High energy density of flexible graphene supercapacitors with discharge times controlled by silica microparticles

Synthetic Metals 261 (2020) 116327

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High energy density of flexible graphene supercapacitors with discharge times controlled by silica microparticles

T

A.I. Mtz-Enriqueza, A.I. Olivab, C. Gomez-Solisc, J. Martinez-Ligasd, M. Velazquez-Manzanaresd, J. Olivae,* a

Cinvestav Unidad Saltillo, Parque Industrial, Ramos Arizpe, Coahuila, 25900, Mexico Cinvestav, Unidad Merida, Depto. de Física Aplicada, Merida, Yucatan, 97310, Mexico c Departmento de Ingeniería Física, Universidad de Guanajuato, 37150, Leon, Mexico d CONACYT-Facultad de Ciencias Quimicas, Universidad Autónoma de Coahuila, 25280 Saltillo, Coahuila, Mexico e División de Materiales Avanzados, Instituto Potosino de Investigación Científica y Tecnológica A. C., Camino a la Presa 2055, 78216, San Luis Potosí, SLP, Mexico b

A R T I C LE I N FO

A B S T R A C T

Keywords: Flexible composite Graphene supercapacitor Silica microparticles Energy density

This work reports the electrochemical performance of flexible symmetric supercapacitors which employs two flexible graphene electrodes (FGEs) and silica microparticles (SiO2-MPs) in the electrolyte solution. The discharging time of the supercapacitors increased up to six times after increasing the SiO2-MPs content from 10 to 30 wt%. The introduction of SiO2-MPs played two main roles: i) to enhance the specific surface area of the electrodes (up to 145 % with respect to the electrodes without SiO2- MPs), which in turn, raises the capacity for the ion storage in the devices; and, ii) to generate the SieO, SiOH, SieOeC chemical bonds and defects sites, which in turn, enhanced the charge storage by redox (faradaic) reactions. The supercapacitors presented a high energy density value of 748 Wh/kg and specific capacitance of 1663.3 F/g due to the charge storage by the redox reactions and to the high specific surface area available in the FGEs. The long discharge times, high energy densities and flexibility of the supercapacitors, suggest that they could be useful for applications of portable devices which need long operating times.

1. Introduction The development of flexible energy storage devices with miniaturized dimensions is a new technological trend which can boost the use of supercapacitors on clothes, textiles or curved surfaces [1,2]. Some reports studied the performance of wearable supercapacitors, where the electrodes are fibers made of graphene or carbon nanotubes (CNTs): Cai et al. [3] reported graphene fibers coated with nickel-cobalt for the fabrication of flexible and light-weight supercapacitors which produced a high voltage of 1.5 V and a maximum areal capacitance of 568 m F/ cm2. Zang et al. [2] fabricated thin-film supercapacitors that use graphene woven-fabric (GWF) films as electrode materials. The GWF films were synthesized by using direct chemical vapor deposition on copper mesh. This device had an area specific capacitance of 267 F/g and 100 % of capacitance retention after 1000 charge-discharge cycles. Other fiber based capacitors used CNTs twisted with nylon threads and decorated with MnO2 fibers to produce capacitance by oxidation-reduction reactions [4]. This device reached a maximum area specific capacitance of 40.9 m F cm−2 while its best energy density was 2.6 mW h/

⁎

cm2, which is comparable to that for other CNTs/graphene yarn, mesoporous carbon/CNTs fiber and polyaniline (PANI)/stainless steel wire supercapacitors (0.95–3.85 mW h/cm2) [4]. The group of Xu et al. [5] deposited nanoflowers of CuO on carbon fiber fabric via a hydrothermal method and their supercapacitor exhibited a specific capacitance of 131.34 F/g, a power density of 145.12 W/kg, and 95.8 % capacitance retention after 2000 charge-discharge cycles [5]. There are other designs that involve the use of CNTs or graphene for the fabrication of electrodes for planar solid state supercapacitors. For example, Ervin et al. [6] printed graphene electrodes on a metallic collector on Kapton tape, obtaining a maximum specific capacitance of 192 F/g and a power density of 10 kW/kg [6]. Gao et al. [7] reported solid state supercapacitors with low cost and environmentally friendly electrodes made of cellulose fibers decorated with reduced graphene oxide (rGO) with specific capacitance of 207 F/g, and capacitance retention of 99 % after 5000 charge-discharge cycles. The combination of graphene with activated carbon for producing electrodes for supercapacitors with maximum capacitance of 59 F/g, power density energy of 65 kW/kg, and capacitance retention of 95 % after 10,000 cycles has been reported [8].

Corresponding author. E-mail address: [email protected] (J. Oliva).

https://doi.org/10.1016/j.synthmet.2020.116327 Received 25 October 2019; Received in revised form 24 January 2020; Accepted 7 February 2020 0379-6779/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. a) Pictures of the FGEs fabricated in this work, b) schematic illustration showing the configuration of the supercapacitors studied in this work. SEM images of the FGEs: c) in-plane surface, and d) cross-section. Insets in c) and d) show the pore size distribution for the in-plane and cross-section views.

