Tampere University of Technology

Printable and disposable supercapacitor from nanocellulose and carbon nanotubes

CitationTuukkanen, S., Lehtimäki, S., Jahangir, F., Eskelinen, A-P., Lupo, D., & Franssila, S. (2014). Printable anddisposable supercapacitor from nanocellulose and carbon nanotubes. In ESTC 2014, 5th Electronics System-Integration Technology Conference, September 16 – 18, 2014, Helsinki, Finland (pp. 1-6). Institute of Electricaland Electronics Engineers IEEE. https://doi.org/10.1109/ESTC.2014.6962740Year2014

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Published inESTC 2014, 5th Electronics System-Integration Technology Conference, September 16 – 18, 2014, Helsinki,Finland

DOI10.1109/ESTC.2014.6962740

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Figure 1: Principle structure of the supercapacitor.

Printable and disposable supercapacitor from nanocellulose and carbon nanotubes

S. Tuukkanen1,2, S. Lehtimäki2, F. Jahangir1, A.-P. Eskelinen1, D. Lupo2, S. Franssila1 1Aalto University, School of Chemical Technology, Department of Materials Science and Engineering,

P.O. Box 16200, 00076 Aalto, Finland. 2Tampere University of Technology, Department of Electronics and Communications Engineering,

P.O. Box 692, Korkeakoulunkatu 3, 33101 Tampere, Finland. Tel: +358 40 541 5276, E-mail: sampo.tuukkanen@gmail.com

Abstract Supercapacitors are promising energy storage devices

providing capacitance much higher than conventional capacitors and higher power density and longer cycle life than Li-batteries. We report printable and disposable supercapacitors fabricated from solution-processed carbon nanotube (CNT) composite material as active electrodes and nanocellulose (NC) as a separator. Use of a highly porous and electrically conducting CNT film as electrode materials eliminates the need of current collector. NC is a robust separator material used instead of conventional polymer separator films. Supercapacitor characterization was done with a galvanostatic discharge method according to an industrial standard. The capacitance of 1.8 cm2 devices was 14.9–16.5 mF (7.4–9.1 mF/cm2 or 2.4–2.9 F/g) and equivalent series resistance (ESR) 74–155 . This type of low-cost energy storage devices fabricated from safe and environmentally friendly materials have obvious applications in autonomous intelligence and disposable low-end products.

Introduction Electric double-layer capacitors (EDLC), also known

as supercapacitors or ultracapacitors, are promising energy storage devices which have the performance between those of batteries and capacitors. In a supercapacitor, the accumulation of ions on the surface of highly porous electrodes yields high capacitance when compared with conventional capacitors [1, 2]. They show higher power density and longer cycle life than Li-batteries and can be fabricated from safe and disposable materials. Since supercapacitors are relatively new electronic components, there is still plenty of room for new innovations related to the selection of materials, architecture and fabrication method.

The supercapacitor structure is described in Fig. 1. The main difference from the conventional metal-dielectric-metal capacitor is that supercapacitor contains an electrolyte which allows positive and negative ions to move and accumulate to the high surface area active electrode surfaces when the device is charged. The separator, which is soaked with the electrolyte, keeps the electrodes separated preventing electron current between them. Current collectors are required to obtain high enough conductivity from the active electrodes to the device output, since active electrodes are conventionally prepared from poorly conducting activated carbon. Finally, the overall device has to be encapsulated to

prevent the evaporation or leakage of the liquid electrolyte.

The fabrication of supercapacitors on flexible substrates from solution-processable materials enables their low-cost and high throughput mass production. Flexible, low-cost supercapacitors are interesting interim power sources for autonomous energy harvesting systems such as ubiquitous sensory networks and disposable low-end products [3, 4].

Carbon nanomaterials, such as carbon nanotubes (CNT) and graphene, are attractive active electrode materials for supercapacitors due to their flexibility, high specific capacitance and high electrical conductivity [5] as well as their solution-processability [6]. The authors have previously reported the use of solution-processed CNT and graphene films as flexible [7, 8], stretchable [9] and transparent [10] electrodes in sensor applications. Solution-processed CNT based supercapacitors using conventional separator films were previously reported [4, 10, 11].

