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diatomite conductive architecture for high performance asymmetric supercapacitor
Journal Pre-proofs Full Length Article Manganese dioxide anchored on hierarchical carbon nanotubes/graphene/ diatomite conductive architecture for high performance asymmetric supercapacitor Zhufeng Hu, Kui Ma, Wen Tian, Feifei Wang, Hualian Zhang, Jing He, Kuan Deng, Yu Xin Zhang, Hairong Yue, Junyi Ji PII: DOI: Reference:
S0169-4332(19)33593-7 https://doi.org/10.1016/j.apsusc.2019.144777 APSUSC 144777
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 August 2019 24 October 2019 17 November 2019
Please cite this article as: Z. Hu, K. Ma, W. Tian, F. Wang, H. Zhang, J. He, K. Deng, Y. Xin Zhang, H. Yue, J. Ji, Manganese dioxide anchored on hierarchical carbon nanotubes/graphene/diatomite conductive architecture for high performance asymmetric supercapacitor, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144777
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Manganese
dioxide
anchored
on
hierarchical
carbon
nanotubes/graphene/diatomite conductive architecture for high performance asymmetric supercapacitor
Zhufeng Hu,a, Kui Ma,a Wen Tian,a Feifei Wang,a Hualian Zhang,a Jing He,a Kuan Deng,a Yu Xin Zhang,c Hairong Yue,a Junyi Jia, b,*
a
b
School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China State Key Laboratory of Polymer Materials Engineering, Sichuan University,
Chengdu 610065, P. R. China c
College of Material Science and Engineering, Chongqing University, Chongqing,
400044, P. R. China
* Corresponding author E-mail addresses: Junyi Ji, E-mail: [email protected]
Abstract: Natural abundant and industrial available biomass diatomite is act as the porous template to in-situ grown graphene layer (G), while the carbon nanotubes (CNTs) arrays are subsequently grown on the graphene surface. The seamless anchored 1D CNTs arrays together with 3D interconnected graphene networks can construct conductive hierarchical framework to enhance the electric conductivity. Furthermore, the enlarged effective surface area of the CNTs/G/diatomite can increase the active material loading amount
and
facilitate
the
electrolyte
infiltration.
Therefore,
the
MnO2@CNTs/G/diatomite hybrid exhibits a highest specific capacitance of 264.0 F g1
(based on the MnO2, the value is ~880.0 F g-1), good rate capability and excellent
cyclic
stability.
Moreover,
the
asymmetric
supercapacitor
assembled
by
MnO2@CNTs/G/diatomite and microwave exfoliated graphite oxide reveals a highest energy density of 64.3 W h kg-1 and maximum power density of 19.8 kW kg-1, respectively.
Keywords: Diatomite biomass; CNTs/graphene hybrid; hierarchical conductive structure; MnO2; Energy storage
1
Introduction Supercapacitor has received much attention due to the environmental friendly
properties such as high theoretical power density, fast charge transfer, and structural stability, which is believed can act as an alternative energy devise to replace the fossil fuels [1-4]. However, the development and practice applications of supercapacitors are still hindered by the relatively low energy density and slow electrons transfer rate [5, 6]. Recently, the design and preparation of nanostructured hybrids composed of active materials and conductive networks has become an important research direction to improve the electrochemical performance [7-12]. Among the metal-based active materials, MnO2 has attracted much attention due to its abundant reserves, non-toxicity and low cost, as well as its good theoretical electrochemical properties [13, 14]. However, the bandgap of the energy storage performance between the theoretical value and practical results is still tremendous for the MnO2-based nanostructures. Therefore, the inexpensive manufacturing of the composite material with large surface area hierarchical conductive carbon structure and the MnO2 nanostructure is an effective method for practically applications. Biomasses, as the natural abundant and renewable resources, can act as the templates or carbon sources to fabricate carbon-based conductive networks or active materials with high electrochemical performance [15, 16]. For example, agricultural by-products (carbohydrate) such as bamboo, silk, cotton, or other sustainable biomasses, can be carbonized into activated carbons with high surface area [17-20]. On the other hand, biogenic minerals, such as diatomite, seashell and bone [21, 22], can act as the catalysts
and 3D porous templates to in-situ grow carbon architectures on the surface. Typically, natural abundant and industrially available diatomite with unique 3D silica shell and hierarchical nanostructure can provide sufficient surface area, while the as-grown graphene coating layer is proved can inherited its complex porous structure and improve the conductivity [15, 23]. However, due to the low coating thickness of the graphene layer, the electric conductivity and effective surface area graphene coated diatomite is still limited. Moreover, diatomite is demonstrated can effectively anchor metal-based active materials and preform improved electrochemical performance [2428]. Therefore, rational design of hierarchical carbon architecture with interconnected conductive networks and increased surface area on the biomass-based diatomite is urgent for the practical application. In this work, diatomite is used as the porous template for the in-situ growth of graphene layer, subsequently the CNTs arrays are uniformly grown on the graphene surface to form a hierarchical conductive framework [29, 30]. Finally, nanostructured MnO2 is grown on the CNTs/graphene surface via a one-step hydrothermal process. The structural evolution, electric conductivity as well as the effective surface area of the CNTs/G/diatomite is investigated. Moreover, the energy storage performance of the MnO2@CNTs/G/diatomite hybrid is also tested, and the composite reveals high specific capacitances, good rate capability and excellent cyclic stability. In addition, the asymmetric supercapacitor assembled by MnO2@CNTs/G/diatomite and microwave exfoliated graphite oxide (a-MEGO) also reveals high energy density and power density.
