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Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride
Accepted Manuscript Title: Morphology Dependent Supercapacitance of Nanostructured NiCo2 O4 on Graphitic Carbon Nitride Author: Bo Guan Qian Yuan Shan Hao Chen Dongfeng Xue Kunfeng Chen Yu Xin Zhang PII: DOI: Reference:
S0013-4686(16)30758-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.175 EA 27005
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-3-2016 28-3-2016 29-3-2016
Please cite this article as: Bo Guan, Qian Yuan Shan, Hao Chen, Dongfeng Xue, Kunfeng Chen, Yu Xin Zhang, Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.175 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride Bo Guan1, Qian Yuan Shan1, Hao Chen1, Dongfeng Xue2*, Kunfeng Chen2, Yu Xin Zhang1* 1
College of Materials Science and Engineering, National Key Laboratory of Fundamental
Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, China 2
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China *
E-mail: [email protected] (Prof. Dr. D. Xue); [email protected] (Prof. Dr. Y. X.
Zhang)
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Graphical abstract
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Highlights
Nickel Cobaltite/graphitic carbon nitride hybrid nanostructure: Self-assembly
Morphology and crystallinity-controlled synthesis
High specific capacitance of nanoneedle-assembled NiCo2O4/g-C3N4:253 F g-1 at a current density of 2 A g-1
Good cycle stability of nanosheet-assembled NiCo2O4/g-C3N4: 101.4% capacitance retention after 1000 cycles at a current rate of 4A g-1
Abstract Low cost, non-toxic nanomaterials and their controllable synthesis are highly demanded in the field of supercapacitors. In this work, nanoneedle and nanosheet-like porous NiCo2O4 architectures were directly fabricated on graphitic carbon nitride (g-C3N4), indicating structure dependence on their capacitive performances. Electrochemical performances show that nanoneedle-assembled NiCo2O4/g-C3N4 exhibits higher specific capacitance (253 F g-1 at a current density of 2 A g-1), while nanosheet-assembled NiCo2O4/g-C3N4 possesses a better cycling durability (101.4% capacitance after 1000 cycles) in a three-electrode configuration. Further studies indicate that these electrochemical differences are related to the structure and electrochemical impedance of nanocomposites. Keywords: NiCo2O4; Graphitic carbon nitride; Supercapacitor; Energy storage; Nanocomposites
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1.
Introduction
Nowadays, much attention has been paid to the supercapacitors to satisfy the increasing demand for sustainable and renewable power sources in modern electronic industries because of their high power density, excellent pulse charge-discharge characteristics and safe operation [1-8]. It is possible to take diverse methods to create extremely excellent materials for supercapacitors [9-15]. One of the most promising electrode materials studied is RuO2 [16]. However, its large-scale application is hindered by high cost and toxicity. As a result, intensive research has been focused on the preparation of metal hydroxides/oxides which are low cost and nontoxic. For instance, Co(OH)2 [17], Ni(OH)2 [18], and metal oxides, such as MnO2 [19, 20], Co3O4 [21], NiO [22], CuO [23] with various structures have demonstrated their potential applications for supercapacitors. Another notable example is spinel nickel cobaltite (NiCo2O4). Lu and his co-workers firstly showed the application of the NiCo2O4 aerogel with largely enhanced capacitance for supercapacitors, which has been in principle conceived promising as it can offer various advantages, including nontoxicity, excellent electronic conductivity. Nevertheless, NiCo2O4 has its detrimental deficiencies: agglomeration, volume changes and low surface areas [10]. To overcome these obstacles, efforts were focused on incorporating NiCo2O4 with other materials to limit the agglomeration and volume changing. Gupta et al. demonstrated high rate charge−discharge capability by depositing NiCo2O4 film on stainless steel substrate. However, the inflexible/rigid nature of stainless steel substrate prevents it from practical applications in harsh environments. Thus, low cost fabrication of multicomponent structure with highly-accessible surface areas and long service life is still full of challenge [11]. 4
Recently, the natural structure of graphitic carbon nitride (g-C3N4) contains large amount of pyrrolic N “hole” defects in the lattice and doubly bonded nitrogen at the edges of the vacancy has attracted much attention, in which pyrrolic “hole” defects should be responsible for the high rate capability of N-doped graphene due to short-distance orderings formed on edges and profound surface defects during discharging [24]. Furthermore, gC3N4 is inexpensive and nontoxic, and it can be rapidly synthesized by the facile and scalable heating method [25-30]. To make full use of pyrrolic N “hole” defect of g-C3N4 for supercapacitors, it is suitable to directly grow NiCo2O4 on g-C3N4. Meanwhile, Studies have indicated that the electrochemical performances of NiCo2O4 were extremely dependent on their structures, various morphologies such as nanosheets, nanoflowers, nanowires/nanorods and thin films have been reported [31-34]. Among these structures, flower structures have obtained greatly scientific and technological interests due to flower structures can possess large surface area, and then enhance the effective interaction between the electrolyte and the electrode [35]. Although much attention has been paid to NiCo2O4 and its hybrids, not only the preparation of NiCo2O4/g-C3N4 hybrids is scarcely reported, but also the NiCo2O4 hybrids with precisely morphological controls in an effective way still remain to be a significant challenge. Herein, we report the facile synthesis of nanostructured NiCo2O4/g-C3N4 electrodes, which exhibit a synergistic effect on the performance of supercapacitors system, In the NiCo2O4/g-C3N4 hybrids, g-C3N4 serves as substrate to enlarge the surface area, while compound benefitts from high electronic conductivity of NiCo2O4. Notably, two different morphologies (nanoneedles and nanosheets) have been fabricated on g-C3N4 through 5
different reaction conditions. Further electrochemical tests point out that nanoneedleassembled NiCo2O4/g-C3N4 exhibits higher specific capacitance while nanosheet assembled NiCo2O4/g-C3N4 processes a better cycling durability.
