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Printed supercapacitors on paperboard substrate
Electrochimica Acta 85 (2012) 302–306
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Printed supercapacitors on paperboard substrate Jari Keskinen a,∗ , Eino Sivonen a , Salme Jussila b , Mikael Bergelin c , Max Johansson c , Anu Vaari b , Maria Smolander b a b c
VTT Technical Research Centre of Finland, PO Box 1300, FI-33101 Tampere, Finland VTT Technical Research Centre of Finland, PO Box 1000, FI-02044 VTT, Finland Åbo Akademi, Process Chemistry Centre, Piispankatu 8, FI-20500 Turku, Finland
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 17 August 2012 Accepted 18 August 2012 Available online 25 August 2012 Keywords: Supercapacitor Electrical double layer capacitor Printed power source Printed electronics
a b s t r a c t Printed supercapacitors were prepared to be applied as a part of a hybrid power source in printed electronics applications. The use of non-toxic materials was preferred. The supercapacitor structure consisting of current collectors and activated carbon electrodes was applied on paperboard substrate using silver, graphite and activated carbon inks. Aqueous electrolytes with NaCl salt limit the maximum potential to about 1.2 V but are environmentally friendly and provide low equivalent series resistance (ESR). The capacitance values of our printed supercapacitors were typically 0.1–0.5 F. With 2 cm2 geometrical active area the ESR was below 1 which is acceptable for 50 mA current output. The efficiency with 50 mA charge and discharge current was typically about 90% and with 10 mA about 95%. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical supercapacitors [1] are intensively developed since they are able to supply high peak power requirements in energy storage applications. An electrochemical supercapacitor consists of two electrodes separated by an ionically conductive electrolyte. The electrodes are typically made of activated carbon powder that is bound using fluorine containing polymers such as PTFE or PVDF. As electrolytes organic solutions such as propylene carbonate or acetonitrile are used, but also aqueous alternatives are possible. With organic electrolytes the maximum potential between the electrodes is about 2.5 V but water based electrolytes limit the maximum voltage to about 1.3 V. In recent years a lot of effort has been put on the development of different integratable identification and data storage/transfer devices as well as miniaturized sensors. Active type tags and sensors require a suitable power source to fully utilize their potential allowing greater memory capacity in the tag and increasing the range from which the tag can be read [2]. Since these tags are often intended to be used as integrated parts of recyclable products, all components of the power source/RFID tag combination should be biodegradable or incineratable when the product is
∗ Corresponding author. Tel.: +358 20 722 3703; fax: +358 20 722 3498. E-mail addresses: jari.keskinen@vtt.fi (J. Keskinen), eino.sivonen@vtt.fi (E. Sivonen), salme.jussila@vtt.fi (S. Jussila), mikael.bergelin@abo.fi (M. Bergelin), max.johansson@abo.fi (M. Johansson), anu.vaari@vtt.fi (A. Vaari), maria.smolander@vtt.fi (M. Smolander). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.076
disposed together with normal household waste. These demands are not easily met using traditional battery technology. Furthermore the materials costs should be low and the manufacturing method inexpensive. The above given requirements can be met using a biological fuel cell as the power source. This type of fuel cell utilizes biological components, such as enzymes or micro-organisms, and no expensive noble metal catalysts are needed [3–5]. The fuel of choice can be different types of sugars or alcohols. The main drawback of these biological fuel cells is that the maximum current output is rather modest. However, relatively high current output is required for short time periods in many applications. Low peak current output of these enzymatic fuel cells can be improved by integrating the cell with a printed supercapacitor. Our half-enzymatic cell having 16 cm2 geometrical area produces 0.6–0.8 V potential and gives an output current of about 0.1 mA [6]. Fully enzymatic bio-fuel cells had the maximum power output of 1.7 W cm−2 at a voltage of 0.35 V which corresponds 78 A current from 16 cm2 geometrical area [7]. The manufacturing of the supercapacitor by printing methods is essential in this case to facilitate inexpensive manufacturing process together with the enzymatic fuel cell. Obviously the same manufacturing route can be applied also when preparing supercapacitors for other applications. The main components needed in a double layer capacitor can be made by printing techniques, and as the structure of the bio-fuel cell current collector components and the supercapacitor electrodes are partly similar, single stage manufacturing and integration is feasible. The manufacturing of supercapacitor electrodes using
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
303
Table 1 Supercapacitor structures. Electrode area (cm2 )
Sample number
Paperboard surface
1 2 3 4 5 6 7 8 9 10 11 12
PE 2 PE 2 Barr 2 Barr 2 2 PE Barr 2 Barr 0.5 Barr 2 Barr 2 Barr 2 2, thin layer Barr 2 samples in series (similar to sample number 10)
Electrolyte, NaCl:H2 O
Graphite coverage
Current collector
Separator
1:5 1:5 1:5 1:5 1:5 1:5 1:5 1:20 1:5 1:5 1:4
Narrow Narrow Narrow Wide Wide Narrow Wide Wide Wide Wide Wide
Acheson Xymara Acheson Acheson Acheson Acheson Acheson Acheson Acheson Acheson Acheson
3 × cellulose 3 × cellulose 3 × cellulose 3 × cellulose 3 × cellulose 1 × polycarbonate 1 × cellulose 3 × cellulose 3 × cellulose 1 × cellulose 1 × cellulose
printing techniques has been described in patents [8,9] and scientific report [10–15]. Solution based process that can be modified to screen printing have been reported in the preparation of polyaniline based supercapacitors [16]. Batteries and supercapacitors of carbon nanotubes and room-temperature ionic liquid electrolytes have been constructed using paper as substrate material [17]. The capacitance as well as other requirements of the supercapacitor was determined by the performance of the enzymatic power source and the requirements set in the beginning of the study. To provide the 300 ms pulse with 50 mA current with acceptable power drop the suitable capacitance is 0.1–0.2 F. The low voltage level of 1.0–1.2 V allowed the use of aqueous electrolyte.