Furthermore, thick electrodes of graphene (400 μm) have been employed to fabricate planar supercapacitors with capacitance as high as 172 F/g [9]. In addition, CNTs based supercapacitors have been reported to improve the stretchability and flexibility of supercapacitors. Some electrodes used in those devices are CNTs/ PANI, CNT/polyimide (PI), RuO2@CNTs or CNTs/MnO2 [10–13]. The conductive PANI and PI polymers provide better flexibility to the CNTs electrodes, while the RuO2 and MnO2 oxides are used to enhance the electrochemical performance of the supercapacitors by pseudocapacitive effects [11–13]. These CNTs based supercapacitors show energy densities in the range of 100–300 W h/kg and specific capacitances of 100−320 F/g. Although the devices mentioned above present high specific capacitance and high electrochemical stability, they have several problems such as: i) complex and high cost procedures for the electrodes’ fabrication, ii) they need nanoparticles to generate charge by oxidation-reduction (faradaic) reactions, iii) their encapsulation is deficient, which could cause a leak of the corrosive electrolyte. Moreover, their charging time is similar to their discharge time; therefore, their charge-discharge profiles have triangular shape, which is not convenient because longer times (1−10 min) are required to completely charge the devices. On the other hand, efficient flexible supercapacitors should have the following characteristics in their structure: i) electrodes with high electrical conductivity to rapidly transfer electrons, ii) high flexibility to produce a stable capacitance under deformations, and iii) a solid electrolyte with high chemical stability after thousands of charge/discharge cycles. In order to satisfy these requirements and to produce higher energy densities in flexible supercapacitors, it has been reported hybrid supercapacitors where one of the electrodes is considered as capacitive while the second one is a battery-type electrode. Zhang et al. [14] fabricated a hybrid device where the negative cathode was made of Fe3O4/graphene and the positive electrode consisted of 3D graphene immersed in an organic electrolyte containing LiPF6. This device with 3 V as operating voltage, exhibited a high specific capacity of 1089 mA h/g, but the imbalance kinetics between these hybrid electrodes produced a low density energy of 30 Wh/kg−1. In general, the flexible supercapacitors have energy densities lower than that for batteries due to the limited specific surface area of their electrodes [15]. Moreover, the flexible supercapacitors have other

problems such as: i) the packaging materials and active electrode materials on current collectors have poor mechanical properties and cannot support high strain; ii) they present heavy, thick, and rigid configurations, which avoid their use for wearable electronics, iii) they use flammable and corrosive electrolytes [16,17]. Therefore, these flexible supercapacitors are far from the requirements for mobile or wearable mobile devices. For these applications, it is necessary a fully flexible and thin supercapacitor which combines the high energy density of the modern LIBs and the power density of the supercapacitors. To reach this goal, several research groups have tried: i) to increase the specific surface area of the supercapacitor electrodes (made of carbon or metal materials), ii) to increase the electrical conductivity of the electrodes by combining them with conductive polymers or microparticles, iii) to increase the ionic conductivity of the electrolytes, iv) to introduce microparticles to obtain fast oxidation-reduction reactions, which in turn, generates high density energy, v) to fabricate hybrid supercapacitors where one electrode is capacitive and the other is a battery-type, and vi) to introduce oxide/hydroxide nanomaterials that avoid the agglomeration and restacking of graphene sheets (in the case of graphene based supercapacitors), which decreases the diffusion of the electrolyte ions between the graphene layers [3–9]. Although those previous attempts are promising to increase the energy density of supercapacitors, there are not reports to increase the energy density of the supercapacitors by prolonging their discharge times to the best of our knowledge. Therefore, this work demonstrates that introducing silica microparticles with high specific surface area in the flexible graphene electrodes (FGEs), prolonged the discharge time of the graphene supercapacitors. The silica microparticles delayed the ion diffusion in the electrolyte, produced Faradaic reactions, and increased the capacity of the graphene electrodes to store ions on their surface, this in turn, enhanced the energy density of the supercapacitors. 2. Experimental 2.1. Fabrication of the flexible graphene electrodes (FGEs) and synthesis of the silica microparticles The FGEs were fabricated using graphene nanoplates and a casting/ 2

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electrodes etc.) varied from 0.77 to 0.80 g.

molding method previously reported [18]. A brief description of their fabrication process would be as follows: Basically, graphene powder was dispersed in a mixture of solvents (acetone and IPA). After this, the graphene solution was dropped on a plastic mold, dried, and an acrilyc monomer was dropped on the dried graphene. Once the monomer percolated the dried graphene, the whole graphene composite was cured with UV light and was removed from the plastic mold. This procedure formed the FGEs. The thickness of the FGEs employed in this work was 1.0 ± 0.1 mm. Fig. 1a shows pictures of the fabricated FGEs with different sizes and rectangular shape. The silica porous microparticles were synthesized by a modified sol-gel method reported by Ravikrishna et al. [19]. This procedure produces silica microparticles (SiO2-MPs) with specific surface area of 300−500 m2/g.

2.6. Electrochemical characterization of solid state graphene supercapacitors The electrochemical performance of the flexible supercapacitors was measured at room temperature. Cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) and electrochemical impedance spectroscopy (EIS) of the solid-state flexible supercapacitors were investigated using an electrochemical station (Gamry Galvanostat/ Potentiostat Reference 600) and two-electrode configuration. The CV curves were recorded at a scan rate of 70 mV/s in the potential range of -1.2 V-1.8 V. EIS were achieved applying an AC voltage with 20 mV amplitude in the frequency range from 10 HZ to 100 KHz under open circuit potential conditions. For the two electrodes configuration employed in this work, the specific gravimetric capacitance (Cs) was calculated from the GCD curves with [20,21]:

2.2. Morphological and surface area characterization of FGEs The morphology and the cross-section views of the FGEs were obtained by scanning electron microscopy (SEM) using a microscope Auriga 3916. All the FGEs were cut with a plasma beam to obtain clean (polished) cross sections before their analysis by SEM. Brunauer–Emmett–Teller (BET) specific surface area, pore volume, and pore size distribution of the FGEs were determined by N2 adsorption at room temperature using a Micromeritics ASAP 2020 physisorption analyzer. Before measurements, the samples were degassed under vacuum at 100 °C for at least 12 h.

Cs =

2I∫ V (t ) dt m (ΔV 2)

(1)

where I is the discharge current, ∫ V (t ) dt is the total area under the discharge curve, m is the active mass, and ΔV represents the potential change after a full discharge. The specific energy density (E) in Wh/kg and specific power density (P) in W/kg of the flexible supercapacitors were evaluated from the charge–discharge curves using the equations [20,21]:

2.3. XPS and FTIR characterization of FGEs

E=

1 ⎡ Cs ∙ΔV 2 ⎤ 2⎢ ⎦ ⎣ 3.6 ⎥

(2)

P=

3600∙E Δt

(3)

X-ray photoelectron spectroscopy (XPS) spectra were obtained by using a Thermo Scientific K-Alpha system. The AlKα source produces xrays of 1486.7 eV that are focused to a 200 × 200 μm2 spot (power density = 66 W/m2). The Fourier transform infrared (FTIR) spectra were recorded in the range of 500–400 cm−1 on an ABA (MB300) spectrometer using the KBr method.

