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Modification of Al current collector surface by sol–gel deposit for carbon–carbon supercapacitor applications
Electrochimica Acta 49 (2004) 905–912
Modification of Al current collector surface by sol–gel deposit for carbon–carbon supercapacitor applications C. Portet, P.L. Taberna, P. Simon∗ , C. Laberty-Robert CIRIMAT, UMR CNRS 5085, 118 Route de Narbonne, Toulouse Cedex 31062, France Received 8 July 2003; received in revised form 19 September 2003; accepted 28 September 2003
Abstract Four square centimeter carbon–carbon supercapacitor cells were assembled with Al current collectors in organic electrolyte. Different treatments of the Al current collectors were made in order to increase the supercapacitor performances. A sol–gel deposit of a conducting carbonaceous material led to the best results. On the basis of electrochemical impedance spectroscopy measurements, the differences observed with the previous treatments were assumed to be linked to the modification of the Al/active material interface. The cell using sol–gel treated current collector presented an activated carbon specific capacitance of 100 F/g and a series resistance of 0.8 cm2 in acetonitrile 1 M NEt4 BF4 , that are characteristics compatible with high power applications. © 2003 Elsevier Ltd. All rights reserved. Keywords: Supercapacitors; Activated carbon; Sol–gel deposit; Impedance spectroscopy
1. Introduction Since the last past 5 years, supercapacitors have focused a lot of interest because of their promising properties in terms of energy storage and power supply. These devices can fill the gap existing between the batteries and the dielectric capacitors. As compared to batteries, they can deliver higher specific power; on the other hand, they store higher energy density than dielectric capacitors. These particular properties already make them suitable for a lot of applications in power electronics and peak saving. But their performances are so interesting that they are now promising candidates in transportation applications, particularly in vehicle and hybrid electric vehicle (HEV). In HEV, they could help the energy storage source to deliver the high power needed to perform the electric boost, the stop and go function or to help the braking energy recovering [1,2]. Three different types of supercapacitors are commonly described in the literature, depending on the active material nature used: activated carbon [3,4], metal oxide [5,6] and electronically conducting polymer [7,8]. Electronically conducting polymer systems are still at the research state. ∗ Corresponding author. Tel.: +33-5-61-55-68-02; fax: +33-5-61-55-61-63. E-mail address: [email protected] (P. Simon).
0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.09.043
Metal oxide-based supercapacitors are limited to applications where costless products are not needed. Activated carbon-based supercapacitors are the most developed technology; these are commercially available systems, using cheap active material, i.e. a highly divided carbon with specific surface area as high as 2000 m2 /g. The charge storage is purely electrostatic; the electrolyte ions are reversibly adsorbed in the electrochemical double layer of the porous carbon electrode structure [9a]. There is no faradaic reaction; the series resistance of the system is then low, leading to high specific power systems. It must be pointed out however that a last type of supercapacitors is now emerging: the hybrid systems, associating faradaic and carbon electrodes, in order to increase their specific energy [10–12]. In this paper, the results obtained with 4 cm2 carbon-based supercapacitors cells are presented. The aim of this study is to try to improve the aluminum current collector/active material interface properties in order to decrease the interface impedance and then the series resistance of the supercapacitor. In this way, a solution, proposed in the literature, consists in the use of a Al current collector covered by a conducting paint, leading to a better Al current collector/active material contact [10,13]. The sol–gel deposits are well known for their highly covering properties. Here, two different surface treatments have
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been achieved on the Al current collectors: an electrochemical etching and a conducting thin film coating via the sol–gel route. The influence of such treatments on the electrochemical properties of the supercapacitors cells will be discussed with the help of the electrochemical impedance spectroscopy technique. An electric equivalent circuit will be proposed to describe the modifications induced by these treatments at the current collector/active material interface.
2. Experimental Supercapacitor electrodes have been described elsewhere [13]. They consist of active layers laminated onto aluminum foil of 200 m thick. The active layer composition is 95 wt.% activated carbon, 3% carboxymethylcellulose (CMC; Prolabo) and 2% polytetrafluoroethylene (PTFE; Dupont de Nemours). The activated carbon used is the PicatifBP10 from the Pica Company (Vierzon, France). The active material films are 150 m thick; the electrode weight density is 15 mg/cm2 . All the cell assembly was made in a glove box with both water and oxygen content lower than 1 mg/kg. Electrodes were made by laminating the active material films onto the current collectors. The stack was assembled by inserting two layers of porous polymeric separators between the two electrodes; two PTFE plates and two stainless steel clamps were placed to press the stack, as can be seen in Fig. 1. The stack was immersed in an organic electrolyte, a solution of acetonitrile (AN, 10 mg/kg water) with 1.5 M NEt4 BF4 dried salt. Electrochemical impedance spectroscopy measurements were achieved with an EGG 6310, between 50 kHz and 10 mHz, at a bias voltage of 0 V. The cycling experiments were performed with a Mac Pile battery cycling apparatus, at a constant current density of ±2.5 mA/cm2 , between 0 and 2.3 V. SEM pictures were obtained with a JEOL JSM 6400 apparatus, and the roughness measurements were carried out with a ZYGO Corporation apparatus.
Fig. 1. Schematic drawing of the two-electrode 4 cm2 cell stack.
3. Results and discussion 3.1. Electrochemical impedance spectroscopy study of 4 cm2 cells 3.1.1. 4 cm2 supercapacitor cell impedance spectroscopy measurements Supercapacitor has been assembled with “as-received” Al foils. Fig. 2 presents the SEM picture of a “as-received” Al foil surface. It can be seen a flat surface with an average roughness of 0.2–0.3 m. Fig. 3 presents the theoretical Nyquist plot of a supercapacitor [9b]. Fig. 4 shows the experimental plot of a two-electrode 4 cm2 cell assembled with two “as-received” Al foils (99.99% Al) between 50 kHz and 10 mHz at 0 V. The plot in Fig. 3 presents three different regions, depending on the frequency. At very high frequency, the supercapacitor behaves like a pure resistance. At low frequency, the imaginary part sharply increases and a vertical line is observed, traducing a pure capacitive behavior. In the middle frequency domain, the influence of the electrode porosity can be seen. When the frequency decreases, starting from the very high frequency, the signal penetrates deeper and deeper inside the porous structure
Fig. 2. SEM picture of a “as-received” Al foil surface.