2. Experimental The materials choice of the supercapacitor was largely defined by the enzymatic fuel cell manufacturing process and the requirement that the total system should be non-toxic, recyclable and incineratable. The current collectors and active material layers were made of printing inks. Aqueous electrolyte was preferred to organic ones and fluorine containing binders were not acceptable. Stora Enso Cupforma Classic Barr 20 + 190 + 42EB56 polymer coated paperboards were used as substrates. Acheson PF410, Spraylat 020 and 073 as well as Xymara Electra SSB111 silver inks were used as current collectors. Silver ink was covered with dense Acheson PF407C graphite ink. In this configuration graphite shields the surface of silver from corrosion caused by the electrolyte. The ink curing temperature was chosen to be 95 ◦ C in order to avoid damages to the polymer coating. Norit DLC Super 30 activated carbon was used in ink form as the electrode material. The water based ink contained activated carbon and chitosan binder in the ratio 20:1. The activated carbon geometrical area was normally 2 cm2 . Also samples with 0.5 cm2 area were used in order to optimize the leakage current. The activated carbon ink was dried at room temperature resulting to a thickness of about 90 m. A laboratory scale bar coater (K-coater) was used for applying the current collector and activated carbon inks. The inks used were also suitable for screen printing. NKK TF40–50 cellulose paper and Whatman Nuclepore (pores 0.6 m) polycarbonate film were used as separators. The electrolyte was made by diluting pro analysis grade NaCl to ion-exchanged water in weight ratios 1:4, 1:5 and 1:20. A schematic cross-section of the supercapacitor structure is shown in Fig. 1. The dimensions were about 70 mm × 50 mm and the total thickness about 0.8 mm. Table 1 contains the manufacturing parameters of the manufactured supercapacitors.
Fig. 1. A schematic cross-section of a printed supercapacitor. Horizontal and vertical dimensions are in different scale.
The electrical properties of the capacitors such as capacitance, equivalent series resistance (ESR), and leakage current were determined as guided in standard [18] using Arbin SCTS instrument. In the measurement procedure the component was first charged and discharged with constant 10 mA or 50 mA current between 0.2 and 1.2 V three times, then the voltage was kept for 30 min at constant value (normally 1.2 V) and after this the capacitance was defined during the constant current discharge step between 0.96 V and 0.48 V potential. After keeping the capacitor for 1 h at constant voltage the leakage current was recorded. The efficiency was defined as the ratio of the discharged and charged energy in the voltage range of 0.2–1.2 V. 3. Results and discussion Supercapacitors having various parameters were compared with each other. In the following the effect of these parameters on the most essential supercapacitor properties is reviewed. Table 2 contains the measured electrical values. Table 2 Supercapacitor properties. Sample number
Capacitance (F)
ESR ()
Efficiency with Leakage current (A) 10 mA (%)
1 2 3 4 5 6 7 8 9 10 11 12
0.28 0.30 0.33 0.33 0.38 0.48 0.14 0.29 0.29 0.29 0.20 0.16
0.73 0.71 0.69 0.84 0.85 0.56 2.8 3.0 0.81 0.43 0.36 1.1
94 94 94 93 93 94 72 78 93 96 97 94
14 17 15 13 14 20 5.3 12 12 9 10 2.8
304
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
a
1.4
Potenal (V)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
8
10
12
Time (s)
b
1.2
3.1. Capacitance For the standard 2 cm2 active area capacitors the capacitance values were more than adequate compared to the set requirements. Thus the series connection of these capacitors or smaller active area in order to decrease the leakage current did still provide reasonable capacitance. The specific capacitance was typically 30–35 F/g when only the activated carbon mass was taken into account. 3.2. Equivalent series resistance Since the motivation for the use of supercapacitor is to be able to supply high power, minimizing ESR is essential. The separator and electrolyte of the supercapacitor can be otherwise quite similar to those of the biofuel cell, but the ion conductivity of the supercapacitor electrolyte needs to be higher to facilitate faster charging and discharging. The electrolyte can also be either organic or aqueous. The advantage of an organic electrolyte is the higher achievable voltage, but they also have significantly higher specific resistance. In our application there was no need for potential exceeding 1.2 V, which made aqueous electrolyte an obvious choice. Fig. 3 shows the behavior of sample 1 when 10 mA and 50 mA charge/discharge currents were used. The effect of ESR to charge/discharge cycling is obvious. Especially at higher currents the IR drop in the beginning of the discharge step is clear. The ESR of the supercapacitor consists of the resistance caused by ohmic resistance due to the current collectors and activated carbon layer and ionic resistance in the electrolyte. In addition there exists also contact resistance between the layers. In the current collector layout used in the components graphite shields the surface of silver from the effect of electrolyte. Since the thickness of the graphite layer is below 0.1 mm, it does not remarkably increase the resistance. In order to keep the efficiency at acceptable level, the ESR should preferably not exceed 1 when the current need is of the order of
Potenal (V)
Fig. 2. Supercapacitor electrodes on paperboard (left) and a supercapacitor made by assembling the electrodes by aligning activated carbon areas against each other.