2
Experimental
Section
2.1 Preparation of graphene/diatomite The graphene on diatomite were prepared via chemical vapor deposition process. Firstly, diatomite (0.4 g) was placed in the tubular furnace, and heated to 1050 °C in 40 min under Ar (67.5 sccm) and H2 (7.5 sccm). Secondly, CH4 (1.0 sccm) was added for the graphene deposition under 1050 °C. After 100 min, the CH4 and H2 were cut off and cooled naturally to obtain the G/diatomite. 2.2 Preparation of CNTs/G/diatomite The carbon nanotubes arrays were prepared via seed deposition and CVD process. Seeds were first grown on the surface of the G/diatomite precursor by a hydrothermal route: Ni(NO3)·6H2O (0.4 mM), Co(NO3)·6H2O (0.8 mM) and urea (4.8 mM) were added into 120 mL deionized water to obtain clear solution, then the G/diatomite precursor was added and stirred for 15 min. Afterward, the solution was transferred to a 200 mL autoclave and kept at 120 °C for 2 h. When cooled naturally, the sample was wash with deionized water and dried at 60 °C overnight. After that, the precursor composite was heated to 800 °C in 15 min under Ar (80.0 sccm) and H2 (10.0 sccm). Then C2H2 (5.0 sccm) was added for 1 h CNT growth, then C2H2 flow and H2 was cut off, and the CNTs/G/diatomite is obtained. 2.3 Preparation of MnO2@CNTs/G/diatomite MnO2@CNTs/G/diatomite was prepared via one-pot hydrothermal method. Briefly, KMnO4 (15 mM) was dissolved into 120 mL deionized water under stirring, the CNTs/G/diatomite was then added into the solution and stirred for another 15 min. The
solution was transferred into 200 mL autoclave and kept at 160 °C for 24 h. The interconnected MnO2@CNTs/G/diatomite was obtained, and the final sample was washed with deionized water and dried at 60 °C overnight. 2.4 Material characterization The morphology was observed by scanning electron microscopy (SEM, JEOL JSM7610F Field Emission) and transmission electron microscopy (TEM, FEI Tenia G20). The element distribution and valance state were measured by X-ray photoelectron spectroscope (PHI5000) and energy dispersive X-ray spectrometer (EDS, Oxford). The crystalline structure was received by X-ray diffraction (Cu K radiation). Raman spectrum was measured (Thermal, XRD spectrometer) under 455 nm laser. 2.5 Electrochemical measurements The three-electrode performance was investigated with 1 M Na2SO4 as electrolyte. The composites, Ag/AgCl electrode and platinum plate were act as the working, reference and counter electrode, respectively. The working electrode was prepared by mixing the composite (80 wt.%), carbon black (10 wt.%), and polytetrafluoroethylene (10 wt.%) to form a slurry, and uniformly coated onto graphite foam (1*1.5 cm). The working electrode contains about 1 mg active material. Cyclic voltammetry (CV) and constant current charge-discharge (GCD) measurements were carried out using the CHI 760E electrochemical workstation. Electrochemical impedance spectroscopy (EIS) testing was measured between 100 kHz and 0.01 Hz (M204, Metrohm Autolab). For the asymmetric supercapacitor, a two-electrode system was fabricated using the MnO2@CNTs/G/diatomite composite and the a-MEGO as the positive and negative
electrode, respectively. The mass ratio was calculated by the m+·C+=m－·C－. The CV, GCD and EIS were measured in 1 M Na2SO4 electrolyte. The energy density and power density of the device were calculated followed the equation: E=0.5CV2 and P=E/t, respectively.
3
Results and Discussion The fabrication strategy of the MnO2@CNTs/G/diatomite composite is shown in
Scheme 1. The SiO2 can act as the catalyst and template to decompose the CH4 carbon source and accumulate carbon atoms on the surface to form the graphene layer [15]. Firstly, the porous biomass diatomite is used as the primary substrate and template to deposit graphene on the surface. Subsequently, the uniformly dispersed carbon nanotubes arrays are in-situ grown onto the G/diatomite substrate by bi-metal seeds deposition and subsequent chemical vapor deposition. Finally, the MnO2 nanosheets are uniformly attached on the surface of the CNTs/graphene structure. The biomass based CNTs/G/diatomite composite can form 3D interconnected conductive networks to facilitate the electron transfer, while porous MnO2 architecture uniformly coated on the conductive networks can facilitate the electrolyte infiltration and decrease ions diffusion path.
Scheme 1. Illustration of the MnO2@CNTs/G/diatomite composite fabrication. The structural evolution of the CNTs/G/diatomite is shown in Figure 1. After growth of the graphene layer, the diatomite template successfully preserves the porous structure without obvious morphology change (Figure 1a-c), indicates the graphene layer is
uniformly coated on the diatomite surface. After seeds deposition (Figure 1d-f), cobalt/nickel bi-metal nanoparticle seeds are tightly immobilized on the graphene surface, and the size distribution of the seeds are around 10-50 nm. These metal seeds can act as the crystalline nuclei for the decompose the C2H2 carbon source and in-situ interfacial growth of the CNTs, while the bi-metal seeds and porous diatomite surface can effectively avoid the aggregation of the nanoparticles under high temperature [31]. After CNTs growth, the surface of the diatomite become fluffy, and the intertwined CNTs are anchored on the graphene/diatomite surface (Figure 1g-i). The diameter of the CNTs are around 10-50 nm, which is in line with the diameter of the bi-metal seeds, further demonstrate no aggregation of the seeds occurs during annealing process. The vertically aligned CNTs and 3D porous graphene composite can form an interconnected conductive network to increase the electrons transfer rate and enlarge the exposed active surface area. Therefore, the electric conductivity as well as the active material loading mass of the CNTs/G/diatomite composite can be greatly improved compare with that of the diatomite template.
Figure 1. SEM images of the a-c) G/diatomite, d-f) seeds/G/diatomite and g-i) CNTs/G/diatomite.
