High-Performance Flexible Solid-State Supercapacitor with an

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High performance flexible solid-state supercapacitor with extended nano regime interface through in situ polymer electrolyte generation Bihag Anothumakkool, Arun Torris A. T., Sajna Veeliyath, Vidyanand Vijayakumar, Manohar Virupax Badiger, and Sreekumar Kurungot ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09677 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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ACS Applied Materials & Interfaces

High performance flexible solid-state supercapacitor with extended nano regime interface through in situ polymer electrolyte generation Bihag Anothumakkool*1,3, Arun Torris A T2, Sajna Veeliyath1,4, Vidyanand Vijayakumar1,3, Manohar V Badiger*2,3 and Sreekumar Kurungot*1,3 1

Physical and Materials Chemistry Division and 2Polymer Science and Engineering Division,

CSIR-National Chemical Laboratory, Pune-411008, Maharashtra, India. 3

Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg,

New Delhi-110001, New Delhi, India. 4

Department of Applied Chemistry, Cochin University of Science and Technology, Cochin-

682022, Kerala, India KEYWORDS: Supercapacitor, in situ polymer generation, polymer electrolyte, interface, impedance analysis, cyclic voltametry.

Abstract Here, we report an efficient strategy by which significantly enhanced electrode-electrolyte interface in an electrode for supercapacitor application could be accomplished by allowing in situ polymer gel electrolyte generation inside the nanopores of the electrodes. This unique and highly

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efficient strategy could be conceived by judiciously maintaining UV-triggered polymerization of a monomer mixture in presence of a high surface area porous carbon. The method is very simple and scalable and a prototype, flexible solid-state supercapacitor could be even demonstrated in an encapsulation-free condition by using the commercial grade electrodes (thickness: 150 µm, area: 12 cm2 and mass loading: 7.3 mg/cm2). This prototype devise shows a capacitance of 130 F/g at a substantially reduced internal resistance of 0.5 Ω and a high capacitance retention of 84 % after 32000 cycles. The present system is found to be clearly outperforming a similar system derived by using the conventional polymer electrolyte (PVA-H3PO4 as the electrolyte), which could display a capacitance of only 95 F/g, and this value falls to nearly 50 % in just 5000 cycles. The superior performance in the present case is credited primarily to the excellent interface formation of the in-situ generated polymer electrolyte inside the nanopores of the electrode. Further, the interpenetrated nature of the polymer also helps the device to show a low ESR, power rate and most importantly excellent shelf-life in the unsealed flexible conditions. Since the nature of the electrode-electrolyte interface is the major performance determining factor in the case of many electrochemical energy storage/conversion systems, along with the supercapacitors, the developed process can also find applications in preparing electrodes for the devices such as Li-ion batteries, metal air batteries, polymer electrolyte membrane fuel cells etc.

Introduction Since modern power electronics demand lighter, safer and flexible systems, in the case of the devices meant for energy conversion and storage applications, there has been an increasing pressure to develop more adaptable designs for futuristic applications. Gaining momentum from this demand, the ongoing efforts to utilize

various polymer

electrolytes1-2 in place of the conventional liquid electrolytes in the electrochemical


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energy storage/conversion devices such as Li-ion battery3-4, supercapacitors5, metal-air batteries6 etc. have also been intensified in these years. The polymer electrolytes are expected to impart more safety to the devices by preventing leakage of harmful electrolytes from the devices, which ultimately ensures safer gadgets for the end user. Apart from the safety aspects, replacement of a liquid electrolyte with an appropriate solid-electrolyte can bring in radical changes in the design aspects of the system, especially in terms of the features like flexibility, size and adaptability during system integration7. Nevertheless, the performances of the systems based on the current solidstate/polymer electrolytes are inferior compared to the corresponding liquid-state counterparts owing to the reasons like the relatively low conductivity and the associated difficulties involved in establishing the extended electrolyte interface within the electrodes with the solid/polymer electrolytes7. Among the various choices of the solid electrolytes, polymer gel electrolytes, which are usually obtained by incorporating large amount of conducting solvents such as acids7-8, alkalis5, 9, salts10-11, ionic liquids12-13 etc. in the polymer matrix, show superior conductivity compared to the systems based on the conventional polymer–salt complexes14. The polymer matrix in polymer gel electrolytes normally consists of the polymers like poly(ethyleneoxide) (PEO), poly(vinylalcohol) (PVA),

poly(methylmethacrylate)