Where Δt is the discharging time (s).

2.4. Mechanical and electrical conductivity characterization of FGEs

3. Results and discussion

The samples were prepared according to the ASTM D-638 standard prior to the mechanical measurements. The tensile properties were measured by using a home-made universal testing machine controlled with LabView software with a cross-head speed of 7 × 10−4 mm/s. A GW Instek (GOM-802) four probe equipment was used for measuring the electrical resistance of the FGEs with sizes of 2 × 2 × 0.4 cm3. The probes were firstly calibrated using an indium tin oxide (ITO) substrate and five readings were taken in 10 different FGEs in order to obtain an average value of the electrical resistance through the volume of the composite.

3.1. Mechanical and electrical characterization of the FGEs The FGEs were fabricated using a polymer/graphene (P/G) ratio of 25/75 wt% [18]. These FGEs had an elastic modulus of 18.2 ± 0.1 MPa, an electrical conductivity of 8.1 S/m and a sheet resistance of 5–7 Ω/□. An advantage of our fabrication method for the FGEs is the fact that it can be easily modified to produce FGEs with different sizes and shapes as illustrated in Fig. 1a. According to the BET measurements, the specific surface area of the FGEs was 112 m2/g, which is comparable with other surface area values (100−200 m2/g) previously reported for graphene electrodes, which were employed in high performance graphene-based energy storage devices [22,23].

2.5. Assembling of solid state graphene supercapacitors (SCs) The fabrication of the SCs was carried out as follows: two FGEs with 2.5 × 1.0 cm2 size were coated on one side with a gel electrolyte formed by mixing polymethylmethacrylate (PMMA), SiO2-MPs, acetone, distilled water and phosphoric acid (70 % conc.) using a weight ratio of 0.4:0.2:1:1:0.3, respectively. Subsequently, a semipermeable acrylate polymer membrane (APM) with 200 μm-thickness was sandwiched between the two electrodes and mechanically pressed. Next, the formed FGE/APM/FGE system was dried at 95 °C for 2 h. The electrodes of the formed structure were connected with copper wires and encapsulated using a polymer acrylate. Fig. 1b depicts a side view of the graphene supercapacitor to visualize each component. The dielectric layer was the APM. Three graphene supercapacitors were fabricated using 3 different percentages of SiO2-MPs in the gel electrolyte: 10 %, 20 % and 30 % wt%. The devices fabricated with these silica percentages were named as GS-10, GS-20 and GS-30, respectively. The total mass of the devices (which includes copper wires, encapsulation,

3.2. Structural and morphological properties of the FGEs The porosity, electrical conductivity, impurities, and defects in the FGEs are intimately related with the performance of the flexible supercapacitors [24–26]. The surface of the FGEs was analyzed by SEM in order to visualize the arrangement of the graphene nanoplates. Fig. 1c shows that the FGEs’ surface is composed by graphene nanoplatelets randomly aligned and overlapped in the in-plane direction. The pore size on the FGEs’ surface was in the range of 15−60 μm, see red circles in Fig. 1c. No presence of polymer on the surface of the FGEs was observed, which means that the entire polymer infiltrated into the graphene electrode to provide strong mechanical support. Fig. 1d presents a cross-section view of the FGEs, where pore sizes in the range of 8−65 μm are observed. These pores have irregular shapes and look like grooves. Insets in Fig. 1c-d show the statistics for the pore size in the inplane and through-plane directions. In both cases, most of pores have 3

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Fig. 2. a) cyclic voltametry curves, and b) charge/discharge curves for the supercapacitors studied in this work; c) capacitance retention percentages for the GS-30 device after bending, twisting, and without bending; d) Ragone plot showing the relationship between energy density and power density of commercial Li batteries, supercapatteries, supercapacitors, and superbatteries. Inset in 2a shows the CV curve for the GS-10 device, inset in Fig. 2b shows the charge curve for all the devices and insets in Fig. 2c show pictures of the flexible supercapacitors.

sizes in the range of 20−50 μm (79 % for the in-plane and 72 % for the trough-plane directions). Regarding the structure of the FGEs: a previous report confirmed the presence of graphene through XRD measurements. This graphene had a multilayer structure and very low content of defects as demonstrated by Raman measurements [18].

energy density with respect to the other devices. It is worthy to mention that all the CV curves showed good capacitance even in the negative window (from 0 to -1.2 V), indicating that the change of polarity does not affect its performance. This occurred because two graphene electrodes of the same type were employed (symmetric supercapacitor). The capacitance values were calculated in the positive window from 0 to 1.8 V using the GCD curves as demonstrated later. Previous reports have used composite materials (made of oxides and carbon or polymer and carbon materials) as electrodes with redox effects because they can increase from 10 to 100 times the capacitance of the devices [29]. Therefore, some research groups have reported the use of composites such as graphene/PANI (conductive polymer), rGO/PANI, or MOMPsgraphene (MOMPs = metal oxides nanoparticles = RuO2, MnO2, Co3O4 and NiO) as redox electrodes and have obtained capacitance values in the range of 200−800 F/g [1,30]. Although the devices made with these electrodes provided high specific capacitances, they are unstable and work only few hundred of cycles. This is due to the poor reproducibility of the nanoparticle's distribution into the carbon electrode. In our case, we observed the redox reactions without incorporating a conductive polymer or oxide nanoparticles, which reduces the fabrication cost with respect to these devices previously reported. Fig. 2b shows de charge/discharge curves for a charging current of 10 mA. All the devices require 10 s to be completely charged (see inset in Fig. 2b) and the discharge time increased with the content of silica. The discharge times were 1922s, 2775s, and 12050s for the GS-10, GS20 and GS-30 devices, respectively. The presence of silica was a key factor to have long discharge times. However, the discharge time lasted only a few seconds in the GS-NS device made without silica, see Figure S1b in supporting information. The discharge times obtained in this work are higher than these reported for high efficient solid state supercapacitors or supercapatteries which normally are in the range of 50–1000 s [1,28,31–35]. This opens the possibility to obtain higher values of capacitance and energy density, because both parameters are directly proportional to the discharge time [36]. Thus, we will discuss from here, how the capacitance and energy density values change in