During MnO2 growth, KMnO4 can react with the carbon and thus form nucleis on the hierarchical structure, then the MnO2 crystalline can in-situ grown on the CNTs/G surface [14]. After growth of the MnO2 architecture, the entire surface of the CNTs/G/diatomite composite is encapsulated by the MnO2 coating layer (Figure 2a). The MnO2 nanosheets with diameter around 100-300 nm are uniformly anchored on the CNTs and the graphene surface (Figure 2b-d), indicates the uniformly nucleation and well growth of the MnO2 nanosheets. The rare amount of the MnO2 nanowires on the surface of the CNTs/G/diatomite may due to the deposition of the nanowires grown in the solution. Moreover, due to the enlarged active surface area of the CNTs/G/diatomite substrate, the MnO2 nanosheets array coating amount can be increased without hinder the electrons/ions transfer efficiency [32-34]. The loading percentage of the MnO2 on the MnO2@CNTs/G/diatomite hybrid is measured to be 30% (Table S1). In addition, the SEM and the corresponding EDS mapping images of MnO2@CNTs/G/diatomite composite confirms the uniformly distribution of the Mn, O and Si element, also reveals the uniform encapsulation of the MnO2 nanosheets.
Figure 2. a) SEM images of the MnO2@CNTs/G/diatomite composite under different magnification, b) SEM image and the corresponding EDS mapping images. Transmission electron microscopy (TEM) are applied to further understand the morphology and crystalline structure of the MnO2 grown on CNTs/G/diatomite substrate. As shown in Figure 3a, both nanosheet and nanowire are detected (illustrated by the red arrows), further confirms the co-exist of the nanosheets and the nanowires on the composite surface. The ultrathin MnO2 nanosheet shows a polycrystalline nature, the plenty of nanocrystals with diameter around 10 nm are suitable for the infiltration and diffusion of the electrolyte ions (Figure 3b, c). Moreover, the interlinear spacing of the nanosheet crystal is calculated to be 0.324 and 0.255 nm (Figure 3d), which is corresponds to the (201) and (301) plane of the MnO2 crystalline, respectively.
Figure 3. TEM images of the MnO2 nanostructure on the MnO2@CNTs/G/diatomite and the SAED pattern (the inset in 3d).
To further understand the crystalline feature, X-ray diffraction (XRD) patterns of the as-prepared composites are shown in Figure 4a. The diatomite, G/diatomite and CNTs/G/diatomite show similar XRD patterns, indicating the crystalline structure of the diatomite remains unchanged after the CVD growth of graphene and CNTs. Furthermore, the diffraction peaks located at 27.7° and 37.2° can be indexed to the (201) and (301) phase of the MnO2 crystalline, respectively (JCPDS 39-0375), which is in line with the SAED pattern [35]. Moreover, the Raman is also carried out to verify the crystalline structure of the composites (Figure 4b). Both the G/diatomite and CNTs/G/diatomite composites reveal the main characteristic of D, G and 2D peaks centered at 1359 cm-1, 1582 cm-1 and 2714 cm-1, respectively, indicates the successfully growth of the high-quality graphene layer on the diatomite. The small 2D/G intensity
ratio and appearance of the D peak reveals the graphene layer is multi-layered with structural defects [36]. After the growth of CNTs, the intensity of the ID/IG peak increases slightly, further confirms the growth of the high-quality CNTs arrays. Furthermore, the Raman spectra of the MnO2@CNTs/G/diatomite and the MnO2@diatomite reveal new strong peak around 600~700 cm-1, which is corresponds to the Mn-O stretching vibration [37]. Therefore, both the Raman and XRD indicate the fabrication of the MnO2@CNTs/G/diatomite composite with relatively good crystalline structure.
Figure 4. a) XRD and b) Raman survey spectrum of the pristine diatomite, G/diatomite, CNTs/G/diatomite, MnO2@CNTs/G/diatomite and MnO2@G/diatomite, respectively. XPS spectrum of the c) full survey, d) C 1s, e) O 1s, f) Mn 2p of the MnO2@CNTs/G/diatomite.
To understand the elemental composition and valence states of the composite, X-ray photoelectron spectroscopy (XPS) is also conducted. The full survey spectrum (Figure 4c) of the MnO2@CNTs/G/diatomite reveals the presence of the C, O and Mn, confirms the successfully grown of the MnO2 architecture on the CNTs/G/diatomite surface. The
deconvolution of the C 1s spectrum (Figure 4d) reveals the main peaks located at 284.8 eV, 285.8 eV and 291.2 eV, corresponding to the C-C, C-O and C=O groups, respectively. The existence of the C-O and C=O groups may due to the partially surface oxidation during hydrothermal reaction. Furthermore, four convoluted O 1s peaks located at 529.9 eV, 531.8 eV, 532.7 eV, 533.4 eV are related to the Mn-O-Mn, Mn-OH, H-O-H and C-O/C=O, respectively (Figure 4e) [38-41]. The convoluted Mn 2p spectra is shown in Figure 4f, the main peaks centered at 642.2 eV and 654.0 eV exhibits a spin-energy separation of 11.8 eV, implying common feature of the MnO2 [42,44]. The electrochemical performance of the MnO2@CNTs/G/diatomite is evaluated in a three-electrode system. The cyclic voltammetry (CV) curves of different diatomitebased samples are measured at 50 mV s-1 under 0-1 V (Figure 5a). The enclosed area of the MnO2@CNTs/G/diatomite curve is obviously larger than that of the other composites, indicates the relatively higher specific capacity. The galvanostatic chargedischarge (GCD) tests are conducted at 0.5 A g-1 (Figure 5b), the quasi-rectangular GCD curves of the electrodes reveals the fast electron/ion diffusion mechanism. The gravimetric
capacitance
of
MnO2@CNTs/G/diatomite,
MnO2@G/diatomite,
G/diatomite and diatomite are calculated to be 264.0 F g-1, 142.4 F g-1, 28.6 F g-1 and 18.1 F g-1, respectively (based on the total mass of the composites). The specific capacitance of the MnO2@CNTs/G/diatomite electrode is comparable or better than most of the MnO2-based electrodes (Table S2). Moreover, the gravimetric capacitance can reach 880.0 F g-1 when calculated based on the MnO2 loading mass (Figure 5f),
which is close to the theoretical capacitance value of the MnO2. Furthermore, the rate capability reveals a capacitance retention of 65.84% for the MnO2@CNTs/G/diatomite hybrid when the current density increases from 0.5 to 5 A g-1 (Figure 5c), while only 27.83%, 25.97% and 58.69% capacitance retention can be reached for the diatomite, G/diatomite and MnO2/G/diatomite, respectively. In addition, the long-term cyclic stability of the MnO2@CNTs/G/diatomite hybrid reveals 84.26% capacitance retention after 4000 cycles (Figure 5e), and the nanosheet morphology and crystalline structure of the MnO2 also remain unchanged (Figure S2). The Nyquist plots of the diatomite, G/diatomite, MnO2/G/diatomite and MnO2/CNTs/G/diatomite
hybrids
are
exhibited
in
Figure
5d.