(PMMA),

poly(vinylidenefluoride)

(PVdF),

poly(vinylidene fluoride-hexafluoroproplene) (PVdF-co-HFP), poly(acrylonitrile) (PAN) etc15. Initial reports on the solid-state supercapacitors and Li-ion batteries come up with a simple sandwich model, where the polymer electrolyte is normally kept as a film between the electrodes5,

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. However, the limitations induced by the poor and less extended


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electrode-electrolyte interface of the model result in high internal resistance, which restricts such systems from delivering high energy and power densities. Improved performance can be observed in cases where the electrode surface is soaked in the polymer liquid solution, followed by evaporation of the excess solvent17. Though it is performing better than the sandwich model, it is hard to utilize the full surface area of the electrode owing to the high molecular weight of the polymer chains. However, the liquid electrolyte-like performance can be achieved by imparting 3-D architecture18-21 to the electrodes and/or by using ultra-thin electrodes, where, attainment of better electrodeelectrolyte interface is possible. Our group recently reported the need of 3-D architecture in the case of the electrode based on conducting polymer for fabricating a solid-state supercapacitor, which helps to establish better interface between the electrode and the PVA-H2SO4 polymer electrolyte7-8. However, with high mass loading, even this approach fails as it is difficult for a highly viscous polymer solution to penetrate into the small pores of the electrodes. Significant deviation from the maximum attainable capacitance values can be easily visualized while dealing with the electrode materials possessing pores in nano regime, such as in case of the high surface area (SSA) carbons, which are the standard electrode materials utilized in the commercial electrodes. Even though large number of publications is available in literature on flexible capacitors21-22, most of them are focussed on improving the surface area, conductivity and capacitance by creating nanomorphological features on the electrode materials. Apparently, there are no serious efforts given to look into the problem of extending the electrode-electrolyte interface into the porous architecture of the electrode materials while dealing with the polymer electrolytes. The issue becomes more worrisome while trying to


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maintain high mass loading of the electro-active material per cm2 such as in the case of the commercial supercapacitors, where the electrode loading of the high surface area carbon will be more than of 5 mg/cm2. There are few literature reports on solid-state state supercapacitors which display excellent charge storage properties in terms of capacitance per active mass such as laser scribed graphene23, cellulose papers coated with carbon nanotubes24-25etc. However, due to the low active mass loading in these systems, the capacitance per active area is normally very less 23-25. In general, the current strategies adopted for fabricating the solid-state supercapacitors fail especially when the electrode thickness is high (100-150 µm) and the electroactive material contains nanopores. Thus, developing a feasible methodology which can help to 3-dimensionally (3-D) extend the electrode-electrolyte interface in porous carbon materials with a polymer electrolyte is highly important for building lighter, safer and flexible supercapacitors for future applications. This is particularly important in the case of supercapacitors, compared to any other energy storage devices, because the performance of a supercapacitor solely depends on how effectively the fast charge-storage process can occur at the electrodeelectrolyte interface26. Further, they are mainly being used in power demanding applications and thus, internal resistance of the device is to be as low as possible, which is mainly contributed by the electrolyte resistance. Thus, developing effective electrodeelectrolyte interface in a solid-state supercapacitor is essential for ensuring both improved energy and power capabilities. To address the challenging issue of the low interface formation in the case of the microporous (< 2 nm) electrodes with the solid polymer electrolytes, we are proposing an entirely new strategy of enginering extended solid electrolyte-electrode interfaces in such cases.


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Instead of manipulating the electrode7-8 for accommodating highly viscous polymer solution, the electrode (carbon) pores are initially soaked with a monomer-electrolyte mixture, followed by polymerization under UV irradiation. Though we have adopted UV irradiation in the present case for accomplishing polymerization, thermal or other chemical initiator based processes also can be used for the polymerization depending on the monomer selection and other conditions. As the size of the polymer monomer is much smaller than the pores present on the electrode material, it is expected that the polymerization happens inside the nanopores as well. Though, UV initiated polymerization is well known in the polymer gel electrolyte chemistry, as per our knowledge, this is the first report on the in situ polymerization of the monomer inside the porous electrode. So far the resaerch reports on polymer electrolytes involve in utlising them as an electrolyte film between the electrodes27-29 meant for Li-ion batteries30-31 and supercpacitors32-33. Unlike the conventional approaches as can be seen in the above refernces, an in situ startegy helps the polymer electrolyte to form highly inter-penetarted network within the micropores of the carbon. This subsequently helps in attaining high energy and power outputs from the system along with the other benefits such as improved mechanical strength, stability and integrity of the electrodes.

