- Email: [email protected]
Pore forming with hemp fiber for magnesium phosphate structural supercapacitor
Journal Pre-proof Pore forming with hemp fiber for magnesium phosphate structural supercapacitor
Cuiqin Fang, Dong Zhang PII:
S0264-1275(19)30760-9
DOI:
https://doi.org/10.1016/j.matdes.2019.108322
Reference:
JMADE 108322
To appear in:
Materials & Design
Received date:
22 July 2019
Revised date:
29 October 2019
Accepted date:
29 October 2019
Please cite this article as: C. Fang and D. Zhang, Pore forming with hemp fiber for magnesium phosphate structural supercapacitor, Materials & Design(2018), https://doi.org/10.1016/j.matdes.2019.108322
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© 2018 Published by Elsevier.
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Pore forming with hemp fiber for magnesium phosphate structural supercapacitor Cuiqin Fang, Dong Zhang* Key laboratory of Advanced Civil Engineering Materials, Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China
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*Corresponding author, Email: [email protected]
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Abstract
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For multifunctional structural supercapacitor, pore structure can store lots of
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aqueous electrolyte in the matrix and interconnected pore structure can offer
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numerous channels for ion movement during charge-discharge. This paper first investigates the effects of pore forming with hemp fiber on the performance of
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structural supercapacitor. Various contents of hemp fiber were employed to alter the
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pore structure of the magnesium phosphate electrolyte containing 2 M KOH. Plenty
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of macropores causing by adding large content of fiber are normally bad for mechanical properties, yet they are beneficial to change the pore structure of structural electrolyte. The results show that porosity and pore connectivity of the structural electrolyte are gradually increasing from 8.3% to 18.6% and from 5.6% to 15.5% with the content of hemp fiber increasing within a certain range, respectively. Thus the specific capacitance of structural supercapacitor gradually increases by 46.8%. The results of multifunctionality of structural supercapacitor show that the sample with the most developed pore structure appears for an optimum balance between mechanical and electrochemical performance.
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Keywords
Structural supercapacitor; Pore structure; Hemp fiber; Magnesium phosphate cement; Multifunctionality
1. Introduction New energy storage systems have been continuously developed for replacement of fossil fuel to meet global energy requirements in the last decades [1]. Multifunctional
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materials [2] with an integration of structural and non-structural functions create new
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areas in the advanced energy storage system, such as structural supercapacitor [3],
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which can maintain its capacitive function under a mechanical loading. At least two
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components of structural supercapacitor are required to satisfy structural function
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such as structural electrode [4] and structural electrolyte or separator [5]. Maybe the structural electrolyte is more challenging, because that it must combine high ionic
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conductivity with good mechanical strength. However, it is generally known that ionic
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conductivity and mechanical performance have an inverse relationship [6], because
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ionic conductivity requires pore structure for ion migration but strength needs compact structure. A bicontinuous structure of electrolyte is designed to keep a good balance of performance, where one phase provides ionic conduction and the other phase is responsible for mechanical properties [7]. The earliest structural electrolyte is polymer resin as structural component and ionic liquid based electrolyte as ionic conductive phase because polymer-based electrolyte has good mechanical properties, long lifetime, short charging time and wide potential window [8]. Generally, polymer resin may contain of epoxy resin with more than two oxirane groups [9], polyethylene oxide (PEO) [10] or polyethylene
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glycon (PEG) [11], while ionic liquid phase can combine several types of cations and anions. Shirshov et al. [12] prepared ionic liquid-epoxy resin composites to obtain a structural electrolyte with ionic conductivity of 0.8 mS·cm-1 and a Young’s modulus of 0.2 GPa. Cole et al. [13] reported polymer electrolyte-based stretchable supercapacitors with a specific capacitance of 5 — 10 F·g-1 and mechanical stress of 0
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— 3 MPa in suit microtensile characterization. Greenhalgh et al. [14] fabricated
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structural supercapacitors with epoxy which exhibited a compressive strength of
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19.44 MPa and a specific capacitance of 4.5 m F·g-1. A novel structural supercapacitor
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based on ionic liquid-polyester resin had a specific capacitance of 2.48 F·g-1, an
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equivalent series resistance Rs of 370 Ω and an in-plane shear strength of 102.4 MPa [15]. However, the polymer electrolytes tend to have a low specific capacitance, and
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as the electrochemical property improves, the mechanical performance typically
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decreases, vice versa. Thus such polymer electrolyte may not well solve the challenge
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of balancing the electrochemical and mechanical properties of structural supercapacitor.
Recently, another kind of structural electrolytes were reported for building applications, cement as structural component and aqueous electrolyte as second phase [16]. Normally cement as gel materials has high mechanical strength as well as prosperous pore structure [17]. The pore size of pore structure can be divided into < 20 nm, 20 — 100 nm, 100 — 200 nm and > 200 nm, and the amount of macropores in cement is greater than 90 % of total pores [18]. As hydrated radius of aqueous electrolyte is generally smaller than 0.5 nm [19], such pore structure in cement is
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large enough to be filled with aqueous electrolyte to form structural electrolyte as well as to provide channels for ions passing, which is similar to the effect of pore size on the performance of electrodes [20]. Zhang et al. [16] reported a cement-based structural electrolyte with a specific capacitance of 10 F·g-1 and a compressive strength of 9.85 MPa. Subsequently, Xu et al. [21] fabricated a structural electrolyte based on geopolymer with a compressive strength of 33.85 MPa and a specific
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capacitance of 33.4 F·g-1. Ma et al. [22] also reported a combining magnesium
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phosphate electrolyte with a compressive strength of 24.5 MPa and a specific
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capacitance of 40.9 F·g-1. Therefore, combining cement with aqueous electrolyte is a
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promising approach to develop structural electrolyte due to its low cost, structural integrity, chemical stability and good compatibility with structural component.
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Thus pore structure has an important role in the performance of cement-based
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structural electrolyte. The porosity and pore interconnectivity are likely to play critical
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roles in characterizing the pore structure of the structural electrolyte. Hemp fiber has been chosen to alter pore structure of cement based structural electrolyte in this work. There are two reasons for choosing the hemp fiber as pore forming agent. The first reason is that unique interconnected partially graphitic carbon nanosheets with significant volume fraction of mesoporosity has been created from hemp fiber, which exhibits excellent electrochemical performance in supercapacitor [23] and sodium ion battery [24]. The other reason is that a large content of hemp fiber often leads to considerable number of macropores [25], which can result in forming interconnected pore structure in cement. Thus the effect of pore structure of structural electrolyte on
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the performance of structural supercapacitor was investigated by adding various volume fractions of hemp fiber in this work. 2. Materials and methods 2.1. Materials A high-conductivity graphene was received from Institute of Coal Chemistry CAS
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of China. Magnesia powder (MgO), potassium dihydrogen phosphate (KDP), borax
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and Potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent
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Co., Ltd. Hemp fiber provided by Sichuang Hemp Industry Co., Ltd was shorten into
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pieces of 1µm diameter and 5 mm in length. The chemical composition of fly ash,
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purchased from Jinghang Mineral Products Source Factory, was listed in Table 1. Ordinary tap water was used as mixing water of magnesium phosphate paste.
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Table 1
SiO2
Content (%)
50.5
Al2O3
Fe2O3
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Component
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Chemical composition of fly ash.