Cellulose based nanomaterials [13, 14], such as nanofibrillated cellulose (NFC) [15] and cellulose nanocrystals (CNC) [16], are interesting renewable bio-based nanomaterials which have potential applications for example in electronics, material sciences and medical engineering. Their nano-scale dimensions and the ability to form a strong entangled nanoporous network makes them interesting materials to be used as supercapacitor separators.

In this paper, we report for the first time to our knowledge printable and ecological supercapacitors fabricated from solution-processed CNT nanocomposite electrodes and NC separator on a flexible polymer substrate.

Fabrication of electrodes Active electrodes were fabricated on a poly(ethylene

terephthalate) (PET, Melinex ST506) substrate from highly viscous CNT based nanocomposite ink (3.5 wt-% in water) using blade-coating with a mechanical mask (Scotch tape mask). Details of CNT nanocomposite ink preparation are described elsewhere [11]. CNT electrode dimensions were 3.2 cm x 1.4 cm. Six layers of CNT ink were deposited and after each layer deposition the film was dried at 50 ºC in a convection oven. The sheet resistance of the CNT electrodes (6 electrode samples) was 10–13 / (determined using four-probe setup which was described previously [7]).

Fabrication of separator layer A double-layer of two cellulose based nanomaterials,

i.e., nanofibrillate cellulose (NFC) and cellulose nanocrystal (CNC), is used as a solution-processed separator layer in the supercapacitor. The NFC material was produced by mechanical homogenizing process [17, 18]), where it underwent 6 passes. The CNC was fabricated using acid hydrolysis method [16, 19].

The quality and morphology of the produced NFC and CNC films were studied using scanning electrode microscopy (SEM). Field-emission SEM (JEOL JSM-6335F) with 5 kV acceleration voltage was used for imaging. SEM images of CNC and NFC materials used in this work are presented in Fig. 2.

NC separator layers were deposited on both CNT electrodes separately before the supercapacitor assembly to minimize the probability of short-circuiting. The separator layer was deposited on each electrode using a two-step deposition process and a mechanical polydimethylsiloxane (PDMS) mask. First, a thin film of CNC solution (0.8 wt-% in water) was spray-coated onto

the electrode through the mechanical mask when the sample was placed on hot-plate at 60 ºC to dry the CNC film. Thin CNC film was used as an adhesion promoter layer between thick electrode and separator films. Second, NFC gel (0.8 wt-% in water) was deposited on the area enclosed by the mask using drop-casting and let dry in room temperature for dehydration leading to a firmly attached thick film of NFC.

The PDMS mask was fabricated from the PDMS elastomer kit (Sylgard 184) containing a polymer base and cross linking agent solutions [20]. The polymer and the agent were mixed manually in the ratio of 10:1, degassed and cured in the oven for 2 h at 50 ºC. The cross-linked mold was then cut into desired mask shape using a scalpel. The PDMS mask was 6 mm thick and it had 1.7 cm x 1.7 cm opening defining the deposition area.

Photograph of one electrode after separator deposition before supercapacitor assembly is presented in Fig. 3. It was observed that the uniformity of the films is increased by decreasing the thermal stress exerted during dehydration.

Supercapacitor assembly In total, three supercapacitor devices were assembled

in a symmetric head-on configuration presented schematically in Fig. 4(a). Electrodes soaked with an aqueous electrolyte (3 drops of 1 M NaCl) were sandwiched to form a supercapacitor device using an adhesive film (from UPM Raflatac). The photographs of assembled supercapacitor are presented in Fig. 4(b) and 4(c). The active area of the assembled supercapacitor was 1.8 cm2. Total electrode masses (including both electrodes) on the active supercapacitor areas were 5.6, 6.2 and 6.9 mg for the fabricated devices SC-1, SC-2 and SC-3, respectively.

Electrical characterization The electrical properties of the supercapacitor were

measured using cyclic voltammetry (CV) and galvanometry. Electrical measurements of the devices were conducted with the potentiostat (Zahner Zennium Electrochemical Workstation). For the electrical

Figure 3: Photograph of one solution processed CNT electrode and NC separator layers on PET substrate before supercapacitor assembly.