3.3. Electrochemical performance of the solid state flexible supercapacitors The FGEs were used to fabricate the flexible supercapacitors. The solution PMMA/AP/acetone/water/ SiO2-MPs /H3PO4 was chosen as the electrolyte and a semipermeable acrylate polymer membrane was employed as the electrode's separator. We used two FGEs as the anode and cathode for the graphene supercapacitor. Fig. 2a presents the cyclic voltammetry (CV) curves of the GS-10, GS-20 and GS-30 devices recorded at a scan rate of 70 mV/s. The potential window ranged from -1.5 to 1.8 V for the GS-10 device and from -1.2 to 1.8 V for the GS-20 and GS-30 devices. The working potential window for the graphene supercapacitors is wider than that reported in previous flexible supercapacitors (1−2 V) [1,27,28]. The CV curve for GS-10 device (made with a gel electrolyte whose silica content was 10 wt%) is presented in the inset of Fig. 2a. This curve presents two shoulders (cathodic and anodic peaks at -0.43 V and 0.17 V, respectively), which suggest the presence of diffusion-controlled redox (faradaic) reactions [15]. The GS-20 device (which contained 20 wt% of silica in its gel electrolyte) had a distorted CV curve, which presented redox peaks more intense than these for the GS-10 device, see Fig. 2a. At this point, it is possible to say that the GS-10 and GS-20 devices store charge by redox reactions. The GS-30 device (which contained 30 wt% of silica in its gel electrolyte) presented prominent Faradaic peaks, suggesting that the redox reactions are now dominant. This is possible by the interaction of the silica with the FGEs. The maximum anodic current reached for GS-10, GS-20 and GS-30 devices were 6.4, 4.9 and 14.2 mA, respectively, see Fig. 2a and its inset. For comparison, a graphene supercapacitor named GS-NS was fabricated without silica and its maximum anodic current was only 3.3 mA (see Figure S1a in supporting information). This device presented a narrower CV curve, which means a lower capacitance/ 4

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rGO/MoO3/PANI [14,35,37,38] as electrodes. It is worthy to mention that the power density values decrease with the silica content, this occurred because the discharge time increases with the content of silica. In other words, a higher content of silica in the devices produces a slower release of charge, which in turn, causes a reduction of the power density. According to the Ragone plots, the commercial Li batteries have energy densities in the range of 100–250 W h/kg and power energies in the range of 10-100 W∙kg−1 [30,31]. The ultimate trend is reaching very high power densities and energy densities simultaneously [40]. The Ragone plot in Fig. 2d was drawn considering each type of device reported in literature as well as their values of power density and energy density [3–40]. It is clear that our best GS-30 device shows high energy density in comparison with the conventional Li ion batteries [30]. Nevertheless, the value of power energy should be enhanced in our devices in order to fall in the red region for the next generation of devices (superbattery region in Fig. 2d). In general, the SCs devices presented in this work have lower charging times, larger discharge times, and higher values of energy density than the previous flexible carbon-based supercapacitors.

Table 1 Electrochemical characteristics of the flexible supercapacitors studied in this work. Silica content (wt%)

Device

Specific capacitance (F/g)

Energy density (Wh/kg)

Power density (W/kg)

0 10 20 30

GS-NS GS-10 GS-20 GS-30

5.7 367.7 417.3 1663.3

2.6 165.4 187.8 748.5

65.9 309.2 243.6 223.6

our graphene supercapacitors with the silica content. The values of specific capacitance, energy density, and power density are summarized in Table 1 (the values in this table were obtained by averaging the results of 10 devices made with different content of silica). The capacitance was calculated considering the mass of both electrodes and the working potential was 1.8 V (since the drop of voltage started at 1.8 V and finished at 0 V). The total mass used for the calculations of capacitance were: 8.7 ± 0.1 mg, 10 ± 0.1 mg, 12.7 ± 0.1 mg and 13.8 ± 0.1 mg for the GS-NS, GS-10, GS-20 and GS-30 devices, respectively. As observed in Table 1, the values of specific capacitance and energy density increase with the content of silica. The highest values of specific capacitance and energy density (at 0.15 A/g) were found for the GS-30 device, that is, 1663.3 F/g and 748.5 Wh/kg, respectively. The specific capacitance of the GS-30 device is higher than these reported for graphene-, rGO- or CNT- based storage devices, which had specific capacitances in the range of 90−400 F/g [14,15]. Moreover, the capacitance retention for the GS-30 device was calculated for 1800 cycles of charge/discharge as depicted in Fig. 2c. This plot shows that the maximum of capacitance (1663.3 F/g for which the capacitance retention = 100 %) is maintained for at least 312 cycles but decreased to 1280.7 F/g (capacitance retention = 77 %) after 1800 cycles of charge/discharge. The capacitance retention is above 90 % for the first 700 cycles of charge/discharge, which is acceptable for energy storage applications. The stability of the GS-30 device was also analyzed when it is subjected to deformations such as bending or twisting (see pictures in the inset of Fig. 2c). The capacitance of the GS-30 device decreases from 1663.3 F/g to 1364.1 F/g (a reduction of capacitance by 18 %) after twisting it 100 times. If the GS-30 device is bended 100 times, its capacitance decreases from 1663.3 F/g to 1546.6 F/g (a reduction of capacitance by 7 %). If these devices previously subjected to twisting or bending (100 times) are now subjected to 1800 cycles of charge/discharge, we obtain final capacitance values of 1082.6 F/g and 763.8 F/g for the devices twisted and bended, respectively. Those last values of capacitance were used to calculate the capacitance retention percentages for the twisted and bended devices. As result, values of 70 % and 56 % were obtained for the bended and twisted devices, see the blue and green curves in Fig. 2c. These values of capacitance retention are lower than that of 77 % obtained for the device without deformation but subjected to 1800 cycles of charge/discharge as well. Probably, the diminution of capacitance retention after the deformation of the device was due to the physical damage (cracks or fractures) of the electrodes, which were produced during twisting or bending. On the other hand, the calculated value of energy density (748.5 Wh/kg) for the GS-30 device is higher than these reported for other graphene based supercapacitors or for flexible lithium batteries, which have energy values in the range of 40–250 W h/kg [1,14,15,30,37]. Our value of energy density is 2.3 times higher than the record value reported by Kim et al. (225 Wh/kg) for a graphene based battery [38], and 3.3 times higher than the value reported by Cao et al. (139 Wh/Kg) for a flexible lithium ion battery which employed flexible graphene/ ZnCo2O4 electrodes [39]. In addition, the power density values for the supercapacitor devices reported here (see Table 1) are comparable with these recently reported for flexible supercapacitors (50−450 W/Kg) which employed rGO, CNT/PANI, rGO/PANI, graphene/PEDOT and