20.2
4.88
CaO
K2O
TiO2
SO3
MgO
Na2O
P2O5
3.79
2.01
1.10
0.79
0.60
0.44
0.30
2.2. Fabrication of structural supercapacitors with hemp fiber The mixture of graphene and 6 wt. % PTFE was homogenized in ethanol by being sonicated for 30 min and dried at 110 °C to paste-like. Then the paste was pressed on a Ni foam current electrode with pressure and dried to constant weight. The mass of each electrode was 1 mg with a diameter of 10 mm. The starting materials mass ratio of magnesium phosphate, including MgO, KH2PO4, borax, fly ash and water, was fixed at 100: 33: 30: 25: 38. Various volume
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fractions of hemp fiber were added to the uncured magnesium phosphate paste, with contents of 0, 5, 10, 15 and 20 vol. %, respectively. The homogenized paste was casted into a circular mold (10 mm×10 mm×1mm) and cured at the condition of 20 °C and 95 RH for 28 days. After forming, all the samples were infused in 2M KOH for 24 h to assemble into structural electrolytes. A structural electrolyte was
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sandwiched between two graphene electrodes to assemble a structural supercapacitor.
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2.3. Characterization
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Electrochemical characterizations were carried out on CHI660E workstation at
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25 °C. Cyclic voltammetry (CV) was at a scan rate of 100 mV·s-1. Galvanostatic
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charge-discharge was conducted at 1.0 A·g-1 current density. The specific capacitance C of structural supercapacitor can be calculated according to the Eq. 1, I∆t m ΔU
(1)
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C=
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where I is the discharging current, Δt is the discharging time, m is the mass of each
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electrode, and ΔU is the voltage change. Electrochemical impedance spectroscopy (EIS) was measured with frequency range 0.1 Hz — 100 kHz and initial voltage was set to open circuit potential.
Pore structure of structural electrolyte with hemp fiber was characterized by FEI field emission environmental scanning electron microscopy (SEM), operating at 20kV. Compressive strength of structural supercapacitor with hemp fiber was conducted six specimens on JES-300 concrete compressive strength tester at 2.4 kN·s-1 loading rate. X-ray photoelectron microscopy (XPS) was used to examine surface composition of graphene using monochromated Mg Kɑ radiation. Raman spectroscopy measurement
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on the graphene was carried out using an excitation line of 532 nm. Powder X-ray diffraction (XRD) pattern was used to performed on D/max 2550VB3+/PC X-ray diffractometer using Kɑ radiation. Mercury intrusion porosimetry (MIP) was used to measure the pore size distribution and porosity of the structural electrolyte. 3. Results and discussion
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3.1. Properties of materials
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The performance of high conductivity graphene materials is shown in Table 2.
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Fig. 1a shows Raman spectrum of the graphene. The spectrum exhibits a typical D
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band (1300 cm-1) and G band (1580 cm-1) corresponding to structural defects and
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graphitic sp2 carbon atoms, respectively [26]. The ID/IG ratio is 0.97, which reveals that the graphene is prepared by reduction of graphene oxide [27]. Fig. 1b displays
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XPS spectrum of the graphene in the binding energy range of 200 — 600 eV, showing
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the presence of C and O elements. The content of C element is 97.73 %, leading to a
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high conductivity of graphene electrode (286 S·cm-1). As a result of excellent physical and chemical properties of the graphene materials, a slid-state supercapacitor assembled with the graphene electrodes separated by filter paper exhibits a specific capacitance of 124.5 F·g-1.
Fig. 1. Raman spectrum (a) and XPS spectrum (b) of graphene materials.
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Table 2 Performance of the graphene materials.
Specific surface area
Average pore size
Pore volume
Unit
m2·g-1
nm
cm3·g-1
Test Method
BET
BET
BET
Value
531.79
1.64
0.22
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Item
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The crystallinity of magnesium phosphate, hemp fiber or the structural component
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with 15 vol. % hemp fiber hemp fiber is determined from the XRD pattern (Fig. 2).
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The crystallinity index CI is calculated by CI = (𝐼002 − 𝐼)⁄𝐼002 . For hemp fiber, the maximum diffraction intensity of lattice peak I002 is located at 22.4° representative of
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both amorphous and crystalline material [28], while the minimum diffraction intensity
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I is located at 16.1° representative of amorphous material. Thus the calculated CI of
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hemp fiber is 31.3%. The XRD pattern of the structural supercapacitor with 15 vol. % of hemp fiber is similar with that of magnesium phosphate, which means that the hemp fiber has no effect on the materialization hydration products and impartially contributes to change of physical and chemical properties of MPC syntheses [29].
Fig. 2. XRD analysis of magnesium phosphate, hemp fiber and the structural component with 15
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vol. % hemp fiber.
3.2. Pore structure of structural electrolytes Typical SEM image of the structural electrolyte containing 15 vol. % of hemp fiber is shown in Fig. 3a and the corresponding EDX analysis result is listed in Fig. 3d. It is clearly seen that hemp fiber is non-uniformly distributed in the matrix due to
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the large fiber content and non-homogeneous mix. Due to the presence of hemp fiber
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seriously interfering with image processing, careful selections of parts of SEM images
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without hemp fiber were used to measure the two parameters of pore structure,
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including porosity and pore connectivity, like Fig. 3b. Image analysis technique [30]
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is employed to process the SEM images to quantitatively characterize pore structure
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of the structural electrolyte with hemp fiber, like Fig. 3c.
Fig. 3. Typical SEM of structural electrolyte with 15 vol. % of hemp fiber: (a) SEM; (b) Chosen part; (c) Chosen part after image processing; (d) EDX analysis (area scanning).
The selected representative SEM images of structural electrolytes with various contents of hemp fiber are presented in Fig. 4. As the content of hemp fiber increases, wider interlayer voids are found in the matrix. The processed images of structural
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electrolyte with various contents of hemp fiber are shown in Fig. 5. Area considered here is defined as the number of pixels. The white part represents pore structure, while the black part is matrix. The pore structure here is mainly macropores, which have the greatest influence on storage of aqueous electrolyte and ionic migration. Interconnected pore structure is consisting of some isolated macropores in the matrix.
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Thus, the porosity P of structural electrolyte is calculated by P = 𝑆1 ⁄𝑆, where S1 is
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the area of all the white parts, and S is the area of the overall area. And the pore
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connectivity PC of structural electrolyte is measured according to PC = 𝑆2 ⁄𝑆, where
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Pr
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S2 is the area of all the interconnected pore structure.
Fig. 4. SEM images of structural electrolytes with various contents of hemp fiber: (a) 0 vol. %; (b) 5 vol. %; (c) 10 vol. %; (d) 20 vol. %.
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Fig. 5. Processing SEM images of structural electrolytes with various contents of hemp fiber: (a) 0
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vol. %; (b) 5 vol. %; (c) 10 vol. %; (d) 20 vol. %.
