Energy Fuels 2010, 24, 3346–3351 Published on Web 03/30/2010
: DOI:10.1021/ef901505c
Carbon/Layered Double Hydroxide (LDH) Composites for Supercapacitor Application† Agnieszka Malak-Polaczyk,‡,§ Cathie Vix-Guterl,§ and Elzbieta Frackowiak*,‡ ‡
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Piotrowo 3, 60965 Poznan, Poland, and § Institut de Sciences des Mat eriaux de Mulhouse, Centre National de la Recherche Scientifique (CNRS) LRC 7228, 15 Rue Starcky, 68057 Mulhouse, France Received December 9, 2009. Revised Manuscript Received March 10, 2010
In this paper, layered double hydroxide (LDH)/activated carbon composite electrodes for use in an electrochemical capacitor have been prepared by a simple chemical precipitation of hydroxy-carbonates from the homogeneous solution of metal salts and urea after the thermally induced hydrolysis of urea. Subsequently, the LDH has been converted into other forms of oxides by thermal treatment. Physical properties, morphology, and specific surface area of prepared materials were characterized by X-ray diffraction and energy-dispersive spectroscopy, scanning and transmission electron microscopy (SEM and TEM), and nitrogen sorption measurements. SEM images confirmed uniform and hexagonal platelets of LDH well-dispersed on the carbon material external surface. The electrochemical performance of the composite electrodes was studied by cyclic voltammetry, galvanostatic charge/discharge measurements, and impedance spectroscopy. Introducing a small amount of LDH to activated carbon improved electrochemical properties of the electrode, which is a result of a combination of the redox reactive property from LDH and good electronic conductivity of the carbon host. High Brunauer-Emmett-Teller (BET) surface area of the LDH/carbon composite can be accessible to the electrolyte in a micro-/ mesoporous network. Moreover, a symmetric capacitor based on carbon/LDH composite electrodes retains the value of specific capacitance after long cycling and at high current density.
be introduced in the carbon network.5,6 In this case, functional groups are involved in fast redox charge-transfer processes, referred to as pseudo-capacitance. On the other hand, carbon can be used as a support of pseudo-capacitive materials, e.g., transition-metal oxides and electronically conducting polymers.7,8 Electrode materials with pseudo-capacitive properties are very promising for enhancing the performance of supercapacitors.9-11 However, their usage is limited by possible swelling and shrinkage that may occur during charging/discharging of the active film, leading to mechanical degradation of the electrodes and fading of the electrochemical performance during cycling. To take full advantage of the doublelayer capacitance and pseudo-capacitance, many metal oxide/ carbon composite electrodes have been investigated for supercapacitors. CoAl layered double hydroxides (LDHs) have already been used as the active material for capacitors.12 In the present study, for the first time, LDHs have been used as a source of metal oxide for reversible Faradaic reactions in composite with activated carbon.
1. Introduction Electrochemical capacitors attract great interest as power sources and energy-storage devices because of fast energy delivery, high power capability, and long durability.1,2 Dependent upon the charge storage mechanism, they can be divided into (1) electrical double-layer capacitors (EDLCs), where capacitance arises from charge separation at the electrode/ electrolyte interface, and (2) redox electrochemical capacitors concerning reversible redox reactions, where the dominant process is of pseudo-capacitive origin. The most used materials for EDLCs are activated carbons.3,4 However, taking into account equation Ce = εS/d, where Ce represents the capacitance of one electrode, S represents the surface area of the electrode/electrolyte interface, ε represents the permittivity of the electrolyte, and d represents the thickness of the electrical double layer, it can be noticed that double-layer capacitance coming from charges/ions stored in the electrode/electrolyte interface is limited by the specific surface area of the electrode material and porosity accessible for ions transport. To overcome this drawback, instead of developing the porosity of carbons, nitrogenated and/or oxygenated functionalities can
(5) Jurewicz, K.; Babel, K.; Ziolkowski, A.; Wachowska, H. Electrochim. Acta 2003, 48, 1491. (6) Lota, G.; Grzyb, B.; Machnikowska, H.; Machnikowski, J.; Frackowiak, E. Chem. Phys. Lett. 2005, 404, 53. (7) He, K.-X.; Wu, Q.-F.; Zhang, X.-g.; Wang, X.-L. J. Electrochem. Soc. 2006, 153, A1568. (8) Chang, J. K.; Lin, Ch. T.; Tsai, W. T. Electrochem. Commun. 2004, 6, 666–671. (9) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2002, 14, 3946. (10) Raymundo-Pi~ nero, E.; Khomenko, V.; Frackowiak, E.; Beguin, F. J. Electrochem. Soc. 2005, 152, A229. (11) Mastragostino, M.; Arbizzani, C.; Soavi, F. Solid State Ionics 2002, 148, 493. (12) Wang, Y. G.; Cheng, L.; Xia, Y. Y. J. Power Sources 2006, 153, 191.
