material for Recently, 3D graphene materials with large surf pore structures are used as electrodes for supon developing different active electrode materials for super- capacitors based on transition metal oxides/hydroxides/sulfides formance supercapacitor applications. Aerogels have been fabricated for various applications such asNickel-cobalt sulfides Hydrothermal method Aerogel nanocomposites Supercapacitors 1. Introduction Supercapacitors have been wide devices over the last few decades density, long cyclic life (>106 time relatively low cost [1e4]. Therefore, mhttps://doi.org/10.1016/j.jsamd.2021.07.007 2468-2179/© 2021 The Authors. Publishing services b (© 2021 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license ( ied as energy storage e of their high power e of maintenance, and searchers have focused Carbon-based materials often have high conductivity and specific surface area. A larger surface area usually requires a larger capac- itance, but microporosity may dramatically lower the capacitance at large current densities [12]. Therefore, the design and synthesis of unique carbon-based composites with proper pore size and high specific surface area remains challenging to their use in high per-Keywords: GrapheneNiCo2S4 aerogel//RGO) showed a Cs of 45.3 F g at 1.0 A g and a suitable remaining capacitance of over 84.3 % after 2000 cycles. The asymmetric supercapacitor had a high energy density of 40.3 Wh kg
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1Original Article A hierarchical porous aerogel nanocomp an active electrode material for superca Nguyen Van Hoa a, *, Pham Anh Dat a, Nguyen Van a Department of Chemical Engineering, Nha Trang University, 2 Nguyen Dinh Chieu, Nh b Coastal Branch, Russia-Vietnam Tropical Center, 30 Nguyen Thien Thuat, Nha Trang, V a r t i c l e i n f o Article history: Received 25 April 2021 Received in revised form 21 July 2021 Accepted 24 July 2021 Available online 31 July 2021 a b s t r a c t A three-dimensional reduc by an efficient and facile on many small nanoparticles porosity and conductivity. exhibited a large capacitan journal homepage: wwwy Elsevier B.V. on behalf of Vietnamsite of graphene/NiCo2S4 as citors i b, Le Hong Quan b ang, Viet Nam am raphene oxide/nickel-cobalt sulfide (RGO/NiCo2S4) aerogel was fabricated tep hydrothermal approach. The NiCo2S4 needle-like structure consisted of l attached to the surface of the RGO sheet. The prepared aerogel had high an active electrode material for supercapacitors, the RGO/NiCo2S4 aerogel of 813 F g
1 at 1.5 A g
1. Besides, the asymmetric supercapacitor (RGO/ le at ScienceDirect ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license spectroscopy (XPS). In addition, the electrochemical performance of the RGO/NiCo2S4 aerogel supercapacitors were also investigated. 2. Experimental Graphite powder (99.995 %, Alfa Aesar) and the other reagents were of analytical grade and used as received. Graphene oxide (GO) was synthesized via the procedure described in our previous report [16]. In a typical experiment, 45 mg of GO was dispersed in 15 mL of DI water and sonicated in an ultrasound bath for 30 min to obtain a homogeneous dispersion (3 mg/mL). After that, 0.20 mmol of Ni(NO3)2.6H2O and 0.40 mmol of Co(NO3)2.6H2O were dissolved in the solution and stirred for another 30 min. Afterward, 1.20 mmol of thiourea was added into the above mixture under stirring for 30 min. The prepared solution was delivered into a 20 mL PTFE- lined hydrothermal stainless steel reactor and reacted at 180 C for 5 h to obtain a hydrogel composite. Finally, the product was soaked in DI water for one week to remove the unreacted compo- nents and by-products. The RGO/NiCo2S4 aerogels were obtained by the freeze-drying method. For comparison, bare RGO aerogels were prepared by the same procedure using a GO (3 mg/mL) sus- pension without adding nickel-cobalt sulfide precursors. The samples were characterized by SEM (Hitachi, S-4200), TEM (Philips, CM-200), and XPS (ULVAC-PHI electron spectrometer Quantera SXM). All electrochemical tests were carried out on Autolab PGSTAT204N (Metrohm, Netherlands) in three-electrode and two-electrode systems. Platinum foil as the counter and an Ag/AgCl electrode were used as a reference electrode. The working electrode was fabricated by a mixture of 80 wt.% aerogel (2 mg) with 15 wt.% carbon black and 5 wt.% Nafion binder. The paste was pressed on a nickel foam collector (1.0 cm  1.0 cm). All electro- chemical tests were done in a 3 M KOH solution at room temperature. The specific capacitance (Cs, F g