3.4. Mechanisms for the charge storage in the solid state flexible supercapacitors There are several reasons for the outstanding performance of our devices. Firstly, the SiO2-MPs incorporated in the electrolyte remains attached to the FGEs due to the presence of a thin PMMA/AP membrane, which binds the SiO2-MPs to the FGEs. Fig. 3a shows a SEM image of the SiO2-MPs deposited on the FGEs. As observed, a porous network with high surface area is formed by the quasi-rounded SiO2MPs, which will benefit the capability of the FGEs to store ions. A closer inspection of the SiO2-MPs clearly shows the presence of pores with sizes in the range of 60−2400 nm, see Fig. 3b. The porous SiO2-MPs have amorphous structure as confirmed by the XRD pattern in Fig. 3c. The smallest SiO2-MPs have sizes of 1−3 μm and the biggest ones have sizes of 10−20 μm. Since the FGEs have pores with sizes of 15−60 μm, it is possible that the SiO2-MPs added in the electrolyte percolates into the pores of the FGEs. Consequently, the SiO2-MPs could remain into the porosity of the FGEs and store electrolyte into their pores. Since the SiO2-MPs have a specific surface area of 315 m2/g, they provide to the electrodes an additional area where the ions of the electrolyte can be stored. In fact, the specific surface area of five FGEs was measured after they absorbed the electrolyte solution with silica (the electrodes were dried prior to the BET measurements). As a result, a specific surface area of 275 m2/g was obtained, which is ≈ 145 % higher than the value obtained for only the FGEs without absorbed electrolyte solution. This last result indicates that the presence of silica on the FGEs increased their total specific surface area, which helps to improve the capacity of the electrodes for storing charge, which in turn, enhances the capacitance and energy density of the supercapacitors. Therefore, a higher value of energy density was observed with the content of the SiO2-MPs, see Table 1. Nyquist plots were also obtained for each device using EIS, see Fig. 3d. These plots were fitted using as model the equivalent electrical circuit showed in the inset of Fig. 3d in order to obtain the Re, Rt, and Rc parameters, which represent the electrolyte resistance, the resistance in the interface electrode/electrolyte and the total cell resistance, respectively [41]. The values of Re were 46, 54 and 650 ohms for GS-10, GS-20 and GS-30 devices, respectively. The values of Rc were 99, 110 and 1326 Ω for for GS-10, GS-20 and GS-30 devices, respectively; while the values of Rt were 53, 56 and 776 for GS-10, GS-20 and GS-30 devices, respectively. It is clear that all the resistances increased with the content of silica, which is reasonable because the SiO2-MPs are a dielectric material with low electrical conductivity. Since the electrolyte resistance increases with the amount of silica, it is expected a slower diffusion of ions (in this case H+ and H2PO4−-/PO4-3 after the dissociation of the phosphoric acid in the aqueous gel electrolyte), therefore, a very long discharge time and high charge storage capability 5

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Fig. 3. a) and b) are SEM images of the microporous SiO2-MPs deposited on the FGEs, c) XRD spectra of the SiO2-MPs and d) impedance of the supercapacitors with different content of SiO2-MPs. Inset in Fig. 3d shows an equivalent electrical circuit for the devices studied in this work.

are observed, see Fig. 2b. In fact, it has been published that the silica nanoparticles with size of ≈200 nm, can delay the electron recombination at the interface of AgPO4/SiO2 (a photocatalytic material for dyes degradation) after electron photogeneration [42]. In our case, the deep pores and defects in the SiO2-MPs could act as trapping centers for ions, which would delay the extraction of charge and would increase the discharging time. As result, the discharge time of the supercapacitors and their capacity to store charge can be controlled with the silica content. During the discharge time, anions and cations are separated from the two electrodes and bulk redox reactions occur at the electrodes. At the same time, the ion separation or rapid charge transfer occurs at the capacitive electrode, generating the electrical current that can be collected by an electrical circuit. Taking into account this mechanism, it would take more time for the ions stored into the pores of the silica MPs (which are in turn located into the porous network of the FGE) to be separated from the electrodes, Thus, it is expected longer discharge times in devices with higher content of silica. This could explain why the discharge time was the longest for the GS-30 device with respect to the other devices with lower silica content. We should mention that another reason for the long discharge time in device GS-30 could be the thermodynamic instability of its electrodes in the charged state. This means that the charged potential would surpass the thermodynamic limitation of the electrodes, causing their Faradaic decomposition [43]. If this effect occurs, additional charge could be provided by the Faradaic decomposition of the electrodes, which prolongs even more the discharge time. Finally, the presence of silica in the composite could also avoid the aggregation of graphene nanoplates, which is considered detrimental for the electrochemical performance [44]. In order to elucidate how the redox process occurs in the FGEs. They were analyzed by FTIR and XPS techniques. Fig. 4 presents the FTIR spectra of the FGE which absorbed the electrolyte solution with silica (such electrode was used to make the GS-30 device), see blue curve.