The
MnO2/CNTs/G/diatomite electrode exhibits a more vertical line in low frequency region, indicating the higher ions/electrons transfer rate and better capacitive behavior. On the other hand, the smallest ESR semi-cycle of the composite also reveals the lowest charge transfer resistance at the electrode/electrolyte interface. Therefore, the enhanced electrochemical performance of the MnO2/CNTs/G/diatomite hybrid may due to: 1) the in-situ grown CNTs/G hierarchical conductive framework on the bio-based diatomite structure can form a seamless electron transfer pathway; 2) the enlarged surface area of the 3D framework can increase the MnO2 nanostructure loading mass without hinder the electrolyte infiltration and diffusion, thus the electrons/ions mass transfer rate can be enhanced.
Figure 5. a) CV curves under 50 mV s-1, b) galvanostatic charge–discharge under 0.5 A g-1, c) rate capability and d) Nyquist plots of the diatomite, G/diatomite, MnO2/G/diatomite and MnO2/CNTs/G/diatomite electrodes. e) The cyclic stability of the MnO2/CNTs/G/diatomite, f) gravimetric capacitances calculated based on the MnO2 loadings mass under different current density.
Furthermore, an asymmetric supercapacitor (ASCs) is fabricated using the MnO2/CNTs/G/diatomite composite and a-MEGO (Figure S3) as positive and negative electrodes, respectively. As shown in Figure 6a, the CV curves of the MnO2/CNTs/G/diatomite//a-MEGO ASCs reveal potential window enlargement, when the operating voltages arise to 2 V, obvious polarization curve is observed. Therefore, the operating voltage is fixed to 1.8 V for further performance evaluation. The quasirectangular CV curves in Figure 6b is in line with that of the MnO2/CNTs/G/diatomite and a-MEGO electrodes. The specific capacitance of the MnO2/CNTs/G/diatomite//aMEGO calculated from the GCD curves in Figure 6c can reaches a high value of 143.0 F g-1 at 0.5 A g-1 based on the total electrode mass (MnO2/CNTs/G/diatomite and aMEGO). Moreover, the capacitance retention of the ASCs can remain 84.59 % after 5000 cycles at 10 A g-1 (Figure 6e), indicating relatively good cyclic stability of the
device. The Nyquist plots shown in Figure 6d reveals a low charge transfer resistance at the electrode/electrolyte interface. The Ragone plots of the ASCs device are also calculated (Figure 6d), a highest energy density of 64.4 W h kg-1 (at power density of 451.5 W kg-1) and a maximum power density of 19.8 kW kg-1 (at energy density of 6.7 W h kg-1) can be reached, which is comparable or superior than many MnO2-based asymmetric supercapacitor [14, 25, 38,44-48].
Figure 6. a) CV curves under various voltage windows, b) CV curves, c) GCD curves, d) EIS curves, e) cyclic capability and f) Ragone plots of the MnO2/CNTs/G/diatomite//a-MEGO ASCs.
4
Conclusions In summary, natural abundant biomass diatomite is act as the porous template to in-
situ grown graphene layer, while the CNTs arrays are subsequently grown on the graphene surface. The seamless grown 1D CNTs arrays together with 3D interconnected graphene can form conductive hierarchical framework to facilitate the electric conductivity. Furthermore, the enlarged effective surface area of the
CNTs/G/diatomite can increase the loading mass and facilitate the electrolyte infiltration. Thus, the MnO2@CNTs/G/diatomite hybrid exhibits a highest specific capacitance of 264.0 F g-1 based on the total mass (based on the MnO2, the value can reach 880.0 F g-1), good rate capability and excellent cyclic stability (capacitance retention of 84.26% after 4000 cycles). The MnO2@CNTs/G/diatomite//a-MEGO asymmetric supercapacitor reveals the maximum energy density of 64.3 W h kg-1 and highest power density of 19.8 kW kg-1, respectively. This cost-effective and biomassbased substrate fabrication method can be massive produced and offers promise for energy storage and conversion applications.
Author Information Corresponding Author *Junyi Ji, E-Mail: [email protected]
Notes The authors declare no competing interest.
Acknowledgment The financial support from the National Natural Science Foundation of China (21776187, 21490582, 21978178), the State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2017-3-01), the Fundamental Research Funds for the Central Universities is appreciated.
Appendix A. Supplementary material Supplementary data to this article (three-electrode test of the as-prepared composite under KOH electrolyte, SEM and TEM images, a-MEGO under Na2SO4 electrolyte and ICP measurement of the composite) can be found online.