Supercapacitor electrodes of 2 cm2 and 0.5 cm2 activated carbon geometrical area were prepared. The graphite ink was applied using two different layouts. The graphite was either applied only to cover the silver ink (referred as narrow in Table 1) or to cover also the paperboard surface that is in contact with the electrolyte (wide). Fig. 2 shows electrodes with wider graphite layers and a supercapacitor made of these.
1.4
1 0.8 0.6 0.4 0.2 0 0
2
4
6
Time (s) Fig. 3. The charge/discharge of supercapacitor sample 1 with (a) 10 mA and (b) 50 mA current.
10–50 mA (with 1 the voltage loss is 10 mV and 50 mV, respectively). The typical ESR obtained was 0.5–0.7 . No practical differences between Acheson, Spraylat and Xymara silver inks were obtained in the conductivity values. Typically the square resistances were of the order of 0.03–0.05 / for 20–30 m thick silver layers. Thus the ESR values obtained for the capacitors with these inks were very near to each other. First the separator cellulose paper was used as 3-fold in order to avoid short-circuits between the electrodes. However, it was found that 1-fold paper did not increase leakage current and the effect on decreasing ESR was quite significant: by using 1-fold separator the ESR value was decreased by about 30% because of shorter distance between the electrodes. Good electrical properties were obtained also with a perforated polycarbonate separator foil having 0.6 m holes (sample 6) manufactured by Whatman. The ESR, efficiency and leakage current values were on the same level as with cellulose paper. The most often used electrolyte was NaCl:water solution in ratio 1:5. It was used as standard alternative in estimating other supercapacitor material and structure alternatives. Also 1:20 and 1:4 solutions were evaluated (samples 8 and 11). When the typical ESR for the 1:5 NaCl:water electrolyte was 0.5–1 , the 1:20 solution resulted to about 3 . By increasing the NaCl content to 1:4, the ESR was slightly decreased because of the higher ionic conductivity. The specific conductances for 1:20, 1:5 and 1:4 solutions are 68, 184 and 204 mS/cm, respectively [19]. To minimize the ESR, 90 m thick activated carbon layer was replaced with 60 m thick layer and NaCl:water concentration was changed to 1:4. In this way the lowest ESR obtained was 0.36 (sample 11). For series connected supercapacitors the ESR was obviously doubled compared with single components. Also for smaller area
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
a
305
1.4 1.2
Potential/V
1.0 0.8 6 0.4 0.2 0.0 0
50
100
150
200
250
b
100.0
Leakage current (μA)
Time/h
10.0
1.0
Fig. 5. Series connected supercapacitors in single package, sample 12 in the tables.
ESR becomes. With low current the leakage current can be the main reason for low efficiency. The efficiency values were measured mainly for 10 mA and 50 mA charge and discharge currents. For the standard structure (as described above in the ESR paragraph) the efficiency for 10 mA current was typically 94–95% and for 50 mA current 89–90%. Increased ESR due to lower NaCl concentration clearly decreased efficiency (sample 8). In the case of series connected supercapacitors the ESR values for 10 mA and 50 mA were about 94 and 80%. The decrease compared with single capacitors was due to the increased ESR. 0.0
0.5
1.0
1.5
3.5. Lifetime
Voltage (V) Fig. 4. The behavior of a disconnected supercapacitor (sample 10 in the tables) after keeping it for 1 h at 1.2 V The voltage as a function of time (a) and momentary leakage current as a function of voltage (b).