Fig. 4. FTIR spectra for the SiO2-MPs, FGE + SiO2-MPs electrode (GS-30 device) and PMMA/acrylate polymer.

This figure also shows the FTIR spectra of the SiO2-MPs and PMMA/ acrylate polymer individually, see red and black curves. As expected, the FTIR spectra confirmed that FGEs contained SiO2-MPs but a shift of the peak corresponding to the SieOeSi (from 1045 to 969 cm−1) bond also occurred, compare the blue and red curves in the range of 450−1200 cm−1. The bands at 808 and 793 cm−1 are attributed to SieOH and OeSieO bonds, respectively. The bands in the range of 1190−1765 cm−1, 2345 cm−1 and the band at 2921 cm−1 correspond to the polymer formed by PMMA/acrylate polymer which binds the SiO2-MPs to the FGEs, while the band centered at 3328 cm-1 is associated to OH groups. Thus, these bands indicate the presence of silica and polymer used for the preparation of the electrolyte on the FGEs. 6

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Fig. 5. a) and b) XPS spectra for the C1s and O1s orbitals of the FGE without absorbed electrolyte (without SiO2-MPs on its surface), c) FTIR spectra of the FGE without absorbed electrolyte, d) and e) XPS spectra for the C1s and O1s orbitals of the FGE with absorbed electrolyte (with SiO2-MPs on its surface) and f) XPS spectra for the Si2p orbital (taken from the FGE + SiO2-MPs electrode).

associated to SieOH bonds [57,58]. Furthermore, the oxidized surface of the FGEs could facilitate the formation of these chemical bonds between the FGEs and the SiO2-MPs because it contains CeOeH, OeH and CeO bonds available, see Fig. 5c. As mentioned above, the supercapacitors store charge by redox reactions. Such reactions could be achieved through the CeO and OeCeO bonds which are considered as redox centers for the faradaic reactions [38]. In our case, there are additional redox centers such as SieO, SieOH and SieOeC bonds as well as the silica defects formed due to the interaction of the SiO2-MPs and H+ [59]. Taking into account the presence of SieO, SieOH and SieOeC bonds as redox centers for charge storage, we propose the following redox reactions for the charge storage:

Figs. 5a and b present the deconvoluted XPS spectra for the C1s and O1s orbitals, respectively. These spectra correspond to the FGEs without absorbed electrolyte. The deconvoluted spectrum of the C1s orbital presents four bonds: OeCeO, C]O, CeO, and CeC while the spectrum of the O1s orbital shows the components for CeO, CeOH and C]O bonds. These bands correspond to the graphene oxide (GO) bonds as previously reported [45,46]. The presence of GO on the FGEs (before its use for the fabrication of the supercapacitors) was corroborated by FTIR measurements, see Fig. 5c. Two peaks at 1083 cm−1 and 1451 cm−1 correspond to GO and are related with the CeOeC, CeOeH, and CeO bonds [47,48]. Thus, it is possible to say at this point that a layer of GO was formed on the surface of the FGEs employed for the fabrication of the devices. Additionally, the band centered at 3440 cm−1 indicates the presence of residual water adsorbed on the FGEs. The peak at 1621 cm−1 corresponds to the skeletal vibrations of graphene (C]C) [49,50]. Fig. 5d and e show the deconvoluted XPS spectra for the C1s and O1s orbitals respectively, which correspond to the FGEs with absorbed electrolyte, respectively. The deconvoluted spectrum of the C1s orbital presents again four bonds: OeCeO, C]O, CeO, and CeC; but they are slightly broader than that in Fig. 5a corresponding to the FGEs without absorbed electrolyte, which suggests the formation of chemical bonds on the FGEs surface. The formation of these chemical bonds is confirmed by the additional band of SieOH (at 533.5 eV [51,52]) observed in Fig. 5e (compare Fig. 5b and e). Additionally, Fig. 5f (deconvoluted spectrum for the Si 2p orbital) shows the bonds of SieO, SiO2 and OeSieO at 101.7, 104.4 and 103.1 eV [53–55], which confirms the presence of the silica on the FGEs with absorbed electrolyte. The peak at 102.6 eV is attributed to the SieOeC bond [56]. The presence of the SieOH and SieOeC bonds suggests that the SiO2-MPs are chemically bonded to the surface of the FGEs, which would decrease the resistance of charge at the interface electrode/electrolyte. The SieOH bond indicates a chemical bond between the PMMA and the SiO2-MPs, since PMMA could contains OH groups. The presence of the SieOH can be further supported by the FTIR spectra in Fig. 4 (blue curve) since the peak at 880 cm−1 and the broad band from 2250−3700 cm−1 are

(4) (oxidation reaction) SiOOH + OH− ↔SiO2+H2O + e− (5) (reduction reaction) SiO2 + xC + x e− ↔ SiO2-xC The left side in Eq. (4) indicates that the reaction of Si hydroxide (this compound contains SieOH bonds which are considered as redox centers) with OH− groups (from the solvents in the gel electrolyte) could produce silica + water + free electrons and this reaction is reversible. The left side in Eq. (5) indicates that the silica (SiO2) could react with certain amount of carbon (xC) and certain amount of electrons (xe-) to produce silica bonded with carbon (SiO2-C]SieOeC), thus, this last equation demonstrates that the SieOeC can work as a redox center. To find the exact amount of electrons and C necessary for the reaction (5), more specialized experiments are needed, which are out of scope for this article. In general, the remarkable performance of the supercapacitors could be associated to several factors: i) the abundant surface area available for the storage of ions (which enhances by the presence of the SiO2-MPs), ii) the high electrical conductivity of the graphene electrodes, and iii) to the presence of redox centers as a consequence of the chemical bonding between the SiO2-MPs and the FGEs.