Fig. 3. Theoretical Nyquist plot of a carbon–carbon supercapacitor.
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Fig. 4. Nyquist plot of a two-electrode 4 cm2 supercapacitor cell assembled with “as-received” Al current collectors between 50 kHz and 10 mHz at 0 V.
of the electrode; more and more activated carbon surface is available for the ion adsorption. This middle frequency range is then related to the electrolyte penetration inside the porous structure of the activated carbon electrode. This point has been extensively studied by de Levie [14,15]. The experimental plot presented in Fig. 4 is slightly different as compared to the theoretical one. When the frequency is decreased, from 7.92 Hz down to 10 mHz, the theoretical behavior is observed with the different zones previously explained. But a semi-circle loop appears between 50 kHz and 7.92 Hz. This loop shifts the capacitive behavior along the real axis, i.e. towards higher resistance value. This loop is undesirable as it decreases the performances of the cell in terms of resistance and power. This semi-circle loop has been observed with carbon supercapacitors systems by numerous authors in the literature. Andrieu et al. associate this high frequency loop to the inter-granular electronic resistance between the activated carbon particles [16], namely Rt ; this resistance is called “pseudo-transfer resistance”. Rt depends mainly on the electrode surface area and the inter-particle resistivity. The realization of thin active layers, as well as the improvement of the electronic percolation can reduce this value. Other authors propose an explanation in relationship with the activated carbon–current collector interface [17]. This semi-circle was suggested to represent the impedance at the interface between the current collector and the carbon fabric that were activated carbon fibers, as well as between the fibers [18,19]. They noticed that when the surface oxide population on the carbon was increased, so did the semi-circle loop, shifting the capacitive behavior along the real axis to higher resistance [17]. Pell et al. attributed this RC loop to the resistance of the passive layer (TiO2 ) developed on the current collector surface (Ti) [20]. This semi-circle loop increases the resistance of the cell; it is then important to find solutions to decrease this contribution. One solution may be to increase the contact surface area between the activated carbon and the Al current collector, by creating a porosity onto the aluminum surface. This
should lead to the decrease of the contact resistance between the active material and the Al current collector foil. Some experiments have been carried out in this way, and results are presented below. 3.1.2. Electrochemical etching of aluminum current collectors Electrochemical etching of the aluminum current collectors foils has been carried out, on the basis of the work published earlier by Alwitt et al. [21]. This electrochemical etching technique consists in the immersion of the Al foil in an aggressive bath under constant anodic current in order to develop a pitting corrosion on the aluminum surface. The anodic current enhances the dissolution process, leading to the creation of deep channels with a diameter size of few microns. In a first step, 4 cm2 Al foil current collectors are immersed in a NaOH 1 M electrolyte during 10 min at room temperature. The aim of this treatment is to degrease the foil and simultaneously to generate nucleation sites for aluminum dissolution. After washing with distilled water, the Al samples are placed in an HCl 1 M solution at 80 ◦ C in a three-electrode electrochemical cell, under constant anodic current of 200 mA/cm2 during 20 s. The potential of the Al foil sharply increases during the first second of the anodic polarization traducing an increasing dissolution of the metal. This mainly occurs on the nucleation sites created in the previous NaOH treatment; the potential stabilizes around −0.74 V/ECS. After anodic polarization, Al foils are removed from the electrolyte and washed with distilled water. Fig. 5 presents the SEM picture of the Al foil surface after the etching treatment. The aluminum surface is porous, as it was expected, due to the anodic dissolution of the aluminum. The nucleation sites created by the NaOH immersion are homogeneously distributed among the surface of the foil even if some aluminum surface areas have not been attacked during the electrochemical etching treatment (that can be seen at the center of the picture). The average roughness was found to be around 2.5–2.6 m, with channels between 10 and 15 m deep. In conclusion and as an expected result,
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Fig. 5. SEM picture of a Al foil surface after electrochemical etching treatment first in NaOH and after in HCl at 80 ◦ C under 200 mA/cm2 during 20 s.
this electrochemical etching treatment allowed the creation of a surface porosity on the aluminum current collectors foils. The specific surface area of the aluminum surface has then been increased as compared to untreated aluminum foil (Fig. 2). 3.1.3. 4 cm2 supercapacitor cell using electrochemically etched Al current collectors foil A 4 cm2 cell was assembled using the electrochemically etched aluminum current collectors foils. The weight as well as the thickness of the active material were the same as previously, i.e. 15 mg/cm2 and 150 m thick. Fig. 6 presents the Nyquist plot of this cell between 50 kHz and 10 mHz. The general shape of the plot is conserved, as compared to Fig. 4. The high frequency resistance is more or less constant, between 1 and 2 cm2 . The high frequency loop is still present; at low frequency, the capacitive behavior appears. In the middle frequency range, the electrolyte penetration inside the porous structure of the activated carbon is also observed. The major difference lies in the loop diameter, that has been sharply decreased. The global resistance of the cell has been decreased in a greater extend as compared to Fig. 4. The capacitive behavior is only slightly shifted along the real axis of the plot. The conclusion is that the global resistance of the cell has been decreased. The electrochemical etching treatment allows the increase of the active material/Al current collector surface contact. The real surface contact between both materials is then higher than the geometric one, leading to the decrease of the contact resistance. It seems then that the impedance interface plays an important role in the formation of the high frequency loop observed in the Nyquist plots: the higher the contact, the lower the loop. The electrochemical etching treatment allowed the decrease of the high frequency loop, that is still present. The average diameter (or width) of the channels created by the treatment was measured around few microns. The activated carbon used in this study has a d50 parameter of 10 m,
Fig. 6. Nyquist plot of the two-electrode 4 cm2 supercapacitor cell assembled with electrochemically treated Al current collectors foil between 50 kHz and 10 mHz. Active material weight: 15 mg/cm2 ; active material thickness: 150 m.