2. Experimental section All the chemical reagents were purchased from Alfa Aesar, which were analytical purity and directly used without any further purification. In a typical synthesis, g-C3N4 was prepared by heating thiourea to 550 oC for 3h under static air, as reported in literature [36]. Pure NiCo2O4 was prepared by hydrothermal method. In a emblematical procedure, nickel nitrate (Ni(NO3)26H2O, 0.5 mmol), cobalt nitrate (Co(NO3)26H2O, 1.0 mmol), ammonium fluoride (NH4F, 1.0 mmol) and urea (2.5 mmol) were dissolved in 35 mL deionized water, respectively, after that the mixture was transferred into a Teflon-lined stainless steel autoclave, then heated to 100 oC for 12 h. The products were collected through centrifugation, washed with deionized water and ethanol for several times, after that dried for 12 h at 80 oC. Finally, pure NiCo2O4 was obtained by annealing at 350 oC for 2 h. Initially, the as-obtained g-C3N4 (70 mg) was dispersed into 70 mL ethanol, afterward, nickel nitrate (Ni(NO3)26H2O, 1.0 mmol), cobalt nitrate (Co(NO3)26H2O, 2.0 mmol), ammonium fluoride (NH4F, 2.0 mmol) and urea (5 mmol) were dissolved in 70 mL deionized water, respectively. Subsequently, the above solutions were mixed by ultrasonication, and then heated to 90 oC in an oil bath for 9 h. After the solutions were naturally cooled down to room temperature, the precipitates were collected through centrifugation, and washed with deionized water and ethanol for several times. Later, the 6
products were dried for 12 h at 80 oC, followed by annealing at 350 oC for 2 h. Finally, nanosheet-assembled NiCo2O4/g-C3N4 (350.1 mg, the mole ratio of NiCo2O4/g-C3N4 is about 1.5) was obtained, denoted as NNC-I. In the meantime, nickel nitrate (Ni(NO3)26H2O, 0.5 mmol), cobalt nitrate (Co(NO3)26H2O, 1.0 mmol), ammonium fluoride (NH4F, 1.0 mmol), urea (2.5 mmol) and g-C3N4 (30 mg) were immersed in the 35 ml deionized water. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated to 100 oC for 12 h, the precipitates were collected through centrifugation, and washed with deionized water and ethanol for several times, then dried for 12 h at 80 oC, followed by annealing at 350 oC for 2 h. Finally, nanoneedle-assembled NiCo2O4/g-C3N4 (160.6 mg, the approximate mole ratio of NiCo2O4/g-C3N4 is 1.5) was obtained, denoted as NNC-II. Crystallographic information of the as-prepared samples was obtained by the X-ray diffraction (XRD, D/max1200, CuKα). Microstructures of the NiCo2O4/g-C3N4 hybrids and pure g-C3N4 were measured by focused ion beam (Zeiss Auriga FIB/SEM). The specific surface area was confirmed by nitrogen adsorption/desorption isotherms at 77 K. Thermal properties of the samples were studied from room temperature (30 oC) to 900 oC in air, using
thermogravimetric
analyzer–differential
scanning
calorimeter
(TGA–DSC,
NETZSCHSTA449C). The electrochemical properties of these electrodes were carried out using an electrochemical workstation (CHI 660E) with three-electrode configuration in a 6 M KOH aqueous solution. The working electrodes were fabricated by coating a slurry containing 70wt% of active materials (NNC-I and NNC-II, respectively), 20wt% of acetylene blacks, 7
and 10wt% of polyvinylidene difluoride (PVDF), and then dried at 120 oC for 12 h in a vacuum. Platinum plate was used as counter electrode and saturated calomel electrode (SCE) as reference electrode. The working electrodes were investigated by cyclic voltammetry (CV) technique with the potential range between 0.0 and 0.5 V at various rates between 5-200 mV s-1. Galvanostatic charge-discharge (CC) experiments were performed with current densities ranged from 1 to 10 A g-1 at the potential of 0.0 to 0.4 V. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range between 100 kHz and 0.01 Hz with a perturbation amplitude of 5 mV versus the opencircuit potential. Moreover, the working electrodes (1×1 cm2) were the NNC-I and NNC-II with a mass loading of 3.0 mg cm-2, respectively.
3.
Results and Discussion
NiCo2O4 directly grown on g-C3N4 were achieved by two steps, involving the synthesis of the metal hydroxide in the aqueous solution and a calcination process for the formation of NiCo2O4 in air atmosphere. The basic reactions may be illustrated as follows [37]: Ni2+ + 2Co2+ + 3xF → [NiCo2F3x]3(x-2)
(1)
CO(NH2)2 + H2O → 2NH3 + CO2
(2)
CO2 + H2O → CO32- + 2H+
(3)
NH3·H2O → NH4+ + OH
(4)
[NiCo2F3x]3(x-2) +1.5(2-y)CO32 +3yOH+nH2O→NiCo2(OH)3y(CO3)1.5(2-y)·nH2O+3xF (5) 2NiCo2(OH)3y(CO3)1.5(2-y)·nH2O + O2 → 2NiCo2O4 + (3y+2n)H2O + 3(2-y)CO2 (6) As illustrated in Fig. 1, the metal hydroxide precursor was prepared by reacting both (Ni2+ 8
and Co2+) ions and OH-, resulting in the uniform precipitation of mixed (Ni, Co) precursor on the surface of g-C3N4. The formation process of NiCo2O4 nanosheet on the g-C3N4 probably through the following reactions [37]: CH3CH2OH + H2O → CH3OH2+ + OH or H2O → H+ + OH
(7)
xNi2+ + 2xCo2+ + 6xOH → NixCo2x(OH)6x
(8)
2NixCo2x(OH)6x + xO2 → 2xNiCo2O4 + 6xH2O
(9)
In order to investigate the suitable calcination temperature, thermogravimetric analysis of the metal hydroxide precursors of NNC-I and NNC-II were carried out. As revealed in Fig. 2, both of the precursors experience weight loss in three stages. Both of the initial weight losses (19.8% of NNC-I and 16.9% of NNC-II) below 200 oC are assigned to the loss of adsorbed water and the evaporation of intercalated water molecules. Then, there is a weight loss of 21.4% and 20.8% of NNC-I and NNC-II at around 339 oC, caused by decomposition of Ni-Co hydroxide to NiCo2O4. The results show that it is reasonable to set the calcination temperature of the metal hydroxide precursors of NNC-I and NNC-II at 350 oC. Strangely, there is a constant reduction (10.3% and 10.6% of NNC-I and NNC-II) when the temperature exceeds 400 oC, comparing with other thermogravimetric analysis results [36]. Lotsch et al. have convincingly suggested that the planar cohesion of the g-C3N4 obtained from thermal condensation of organic precursors is mainly contributed by hydrogen bonding between strands of polymeric melon units with NH/NH2 groups, which are not stable enough against oxidation process in air, and will be gradually oxidized away from gC3N4 [36, 38-40]. So there is a constant decrement of weight caused by the thermal oxidation etching of g-C3N4, as the temperature exceeds 400 oC. 9
Fig. 3 shows the wide-angle XRD patterns of the pure NiCo2O4, g-C3N4, NNC-I and NNC-II, respectively. The XRD pattern of g-C3N4 presents a main diffraction peak at 27.4°, which indicates that the as-prepared g-C3N4 is graphite-like carbon nitride with exposed (002) surfaces. The diffraction peaks of as-prepared pure NiCo2O4 are observed at 2θ values of 18.9°, 31.1°, 36.6°, 44.6°, 59.1°, 64.9° and 68.3° [24]. All of these peaks can be successfully indexed to (111), (220), (311), (400), (511), (440) and (531) plane reflections of the spinel NiCo2O4 crystalline structure (JCPDF file no.20-0781; space group: F*3 (202)) [24]. In the XRD patterns of NNC-I and NNC-II, there are obvious (002) diffraction peaks of g-C3N4 except the standard peaks of NiCo2O4, demonstrating that the as-prepared NNC-I and NNC-II are the mixture of graphite-like carbon nitride (g-C3N4) and the spinel NiCo2O4. The morphologies of the as-prepared g-C3N4, NNC-I and NNC-II were analyzed by focused ion beam, as shown in Fig. 4. The effect of reaction conditions on the morphology of NiCo2O4/g-C3N4 hybrids is investigated and the result shows that NNC-I and NNC-II possess different flower structures. Smart hybrids of NiCo2O4 and g-C3N4 are realized through an “self-assembly” processes, where g-C3N4 acts as an effective substrate for the nucleation and subsequently in situ growth of NiCo2O4. As illustrated in the Fig. 4a, the morphology of the obtained g-C3N4 is bulk，but after thermal oxidation etching of g-C3N4 in the calcination process, some g-C3N4 on the edge has been exfoliated effectively. It is clearly seen in Fig.4b-d that for NNC-I the NiCo2O4 nanosheets are staggered on the substrate, while the NiCo2O4 nanoneedles are uniformly distributed on the substrate for NNC-II. It is proposed that both of such flower structures are crucial to enlarge the specific surface area. 10
The specific surface area, average pore size and mesoporous volume are important factors affecting the electrochemical performance [36, 41, 42]. In order to more clearly describe their structures of hybrids, the N2 adsorption/desorption isotherms of NNC-I and NNC-II are depicted in the Fig. 5. The Brunauer-Emmett-Teller (BET) surface area values of NNC-I and NNC-II are calculated to be 77.5 and 105.7 m2 g-1, respectively. Both of them are larger than the BET surface area of pure NiCo2O4 (35.2 m2 g-1) [35]. Distinct hysteresis loop can be observed in the range of 0.5-1.0 p/p0. This result suggests that the as-proposed hybrids have a typical mesoporous structure. The mesoporous structure and large specific surface area not only greatly improve the electrode-electrolyte contact area but also afford enough active sites for electrochemical reaction, which are beneficial for a fast electrochemical reaction. The Barrett-Joyner-Halenda (BJH) pore size distribution (PSD) data (see the Supplementary Information, Figure. S1) further confirm the mesoporous structure of the as proposed hybrids. The pore distributions are relatively 5 nm (NNC-I) and 8 nm (NNC-II), both of them are the optimal pore size for the diffusion of active species in electrode materials [43, 44]. It is expected that NNC-II will have great electrochemical performance. To evaluate the properties of NNC-I and NNC-II as supercapacitor electrodes, the electrochemical performances of different samples were carried out through galvanostatic charge-discharge (CC), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests. The CV curves of NNC-I and NNC-II in three electrode configuration were measured at different scan rates ranged from 5 to 200 mV s-1, and it is obvious that a pair of well-defined redox peaks within 0-0.5 V is visible in all CV curves (Fig. 6a and b). These pairs of peaks are mainly attributed to the faradaic redox reaction 11
described as follows [45]： NiCo2O4 +OH +H2O ↔ NiOOH +2CoOOH +e CoOOH+ OH ↔ CoO2 + H2O +e
(10) (11)
Apparently, all curves exhibit a similar shape, and the current density increases with the increasing scan rate. Even at a high scan rate of 200 mV s-1, the CV curve still shows a pair of redox peaks, indicating that the structure of NNC-I and NNC-II are beneficial to fast redox reaction. As compared with CV curves of NNC-I, NNC-II has more CV curve area showing a better capacitive behavior, proving the hypothesis raised from porous properties of the hybrids. To further evaluate the application potential of the as-synthesized NNC-I and NNC-II as an electrode material for supercapacitors, galvanostatic charge-discharge measurements were carried out between 0 and 0.4 V at various current densities ranging from 1 to 10 A g−1, as shown in Fig. 6c-d, respectively. The specific capacitance can be calculated by the following equation：
Cm
I t mV
(12)
where I is the discharging current, t is the discharging time, V is the potential drop during discharge, and m is the mass of active material in a single electrode. The specific capacitances of NNC-II are 274.8, 253.1, 222.3 and 152.5 F g−1 at 1, 2, 4 and 10 A g−1, respectively (Fig. 6b). In contrast, the specific capacitances of NNC-I are 118.2, 100.6, 69.4 and 25.0 F g−1 at 1, 2, 4 and 10 A g−1, respectively (Figure. S2). This is in agreement with the result of the CV curves. NNC-II possesses such greater specific capacitance, which may be attributed to the structure of the present electrode. Specifically, the main advantages of 12
NNC-II are: i) the flower-like and mesoporous characteristics give rise to very high surface area, and thus provide more electroactive sites for redox reaction; ii) the open space between these nanoneedles can greatly enhance the diffusion kinetics within the electrode; iii) the needle-like structure ensures efficient contact between electrode and electrolyte even at high rates; iv) NNC-II can ensure more excellent electric conductivity, which can be verified by the electrochemical impedance spectroscopy measurement [46-48]. As shown in Fig. 6e, both electrolyte resistance and charge transfer resistance of NNC-II are smaller than that of NNC-I. Specifically, the slopes for NNC-1 are higher than that of NNC-2 at low frequency, indicating that the NNC-II had larger diffusive resistance. The intercept at real part (Z’) represents Rs (0.68 Ω for NNC-I and 0.52Ω for NNC-II) which is the combined resistance of ionic resistance of electrolytes, electronicresistance of electrode materials, and contact resistance at various phase interfaces. The semicircles in the high-frequencyrange correspond to the charge-transfer resistance (Rct), caused by the Faradaic reactions and the electric double-layer capacitance (Cdl) at the electrode/electrolytes interfaces. The Rct values of NNC-I and NNC-II are about 5.48 and 1.56 Ω. Various criteria of electrode materials such as large enough specific capacitance, low resistance, long cycling stability, and low cost, have been widely used to guide the searching for novel supercapacitor system, even many efforts have been done and much work has to be carried out in many aspects [49-53]. Furthermore, the long-term cycling performances of NNC-I and NNC-II at 4 A g−1 are recorded. As shown in Fig. 6f, the specific capacitance of NNC-I increases gradually up to 80 F g− 1 in the course of initial 200 cycles, which can be attributed to the full activation of the present electrode. After a 100013
cycle test, the specific capacitance reaches a high value of 70 F g−1, which is higher than its initial value (69 F g−1) may be benefit from NiCo2O4 nanosheet crisscross on the g-C3N4. As for NNC-II, the decay in specific capacitance of NNC-II after a 1000-cycle test is 10.6%, which is superior than the reported results [5,54].
4. Conclusion In summary, the nanoneedle-like NiCo2O4 and nanosheet-like NiCo2O4 were successfully grown on the g-C3N4 through a facile two-step method, and applied as supercapacitor electrode materials. Our findings on the morpholoy-dependent supercapcitance of nanostructured NiCo2O4 indicate that nanoneedle-like NiCo2O4 can show higher capacitive behaviors than nanosheet-like NiCo2O4. Two main factors which endow nanoneedle-like NiCo2O4 with superior electrochemical performance were proposed: 1) larger surface area and larger pore volume can provide more electron/ion paths for more feasible ion transportion and faster redox reaction; 2) the higher capacitance for nanoneedle-like NiCo2O4 is due to the lower intrinsic resistance of the electroactive materials and contact resistance at the interface between electroactive materials and current collector. Our current work shows that the morphology energineering of inorganic materials is one of key respect in searching for novel electrode materials toward high performance supercapacitors.
Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant nos. 21576034, 51125009, 91434118, 21521092), International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001), National 14
Training Program of Innovation and Entrepreneurship for Undergraduates (201510611102) and Innovation Training Programs for Undergraduates, the External Cooperation Program of BIC (Grant No. 121522KYS820150009), and the Hundred Talents Program, Chinese Academy of Sciences. The authors also thank Electron Microscopy Center of Chongqing University for materials characterizations.
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composite photocatalyst with enhanced photocatalytic activity towards water oxidation and reduction reaction, Journal Materials Chemistry A 3 (2015) 18622-18635. [37] H. Wang, X. Wang, Growing nickel cobaltite nanowires and nanosheets on carbon cloth with different pseudocapacitive performance, ACS Applied Materials & Interfaces 5 (2013) 6255-6260. [38] P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities, Advanced Functional Materials 22 (2012) 4763-4770. [39] M. Kuang, W. Zhang, X.L. Guo, L. Yu, Y.X. Zhang, Template-free and large-scale synthesis of hierarchical dandelion-like NiCo2O4 microspheres for high-performance supercapacitors, Ceramics International 40 (2014) 10005-10011. [40] X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, Enhanced photoresponsive ultrathin graphitic-phase C3N4 nanosheets for bioimaging, Journal of the Amican Chemistry Society 135 (2013) 18-21. [41] H. Xia, C. Hong, X. Shi, B. Li, G. Yuan, Q. Yao, J. Xie, Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors, Journal of Materials Chemistry A, 3 (2015) 1216-1221. [42] J. Liu, M. Zheng, X. Shi, H. Zeng, H. Xia, You have full text access to this content Amorphous FeOOH Quantum Dots Assembled Mesoporous Film Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors, Advanced Functional Materials, 26 (2016) 919-930. [43] R. Ding, L. Qi, M. Jia, H. Wang, Facile and large-scale chemical synthesis of highly porous secondary submicron/micron-sized NiCo2O4 materials for high-performance 21
aqueous hybrid AC-NiCo2O4 electrochemical capacitors, Electrochimica Acta 107 (2013) 494-502. [44] K. Song, W. L. Song, L. Z. Fan, Scalable fabrication of exceptional 3D carbon networks for supercapacitors, Journal of Materials Chemistry A, 3 (2015) 16104-16111. [45] X.Y. Liu, Y.Q. Zhang, X.H. Xia, S.J. Shi, Y. Lu, X.L. Wang, C.D. Gu, J.P. Tu, Selfassembled porous NiCo2O4 hetero-structure array for electrochemical capacitor, Journal of Power Sources 239 (2013) 157-163. [46] L. Huang, D. Chen, Y. Ding, S. Feng, Z.L. Wang, M. Liu, Nickel-cobalt hydroxide nanosheets coated on NiCo2O4 nanowires grown on carbon fiber paper for highperformance pseudocapacitors, Nano Letters 13 (2013) 3135-3139. [47] Z. Zhang, Y. Wang, M. Zhang, Q. Tan, X. Lv, Z. Zhong, F. Su, Mesoporous CoFe2O4 nanospheres cross-linked by carbon nanotubes as high-performance anodes for lithium-ion batteries, Journal of Materials Chemistry A 1 (2013) 7444-7450. [48] R. Zou, K. Xu, T. Wang, G. He, Q. Liu, X. Liu, Z. Zhang, J. Hu, Chain-like NiCo2O4 nanowires with different exposed reactive planes for high-performance supercapacitors, Journal of Materials Chemistry A 1 (2013) 8560-8566. [49] K. Chen, D. Xue, Colloidal ionic supercapcitors, Journal of Electrochemistry 21 (2015) 534-542. [50] K. Chen, D. Xue, Rare earth and transitional metal colloidal supercapacitors, Science China-Technological Sciences 58 (2015) 1768-1778. [51] F. Liu, D. Xue, Electrochemical energy storage applications of “pristine” graphene produced by non-oxidative routes, Science China-Technological Sciences 58 (2015) 184122
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Fig. 1. Schematic illustration for the possible formation mechanism of NNC-I and NNC-II
24
Fig. 2. TGA and DSC curves for the precursors of NNC-I (a) and NNC-II (b)
25
Fig. 3. XRD patterns of g-C3N4, NNC-I, NNC-II and pure NiCo2O4
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Fig. 4. (a) Low and high (the inset) magnification SEM images of the g-C3N4 ; (b) low and high (the inset) magnification SEM images of NNC-I; (c) Low and (d) high magnification SEM images of NNC-II.
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Fig. 5. (a) Nitrogen adsorption and desorption isotherms measured at 77 K for NNC-II and (b) NNC-I. The insets show the corresponding BJH pore size distributions.
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Fig. 6. (a, b) CV curves, (c, d) charge-discharge curves, (e) electrochemical impedance spectra, and (f) variation of capacitance with cycle number at 4 A g-1 of NNC-I (a, c) and NNC-II (b, d).
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S0013-4686(16)30758-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.175 EA 27005
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-3-2016 28-3-2016 29-3-2016
Please cite this article as: Bo Guan, Qian Yuan Shan, Hao Chen, Dongfeng Xue, Kunfeng Chen, Yu Xin Zhang, Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.175 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride Bo Guan1, Qian Yuan Shan1, Hao Chen1, Dongfeng Xue2*, Kunfeng Chen2, Yu Xin Zhang1* 1
College of Materials Science and Engineering, National Key Laboratory of Fundamental
Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, China 2
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China *
E-mail: [email protected] (Prof. Dr. D. Xue); [email protected] (Prof. Dr. Y. X.
Zhang)
1
Graphical abstract
2
Highlights
Nickel Cobaltite/graphitic carbon nitride hybrid nanostructure: Self-assembly
Morphology and crystallinity-controlled synthesis
High specific capacitance of nanoneedle-assembled NiCo2O4/g-C3N4:253 F g-1 at a current density of 2 A g-1
Good cycle stability of nanosheet-assembled NiCo2O4/g-C3N4: 101.4% capacitance retention after 1000 cycles at a current rate of 4A g-1
Abstract Low cost, non-toxic nanomaterials and their controllable synthesis are highly demanded in the field of supercapacitors. In this work, nanoneedle and nanosheet-like porous NiCo2O4 architectures were directly fabricated on graphitic carbon nitride (g-C3N4), indicating structure dependence on their capacitive performances. Electrochemical performances show that nanoneedle-assembled NiCo2O4/g-C3N4 exhibits higher specific capacitance (253 F g-1 at a current density of 2 A g-1), while nanosheet-assembled NiCo2O4/g-C3N4 possesses a better cycling durability (101.4% capacitance after 1000 cycles) in a three-electrode configuration. Further studies indicate that these electrochemical differences are related to the structure and electrochemical impedance of nanocomposites. Keywords: NiCo2O4; Graphitic carbon nitride; Supercapacitor; Energy storage; Nanocomposites
3
1.
Introduction
Nowadays, much attention has been paid to the supercapacitors to satisfy the increasing demand for sustainable and renewable power sources in modern electronic industries because of their high power density, excellent pulse charge-discharge characteristics and safe operation [1-8]. It is possible to take diverse methods to create extremely excellent materials for supercapacitors [9-15]. One of the most promising electrode materials studied is RuO2 [16]. However, its large-scale application is hindered by high cost and toxicity. As a result, intensive research has been focused on the preparation of metal hydroxides/oxides which are low cost and nontoxic. For instance, Co(OH)2 [17], Ni(OH)2 [18], and metal oxides, such as MnO2 [19, 20], Co3O4 [21], NiO [22], CuO [23] with various structures have demonstrated their potential applications for supercapacitors. Another notable example is spinel nickel cobaltite (NiCo2O4). Lu and his co-workers firstly showed the application of the NiCo2O4 aerogel with largely enhanced capacitance for supercapacitors, which has been in principle conceived promising as it can offer various advantages, including nontoxicity, excellent electronic conductivity. Nevertheless, NiCo2O4 has its detrimental deficiencies: agglomeration, volume changes and low surface areas [10]. To overcome these obstacles, efforts were focused on incorporating NiCo2O4 with other materials to limit the agglomeration and volume changing. Gupta et al. demonstrated high rate charge−discharge capability by depositing NiCo2O4 film on stainless steel substrate. However, the inflexible/rigid nature of stainless steel substrate prevents it from practical applications in harsh environments. Thus, low cost fabrication of multicomponent structure with highly-accessible surface areas and long service life is still full of challenge [11]. 4
Recently, the natural structure of graphitic carbon nitride (g-C3N4) contains large amount of pyrrolic N “hole” defects in the lattice and doubly bonded nitrogen at the edges of the vacancy has attracted much attention, in which pyrrolic “hole” defects should be responsible for the high rate capability of N-doped graphene due to short-distance orderings formed on edges and profound surface defects during discharging [24]. Furthermore, gC3N4 is inexpensive and nontoxic, and it can be rapidly synthesized by the facile and scalable heating method [25-30]. To make full use of pyrrolic N “hole” defect of g-C3N4 for supercapacitors, it is suitable to directly grow NiCo2O4 on g-C3N4. Meanwhile, Studies have indicated that the electrochemical performances of NiCo2O4 were extremely dependent on their structures, various morphologies such as nanosheets, nanoflowers, nanowires/nanorods and thin films have been reported [31-34]. Among these structures, flower structures have obtained greatly scientific and technological interests due to flower structures can possess large surface area, and then enhance the effective interaction between the electrolyte and the electrode [35]. Although much attention has been paid to NiCo2O4 and its hybrids, not only the preparation of NiCo2O4/g-C3N4 hybrids is scarcely reported, but also the NiCo2O4 hybrids with precisely morphological controls in an effective way still remain to be a significant challenge. Herein, we report the facile synthesis of nanostructured NiCo2O4/g-C3N4 electrodes, which exhibit a synergistic effect on the performance of supercapacitors system, In the NiCo2O4/g-C3N4 hybrids, g-C3N4 serves as substrate to enlarge the surface area, while compound benefitts from high electronic conductivity of NiCo2O4. Notably, two different morphologies (nanoneedles and nanosheets) have been fabricated on g-C3N4 through 5
different reaction conditions. Further electrochemical tests point out that nanoneedleassembled NiCo2O4/g-C3N4 exhibits higher specific capacitance while nanosheet assembled NiCo2O4/g-C3N4 processes a better cycling durability.
2. Experimental section All the chemical reagents were purchased from Alfa Aesar, which were analytical purity and directly used without any further purification. In a typical synthesis, g-C3N4 was prepared by heating thiourea to 550 oC for 3h under static air, as reported in literature [36]. Pure NiCo2O4 was prepared by hydrothermal method. In a emblematical procedure, nickel nitrate (Ni(NO3)26H2O, 0.5 mmol), cobalt nitrate (Co(NO3)26H2O, 1.0 mmol), ammonium fluoride (NH4F, 1.0 mmol) and urea (2.5 mmol) were dissolved in 35 mL deionized water, respectively, after that the mixture was transferred into a Teflon-lined stainless steel autoclave, then heated to 100 oC for 12 h. The products were collected through centrifugation, washed with deionized water and ethanol for several times, after that dried for 12 h at 80 oC. Finally, pure NiCo2O4 was obtained by annealing at 350 oC for 2 h. Initially, the as-obtained g-C3N4 (70 mg) was dispersed into 70 mL ethanol, afterward, nickel nitrate (Ni(NO3)26H2O, 1.0 mmol), cobalt nitrate (Co(NO3)26H2O, 2.0 mmol), ammonium fluoride (NH4F, 2.0 mmol) and urea (5 mmol) were dissolved in 70 mL deionized water, respectively. Subsequently, the above solutions were mixed by ultrasonication, and then heated to 90 oC in an oil bath for 9 h. After the solutions were naturally cooled down to room temperature, the precipitates were collected through centrifugation, and washed with deionized water and ethanol for several times. Later, the 6
products were dried for 12 h at 80 oC, followed by annealing at 350 oC for 2 h. Finally, nanosheet-assembled NiCo2O4/g-C3N4 (350.1 mg, the mole ratio of NiCo2O4/g-C3N4 is about 1.5) was obtained, denoted as NNC-I. In the meantime, nickel nitrate (Ni(NO3)26H2O, 0.5 mmol), cobalt nitrate (Co(NO3)26H2O, 1.0 mmol), ammonium fluoride (NH4F, 1.0 mmol), urea (2.5 mmol) and g-C3N4 (30 mg) were immersed in the 35 ml deionized water. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated to 100 oC for 12 h, the precipitates were collected through centrifugation, and washed with deionized water and ethanol for several times, then dried for 12 h at 80 oC, followed by annealing at 350 oC for 2 h. Finally, nanoneedle-assembled NiCo2O4/g-C3N4 (160.6 mg, the approximate mole ratio of NiCo2O4/g-C3N4 is 1.5) was obtained, denoted as NNC-II. Crystallographic information of the as-prepared samples was obtained by the X-ray diffraction (XRD, D/max1200, CuKα). Microstructures of the NiCo2O4/g-C3N4 hybrids and pure g-C3N4 were measured by focused ion beam (Zeiss Auriga FIB/SEM). The specific surface area was confirmed by nitrogen adsorption/desorption isotherms at 77 K. Thermal properties of the samples were studied from room temperature (30 oC) to 900 oC in air, using
thermogravimetric
analyzer–differential
scanning
calorimeter
(TGA–DSC,
NETZSCHSTA449C). The electrochemical properties of these electrodes were carried out using an electrochemical workstation (CHI 660E) with three-electrode configuration in a 6 M KOH aqueous solution. The working electrodes were fabricated by coating a slurry containing 70wt% of active materials (NNC-I and NNC-II, respectively), 20wt% of acetylene blacks, 7
and 10wt% of polyvinylidene difluoride (PVDF), and then dried at 120 oC for 12 h in a vacuum. Platinum plate was used as counter electrode and saturated calomel electrode (SCE) as reference electrode. The working electrodes were investigated by cyclic voltammetry (CV) technique with the potential range between 0.0 and 0.5 V at various rates between 5-200 mV s-1. Galvanostatic charge-discharge (CC) experiments were performed with current densities ranged from 1 to 10 A g-1 at the potential of 0.0 to 0.4 V. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range between 100 kHz and 0.01 Hz with a perturbation amplitude of 5 mV versus the opencircuit potential. Moreover, the working electrodes (1×1 cm2) were the NNC-I and NNC-II with a mass loading of 3.0 mg cm-2, respectively.
3.