Result and Discussion Polyhydroxyethyl methacrylate-co-trimethylolpropane allyl ether copolymer (PHEMA-coTMPA) based gel containing phosphoric acid as the electrolyte was prepared by the free radical co-polymerization of hydroxyethyl methacrylate (HEMA) and trimethylolpropane ally ether (TMPA) in the presence of hydroxyl methylpropiophenone (HMPP) as the initiator, ethylene glycol dimethacrylate (EGDMA) as the cross-linker and phosphoric acid (H3PO4) as the electrolyte under ultraviolet radiation (UV) for 18 min. From here onwards, the formed gels are named as H-T-Ac in general and, otherwise, the relative acid percentage is given as the last digits


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after a hyphen. The reaction scheme is given in Figure 1a. TMPA monomer was used to copolymerize with HEMA to modulate the hydrophobicity of the network structure of poly hydroxyethyl methacrylate (PHEMA). PHEMA-co-TMPA (i.e. H-T-Ac) is expected to be more flexible than PHEMA which could be attributed to the plasticizing effect of TMPA. A

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C CP-MAS NMR spectrum of PHEMA-co-TMPA along with the peak assignments is

shown in Figure S1 in the Supporting Information (SI). The signal at δ = 180 ppm corresponds to the carbonyl carbon in HEMA, whereas the signals at δ = 81.6, 73.8, 68.89 and 61.86 ppm pertain to the -CH2 groups in the co-polymer. The two main groups of signals at 46.77 ppm and 32.4 to 18.00 ppm correspond to the tertiary carbon atom and –CH- groups in the co-polymer, respectively. The signal at δ = 9.75 ppm corresponds to the –CH3 group in TMPA. The image of the formed gels is shown in Figure 1b-d. Figure 1c shows the compressibility of PHEMA-coTMPA containing 60 % H3PO4, named as H-T-Ac-60 %, which is found to be retaining its shape after removing the force. Quantitative compression and tensile studies of H-T-Ac-60 % gels were carried out and the corresponding results are shown in Figure 1e and Figure S2a, respectively. These gels possess 0.6 M Pa of compressive strength and the material can withstand a compression of up to 80 % of its total volume. The tensile results show 25% stretchability for these gels and they are found to be withstanding a tension of 0.04 MPa. For comparison, the conventional polyvinyl alcohol (PVA) gelated with phosphoric acid34 with the same polymer to acid ratio also was prepared (PVA-Ac) and the details are given in the experimental section in SI. Thermal stability of H-T-Ac-30 % over PVA-Ac-30 % is confirmed from the thermogravimetric analysis (TGA) (Figure S2b), where, the H-T-Ac sample starts degrading at around 250 oC in comparison to an early polymer degradation observed in the case of PVA-Ac30% at around 180 oC. The enhanced thermal stability in the former case is attributed to the cross


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linked nature of the PHEMA-co-TMPA matrix compared to the PVA matrix. The initial weight loss before the polymer loss in each case is due to the evaporation H3PO4 from the matrix. Figure 1b represents the various gels containing the different amounts of the acid. Conductivity of the prepared gels was measured using electrochemical impedance spectroscopy. Nyquist plots of the gel films are shown in Figure S3a-c. PHEMA containing 63 % H3PO4 (H-Ac-63%) shows a conductivity of 1.28 x 10-2 S/cm, whereas, after addition of 20 % of TMPA to the PHEMA and with the same amount of H3PO4 (H-T-Ac-63 %), the conductivity of the gel has been improved to 1.82 x 10-2 S cm-1 (Figure S3a). In this context, the improved conductivity in the latter case can be clearly credited to the more plasticizing effect of TMPA, which reduces the crystallinity of the polymer matrix and helps the system for facilitating easy movement of ions through segmental motion inside the gel. Conductivity in all the cases of the H-T-Ac samples is found to be increasing with respect to the amount of H3PO4 (Figure 2a). The conductivities of the H-T-Ac gels with 38, 50, 63 and 75 % of H3PO4 are respectively 0.13, 1.31, 1.80 and 2.80 x 10-2 S/cm. The Nyquist plots showing the comparative study are given in Figure S3b. Even with the 75 % of H3PO4, we observed that the gel formation in H-T-Ac was quite high compared to the literature reports. However, large amount of H3PO4 will adversely affect the physical properties of the gel. Conductivity of H-T-Ac is also compared with the conventional PVA-Ac system. However, compared to H-Ac and H-T-Ac, the gelation or the acid holding capacity in the solid/polymer form is very poor in the case of PVA and our experiment indicates that only up to 50 % of H3PO4 can be held by PVA. We compared the conductivity of H-T-Ac-50 % with PVA-Ac-50 % (Figure 2b and Figure S3c), in which, both the systems contain 50 % of H3PO4. The study clearly reveals that H-T-Ac-50 % displays higher conductivity (1.31 x 10-2 S/cm) compared to


