Electrochemical Supercapacitors. Oleg A. ... materials for supercapacitors are essentially nanocrystalline .... the prototype symmetric supercapacitor with two.
Proceedings of the 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems January 16 - 19, 2007, Bangkok, Thailand
New Nanostructured Electrode Material for Electrochemical Supercapacitors Oleg A. Shlyakhtin"2, Min Seob Song', and Young-Jei Ohl
'Thin Films Materials Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, 136-791 Seoul, Korea 2Kinetics and Catalysis Division, Institute of Chemical Physics, Russian Academy of Sciences, Kosygina st., 4, 119991 Moscow, Russia.
oxide/hydroxide powders in the aqueous electrolytes is not described yet. Present paper summarizes our first results of their synthesis and electrochemical performance studies.
Abstract-Freeze drying product of the coprecipitated Ni-Mn hydroxides is comprised of the platelike 10 nm thick particles demonstrating considerable electrochemical activity in the aqueous solutions (specific capacitance 140-150 Fg-1). Formation of the complex oxides during thermal processing of freeze dried hydroxides is accompanied by the nonmonotonic decrease of capacitance in the lack of morphological evolution. Keywords-capacitors, electrochemical storage, materials processing
electrodes,
EXPERIMENTAL Nanocrystalline Ni-Mn hydroxides were precipitated under intense stirring from the aqueous acetate solution by IM NaOH, filtered and carefully washed. In order to retain the native micromorphology of the hydroxide particles, the residue was fast frozen and freeze dried at P = 10-2 mbar. Following heat treatment was performed in air with 10 K min-' ramp and 3 hrs dwell at desired temperature. In order to produce electrodes for electrochemical studies, as-obtained oxide/hydroxide powders were mixed with carbon black and PTFE in the 80:15:5 weight ratio. After adding several drops of EtOH, the slurry was pasted onto Ni foam (1015 mg cm- ), carefully dried and pressed at 300 kg cm2 Cyclic voltammetry and galvanostatic charge-discharge studies were performed in 0.1M Na2SO4 solution with a platinum counter electrode and Ag/AgCl reference electrode using WBCS 3000 automatic battery cycler. II.
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INTRODUCTION Chemical supercapacitors are considered now as an attractive energy source for the portable electronic devices with pulsed character of energy consumption. All known electrode materials for supercapacitors are essentially nanocrystalline that allows to reduce the distance of the possible ion transport and to extend the dimensions of the double electric layer due to high specific surface area of these materials. The electrodes of the modern industrially produced supercapacitors are usually made of the various kinds of carbon-based materials [1]. Substantially higher capacitance is demonstrated by hydrated RuOx-based electrodes [2,3] though their higher cost and considerable toxicity promote further search of moderately priced candidates. I.
RESULTS AND DISCUSSION Analysis of the electrochemical behavior of freeze dried hydroxides demonstrated their considerable electrochemical activity in Na2SO4 electrolyte comparable with the product of electrochemical deposition; contribution of the Ni foam and the carbon black was found negligible. Cyclic voltammograms of the Ni0.35Mno.65(OH)x nH2O at various sweep rates (Fig. IA) show at its true capacitive behavior with perfectly symmetric cathodic and anodic branches of charge-discharge curves and the absence of distinct peaks and anomalies usually associated with bulk electrochemical processes. The character of observed CV curves is similar to the potentiostatic plots of the corresponding thin films [10] except substantially larger capacity fall at higher sweep rates. Replotting of these CV data in the normalized coordinates (Fig. iB) indicates that the increase in sweep rate causes systematic decrease of the electrochemical availability of (Ni,Mn)(OH)x particles that can be related to their low electrical conductivity and poorer electric contact with Ni substrate. III.
A promising alternative to RuO2 *nH2O can be found among nanocrystalline hydroxides and oxyhydroxides of transition elements [4], first of all nickel and manganese hydroxides widely used before in various kinds of primary and secondary batteries. Thin MnOx nH2O films often demonstrate high specific capacitance and excellent pseudocapacitive behavior [5,6]; however, an increase in film thickness is usually accompanied by progressive falling down their specific capacitance. Apart from oc- and 3-Ni(OH)2 demonstrating high electrochemical capacitance mostly at low discharge rates, pseudocapacitive behavior of mesoporous NiO powders and coatings obtained by various chemical and electrochemical methods allows them to retain specific capacitance over 100 Fg 'even at rather high discharge currents and sweep rates (100 - 500 mV s-1) [7-9]. Recent studies revealed a promising supercapacitor behavior of electrochemically deposited thin films of double Ni-Mn hydroxides in IM Na2SO4 solution [10]. However, electrochemical activity of the corresponding complex This work was supported by the Korea Research Foundation and Ministry of Science and Technology, Republic of Korea. *Corresponding author: Dr. Young-Jei Oh. Fax +82-2-958-5554; e-mail
youngjei*kist.re.kr.
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considerable electrolyte resistance in the pores [11] and, thus, an increase in the IR drop.
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Figure 1. Cyclic voltammograms of FD Nio.35Mn0.65(0H)x nH2O electrode at various sweep rates (A) and the same curves normalized to discharge conditions at 10 mV/sec (B).