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4. Conclusions

[8] I.L. Tsaia, J. Cao, Ll. Fevrea, B. Wang, R. Todd, R.A.W. Dryfe, A.J. Forsyth, Electrochim. Acta 257 (2017) 372–379. [9] H. Li, Y. Tao, X. Zheng, J. Luo, F. Kang, H.M. Cheng, Q.H. Yang, Energy Environ. Sci. 9 (3135) (2016) 31–42. [10] S. Zeng, H. Chen, F. Cai, Y. Kang, M. Chen, Q. Li, J. Mater. Chem. A 3 (2015) 23864–23870. [11] T. Liu, J.J. Xu, Q.C. Liu, Z.W. Chang, Y.B. Yin, X.Y. Yang, X.B. Zhang, Small 13 (2017) 1602952–1602957. [12] G. Wu, P. Tan, D. Wang, Z. Li, L. Peng, Y. Hu, C. Wang, W. Zhu, S. Chen, W. Chen, Sci. Rep. 7 (2017) 43676. [13] S.I. Wong, J. Sunarsoa, B.T. Wong, H. Lin, A. Yu, B. Jia, J. Power Sources 396 (2018) 182–206. [14] Y. Zhang, Y. Jiao, M. Liao, B. Wang, H. Peng, Carbon 124 (2017) 79–88. [15] A. Sing, A. Chandra, Sci. Rep. 6 (2016) 25973. [16] K. Chen, Q. Wang, Z. Niu, J. Chen, J. Energy Chem. 27 (2018) 12–24. [17] A. Sepúlveda, J. Speulmanns, P.M. Vereecken, Sci. Tech. Adv. Mat. 19 (2018) 454–461. [18] J. Oliva, A.I. Mtz-Enriquez, A.I. Oliva, R. Ochoa-Valiente, C.R. Garcia, Q. Pei, J. Phys. D: Appl. Phys. 52 (2019) 025103. [19] R. Ravikhrisna, R. Green, K.T. Valsaraj, J. Sol-Gel Sci. Tech. 34 (2005) 111–122. [20] B. Pandit, D.P. Dubal, B.R. Sankapal, Electrochim. Acta 242 (2017) 382–389. [21] S. Roldan, D. Barreda, M. Granda, R. Menendez, R. Santamaria, C. Blanco, Phys. Chem. Chem. Phys. 17 (2015) 1084–1092. [22] V. Sahu, S. Shekhar, R.K. Sharma, G. Singh, ACS Appl. Mater. Interfaces 7 (2015) 3110–3116. [23] Y. Huang, J. Liang, Y. Chen, Small 8 (2012) 1805–1834. [24] R.E. Triambulo, J. Park, Org. Electron. 28 (2016) 123–134. [25] Z. Gao, Y. Fu, M.M.F. Yuen, J. Liu, Carbon 61 (2013) 342–348. [26] H. Han, Y. Zhang, N. SWang, M.K. Samani, Y. Ni, Z.Y. Mijbil, M. Edwards, S. Xiong, K. Saaskilahti, M. Murugesan, Y. Fu, L. Ye, H. Sadeghi, S. Bailey, Y.A. Kosevich, C.J. Lamber, J. Liu, S. Volz, Nat. Comm. 7 (2016) 11281. [27] J. Kim, W.H. Khoh, B.H. Wee, J.D. Hong, RSC Adv. 5 (2015) 9904–9911. [28] S. Hu, R. Rajamani, X. Yu, Appl. Phys. Lett. 100 (2012) 104103. [29] Handbook of Clean Energy Systems, John Wiley & Sons, Ltd., 2015. [30] Y. Zhao, J. Liu, B. Wang, J. Sha, Y. Li, D. Zheng, M. Amjadipour, J. Mac Leod, N. Motta, ACS Appl. Mater. Interfaces 9 (2017) 22588–22596. [31] M.N. Rantho, M.J. Madito, N. Manyala, Electrochim. Acta 262 (2018) 82–96. [32] K.O. Oyedotun, M.J. Madito, D.Y. Momodu, A.A. Mirghni, T.M. Masikhwa, N. Manyala, Chem. Eng. J. 335 (2018) 416–427. [33] X. Peng, H. Chai, Y. Cao, Yucheng Wang, H. Dong, D. Jia, W. Zhou, Mat. Today Energy 7 (2018) 129. [34] Q. Ke, J. Wang, J. Materiomics 2 (2016) 37–54. [35] Y. Shao, M.F. El-Kady, L.J. Wang, Q. Zhang, Y. Li, H. Wang, M.F. Mousavi, R.B. Kaner, Chem. Soc. Rev. 44 (2015) 3639–3665. [36] G.T. Pan, S. Chong, T.C.K. Yang, C.M. Huang, Materials 10 (2017) 370. [37] V.L. Pushparaj, M.M. Shaijumon, A. Kumar, S. Murugesan, L. Ci, R. Vajtai, R.J. Linhardt, O. Nalamasu, P.M. Ajayan, PNAS 104 (2007) 13574–13577. [38] H. Kim, K.Y. Park, J. Hong, K. Kang, Sci. Rep. 4 (2014) 5278. [39] H. Cao, X. Zhou, W. Deng, Z. Ma, Y. Liu, Z. Liu, Chem. Eng. J. 343 (2018) 654–661. [40] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017) 1600539. [41] S. Zhang, N. Pan, Adv. Energy Mater. 5 (2015) 1401401. [42] M. Sharma, K. Ojha, A. Ganguly, A.K. Ganguli, New J. Chem. 39 (2015) 9242–9248. [43] B.K. Kim, S. Sy, A. Yu, J. Zhang, Electrochemical supercapacitors for energy storage and conversion, Handb. Clean Energy Syst. (2015), pp. 1–25. [44] K. Gao, Z. Shao, J. Li, X. Wang, X. Peng, W. Wang, F. Wang, J. Mater. Chem. A 1 (2013) 63–67. [45] J.R. Rani, J. Lim, J. Oh, D. Kim, D. Lee, J.-W. Kim, H.S. Shin, J.H. Kim, S.C. Jun, RSC Adv. 3 (2013) 5926–5936. [46] X. Peng, J. An, S. Xu, W. Chen, Z. Xu, J. Solid State Sci. Technol. 5 (2016) M51–M57. [47] T. Xian, H. Yang, L. Di, J. Ma, H. Zhang, L. Dai, Nanoscale Res. Lett. 9 (2014) 327. [48] M. Hayyan, A. Abo-Hamad, M.A. AlSaadi, M.A. Hashim, Nanoscale Res. Lett. 10 (2015) 324. [49] J. Zhang, X. Xie, C. Li, H. Wang, L. Wang, RSC Adv. 5 (2015) 29757–29765. [50] N. Jiang, Z. Xiu, Z. Xie, H. Li, G. Zhao, W. Wang, Y. Wu, X. Hao, New J. Chem. 38 (2014) 4312–4320. [51] N. Liu, X. Huang, J.J. Dubowski, J. Phys. D: Appl. Phys. 47 (2014) 385106. [52] A.U. Alam, M.M.R. Howlader, M.J. Deen, ECS J. Solid State Sci. Tech. 2 (2013) P515–P538. [53] J.C. Woo, C.A. Choi, C.I. Kim, Trans. Electr. Electr. Mat. 14 (2013) 216. [54] S.R. Darmakkolla, H. Tran, A. Gupta, S.B. Rananavare, RSC Adv. 6 (2016) 93219–93230. [55] P.H. Wu, X.Y. Yu, C.W. Cheng, C.H. Liao, S.W. Feng, H.C. Wang, Opt. Expr. 19 (2011) 16390–16400. [56] Y.L. Khung, S.H. Ngalim, A. Scaccabarozz, D. Narducci, Beilstein J. Nanotechnol. 6 (2015) 19–26. [57] N. Arad-Vosk, A. Sa’ar, Nanoscale Res. Lett. 9 (2014) 47. [58] A.L. Balieiro, R.A. Santos, M.M. Pereira, R.T. Figueiredo, L.S. Freitas, O.L.S. de Alsina, A.S. Lima, C.M.F. Soares, Braz. J. Chem. Eng. 33 (2016) 361–372. [59] M. Canna, S. Agnello, R. Boscaino, S. Costa, F.M. Gelardi, Nucl. Instr. Meth. Phys. Res. 191 (2002) 401–405.