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PVA- Ac-50% (4.4 x 10-4 S/cm). We believe that this difference in the conductivity between the samples could be originated from the proper segmental motion of the H+ ions in H-T-Ac4 as they are entrapped in the polymer matrix uniformly due to the in situ polymer generation process in comparison to PVA-Ac, where the moieties exist as simple physical mixture. Such an in situ polymer growth as in the case of H-T-Ac can help to form widely interpenetrated polymer backbone by gaining an ability to hold the electrolyte in large amount without leakage or even evaporation. Further, when the desired amount of H3PO4 was added to the PVA aqueous solution to form the gel, the density measurements in this case gave an indication of a lower amount of acid retained by the system from the added amount of 50 %. The density of the pure PVA film is 0.2 g/cm3, which is increased to only 0.5 g/cm3 after the 50 % (expected) acid addition. Whereas, the density change of HEMA-TMPA dry gel from 1.0 to 1.4 g/cm3 indicates that the polymer is holding the desired amount of the acid. This also is substantiated by the TGA results in Figure S2a, where the analysis was made with H-T-Ac and PVA by adding 30 % H3PO4 in the total weight of the gel during their synthesis. However, the TGA profile indicates that PVA holds only 12 % acid, as evident from the weight loss up to 180 oC, by letting the remaining acid to evaporate during the drying process included to remove the excess water. Compared to PVA, HT-Ac holds around 27 % acid as evident from the weight loss up to 250 oC from the TGA profile. The efficient acid holding ability of the H-T-Ac is expected to be originated from the interaction between the gel polymer matrix and the acid groups. This enhanced acid retaining capacity in the case of H-T-Ac can be further validated from the IR spectra of the samples shown in Figure 2c. The carbonyl peak originated from the ester functionality in the HEMA monomer has been shifted from 1720 cm-1 to 1694 cm-1, indicating the (PO4)3- interaction, which is giving electron


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releasing effect for the CO groups. The same interaction is expected to be helping for holding the acid in the polymer matrix compared to the PVA system where any such interaction is absent. For the study of the interface formation in the solid-state supercapacitor, selection of the electrode material is highly critical. To ensure the formation of pure double layer capacitance and robustness of the electrode during the stability cycling, a clean high surface area carbon (YP80F) was selected as the electrode material, which was procured from Kuraray Chemical Co. Figure S4a shows the SEM image of the carbon and the EDX mapping presented in Figure S4b quantifies the elements present in the material. The carbon powder is found to be very hard having an average grain size of 2-5 µm. EDX mapping shows nearly 99.9 % of carbon, indicating absence of any heteroatoms such as N, O or P. This also rules out the presence of functional groups on the surface and, hence, any pseudocapacitive contribution towards the capacitance35. The pore distribution profile of the carbon is presented in Figure 3a (N2adsorption isotherm of YP-80F is given in Figure S5 in SI), which shows an average size distribution of the pores between 1 to 3 nm with a major contribution from the pores in the range of 1.0 to 1.5 nm. Pores of this size (1-3 nm) are pretty enough to accommodate the molecules of the monomer (HEMA and TMPA) which are having a maximum dimension of 1 nm (length of TMPA) as represented in Figure 3b along with the smaller molecules of H3PO4. The specific surface area (SSA) of the carbon measured by the BET analysis is 2044 m2/g. Solid-state supercapacitors were fabricated by using the carbon coated Grafoil® electrode of 4 cm2 area with H-T-Ac-63 % as the gel polymer electrolyte by the method of in situ gel formation. Initially, the carbon coated Grafoil® sheets were soaked with optimized ratio and volume of the monomer mixture in H3PO4. Further, two such electrodes of 4 cm2 area were kept face to face by keeping a polypropylene separator between them and the edges were sealed using