Figure 2: SEM micrographs of the spray-coated (a) NFCand (b) CNC films.

measurements pieces of graphite foil were placed between the supercapacitor electrode end and the potentiostat alligator clip to ensure good electrical contact between graphitic electrode material (CNT film) and metal clip (see Fig. 4(b)).

CV measurements were conducted immediately after the assembly. CV curves from different sweep rates (5, 10, 50 and 100 mV/s) are presented in Fig. 5. The last cycle of 10 cycles measured with each rate was used for plotting the CV curves. The box-like shape of the CV curves indicates good capacitive behavior of the device. The lower normalized current in the case of faster sweep rate is mostly caused by the series resistance, but also from the shorter time for the electrolyte ions to migrate into the pores of the active electrodes.

After performing the qualitative analysis of supercapacitor using cyclic voltammetry, the capacitance and ESR were determined from galvanostatic discharge measurements according to an international standard [21], Class 3 (Power). In galvanostatic measurements the

supercapacitor was charged up to 0.9 V in 1 min, then held at 0.9 V for 30 min and discharged with a constant current. To ensure stabilization of device properties, the capacitor underwent an additional 50 charge/discharge cycles at 50 mV/s rate before collecting the parameter values. After each galvanostatic measurement the sample was kept at 0 V for 30 min.

The capacitance was calculated from the voltage decrease rate between 80 % and 40 % voltage points of 0.9 V through C = I/(dV/dt). The ESR was determined from the initial IR drop at the start of the discharge phase. The ESR measurement current was 10 times that of the

Figure 4: (a) A schematic side-view of used supercapacitorarchitecture used in this work, (b) a top-view of theassembled supercapacitor during the electricalcharacterization and (c) a photo of bent device SC-1.

Figure 5: (a) Typical cyclic voltammetry curves at different voltage sweep rates. Normalization to sweep rate allows comparison of curves. The current divided by sweep rate has units of mA/(V/s), which is equal to mF. (b)-(c) Typical galvanostatic discharge curves. The capacitance is determined from the slope of the galvanostatic discharge curve at lower current (b) and the ESR from the IR drop of the voltage in the beginning of the discharge at higher current (c). The initial downward spike at the beginning of discharge is due to stray capacitances in the measurement system and not related to supercapacitor behaviour. Plotted curves here are for sample SC-1.

Table 1: Electrical characteristics of the supercapacitors SC-1, SC-2 and SC-3 fabricated in this work with the recent results from the literature where CNT films are used both as active layer and current collector. Some values from the literature are converted to specific capacitance of the whole device (and not for the single electrode) [11, 12, 23-27].

Parameter (unit) SC-1 SC-2 SC-3 [11] [12] [23] [24] [25] [26] [27] C (mF) 16.5 14.9 16.3 12 16.6 - - 10 - - Csc (F/g) 2.9 2.4 2.4 2.3 - 21 55 33 1.5 116 C/A (mF/cm2) 9.1 7.5 8.1 6 8.3 - - 10 9 - ESR (Ω) 155 74 120 80 74 2.1 80 30 28 118 Ileakage (A) 7 <1* <1* 7 6 - - - - -

*The leakage currents for samples SC-2 and SC-3 were measured after the CV cycling experiment.

current for the capacitance measurement [21]. Leakage current was determined from the galvanostatic measurement at the end of the 30 min hold at 0.9 V, just before discharge. Results of the electrical measurements are reported in the Table I, where the C is total device capacitance, Csc specific capacitance (i.e. device capacitance divided by mass of both electrodes on active area), C/A capacitance per active area, ESR equivalent series resistance and Ileakage leakage current. Discharge currents used for C and ESR measurements were 50/60 A and 500/600 A, respectively.