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Highlights Cost-effective 3D porous diatomite biomass is used to fabricate conductive networks. The CNTs/graphene hybrid can enhance the electric conductivity and surface area. The MnO2@CNTs/G/diatomite shows high specific capacitance and good cyclic stability. The ASC exhibits high energy density (64.4 W h kg-1) and power density (19.8 kW kg-1).
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0169-4332(19)33593-7 https://doi.org/10.1016/j.apsusc.2019.144777 APSUSC 144777
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 August 2019 24 October 2019 17 November 2019
Please cite this article as: Z. Hu, K. Ma, W. Tian, F. Wang, H. Zhang, J. He, K. Deng, Y. Xin Zhang, H. Yue, J. Ji, Manganese dioxide anchored on hierarchical carbon nanotubes/graphene/diatomite conductive architecture for high performance asymmetric supercapacitor, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144777
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Manganese
dioxide
anchored
on
hierarchical
carbon
nanotubes/graphene/diatomite conductive architecture for high performance asymmetric supercapacitor
Zhufeng Hu,a, Kui Ma,a Wen Tian,a Feifei Wang,a Hualian Zhang,a Jing He,a Kuan Deng,a Yu Xin Zhang,c Hairong Yue,a Junyi Jia, b,*
a
b
School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China State Key Laboratory of Polymer Materials Engineering, Sichuan University,
Chengdu 610065, P. R. China c
College of Material Science and Engineering, Chongqing University, Chongqing,
400044, P. R. China
* Corresponding author E-mail addresses: Junyi Ji, E-mail: [email protected]
Abstract: Natural abundant and industrial available biomass diatomite is act as the porous template to in-situ grown graphene layer (G), while the carbon nanotubes (CNTs) arrays are subsequently grown on the graphene surface. The seamless anchored 1D CNTs arrays together with 3D interconnected graphene networks can construct conductive hierarchical framework to enhance the electric conductivity. Furthermore, the enlarged effective surface area of the CNTs/G/diatomite can increase the active material loading amount
and
facilitate
the
electrolyte
infiltration.
Therefore,
the
MnO2@CNTs/G/diatomite hybrid exhibits a highest specific capacitance of 264.0 F g1
(based on the MnO2, the value is ~880.0 F g-1), good rate capability and excellent
cyclic
stability.
Moreover,
the
asymmetric
supercapacitor
assembled
by
MnO2@CNTs/G/diatomite and microwave exfoliated graphite oxide reveals a highest energy density of 64.3 W h kg-1 and maximum power density of 19.8 kW kg-1, respectively.
Keywords: Diatomite biomass; CNTs/graphene hybrid; hierarchical conductive structure; MnO2; Energy storage
1
Introduction Supercapacitor has received much attention due to the environmental friendly
properties such as high theoretical power density, fast charge transfer, and structural stability, which is believed can act as an alternative energy devise to replace the fossil fuels [1-4]. However, the development and practice applications of supercapacitors are still hindered by the relatively low energy density and slow electrons transfer rate [5, 6]. Recently, the design and preparation of nanostructured hybrids composed of active materials and conductive networks has become an important research direction to improve the electrochemical performance [7-12]. Among the metal-based active materials, MnO2 has attracted much attention due to its abundant reserves, non-toxicity and low cost, as well as its good theoretical electrochemical properties [13, 14]. However, the bandgap of the energy storage performance between the theoretical value and practical results is still tremendous for the MnO2-based nanostructures. Therefore, the inexpensive manufacturing of the composite material with large surface area hierarchical conductive carbon structure and the MnO2 nanostructure is an effective method for practically applications. Biomasses, as the natural abundant and renewable resources, can act as the templates or carbon sources to fabricate carbon-based conductive networks or active materials with high electrochemical performance [15, 16]. For example, agricultural by-products (carbohydrate) such as bamboo, silk, cotton, or other sustainable biomasses, can be carbonized into activated carbons with high surface area [17-20]. On the other hand, biogenic minerals, such as diatomite, seashell and bone [21, 22], can act as the catalysts
and 3D porous templates to in-situ grow carbon architectures on the surface. Typically, natural abundant and industrially available diatomite with unique 3D silica shell and hierarchical nanostructure can provide sufficient surface area, while the as-grown graphene coating layer is proved can inherited its complex porous structure and improve the conductivity [15, 23]. However, due to the low coating thickness of the graphene layer, the electric conductivity and effective surface area graphene coated diatomite is still limited. Moreover, diatomite is demonstrated can effectively anchor metal-based active materials and preform improved electrochemical performance [2428]. Therefore, rational design of hierarchical carbon architecture with interconnected conductive networks and increased surface area on the biomass-based diatomite is urgent for the practical application. In this work, diatomite is used as the porous template for the in-situ growth of graphene layer, subsequently the CNTs arrays are uniformly grown on the graphene surface to form a hierarchical conductive framework [29, 30]. Finally, nanostructured MnO2 is grown on the CNTs/graphene surface via a one-step hydrothermal process. The structural evolution, electric conductivity as well as the effective surface area of the CNTs/G/diatomite is investigated. Moreover, the energy storage performance of the MnO2@CNTs/G/diatomite hybrid is also tested, and the composite reveals high specific capacitances, good rate capability and excellent cyclic stability. In addition, the asymmetric supercapacitor assembled by MnO2@CNTs/G/diatomite and microwave exfoliated graphite oxide (a-MEGO) also reveals high energy density and power density.