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an adhesive sellotape to prevent solvent leakage. Subsequently, the system was subjected to UV irradiation for accomplishing polymerization. Subsequent to the UV irradiation, the outer sellotape sealing was removed while the electrodes are held together by the in situ formed polymer gel (details are given in the experimental section in the SI). The solid device thus derived is hereafter termed as H-T-Ac-S, where ‘S’ stands for solid device. To see whether the in situ polymerization has any effect on the device performance, the devices were also tested before subjecting to the UV-polymerization in the sealed condition so that liquid cannot come out. Hence, this second device contains the monomer and H3PO4, which is henceforth represented as H-T-Ac-L, where ‘L’ stands for the liquid device. The schematics of the strategy adopted for the device fabrication are shown in Figure 3c. The devices were fabricated using the electrodes having a mass loading of 1.9, 3.3 and 5.7 mg/cm2 for various comparative studies. Similarly, devices based on PVA-Ac (hereinafter termed as PVA-Ac-S) were also fabricated for comparison with the H-T-Ac-S. In all the cases, the amount of H3PO4 is deliberately kept as 63 %, even though PVA-Ac-63 % shows a semi-liquid nature compared to H-T-Ac-63 %. It is also worthy to mention that all the studies were done on the devices without any sealing and the electrodes were held together simply by the polymer electrolyte. Such encapsulation-free devices will tell about the practically usable performance and stability aspects of the systems. As mentioned before, the carbon used for the present study possesses a major contribution from the micropores in the range of 1.0 to 1.5 nm and these are expected to contribute immensely towards the capacitance. The remaining pores are in the range of 1.5-3.0 nm. When we look at the size of the monomer and H3PO4 molecules (Figure 3b), the smaller sized H3PO4 can easily sit inside the micropores of 1.5 nm36. Whereas, larger size of the monomers (HEMA: 0.85 nm and TMPA: 1.0 nm) can restrict their entry inside the small pores


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for more than one molecule. However, remaining larger pores between 1.5 nm and 3.0 nm can be easily filled by the monomers and further polymerization can be achieved inside these pores (Figure 3e). Such pore filled polymer will effectively restrict the H3PO4 moiety from its evaporation from the micropores even in unsealed devices. Contrary to this, in case of PVA-AcS, due to the high molecular weight of the PVA polymer, entry inside the pores having size < 10 nm is difficult (Figure 3d). Thus, the capacitance obtained in this case can be attributed primarily to the free H3PO4 entered inside the micro and mesopores (< 5 nm), which possesses no chemical interaction with the PVA network. However, in such cases, acid evaporation can occur easily, which can end up with the issue of the capacitance degradation during the long term cycling. To confirm the formation of the desired carbon-H3PO4 electrode-electrolyte interface, a small carbon portion in the device (H-T-Ac-S) after the in situ polymerization was probed using EDX elemental mapping. The uniform distribution of P and O all over the carbon region (Figure S6) stands out as strong evidence on the uniform gel formation inside the carbon pores. Higher resolution imaging, however, was disturbed by the polymer burning under the electron beam and this issue restricts the imaging at µm resolution. To analyze the capacitance, cyclic voltammetry (CV) and constant charge-discharge (CD) measurements have been carried out. Figure 4a shows the CV profiles of the various devices with a mass loading from 5.7 mg/cm2 recorded at a voltage scan rate of 50 mV/s. Remaining CV profiles are shown in Figure S7 and S8. In all the cases, H-T-Ac-L and H-T-AcS show nearly similar current-voltage profiles, indicating the desired interface achieved in the solid device is matching with that of its liquid-state counterpart. On the other hand, the CV behavior of PVA-Ac-S deviates from the ideal capacitive nature especially with high mass