† This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed: Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Piotrowo 3, 60965 Poznan, Poland. Telephone: þ48-61-665-36-32. Fax: þ48-61-665-37-91. E-mail:
[email protected]. (1) Conway, B. E. Electrochemical Supercapacitors;Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Publishers: New York, 1999. (2) K€ otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (3) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (4) Kierzek, K.; Frackowiak, E.; Lota, G.; Gryglewicz, G.; Machnikowski, J. Electrochim. Acta 2004, 49, 515.
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Energy Fuels 2010, 24, 3346–3351
: DOI:10.1021/ef901505c
Malak-Polaczyk et al.
LDHs are a class of ionic lamellar compounds that consist of positively charged host layers with two kinds of metallic cations and exchangeable hydrated anions located in the interlayer gallery for charge balance. The charge of the layers arises from the substitution of a part of the divalent metal ions with trivalent ones. Metal ions are octahedrally coordinated by six oxygen atoms belonging to six OH groups. Each OH group is shared by three octahedral cations.13,14 The general formula is
metal oxides and, depending upon the temperature, carbon residue. 2. Experimental Section 2.1. Material Preparation. Steam-activated carbon material was supplied by NORIT. Urea, cobalt nitrate, and aluminum nitrate were commercially available reagents (p.a.) from Aldrich and Fluka. LDH/carbon composite was prepared via a method of homogeneous precipitation described previously in ref 27, except replacing chlorides with nitrates. Briefly, 1000 cm3 of water solution containing 10, 5, and 35 mM Co(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and urea, respectively, and 2 g of activated carbon was heated at 95 °C and under continuous magnetic stirring for 48 h. After filtering and washing with distilled water and ethanol for several times, the resulting powder was dried first at room temperature and after that at 80 °C overnight. This material is denoted as RCoAU (R, activated carbon residue; Co, cobalt; A, aluminum; and U, urea). Synthesis of LDHs was followed by thermal treatment in synthetic air (10 L/h) for 3 h at different temperatures ranging from 160 to 500 °C. Samples after heat treatment are denoted as RCoAUx, where letters correspond to the carbon composite and x (x = 200, 300, etc.) is the temperature of the heat treatment. 2.2. Structural Characterization. The textural properties of the materials were determined by nitrogen adsorption at 77 K using a Micromeritics ASAP2020 setup. The specific surface area was evaluated according to the Brunauer-Emmett-Teller (BET) equation in the region of a relative pressure of 0.05-0.3 for composites and 0.02-0.2 for the carbon material. The total pore volume was determined at a relative pressure P/P0 = 0.95, while the pore sizes and pore size distributions were calculated by the density functional theory (DFT) method from the adsorption branches. The crystallinity and the structure of the samples were evaluated by the X-ray diffraction (XRD) technique. XRD analysis of each sample was carried out using a Philips X’pert MPD diffractometer equipped with a Cu anode to generate Cu KR radiation (λ = 1.5406 A˚). Each diffraction pattern was collected in the 2θ range of 10-70°, with the step size of 0.05° and a count time of 5.0 s/step. 2.3. Electrochemical Characterization. The capacitor electrodes for evaluating the electrochemical properties were formed as pellets consisting of 80% composite material, 10% acetylene black, and 10% binder (PVDF, Kynar Flex 2801). These pellets were 10 mm in diameter and 0.20 mm ((0.02 mm) in thickness and were prepared under pressure of 10 MPa. The typical mass of the electrode material ranged from 9.0 to 11.0 mg. Symmetric supercapacitors operated in 6 mol L-1 KOH electrolytic solution. All experiments were carried out at room temperature. The capacitance values were determined in a two-electrode Swagelok system by galvanostatic charge/discharge and cyclic voltammetry with different scan rates using a VMP3 (Bio-Logic, France) multichannel potentiostat. The capacitance values of material were calculated per mass of active material in one electrode. The specific capacitance has been evaluated using the formula C = IΔt/ (mΔV), where I is the current used for discharge, Δt is the time elapsed for the discharge, m is the mass of the composite, and ΔV is the voltage range of the discharge.
½M12þ- x Mx3þ ðOHÞ2 xþ ðAx=n n- Þ 3 nH2 O where M2þ indicates divalent cation (Mg2þ, Fe2þ, Co2þ, etc.), M3þ indicates trivalent cation (Al3þ, Fe3þ, Cr3þ, etc.), Anindicates interlayer anion, and x is equal to the molar ratio M3þ/(M3þ þ M2þ). The parent material of those compounds is the naturally occurring mineral hydrotalcite, which has the formula Mg6Al2(OH)16CO3 3 4H2O. The most common anion found in the naturally occurring LDHs is carbonate. In practice, however, different charge balancing anions may be incorporated, mainly oxyanions, halides, silicates, polyoxometalate, and complex and organic anions.15 The simplest and most commonly used preparation method is co-precipitation of the chosen M2þ and M3þ hydroxides with diluted NaOH and/or Na2CO3 or NaHCO3. However, they can also be synthesized using urea hydrolysis, a hydrothermal or ion-exchange method, etc.16,17 Because of some interesting properties, such as large surface area, high anionexchange capacity that is comparable to those of anionexchange resins, and good thermal stability,13,18 these compounds have been extensively studied for many possible applications, such as catalysts and their supports, anion exchangers, adsorbents, bioactive nanocomposites, and reaction media for photochemical and electrochemical processes.19-24 In the field of electrochemistry, they have been applied as the electrode for alkaline secondary cells25 and materials for electrodes in asymmetric capacitors.26 In the present work, synthesis of LDH/carbon composite has been achieved through co-precipitation under refluxing conditions from the solution containing a proper amount of metal salts and urea and activated carbon. In this approach, the carbon surface active sites and defects serve as nucleation centers for the growth of LDH crystallites during precipitation. Synthesis of LDHs was followed by thermal treatment at different temperatures in synthetic air. During annealing, LDHs were converted to a composite consisting of mixed (13) Cavani, F.; Trifirb, F.; Vaccari, A. Catal. Today 1991, 11, 173– 301. (14) Li, F.; Duan, X. Struct. Bonding 2005, 119, 193–223. (15) Newman, S. P.; Jones, W. New J. Chem. 1998, 22. (16) He, J.; Wei, M.; Li, B.; Kang, Y.; Evans, D. G.; Duan, X. Struct. Bonding 2005, 119, 89–119. (17) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg. Chem. 1998, 10, 1439–1446. (18) Vaccari, A. Catal. Today 1998, 41, 53–71. (19) Rahman, M. B. A.; Basri, M.; Hussein, M. Z.; Idris, M. N. H.; Rahman, R. N. Z. R. A.; Salleh, A. B. Catal. Today 2004, 94-95, 405– 410. (20) Sels, B. F.; De Vos, D. E.; Jacobs, P. A. Catal. Rev. 2001, 43, 443– 488. (21) Palomares, A. E.; L opez-Nieto, J. M.; Lazaro, F. J.; L opez, A.; Corma, A. Appl. Catal., B 1999, 20, 257–266. (22) Shimizu, H.; Okubo, M.; Nakamoto, A.; Enomoto, M.; Kojima, N. Inorg. Chem. 2006, 45, 10240–10247. (23) Li, F.; Duan, X. Struct. Bonding 2005, 119, 193–223. (24) Evans, D. G.; Duan, X. Chem. Commun. 2006, 5, 485–496. (25) Jayashree, R. S.; Vishnu Kamath, P. J. Power Sources 2002, 107. (26) Su, L. H.; Zhang, X. G.; Mi, Ch. H.; Liu, Y. J. Power Sources 2008, 179, 388–395.
3. Results and Discussion In this paper, for the preparation of carbon/LDH composite, the urea method was chosen because of its advantages compared to the most commonly applied co-precipitation (27) Liu, Z.; Ma, R.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872–4880.
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: DOI:10.1021/ef901505c
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Figure 1. SEM images of the composite RCoAU consisting of LDH particles dispersed on the surface of activated carbon.