2
Experimental
Section
2.1 Preparation of graphene/diatomite The graphene on diatomite were prepared via chemical vapor deposition process. Firstly, diatomite (0.4 g) was placed in the tubular furnace, and heated to 1050 °C in 40 min under Ar (67.5 sccm) and H2 (7.5 sccm). Secondly, CH4 (1.0 sccm) was added for the graphene deposition under 1050 °C. After 100 min, the CH4 and H2 were cut off and cooled naturally to obtain the G/diatomite. 2.2 Preparation of CNTs/G/diatomite The carbon nanotubes arrays were prepared via seed deposition and CVD process. Seeds were first grown on the surface of the G/diatomite precursor by a hydrothermal route: Ni(NO3)·6H2O (0.4 mM), Co(NO3)·6H2O (0.8 mM) and urea (4.8 mM) were added into 120 mL deionized water to obtain clear solution, then the G/diatomite precursor was added and stirred for 15 min. Afterward, the solution was transferred to a 200 mL autoclave and kept at 120 °C for 2 h. When cooled naturally, the sample was wash with deionized water and dried at 60 °C overnight. After that, the precursor composite was heated to 800 °C in 15 min under Ar (80.0 sccm) and H2 (10.0 sccm). Then C2H2 (5.0 sccm) was added for 1 h CNT growth, then C2H2 flow and H2 was cut off, and the CNTs/G/diatomite is obtained. 2.3 Preparation of MnO2@CNTs/G/diatomite MnO2@CNTs/G/diatomite was prepared via one-pot hydrothermal method. Briefly, KMnO4 (15 mM) was dissolved into 120 mL deionized water under stirring, the CNTs/G/diatomite was then added into the solution and stirred for another 15 min. The
solution was transferred into 200 mL autoclave and kept at 160 °C for 24 h. The interconnected MnO2@CNTs/G/diatomite was obtained, and the final sample was washed with deionized water and dried at 60 °C overnight. 2.4 Material characterization The morphology was observed by scanning electron microscopy (SEM, JEOL JSM7610F Field Emission) and transmission electron microscopy (TEM, FEI Tenia G20). The element distribution and valance state were measured by X-ray photoelectron spectroscope (PHI5000) and energy dispersive X-ray spectrometer (EDS, Oxford). The crystalline structure was received by X-ray diffraction (Cu K radiation). Raman spectrum was measured (Thermal, XRD spectrometer) under 455 nm laser. 2.5 Electrochemical measurements The three-electrode performance was investigated with 1 M Na2SO4 as electrolyte. The composites, Ag/AgCl electrode and platinum plate were act as the working, reference and counter electrode, respectively. The working electrode was prepared by mixing the composite (80 wt.%), carbon black (10 wt.%), and polytetrafluoroethylene (10 wt.%) to form a slurry, and uniformly coated onto graphite foam (1*1.5 cm). The working electrode contains about 1 mg active material. Cyclic voltammetry (CV) and constant current charge-discharge (GCD) measurements were carried out using the CHI 760E electrochemical workstation. Electrochemical impedance spectroscopy (EIS) testing was measured between 100 kHz and 0.01 Hz (M204, Metrohm Autolab). For the asymmetric supercapacitor, a two-electrode system was fabricated using the MnO2@CNTs/G/diatomite composite and the a-MEGO as the positive and negative
electrode, respectively. The mass ratio was calculated by the m+·C+=m－·C－. The CV, GCD and EIS were measured in 1 M Na2SO4 electrolyte. The energy density and power density of the device were calculated followed the equation: E=0.5CV2 and P=E/t, respectively.
3
Results and Discussion The fabrication strategy of the MnO2@CNTs/G/diatomite composite is shown in
Scheme 1. The SiO2 can act as the catalyst and template to decompose the CH4 carbon source and accumulate carbon atoms on the surface to form the graphene layer [15]. Firstly, the porous biomass diatomite is used as the primary substrate and template to deposit graphene on the surface. Subsequently, the uniformly dispersed carbon nanotubes arrays are in-situ grown onto the G/diatomite substrate by bi-metal seeds deposition and subsequent chemical vapor deposition. Finally, the MnO2 nanosheets are uniformly attached on the surface of the CNTs/graphene structure. The biomass based CNTs/G/diatomite composite can form 3D interconnected conductive networks to facilitate the electron transfer, while porous MnO2 architecture uniformly coated on the conductive networks can facilitate the electrolyte infiltration and decrease ions diffusion path.
Scheme 1. Illustration of the MnO2@CNTs/G/diatomite composite fabrication. The structural evolution of the CNTs/G/diatomite is shown in Figure 1. After growth of the graphene layer, the diatomite template successfully preserves the porous structure without obvious morphology change (Figure 1a-c), indicates the graphene layer is
uniformly coated on the diatomite surface. After seeds deposition (Figure 1d-f), cobalt/nickel bi-metal nanoparticle seeds are tightly immobilized on the graphene surface, and the size distribution of the seeds are around 10-50 nm. These metal seeds can act as the crystalline nuclei for the decompose the C2H2 carbon source and in-situ interfacial growth of the CNTs, while the bi-metal seeds and porous diatomite surface can effectively avoid the aggregation of the nanoparticles under high temperature [31]. After CNTs growth, the surface of the diatomite become fluffy, and the intertwined CNTs are anchored on the graphene/diatomite surface (Figure 1g-i). The diameter of the CNTs are around 10-50 nm, which is in line with the diameter of the bi-metal seeds, further demonstrate no aggregation of the seeds occurs during annealing process. The vertically aligned CNTs and 3D porous graphene composite can form an interconnected conductive network to increase the electrons transfer rate and enlarge the exposed active surface area. Therefore, the electric conductivity as well as the active material loading mass of the CNTs/G/diatomite composite can be greatly improved compare with that of the diatomite template.
Figure 1. SEM images of the a-c) G/diatomite, d-f) seeds/G/diatomite and g-i) CNTs/G/diatomite.