meaning that 50% of activated particles have a medium size below 10 m; only 10% of the particle has a medium size below 3 m. This means that an important part of the activated carbon particles has a larger size than the average channels diameter (or width). It is then difficult to generate a continuous interface between the active material and the aluminum current collector through the porosity of the treated aluminum surface. In the same way, the active material is laminated onto the Al current collector. This process is here again not in favor of the development of a continuous interface between both materials. To decrease the impedance contact between the active material and the current collector, another solution can be proposed: a thin film deposit of a conducting material, electrochemically inert in the organic electrolyte used, with a particle size of less than the channel diameter (or width). It consists in a suspension deposit by a dip-coating method. The suspension is a mixture of oxide and polymeric sol. This method has been used for the preparation of electrode material for solid oxide fuel cell devices [22]. The deposit of conducting carbonaceous material particles via a sol–gel route has been studied and the results are presented below. 3.1.4. Conducting coating deposition via the sol–gel route The films were obtained by the dip-coating method using slurry. Slurry was formed by stirring a few percent of
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Fig. 7. SEM picture of an electrochemically etched aluminum current collector with a conducting carbonaceous material sol–gel deposit.
carbonaceous material powder in polymeric sol. The polymeric sol was prepared by condensation reactions between hexamethylenetetraamine (HMTA) and acetylacetone (acac) in acetic acid. The sol–gel matrix allows first to obtain a stable suspension of the carbonaceous powder and second to adjust the viscosity of the suspension (20 cP), for an optimized deposit. The films were prepared on aluminum foils, that will be used as current collector in supercapacitor systems. The slurry was deposited onto the Al substrate with a controlled withdrawal speed of 50 cm/min. The coated electrodes were thermally treated in air in order to remove the polymeric sol. After thermal treatment, the Al current collector surface is covered by the conducting carbonaceous particles. A carbonaceous material with high electronic conductivity was chosen to prepare the conductive layer. The average of the particle size was in the range of 50 nm. As a carbon-based material, its electrochemical potential stability window is in the same range as the activated carbon. The elaboration of such thin film at the current collector surface may lead to a stable, electronically conductive and high surface area Al current collector/active material interface. The objective was to cover the porous surface of electrochemically treated Al foils in order to increase the surface contact at the Al/active material interface. Fig. 7 presents the SEM picture of the electrochemically treated Al current collector foil covered by the conducting layer deposited via the sol–gel route. As compared to Fig. 5, it can be seen now that the most important part of the Al surface is covered by a thin layer of agglomerated particles. The carbonaceous material deposit seems to be homogenously distributed onto the surface. Fig. 8 presents the Nyquist plot for a 4 cm2 cell assembled with treated Al current collectors foils. The treatment consists in the previous electrochemical etching followed by the sol–gel deposit of a conducting layer as described above. The main difference that can be observed as compared to Fig. 6 is that the RC loop is no longer present. In the same time, the high frequency resistance of the cell was kept down to a low value.
Fig. 8. Nyquist plot of a 4 cm2 cell assembled with treated current collector with a conductive film deposited by the sol–gel route.
4. Discussion Four square centimeter cell assembled with aluminum current collectors exhibits a very high internal resistance, around 50 cm2 ; this is due to the presence of a large RC loop. The development of a porosity onto the Al current collector foil has led to a sharp decrease of the resistance (around 5 cm2 ), linked to the diminution of the RC loop. The elaboration of a conducting carbonaceous material thin layer by the sol–gel route allows the complete disappearance of the RC loop; the supercapacitor behavior tends to be purely capacitive. From these results, an hypothesis can be proposed: the RC loop could be linked to the Al current collector/active material interface electronic properties. This is what is going to be discussed now. To describe the impedance behavior on the whole frequency range of the supercapacitors cells, it is possible to use the equivalent circuit presented in Figs. 9 and 10.
Fig. 9. Equivalent circuit representing the supercapacitor cell behavior on the whole frequency range, where Rs is the high frequency resistance, Ri the resistance of the active material/current collector interface, Ci the interface capacitance with the dispersion parameter αi , R(ω) is a part of the supercapacitor resistance depending on the frequency and C(ω) is the supercapacitor cell capacitance.
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Fig. 10. Nyquist plot and equivalent circuit representing the high frequency behavior of the cell with the Ri –Ci loop (left) and the low frequency behavior (right) with the series resistance Rsc (ω) and the cell capacitance C(ω); note that Rsc (ω) = Rs + Ri + R(ω).
At high frequency (between 50 and 1 kHz), the cell behavior consists in the loop observed in the previous Nyquist plots. It can be represented by a parallel “RC” circuit, namely Ri –Ci , associated with a series resistance Rs . As the previous results, the “Ri Ci ” circuit could describe the Al current collector/active material interface electronic properties. In this way, Ri represents the interface resistance, including the contact resistance between the active material and the current collector, and the resistance of the dielectric Al2 O3 barrier film present onto the Al surface [17,20]. Ci represents the interfacial capacitance at the current collector/active material interface; αi traduces the non ideal behavior of this capacitance (0 < αi < 1). Depending on the Ri and Ci values, the loop delays or blocks the capacitive behavior. At low frequency (lower than 1 kHz), the cell behavior can be represented by a series Rsc (ω)–C(ω) circuit, traduc-
ing the supercapacitive behavior [13]. Rsc (ω) is the series resistance of the cell, depending on the frequency; C(ω) is the cell capacitance also depending on the frequency. The series resistance Rsc of the supercapacitor is given by: Rsc = Rs + Ri + R(ω). This value strongly depends on the impedance contact at the Al/active material interface. Depending on the interface Al/active material, many cases are possible. From Fig. 9, in the limit case where Ri is infinite, the cell behaves like a dielectric capacitor; the interface is blocked, meaning that the active material cannot be charged. The capacitive behavior of the supercapacitor would not be observed on the Nyquist plot. This could happen for instance if anodized Al foils were used as current collectors: the thick alumina layer onto the Al surface would block the electron transfer. The other limit case occurs when Ri tends to a very low value: the Ri –Ci loop disappears and the Nyquist plot has the same shape as the ideal one [9b]. Between these two limit cases, there are a lot of possibilities, depending on the Ri –Ci values. Fig. 11 presents the Nyquist plots at the same scale of the three cells that have been studied in this paper: the cell assembled with untreated (as-received) Al current collector (cell A), with electrochemically etched Al current collectors (cell B), and with electrochemically etched + sol–gel conductive layer deposit Al current collectors (cell C). For the cell A (untreated Al collectors), the Ri is in the range of 50 cm2 . It is a very high value, traducing a poor Al collector/active material contact: the surface contact is not optimum, due to a lack of adhesion of the active layer onto the aluminum surface. The electrolyte can then reach the Al/active material and a thin electrolyte film can locally isolate the Al collector from the active mass that cuts off the electronic contact between the two materials. The native alumina layer present onto the Al surface also hinders the electronic transfer between the aluminum collector and the active material. These two effects lead to an increase of the Ri interface resistance. The current collector/active material interface capacitance Ci , associated with the interface resistance Ri , leads to the high frequency loop (Fig. 11).