However, at moderate sweep rates and discharge currents the freeze dried Ni-Mn hydroxides demonstrate sufficient capacitance values and good cyclability essential for the.=.= supercapacitor electrode materials. Chronopotentiometric studies (Fig. 2) allowed to estimate the specific capacitance of FD hydroxide as 140-150 Fg'1 at I 5 mA cm2and 115-120F g at 10 mA cm2 Discharge process is accompanied by a significant ohmic voltage (IR) drop that can be attributed to the considerable resistance of the electrolyte in the pores of electrode material [11, 12]. It should be noted, however, that the voltage drop is almost absent at the galvanostatic curves of the prototype symmetric supercapacitor with two (Ni,Mn)(OH)x paste electrodes (Fig. 2, inset). TEM analysis of the freeze dried Ni-Mn hydroxide powder revealed that the product consists of the platelike grains 50xlO nm (Fig. 3A) that are often observed during synthesis of the layered double hydroxides (LDH) [13,14]. According to SEM (Fig. 4A), the grains are packed into soft agglomerates with extended porous structure being feasible for the electrolyte penetration into particles but promoting also an enhancement of the ESR (equivalent series resistance) due to
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3 Figure 3. TEM micrographs and SAED patterns of the FD hydroxide (A) and the product of its thermal processing at 400°C in air (B).
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Figure 4. SEM micrographs of Ni-Mn hydroxides after FD (A) and following thermal dehydration at 100°C (B).
The information about the impact of crystallographic ordering on the supercapacitor behaviour of transition metal hydroxides is rather controversial. MnOx-based electrodes usually demonstrate the maximum capacitance in the materials show better amorphous state while NiOJ(OH)y-based I~~~~ performance after crystallization at elevated temperatures [5-9]. Comparison of the electrochemical performance of powders obtained by thermal processing of the Ni-Mn hydroxide shows at the nonmonotonic evolution of their specific capacitance, with intermediate minimum and maximum at 200°C and 3000C, respectively (Fig. 5). According to [10], a thin layer of electrochemically deposited Ni-Mn hydroxides was completely amorphous. XRD analysis of the freeze drying product (Fig. 6) demonstrated significantly broadened but clearly detectable set of peaks that corresponds neither to calcite- [15] nor to the hydrotalcite-like [16] layered double hydroxides usually formed during precipitation by carbonates. Instead, observed product is similar to the metastable tetragonal [17] or cubic [18] spinels observed in these papers at T > 2000C. Thermal processing of the freeze dried powders at 100 2000C causes systematic decrease in electrochemical capacitance (Fig. 5) and results in amorphization and disappearance of the spinel-like phase that allows to attribute initially observed high electrochemical activity to this particular compound.
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2 theta Figure 6. XRD patterns of the FD Ni-Mn hydroxide and the products of its thermal processing. x- spinel-like hydroxide phase; 0 - ilmeniite-type complex oxide.
Thermogravimetric analysis of the FD hydroxide powder (Fig. 7) shows at the correlation of observed amorphization with significant weight loss caused by thermal dehydration of the spinel-type hydroxide compound. Increase in processing temperature to 300 400°C leads to the formation of the ilmenite-type complex Ni-Mn oxide at 300 - 400°C (Fig. 6). Similar process was also observed in [15,17,18] but at T > 400°C. Taking into account that cation stoichiometry of the original ilmenite FeTiO3 is close to 1/1, the products obtained at 300 - 400°C may contain also oc-Mn203 [18] poorly distinguishable with NiTiO3 by powder XRD. Hence, moderate increase in the electrochemical capacitance at 300°C can be attributed to the formation of NiTiO3 and Mn203. -
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insufficient conductivity of the newly discovered electrode material and significant IR drop during charge-discharge cycling has to be solved during further studies and optimization of the powder synthesis process.
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REFERENCES Cu1
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Figure 7. Thermogravimetric curves of FD Ni-Mn hydroxide at various P(02); dT/dt 10 °/min.
[5] [6]
Systematic changes of the electrochemical performance of obtained material during thermal processing could be caused also by the morphological reasons, as topochemical transformation of the chemical systems during thermal decomposition is usually accompanied by the corresponding morphological (microstructural) evolution. TEM analysis of the thermal processing products (Fig. 3B) demonstrated that dramatic changes in the crystal structure, reflected in the corresponding SAED patterns, had rather moderate influence on the size and morphology of the platelike crystallites and their packing mode, except better mutual orientation and more intense particle intergrowth within agglomerates. Such a discrepancy can be explained by a small difference in specific molar volumes of transition metal hydroxides and corresponding oxides causing small internal stresses and, hence, retention of the initial crystallites during dehydration. SEM data (Fig. 4A, 4B) illustrate more visually the process of particle orientation within agglomerates and formation of the close packing that can be responsible for the capacitance decrease at the last stages of thermal processing. However, the most part of observed evolution of electrochemical performance with processing temperature could be explained by crystallochemical rather than morphological reasons.
[7] [8] [9] [10]
[11] [12]
[13]
[14]
IV. CONCLUSION Comparison of the electrochemical performance of (Ni,Mn) hydroxide thin films obtained by electrochemical deposition and corresponding nanocrystalline powders obtained by authors shows at the higher specific capacitance values and better rate capability of thin film coating, probably due to better particle packing and better electrical contact with conducting substrate. However, the bulk Nio.35Mno.65(OH)x nH2O powders, evaluated for the first time as electrode material for supercapacitors, demonstrated sufficient electrochemical activity and rather high specific capacitance values (140 - 150 F g-1) at much higher electrode material loading per cm2 compared to thin films. The problem of
[15]
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[18]
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