During MnO2 growth, KMnO4 can react with the carbon and thus form nucleis on the hierarchical structure, then the MnO2 crystalline can in-situ grown on the CNTs/G surface [14]. After growth of the MnO2 architecture, the entire surface of the CNTs/G/diatomite composite is encapsulated by the MnO2 coating layer (Figure 2a). The MnO2 nanosheets with diameter around 100-300 nm are uniformly anchored on the CNTs and the graphene surface (Figure 2b-d), indicates the uniformly nucleation and well growth of the MnO2 nanosheets. The rare amount of the MnO2 nanowires on the surface of the CNTs/G/diatomite may due to the deposition of the nanowires grown in the solution. Moreover, due to the enlarged active surface area of the CNTs/G/diatomite substrate, the MnO2 nanosheets array coating amount can be increased without hinder the electrons/ions transfer efficiency [32-34]. The loading percentage of the MnO2 on the MnO2@CNTs/G/diatomite hybrid is measured to be 30% (Table S1). In addition, the SEM and the corresponding EDS mapping images of MnO2@CNTs/G/diatomite composite confirms the uniformly distribution of the Mn, O and Si element, also reveals the uniform encapsulation of the MnO2 nanosheets.
Figure 2. a) SEM images of the MnO2@CNTs/G/diatomite composite under different magnification, b) SEM image and the corresponding EDS mapping images. Transmission electron microscopy (TEM) are applied to further understand the morphology and crystalline structure of the MnO2 grown on CNTs/G/diatomite substrate. As shown in Figure 3a, both nanosheet and nanowire are detected (illustrated by the red arrows), further confirms the co-exist of the nanosheets and the nanowires on the composite surface. The ultrathin MnO2 nanosheet shows a polycrystalline nature, the plenty of nanocrystals with diameter around 10 nm are suitable for the infiltration and diffusion of the electrolyte ions (Figure 3b, c). Moreover, the interlinear spacing of the nanosheet crystal is calculated to be 0.324 and 0.255 nm (Figure 3d), which is corresponds to the (201) and (301) plane of the MnO2 crystalline, respectively.
Figure 3. TEM images of the MnO2 nanostructure on the MnO2@CNTs/G/diatomite and the SAED pattern (the inset in 3d).
To further understand the crystalline feature, X-ray diffraction (XRD) patterns of the as-prepared composites are shown in Figure 4a. The diatomite, G/diatomite and CNTs/G/diatomite show similar XRD patterns, indicating the crystalline structure of the diatomite remains unchanged after the CVD growth of graphene and CNTs. Furthermore, the diffraction peaks located at 27.7° and 37.2° can be indexed to the (201) and (301) phase of the MnO2 crystalline, respectively (JCPDS 39-0375), which is in line with the SAED pattern [35]. Moreover, the Raman is also carried out to verify the crystalline structure of the composites (Figure 4b). Both the G/diatomite and CNTs/G/diatomite composites reveal the main characteristic of D, G and 2D peaks centered at 1359 cm-1, 1582 cm-1 and 2714 cm-1, respectively, indicates the successfully growth of the high-quality graphene layer on the diatomite. The small 2D/G intensity
ratio and appearance of the D peak reveals the graphene layer is multi-layered with structural defects [36]. After the growth of CNTs, the intensity of the ID/IG peak increases slightly, further confirms the growth of the high-quality CNTs arrays. Furthermore, the Raman spectra of the MnO2@CNTs/G/diatomite and the MnO2@diatomite reveal new strong peak around 600~700 cm-1, which is corresponds to the Mn-O stretching vibration [37]. Therefore, both the Raman and XRD indicate the fabrication of the MnO2@CNTs/G/diatomite composite with relatively good crystalline structure.
Figure 4. a) XRD and b) Raman survey spectrum of the pristine diatomite, G/diatomite, CNTs/G/diatomite, MnO2@CNTs/G/diatomite and MnO2@G/diatomite, respectively. XPS spectrum of the c) full survey, d) C 1s, e) O 1s, f) Mn 2p of the MnO2@CNTs/G/diatomite.
To understand the elemental composition and valence states of the composite, X-ray photoelectron spectroscopy (XPS) is also conducted. The full survey spectrum (Figure 4c) of the MnO2@CNTs/G/diatomite reveals the presence of the C, O and Mn, confirms the successfully grown of the MnO2 architecture on the CNTs/G/diatomite surface. The
deconvolution of the C 1s spectrum (Figure 4d) reveals the main peaks located at 284.8 eV, 285.8 eV and 291.2 eV, corresponding to the C-C, C-O and C=O groups, respectively. The existence of the C-O and C=O groups may due to the partially surface oxidation during hydrothermal reaction. Furthermore, four convoluted O 1s peaks located at 529.9 eV, 531.8 eV, 532.7 eV, 533.4 eV are related to the Mn-O-Mn, Mn-OH, H-O-H and C-O/C=O, respectively (Figure 4e) [38-41]. The convoluted Mn 2p spectra is shown in Figure 4f, the main peaks centered at 642.2 eV and 654.0 eV exhibits a spin-energy separation of 11.8 eV, implying common feature of the MnO2 [42,44]. The electrochemical performance of the MnO2@CNTs/G/diatomite is evaluated in a three-electrode system. The cyclic voltammetry (CV) curves of different diatomitebased samples are measured at 50 mV s-1 under 0-1 V (Figure 5a). The enclosed area of the MnO2@CNTs/G/diatomite curve is obviously larger than that of the other composites, indicates the relatively higher specific capacity. The galvanostatic chargedischarge (GCD) tests are conducted at 0.5 A g-1 (Figure 5b), the quasi-rectangular GCD curves of the electrodes reveals the fast electron/ion diffusion mechanism. The gravimetric
capacitance
of
MnO2@CNTs/G/diatomite,
MnO2@G/diatomite,
G/diatomite and diatomite are calculated to be 264.0 F g-1, 142.4 F g-1, 28.6 F g-1 and 18.1 F g-1, respectively (based on the total mass of the composites). The specific capacitance of the MnO2@CNTs/G/diatomite electrode is comparable or better than most of the MnO2-based electrodes (Table S2). Moreover, the gravimetric capacitance can reach 880.0 F g-1 when calculated based on the MnO2 loading mass (Figure 5f),
which is close to the theoretical capacitance value of the MnO2. Furthermore, the rate capability reveals a capacitance retention of 65.84% for the MnO2@CNTs/G/diatomite hybrid when the current density increases from 0.5 to 5 A g-1 (Figure 5c), while only 27.83%, 25.97% and 58.69% capacitance retention can be reached for the diatomite, G/diatomite and MnO2/G/diatomite, respectively. In addition, the long-term cyclic stability of the MnO2@CNTs/G/diatomite hybrid reveals 84.26% capacitance retention after 4000 cycles (Figure 5e), and the nanosheet morphology and crystalline structure of the MnO2 also remain unchanged (Figure S2). The Nyquist plots of the diatomite, G/diatomite, MnO2/G/diatomite and MnO2/CNTs/G/diatomite
hybrids
are
exhibited
in
Figure
5d.