Fig. 11. Nyquist plots at the same scale of the three cells studied : the cell A assembled with untreated (as-received) Al current collector; the cell B using electrochemically etched Al current collectors, and the cell C using electrochemically etched + sol–gel conductive layer deposit Al current collectors.
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For the cell B, the high frequency loop is sharply decreased. It can be explained, as previously mentioned, by an increase of the contact surface between the active material and the Al collector: the Ri parameter value is divided by approximately 10 (5 cm2 ). In the same time, the fresh alumina layer induced by the electrochemical etching treatment is thinner [23] and then the electronic transfer through this layer is easier; the interface capacitance value Ci is increased, and the supercapacitor behavior appears at higher frequencies and lower Rsc resistance. For the cell C, the high frequency Ri –Ci loop no longer exists. It can be attributed to a very low Ri resistance induced by the high covering sol–gel deposited conducting film. The surface contact between the current collector and the active material has been then increased and the electronic contact improved. As a matter of fact, the impedance behavior of porous electrochemically inert supercapacitor electrodes can be seen in the whole frequency range, i.e. from 10 kHz down to 13 mHz. The Rs value now corresponding to the high frequency resistance of the supercapacitor cell was measured at 0.8 cm2 , lower than the one obtained when using conductive paint [13]. It makes the supercapacitor cell able to deliver higher power. Fig. 12 shows a cycling curve of the cell C at ±10 mA between 0 and 2.3 V. The linear shape of the V(t) plot at constant current traduces the absence of faradaic reaction in this potential window. The cell capacitance is deduced from the slope of the discharge curve according to: C=
I dV/dt
(1)
where C is the capacitance of the cell in farad, I the discharge current in ampere (A), and dV/dt the slope in volt per second (V/s). In a symmetrical system where the active material weight is the same for the two electrodes, the specific capacitance CmAC in farad per gram of activated carbon (F/g) is related to the capacitance of the cell C by: CmAC =
2C mAC
(2)
Fig. 12. Charge-discharge of a 4 cm2 cell assembled with treated current collector with a deposition of black acetylene between 0 and 2.3 V at I = ±10 mA.
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where mAC is the weight (g) per electrode of activated carbon. Specific capacitance calculated according to Eq. (2) gives 100 F/g of activated carbon Picactif BP10. Maximum specific power and energy can be estimated at 2.3 V from this plot, using Eqs. (3) and (4): Pmax =
V 4(ESR) × mAM
(3)
Emax =
CV 2mAM
(4)
where mAM is the total active material weight on the two electrodes (30 mg/cm2 ). Calculations give Pmax = 55 kW/kg and Emax = 17 Wh/kg of active material for the whole cell.
5. Conclusion This paper has presented the results obtained on 4 cm2 carbon–carbon supercapacitors assembled with aluminum current collectors in organic electrolyte. Firstly, a comparison has been done between three different cells respectively using: “as-received” Al foil, etched Al foil and etched Al foil coated with a conductive carbonaceous material via sol–gel route. With untreated Al foil, the Nyquist plot presented a RC loop which transferred the capacitive behavior to low frequency and the internal resistance was very high (50 cm2 ). The etching treatment achieved on Al surface has permitted to decrease the RC loop in increasing the interface contact between the Al and the active material; the internal resistance value was 5 cm2 . The solution proposed, in order to eliminate this loop, was to create a conducting and continuous interface between the collector and the active material in achieving a conductive carbonaceous material coating by the sol–gel route. The loop disappeared and the internal resistance decreased (0.8 cm2 ). The high frequency RC loop has been linked to the Al/active material interface. An electric equivalent circuit has been suggested to describe the Nyquist plots. The equivalent circuit consists of a parallel Ri –Ci branch in series with a R(ω)C(ω) branch representing the supercapacitance as previously described [13]. An interface resistance Ri and an interface capacitance Ci linked to this loop have been defined. The loop is more or less important depending on Ri and Ci values. Using untreated aluminum, the alumina layer is thick and the electron transfer was more difficult or blocked. Moreover, the electrolyte hindered the contact between the collector and the active material. In this case, Ri was high and contribute to the global resistance increase. When the contact was improved in creating a porous surface on the Al, Ri decreased and the global resistance was divided by 10. In increasing the interface conductivity with acetylene black which covered the whole surface, Ri tended to zero because of the electron transfer enhancement. So, it became negligible; the high frequency loop removed.
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The improvement of the Al/active material interface leads to the increase of the performance of the carbon–carbon supercapacitors.
Acknowledgements The authors want to thank the “Délégation Générale pour l’Armement” for financial support of this work.
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[9] (a) B.E. Conway, Electrochemical Capacitor, Scientific, Fundamentals and Technological Applications, Plenum Press, New York, 1999, p. 105; (b) B.E. Conway, Electrochemical Capacitor, Scientific, Fundamentals and Technological Applications, Plenum Press, New York, 1999, p. 545. [10] A. Du Pasquier, I. Plitz, J. Gural, S. Menocal, G. Amatucci, J. Power Sources 113 (2003) 62. [11] A. Laforgue, P. Simon, J.F. Fauvarque, M. Mastragostino, F. Soavi, J.F. Sarrau, P. Lailler, M. Conte, E. Rossi, S. Saguatti, J. Electrochem. Soc. 150 (2003) A645. [12] A. Burke, in: Proceedings of the Third International Advanced Automotive Battery Conference, Nice, France, 10 June 2003. [13] P.L. Taberna, P. Simon, J.F. Fauvarque, J. Electrochem. Soc. 150 (2003) A292. [14] R. de Levie, Electrochem. Acta 8 (1963) 751. [15] R. de Levie, Electrochem. Acta 9 (1964) 1231. [16] X. Andrieu, G. Crépy, L. Josset, in: Proceedings of the Third International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Florida Educational Seminar Editors, Deerfield Beach, FL, USA, 3–5 December 1993. [17] Y.-R. Nian, H. Teng, J. Electroanal. Chem. 540 (2003) 119. [18] Y.-R. Nian, H. Teng, J. Electrochem. Soc. 149 (2002) A108. [19] T. Momma, X. Liu, T. Osaka, Y. Ushio, Y. Sawada, J. Power Sources 60 (1996) 249. [20] W.G. Pell, B.E. Conway, N. Marincic, J. Electroanal. Chem. 491 (2000) 9. [21] R.S. Alwitt, H. Uchi, T.R. Beck, R.C. Alkire, J. Electrochem. Soc. 131 (1984) 13. [22] M. Gaudon, C. Laberty, F. Ansart, P. Stevens, French Patent No. EN 03,50,233. [23] G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2186.
Modification of Al current collector surface by sol–gel deposit for carbon–carbon supercapacitor applications C. Portet, P.L. Taberna, P. Simon∗ , C. Laberty-Robert CIRIMAT, UMR CNRS 5085, 118 Route de Narbonne, Toulouse Cedex 31062, France Received 8 July 2003; received in revised form 19 September 2003; accepted 28 September 2003
Abstract Four square centimeter carbon–carbon supercapacitor cells were assembled with Al current collectors in organic electrolyte. Different treatments of the Al current collectors were made in order to increase the supercapacitor performances. A sol–gel deposit of a conducting carbonaceous material led to the best results. On the basis of electrochemical impedance spectroscopy measurements, the differences observed with the previous treatments were assumed to be linked to the modification of the Al/active material interface. The cell using sol–gel treated current collector presented an activated carbon specific capacitance of 100 F/g and a series resistance of 0.8 cm2 in acetonitrile 1 M NEt4 BF4 , that are characteristics compatible with high power applications. © 2003 Elsevier Ltd. All rights reserved. Keywords: Supercapacitors; Activated carbon; Sol–gel deposit; Impedance spectroscopy
1. Introduction Since the last past 5 years, supercapacitors have focused a lot of interest because of their promising properties in terms of energy storage and power supply. These devices can fill the gap existing between the batteries and the dielectric capacitors. As compared to batteries, they can deliver higher specific power; on the other hand, they store higher energy density than dielectric capacitors. These particular properties already make them suitable for a lot of applications in power electronics and peak saving. But their performances are so interesting that they are now promising candidates in transportation applications, particularly in vehicle and hybrid electric vehicle (HEV). In HEV, they could help the energy storage source to deliver the high power needed to perform the electric boost, the stop and go function or to help the braking energy recovering [1,2]. Three different types of supercapacitors are commonly described in the literature, depending on the active material nature used: activated carbon [3,4], metal oxide [5,6] and electronically conducting polymer [7,8]. Electronically conducting polymer systems are still at the research state. ∗ Corresponding author. Tel.: +33-5-61-55-68-02; fax: +33-5-61-55-61-63. E-mail address: [email protected] (P. Simon).
0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.09.043
Metal oxide-based supercapacitors are limited to applications where costless products are not needed. Activated carbon-based supercapacitors are the most developed technology; these are commercially available systems, using cheap active material, i.e. a highly divided carbon with specific surface area as high as 2000 m2 /g. The charge storage is purely electrostatic; the electrolyte ions are reversibly adsorbed in the electrochemical double layer of the porous carbon electrode structure [9a]. There is no faradaic reaction; the series resistance of the system is then low, leading to high specific power systems. It must be pointed out however that a last type of supercapacitors is now emerging: the hybrid systems, associating faradaic and carbon electrodes, in order to increase their specific energy [10–12]. In this paper, the results obtained with 4 cm2 carbon-based supercapacitors cells are presented. The aim of this study is to try to improve the aluminum current collector/active material interface properties in order to decrease the interface impedance and then the series resistance of the supercapacitor. In this way, a solution, proposed in the literature, consists in the use of a Al current collector covered by a conducting paint, leading to a better Al current collector/active material contact [10,13]. The sol–gel deposits are well known for their highly covering properties. Here, two different surface treatments have
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been achieved on the Al current collectors: an electrochemical etching and a conducting thin film coating via the sol–gel route. The influence of such treatments on the electrochemical properties of the supercapacitors cells will be discussed with the help of the electrochemical impedance spectroscopy technique. An electric equivalent circuit will be proposed to describe the modifications induced by these treatments at the current collector/active material interface.
2. Experimental Supercapacitor electrodes have been described elsewhere [13]. They consist of active layers laminated onto aluminum foil of 200 m thick. The active layer composition is 95 wt.% activated carbon, 3% carboxymethylcellulose (CMC; Prolabo) and 2% polytetrafluoroethylene (PTFE; Dupont de Nemours). The activated carbon used is the PicatifBP10 from the Pica Company (Vierzon, France). The active material films are 150 m thick; the electrode weight density is 15 mg/cm2 . All the cell assembly was made in a glove box with both water and oxygen content lower than 1 mg/kg. Electrodes were made by laminating the active material films onto the current collectors. The stack was assembled by inserting two layers of porous polymeric separators between the two electrodes; two PTFE plates and two stainless steel clamps were placed to press the stack, as can be seen in Fig. 1. The stack was immersed in an organic electrolyte, a solution of acetonitrile (AN, 10 mg/kg water) with 1.5 M NEt4 BF4 dried salt. Electrochemical impedance spectroscopy measurements were achieved with an EGG 6310, between 50 kHz and 10 mHz, at a bias voltage of 0 V. The cycling experiments were performed with a Mac Pile battery cycling apparatus, at a constant current density of ±2.5 mA/cm2 , between 0 and 2.3 V. SEM pictures were obtained with a JEOL JSM 6400 apparatus, and the roughness measurements were carried out with a ZYGO Corporation apparatus.
Fig. 1. Schematic drawing of the two-electrode 4 cm2 cell stack.
3. Results and discussion 3.1. Electrochemical impedance spectroscopy study of 4 cm2 cells 3.1.1. 4 cm2 supercapacitor cell impedance spectroscopy measurements Supercapacitor has been assembled with “as-received” Al foils. Fig. 2 presents the SEM picture of a “as-received” Al foil surface. It can be seen a flat surface with an average roughness of 0.2–0.3 m. Fig. 3 presents the theoretical Nyquist plot of a supercapacitor [9b]. Fig. 4 shows the experimental plot of a two-electrode 4 cm2 cell assembled with two “as-received” Al foils (99.99% Al) between 50 kHz and 10 mHz at 0 V. The plot in Fig. 3 presents three different regions, depending on the frequency. At very high frequency, the supercapacitor behaves like a pure resistance. At low frequency, the imaginary part sharply increases and a vertical line is observed, traducing a pure capacitive behavior. In the middle frequency domain, the influence of the electrode porosity can be seen. When the frequency decreases, starting from the very high frequency, the signal penetrates deeper and deeper inside the porous structure
Fig. 2. SEM picture of a “as-received” Al foil surface.
Fig. 3. Theoretical Nyquist plot of a carbon–carbon supercapacitor.
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Fig. 4. Nyquist plot of a two-electrode 4 cm2 supercapacitor cell assembled with “as-received” Al current collectors between 50 kHz and 10 mHz at 0 V.
of the electrode; more and more activated carbon surface is available for the ion adsorption. This middle frequency range is then related to the electrolyte penetration inside the porous structure of the activated carbon electrode. This point has been extensively studied by de Levie [14,15]. The experimental plot presented in Fig. 4 is slightly different as compared to the theoretical one. When the frequency is decreased, from 7.92 Hz down to 10 mHz, the theoretical behavior is observed with the different zones previously explained. But a semi-circle loop appears between 50 kHz and 7.92 Hz. This loop shifts the capacitive behavior along the real axis, i.e. towards higher resistance value. This loop is undesirable as it decreases the performances of the cell in terms of resistance and power. This semi-circle loop has been observed with carbon supercapacitors systems by numerous authors in the literature. Andrieu et al. associate this high frequency loop to the inter-granular electronic resistance between the activated carbon particles [16], namely Rt ; this resistance is called “pseudo-transfer resistance”. Rt depends mainly on the electrode surface area and the inter-particle resistivity. The realization of thin active layers, as well as the improvement of the electronic percolation can reduce this value. Other authors propose an explanation in relationship with the activated carbon–current collector interface [17]. This semi-circle was suggested to represent the impedance at the interface between the current collector and the carbon fabric that were activated carbon fibers, as well as between the fibers [18,19]. They noticed that when the surface oxide population on the carbon was increased, so did the semi-circle loop, shifting the capacitive behavior along the real axis to higher resistance [17]. Pell et al. attributed this RC loop to the resistance of the passive layer (TiO2 ) developed on the current collector surface (Ti) [20]. This semi-circle loop increases the resistance of the cell; it is then important to find solutions to decrease this contribution. One solution may be to increase the contact surface area between the activated carbon and the Al current collector, by creating a porosity onto the aluminum surface. This
should lead to the decrease of the contact resistance between the active material and the Al current collector foil. Some experiments have been carried out in this way, and results are presented below. 3.1.2. Electrochemical etching of aluminum current collectors Electrochemical etching of the aluminum current collectors foils has been carried out, on the basis of the work published earlier by Alwitt et al. [21]. This electrochemical etching technique consists in the immersion of the Al foil in an aggressive bath under constant anodic current in order to develop a pitting corrosion on the aluminum surface. The anodic current enhances the dissolution process, leading to the creation of deep channels with a diameter size of few microns. In a first step, 4 cm2 Al foil current collectors are immersed in a NaOH 1 M electrolyte during 10 min at room temperature. The aim of this treatment is to degrease the foil and simultaneously to generate nucleation sites for aluminum dissolution. After washing with distilled water, the Al samples are placed in an HCl 1 M solution at 80 ◦ C in a three-electrode electrochemical cell, under constant anodic current of 200 mA/cm2 during 20 s. The potential of the Al foil sharply increases during the first second of the anodic polarization traducing an increasing dissolution of the metal. This mainly occurs on the nucleation sites created in the previous NaOH treatment; the potential stabilizes around −0.74 V/ECS. After anodic polarization, Al foils are removed from the electrolyte and washed with distilled water. Fig. 5 presents the SEM picture of the Al foil surface after the etching treatment. The aluminum surface is porous, as it was expected, due to the anodic dissolution of the aluminum. The nucleation sites created by the NaOH immersion are homogeneously distributed among the surface of the foil even if some aluminum surface areas have not been attacked during the electrochemical etching treatment (that can be seen at the center of the picture). The average roughness was found to be around 2.5–2.6 m, with channels between 10 and 15 m deep. In conclusion and as an expected result,
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Fig. 5. SEM picture of a Al foil surface after electrochemical etching treatment first in NaOH and after in HCl at 80 ◦ C under 200 mA/cm2 during 20 s.
this electrochemical etching treatment allowed the creation of a surface porosity on the aluminum current collectors foils. The specific surface area of the aluminum surface has then been increased as compared to untreated aluminum foil (Fig. 2). 3.1.3. 4 cm2 supercapacitor cell using electrochemically etched Al current collectors foil A 4 cm2 cell was assembled using the electrochemically etched aluminum current collectors foils. The weight as well as the thickness of the active material were the same as previously, i.e. 15 mg/cm2 and 150 m thick. Fig. 6 presents the Nyquist plot of this cell between 50 kHz and 10 mHz. The general shape of the plot is conserved, as compared to Fig. 4. The high frequency resistance is more or less constant, between 1 and 2 cm2 . The high frequency loop is still present; at low frequency, the capacitive behavior appears. In the middle frequency range, the electrolyte penetration inside the porous structure of the activated carbon is also observed. The major difference lies in the loop diameter, that has been sharply decreased. The global resistance of the cell has been decreased in a greater extend as compared to Fig. 4. The capacitive behavior is only slightly shifted along the real axis of the plot. The conclusion is that the global resistance of the cell has been decreased. The electrochemical etching treatment allows the increase of the active material/Al current collector surface contact. The real surface contact between both materials is then higher than the geometric one, leading to the decrease of the contact resistance. It seems then that the impedance interface plays an important role in the formation of the high frequency loop observed in the Nyquist plots: the higher the contact, the lower the loop. The electrochemical etching treatment allowed the decrease of the high frequency loop, that is still present. The average diameter (or width) of the channels created by the treatment was measured around few microns. The activated carbon used in this study has a d50 parameter of 10 m,
Fig. 6. Nyquist plot of the two-electrode 4 cm2 supercapacitor cell assembled with electrochemically treated Al current collectors foil between 50 kHz and 10 mHz. Active material weight: 15 mg/cm2 ; active material thickness: 150 m.
meaning that 50% of activated particles have a medium size below 10 m; only 10% of the particle has a medium size below 3 m. This means that an important part of the activated carbon particles has a larger size than the average channels diameter (or width). It is then difficult to generate a continuous interface between the active material and the aluminum current collector through the porosity of the treated aluminum surface. In the same way, the active material is laminated onto the Al current collector. This process is here again not in favor of the development of a continuous interface between both materials. To decrease the impedance contact between the active material and the current collector, another solution can be proposed: a thin film deposit of a conducting material, electrochemically inert in the organic electrolyte used, with a particle size of less than the channel diameter (or width). It consists in a suspension deposit by a dip-coating method. The suspension is a mixture of oxide and polymeric sol. This method has been used for the preparation of electrode material for solid oxide fuel cell devices [22]. The deposit of conducting carbonaceous material particles via a sol–gel route has been studied and the results are presented below. 3.1.4. Conducting coating deposition via the sol–gel route The films were obtained by the dip-coating method using slurry. Slurry was formed by stirring a few percent of
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Fig. 7. SEM picture of an electrochemically etched aluminum current collector with a conducting carbonaceous material sol–gel deposit.
carbonaceous material powder in polymeric sol. The polymeric sol was prepared by condensation reactions between hexamethylenetetraamine (HMTA) and acetylacetone (acac) in acetic acid. The sol–gel matrix allows first to obtain a stable suspension of the carbonaceous powder and second to adjust the viscosity of the suspension (20 cP), for an optimized deposit. The films were prepared on aluminum foils, that will be used as current collector in supercapacitor systems. The slurry was deposited onto the Al substrate with a controlled withdrawal speed of 50 cm/min. The coated electrodes were thermally treated in air in order to remove the polymeric sol. After thermal treatment, the Al current collector surface is covered by the conducting carbonaceous particles. A carbonaceous material with high electronic conductivity was chosen to prepare the conductive layer. The average of the particle size was in the range of 50 nm. As a carbon-based material, its electrochemical potential stability window is in the same range as the activated carbon. The elaboration of such thin film at the current collector surface may lead to a stable, electronically conductive and high surface area Al current collector/active material interface. The objective was to cover the porous surface of electrochemically treated Al foils in order to increase the surface contact at the Al/active material interface. Fig. 7 presents the SEM picture of the electrochemically treated Al current collector foil covered by the conducting layer deposited via the sol–gel route. As compared to Fig. 5, it can be seen now that the most important part of the Al surface is covered by a thin layer of agglomerated particles. The carbonaceous material deposit seems to be homogenously distributed onto the surface. Fig. 8 presents the Nyquist plot for a 4 cm2 cell assembled with treated Al current collectors foils. The treatment consists in the previous electrochemical etching followed by the sol–gel deposit of a conducting layer as described above. The main difference that can be observed as compared to Fig. 6 is that the RC loop is no longer present. In the same time, the high frequency resistance of the cell was kept down to a low value.
Fig. 8. Nyquist plot of a 4 cm2 cell assembled with treated current collector with a conductive film deposited by the sol–gel route.
4. Discussion Four square centimeter cell assembled with aluminum current collectors exhibits a very high internal resistance, around 50 cm2 ; this is due to the presence of a large RC loop. The development of a porosity onto the Al current collector foil has led to a sharp decrease of the resistance (around 5 cm2 ), linked to the diminution of the RC loop. The elaboration of a conducting carbonaceous material thin layer by the sol–gel route allows the complete disappearance of the RC loop; the supercapacitor behavior tends to be purely capacitive. From these results, an hypothesis can be proposed: the RC loop could be linked to the Al current collector/active material interface electronic properties. This is what is going to be discussed now. To describe the impedance behavior on the whole frequency range of the supercapacitors cells, it is possible to use the equivalent circuit presented in Figs. 9 and 10.
Fig. 9. Equivalent circuit representing the supercapacitor cell behavior on the whole frequency range, where Rs is the high frequency resistance, Ri the resistance of the active material/current collector interface, Ci the interface capacitance with the dispersion parameter αi , R(ω) is a part of the supercapacitor resistance depending on the frequency and C(ω) is the supercapacitor cell capacitance.
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Fig. 10. Nyquist plot and equivalent circuit representing the high frequency behavior of the cell with the Ri –Ci loop (left) and the low frequency behavior (right) with the series resistance Rsc (ω) and the cell capacitance C(ω); note that Rsc (ω) = Rs + Ri + R(ω).