1) was calculated from discharge curves using the following equation: Cs¼ It/mDV,where I, t, m, and DV are discharge current (A), discharge time (s), active material mass (g), and discharging potential (V), respectively. The energy density (E, W h kg
1) and power density (P, kW kg
1) are calculated from the following equations, respectively: E¼ 0.5CsDV2; P ¼ E/t, where Cs, DV, and t are specific capacitance (F g
1), discharge potential (V), and discharge time (s). 3. Results and discussion 3.1. Characteristics of prepared aerogel composites Fig. 1 displays schematics of the GO and the precursors' sus- pension before their addition into the reactor and formation of RGO/NiCo2S4 hydrogels after hydrothermal treatment. The mor- phologies of the samples were observed more clearly by SEM and TEM images. Fig. 2 shows SEM images of bare RGO and RGO/ NiCo2S4 aerogels. The RGO aerogel presented a 3D porous structure, in which interconnected graphene sheets had many distinct edges, wrinkled surfaces, and foldings (Fig. 2a). The sample's morphology showed an excellent distribution of needle-like NiCo2S4 attached to d af N.V. Hoa, P.A. Dat, N. Van Chi et al. Journal of Science: Advanced Materials and Devices 6 (2021) 569e577Fig. 1. Schematics of the suspensions before anFig. 2. SEM images of (a) RGO aerog 570ter hydrothermal treatment to form hydrogels.el and (b) RGO/NiCo2S4 aerogel. N.V. Hoa, P.A. Dat, N. Van Chi et al. Journal of Science: Advanced Materials and Devices 6 (2021) 569e577graphene sheets in 3D network structures (Fig. 2b). This porous structure is believed to enhance electrolyte ion access and trapping into the aerogel-modified electrode surface during supercapacitor charge/discharge processes. The RGO and RGO/NiCo2S4 aerogels were measured by TEM and the morphology is observed more clearly at different magnifications in Fig. 3. Indeed, the RGO/ NiCo2S4 aerogel showed an even deposition of NiCo2S4 nano- needles on the RGO sheets. Interestingly, the NiCo2S4 nanoneedles possessed high porosity and consisted of many smaller nano- particles with a size of approximately 5 nm. The elemental distri- bution of the RGO/NiCo2S4 aerogel is shown in Fig. 4. The high- angle annular dark-field scanning transmission electron micro- scopy (HAADF-STEM) image presents uniformly interconnected NiCo2S4 nanoneedles coated on the RGO sheets (Fig. 4a and b), which is similar to observations made from the TEM images (Fig. 3bed). The elemental mapping showed the K-edge signals of Co, Ni, S, and C (Fig. 4cef). An even distribution of Co, Ni, and S was observed, which confirms a uniform deposition of the NiCo2S4 nanoneedles on the RGO sheets. The XPS spectra was examined to provide surface information of the samples and the elemental oxidation state of the detected ele- ments. Fig. 5 presents the core-level C 1s, Ni 2p, Co 2p, and S 2p peaks. The C1s peak was prominent at 284.5 eV and another weak peak was detected at 288.7 eV (Fig. 5a). The peaks at 284.5 and 288.7 eV correspond to the binding energy of sp2 (CeC bonds) and the characteristic C]O bonds of ketone/carboxylic groups. The Ni 2p peak was best fit to two spineorbit doublets using a Gaussian profile at the low energy band (2p3/2) for Ni3þ, the high energy band (2p1/2) Fig. 3. TEM images of (a) RGO aerogel and (bed) RG 571for Ni2þ, and two shake-up satellites designated as “Sat.” (Fig. 5b). Similarly, the Co 2p peak (Fig. 5c) was best fit to two spineorbit doublets, which are 2p3/2 (Co3þ) and 2p1/2 (Co2þ), and two shake- up satellites [17]. Fig. 5d presented the S 2p core-level peak, including the binding energies of 163.5 eV (S 2p1/2) and 161.9 eV (S 2p3/2). The first peak at 163.5 eV corresponded to sulfur-metal bonds [18,19], while the second peak at 161.9 eV corresponded to the S2
ion of poor coordination at the surface [20]. Therefore, the near- surface of the prepared aerogel consisted of Co3þ, Co2þ, Ni3þ, Ni2þ, and S2