supercapacitors the ESR increased being approximately inversely proportional to the geometrical active area. 3.3. Leakage current In the case of current collectors made of silver ink coated with graphite ink the leakage current values for normal 2 cm2 electrode area were of the order of 10–20 A and for 0.5 cm2 area less than 10 A was obtained. The self-discharge rate of sample number 10 is shown in Fig. 4a. The voltage decrease in open circuit case reveals the momentary leakage current. The momentary leakage current calculated from the data in Fig. 4a is shown in Fig. 4b. Since the leakage current increases faster than linearly when voltage is increased, using a series connected supercapacitor pair can result to very low leakage current. When the total voltage is 1.2 V, there is only 0.6 V potential over each supercapacitor. With this configuration the leakage current was only 2–3 A. The drawback is the same as with smaller area electrodes: the ESR value is increased and thus the efficiency decreases especially with higher current. Series connected supercapacitors were printed on single substrate shown in Fig. 5. 3.4. Efficiency The efficiency of a supercapacitor is mainly dictated by ESR and leakage current. The higher the current is, the more important the
In the lifetime tests a clear correlation between the mass decrease caused by electrolyte evaporation and the deterioration of electrical properties was observed. Fig. 6 shows the mass change as a function of time. The efficiency of the capacitors is marked in the figure. It can be estimated that about 20–30% of the electrolyte mass can be lost before remarkable effect on the efficiency take place. From the time scale it can be seen that the lifetime of the supercapacitors clearly exceeds 3 months. Two different graphite ink layouts were used. In the narrower configuration the ink covered only the silver ink and in the wider the graphite layer was wide enough to prevent the electrolyte from being in contact with the surface of the paperboard. The majority of the supercapacitors were made on the barrier side of the paperboard. The main reason for this was that the major 0.65
η=90-93
0.6
Electrolyte mass (g)
0.1
η=85-91
0.55
1
0.5
η=82-88
2
η=76-86
0.45
η=93
3
0.4
η=41-65
4
η=89
0.35
5
0.3 0.25 0.2
0
20
40
60
80
100
120
140
Time (days) Fig. 6. Electrolyte mass decrease and supercapacitor efficiency percentage as functions of time for selected components. The numbers 1–5 refer to sample numbers in Tables 1 and 2.
306
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
lifetime limiting factor was the evaporation of electrolyte. When made on the PE side the electrolyte can evaporate first through the PE layer and then migrate along the porous paperboard layer further. Thus to exploit the barrier layer the electrolyte must be on the barrier side of the paperboard. However, by applying graphite ink on larger area (samples 4 and 5), the electrolyte evaporation can be retarded, which allows the use of a substrate with higher water permeability. Further improvement could be obtained by using a substrate material that contains e.g. also a metal foil to prevent electrolyte evaporation. 4. Conclusions Different printable supercapacitor structures were prepared. The design was guided by the demand of non-toxic materials and easy manufacturing method using printing methods. The capacitance was defined by the need to provide 50 mA pulses for the period of 300 ms. The capacitance of the manufactured supercapacitors was 0.15–0.5 F. The efficiency level obtained with 10 mA current exceeded 95% and with 50 mA about 90% was reached. As current collector the combination of silver and graphite inks was found good enough to be used in practical components. The use of inks made it possible to partly combine the manufacturing process of enzymatic fuel cell and supercapacitor on the same substrate. It would be advantageous to decrease the leakage current further in order to make the combination of enzymatic fuel cell and supercapacitor work more efficiently. The leakage current of series connected supercapacitor pairs was about 2–3 A. The lifetime for the supercapacitors was measured to be over 3 months. This was adequate although longer lifetime would be preferred to lengthen the supercapacitor shelf-life.