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The porosity and pore connectivity of structural electrolytes with various contents of hemp fiber are depicted in Fig. 6. It is found that the volume of hemp fiber is
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between 5 vol. % and 15 vol. %, a gradual increase from 8.3 % to 18.6 % in porosity
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and from 5.6 % to 15.5 % in pore connectivity of the structural electrolyte. The
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increase in porosity and pore connectivity attributes to the interlayer voids trapped beneath the fiber during casting [27]. However, as the content of hemp fiber increases to 5 vol. %, the porosity of the sample decreases from 10.8 % and the pore connectivity decreases from 7.2 %. It is consistent with that low volume of fiber is effective in reducing free plastic shrinkage and pore structure development [31]. Moreover, as the content of hemp fiber is higher than 15 vol. %, the porosity and pore connectivity of the samples decreases due to the disorganized arrangement and massive stacking dispersion of hemp fiber in the matrix [32]. Thus when compared with foaming method by adding air entraining agent [33], synchronized changes of
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porosity and pore connectivity of the structural electrolyte by adding hemp fiber show better. This is mainly due to that the macropores created by air entraining agent are independent bubbles and difficult to connect with each other, but lots of
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interconnected pore structure can be formed by adding large content of hemp fiber.
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Fig. 6. Porosity and pore connectivity of structural electrolytes with various contents of fiber.
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Fig. 7 shows the pore size distribution curve and porosity of structural electrolytes with various contents of fibers tested by MIP. It is clear that most probably aperture
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diameter is about 115µm, 54.6 µm and 86.5µm for the structural electrolyte with 0
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vol. %, 15 vol. % and 20 vol. % of hem fiber, respectively. As the content of hemp
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fiber increases to 15 vol. %, the pore number of structural electrolyte dramatically increases. The structural electrolyte has a total porosity of 13.0%, 24.2 % and 21.1% with 0 vol. %, 15 vol. % and 20 vol. % of hemp fiber, respectively. The pore content obtained by MIP is a little higher than by image analysis, which is mainly due to sampling, but the variation trend of porosity is consistent with that of image analysis.
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Fig. 7. Pore size distribution curve (a) and porosity (b) of structural electrolytes with various contents of fibers tested by MIP.
3.3. Electrochemical performance of structural supercapacitors Fig.8 displays the CV curves of structural supercapacitors with different contents of hemp fiber. All the CV curves are relatively in rectangular shapes and exhibit near
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mirror-image current response on voltage reversal, meaning ideal capacitive behavior.
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Based on the area of CV curves, the specific capacitance of the structural
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supercapacitor initially increases with the increasing content of hemp fiber and then
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decreases, which are resulted from the porosity and pore connectivity of structural
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electrolytes. Here, as the volume content of hemp fiber increases from 15 vol. % to 20 vol. %, the area of CV curves decreases, which is mainly due to that the pore structure
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of structural electrolyte decreases by 33.5%. Thus the pore structure of structural
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electrolyte significantly affects the specific capacitance of structural supercapacitor.
Fig. 8. CV curves of structural supercapacitors with various contents of hemp fiber.
From Fig. 9a, the typical triangular-shaped galvanostatic charge-discharge curves of the structural supercapacitors with different contents of hemp fiber are highly linear and symmetrical without obvious iR drop, exhibiting nearly perfect capacitive behavior. In Fig. 9b, as the content of hemp fiber increases from 5 vol. % to 15 vol. %,
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connectivity of structural electrolyte. It can be deduced that aqueous electrolyte can
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be stored in the pores of matrix to assemble structural electrolyte, and aqueous
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electrolyte can pass quickly from the surface to underlying layers of the matrix
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through the interconnected pore channels during charge-discharge. Similar findings
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have been reported by J.M. Xu, W.Y. Ma and N. Shirshova, they all agreed that ionic conductive phase was not only stored in the pore structure of matrix, but also
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transmitted to the electrode surface through the pore structure during charge-discharge.
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Yet they didn’t discuss the effect of pore structure on the specific capacitance of the
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structural supercapacitor in depth. The higher porosity in the matrix, the more KOH aqueous stored. And the higher pore connectivity in the matrix, the more passageways provided for ion movement. Thus the specific capacitance of structural supercapacitor is mainly affected by the porosity and pore connectivity of structural electrolyte. Furthermore, the specific capacitance (51.4 F·g-1) of the structural supercapacitor reaches to 41% of solid-state supercapacitor (124.5 F·g-1) under the same test conditions, which is promising to develop structural electrolyte combining magnesium phosphate with KOH aqueous. Fig. 9c shows the Nyquist plots of structural supercapacitors with various contents
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of hemp fiber. The nearly vertical line in the low-frequency region is a result of ion diffusion in the electrolyte to the electrode surface, signifying an ideal capacitive behavior and the high frequency region represents an arc-like shape [34]. The internal resistance Rs of the structural supercapacitor can be obtained from Nyquist curves by the intercept at real part of horizontal axis. The charge transfer resistance Rct at the
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electrolyte/ electrode interface is represented by the arc formed in a high frequency
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region. Seen from Fig. 9d, as the volume fraction of hemp fiber increases from 5 vol. %
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to 20 vol. %, the curve trend of Rs and Rct are negatively related with that of porosity
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and pore connectivity in structural electrolyte. In addition, as the content of hemp fiber increases to 5 vol. %, Rs of the structural supercapacitor sharply reduces from
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40.2 Ω to 17.4 Ω and Rct decreases from 21.2 to 4.4 Ω due to the strong absorption
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capacity of hemp fiber for aqueous electrolyte. Thus the internal resistance Rs and
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charge transfer resistance Rct are mainly affected by absorption capacity of hemp fiber
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to aqueous electrolyte when small content of hemp fiber is added, yet they are mainly affected by the porosity and pore connectivity of matrix when larger content of hemp fiber is added. Rs data determines the power density of a structural supercapacitor. Low Rct value of the structural supercapacitor reflects the improved transfer mechanism at the interface of electrode/electrolyte. Therefore, structural supercapacitors in this work show better electrochemical performance than that of epoxy system.
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Fig. 9. Electrochemical properties of structural supercapacitors with various contents of fiber: (a)
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Galvanostatic charge-discharge curves; (b) Specific capacitance; (c) Nyquist plots; (d) Rs.
The long-term cycling stability of the structural supercapacitor is evaluated by CV
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method with a scan rate of 100 mV·s-1 for 2000 cycles. From Fig. 10, the specific
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capacitance of the structural supercapacitors with 0 vol. % and 15 vol. % of hemp
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fiber retain about 92.2 % and 88.2 % after 2000 cycles, respectively. The cycling
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stability of the structural supercapacitor slightly decreases after adding 15 vol. % of hemp fiber, which is due to the increase of pore structure of the structural electrolyte. It can be concluded that the structural supercapacitors in this work have good cycling stability during long-term charge-discharge processes.
Fig. 10. Cycling stability of structural supercapacitors for 2000 cycles.
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3.4. Multifunctional analysis of structural supercapacitors Fig. 11a depicted the compressive strength of structural supercapacitors with various contents of hemp fiber. It is seen that an increase in the content of hemp fiber causes a gradual reduction in compressive strength of structural supercapacitor. Although hemp fiber has been used to enhance the mechanical properties of cement
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pastes, it has been reported that hemp fiber has a negative effect on the compressive
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strength of cement matrix, even a small content [35]. In general, high porosity of
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structural electrolyte can reduce the compressive strength of structural supercapacitor,
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but it is not the only factor. Except for porosity, there are still many reasons for the
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reduction in compressive of structural supercapacitor, such as disorganized arrangement of fiber, agglomeration of fiber and surface interactions between matrix
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and fiber.
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To analyze multifunctional properties of the structural supercapacitor, two
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performance parameters are employed in this work, specific capacitance and compressive strength. However, as the mechanical performance and ion conductivity are in a trade-off relationship, an ideal structural supercapacitor is designed in this work. The ideal structural supercapacitor exhibits the highest values of the two parameters among all the samples, with a compressive strength of 24.1 MPa and a specific capacitance of 51.4 F·g-1. Fig. 11b presents the multifunctionality of structural supercapacitors with various contents of hemp fiber. It is known that the shorter the distance of scatter to the ideal point, the closer to the ideal structural supercapacitor. As the content of hemp fiber
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increases from 5 vol. % to 15 vol. %, multifunctionality of the structural supercapacitor gradually increases. Nevertheless, as the content of hemp fiber is smaller than 5 vol. % or greater than 15 vol. %, the multifunctionality of structural supercapacitor decreases with the increasing content of hemp fiber. It can be inferred that the multifunctional properties of the structural supercapacitor is mainly affected
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by the porosity and pore connectivity of structural electrolyte. Lots of pore structure
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causing by adding large content of hemp fiber result in low compressive strength of
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structural supercapacitor, yet such pore structure are beneficial to improve
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electrochemical properties of structural supercapacitor. Moreover, when specific
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capacitance and compressive strength are combined for evaluation of structural supercapacitor, the porosity and pore connectivity are positively correlated with its
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multifunctionality. The sample containing 15 vol. % of hemp fiber appears for the
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strength of 13.2 MPa.
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optimum balance, displaying a specific capacitance of 51.4 F·g-1 and a compressive
Fig. 11. Compressive strength (a) and Multifunctionality (b) of structural supercapacitors with various contents of hemp fiber.
The comparison of this study with other relevant structural supercapacitors is listed in Table 3. It can be seen that the structural supercapacitor with 15 vol. % of
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hemp fiber in this work shows good electrochemical and mechanical properties. In fact, perfect performance of structural supercapacitor is still difficult to achieve, but this work is highly encouraging to improve the multifunctionality of structural supercapacitor by developing pore structure of structure electrolyte with hemp fiber. Table 3 Comparison of this study with other relevant structural supercapacitors. Specific capacitance F/g
Strength MPa
15 vol. % hemp fiber (this work)
51.4
13.2
10
9.5
CAG-CF // PEGDGE-IL
[22]
[36]
Nanoporous Si//epoxy -BMIBF4 CFRP// Li1.4Al0.4Ti1.6(PO4)3
[37]
[38]
33.85
40.92
24.59
0.6025
8.7
3.2
0.2
3.6×10
[11]
50
-5
651 2
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CCNC Nws-CF //PEO-BMITFSI
36.5
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G//Magnesium phosphate-KOH
4. Conclusions
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[21]
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G//Geopolymer-KOH
[16]
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G//Portland cement-KOH
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Structural supercapacitor
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The properties of structural supercapacitor are mainly affected by the pore structure
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of matrix. The pore structure can store aqueous electrolyte as well as provide channels for ion migration during charge-discharge. Large content of fiber often leads to lots of pore structure which are harmful to mechanical properties, yet such pore structure is conducive to improving electrochemical properties of the structural supercapacitor. As the content of hemp fiber increases from 5 vol. % and 15 vol. %, a gradual increase from 8.3 % to 18.6 % in porosity and a gradual increase from 5.6 % to 15.5 % in pore connectivity of the sample, indicating that hemp fiber effectively introduces an increasing number of pore structure in the matrix for storage of KOH electrolyte and enough channels for ion migration. As a result, the electrochemical properties of
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structural supercapacitors gradually increase with the increasing of pore structure, exhibiting the specific capacitance increasing from 35 F·g-1 to 51.4 F·g-1. The multifunctionality of structural supercapacitor was also analyzed, and the sample with the most developed pore structure appears for the optimum balance, displaying a specific capacitance of 51.4 F·g-1 and a compressive strength of 13.2 MPa. Thus this
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work is highly encouraging to improve multifunctionality of structural supercapacitor
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by developing pore structure of structure electrolyte with hemp fiber.
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Acknowledgements
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This work was supported by the NSAF Foundation of National Natural Science
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Foundation of China and Chinese Academy of Engineering Physics under Grant [No. U1730117].
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[9] B.K. Deka, A. Hazarika, O. Kwon, D.Y. Kim, Y.B. Park, H.W. Park, Multifunctional enhancement of woven carbon fiber/ZnO nanotube-based structural supercapacitor and polyester resin-domain solid-polymer electrolytes, Chem. Eng. J. 325 (2017) 672-680. [10] B. Sun, T.L. Shi, Z.Y. Liu, Y.N. Wu, J.X. Zhou, G.L. Liao, Large-area flexible photodetector based on atomically thin MoS2/graphene film, Mater. Design 154 (2018) 1-7.
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[13] D.P. Cole, A.L.M. Reddy, M.G. hahm, R. McCotter, A.H.C. Hart, R. Vajtai, P.M. Ajayan, S.P. Karna, M.L. Bundy, Electromechanical properties of polymer
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[14] E.S. Greenhalgh, J. Ankersen, L.E. Asp, A. Bismarck, O.P.V. Fontana, M. Houlle, G. Kalinka, A. Kucernak, M. Misty, S. Nguyen, H. Qian, M.S.P. Shirshova, J.H.G. Steinke, M. Wienrich, Mechanical, electrical and microstructural characterization of multifunctional structural power composites, J. Compos. Mater. 0 (2014) 1-12. [15] B.K. Deka, A. Hazarika, J. Kim, Y.B. Park, H.W. Park, Multifunctional CuO nanowire embodied structural supercapacitor based on woven carbon fiber/ionic liquid-polyester resin, Composi: PartA 87 (2016) 256-262. [16] J.J. Zhang, J.M. Xu, D. Zhang, A structural supercapacitor based on graphene
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and hardened cement paste, J. Electrochem. Soc. 163 (3) (2016) E83-E87. [17] T.C. Hansen, Physical structure of hardened cement paste: A classical approach, Mater. Struct. 19 (6) (1986) 423-436. [18] Y. Li, H. Lin, Experimental study on the effect of different dispersed degrees carbon nanotubes on the modification of magnesium phosphate cement, Constr.
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Build Mater. 200 (2019) 240-247.
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[19] B. Tansel, J. Sager, T. Rector, J. Garland, R.F. Strayer, L.F. Levine, M. Roberts,
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[21] J.M. Xu, D. Zhang, Multifunctional structural supercapacitor based on graphene and geopolymer, Electrochim. Acta 224 (2017) 105-112. [22] W.Y. Ma, D. Zhang, Multifunctional structural supercapacitor based on graphene and magnesium phosphate cement, J. Compos. Mater. 53 (6) (2019) 719-730. [23] H.L. Wang, Z.W. Xu. A. Kohandehghan, Z. Li, K. Cui, X.H.Tan, T.J. Stephenson, C.K. King’ondu, C.M.B. Holt, B.C. Olsen, J.K. Tak, D. Harfield, A.O. Anyia, D. Mitlin, Interconnected carbon nanosheets derived from hemp for ultrafast supercapacitors with high energy, ACS Nano 7 (2013) 5131-5141. [24] H.L. Wang, W.H. Yu, N. Mao, J. Shi, W. Liu, Effect of surface modification on
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properties, J. Clean. Prod. 229 (2019) 138-143. [29] M.A. Haque, B. Chen, M.R. Ahmad, S.F. Shah, Evaluating the physical and strength properties of fibre reinforced magnesium phosphate cement mortar considering mass loss, Constr. Build. Mater. 217 (2019) 427-440. [30] H.S. Wong, R.W. Zimmerman, N.R. Buenfeld, Estimating the permeability of cement pastes and mortars using image analysis and effective medium theory, Cement Concrete Res. 42 (2012) 476-483. [31] O. Onuaguluchi, N. Banthia, Planted-based natural fibre reinforced cement composites: A review, Cement Concrete Comp. 68 (2016) 96-108.
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[32] N. Mostefai, R. Hamzaoui, S. Guessasma, A. Aw, H. Nouri, Microstructure and mechanical performance of modified hemp fibre and shiv mortars: Discovering the optimal formulation, Mater. Design 84 (2015) 359-371. [33] W.Y. Ma, D. Zhang, Preparation of porous magnesium phosphate material and its application in structural supercapacitors, CIESC J. 69 (10) (2018) 4438-4448.
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[34] M. Minakshi, D.R.G. Mitchell, A.R. Munnangi, A.J. Barlow, M. Fichtner, New
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insights into the electrochemistry f magnesium molybdate hierarchical
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carbon fabric and a solid electrolyte, Mater. Sci. Tech. 35 (2019).
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[37] A.S. Westover, B. Baer, B.H. Bello, H.T. Sun, L. Oakes, L.M. Bellan, C.L. Pint, Multifunctional high strength and high energy epoxy composite structural supercapacitors with wet-dry operational stability, J. Mater. Chem. A 3 (2015) 20097. [38] T.J. Adam, G.Y. Liao, J. Petersen, S. Geier, B. Finke, P. Wierach, A. Kwade, M. Wiedemann, Multifunctional composites for future energy storage in aerospace structures, Energies 11 (2018) 335.
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Author Contribution Statement
Cuiqin Fang: Investigation, Data Curation, Formal Analysis Software, Writing- Original draft
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preparation, Visualization, Validation
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Dong Zhang:
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Methodology, Conceptualization, Resources, Writing- Reviewing and Editing,
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Supervision, Project Administration, Funding Acquisition
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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.
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Graphical abstract
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Highlights 1. The pore structure affects the performance of structural supercapacitor. 2. Porosity of structural electrolyte increases from 8.3% to 18.6% by adding hemp fiber. 3. Pore connectivity of structural electrolyte increases by 176% after adding hemp
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fiber.
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4. Multifunctionality of the sample with 15 vol. % of hemp fiber shows the best.
Cuiqin Fang, Dong Zhang PII:
S0264-1275(19)30760-9
DOI:
https://doi.org/10.1016/j.matdes.2019.108322
Reference:
JMADE 108322
To appear in:
Materials & Design
Received date:
22 July 2019
Revised date:
29 October 2019
Accepted date:
29 October 2019
Please cite this article as: C. Fang and D. Zhang, Pore forming with hemp fiber for magnesium phosphate structural supercapacitor, Materials & Design(2018), https://doi.org/10.1016/j.matdes.2019.108322
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© 2018 Published by Elsevier.
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Pore forming with hemp fiber for magnesium phosphate structural supercapacitor Cuiqin Fang, Dong Zhang* Key laboratory of Advanced Civil Engineering Materials, Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China
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*Corresponding author, Email: [email protected]
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Abstract
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For multifunctional structural supercapacitor, pore structure can store lots of
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aqueous electrolyte in the matrix and interconnected pore structure can offer
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numerous channels for ion movement during charge-discharge. This paper first investigates the effects of pore forming with hemp fiber on the performance of
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structural supercapacitor. Various contents of hemp fiber were employed to alter the
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pore structure of the magnesium phosphate electrolyte containing 2 M KOH. Plenty
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of macropores causing by adding large content of fiber are normally bad for mechanical properties, yet they are beneficial to change the pore structure of structural electrolyte. The results show that porosity and pore connectivity of the structural electrolyte are gradually increasing from 8.3% to 18.6% and from 5.6% to 15.5% with the content of hemp fiber increasing within a certain range, respectively. Thus the specific capacitance of structural supercapacitor gradually increases by 46.8%. The results of multifunctionality of structural supercapacitor show that the sample with the most developed pore structure appears for an optimum balance between mechanical and electrochemical performance.
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Keywords
Structural supercapacitor; Pore structure; Hemp fiber; Magnesium phosphate cement; Multifunctionality
1. Introduction New energy storage systems have been continuously developed for replacement of fossil fuel to meet global energy requirements in the last decades [1]. Multifunctional
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materials [2] with an integration of structural and non-structural functions create new
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areas in the advanced energy storage system, such as structural supercapacitor [3],
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which can maintain its capacitive function under a mechanical loading. At least two
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components of structural supercapacitor are required to satisfy structural function
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such as structural electrode [4] and structural electrolyte or separator [5]. Maybe the structural electrolyte is more challenging, because that it must combine high ionic
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conductivity with good mechanical strength. However, it is generally known that ionic
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conductivity and mechanical performance have an inverse relationship [6], because
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ionic conductivity requires pore structure for ion migration but strength needs compact structure. A bicontinuous structure of electrolyte is designed to keep a good balance of performance, where one phase provides ionic conduction and the other phase is responsible for mechanical properties [7]. The earliest structural electrolyte is polymer resin as structural component and ionic liquid based electrolyte as ionic conductive phase because polymer-based electrolyte has good mechanical properties, long lifetime, short charging time and wide potential window [8]. Generally, polymer resin may contain of epoxy resin with more than two oxirane groups [9], polyethylene oxide (PEO) [10] or polyethylene
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glycon (PEG) [11], while ionic liquid phase can combine several types of cations and anions. Shirshov et al. [12] prepared ionic liquid-epoxy resin composites to obtain a structural electrolyte with ionic conductivity of 0.8 mS·cm-1 and a Young’s modulus of 0.2 GPa. Cole et al. [13] reported polymer electrolyte-based stretchable supercapacitors with a specific capacitance of 5 — 10 F·g-1 and mechanical stress of 0
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— 3 MPa in suit microtensile characterization. Greenhalgh et al. [14] fabricated
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structural supercapacitors with epoxy which exhibited a compressive strength of
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19.44 MPa and a specific capacitance of 4.5 m F·g-1. A novel structural supercapacitor
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based on ionic liquid-polyester resin had a specific capacitance of 2.48 F·g-1, an
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equivalent series resistance Rs of 370 Ω and an in-plane shear strength of 102.4 MPa [15]. However, the polymer electrolytes tend to have a low specific capacitance, and
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as the electrochemical property improves, the mechanical performance typically
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decreases, vice versa. Thus such polymer electrolyte may not well solve the challenge
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of balancing the electrochemical and mechanical properties of structural supercapacitor.
Recently, another kind of structural electrolytes were reported for building applications, cement as structural component and aqueous electrolyte as second phase [16]. Normally cement as gel materials has high mechanical strength as well as prosperous pore structure [17]. The pore size of pore structure can be divided into < 20 nm, 20 — 100 nm, 100 — 200 nm and > 200 nm, and the amount of macropores in cement is greater than 90 % of total pores [18]. As hydrated radius of aqueous electrolyte is generally smaller than 0.5 nm [19], such pore structure in cement is
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large enough to be filled with aqueous electrolyte to form structural electrolyte as well as to provide channels for ions passing, which is similar to the effect of pore size on the performance of electrodes [20]. Zhang et al. [16] reported a cement-based structural electrolyte with a specific capacitance of 10 F·g-1 and a compressive strength of 9.85 MPa. Subsequently, Xu et al. [21] fabricated a structural electrolyte based on geopolymer with a compressive strength of 33.85 MPa and a specific
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capacitance of 33.4 F·g-1. Ma et al. [22] also reported a combining magnesium
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phosphate electrolyte with a compressive strength of 24.5 MPa and a specific
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capacitance of 40.9 F·g-1. Therefore, combining cement with aqueous electrolyte is a
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promising approach to develop structural electrolyte due to its low cost, structural integrity, chemical stability and good compatibility with structural component.
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Thus pore structure has an important role in the performance of cement-based
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structural electrolyte. The porosity and pore interconnectivity are likely to play critical
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roles in characterizing the pore structure of the structural electrolyte. Hemp fiber has been chosen to alter pore structure of cement based structural electrolyte in this work. There are two reasons for choosing the hemp fiber as pore forming agent. The first reason is that unique interconnected partially graphitic carbon nanosheets with significant volume fraction of mesoporosity has been created from hemp fiber, which exhibits excellent electrochemical performance in supercapacitor [23] and sodium ion battery [24]. The other reason is that a large content of hemp fiber often leads to considerable number of macropores [25], which can result in forming interconnected pore structure in cement. Thus the effect of pore structure of structural electrolyte on
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the performance of structural supercapacitor was investigated by adding various volume fractions of hemp fiber in this work. 2. Materials and methods 2.1. Materials A high-conductivity graphene was received from Institute of Coal Chemistry CAS
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of China. Magnesia powder (MgO), potassium dihydrogen phosphate (KDP), borax
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and Potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent
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Co., Ltd. Hemp fiber provided by Sichuang Hemp Industry Co., Ltd was shorten into
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pieces of 1µm diameter and 5 mm in length. The chemical composition of fly ash,
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purchased from Jinghang Mineral Products Source Factory, was listed in Table 1. Ordinary tap water was used as mixing water of magnesium phosphate paste.
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Table 1
SiO2
Content (%)
50.5
Al2O3
Fe2O3
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Component
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Chemical composition of fly ash.
20.2
4.88
CaO
K2O
TiO2
SO3
MgO
Na2O
P2O5
3.79
2.01
1.10
0.79
0.60
0.44
0.30
2.2. Fabrication of structural supercapacitors with hemp fiber The mixture of graphene and 6 wt. % PTFE was homogenized in ethanol by being sonicated for 30 min and dried at 110 °C to paste-like. Then the paste was pressed on a Ni foam current electrode with pressure and dried to constant weight. The mass of each electrode was 1 mg with a diameter of 10 mm. The starting materials mass ratio of magnesium phosphate, including MgO, KH2PO4, borax, fly ash and water, was fixed at 100: 33: 30: 25: 38. Various volume
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fractions of hemp fiber were added to the uncured magnesium phosphate paste, with contents of 0, 5, 10, 15 and 20 vol. %, respectively. The homogenized paste was casted into a circular mold (10 mm×10 mm×1mm) and cured at the condition of 20 °C and 95 RH for 28 days. After forming, all the samples were infused in 2M KOH for 24 h to assemble into structural electrolytes. A structural electrolyte was
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sandwiched between two graphene electrodes to assemble a structural supercapacitor.
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2.3. Characterization
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Electrochemical characterizations were carried out on CHI660E workstation at
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25 °C. Cyclic voltammetry (CV) was at a scan rate of 100 mV·s-1. Galvanostatic
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charge-discharge was conducted at 1.0 A·g-1 current density. The specific capacitance C of structural supercapacitor can be calculated according to the Eq. 1, I∆t m ΔU
(1)
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C=
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where I is the discharging current, Δt is the discharging time, m is the mass of each
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electrode, and ΔU is the voltage change. Electrochemical impedance spectroscopy (EIS) was measured with frequency range 0.1 Hz — 100 kHz and initial voltage was set to open circuit potential.
Pore structure of structural electrolyte with hemp fiber was characterized by FEI field emission environmental scanning electron microscopy (SEM), operating at 20kV. Compressive strength of structural supercapacitor with hemp fiber was conducted six specimens on JES-300 concrete compressive strength tester at 2.4 kN·s-1 loading rate. X-ray photoelectron microscopy (XPS) was used to examine surface composition of graphene using monochromated Mg Kɑ radiation. Raman spectroscopy measurement
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on the graphene was carried out using an excitation line of 532 nm. Powder X-ray diffraction (XRD) pattern was used to performed on D/max 2550VB3+/PC X-ray diffractometer using Kɑ radiation. Mercury intrusion porosimetry (MIP) was used to measure the pore size distribution and porosity of the structural electrolyte. 3. Results and discussion
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3.1. Properties of materials
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The performance of high conductivity graphene materials is shown in Table 2.
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Fig. 1a shows Raman spectrum of the graphene. The spectrum exhibits a typical D
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band (1300 cm-1) and G band (1580 cm-1) corresponding to structural defects and
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graphitic sp2 carbon atoms, respectively [26]. The ID/IG ratio is 0.97, which reveals that the graphene is prepared by reduction of graphene oxide [27]. Fig. 1b displays
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XPS spectrum of the graphene in the binding energy range of 200 — 600 eV, showing
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the presence of C and O elements. The content of C element is 97.73 %, leading to a
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high conductivity of graphene electrode (286 S·cm-1). As a result of excellent physical and chemical properties of the graphene materials, a slid-state supercapacitor assembled with the graphene electrodes separated by filter paper exhibits a specific capacitance of 124.5 F·g-1.
Fig. 1. Raman spectrum (a) and XPS spectrum (b) of graphene materials.
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Table 2 Performance of the graphene materials.
Specific surface area
Average pore size
Pore volume
Unit
m2·g-1
nm
cm3·g-1
Test Method
BET
BET
BET
Value
531.79
1.64
0.22
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Item
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The crystallinity of magnesium phosphate, hemp fiber or the structural component
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with 15 vol. % hemp fiber hemp fiber is determined from the XRD pattern (Fig. 2).
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The crystallinity index CI is calculated by CI = (𝐼002 − 𝐼)⁄𝐼002 . For hemp fiber, the maximum diffraction intensity of lattice peak I002 is located at 22.4° representative of
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both amorphous and crystalline material [28], while the minimum diffraction intensity
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I is located at 16.1° representative of amorphous material. Thus the calculated CI of
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hemp fiber is 31.3%. The XRD pattern of the structural supercapacitor with 15 vol. % of hemp fiber is similar with that of magnesium phosphate, which means that the hemp fiber has no effect on the materialization hydration products and impartially contributes to change of physical and chemical properties of MPC syntheses [29].
Fig. 2. XRD analysis of magnesium phosphate, hemp fiber and the structural component with 15
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vol. % hemp fiber.
3.2. Pore structure of structural electrolytes Typical SEM image of the structural electrolyte containing 15 vol. % of hemp fiber is shown in Fig. 3a and the corresponding EDX analysis result is listed in Fig. 3d. It is clearly seen that hemp fiber is non-uniformly distributed in the matrix due to
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the large fiber content and non-homogeneous mix. Due to the presence of hemp fiber
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seriously interfering with image processing, careful selections of parts of SEM images
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without hemp fiber were used to measure the two parameters of pore structure,
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including porosity and pore connectivity, like Fig. 3b. Image analysis technique [30]
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is employed to process the SEM images to quantitatively characterize pore structure
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of the structural electrolyte with hemp fiber, like Fig. 3c.
Fig. 3. Typical SEM of structural electrolyte with 15 vol. % of hemp fiber: (a) SEM; (b) Chosen part; (c) Chosen part after image processing; (d) EDX analysis (area scanning).
The selected representative SEM images of structural electrolytes with various contents of hemp fiber are presented in Fig. 4. As the content of hemp fiber increases, wider interlayer voids are found in the matrix. The processed images of structural
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electrolyte with various contents of hemp fiber are shown in Fig. 5. Area considered here is defined as the number of pixels. The white part represents pore structure, while the black part is matrix. The pore structure here is mainly macropores, which have the greatest influence on storage of aqueous electrolyte and ionic migration. Interconnected pore structure is consisting of some isolated macropores in the matrix.
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Thus, the porosity P of structural electrolyte is calculated by P = 𝑆1 ⁄𝑆, where S1 is
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the area of all the white parts, and S is the area of the overall area. And the pore
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connectivity PC of structural electrolyte is measured according to PC = 𝑆2 ⁄𝑆, where
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Pr
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S2 is the area of all the interconnected pore structure.
Fig. 4. SEM images of structural electrolytes with various contents of hemp fiber: (a) 0 vol. %; (b) 5 vol. %; (c) 10 vol. %; (d) 20 vol. %.
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Fig. 5. Processing SEM images of structural electrolytes with various contents of hemp fiber: (a) 0
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vol. %; (b) 5 vol. %; (c) 10 vol. %; (d) 20 vol. %.
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The porosity and pore connectivity of structural electrolytes with various contents of hemp fiber are depicted in Fig. 6. It is found that the volume of hemp fiber is
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between 5 vol. % and 15 vol. %, a gradual increase from 8.3 % to 18.6 % in porosity
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and from 5.6 % to 15.5 % in pore connectivity of the structural electrolyte. The
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increase in porosity and pore connectivity attributes to the interlayer voids trapped beneath the fiber during casting [27]. However, as the content of hemp fiber increases to 5 vol. %, the porosity of the sample decreases from 10.8 % and the pore connectivity decreases from 7.2 %. It is consistent with that low volume of fiber is effective in reducing free plastic shrinkage and pore structure development [31]. Moreover, as the content of hemp fiber is higher than 15 vol. %, the porosity and pore connectivity of the samples decreases due to the disorganized arrangement and massive stacking dispersion of hemp fiber in the matrix [32]. Thus when compared with foaming method by adding air entraining agent [33], synchronized changes of
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porosity and pore connectivity of the structural electrolyte by adding hemp fiber show better. This is mainly due to that the macropores created by air entraining agent are independent bubbles and difficult to connect with each other, but lots of
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interconnected pore structure can be formed by adding large content of hemp fiber.
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Fig. 6. Porosity and pore connectivity of structural electrolytes with various contents of fiber.
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Fig. 7 shows the pore size distribution curve and porosity of structural electrolytes with various contents of fibers tested by MIP. It is clear that most probably aperture
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diameter is about 115µm, 54.6 µm and 86.5µm for the structural electrolyte with 0
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vol. %, 15 vol. % and 20 vol. % of hem fiber, respectively. As the content of hemp
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fiber increases to 15 vol. %, the pore number of structural electrolyte dramatically increases. The structural electrolyte has a total porosity of 13.0%, 24.2 % and 21.1% with 0 vol. %, 15 vol. % and 20 vol. % of hemp fiber, respectively. The pore content obtained by MIP is a little higher than by image analysis, which is mainly due to sampling, but the variation trend of porosity is consistent with that of image analysis.
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Fig. 7. Pore size distribution curve (a) and porosity (b) of structural electrolytes with various contents of fibers tested by MIP.
3.3. Electrochemical performance of structural supercapacitors Fig.8 displays the CV curves of structural supercapacitors with different contents of hemp fiber. All the CV curves are relatively in rectangular shapes and exhibit near
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mirror-image current response on voltage reversal, meaning ideal capacitive behavior.
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Based on the area of CV curves, the specific capacitance of the structural
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supercapacitor initially increases with the increasing content of hemp fiber and then
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decreases, which are resulted from the porosity and pore connectivity of structural
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electrolytes. Here, as the volume content of hemp fiber increases from 15 vol. % to 20 vol. %, the area of CV curves decreases, which is mainly due to that the pore structure
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of structural electrolyte decreases by 33.5%. Thus the pore structure of structural
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electrolyte significantly affects the specific capacitance of structural supercapacitor.
Fig. 8. CV curves of structural supercapacitors with various contents of hemp fiber.
From Fig. 9a, the typical triangular-shaped galvanostatic charge-discharge curves of the structural supercapacitors with different contents of hemp fiber are highly linear and symmetrical without obvious iR drop, exhibiting nearly perfect capacitive behavior. In Fig. 9b, as the content of hemp fiber increases from 5 vol. % to 15 vol. %,
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connectivity of structural electrolyte. It can be deduced that aqueous electrolyte can
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be stored in the pores of matrix to assemble structural electrolyte, and aqueous
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electrolyte can pass quickly from the surface to underlying layers of the matrix
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through the interconnected pore channels during charge-discharge. Similar findings
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have been reported by J.M. Xu, W.Y. Ma and N. Shirshova, they all agreed that ionic conductive phase was not only stored in the pore structure of matrix, but also
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transmitted to the electrode surface through the pore structure during charge-discharge.
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Yet they didn’t discuss the effect of pore structure on the specific capacitance of the
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structural supercapacitor in depth. The higher porosity in the matrix, the more KOH aqueous stored. And the higher pore connectivity in the matrix, the more passageways provided for ion movement. Thus the specific capacitance of structural supercapacitor is mainly affected by the porosity and pore connectivity of structural electrolyte. Furthermore, the specific capacitance (51.4 F·g-1) of the structural supercapacitor reaches to 41% of solid-state supercapacitor (124.5 F·g-1) under the same test conditions, which is promising to develop structural electrolyte combining magnesium phosphate with KOH aqueous. Fig. 9c shows the Nyquist plots of structural supercapacitors with various contents
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of hemp fiber. The nearly vertical line in the low-frequency region is a result of ion diffusion in the electrolyte to the electrode surface, signifying an ideal capacitive behavior and the high frequency region represents an arc-like shape [34]. The internal resistance Rs of the structural supercapacitor can be obtained from Nyquist curves by the intercept at real part of horizontal axis. The charge transfer resistance Rct at the
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electrolyte/ electrode interface is represented by the arc formed in a high frequency
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region. Seen from Fig. 9d, as the volume fraction of hemp fiber increases from 5 vol. %
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to 20 vol. %, the curve trend of Rs and Rct are negatively related with that of porosity
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and pore connectivity in structural electrolyte. In addition, as the content of hemp fiber increases to 5 vol. %, Rs of the structural supercapacitor sharply reduces from
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40.2 Ω to 17.4 Ω and Rct decreases from 21.2 to 4.4 Ω due to the strong absorption
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capacity of hemp fiber for aqueous electrolyte. Thus the internal resistance Rs and
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charge transfer resistance Rct are mainly affected by absorption capacity of hemp fiber
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to aqueous electrolyte when small content of hemp fiber is added, yet they are mainly affected by the porosity and pore connectivity of matrix when larger content of hemp fiber is added. Rs data determines the power density of a structural supercapacitor. Low Rct value of the structural supercapacitor reflects the improved transfer mechanism at the interface of electrode/electrolyte. Therefore, structural supercapacitors in this work show better electrochemical performance than that of epoxy system.
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Fig. 9. Electrochemical properties of structural supercapacitors with various contents of fiber: (a)
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Galvanostatic charge-discharge curves; (b) Specific capacitance; (c) Nyquist plots; (d) Rs.
The long-term cycling stability of the structural supercapacitor is evaluated by CV
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method with a scan rate of 100 mV·s-1 for 2000 cycles. From Fig. 10, the specific
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capacitance of the structural supercapacitors with 0 vol. % and 15 vol. % of hemp
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fiber retain about 92.2 % and 88.2 % after 2000 cycles, respectively. The cycling
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stability of the structural supercapacitor slightly decreases after adding 15 vol. % of hemp fiber, which is due to the increase of pore structure of the structural electrolyte. It can be concluded that the structural supercapacitors in this work have good cycling stability during long-term charge-discharge processes.
Fig. 10. Cycling stability of structural supercapacitors for 2000 cycles.
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3.4. Multifunctional analysis of structural supercapacitors Fig. 11a depicted the compressive strength of structural supercapacitors with various contents of hemp fiber. It is seen that an increase in the content of hemp fiber causes a gradual reduction in compressive strength of structural supercapacitor. Although hemp fiber has been used to enhance the mechanical properties of cement
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pastes, it has been reported that hemp fiber has a negative effect on the compressive
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strength of cement matrix, even a small content [35]. In general, high porosity of
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structural electrolyte can reduce the compressive strength of structural supercapacitor,
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but it is not the only factor. Except for porosity, there are still many reasons for the
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reduction in compressive of structural supercapacitor, such as disorganized arrangement of fiber, agglomeration of fiber and surface interactions between matrix
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and fiber.
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To analyze multifunctional properties of the structural supercapacitor, two
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performance parameters are employed in this work, specific capacitance and compressive strength. However, as the mechanical performance and ion conductivity are in a trade-off relationship, an ideal structural supercapacitor is designed in this work. The ideal structural supercapacitor exhibits the highest values of the two parameters among all the samples, with a compressive strength of 24.1 MPa and a specific capacitance of 51.4 F·g-1. Fig. 11b presents the multifunctionality of structural supercapacitors with various contents of hemp fiber. It is known that the shorter the distance of scatter to the ideal point, the closer to the ideal structural supercapacitor. As the content of hemp fiber
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increases from 5 vol. % to 15 vol. %, multifunctionality of the structural supercapacitor gradually increases. Nevertheless, as the content of hemp fiber is smaller than 5 vol. % or greater than 15 vol. %, the multifunctionality of structural supercapacitor decreases with the increasing content of hemp fiber. It can be inferred that the multifunctional properties of the structural supercapacitor is mainly affected
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by the porosity and pore connectivity of structural electrolyte. Lots of pore structure
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causing by adding large content of hemp fiber result in low compressive strength of
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structural supercapacitor, yet such pore structure are beneficial to improve
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electrochemical properties of structural supercapacitor. Moreover, when specific
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capacitance and compressive strength are combined for evaluation of structural supercapacitor, the porosity and pore connectivity are positively correlated with its
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multifunctionality. The sample containing 15 vol. % of hemp fiber appears for the
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strength of 13.2 MPa.
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optimum balance, displaying a specific capacitance of 51.4 F·g-1 and a compressive
Fig. 11. Compressive strength (a) and Multifunctionality (b) of structural supercapacitors with various contents of hemp fiber.
The comparison of this study with other relevant structural supercapacitors is listed in Table 3. It can be seen that the structural supercapacitor with 15 vol. % of
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hemp fiber in this work shows good electrochemical and mechanical properties. In fact, perfect performance of structural supercapacitor is still difficult to achieve, but this work is highly encouraging to improve the multifunctionality of structural supercapacitor by developing pore structure of structure electrolyte with hemp fiber. Table 3 Comparison of this study with other relevant structural supercapacitors. Specific capacitance F/g
Strength MPa
15 vol. % hemp fiber (this work)
51.4
13.2
10
9.5
CAG-CF // PEGDGE-IL
[22]
[36]
Nanoporous Si//epoxy -BMIBF4 CFRP// Li1.4Al0.4Ti1.6(PO4)3
[37]
[38]
33.85
40.92
24.59
0.6025
8.7
3.2
0.2
3.6×10
[11]
50
-5
651 2
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CCNC Nws-CF //PEO-BMITFSI
36.5
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G//Magnesium phosphate-KOH
4. Conclusions
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[21]
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G//Geopolymer-KOH
[16]
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G//Portland cement-KOH
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Structural supercapacitor
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The properties of structural supercapacitor are mainly affected by the pore structure
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of matrix. The pore structure can store aqueous electrolyte as well as provide channels for ion migration during charge-discharge. Large content of fiber often leads to lots of pore structure which are harmful to mechanical properties, yet such pore structure is conducive to improving electrochemical properties of the structural supercapacitor. As the content of hemp fiber increases from 5 vol. % and 15 vol. %, a gradual increase from 8.3 % to 18.6 % in porosity and a gradual increase from 5.6 % to 15.5 % in pore connectivity of the sample, indicating that hemp fiber effectively introduces an increasing number of pore structure in the matrix for storage of KOH electrolyte and enough channels for ion migration. As a result, the electrochemical properties of
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structural supercapacitors gradually increase with the increasing of pore structure, exhibiting the specific capacitance increasing from 35 F·g-1 to 51.4 F·g-1. The multifunctionality of structural supercapacitor was also analyzed, and the sample with the most developed pore structure appears for the optimum balance, displaying a specific capacitance of 51.4 F·g-1 and a compressive strength of 13.2 MPa. Thus this
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work is highly encouraging to improve multifunctionality of structural supercapacitor
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by developing pore structure of structure electrolyte with hemp fiber.
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Acknowledgements
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This work was supported by the NSAF Foundation of National Natural Science
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Foundation of China and Chinese Academy of Engineering Physics under Grant [No. U1730117].
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Author Contribution Statement
Cuiqin Fang: Investigation, Data Curation, Formal Analysis Software, Writing- Original draft
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preparation, Visualization, Validation
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Dong Zhang:
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Methodology, Conceptualization, Resources, Writing- Reviewing and Editing,
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Supervision, Project Administration, Funding Acquisition
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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.
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Graphical abstract
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Highlights 1. The pore structure affects the performance of structural supercapacitor. 2. Porosity of structural electrolyte increases from 8.3% to 18.6% by adding hemp fiber. 3. Pore connectivity of structural electrolyte increases by 176% after adding hemp
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fiber.
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4. Multifunctionality of the sample with 15 vol. % of hemp fiber shows the best.