WO2011078901A1

WO2011078901A1 - SUPERCAPACITOR BASED ON MnO2 AND TiO2 COMPOSITES

Info

Publication number: WO2011078901A1
Application number: PCT/US2010/051455
Authority: WO
Inventors: Raju Raghurama
Original assignee: Honeywell International Inc.
Priority date: 2009-12-21
Filing date: 2010-10-05
Publication date: 2011-06-30

Abstract

A capacitor includes first and second electrodes and a porous dielectric between the first and second electrodes and an MgNO₃ based aqueous electrolyte. At least one of the electrodes comprises a composite of MnO₂ and TiO₂ nanoparticles or a composite of α-MnO₂ and TiO₂ nanoparticles or a composite of α-MnO₂, TiO₂, and carbon nanoparticles.

Description

SUPERCAPACITOR BASED ON Mn02 AND Ti02 COMPOSITES

Technical Field

[001] The technical field of the present disclosure relates to capacitors such as redox supercapacitors.

Background

[002] A supercapacitor is an electrochemical capacitor that has an unusually high energy density when compared to conventional capacitors. A supercapacitor is of particular interest in automotive applications for hybrid vehicles and as supplementary storage systems for battery operated electric vehicles. A supercapacitor also capable of being charged very quickly (for example, in about sixty seconds) compared to a few hours typically required for charging traditional rechargeable storage batteries.

[003] Supercapacitors have many advantages. They can withstand a high number of charge-discharge cycles (on the order of a million or more cycles compared to 200-1000 for most commercially available rechargeable batteries). They have no disposable parts during the whole of their operating life, which makes them environmentally friendly. Further, the materials used in a supercapacitor are themselves environmentally friendly so that disposal of a supercapacitor is not be a threat to the environment, unlike other batteries such as lead acid and nickel/cadmium batteries.

[004] Supercapacitors have other advantages as well. For example, as compared to rechargeable batteries, supercapacitors have an extremely low internal resistance or ESR, they are highly efficient (> 95%), they have high output power, they produce an extremely low level of heating, and they have improved safety. According to ITS (Institute of Transportation Studies, Davis, CA) test results, the specific power of supercapacitors can exceed 6 kW/kg at 95 % efficiency.

[005] The idea of replacing batteries with capacitors in conjunction with novel alternative energy sources became a concept of the Green Electricity (GEL) Initiative introduced by Dr. Alexander Bell. One particularly successful implementation of the GEL Initiative concept was introduced in the article "Muscle power drives battery-free electronics", describing a muscle-driven autonomous, environmentally- friendly solution, which employs a multi-Farad supercapacitor (hecto- and kilo- Farad range capacitors are now widely available) as intermediate energy storage devices to power a variety of portable electrical and electronic devices such as MP3 players, AM/FM radios, flashlights, cell phones, and emergency kits. As the energy density of supercapacitors bridges the gap with batteries, it is expected that in the near future the automotive industry will deploy supercapacitors as a replacement for chemical batteries.

[006] The first trials of using supercapacitors in industrial applications to power robots have been carried out. Also, China is experimenting with a new form of electric bus that runs without power lines using power stored in large onboard supercapacitors, which are quickly recharged whenever the electric bus stops at any bus stop, and are fully charged in the terminus. A few prototypes were tested in Shanghai in early 2005. In 2006, two commercial bus routes began to use supercapacitor buses, one of them over route 11 in Shanghai.

[007] Unfortunately, present supercapacitors have a low density and their capacitances decrease with increasing temperature and charge/discharge cycles. Third generation supercapacitors are electric double layer capacitors, where the electric charge stored at the metal/electrode interface is exploited in constructing the storage device. The interface can store electrical charge on the order of 106 F.

[008] The main component in the electrode construction of this capacitor is activated carbon. The use of carbon as the main electrode material for both the anode and cathode with organic and aqueous electrolytes are commercialized and used in day-to-day applications. Typical charge storage for these carbon based supercapacitors is around 100 F/gram. When organic electrolytes are used, the capacity of the capacitors decreases drastically with increasing temperature at around 50° C.

[009] The size of this supercapacitor is another factor due to its lower storage capability per gram. There are a few reports that the use of carbon nanotubes and RuC>₂ in supercapacitors increases the storage capacitance to around 1000 F/gram. However, the cost of these materials is exceptionally high.

[0010] To rectify these problems, researchers have recently tried to incorporate transition metal oxides along with carbon as the electrode materials. When the electrode materials consist of transition metal oxides, the electro sorption or redox processes enhance the value of the specific capacitance by 10 to 100 times, depending on the nature of the oxides.

[0011] Fourth generation supercapacitors use T1O₂ which is a well known dielectric material and holds a high charge because of the inherent nature of this material. Unfortunately, T1O₂ is highly insulating and is not suitable for use as an electrode material for a supercapacitor.