Results and Discussion
NiCo2O4 directly grown on g-C3N4 were achieved by two steps, involving the synthesis of the metal hydroxide in the aqueous solution and a calcination process for the formation of NiCo2O4 in air atmosphere. The basic reactions may be illustrated as follows [37]: Ni2+ + 2Co2+ + 3xF → [NiCo2F3x]3(x-2)
(1)
CO(NH2)2 + H2O → 2NH3 + CO2
(2)
CO2 + H2O → CO32- + 2H+
(3)
NH3·H2O → NH4+ + OH
(4)
[NiCo2F3x]3(x-2) +1.5(2-y)CO32 +3yOH+nH2O→NiCo2(OH)3y(CO3)1.5(2-y)·nH2O+3xF (5) 2NiCo2(OH)3y(CO3)1.5(2-y)·nH2O + O2 → 2NiCo2O4 + (3y+2n)H2O + 3(2-y)CO2 (6) As illustrated in Fig. 1, the metal hydroxide precursor was prepared by reacting both (Ni2+ 8
and Co2+) ions and OH-, resulting in the uniform precipitation of mixed (Ni, Co) precursor on the surface of g-C3N4. The formation process of NiCo2O4 nanosheet on the g-C3N4 probably through the following reactions [37]: CH3CH2OH + H2O → CH3OH2+ + OH or H2O → H+ + OH
(7)
xNi2+ + 2xCo2+ + 6xOH → NixCo2x(OH)6x
(8)
2NixCo2x(OH)6x + xO2 → 2xNiCo2O4 + 6xH2O
(9)
In order to investigate the suitable calcination temperature, thermogravimetric analysis of the metal hydroxide precursors of NNC-I and NNC-II were carried out. As revealed in Fig. 2, both of the precursors experience weight loss in three stages. Both of the initial weight losses (19.8% of NNC-I and 16.9% of NNC-II) below 200 oC are assigned to the loss of adsorbed water and the evaporation of intercalated water molecules. Then, there is a weight loss of 21.4% and 20.8% of NNC-I and NNC-II at around 339 oC, caused by decomposition of Ni-Co hydroxide to NiCo2O4. The results show that it is reasonable to set the calcination temperature of the metal hydroxide precursors of NNC-I and NNC-II at 350 oC. Strangely, there is a constant reduction (10.3% and 10.6% of NNC-I and NNC-II) when the temperature exceeds 400 oC, comparing with other thermogravimetric analysis results [36]. Lotsch et al. have convincingly suggested that the planar cohesion of the g-C3N4 obtained from thermal condensation of organic precursors is mainly contributed by hydrogen bonding between strands of polymeric melon units with NH/NH2 groups, which are not stable enough against oxidation process in air, and will be gradually oxidized away from gC3N4 [36, 38-40]. So there is a constant decrement of weight caused by the thermal oxidation etching of g-C3N4, as the temperature exceeds 400 oC. 9
Fig. 3 shows the wide-angle XRD patterns of the pure NiCo2O4, g-C3N4, NNC-I and NNC-II, respectively. The XRD pattern of g-C3N4 presents a main diffraction peak at 27.4°, which indicates that the as-prepared g-C3N4 is graphite-like carbon nitride with exposed (002) surfaces. The diffraction peaks of as-prepared pure NiCo2O4 are observed at 2θ values of 18.9°, 31.1°, 36.6°, 44.6°, 59.1°, 64.9° and 68.3° [24]. All of these peaks can be successfully indexed to (111), (220), (311), (400), (511), (440) and (531) plane reflections of the spinel NiCo2O4 crystalline structure (JCPDF file no.20-0781; space group: F*3 (202)) [24]. In the XRD patterns of NNC-I and NNC-II, there are obvious (002) diffraction peaks of g-C3N4 except the standard peaks of NiCo2O4, demonstrating that the as-prepared NNC-I and NNC-II are the mixture of graphite-like carbon nitride (g-C3N4) and the spinel NiCo2O4. The morphologies of the as-prepared g-C3N4, NNC-I and NNC-II were analyzed by focused ion beam, as shown in Fig. 4. The effect of reaction conditions on the morphology of NiCo2O4/g-C3N4 hybrids is investigated and the result shows that NNC-I and NNC-II possess different flower structures. Smart hybrids of NiCo2O4 and g-C3N4 are realized through an “self-assembly” processes, where g-C3N4 acts as an effective substrate for the nucleation and subsequently in situ growth of NiCo2O4. As illustrated in the Fig. 4a, the morphology of the obtained g-C3N4 is bulk，but after thermal oxidation etching of g-C3N4 in the calcination process, some g-C3N4 on the edge has been exfoliated effectively. It is clearly seen in Fig.4b-d that for NNC-I the NiCo2O4 nanosheets are staggered on the substrate, while the NiCo2O4 nanoneedles are uniformly distributed on the substrate for NNC-II. It is proposed that both of such flower structures are crucial to enlarge the specific surface area. 10
The specific surface area, average pore size and mesoporous volume are important factors affecting the electrochemical performance [36, 41, 42]. In order to more clearly describe their structures of hybrids, the N2 adsorption/desorption isotherms of NNC-I and NNC-II are depicted in the Fig. 5. The Brunauer-Emmett-Teller (BET) surface area values of NNC-I and NNC-II are calculated to be 77.5 and 105.7 m2 g-1, respectively. Both of them are larger than the BET surface area of pure NiCo2O4 (35.2 m2 g-1) [35]. Distinct hysteresis loop can be observed in the range of 0.5-1.0 p/p0. This result suggests that the as-proposed hybrids have a typical mesoporous structure. The mesoporous structure and large specific surface area not only greatly improve the electrode-electrolyte contact area but also afford enough active sites for electrochemical reaction, which are beneficial for a fast electrochemical reaction. The Barrett-Joyner-Halenda (BJH) pore size distribution (PSD) data (see the Supplementary Information, Figure. S1) further confirm the mesoporous structure of the as proposed hybrids. The pore distributions are relatively 5 nm (NNC-I) and 8 nm (NNC-II), both of them are the optimal pore size for the diffusion of active species in electrode materials [43, 44]. It is expected that NNC-II will have great electrochemical performance. To evaluate the properties of NNC-I and NNC-II as supercapacitor electrodes, the electrochemical performances of different samples were carried out through galvanostatic charge-discharge (CC), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests. The CV curves of NNC-I and NNC-II in three electrode configuration were measured at different scan rates ranged from 5 to 200 mV s-1, and it is obvious that a pair of well-defined redox peaks within 0-0.5 V is visible in all CV curves (Fig. 6a and b). These pairs of peaks are mainly attributed to the faradaic redox reaction 11
described as follows [45]： NiCo2O4 +OH +H2O ↔ NiOOH +2CoOOH +e CoOOH+ OH ↔ CoO2 + H2O +e
(10) (11)