The charge/discharge cycling stability of the supercapacitors (SC-2 and SC-3) was studied by repeating CV charge/discharge cycle 2000 times. The capacitance calculated from CV curve by integrating over the enclosed area as a function of time is presented in Fig. 6. Galvanostatic measurements were performed before and after the CV cycling and they revealed that the device capacitance was the same or even higher after the cycling. This can be due to improved wetting of the deepest pores of the CNT electrodes. It was also observed that the leakage current of the devices SC-2 and SC-3 was lower than 1 A after the cycling, indicating leakage due to irreversible reactions of impurities, which were depleted during cycling.

Microscopic analysis SEM analysis was performed to further study the

microscopic structure of the supercapacitor SC-1. For SEM imaging the supercapacitor was cut mechanically by using a microtome. The SEM image of supercapacitor cross-section presented in Fig. 7 shows that the layers are quite homogeneous. Also, no large gaps appear between adjacent layers which indicated good adhesion between them. Based on the SEM analysis the average thickness of electrodes and separators on each side were (10 ± 2) m and (2.6 ± 0.6) m, respectively. Thickness of the total device was about 30 m.

Discussion The electrical characteristics of the fabricated

supercapacitor are summarized in Table I together with the results from recent studies in the literature. There are only a few very recent studies using CNT electrodes both as the active layer and the current collectors in the device

with aqueous [4, 11, 12, 22] organic [23-26], or aqueous gel electrolyte [23, 25, 26]. The capacitance values obtained in this work are comparable or even better that was reported recently by the authors using similar CNT electrodes in combination with a conventional paper separator [4, 11, 12]. The ESR values are same order of magnitude in all studies. The ESR can be decreased by increasing the layer thickness or by optimizing the amount and type of CNT dispersing agent.

The CNT type (single or multi-walled) and quality, active layer thickness and structure as well as CNT material processability vary a lot between these studies and they are difficult to compare convincingly. Even though the gravimetric specific capacitance (F/g) for our device is lower than in some recent reports [22-24, 26], the specific capacitance per surface area (F/cm2) is very similar in all studies as can be seen from Fig. 8 where the device performances are compared. When considering the supercapacitor dimensions for the practical applications the capacitance per surface area is the most important parameter. The specific capacitance can be increased for instance by using thinner CNT film ensuring thorough wetting of the film or by optimizing the pore size and access for ions in CNT film.

The cycle life-time of the CNT-NC is expected to be very high because the performed 2000 times repeated charge/discharge cycling experiment did not decrease the device performance at all. Further, the leakage current of the devices was decreased during the cycling below 1 A level.

To our knowledge, the use of solution-processed NC films as supercapacitor separators has not been reported

Figure 7: SEM micrograph of the cross-section ofsupercapacitor SC-1. The resolved supercapacitor layersare named in a schematic view on the left.

Figure 6: Cycle retention of supercapacitors SC-2 and SC-3.

0 500 1000 1500 20008

9

10

11

Device SC-2 Device SC-3

C (

mF

)

Cycle number

before this study. The proposed supercapacitor architecture enables fast and simple manufacturing using printing techniques from cheap and disposable materials, i.e., carbon-based materials, water and salt, which makes these devices interesting from ecological and economical perspectives, especially for disposable low-end products.

Conclusions Printable, flexible and disposable supercapacitor was

fabricated on plastic PET substrate using solution-processed materials. The active electrodes were fabricated from a highly viscous CNT ink and the separator from an aqueous NC dispersion. This is the first study where the solution-processed NC film is used as a separator in a supercapacitor. Microstructure of the supercapacitor was studies by SEM analysis of the device cross-section. A relatively high capacitance (14.9–16.5 mF) was obtained for the devices which had an active area of 1.8 cm2, giving specific capacitances of 7.4–9.1 mF/cm2 and 2.4–2.9 F/g. ESR is relatively large (74–155 ) due to the use of non-optimized CNT film conductivity, but is still low enough for some applications. The CNT-NC supercapacitors capacitance remained the same or even increased during the 2000 times charge/discharge cycling and further the device leakage current was below 1 A in the end of cycling. Supercapacitors fabricated from safe and solution-processable nanomaterials are promising candidates for disposable low-end products, for example for autonomously powered short life-time personal electronics devices or medical diagnostics systems.

Acknowledgments The authors acknowledge funding from the Academy of Finland (Dec. No. 138146, 139881 and 264743).

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