[0012] What is needed is a solution to one or more of these or other problems. Brief Description of the Drawing

[0013] The single drawing figure shows a side view of a supercapacitor having an electrode including MnC>₂ and T1O₂ nanoparticles.

Detailed Description

[0014] Like MnC>₂, T1O₂ is a tetravalent ion. Manganese dioxide (MnC ) is a well known material for supercapacitor applications. Forming a composite of ΤΊΟ₂ and MnC>₂ at the nanoscale level can be used to enhance the capacity of a supercapacitor. With the addition of a conducting polymer, a system suitable for large capacity devices can be created. Such a redox based supercapacitor has the advantage of storing 300 Farads/gram.

[0015] The capacity of a supercapacitor with electrodes of this material increases with increasing temperature from 0° C to 70° C and is very useful for places where the temperatures are above 40° C. By contrast, current known double layer supercapacitors experience a decrease in capacitance with increasing temperature above 40° C.

[0016] The basic equation for a capacitor is given as follows:

[0017] C = kS/d

[0018] where C is in farads and is the electrostatic capacity, k is the dielectric constant of the dielectric, S is in square centimeters and is the surface area of the electrode, and d is in centimeters and is the thickness of the dielectric.

[0019] The charge accumulating principle of a capacitor with MnC>₂ based electrodes can be described as follows. When a battery is connected to the capacitor, the flow of current induces the flow of electrons so that electrons are attracted to the positive terminal of the battery and flow towards the power source. As a result, an electron deficiency develops at the positive side, which becomes positively charged, and an electron surplus develops at the negative side, which becomes negatively charged. This electron flow continues until the potential difference between the two electrodes becomes equal to the battery voltage. Thus, the capacitor is charged.

[0020] Once the battery is removed from the capacitor, the electrons flow from the negative side to the side with the electron deficiency, which leads to the discharging of the capacitor. The charge/discharge mechanism of this capacitor is given as follows:

[0021] Mn0₂ + H₂0 + e→ MnOOH + OH [0022] MnOOH + e→ HMn0₂ -

[0023] MnOOH + HMn0₂ -→ Mn₂0₃ + H₂0 + OH

[0024] A supercapacitor 10 according to this construction is shown in the drawing figure. The supercapacitor 10 includes an anode 12, a cathode 14, and a porous glass separator 16 between the anode 12 and the cathode 14 of the supercapacitor 10 to pass electrolyte from one electrode to the other. The electrolyte is soaked into the porous glass separator 16 as well as in the composite electrodes 12 and 14.

[0025] The electrode 12 is a metal such as stainless steel and has a coating 18 of a composite of Ti0₂ and Mn0₂ or a composite of -Mn0₂ and Ti0₂ or other composite prepared at the nanoscale level. The electrode 14 also may be a metal such as stainless steel and has a similar coating 20. The coatings 18 and 20 are on the inside of the electrodes 12 and 14 so that they face the porous glass separator 16. As stated above, this composite material enhances the capacity of a supercapacitor. With addition of a conducting polymer, a system suitable for large capacity devices can be created.

[0026] The composite of Ti0₂ and Mn0₂ can comprise, for example, 99-90% Mn0₂ and 1- 10% Ti0₂. One good candidate for electrodes 12 and 14 is a composite of 95% Mn0₂ and 5% Ti0₂.

[0027] For this composite of Ti0₂ and Mn0₂ , polymer is added. The polymer helps to act as a binder and also reduces the electrical series resistance. Carbon black is added for the same purpose. Accordingly, the resulting electrode, for example, comprises 20% of polymer, 75% of the composite of Ti0₂ and Mn0₂, and 5% carbon black. These percentages are by weight.

[0028] Porous fiberglass, for example, may be used for the dielectric 16 to separate the electrodes 12 and 14 and to hold the electrolyte of the supercapacitor.

[0029] Various chemical routes can be used to make the Ti0₂ and Mn0₂ nanomaterials, and the electrodes 12 and 14 may be fabricated as thick films as well as from pellets of Ti0₂ and Mn0₂ nanomaterials.

[0030] The preparation of the Mn0₂ + Ti0₂ composite nanopowder material is carried out by the polyol method. In a typical synthesis of Mn0₂ + 5% Ti0₂, 0.39g of Ti(OBu)₄ is taken into a 100 ml beaker. To these, 5 ml of ethylene glycol is added while stirring [Beaker 1]. 3.2 g of KMn0₄ is added to 200 ml DD water in 250 ml beaker while stirring [Beaker 2]. After dissolving the KMn0₄ in the water from beaker 1 (beaker 1 contains 0.39g of Ti(OBu)4+5ml EG), the solution is added drop wise to beaker 2 using a pipette (beaker 2 contain 3.2g of KMnO₄+200ml DD water) while stirring. A precipitate will form (in about 1 hour). The precipitate is separated by the filtration, is washed with DD water (such as three times), and is finally washed with ethanol. The resulting material is dried in the oven at 60° C. The final product is ground using motor and taken the total weight.