Apparently, all curves exhibit a similar shape, and the current density increases with the increasing scan rate. Even at a high scan rate of 200 mV s-1, the CV curve still shows a pair of redox peaks, indicating that the structure of NNC-I and NNC-II are beneficial to fast redox reaction. As compared with CV curves of NNC-I, NNC-II has more CV curve area showing a better capacitive behavior, proving the hypothesis raised from porous properties of the hybrids. To further evaluate the application potential of the as-synthesized NNC-I and NNC-II as an electrode material for supercapacitors, galvanostatic charge-discharge measurements were carried out between 0 and 0.4 V at various current densities ranging from 1 to 10 A g−1, as shown in Fig. 6c-d, respectively. The specific capacitance can be calculated by the following equation：
Cm
I t mV
(12)
where I is the discharging current, t is the discharging time, V is the potential drop during discharge, and m is the mass of active material in a single electrode. The specific capacitances of NNC-II are 274.8, 253.1, 222.3 and 152.5 F g−1 at 1, 2, 4 and 10 A g−1, respectively (Fig. 6b). In contrast, the specific capacitances of NNC-I are 118.2, 100.6, 69.4 and 25.0 F g−1 at 1, 2, 4 and 10 A g−1, respectively (Figure. S2). This is in agreement with the result of the CV curves. NNC-II possesses such greater specific capacitance, which may be attributed to the structure of the present electrode. Specifically, the main advantages of 12
NNC-II are: i) the flower-like and mesoporous characteristics give rise to very high surface area, and thus provide more electroactive sites for redox reaction; ii) the open space between these nanoneedles can greatly enhance the diffusion kinetics within the electrode; iii) the needle-like structure ensures efficient contact between electrode and electrolyte even at high rates; iv) NNC-II can ensure more excellent electric conductivity, which can be verified by the electrochemical impedance spectroscopy measurement [46-48]. As shown in Fig. 6e, both electrolyte resistance and charge transfer resistance of NNC-II are smaller than that of NNC-I. Specifically, the slopes for NNC-1 are higher than that of NNC-2 at low frequency, indicating that the NNC-II had larger diffusive resistance. The intercept at real part (Z’) represents Rs (0.68 Ω for NNC-I and 0.52Ω for NNC-II) which is the combined resistance of ionic resistance of electrolytes, electronicresistance of electrode materials, and contact resistance at various phase interfaces. The semicircles in the high-frequencyrange correspond to the charge-transfer resistance (Rct), caused by the Faradaic reactions and the electric double-layer capacitance (Cdl) at the electrode/electrolytes interfaces. The Rct values of NNC-I and NNC-II are about 5.48 and 1.56 Ω. Various criteria of electrode materials such as large enough specific capacitance, low resistance, long cycling stability, and low cost, have been widely used to guide the searching for novel supercapacitor system, even many efforts have been done and much work has to be carried out in many aspects [49-53]. Furthermore, the long-term cycling performances of NNC-I and NNC-II at 4 A g−1 are recorded. As shown in Fig. 6f, the specific capacitance of NNC-I increases gradually up to 80 F g− 1 in the course of initial 200 cycles, which can be attributed to the full activation of the present electrode. After a 100013
cycle test, the specific capacitance reaches a high value of 70 F g−1, which is higher than its initial value (69 F g−1) may be benefit from NiCo2O4 nanosheet crisscross on the g-C3N4. As for NNC-II, the decay in specific capacitance of NNC-II after a 1000-cycle test is 10.6%, which is superior than the reported results [5,54].
4. Conclusion In summary, the nanoneedle-like NiCo2O4 and nanosheet-like NiCo2O4 were successfully grown on the g-C3N4 through a facile two-step method, and applied as supercapacitor electrode materials. Our findings on the morpholoy-dependent supercapcitance of nanostructured NiCo2O4 indicate that nanoneedle-like NiCo2O4 can show higher capacitive behaviors than nanosheet-like NiCo2O4. Two main factors which endow nanoneedle-like NiCo2O4 with superior electrochemical performance were proposed: 1) larger surface area and larger pore volume can provide more electron/ion paths for more feasible ion transportion and faster redox reaction; 2) the higher capacitance for nanoneedle-like NiCo2O4 is due to the lower intrinsic resistance of the electroactive materials and contact resistance at the interface between electroactive materials and current collector. Our current work shows that the morphology energineering of inorganic materials is one of key respect in searching for novel electrode materials toward high performance supercapacitors.
Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant nos. 21576034, 51125009, 91434118, 21521092), International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001), National 14
Training Program of Innovation and Entrepreneurship for Undergraduates (201510611102) and Innovation Training Programs for Undergraduates, the External Cooperation Program of BIC (Grant No. 121522KYS820150009), and the Hundred Talents Program, Chinese Academy of Sciences. The authors also thank Electron Microscopy Center of Chongqing University for materials characterizations.
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Fig. 1. Schematic illustration for the possible formation mechanism of NNC-I and NNC-II
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Fig. 2. TGA and DSC curves for the precursors of NNC-I (a) and NNC-II (b)
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Fig. 3. XRD patterns of g-C3N4, NNC-I, NNC-II and pure NiCo2O4
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Fig. 4. (a) Low and high (the inset) magnification SEM images of the g-C3N4 ; (b) low and high (the inset) magnification SEM images of NNC-I; (c) Low and (d) high magnification SEM images of NNC-II.
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Fig. 5. (a) Nitrogen adsorption and desorption isotherms measured at 77 K for NNC-II and (b) NNC-I. The insets show the corresponding BJH pore size distributions.
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Fig. 6. (a, b) CV curves, (c, d) charge-discharge curves, (e) electrochemical impedance spectra, and (f) variation of capacitance with cycle number at 4 A g-1 of NNC-I (a, c) and NNC-II (b, d).
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