This work reports flexible SCs with energy density values higher than these previously reported for flexible SCs or Li ion batteries. The supercapacitors stored charge by redox (faradaic) processes in the graphene electrodes. The redox reaction was favored by the chemical bond between the SiO2-MPs and the FGEs or between the SiO2-MPs and the PMMA polymer. The discharging times of the SCs were controlled with the content of SiO2-MPs, since longer discharge times were observed as the content of SiO2-MPs increases. The presence of the SiO2MPs helped to control the diffusion processes of the ions moving through the electrolyte and the interface electrolyte/electrode. This was confirmed by the increase of the parameters Re, Rt and Rc with the content of silica. Moreover, the presence of SiO2-MPs enhanced the available sites for redox reactions in the electrodes, rising the charge storage and energy density. The best graphene supercapacitor has an energy density of 748 Wh/kg which overcomes the energy density values previously reported for Li based batteries. For this reason, we believe that our supercapacitors can pave the way toward the next generation of energy storage devices with high energy density/power density values. Those devices could be used to power portable devices during several days. CRediT authorship contribution statement A.I. Mtz-Enriquez: Resources, Methodology, Writing - original draft. A.I. Oliva: Resources, Methodology, Writing - original draft. C. Gomez-Solis: Investigation, Resources, Formal analysis. J. MartinezLigas: Investigation, Methodology. M. Velazquez-Manzanares: Formal analysis, Methodology. J. Oliva: Validation, Visualization, Writing - review & editing. Declaration of Competing Interest 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. Acknowledgments J. Oliva thanks the financial support of CONACYT for the CATEDRAS-CONACYT program. Authors appreciate the technical work performed by Alejandro May-Pat and Emilio Corona for the mechanical tests and by Felipe Marquez for the SEM images. We thank to LANNBIO (Cinvestav-Mérida) for the XPS measurements. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2020. 116327. References [1] Z. Su, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang, C.P. Wong, Energy Environ. Sci. 7 (2014) 2652–2659. [2] X. Zang, Q. Chen, P. Li, Y. He, X. Li, M. Zhu, X. Li, K. Wang, M. Zhong, D. Wu, H. Zhu, Small 10 (2014) 2583–2588. [3] W. Cai, T. Lai, J. Lai, H. Xie, L. Ouyang, J. Ye, C. Yu, Sci. Rep. 6 (2016) 26890. [4] C. Choi, S.H. Kim, H.J. Sim, J.A. Lee, A.Y. Choi, Y.T. Kim, X. Lepro, G.M. Spinks, R.H. Baughman, S.J. Kim, Sci. Rep. 5 (2016) 9387. [5] W. Xu, S. Dai, G. Liu, Y. Xi, C. Hu, X. Wang, Electrochim. Acta 203 (2016) 1–8. [6] M.H. Ervin, L.T. Le, W.Y. Lee, Electrochim. Acta 147 (2014) 610–616. [7] K. Gao, Z. Shao, J. Li, X. Wang, X. Peng, W. Wang, F. Wang, J. Mater. Chem. A 1 (2013) 63–67.

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