At high frequency (between 50 and 1 kHz), the cell behavior consists in the loop observed in the previous Nyquist plots. It can be represented by a parallel “RC” circuit, namely Ri –Ci , associated with a series resistance Rs . As the previous results, the “Ri Ci ” circuit could describe the Al current collector/active material interface electronic properties. In this way, Ri represents the interface resistance, including the contact resistance between the active material and the current collector, and the resistance of the dielectric Al2 O3 barrier film present onto the Al surface [17,20]. Ci represents the interfacial capacitance at the current collector/active material interface; αi traduces the non ideal behavior of this capacitance (0 < αi < 1). Depending on the Ri and Ci values, the loop delays or blocks the capacitive behavior. At low frequency (lower than 1 kHz), the cell behavior can be represented by a series Rsc (ω)–C(ω) circuit, traduc-
ing the supercapacitive behavior [13]. Rsc (ω) is the series resistance of the cell, depending on the frequency; C(ω) is the cell capacitance also depending on the frequency. The series resistance Rsc of the supercapacitor is given by: Rsc = Rs + Ri + R(ω). This value strongly depends on the impedance contact at the Al/active material interface. Depending on the interface Al/active material, many cases are possible. From Fig. 9, in the limit case where Ri is infinite, the cell behaves like a dielectric capacitor; the interface is blocked, meaning that the active material cannot be charged. The capacitive behavior of the supercapacitor would not be observed on the Nyquist plot. This could happen for instance if anodized Al foils were used as current collectors: the thick alumina layer onto the Al surface would block the electron transfer. The other limit case occurs when Ri tends to a very low value: the Ri –Ci loop disappears and the Nyquist plot has the same shape as the ideal one [9b]. Between these two limit cases, there are a lot of possibilities, depending on the Ri –Ci values. Fig. 11 presents the Nyquist plots at the same scale of the three cells that have been studied in this paper: the cell assembled with untreated (as-received) Al current collector (cell A), with electrochemically etched Al current collectors (cell B), and with electrochemically etched + sol–gel conductive layer deposit Al current collectors (cell C). For the cell A (untreated Al collectors), the Ri is in the range of 50 cm2 . It is a very high value, traducing a poor Al collector/active material contact: the surface contact is not optimum, due to a lack of adhesion of the active layer onto the aluminum surface. The electrolyte can then reach the Al/active material and a thin electrolyte film can locally isolate the Al collector from the active mass that cuts off the electronic contact between the two materials. The native alumina layer present onto the Al surface also hinders the electronic transfer between the aluminum collector and the active material. These two effects lead to an increase of the Ri interface resistance. The current collector/active material interface capacitance Ci , associated with the interface resistance Ri , leads to the high frequency loop (Fig. 11).
Fig. 11. Nyquist plots at the same scale of the three cells studied : the cell A assembled with untreated (as-received) Al current collector; the cell B using electrochemically etched Al current collectors, and the cell C using electrochemically etched + sol–gel conductive layer deposit Al current collectors.
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For the cell B, the high frequency loop is sharply decreased. It can be explained, as previously mentioned, by an increase of the contact surface between the active material and the Al collector: the Ri parameter value is divided by approximately 10 (5 cm2 ). In the same time, the fresh alumina layer induced by the electrochemical etching treatment is thinner [23] and then the electronic transfer through this layer is easier; the interface capacitance value Ci is increased, and the supercapacitor behavior appears at higher frequencies and lower Rsc resistance. For the cell C, the high frequency Ri –Ci loop no longer exists. It can be attributed to a very low Ri resistance induced by the high covering sol–gel deposited conducting film. The surface contact between the current collector and the active material has been then increased and the electronic contact improved. As a matter of fact, the impedance behavior of porous electrochemically inert supercapacitor electrodes can be seen in the whole frequency range, i.e. from 10 kHz down to 13 mHz. The Rs value now corresponding to the high frequency resistance of the supercapacitor cell was measured at 0.8 cm2 , lower than the one obtained when using conductive paint [13]. It makes the supercapacitor cell able to deliver higher power. Fig. 12 shows a cycling curve of the cell C at ±10 mA between 0 and 2.3 V. The linear shape of the V(t) plot at constant current traduces the absence of faradaic reaction in this potential window. The cell capacitance is deduced from the slope of the discharge curve according to: C=
I dV/dt
(1)
where C is the capacitance of the cell in farad, I the discharge current in ampere (A), and dV/dt the slope in volt per second (V/s). In a symmetrical system where the active material weight is the same for the two electrodes, the specific capacitance CmAC in farad per gram of activated carbon (F/g) is related to the capacitance of the cell C by: CmAC =
2C mAC
(2)
Fig. 12. Charge-discharge of a 4 cm2 cell assembled with treated current collector with a deposition of black acetylene between 0 and 2.3 V at I = ±10 mA.
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where mAC is the weight (g) per electrode of activated carbon. Specific capacitance calculated according to Eq. (2) gives 100 F/g of activated carbon Picactif BP10. Maximum specific power and energy can be estimated at 2.3 V from this plot, using Eqs. (3) and (4): Pmax =
V 4(ESR) × mAM
(3)
Emax =
CV 2mAM
(4)
where mAM is the total active material weight on the two electrodes (30 mg/cm2 ). Calculations give Pmax = 55 kW/kg and Emax = 17 Wh/kg of active material for the whole cell.
5. Conclusion This paper has presented the results obtained on 4 cm2 carbon–carbon supercapacitors assembled with aluminum current collectors in organic electrolyte. Firstly, a comparison has been done between three different cells respectively using: “as-received” Al foil, etched Al foil and etched Al foil coated with a conductive carbonaceous material via sol–gel route. With untreated Al foil, the Nyquist plot presented a RC loop which transferred the capacitive behavior to low frequency and the internal resistance was very high (50 cm2 ). The etching treatment achieved on Al surface has permitted to decrease the RC loop in increasing the interface contact between the Al and the active material; the internal resistance value was 5 cm2 . The solution proposed, in order to eliminate this loop, was to create a conducting and continuous interface between the collector and the active material in achieving a conductive carbonaceous material coating by the sol–gel route. The loop disappeared and the internal resistance decreased (0.8 cm2 ). The high frequency RC loop has been linked to the Al/active material interface. An electric equivalent circuit has been suggested to describe the Nyquist plots. The equivalent circuit consists of a parallel Ri –Ci branch in series with a R(ω)C(ω) branch representing the supercapacitance as previously described [13]. An interface resistance Ri and an interface capacitance Ci linked to this loop have been defined. The loop is more or less important depending on Ri and Ci values. Using untreated aluminum, the alumina layer is thick and the electron transfer was more difficult or blocked. Moreover, the electrolyte hindered the contact between the collector and the active material. In this case, Ri was high and contribute to the global resistance increase. When the contact was improved in creating a porous surface on the Al, Ri decreased and the global resistance was divided by 10. In increasing the interface conductivity with acetylene black which covered the whole surface, Ri tended to zero because of the electron transfer enhancement. So, it became negligible; the high frequency loop removed.
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The improvement of the Al/active material interface leads to the increase of the performance of the carbon–carbon supercapacitors.
Acknowledgements The authors want to thank the “Délégation Générale pour l’Armement” for financial support of this work.
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