Acknowledgements Financial support from Tekes – the Finnish Funding Agency for Technology and Innovation and industrial project partners is gratefully acknowledged. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Plenum, New York, 1999. [2] R. Moscatiello, SGIA Journal (1st quarter) (2005) 25. [3] S.D. Minteer, B.Y. Liaw, M.J. Cooney, Current Opinion in Biotechnology 18 (2007) 228. [4] F. Davis, S.P.J. Higson, Biosensors and Bioelectronics 22 (2007) 1224. [5] R.A. Bullen, T.C. Arnot, J.B. Lakeman, F.C. Walsh, Biosensors and Bioelectronics 21 (2006) 2015. [6] M. Smolander, H. Boer, M. Valkiainen, R. Roozeman, M. Bergelin, J.-E. Eriksson, X.-C. Zhang, A. Koivula, L. Viikari, Enzyme and Microbial Technology 43 (2008) 93. [7] P. Jenkins, S. Tuurala, A. Vaari, M. Valkiainen, M. Smolander, D. Leech, Bioelectrochemistry 87 (2012) 172. [8] O.S. Nissen, H.C. Beck, M. Schou, Patent US 6,341,057 (2002). [9] J. Lang, Patent US 2005/0,007,727 (2005). [10] D. Wei, S. Wakeham, T. Ng, M. Thwaites, H. Brown, P. Beecher, Electrochemistry Communications 11 (2009) 2285. [11] M. Kaempgen, C.K. Chan, J. Ma, Y. Cui, G. Gruner, Nano Letters 9 (2009) 1872. [12] K. Jost, C.R. Perez, J.K. McDonough, V. Presser, M. Heon, G. Dion, Y. Gogotsi, Energy Environmental Science 4 (2011) 5060. [13] L. Hu, H. Wu, Y. Cui, Applied Physics Letters 96 (2010) 183502-1. [14] L.T. Le, M.H. Erwin, H. Qui, B.E. Fuchs, W.Y. Lee, Electrochemistry Communications 13 (2011) 355. [15] P. Kossyrev, Journal of Power Sources 201 (2012) 347. [16] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Nano Letters 10 (2010) 4025. [17] V.L. Pushparaj, M.M. Shaijumon, A. Kumar, S. Murugesan, L. Ci, R. Vajtai, R.J. Linhardt, O. Nalamasu, M. Ajayan, Proceedings of the National Academy of Sciences of the United States of America 104 (2007) 13574. [18] Standard IEC 62391 (2006). [19] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 64th ed., CRC Press, 1983, p. D-258.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Printed supercapacitors on paperboard substrate Jari Keskinen a,∗ , Eino Sivonen a , Salme Jussila b , Mikael Bergelin c , Max Johansson c , Anu Vaari b , Maria Smolander b a b c
VTT Technical Research Centre of Finland, PO Box 1300, FI-33101 Tampere, Finland VTT Technical Research Centre of Finland, PO Box 1000, FI-02044 VTT, Finland Åbo Akademi, Process Chemistry Centre, Piispankatu 8, FI-20500 Turku, Finland
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 17 August 2012 Accepted 18 August 2012 Available online 25 August 2012 Keywords: Supercapacitor Electrical double layer capacitor Printed power source Printed electronics
a b s t r a c t Printed supercapacitors were prepared to be applied as a part of a hybrid power source in printed electronics applications. The use of non-toxic materials was preferred. The supercapacitor structure consisting of current collectors and activated carbon electrodes was applied on paperboard substrate using silver, graphite and activated carbon inks. Aqueous electrolytes with NaCl salt limit the maximum potential to about 1.2 V but are environmentally friendly and provide low equivalent series resistance (ESR). The capacitance values of our printed supercapacitors were typically 0.1–0.5 F. With 2 cm2 geometrical active area the ESR was below 1 which is acceptable for 50 mA current output. The efficiency with 50 mA charge and discharge current was typically about 90% and with 10 mA about 95%. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical supercapacitors [1] are intensively developed since they are able to supply high peak power requirements in energy storage applications. An electrochemical supercapacitor consists of two electrodes separated by an ionically conductive electrolyte. The electrodes are typically made of activated carbon powder that is bound using fluorine containing polymers such as PTFE or PVDF. As electrolytes organic solutions such as propylene carbonate or acetonitrile are used, but also aqueous alternatives are possible. With organic electrolytes the maximum potential between the electrodes is about 2.5 V but water based electrolytes limit the maximum voltage to about 1.3 V. In recent years a lot of effort has been put on the development of different integratable identification and data storage/transfer devices as well as miniaturized sensors. Active type tags and sensors require a suitable power source to fully utilize their potential allowing greater memory capacity in the tag and increasing the range from which the tag can be read [2]. Since these tags are often intended to be used as integrated parts of recyclable products, all components of the power source/RFID tag combination should be biodegradable or incineratable when the product is
∗ Corresponding author. Tel.: +358 20 722 3703; fax: +358 20 722 3498. E-mail addresses: jari.keskinen@vtt.fi (J. Keskinen), eino.sivonen@vtt.fi (E. Sivonen), salme.jussila@vtt.fi (S. Jussila), mikael.bergelin@abo.fi (M. Bergelin), max.johansson@abo.fi (M. Johansson), anu.vaari@vtt.fi (A. Vaari), maria.smolander@vtt.fi (M. Smolander). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.076
disposed together with normal household waste. These demands are not easily met using traditional battery technology. Furthermore the materials costs should be low and the manufacturing method inexpensive. The above given requirements can be met using a biological fuel cell as the power source. This type of fuel cell utilizes biological components, such as enzymes or micro-organisms, and no expensive noble metal catalysts are needed [3–5]. The fuel of choice can be different types of sugars or alcohols. The main drawback of these biological fuel cells is that the maximum current output is rather modest. However, relatively high current output is required for short time periods in many applications. Low peak current output of these enzymatic fuel cells can be improved by integrating the cell with a printed supercapacitor. Our half-enzymatic cell having 16 cm2 geometrical area produces 0.6–0.8 V potential and gives an output current of about 0.1 mA [6]. Fully enzymatic bio-fuel cells had the maximum power output of 1.7 W cm−2 at a voltage of 0.35 V which corresponds 78 A current from 16 cm2 geometrical area [7]. The manufacturing of the supercapacitor by printing methods is essential in this case to facilitate inexpensive manufacturing process together with the enzymatic fuel cell. Obviously the same manufacturing route can be applied also when preparing supercapacitors for other applications. The main components needed in a double layer capacitor can be made by printing techniques, and as the structure of the bio-fuel cell current collector components and the supercapacitor electrodes are partly similar, single stage manufacturing and integration is feasible. The manufacturing of supercapacitor electrodes using
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
303
Table 1 Supercapacitor structures. Electrode area (cm2 )
Sample number
Paperboard surface
1 2 3 4 5 6 7 8 9 10 11 12
PE 2 PE 2 Barr 2 Barr 2 2 PE Barr 2 Barr 0.5 Barr 2 Barr 2 Barr 2 2, thin layer Barr 2 samples in series (similar to sample number 10)
Electrolyte, NaCl:H2 O
Graphite coverage
Current collector
Separator
1:5 1:5 1:5 1:5 1:5 1:5 1:5 1:20 1:5 1:5 1:4
Narrow Narrow Narrow Wide Wide Narrow Wide Wide Wide Wide Wide
Acheson Xymara Acheson Acheson Acheson Acheson Acheson Acheson Acheson Acheson Acheson
3 × cellulose 3 × cellulose 3 × cellulose 3 × cellulose 3 × cellulose 1 × polycarbonate 1 × cellulose 3 × cellulose 3 × cellulose 1 × cellulose 1 × cellulose
printing techniques has been described in patents [8,9] and scientific report [10–15]. Solution based process that can be modified to screen printing have been reported in the preparation of polyaniline based supercapacitors [16]. Batteries and supercapacitors of carbon nanotubes and room-temperature ionic liquid electrolytes have been constructed using paper as substrate material [17]. The capacitance as well as other requirements of the supercapacitor was determined by the performance of the enzymatic power source and the requirements set in the beginning of the study. To provide the 300 ms pulse with 50 mA current with acceptable power drop the suitable capacitance is 0.1–0.2 F. The low voltage level of 1.0–1.2 V allowed the use of aqueous electrolyte.
2. Experimental The materials choice of the supercapacitor was largely defined by the enzymatic fuel cell manufacturing process and the requirement that the total system should be non-toxic, recyclable and incineratable. The current collectors and active material layers were made of printing inks. Aqueous electrolyte was preferred to organic ones and fluorine containing binders were not acceptable. Stora Enso Cupforma Classic Barr 20 + 190 + 42EB56 polymer coated paperboards were used as substrates. Acheson PF410, Spraylat 020 and 073 as well as Xymara Electra SSB111 silver inks were used as current collectors. Silver ink was covered with dense Acheson PF407C graphite ink. In this configuration graphite shields the surface of silver from corrosion caused by the electrolyte. The ink curing temperature was chosen to be 95 ◦ C in order to avoid damages to the polymer coating. Norit DLC Super 30 activated carbon was used in ink form as the electrode material. The water based ink contained activated carbon and chitosan binder in the ratio 20:1. The activated carbon geometrical area was normally 2 cm2 . Also samples with 0.5 cm2 area were used in order to optimize the leakage current. The activated carbon ink was dried at room temperature resulting to a thickness of about 90 m. A laboratory scale bar coater (K-coater) was used for applying the current collector and activated carbon inks. The inks used were also suitable for screen printing. NKK TF40–50 cellulose paper and Whatman Nuclepore (pores 0.6 m) polycarbonate film were used as separators. The electrolyte was made by diluting pro analysis grade NaCl to ion-exchanged water in weight ratios 1:4, 1:5 and 1:20. A schematic cross-section of the supercapacitor structure is shown in Fig. 1. The dimensions were about 70 mm × 50 mm and the total thickness about 0.8 mm. Table 1 contains the manufacturing parameters of the manufactured supercapacitors.
Fig. 1. A schematic cross-section of a printed supercapacitor. Horizontal and vertical dimensions are in different scale.
The electrical properties of the capacitors such as capacitance, equivalent series resistance (ESR), and leakage current were determined as guided in standard [18] using Arbin SCTS instrument. In the measurement procedure the component was first charged and discharged with constant 10 mA or 50 mA current between 0.2 and 1.2 V three times, then the voltage was kept for 30 min at constant value (normally 1.2 V) and after this the capacitance was defined during the constant current discharge step between 0.96 V and 0.48 V potential. After keeping the capacitor for 1 h at constant voltage the leakage current was recorded. The efficiency was defined as the ratio of the discharged and charged energy in the voltage range of 0.2–1.2 V. 3. Results and discussion Supercapacitors having various parameters were compared with each other. In the following the effect of these parameters on the most essential supercapacitor properties is reviewed. Table 2 contains the measured electrical values. Table 2 Supercapacitor properties. Sample number
Capacitance (F)
ESR ()
Efficiency with Leakage current (A) 10 mA (%)
1 2 3 4 5 6 7 8 9 10 11 12
0.28 0.30 0.33 0.33 0.38 0.48 0.14 0.29 0.29 0.29 0.20 0.16
0.73 0.71 0.69 0.84 0.85 0.56 2.8 3.0 0.81 0.43 0.36 1.1
94 94 94 93 93 94 72 78 93 96 97 94
14 17 15 13 14 20 5.3 12 12 9 10 2.8
304
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
a
1.4
Potenal (V)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
8
10
12
Time (s)
b
1.2
3.1. Capacitance For the standard 2 cm2 active area capacitors the capacitance values were more than adequate compared to the set requirements. Thus the series connection of these capacitors or smaller active area in order to decrease the leakage current did still provide reasonable capacitance. The specific capacitance was typically 30–35 F/g when only the activated carbon mass was taken into account. 3.2. Equivalent series resistance Since the motivation for the use of supercapacitor is to be able to supply high power, minimizing ESR is essential. The separator and electrolyte of the supercapacitor can be otherwise quite similar to those of the biofuel cell, but the ion conductivity of the supercapacitor electrolyte needs to be higher to facilitate faster charging and discharging. The electrolyte can also be either organic or aqueous. The advantage of an organic electrolyte is the higher achievable voltage, but they also have significantly higher specific resistance. In our application there was no need for potential exceeding 1.2 V, which made aqueous electrolyte an obvious choice. Fig. 3 shows the behavior of sample 1 when 10 mA and 50 mA charge/discharge currents were used. The effect of ESR to charge/discharge cycling is obvious. Especially at higher currents the IR drop in the beginning of the discharge step is clear. The ESR of the supercapacitor consists of the resistance caused by ohmic resistance due to the current collectors and activated carbon layer and ionic resistance in the electrolyte. In addition there exists also contact resistance between the layers. In the current collector layout used in the components graphite shields the surface of silver from the effect of electrolyte. Since the thickness of the graphite layer is below 0.1 mm, it does not remarkably increase the resistance. In order to keep the efficiency at acceptable level, the ESR should preferably not exceed 1 when the current need is of the order of
Potenal (V)
Fig. 2. Supercapacitor electrodes on paperboard (left) and a supercapacitor made by assembling the electrodes by aligning activated carbon areas against each other.
Supercapacitor electrodes of 2 cm2 and 0.5 cm2 activated carbon geometrical area were prepared. The graphite ink was applied using two different layouts. The graphite was either applied only to cover the silver ink (referred as narrow in Table 1) or to cover also the paperboard surface that is in contact with the electrolyte (wide). Fig. 2 shows electrodes with wider graphite layers and a supercapacitor made of these.
1.4
1 0.8 0.6 0.4 0.2 0 0
2
4
6
Time (s) Fig. 3. The charge/discharge of supercapacitor sample 1 with (a) 10 mA and (b) 50 mA current.
10–50 mA (with 1 the voltage loss is 10 mV and 50 mV, respectively). The typical ESR obtained was 0.5–0.7 . No practical differences between Acheson, Spraylat and Xymara silver inks were obtained in the conductivity values. Typically the square resistances were of the order of 0.03–0.05 / for 20–30 m thick silver layers. Thus the ESR values obtained for the capacitors with these inks were very near to each other. First the separator cellulose paper was used as 3-fold in order to avoid short-circuits between the electrodes. However, it was found that 1-fold paper did not increase leakage current and the effect on decreasing ESR was quite significant: by using 1-fold separator the ESR value was decreased by about 30% because of shorter distance between the electrodes. Good electrical properties were obtained also with a perforated polycarbonate separator foil having 0.6 m holes (sample 6) manufactured by Whatman. The ESR, efficiency and leakage current values were on the same level as with cellulose paper. The most often used electrolyte was NaCl:water solution in ratio 1:5. It was used as standard alternative in estimating other supercapacitor material and structure alternatives. Also 1:20 and 1:4 solutions were evaluated (samples 8 and 11). When the typical ESR for the 1:5 NaCl:water electrolyte was 0.5–1 , the 1:20 solution resulted to about 3 . By increasing the NaCl content to 1:4, the ESR was slightly decreased because of the higher ionic conductivity. The specific conductances for 1:20, 1:5 and 1:4 solutions are 68, 184 and 204 mS/cm, respectively [19]. To minimize the ESR, 90 m thick activated carbon layer was replaced with 60 m thick layer and NaCl:water concentration was changed to 1:4. In this way the lowest ESR obtained was 0.36 (sample 11). For series connected supercapacitors the ESR was obviously doubled compared with single components. Also for smaller area
J. Keskinen et al. / Electrochimica Acta 85 (2012) 302–306
a
305
1.4 1.2
Potential/V
1.0 0.8 6 0.4 0.2 0.0 0
50
100
150
200
250
b
100.0
Leakage current (μA)
Time/h
10.0
1.0
Fig. 5. Series connected supercapacitors in single package, sample 12 in the tables.
ESR becomes. With low current the leakage current can be the main reason for low efficiency. The efficiency values were measured mainly for 10 mA and 50 mA charge and discharge currents. For the standard structure (as described above in the ESR paragraph) the efficiency for 10 mA current was typically 94–95% and for 50 mA current 89–90%. Increased ESR due to lower NaCl concentration clearly decreased efficiency (sample 8). In the case of series connected supercapacitors the ESR values for 10 mA and 50 mA were about 94 and 80%. The decrease compared with single capacitors was due to the increased ESR. 0.0
0.5
1.0
1.5
3.5. Lifetime
Voltage (V) Fig. 4. The behavior of a disconnected supercapacitor (sample 10 in the tables) after keeping it for 1 h at 1.2 V The voltage as a function of time (a) and momentary leakage current as a function of voltage (b).
supercapacitors the ESR increased being approximately inversely proportional to the geometrical active area. 3.3. Leakage current In the case of current collectors made of silver ink coated with graphite ink the leakage current values for normal 2 cm2 electrode area were of the order of 10–20 A and for 0.5 cm2 area less than 10 A was obtained. The self-discharge rate of sample number 10 is shown in Fig. 4a. The voltage decrease in open circuit case reveals the momentary leakage current. The momentary leakage current calculated from the data in Fig. 4a is shown in Fig. 4b. Since the leakage current increases faster than linearly when voltage is increased, using a series connected supercapacitor pair can result to very low leakage current. When the total voltage is 1.2 V, there is only 0.6 V potential over each supercapacitor. With this configuration the leakage current was only 2–3 A. The drawback is the same as with smaller area electrodes: the ESR value is increased and thus the efficiency decreases especially with higher current. Series connected supercapacitors were printed on single substrate shown in Fig. 5. 3.4. Efficiency The efficiency of a supercapacitor is mainly dictated by ESR and leakage current. The higher the current is, the more important the
In the lifetime tests a clear correlation between the mass decrease caused by electrolyte evaporation and the deterioration of electrical properties was observed. Fig. 6 shows the mass change as a function of time. The efficiency of the capacitors is marked in the figure. It can be estimated that about 20–30% of the electrolyte mass can be lost before remarkable effect on the efficiency take place. From the time scale it can be seen that the lifetime of the supercapacitors clearly exceeds 3 months. Two different graphite ink layouts were used. In the narrower configuration the ink covered only the silver ink and in the wider the graphite layer was wide enough to prevent the electrolyte from being in contact with the surface of the paperboard. The majority of the supercapacitors were made on the barrier side of the paperboard. The main reason for this was that the major 0.65
η=90-93
0.6
Electrolyte mass (g)
0.1
η=85-91
0.55
1
0.5
η=82-88
2
η=76-86
0.45
η=93
3
0.4
η=41-65
4
η=89
0.35
5
0.3 0.25 0.2
0
20
40
60
80
100
120
140
Time (days) Fig. 6. Electrolyte mass decrease and supercapacitor efficiency percentage as functions of time for selected components. The numbers 1–5 refer to sample numbers in Tables 1 and 2.
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lifetime limiting factor was the evaporation of electrolyte. When made on the PE side the electrolyte can evaporate first through the PE layer and then migrate along the porous paperboard layer further. Thus to exploit the barrier layer the electrolyte must be on the barrier side of the paperboard. However, by applying graphite ink on larger area (samples 4 and 5), the electrolyte evaporation can be retarded, which allows the use of a substrate with higher water permeability. Further improvement could be obtained by using a substrate material that contains e.g. also a metal foil to prevent electrolyte evaporation. 4. Conclusions Different printable supercapacitor structures were prepared. The design was guided by the demand of non-toxic materials and easy manufacturing method using printing methods. The capacitance was defined by the need to provide 50 mA pulses for the period of 300 ms. The capacitance of the manufactured supercapacitors was 0.15–0.5 F. The efficiency level obtained with 10 mA current exceeded 95% and with 50 mA about 90% was reached. As current collector the combination of silver and graphite inks was found good enough to be used in practical components. The use of inks made it possible to partly combine the manufacturing process of enzymatic fuel cell and supercapacitor on the same substrate. It would be advantageous to decrease the leakage current further in order to make the combination of enzymatic fuel cell and supercapacitor work more efficiently. The leakage current of series connected supercapacitor pairs was about 2–3 A. The lifetime for the supercapacitors was measured to be over 3 months. This was adequate although longer lifetime would be preferred to lengthen the supercapacitor shelf-life.
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