The
MnO2/CNTs/G/diatomite electrode exhibits a more vertical line in low frequency region, indicating the higher ions/electrons transfer rate and better capacitive behavior. On the other hand, the smallest ESR semi-cycle of the composite also reveals the lowest charge transfer resistance at the electrode/electrolyte interface. Therefore, the enhanced electrochemical performance of the MnO2/CNTs/G/diatomite hybrid may due to: 1) the in-situ grown CNTs/G hierarchical conductive framework on the bio-based diatomite structure can form a seamless electron transfer pathway; 2) the enlarged surface area of the 3D framework can increase the MnO2 nanostructure loading mass without hinder the electrolyte infiltration and diffusion, thus the electrons/ions mass transfer rate can be enhanced.
Figure 5. a) CV curves under 50 mV s-1, b) galvanostatic charge–discharge under 0.5 A g-1, c) rate capability and d) Nyquist plots of the diatomite, G/diatomite, MnO2/G/diatomite and MnO2/CNTs/G/diatomite electrodes. e) The cyclic stability of the MnO2/CNTs/G/diatomite, f) gravimetric capacitances calculated based on the MnO2 loadings mass under different current density.
Furthermore, an asymmetric supercapacitor (ASCs) is fabricated using the MnO2/CNTs/G/diatomite composite and a-MEGO (Figure S3) as positive and negative electrodes, respectively. As shown in Figure 6a, the CV curves of the MnO2/CNTs/G/diatomite//a-MEGO ASCs reveal potential window enlargement, when the operating voltages arise to 2 V, obvious polarization curve is observed. Therefore, the operating voltage is fixed to 1.8 V for further performance evaluation. The quasirectangular CV curves in Figure 6b is in line with that of the MnO2/CNTs/G/diatomite and a-MEGO electrodes. The specific capacitance of the MnO2/CNTs/G/diatomite//aMEGO calculated from the GCD curves in Figure 6c can reaches a high value of 143.0 F g-1 at 0.5 A g-1 based on the total electrode mass (MnO2/CNTs/G/diatomite and aMEGO). Moreover, the capacitance retention of the ASCs can remain 84.59 % after 5000 cycles at 10 A g-1 (Figure 6e), indicating relatively good cyclic stability of the
device. The Nyquist plots shown in Figure 6d reveals a low charge transfer resistance at the electrode/electrolyte interface. The Ragone plots of the ASCs device are also calculated (Figure 6d), a highest energy density of 64.4 W h kg-1 (at power density of 451.5 W kg-1) and a maximum power density of 19.8 kW kg-1 (at energy density of 6.7 W h kg-1) can be reached, which is comparable or superior than many MnO2-based asymmetric supercapacitor [14, 25, 38,44-48].
Figure 6. a) CV curves under various voltage windows, b) CV curves, c) GCD curves, d) EIS curves, e) cyclic capability and f) Ragone plots of the MnO2/CNTs/G/diatomite//a-MEGO ASCs.
4
Conclusions In summary, natural abundant biomass diatomite is act as the porous template to in-
situ grown graphene layer, while the CNTs arrays are subsequently grown on the graphene surface. The seamless grown 1D CNTs arrays together with 3D interconnected graphene can form conductive hierarchical framework to facilitate the electric conductivity. Furthermore, the enlarged effective surface area of the
CNTs/G/diatomite can increase the loading mass and facilitate the electrolyte infiltration. Thus, the MnO2@CNTs/G/diatomite hybrid exhibits a highest specific capacitance of 264.0 F g-1 based on the total mass (based on the MnO2, the value can reach 880.0 F g-1), good rate capability and excellent cyclic stability (capacitance retention of 84.26% after 4000 cycles). The MnO2@CNTs/G/diatomite//a-MEGO asymmetric supercapacitor reveals the maximum energy density of 64.3 W h kg-1 and highest power density of 19.8 kW kg-1, respectively. This cost-effective and biomassbased substrate fabrication method can be massive produced and offers promise for energy storage and conversion applications.
Author Information Corresponding Author *Junyi Ji, E-Mail: [email protected]
Notes The authors declare no competing interest.
Acknowledgment The financial support from the National Natural Science Foundation of China (21776187, 21490582, 21978178), the State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2017-3-01), the Fundamental Research Funds for the Central Universities is appreciated.
Appendix A. Supplementary material Supplementary data to this article (three-electrode test of the as-prepared composite under KOH electrolyte, SEM and TEM images, a-MEGO under Na2SO4 electrolyte and ICP measurement of the composite) can be found online.
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Highlights Cost-effective 3D porous diatomite biomass is used to fabricate conductive networks. The CNTs/graphene hybrid can enhance the electric conductivity and surface area. The MnO2@CNTs/G/diatomite shows high specific capacitance and good cyclic stability. The ASC exhibits high energy density (64.4 W h kg-1) and power density (19.8 kW kg-1).
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: