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Inamori School of Engineering Faculty Scholarship

2021-08

rGO–CMC fiber supercapacitors with core-sheath structure manufactured by coaxial extrusion printing

Ding, Junjun

Gao, Y., Ding, J. rGO–CMC fiber supercapacitors with core-sheath structure manufactured by

coaxial extrusion printing. Journal of Materials Research (2021). https://doi.org/10.1557/s43578-021-00357-5 Springer Nature

https://doi.org/10.1557/s43578-021-00357-5 http://creativecommons.org/licenses/by-nc-nd/4.0

This is the Accepted Manuscript of the following article: Gao, Y., Ding, J. rGO–CMC fiber supercapacitors with core-sheath structure manufactured by coaxial extrusion printing. Journal of Materials Research (2021), which has been published in final form at

https://doi.org/10.1557/s43578-021-00357-5. This manuscript version is made available under the CC BY-NC-ND 4.0 license

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rGO–CMC fiber supercapacitors with core-sheath structure manufactured by coaxial extrusion printing

ArticleCategory : Invited Feature Paper

ArticleSubCategory : Focus Issue: 3D-Printed Electrodes for Energy Storage

Copyright-Holder: The Author(s), under exclusive licence to The Materials Research Society Copyright-Year: 2021

Yuqi Gao ¹, Junjun Ding ^1*

1Materials Science and Engineering, New York State College of Ceramics, Alfred University, Alfred, NY, USA

Rec: 6 July 2021, Accp: 16 August 2021,

Abstract

Fiber-shaped supercapacitors are attractive as an energy storage unit due to their excellent flexibility. However, fabricating robust fibers with large yields remains a challenge. In this work, we prepare flexible core-sheath fibers via coaxial extrusion printing. Carboxymethylcellulose sodium salt (CMC) slurry with controlled rheological properties is extruded from the outer channel, while the graphene oxide (GO) slurry is extruded from the inner channel simultaneously. The followed freeze-drying process protects GO sheets from agglomeration, providing more efficient chemical reduction. The reduced GO (rGO) sheets are separated and expanded to fill in the CMC sheath, which eliminates the delamination between the CMC sheath and rGO core. We study the influences of the freeze-drying process on the fiber microstructures, and explore the slurry design, fiber quality, reduction condition, and electrochemical performance. The fabrication method allows scalable manufacturing of the core- sheath electrodes and fiber-shaped supercapacitors with more efficient conductive networks.

Graphic Abstract

43578_2021_357_Figa_print.png

Keywords: Energy storage, Extrusion, Flexible, Fiber, Freeze drying

Introduction

Energy storage devices have a spreading application in various fields, such as soft robotics, human motion monitoring, and smart textiles [1]. The integration of electronic components with stretchability, lightweight and softness is emerging in the aforementioned applications [2]. Fiber-shaped devices are promising through imparting digit functions on various materials [3]. Core-sheath structure endows the fiber-shaped supercapacitors with robustness and flexibility, and allows the smart designs on multifunctional components [4]. Traditional techniques, such as cast and dip-coating, are costly and time-consuming and limited due to the dependence on the complicated models and unprecise controlling in fiber sizes [5]. The scalable manufacturing of sustainable and uniform fibers with core-sheath structures still needs to be explored and developed. Material extrusion is a flexible technique allowing versatile developments [6]. The modification of a coaxial nozzle and corresponding extrusion controlling systems expand more applications of materials, structures, and functions. Two slurries can be simultaneously extruded through the inner and external channels, resulting the core and sheath layers in one step with controlled dimensions. Given two slurries with desired rheological properties, the core-sheath fiber is fabricated by adjusting the extrusion volume, extrusion velocity speed, and nozzle diameters. A nd the length of continuous fibers is merely dependent on programming extrusion, contributing to the production of homogenous and continuous fibers [7].

Tremendous researchers reported their studies and results [8, 9]. For instance, the carbon nanotubes and silk fibroin core-sheath fibers have been directly printed through the coaxial spinneret via a 3D printer [10].

The advancement of coaxial extrusion comes from building the core-sheath or hollow structure under the programmed control [11]. Extrusion technique is considered to be promising to fabricate the core-sheath

fiber with a large-yield.

Carbon nanomaterials show remarkable thermal, electric and mechanical properties, developing various applications in soft electronics [12]. GO is derivative graphite decorated with oxygen-rich groups, at the ease which the GO suspensions could be stable and homogenous colloidal [13]. The rheological behavior of GO suspensions is highly dependent on GO concentration and sheet size, exhibiting significant compatibility with coaxial extrusion [14]. Moreover, the 2D structure with high surface area and lightweight advantages endows devices with flexibility and robustness. A s a result of inducing oxygen functional groups in the exfoliating process, GO sheets are electrically insulating and tend to aggerate together. rGO is developed to increase the conductivity and porosity. Chemical reaction is an effective method to remove the oxidized groups in GO, resulting rGO at large scale [15]. CMC is an anionic cellulose ether with the characteristics of conducting ionically and insulating electrically [16]. CMC sheath permits ions to penetrate from the electrolyte to active materials and prevents the anodic and cathodic rGO from short circuits. The negatively charged carboxylate ion and hydroxy groups distributed in the polymer chains are responsible for the excellent hydrophilicity of CMC [17]. However, the different hydrophilicities from core and sheath materials result in the different shrinking conditions during the drying process. The separation between core and sheath materials limits the mechanical and electronic properties of fibers [18]. While reports have shown that appropriate slurry concentrations and drying processes are helpful to achieve core-sheath structure with contacted interface [19, 20], coaxial extrusion printing of various materials for energy storage applications are still to be explored.

In this work, core-sheath structural GO–CMC fibers are prepared by coaxial extrusion printing. GO suspension and CMC slurry are extruded through the internal and external syringe nozzles, respectively, realizing the freeform of coaxial fibers. The core GO channel is then chemically reduced into rGO as a conductive electrode. CMC is designed as a sheath to protect the inner conductive rGO core, as well as a separator to avoid short circuits between two electrodes in the fiber supercapacitor. The freeze-drying process and degree of GO reduction show a strong impact in the microstructures and electrochemical performances. With the controlled process, the air gap between rGO and CMC is eliminated to ensure a high- performance fiber supercapacitor. The coaxial extrusion printing broadens the range of extrudable core materials with a broad slurry rheological property. The rGO–CMC fiber supercapacitor shows 51.94 mF/cm² at 0.026 mA /cm². 90.8% of the specific area capacitance remains at the bending angle of 45°, representing the flexibility and stability of the rGO–CMC supercapacitors.

Results and discussion

Coaxial extrusion printing is a promising approach to produce core-sheath structures with precise control over fiber size by slurry volumes in the core and sheath and extrusion speed. A s depicted in Fig. 1, GO and CMC slurries are loaded into two syringes and extruded through a custom-made coaxial nozzle. The core nozzle is located in the center of the outer nozzle, so the core materials are fully enveloped in the outer materials during extrusion. The precious control of volume, extrusion speed, and duration promises a scalable yield of uniform and sustainable coaxial fibers. To maintain the core-sheath structure, the GO–CMC fibers are cross-linked in a coagulation bath. The dense GO is then treated by freeze-drying to increase the surface area. In the freeze drying, ice crystals grow between GO sheets, reducing the interconnections of adjacent GO sheets during the freeze-drying process [21]. Compared to drying at room temperature, freeze- dried GO sheets are better exposed to the reducing agent during the chemical reduction process. The conducting networks composed of porous rGO as the core of fibers. rGO–CMC supercapacitors are then assembled by the prepared rGO–CMC fibers.

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Figure 1 Schematically illustrations of fabricating GO–CMC fibers and assembling supercapacitors.

Extrudable slurries are supposed to be in a certain range of viscosity with shear-thinning rheological behavior [22]. To evaluate the printability, the rheological properties of GO and CMC slurries are characterized and investigated. The viscosity of the applied CMC slurry is approximately 1000 Pa s at the shear rate of 0.01 s⁻¹ as shown in Fig. 2a. The viscosity of GO slurry, attributed to the interfacial interactions [23], reaches 1608 Pa s at the low shear rate of 0.01 s⁻¹ (Fig. 2b). Moreover, the viscosities of CMC and GO slurries decrease at a higher shear rate, which is favorable to a continuous and smooth flow of viscous slurries from the needle.

The storage modulus (G’) and loss modulus (G″) indicate the solid-like behavior and the liquid-like behavior,

respectively. In Fig. 2c, the GO slurry shows that G′ is higher than G″ at the low shear stress, exhibiting the good capability to maintain the geometry after extruding GO fibers. While CMC slurry shows that G′ (150 ~  200 Pa) is bigger than G″ (120 ~ 150 Pa) between 0.1 and 100 Pa stress, and slightly lower than G′ of GO slurry at the stress range of 0.1 ~ 1 Pa. The small difference between G′ and G″ represents the CMC layer is probably deformed after the extrusion process. In consideration of fiber shape retention, a coagulation bath is applied to quickly crosslink the sheath materials (Fig. 2d). The quick solidification contributes to the robustness of sheath, providing support to core materials. The rheology of the core material has a limited influence on the extrusion process. The slurry with a large range of viscosity is allowed to be extruded as a core material.

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Figure 2 (a) The viscosity of CMC slurry; (b) The viscosity of GO slurry; (c) The modulus of CMC and GO slurries; (d) Photos of wet GO–CMC fibers.

SEM is utilized to characterize the morphology of GO–CMC and rGO–CMC fibers. Figure 3a shows the cross- sectional area of non-freeze-dried GO–CMC fibers. Compared to the round shape of coaxial nozzle, the slit shape of the core and sheath suggests a huge deformation in the drying process. The strong interconnections of hydrogen bonding, π-π stacking, and electrostatic interactions contribute to the homogenous GO colloids with the rheological property [24]. When the water is removed, π–π conjugated interaction dominates and forces irreversible aggregations [25]. GO sheets are closely stacking at the higher magnification (Fig. 3b).

CMC also shows high degree of shrinkage and deformation due to the evaporation of water. The different shrinkages cause the separation of the GO core and CMC sheath layer. The non-uniform shrinkage in drying process at room temperature shows a negative influence on the coaxial structure. A lthough a gap between core and sheath still appears after freeze-drying process, the cross-sectional image of freeze-dried GO–CMC fibers shows a near-round and hollow microstructure in Fig. 3c. The diameter of the GO core ranging from 

~ 0.25 to ~ 0.325 mm is slightly smaller tothan than the diameter of the inner nozzle (0.33 mm). The cross- sectional area of CMC (~ 1.13 mm²) is slightly bigger than that of the outer nozzle (1.11 mm²). In terms of the near-round shape and similar diameters between coaxial fiber and nozzle, the freeze-drying process helps to maintain the coaxial fiber at a near-round shape and similar fiber diameters to the size of nozzles by prohibiting materials from non-uniform shrinking. A s a result, the fiber diameter could be designed by the choice of nozzle diameters. Moreover, GO sheets are found unfolded in Fig. 3d. Ice crystals grown in the freezing process impede the interconnections of GO sheets, resulting uniform microstructures and high surface area. The hierarchal GO sheets are achieved after the freeze-drying, providing more opened channels for the subsequent treatment.

43578_2021_357_Fig3_print.png

Figure 3 Scanning Electron Microscopy (SEM) of (a) GO–CMC and (b) GO drying at room temperature; The SEM of (c) GO–CMC and (d) GO treated with freeze-drying.

rGO–CMC fiber is obtained after the chemical reduction process. The stacking GO sheets are separated and expanded in the core channel (Fig. 4a). However, numerous rGO sheets remain overlapping, and air gaps evidently separate GO and CMC when the mass ratio of GO–CMC to reductant is 1:0.5. More GO sheets are expanded when the mass ratio of GO–CMC to reductant is decreased to 1:1 (Fig. 4b). However, further increasing the reductant results less porous microstructures when the mass ratio is at 1:2 (Fig. 4c). The interpretation is that the excessive vapor is produced cause the poor encapsulation. The thickness of CMC sheath is between 40 and 82 µm in Fig. 4a–c. The cross-sections of rGO exhibit more homogenous and significant macropores from the supercritical treatment of freeze-drying in Fig. 4d–f. Thinner GO sheets are produced when the mass ratio is 1:1 comparing to Fig. 4g–j. Besides, the thickness range of CMC sheath is 20  ~  40 µm, indicating the shorter path of ion diffusions created by freeze-drying. Therefore, the freeze- drying process prevents CMC from non-uniform shrinkage and structure collapse, which is supported by the thinner thicknesses of CMC obtained with freeze-drying. A lso, rGO fills in the core of the fiber and eliminates the air gap between the core and sheath. The interconnection between rGO and CMC promises the effective ion diffusions for the rGO and gel electrolyte. The cross-sectional area of rGO–CMC fiber coated by gel electrolyte is shown in Fig. 3k, l. rGO is in contact with CMC sheath coated gel electrolyte, where the gel electrolyte thickness is approximately 30 ~ 40 µm. The coaxial structure is evidently preserved after coating the gel electrolyte.

43578_2021_357_Fig4_print.png

Figure 4 SEM of (a) rGO–CMC (1:0.5), (b, j) rGO-CMC (1:1) and (c) rGO–CMC (1:2) prepared by unfreeze- dried GO–CMC fibers; The SEM of (d, g) rGO–CMC (1:0.5), (e, h) rGO–CMC (1:1) and (f, i) rGO–CMC (1:2) prepared by freeze-dried GO–CMC fibers. The SEM of (k, l) rGO–CMC (1:0.5) supercapacitors.

The reduction process constructs the porous morphology of core materials and increases the conductivity by reducing oxygen-containing groups. The removal of oxidized groups from GO to rGO can be affected by the weight ratio of rGO–CMC fiber to the reducing agent. FTIR spectra of rGO (1:0.5), rGO (1:1), rGO (1:2) are employed to confirm the degree of loss of oxygen - functional groups in Fig. 5. The typical peaks of GO include –OH stretching groups at 3700–3000 cm⁻¹, carbonyl C=O at 1612 cm⁻¹, aromatic C=C at 1427 cm⁻¹, and epoxide C–O–C at 1028 cm⁻¹ [15, 26, 27]. The peak at 1427 cm⁻¹ disappears when GO is treated by hydrazine, confirming the successful reduction. For the rGO from rGO–CMC (1:0.5) and rGO–CMC (1:1) electrodes, the intensities of characteristic peaks from –OH, C=C and C–O–C are decreased. The intensity of rGO–CMC (1:1) is lower than that of rGO–CMC (1:0.5), which demonstrated the lower containing of oxidized groups and a higher level of reduction. The remaining oxidized groups come from the unreacted hydroxyl and epoxy groups, and carboxyl groups that are not able to be reduced [25]. rGO–CMC fibers reduced by more reducing agent shows the strong bands of vibration, while the more reducing agent has a higher requirement for the encapsulation of chemical reaction. When the weight ratio of GO–CMC to reducing agent is 1:2, the encapsulation of vapor reaction was lost under the high pressure from the vapor expansion.

A greeing with the cross-sectional morphologies, FTIR spectra show that the synthesized GO are partially reduced in the reaction environment, and the rGO-CMC (1:1) material performs a higher reduction degree compared to rGO-CMC (1:0.5) and rGO-CMC (1:2) fibers.

43578_2021_357_Fig5_print.png

Figure 5 Fourier Transform Infrared Spectroscopy (FTIR) spectra of rGO exfoliated from rGO–CMC (1:0.5), rGO–CMC (1:1), rGO-CMC (1:2) electrodes.

With the protection of CMC sheath, the symmetrical supercapacitors could be directly assembled by intertwisting two segments of rGO–CMC fibers and coating a layer of Poly(vinyl alcohol)/H₂SO₄ gel electrolyte on the surface. Their electrochemical performances are evaluated and graphically described in Fig. 6. The GCD curves of the rGO–CMC (1:1) supercapacitor are straightforward to show the charging and discharging curves (Fig. 6a), which show some asymmetrical triangular shapes at the different current densities, and a voltage drop is observed due to the internal resistance. It comes from the remaining oxygen- containing groups and the slow migration of ions in gel electrolytes. The discharging time is increased as the current density is decreased, indicating a higher capacitance at 0.026 mA /cm². A s shown in Fig. 6b, the maximum area capacitance of rGO-CMC (1:1) is 51.94 mF/cm² at 0.026 mA /cm², corresponding to the volumetric capacitance of 858.17 mF/cm³. Besides, the area capacitances are 34.13 mF/cm², 23.08 mF/cm², 17.67 mF/cm² and 0.62 mF/cm² at 0.042 mA /cm², 0.052 mA /cm², 0.062 mA /cm², and 0.078 mA /cm², respectively. The decreasing trend of capacitance reflects the low effective diffusion of ions at the higher scanning rate. The CV curves of the rGO–CMC (1:1) supercapacitor exhibit the shapes of oblique fusiform (Fig. 6c). A s the scanning voltage becomes smaller, CV curves are more symmetrical. A lso, the current responses for conversing voltage were more sensitive, contributing to the more rectangular curves. The GCD curves of rGO–CMC (1:1), rGO–CMC (1:2), and rGO-CMC (1:0.5) supercapacitors are compared in Fig. 6d.

The area capacitances of rGO–CMC (1:1), rGO–CMC (1:2) and rGO–CMC (1:0.5) supercapacitor are 51.94 mF/cm², 42.46 mF/cm² and 24.35 mF/cm² at 0.026 mA /cm², which agrees with the microstructures and reduction level. rGO–CMC (1:1) supercapacitor has a higher capacitance for the higher surface area and conductivity.

43578_2021_357_Fig6_print.png

Figure 6 (a) Galvanostatic charge–discharge (GCD) of rGO–CMC (1:1) supercapacitor at 0.026 mA /cm², 0.042 mA /cm², 0.052 mA /cm², 0.062 mA /cm²; (b) A rea capacitance versus current density; (c) Cyclic Voltammetry (CV) of rGO–CMC (1:1) supercapacitor at 200 mV/s, 100 mV/s, 50 mV/s, 20 mV/s; (d) GCD of rGO–CMC (1:1), rGO–CMC (1:2) and rGO–CMC (1:0.5) supercapacitor at the current density of 0.026 mA /cm².

The cycling stability is measured and summarized by the capacitance variation versus 1800 cycles in Fig. 6a,

the capacitance drops at the beginning of 250 cycles, and an increase is followed as the conducting networks are activated. The capacitance retention is 95% after 700 cycles and maintains to above 80% until 1600 cycles. The EIS plot reveals the equivalent resistance in supercapacitors (Fig. 7b). The intrinsic ohmic resistance (R_s) and interfacial charge transfer resistance (R_ct) of the rGO-CMC supercapacitors are in kilohms according to the intercept of the plot and semicircles, which is probably caused by the limited reduction degree and CMC thickness [19, 20]. Decreasing the core diameter and sheath diameter is a strategy to improve the reduction degree and enhance more effective transitions of charges. The tail of rGO–CMC (1:1) has a steep slope than that of rGO–CMC (1:0.5), it represents the fast ion diffusion process resulted from the higher porosity and conductivity.

43578_2021_357_Fig7_print.png

Figure 7 (a) The area capacitance retention after 1800 cycles. (b) Electrochemical Impedance Spectroscopy (EIS) of rGO–CMC (1:1) and rGO–CMC (1:0.5) supercapacitors; (c) The third charging and discharging curves of rGO–CMC (1:1) at the bending angles of 0°, 45°, and 90°; (d) The area capacitance retention at the bending angles of 0°, 45°, and 90°, the error bar is based on three data.

The flexibility of the fiber-shaped supercapacitor is incompatible with the electrochemical performance, because the associated deformations can cause the rearrangement of microstructures and the rupture of conducting channels. The flexibility of the fiber-shaped supercapacitors was characterized and displayed in Fig. 7c, d. The GCD curves of rGO–CMC (1:1) supercapacitors are recorded at the bending angles of 0°, 45°, and 90°, where the bending angle is defined as the supplementary angle of one of the ends. A pproximate 9.2% of C_A was lost when the bending angle is 45°. A slight split on the CMC sheath and rGO core was visible at 90°, but the porous networks keep work and 68.1% of C_A remained. The as-prepared supercapacitors (4 cm) were proved to work at blending and stayed 90.8% of the original C_A at 45°. The stability, contributed by the CMC sheath, can protect the fiber supercapacitors from worn and short circuits during the practical usage. Thus, the flexible rGO-CMC supercapacitors are favorable for the promising applications on soft, textile, and wearable electronics [20], and the manufacturing method provides a smart strategy to produce the controlled structure of the core and sheath layer.

Conclusion

The GO–CMC fibers with core-sheath structure were successfully fabricated by coaxial extrusion. The precise control of the nozzle size, extrusion volume, and speed enables the large-scalable production of uniform coaxial fibers. The GO core was reduced to rGO by the chemical removal of oxygen-containing groups. The rGO–CMC fibers were fabricated as electrodes, where rGO and CMC were designed as conductive core and a protective sheath. The freeze-drying process contributed to the separation of GO sheets, effectively promoting the reduction results. rGO sheets were expanded and contact with CMC sheath. The fiber supercapacitors, assembled by two rGO–CMC fibers, performed an improved capacitance of 51.94 mF/cm² at 0.026 mA /cm². The capacitance retention maintains above 80% within 1600 cycles, and 90.2% capacitance was remaining when the fiber supercapacitor was bent at 45°.

Materials and methods Materials

Graphite powder (A cros organics), Sulfuric acid (H₂SO₄, 95–98%, A ldrich), Potassium permanganate (KMnO₄, 99%, A lfa A esar), Hydrogen peroxide (H₂O₂, 34.37%, Fisher Scientific), Muriatic acid (HCl, 31.45%, Sunnyside), Carboxymethylcellulose sodium salt (CMC, Sigma), Calcium Chloride (CaCl₂, Fisher Scientific), Hydrate Monodrazine (H₄N₂·H₂O, Tokyo Chemical Industry), Poly(vinyl alcohol) (PVA , M_w 89,000–98,000, A ldrich).

Methods

Preparation of GO and CMC slurries

GO slurry was prepared by modified Hummer’s method [13]. 1 g graphite powders were dispersed to 10 ml H₂SO₄. 7 g KMnO₄ were mixed with 30 ml concentrated H₂SO₄ and added into the graphite dispersion. A fter stirring for 2 h in the ice bath, the mixtures were kept stirring in a water bath between 35 °C and 40 °C.

150 ml deionized water was dripped to the mixtures in 12 h. 5 ml H₂O₂ was added to the mixtures. The precipitate was collected after the centrifugation and then washed with 5% HCl 5 times and deionized water 4 times, respectively. The graphene oxide solution with a concentration of 17.8 mg/ml was prepared and collected.

Preparation of rGO–CMC fibers

To obtain uniform GO and CMC slurries, the GO solution was stirred at 5000 rpm for 15 min before extrusion. 0.4 g CMC was solvent by 10 g deionized water and magnetically stirred at 200 rpm for 3 h. GO (17.8 mg/ml) and CMC slurries (40 mg/ml) were loaded to two separated syringes and extruded through the internal channel and external channel simultaneously, where the coaxial nozzle was assembled by a 23-gauge needle and 16-gauge needle. The extrusion speed was 6 mm/s. A fter curing in the coagulation bath of 5 wt% CaCl₂ and ethanol/water (V:5/1) for 30 min and washing with ethanol, the wet fibers were frozen by liquid nitrogen for 1 h and dried at vacuum condition for 24 h. 0.06 g GO–CMC fibers and 0.03 g H₄N₂·H₂O were put into a glass valval with a cover at 90 ℃, coaxial rGO–CMC (1:0.5) fibers were produced and collected in 30 min. Similarly, rGO–CMC (1:1) fibers were prepared by 0.06 g GO–CMC fibers and 0.06 g H₄N₂·H₂O, rGO–CMC (1:2) fibers were prepared by 0.06 g GO–CMC fibers and 0.12 g H₄N₂ ·H₂O. A ll the processes were conducted under the fume hood.

Preparation of gel electrolyte and rGO–CMC fiber supercapacitors

1 g PVA was solvent to 5 g deionized water at 95 ℃. 1 g H₂SO₄ was diluent by 5 g deionized water and added to the clear PVA solution. PVA /H₂SO₄ gel electrolyte was prepared. Two segments of rGO-CMC fibers were twisted and coated with PVA /H₂SO₄ gel electrolyte at room temperature. The rGO–CMC fiber-shaped supercapacitors were fabricated after drying at room temperature.

Characterizations

The rheological properties of GO and CMC slurries were determined by a DISCOVERY HR02 hybrid rheometer (TA Instruments). A diameter of 40 mm parallel steel plate was used for the test at room temperature, where the shear rate was in the range of 0.01  ~  100 s⁻¹, the stress was between 0.1 and 1000 Pa. The microstructures were observed by the SEM (FEI Quanta 200). The infrared spectra were characterized by a Bruker Invenio R-FTIR Spectrometer with A 225/Q-Pt ATR Multiple Crystal CRY Diamond accessory, where the wavenumber was from 4500 to 400 cm⁻¹ with a resolution of 2 cm⁻¹.

Electrochemical measurement

The electrochemical performance was based on a two-electrode system. The length of rGO–CMC fiber supercapacitors was 4 cm. CV and GCD analysis was conducted by a CHI 6015E (Shanghai Chenhua). EIS was tested by a CHI650E (Shanghai Chenhua) between 100 kHz to 0.01 Hz. In the electrochemical measurement process, the anode and cathode were fixed on glass slide with silver paste and copper paste, respectively.

Calculation of the specific area capacitance for supercapacitors

The specific area capacitance (C_A) was calculated based on GCD curves. The equation is

\ (

[C_{{\text{A }}} = \frac{2 \cdot I \cdot t}{{A \cdot U}}\

) ]

where I is current, t is the discharging time, A is the product of the working length and cross-sectional

circumstance, U is the potential window [28].

The volumetric capacitance (C_V) was calculated using the equation

\[ C_{{\text{V}}} = \frac{I \cdot t}{{2 \cdot V \cdot U}} \]

where V is the volume of single electrode, which is equal to the product of surface area and electrode length [29].

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Acknowledgments

This work is supported in New York State College of Ceramics at Alfred University. FundingFaculty Startup Fund in New York State College of Ceramics at Alfred University.

Declarations Conflict of interest

The authors declare that there is no conflict of interest.

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