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loading (Figure S7a-c). This is clearly an indication of the inability of PVA-Ac in PVA-Ac-S to form the desired extended interfaces with the carbon surface and into the pores especially in the cases involving high mass loading, which subsequently results into low power capabilities. The CD profiles at low current (Figure 4b) as well as high current (Figure 4c) densities also show superimposed profiles for H-T-Ac-L and H-T-Ac-S, successfully validating the technical viability of the strategy used to form the efficient interface in the later case. The capacitance is calculated to be 130 F/g for H-T-Ac-S in comparison to 140 F/g obtained with HT-Ac-L (Figure S8). At the same time, the PVA-Ac-S system shows a capacitance value of only 90 F/g, which is an expected deviation owing to the less extended interface formation. A comparison with a solid-state supercapacitor having an electrolyte film made out of H-T-Ac in between the electrodes is avoided as this will produce even less interface than PVA-Ac, which we had proved in our earlier studies7. The obtained capacitance of H-T-Ac-L is well matching with the capacitance obtained using other standard electrolytes such as 0.5 M H2SO4 (128 F/g at 1 mA/cm2, Figure S9c) and 1 M TEABF4/propylene carbonate (118 F/g at 1 mA/cm2; Figure S10) with the same electrode thickness (150 µm). The slightly lower capacitance in case of 0.5 M H2SO4 is due to the hydrophobic nature of the carbon, which can be observed from the slightly higher ESR (Figure S9b) and higher IR drop at a high current discharge rate of 10 mA/cm2 (Figure S9d). On the other hand, the lower capacitance in TEABF4/propylene carbonate is due to the bigger size of the ions compared to the ions in H-T-Ac. The capacitance retention in both H-T-Ac-S and H-T-Ac-L follows same trend as it can be observed in Figure 4d and also from the similar IR drop behavior displayed by the systems at 20 mA/cm2 as shown in Figure 4c. H-T-Ac-S retains 106 F/g even at 20 mA/cm2, which is comparable to 114 F/g retained by HT-Ac-L. Whereas in the case of PVA-Ac-S, a drastic drop in capacitance has been observed (95


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to 22 F/g; Figure 4d) while moving to the high current dragging conditions (1 to 20 mA/cm2) as a penalty caused due to the high ESR of the system as measured from the EIS studies. This is supplemented by a huge IR drop of 540 mV observed for this system at 20 mA/cm2 (Figure 4c). Along with the mass specific capacitance, areal and volumetric capacitances are calculated by considering the active area (4 cm2) and the thickness of the carbon film (70, 90 and 120 µm, respectively, for the mass loading corresponding to 1.9, 3.3, 5.7 mg/cm2). The volumetric capacitance obtained is 51 F/cm3 at a discharge rate of 20 mA/cm2 for H-T-Ac-S, while the conventional PVA-Ac-S shows a value of only 10 F/cm3 (details are shown in Figure S11). The formed interface can be clearly explored by the EIS measurement by analyzing the ESR value, which is taken from the Rs value of the equivalent circuit (inset of Figure 4e) and is nearly equal to the high frequency x-intercept at the real axis of the Nyquist plot. The Nyquist plots of the devices with a mass loading 1.9 mg/cm2 are shown in Figure 3e and the corresponding ESR values extracted for the devices with increased mass loading are shown in Figure 4f. In all the cases, the PVA systems are showing higher ESR values due to the less extended interfaces formed inside the pores of the electrode; the ESR in this case is found to be increased significantly at higher mass loading conditions of 5.7 mg/cm2 (2.2 Ω to 4 Ω). As explained before, the high molecular weight and viscosity of PVA prevent the infiltration of the PVA molecules inside the pores of the carbon, which are mainly falling in the range of 1-3 nm. Contrary to this, in case of H-T-Ac-S and H-T-Ac-L, the ESR is quite low, which comes around 0.5 Ω for the 4 cm2 devices and it is found to be stable even up to a loading of 6.0 mg/cm2. This control on the ESR even at higher mass loading conditions in the case of H-T-Ac-S could be attained as the adopted strategy works well to create the electrode-electrolyte interface, even