technique. In the synthesis with urea (on the contrary to the synthesis with the addition of base during processing), the controlled supply of carbonate and hydroxide by the decomposition of urea in the presence of the M2þ and M3þ salt mixture successfully leads to the formation of monodisperse hydrotalcite particles with a good crystallinity degree and a narrow distribution of particle size. The mechanism of hydrolysis of urea has been studied previously,28 and the proposed reaction is as follows: COðNH2 Þ2 f NH4 CNO NH4 CNO þ 2H2 O f ðNH4 Þ2 CO3 This mechanism includes the formation of cyanate followed by fast hydrolysis of the cyanate to ammonium carbonate. Urea liberates hydroxide and carbonate ions when its aqueous solution is heated. The decomposition rate of urea in aqueous solutions depends upon the temperature, and thus, controlling the temperature of the reaction may easily control the hydrolysis rate. Moreover, the hydrolysis of ammonium to ammonia and carbonate to hydrogenocarbonate gives a pH of about 9, which is suitable for precipitating a large number of metal hydroxides. Figure 1 shows the scanning electron microscopy (SEM) images of the as-received LDH/carbon composite, from which the formation of LDH particles, with the size of about 5 μm, dispersed on the surface of activated carbon seems to be apparent. It can be seen that the sample consists of uniform and plate-like particles with roughly hexagonal shapes and more or less sharp edges. The hexagonal morphology is believed to be the result of the natural growth of LDH particles according to the crystallographic habit (i.e., rhomboedral symmetry). Their uniformity can be attributed to a slow and homogeneous nucleation process because of the slow hydrolysis of urea. The existence of Co and Al has been
Figure 2. Nitrogen adsorption isotherms for carbon material and composite RCoAU.
confirmed by energy-dispersive analysis of the X-ray spectrum, which indicated the presence of ∼15% of Co. LDH particles have been deposited on the surface of activated carbon and some could also fill in the inner big pores or block the smaller pores of activated carbon. This suggestion matches well with the results of nitrogen sorption measurements, which are presented in Figures 2 and 3. From the obtained curves, it is clearly seen that material changes from purely microporous to micro- and mesoporous after co-precipitation. The specific surface area calculated with the BET equation decreases from 1720 m2 g-1 for carbon material to 1120 m2 g-1 for RCoAU composite. A decrease of both the specific surface area and pore volume for the composite is believed to be caused by blocking the porosity by LDH particles; however, some filling of pores can also be assumed. For both materials, different isotherms can be observed. Carbon material gives a type-I isotherm,29 which (29) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Roquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.
(28) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 4729.
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Figure 3. DFT pore size distribution for carbon material and composite RCoAU.
Figure 5. Relation between the heating temperature and specific surface area of RCoAU composite.
Figure 6. Comparison of the pore size distribution of RCoAU composite as prepared and thermally treated.
During calcinations, as the heating temperature increased, the characteristic peaks of LDHs gradually disappeared. Broadening of the crystalline reflections of the diffraction peaks in the thermally treated material indicates a loss of some degree of crystallinity. The intensities of the peaks of the sample heated at 160 °C are lower than those corresponding peaks for material dried at 80 °C, but the layered structure is still retained, as shown by the presence of the low angle basal reflections. Samples heated at 160 and 200 °C became essentially amorphous. As the heating temperature was raised further, the intensities of the peaks increased and line widths decreased, indicating an increase in the crystallinity. After calcination at higher temperatures, their XRD patterns exhibited the characteristic peaks of both Co3O4 and CoAl2O4. The BET specific surface areas of samples as a function of the temperature are shown in Figure 5. Isotherms observed for the calcinated materials were of the same form as those observed for as-prepared RCoAU, showing characteristics of type IV. The specific surface area increases slightly with the temperature and reaches a maximum of 1250 m2 g-1 for material prepared at 300 °C. This behavior is attributed to the widening of smaller pores because of the calcination of carbon, gas evolution, and creation of a more open macrostructure with additional interparticle pore networks (Figure 6). Further thermal treatment results in a decrease of the surface area, caused by a carbon burnoff, giving a calculated value of only 235 m2 g-1 for the sample heated at 450 °C. Electrochemical properties of synthesized composites have been evaluated in a two electrode system. As seen from Table 1,
Figure 4. XRD patterns of composite RCoAU as prepared and after thermal treatment (80 °C; 160 °C; 200 °C; 300 °C; 450 °C; 500 °C).
is characteristic for microporous solids having a relatively small external surface. On the contrary, the isotherm given by RCoAU is of type IV, with a hysteresis loop associated with capillary condensation taking place in mesopores. The hysteresis loop classified according to International Union of Pure and Applied Chemistry (IUPAC) classification can be ascribed to H3 type, suggesting aggregates of plate-like particles giving rise to slit-shape pores. Powder XRD patterns of LDH/carbon composite heated at different temperatures are shown in Figure 4. The structure evaluated by XRD proved the formation of crystalline LDH. The pattern of the as-prepared composite is well-matched to the reported data for the compound Co6Al2CO3(OH)16 3 H2O [Joint Committee on Powder Diffraction Standards (JCPDS): 51-0045]. The peaks of [003], [006], [012], and [018] planes with a constant distance are typical for LDHs, and all could be indexed as rhomboedral. The two strong diffraction peaks appearing at 11.61° and 23.48° ascribed to [003] and [006] planes, respectively, indicate the growth of crystallites in the definite direction. 3349
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: DOI:10.1021/ef901505c
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Table 1. Capacitance Values Estimated in a Two-Electrode Cell at Different Current Loads for Carbon Materials and Synthesized Composites capacitance (F g-1) material
50 mA g-1
100 mA g-1
200 mA g-1
500 mA g-1
1000 mA g-1
carbon RCoAU RCoAU200 RCoAU300 RCoAU450
98 70 74 90 5
96 68 73 85 2
90 66 70 82
75 61 66 79
55 55 60 76
Figure 8. Galvanostatic charge/discharge curves of the symmetric capacitor at different current densities in 6 mol L-1 KOH aqueous solution.
Figure 9. Influence of the constant current density on capacitance for carbon material and RCoAU composite.
kind of composite. The shape remains almost the same, indicating only the slight decrease in capacitance, even at higher scan rates. The same properties can be observed for measurements performed in different potential ranges (Figure 7b). The typical charge/discharge plots of the symmetric capacitor at various current densities in 6 M KOH solution are shown in Figure 8. The presented curves are basically symmetric. However, the discharge time decreases significantly with an increased applied current; hence, the specific gravimetric capacitance fades. It is worth noting that the decrease is much smaller for the composite compared to the pure carbon material, which is illustrated in Figure 9. The carbon electrode is able to work at a constant current of 1 A g-1, while RCoAU300 maintains capacitance up to 5 A g-1. The synergetic effect of the conducting carbon framework and the redox properties of the LDH may explain the improved performance. A high surface area of LDH/carbon composite and additional porosity, which arises between particles, can be accessible to the electrolyte in a mesoporous network. The carbon framework also improves the conductivity of the material, which is a key factor for obtaining materials possessing good electrochemical properties. Another advantage of this new type of electrode is its longterm life. A 15% loss of the capacitance has been observed after 15 000 cycles at 1 A g-1 (Figure 10). Once again, it can be assumed that the improvement in performance of the composite compared to the pure carbon
Figure 7. Cyclic voltammogram for the RCoAU300 electrodes (a) at different scan rates and (b) within different potential ranges at 5 mV s-1 in 6 mol L-1 KOH solution.
there was only a moderate difference in capacitance values between as prepared material and those obtained by thermal treatment at intermediate temperatures. On the contrary, a significant decrease of capacitance occurs in the case of the sample heated at 450 °C. This fading is believed to be caused by carbon loss and a decrease in surface area. For all of the further investigations, the composite RCoAU300, which exhibited the best properties, was chosen. In Figure 7, cyclic voltammograms of RCoAU300 material at different scan rates and within different potential windows (panels a and b of Figure 7, respectively) are presented. Twoelectrode cells built from the RCoAU300 composite pellets in 6 mol L-1 KOH show the typical electrochemical characteristics of a capacitor. The voltammograms have nearly rectangular shape, which indicates a quick dynamic of charge propagation with this 3350
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: DOI:10.1021/ef901505c
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4. Conclusion A composite of activated carbon with cobalt-alumina LDH with extended surface area has been prepared by co-precipitation from solution containing nitrates, carbon, and urea. The controlled supply of carbonate and hydroxide by the thermal decomposition of urea successfully led to the formation of monodisperse LDH particles. In this approach, the carbon surface active sites and defects serve as nucleation centers for the growth of LDH crystallites. Synthesis was followed by thermal treatment at different temperatures, leading to conversion of as-prepared samples into amorphous material, consisting of mixed metal oxides and, depending upon the temperature, carbon residue. LDH/carbon composites were found to be promising material of the electrode for the supercapacitor working at high current density. The symmetric capacitor based on carbon/LDH composite electrodes retains the value of specific capacitance after long cycling at high current density. A combination of the redox reactive property from LDH and good electronic conductivity of the carbon host can be at the origin of the improvement of electrochemical performance.
Figure 10. Galvanostatic charge/discharge curves of the symmetric capacitor based on RCoAU300 composite after cycling measurements in 6 mol L-1 KOH aqueous solution.
electrode lies in two aspects: a combination of the redox reactive property from LDH and good electronic conductivity of carbon. The porosity and, thus, accessibility of active materials also play an important role. Electrochemical impedance spectra (EIS) performed in the frequency range from 100 kHz to 1 mHz confirm a decrease of resistance for material with carbon (not presented here).
Acknowledgment. The authors acknowledge the financial support from the Ministry of Science and Higher Education (Poland) (Grant COST 1199/2007/02).
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