[0031] Powders of MnC>₂ and T1O₂ composite and Ketjan Black (KB) are ground well in a mortar and PTFE suspension is added and mixed thoroughly. The weight ratio of MnC>₂, KB and PTFE is 75 : 20 : 5. The mix becomes dough, which is rolled on a glass plate into a thin layer. A stainless steel mesh (commercial) is cut to the required size (such as 30 mm x 30 mm, or 15 mm xl5 mm), cleaned thoroughly with a detergent, rinsed in 20% HNO₃ and 20% HC1 solutions, and rinsed with acetone to dry. The mass is placed in a die, the layer of electrode material is placed over the SS mesh and compacted using a hydraulic press. The electrodes are dried in an oven at 90-100°C for 2 hours. The electrodes are weighed and dipped in a saturated solution of magnesium nitrate for about 30 min. Absorbent Glass Mat (AGM) of 30 mmx30 mm size can be cut from a sheet, and soaked in saturated Mg(NC>3)₂ solution. Capacitors are assembled by placing the soaked AGM in between the two MnC>₂ and I1O₂ electrodes. The supercapacitor assembled in a suitable case is sealed to avoid electrode evaporation.

[0032] Thus, a supercapacitor includes first and second electrodes and a porous dielectric between the first and second electrodes and an MgNC>3 based aqueous electrolyte. At least one of the electrodes comprises a composite of MnC>₂ and T1O₂ nanoparticles or a composite of a -MnC>₂ and T1O₂ nanoparticles or a composite of a -MnC>₂, T1O₂, and carbon nanoparticles.

[0033] Certain modifications of the present invention have been discussed above. Other modifications of the present invention will occur to those practicing in the art of the present invention. Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.

Claims

I/WE CLAIM:

1. A capacitor comprising:

first and second electrodes, wherein at least one of the electrodes comprises a composite of MnO₂ and TiO₂ nanoscale particles; and,

a dielectric between the first and second electrodes.

2. The capacitor of claim 1 wherein the composite comprises 99-90% MnO₂ and 1-10% TiO₂.

3. The capacitor of claim 1 wherein the composite comprises 95% MnO₂ and

5% TiO₂.

4. The capacitor of claim 1 wherein the MnO₂ nanoscale particles comprises nanoscale - MnO₂ particles.

5. The capacitor of claim 4 wherein the composite comprises 99-90% α - MnO₂ and 1-10% TiO₂.

6. The capacitor of claim 4 wherein the composite comprises 95% a - MnO₂ and 5% TiO₂.

7. The capacitor of claim 1 wherein both the first and second electrodes a composite of MnO₂ and TiO₂ nanoscale particles.

8. The capacitor of claim 7 wherein the composite comprises 99-90% MnO₂ and 1-10% TiO₂.

9. The capacitor of claim 7 wherein the composite comprises 95% Mn0₂ and

5% Ti0₂.

10. The capacitor of claim 7 wherein the Mn0₂ nanoscale particles comprises nanoscale a -Mn0₂ particles.

11. The capacitor of claim 10 wherein the composite comprises 99-90% a - Mn0₂ and 1-10% Ti0₂.

12. The capacitor of claim 10 wherein the composite comprises 95% a - Mn0₂ and 5% Ti0₂.

13. The capacitor of claim 1 wherein the composite further comprises carbon nanoscale particles.

14. The capacitor of claim 1 wherein both of the electrodes comprise a composite of Mn0₂, Ti0₂, and carbon nanoscale particles.

15. The capacitor of claim 1 wherein both of the first and second electrodes comprise a composite of a -Mn0₂, Ti0₂, and carbon nanoscale particles.

16. The capacitor of claim 1 wherein at least one of the first and second electrodes comprises a composite of Ti0₂ and Mn0₂ nanoparticles, carbon, and a polymer, and wherein the at least one of the first and second electrodes comprises 20% of the polymer, 75% of the composite of Ti0₂ and Mn0₂, and 5% of the carbon.

17. The capacitor of claim 1 wherein both of the first and second electrodes comprise a composite of TiO₂ and α-MnO₂ nanopartieies, carbon, and a polymer, and wherein the both of the first and second electrodes comprises 20% of the polymer. 75% of the composite of TiO₂ and MnO₂, and 5% of the carbon.

18. The capacitor of claim 1 wherein the dielectric includes art electrolyte.

19. The capacitor of claim 1 wherein the dieiectric comprises porous glass.

20. The capacitor of claim 19 wherein the porous glass includes an electrolyte.