, which agrees with the formula of NiCo2S4. Fig. 6 shows the wide-angle XRD patterns of the RGO/NiCo2S4 aerogel and GO (inset in Fig. 6). GO has a strong peak at a 2q value of 11.6. Inaddition, therewerewell-defineddiffractionpeaksappearing at 2q values of 18.0, 26.7, 31.5, 38.3, 45.4, 50.3, and55.3with hkl values of (111), (220), (311), (400), (422), (511), and (440), respectively, which corresponds to the NiCo2S4 crystallinity (JCPDF Card No. 20e0782, as showed by the brown lines in Fig. 4). There are no clear peaks at a 2q value of 11.6, which confirms the reduction of GO after the hydrothermal treatment. The BET surface area and pore size dis- tributionsof theRGO/NiCo2S4aerogel are showninFig. 7.According to the IUPAC classification, the N2 adsorptionedesorption isotherms (Fig. 7a) exhibited type-IV isotherms with a pronounced hysteresis loop. The BrunauereEmmetteTeller (BET) surface area was about 62.2 m2 g
1. The pore size distributions were determined by Bar- retteJoynereHalenda (BJH) analysis. Fig. 7b presents the corre- sponding BJH pore size distribution plots of the RGO/NiCo2S4 aerogel. The pore distributionwasmainly mesoporous and ranged from 12 to 15 nmwithout any macroporous features. O/NiCo2S4 aerogel at different magnifications. N.V. Hoa, P.A. Dat, N. Van Chi et al.3.2. Electrochemical performance The RGO/NiCo2S4 aerogel was coated onto nickel foam collectors and used as the working electrode (3-electrode system) or the positive electrode (2-electrode system). The electro- chemical measurements were performed for electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronopotentiometry (CP). The specific capacitance (Cs) of different electrodes can be roughly compared using the CV curves. In Fig. 8a, the RGO/NiCo2S4 electrode presents a much higher integrated area than NiCo2S4 and RGO electrodes at the same scan rate of 5 mV s
1. This can be explained as the highly porous and conductive RGO aerogel matrix supported by the highly capacitive NiCo2S4 nanoneedles accelerated Fig. 4. (a,b) HAADF-STEM images of RGO/NiCo2S4 and EDX m 572Journal of Science: Advanced Materials and Devices 6 (2021) 569e577the ion and electron transport, which resulted in an improvement of the electrochemical performance. EIS curves determined the electric conductivity of the electrodes and the electrodeeelectrolyte inter- face. High-frequency semicircle lines in the Nyquist plots are the charge transfer resistance (Rct), while low-frequency lines are capacitance behavior. The equivalent series resistance (ESR) is the intercept of the semicircle at the real axis. The ESR consists of three resistances including (i) the electrolyte solution resistance, (ii) the active material resistance, and (iii) the interface resistance between the active material and the current collector. Fig. 8b presents the EIS plots of RGO, NiCo2S4, and RGO/NiCo2S4 aerogel electrodes at 5 mV over a frequency range of 10 kHz
10 MHz. The ESR values of the RGO/NiCo2S4 aerogel, RGO, and NiCo2S4 are 0.17, 0.23, and 0.47 U, respectively, as seen clearly in the inset plots of Fig. 8b. This figure apping of (c) carbon, (d) cobalt, (e) nickel, and (f) sulfur. N.V. Hoa, P.A. Dat, N. Van Chi et al.confirmed the lowest diffusion resistance and Rct between the active material and the electrolyte in the aerogel composite structure. Besides, the RGO/NiCo2S4 curve's semicircle diameter is much smaller than that of bare RGO and bare NiCo2S4 curves, which indicates a lower charge-transfer resistance in the RGO/NiCo2S4 composite. Fig. 5. XPS results of the core-levels of (a) C 1s, (b) Ni 2p Fig. 6. XRD patterns of RGO/NiCo2S4 aerogel and JCPDS card No. 20e0782. The inset is the XRD pattern of GO. 573Journal of Science: Advanced Materials and Devices 6 (2021) 569e577Fig. 8c presents the CV diagram of the RGO/NiCo2S4 composite scanned from 2 to 100 mV s
1 in the voltage window of
0.1 to 0.5 V. There are two redox peaks in all the CV curves and still appeared at 100 mV s
1. This confirms the reversible Faradaic re- actions and pseudo-capacitive mechanism. The redox peaks come from the redox reactions of Ni3þ/Ni2þ and Co3þ/Co2þ couples based on the following [21e23]: NiS þ OH
4 NiSOH þ e
(1) CoS þ OH
4 CoSOH þ e
(2) CoSOH þ OH
4 CoSO þ H2O þ e
