Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-09-08
2003-02-04
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S512000, C361S523000, C361S518000, C438S402000, C438S408000
Reexamination Certificate
active
06515845
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the fabrication method of preparing nanoporous carbon materials with pore sizes ranging from 2 nanometer to 20 nanometer which can be used as electrode materials for supercapacitors and electric double layer capacitors being a kind of supercapacitor. The present invention also relates to electric double layer capacitors utilizing these carbon materials as electrodes
Recently, the development of supercapacitors is intensively pursued for the high pulse-power energy source and load-leveling devices for secondary batteries and fuel cells, which are the power sources for the next-generation mobile telecommunication system, IMT-2000 and electric vehicle.
In general, supercapcitors can be classified into electric double layer capacitor and pseudocapacitor. The former utilizes electric double layer formed in the interface of electrode surface and electrolyte. The latter utilizes pseudocapacitance developed inside the electrode from electrochemical reactions. The application of pseudocapacitors is limited because expensive RuO
2
or IrO
2
is utilized as electrode material.
In this connection, when describing more details on electric double layer capacitors, in the electric double layers formed in the interface between electrode and electrolyte, ions are accumulated in the electrolyte phase, and electric charges, which can be released during discharge cycle, are stored on the electrode. Secondary batteries are similar charge storage devices as supercapacitors; however, compared to secondary batteries, supercapacitors can be operated at high current condition and have longer lifetime.
In general, the equivalent circuit for electric double layer capacitor electrodes can be represented as a serial combination of equivalent series resistance (ESR) and double-layer capacitance. The double-layer capacitance is proportional to the surface area of the electrode and the equivalent series resistance is the summation of resistance from electrode, electrolyte bulk solution, and electrolyte in the electrode pores. Capacitance stored in electric double layer capacitors decrease as charging/discharging rate increases and is determined by ESR.
Therefore, the electrode materials for electric double layer capacitors should satisfy the following characteristics: (1) high surface area for high double-layer capacitance, (2) high electrical conductivity for low resistance of electrode, (3) low resistance from the electrolyte in the pores of electrode.
So far, activated carbon powder and activated carbon fiber were utilized as electrode materials for electric double layer capacitors. These activated carbons are produced from the physical or chemical activation of precursors such as wood, peat, charcoal, coal, brown coal, coconut shell, and petroleum coke. However, compared to the requirements for electrode materials for electric double layer capacitors, the following problems exist for the activated carbons.
First, these activated carbons possess irregularly connected pores composed of micropores (below 2 nm), mesopores (2 nm~50 nm) and macropores (over 50 nm), which limit them for the successful application as electrode materials for electric double layer capacitors. The micropores are not so easily wetted by electrolytes, and the surface exposed in micropores may not be utilized for charge storage. Moreover, even in the situation where micropores are wetted by electrolytes, ionic transfer in such small pores are not so facilitated that the high rate capability, which is one of the advantages belonging to electric double layer capacitors, may not be realized. Both charge storage and rate capability is further limited if pores are randomly connected. It is generally accepted that pore sizes bigger than 2 nm is desirable for the electrode materials for the electric double layer capacitors in aqueous electrolyte media, and pore sizes bigger than 5 nm for those in organic electrolyte media.
Second, these activated carbons have low electrical conductivity because micrometer-sized particles are irregularly interconnected resulting from the poor pore connectivity. Conducting additives such as carbon black can be added to activated carbons to increase electric conductivity and to decrease ESR as a result, which, however, will decrease capacitance per weight or volume. On the other hand, electrolyte cannot penetrate into poorly connected isolated pores and charge cannot be stored. In addition, the movement of electrolyte ions will be limited in the poorly connected pores and thus resistance is large.
Accordingly, for achieving high power density for electric double layer capacitors, ESR of the electrode materials must be small and it should have high capacitance as well. For small ESR of the electrode materials, they should possess high electrical conductivity and large pores as mesopores. It is more preferable to have well-connected pores to achieve small ESR.
In this connection, Y. Z. Zhang and coworkers tried to control the pore structure of activated carbons and activated carbon fibers through the treatment with NaOH combined with the activation by CO
2
for their application to electrode materials for electric double layer capacitors (Carbon 24th Biennial Conference on Carbon 11-16, p.434 (1999)). However, they could control the pore size through the research, whereas they could not control pore connectivity of the materials.
Meanwhile, Ryong Ryoo and coworkers used cubic MCM-48 mesoporous silica molecular sieve as template for the synthesis of mesoporous carbon materials. They put sucrose into the pores of MCM-48 silica in the presence of acid catalyst and carbonized sucrose by heating at 800~1100 ° C. under inert atmosphere followed by removing the template material using sodium hydroxide.
This process has some problems because expensive mesoporous silica molecular sieve is used as template and the pore structure of nanoporous carbon produced is inevitably determined by the pore structure of the template and cannot easily controlled.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the above drawbacks. That is, the aim of the present invention is to develop new preparative method to solve all the problems associated with the synthesis of nanoporous carbon materials.
Accordingly, an object of the present invention is to provide the method of preparing nanoporous carbon materials which can be used as electrochemical materials such as electrodes for electric double layer capacitors. More specifically, carbon precursors will be formed in the presence of inorganic template particles to generate template/carbon-precursor composites. These template/carbon-precursor composites will be carbonized, and after the removal of the template, nanoporous carbons will be produced.
The key idea in the process is that the structure of templates will eventually determine the pore structure of the resulting carbon materials. Keeping in mind this idea, we could fabricate nanoporous carbon materials with pore sizes bigger than 2 nm using inorganic templates and further we could also produce nanoporous carbon materials with well-interconnected 2 nm to 20 nm pores, suitable for the electrodes of electric double layer capacitors, which completes the present invention.
Another object of the invention is to provide the method for preparing nanoporous carbon materials with well-interconnected 2 nm to 20 nm pores and high electrical conductivity. By our earnest work, it wad found that the electrode materials made of these nanoporous carbons exhibited excellent charge storage capacity for their applications to electric double layer capacitors at high charging/discharging conditions, which finally completed the present invention. So, the nanoporous carbon materials with well-interconnected 2 nm to 20 nm pores and high electrical conductivity can be applied for the fabrication of electric double layer capacitors with excellent charge storage capacity at high charging/discharging conditions by minimizing equivalent series resistance.
Therefore, a further obj
Han Sang-Jin
Hyeon Taeg-Hwan
Oh Seung-Mo
Fincell Co., Ltd.
Ha Nguyen
Reichard Dean A.
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