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inside the nanopores, to establish a situation well mimicking that of a liquid system (Figure S12). Apart from the improved capacitance obtained from the H-T-Ac system, an evaluation of the long-term cycling stability of the devices made out of the above electrolyte is a mandatory requirement to understand the overall integrity and durability of the system under the testing conditions. Here, the system stability will mainly be attributed by the electrolyte robustness in term of the polymer network stability as well as the H3PO4 holding ability without allowing the acid moiety to evaporate. This is particularly important in the present case since the devices are tested in unsealed conditions. In case of PVA-Ac-S, the capacitance is found to be decreased after 2 days by simply keeping under ambient conditions without subjecting any potential cycling. Figure 5a shows the cyclic voltammograms of PVA-Ac-S, which show a huge loss in the current in the CV profile subsequent to the storage of 2 days duration. This is further confirmed from the EIS measurement (Figure 5b), where the spectrum deviates from its ideal nature and ESR is found to be increased to 7.5 Ω from 3.2 Ω during the storage period. This decrease in the capacitance value is further confirmed by the CD stability cycling, where 50 % loss in its initial capacitance within 5000 cycles (Figure 5c) has been observed. This is clearly due to the loss of liquid (H3PO4) from the polymer matrix as the device is not sealed and the polymer doesn’t have any interaction with the acid to hold it from the evaporation losses. The stability aspect of H-T-Ac-S was also monitored by continuous CD cycling executed at a current density of 20 mA/cm2. The plot representing the variation in the capacitance with respect to the number of the CD cycling is presented in Figure 5d. Even after the 32000 cycles, H-T-Ac-S is found to be retaining 84 % of its initial capacitance (more stability cycling data are provided in SI, Figure S13). Such a superior stability can be credited solely to the improved stability of the


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electrolyte in the system as the other part of the devices are based on the well stable components such as commercial grade carbon, current collector etc. To see the change in ESR during the cycling process, after the 1000 and 2000 cycles, the device was subjected to EIS measurement and it is found to have stable ESR at these positions, which re-assures the structural and morphological stability of the devices (Figure 5e). The robustness of the devices closely relates to the proper selection of the polymer which interacts with the electrolyte and prevents its evaporation. Along with this, the in situ formed polymer creates better inter-penetrated network of the polymer chains which extends even to the micropores of the carbon and retains acid even in the remote areas of the electrode matrix. Figure S14 shows a gel disc in which the carbon is integrated with the gel electrolyte via in situ polymerization as similar to the strategy adopted in the device making. The above gel carbon disc contains nearly 1 % of carbon in the total weight of the disc, which indicates the integrity of the electrolyte with the carbon particle at a macroscopic level. A direct evidence as a proof for the presence of the polymer at the nanopore interface could not be obtained as the normal characterisation techniques are not sufficient probe the situation inside the nanopores. However, analysis of the electrochemical parameters such as capacitance, ESR and stability can give indirect evidences for the formation the polymer electrolyte interface inside the carbon pores. Lastly and mostly, the strategy adopted here is applied for making flexible solid-state supercapacitors. We have made 12 cm2 devices with a mass loading of 7.3 mg/cm2, which is comparable to the commercial electrode standards. Figure 6a shows the CV profile of the device at various flexible conditions as shown in the inset images of the figure. The superimposed CV profiles clearly reveal the robustness of the devices, which is expected to be originated from the inter-penetrated polymer electrolyte with the porous carbon electrode. As the method is highly


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scalable, flexible electrodes in larger area can be prepared with less complexity, which makes this strategy more viable than the conventional methodologies used for fabricating solid-state supercapacitors. For the above prototype flexible capacitor, a mass specific capacitance of 95 F/g has been obtained at a current drag of 1 mA/cm2 (Figure 6b). Further, long term cycle stability evaluation for around 9000 cycles has been performed while the device is bended 100 times between each 2000 cycles (Figure 6c). The device is found to be showing superior capacitance retention of 90 %, while the coulombic efficiency is nearly maintained to 100 % during the whole cycling. Stability of the device is further confirmed from the stable ESR displayed by the device even after the cycling test as shown in Figure S15, where an increment in ESR by only 0.2 Ω is observed during the entire cycle. Further, we have connected three 12 cm2 devices in series to achieve a 3 V system as shown in Figure 6d and SI video. The CD profile of the 3-cell assembly is given in Figure 6d. An LED could be glowed by the above device and the corresponding image is given in the inset of Figure 6d.

Conclusion An in situ polymer gel electrolyte generation strategy is used to enhance the electrodeelectrolyte interface of a solid-state supercapacitor which clearly outperformed the device made by employing the conventional polymer electrolyte strategies and displayed matching performance characteristics with the one made from the liquid electrolyte. The key aspect of the strategy which helped to position the present system in a better side is the adopted UV-triggered polymerization of the monomer mixture after soaking with the high surface carbon. The basic matrix of the poly-hydroxyethyl methacrylate-co-trimethylol propane allyl ether copolymer shows excellent thermal stability in acid and the polymer further interacts with the acid moiety through the CO functional groups of the polymer chain. This helps the system to hold more


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amount of the acid, an ability which is found to be superior to the conventionally used polyvinylalcohol-acid gels. The method is very simple and scalable and a prototype, flexible solidstate supercacitor could be even demonstrated in an encapsulation-free condition by using the commercial grade electrodes (thickness: 150 µm, area: 12 cm2 and mass loading: 7.3 mg/cm2). The prototype shows high capacitance (130 F/g), low internal resistance (0.5 Ω) and enhanced stability (84 % capacitance retention after 32000 cycles in unsealed condition). With respect to these aspects, the present system clearly outperformed the system derived by using the conventional polymer electrolyte (PVA-H3PO4 as the electrolyte). Due to the unique interface formation, which is comparable to the liquid system, very low ESR is achieved in the present case and subsequently this helped for attaining high power capabilities by the system.

Figure 1: a) Synthetic scheme illustrating the preparation of H-T-Ac; b) images of H-TAc gel containing different amounts of H3PO4; c) images of H-T-Ac-60% under the


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conditions of before the compression, during the compression and after the compression; d) disc shaped H-T-Ac-60%; e) Compressive stress – strain plot of H-T-Ac-60 %.

Figure 2:

a) and b) Comparative conductivity data of the H-T-Ac samples having

different acid content and c) comparative IR-spectra of the H-T and H-T-Ac samples with the zoomed frequency range of interest shown separately in the same image.


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Figure 3: a) Pore-size distribution profile of the carbon powder used for the device making; b) average molecular dimension of the polymer monomers used for the preparation of the gel; c) schematic representation of the stages involved in the preparation of the solid-state supercapacitor via in situ polymer gel generation; d) a representation of the expected nature of the carbon-electrolyte interface while using PVAAc; e) a highly interpenetrated and extended carbon-electrolyte interface expected in H-TAc-S due to the in-situ polymer generation from the monomer.

Figure 4: Comparative electrochemical performance of the H-T-Ac-S, H-T-Ac-L and PVA-Ac-S devices having an electrode mass loading of 5.7 mg/cm2 in each case: a) cyclic voltammograms recorded at a scan rate of 50 mV/cm2; b) charge-discharge profiles recorded at a current density of 1 mA/cm2; c) charge-discharge profiles recorded at a current density of 20 mA/cm2; d) capacitance retention profiles of the samples as a function of the varied current density values; e) Nyquist plots (1.9 mg/cm2) fitted using


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the equivalent circuit shown in the inset figure; f) the measured ESR values of the samples as a function of the electrode mass loading.

Figure 5: a) CV profiles at 50 mV/s of the PVA-Ac-S unsealed device before and after storing in ambient condition; b) the corresponding Nyquist plots of the system; c) capacitance retention of the PVA-Ac-S unsealed device; d) plots representing the capacitance retention and coulombic efficiency during the continues charge-discharge cycling of H-T-Ac-S unsealed device; e) Nyquist plots of H-T-Ac-S taken before and after the cycling test.


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Figure 6: a) CV profiles recorded at 50 mV/s of the 12 cm2 H-T-Ac-S device with an electrode mass loading of 7.3 mg/cm2 under various flexible conditions as shown in the inset images; b) the measured capacitance values of the 12 cm2 H-T-Ac-S device, as shown in the inside image, as a function of the increasing current density values; c) capacitance stability and coulombic efficiency of the above device during 12000 CD cycles; d) a three-cell assembly sandwich model devised to achieve 3 V and the corresponding charge-discharge profile (the inset image shows the glowing of an LED powered by the above device). ASSOCIATED CONTENTS Supporting information 1 contain: Experimental section. NMR spectra, mechanical studies, conductivity measurement, SEM and EDAX mopping, supporting electrochemical plots of gel electrolytes. Supporting information 2 contain: video record of LED powered by solid-state


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flexible capacitor. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected], [email protected], Notes The authors declare no competing financial interest. ACKNOWLEDGMENT BA and KS acknowledge CSIR for the research fellowship and project funding (Project No. CSC0122), respectively. References (1) (2) (3) (4) (5) (6) (7)

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TOC


High-Performance Flexible Solid-State Supercapacitor with an

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