(3) The anodic peaks are the oxidations of NiS to NiSOH or/and CoS to CoSOH or/and CoSOH to CoSO. The cathodic peaks are the reverse reduction reactions. Fig. 8d shows the plots of anodic and cathodic peak currents against the potential scan rates. Both curves are linearly proportional to the scan rate indicating that the pre- pared aerogel had richeredox reactions during the charge-storage process. Moreover, clear redox peaks were observed at a high scan rate (150 mV s
1), which suggests the fast transport of elec- trons and ions as well as the prepared electrode's excellent rate capability [24,25]. Fig. 8e presents the RGO/NiCo2S4 composite electrode discharge plots at a current density range of 1.5e7.5 A g
1. The plateau shape and a lack of IR drop were observed in all discharge curves, which indicated the existence of Faradaic re- actions, excellent electrochemical reversibility, and a rapid IeV response of the RGO/NiCo2S4 electrode [9,26]. Based on the , (c) Co 2p, and (d) S 2p in the RGO/NiCo2S4 aerogel. Fig. 7. (a) BET nitrogen adsorption isotherm plot and (b) the pore size distribution of the RGO/NiCo2S4 aerogel. Fig. 8. (a) CV curves of the different electrodes at a scan rate of 5 mV s
1 in 3 M KOH, (b) Nyquist plots of the various electrodes, (c) CV curves of the RGO/NiCo2S4 aerogel electrode at different scan rates, (d) the plots of anodic and cathodic peak currents against the potential scan rates, (e) galvanostatic discharge plots of the RGO/NiCo2S4 aerogel electrode at different current densities, and (f) the specific capacitance at various current densities. N.V. Hoa, P.A. Dat, N. Van Chi et al. Journal of Science: Advanced Materials and Devices 6 (2021) 569e577 574 galvanostatic discharge plots, the Cs values were calculated to be 813, 720, 576, 430, and 405 F g
1 at current densities of 1.5, 2.0, 3.0, 5.0, and 7.5 A g
1, respectively, as plotted in Fig. 8f. The performance degradation at high current densities may be attributed to the large mesopores of the composite with diameters of 5e40 nm [27] and the partial destruction of the NiCo2S4 nano-needles [28]. The rough comparison of our prepared electrode's electrochemical performance to some previous related reports are shown in Table 1. The Cs value of the RGO/NiCo2S4 aerogel electrode is higher than those of some nickel-cobalt sulfides. We also investigate the electrochemical performance of the RGO/NiCo2S4 electrode for real applications. An asymmetric supercapacitor (ASC) was assembled using a positive active elec- trode (RGO/NiCo2S4 aerogel), a negative electrode (RGO aerogel), Table 1 Comparison of the electrochemical performance between the RGO/NiCo2S4 aerogel electrode and other previously reported related electrodes. Electrode materials Specific capacitance (F g
1) Current density (A g
1) Electrolyte Ref. NiCo2S4/graphene hydrogel 1000 0.5 3 M KOH [30] NiCo2S4/graphene aerogel 704.34 1.0 3 M KOH [31] NiCo2S4/carbon fiber paper 1154 1.0 2 M KOH [20] NiCo2S4/RGO 1000.5 1.0 6 M KOH [32] NiCo2S4-rGO 1107 1.0 6 M KOH [33] NiCo2S4 nanosheets 744 1.0 3 M KOH [34] RGO/NiCo2S4 aerogel 813 1.5 3 M KOH This study Fig. 9. (a) CV plots of the RGO/NiCo2S4//RGO device at various scan rates in 3 M KOH, (b) galvanostatic discharge plots of the RGO/NiCo2S4//RGO device in 3 M KOH solution at various current densities, (c) Ragone plot of this device, and (d) the specific capacitance versus the cyclic number of the RGO/NiCo2S4//RGO device at 5 A g
1 (the inset is Nyquist plots of the device before and after 2000 cycles). Table 2 Comparison of the energy density and power density between this study and previously reported ASC devices. y de N.V. Hoa, P.A. Dat, N. Van Chi et al. Journal of Science: Advanced Materials and Devices 6 (2021) 569e577Positive materials Negative materials Energ NiCo2S4/graphene hydrogel AC 19 NiCo2S4/graphene aerogel AC 20.9 NiCo2S4/carbon fiber paper AC 17.3 NiCo2S4/RGO AC 15.4 NiCo2S4 nanosheets AC 25.5 Ni3S2/MWCNT-NC AC 19.8 NiCo2S4@NiCo2O4@rGO rGO 32 EEG@NiCo2S4 NMCS 30.4 RGO/NiCo2S4 aerogel RGO 40.3 575nsity (Wh kg
1) Power density (W kg
1) Ref. 703 [30] 800.2 [31] 180 [20] 2227.3 [32] 334 [34] 798 [35] 375 [36] 800.0 [37] 375.0 This study [3] D. Feng, X. Pan, Q. Xia, J. Qin, Y. Zhang, X. Chen, Metallic MoS nanosphere N.V. Hoa, P.A. Dat, N. Van Chi et al. Journal of Science: Advanced Materials and Devices 6 (2021) 569e577and an electrolyte (3 M KOH solution). The charge balance of qþ ¼ q