Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2002-02-04
2003-04-29
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S517000
Reexamination Certificate
active
06556424
ABSTRACT:
STATEMENT RE. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the electrical device to which the present invention relates belongs to the “liquid electrolytic capacitor” category. Technical jargon amongst chemical engineers recently, for example at the Third Hawaii Battery Conference, manifests their preference to refer to that category as ‘electrochemical capacitors’. Subgrouping of electrochemical capacitors is based on mode of storing electrical charges, and includes ‘electrical double-layer capacitors’ (EDLC) and ‘pseudo-capacitors’ (PC). Electrode materials commonly utilized are high surface area carbons for EDLC, and transition metal oxides such as hydrous ruthenium oxide for PC.
To define what ‘supercapacitor’ means in the TITLE above and hereinafter, the term—sometimes encountered in print with a space between ‘super’ and ‘capacitor’—refers to a composite type electric charges storing device combining features of the aforesaid two subgroups of electrochemical capacitors, EDLC and PC, that owes its capacitance partly to charge storage in electrical double layers formed at the phase boundary between electrode and electrolyte, and partly to a transient change of oxidation state in a pseudo-capacitance material like the ruthenium oxide already mentioned.
One factor motivating attempts to devise superior supercapacitors is the prospect of using them as power sources for self-propelled electric vehicles. Today's leading edge supercapacitors can store about one-fourth as much energy for a given volume and weight as a lead acid battery. If compared only in terms of weight and size needed to store a given amount of energy, therefore, lead acid batteries are considerably lighter weight and more compact than the best super-capacitors. However, the ability of supercapacitors to release substantially all their stored energy within a very short period, say five to ten seconds, is unmatchable by batteries. Thus are supercapacitors especially fitting to contemplate for onboard auxilliary power supply in electric vehicles, as an energy source procuring, on demand, the fast accelerating performance that most electric vehicles based solely on power from lead acid batteries lack. By suitably incorporating supercapacitors into the power system of electric vehicles that use lead acid or any other secondary batteries for basic cruising power, the results would be to improve acceleration for passing in traffic, and to achieve fast starts reaching say 96 kmh (60 mph) in under ten seconds.
Both secondary batteries, and supercapacitors in a complementary role to the batteries in an electric vehicle powering system, would be rechargeable by d.c. power input to them from any known system already devised to recharge the batteries. A petrol-fueled combustion engine driving a d.c. generator can be aboard the vehicle. Hybrid vehicle proposals combining fuel cells and secondary batteries are known to suggest recharging batteries with fuel cell generated power, in that case utilizing the batteries in a kick-in-when-needed type auxilliary role to fuel cell power for the basic cruising. There would be no obstacle to charging capacitors the same way. Another highly pertinent system in this context is the current-producing type of regenerative braking system that recovers energy of braking and converts it to d.c. current for storage. Usually that proposal is directed to recharging batteries, but again the recharged item could as well be a supercapacitor. The present invention is considered particularly applicable to supercapacitors used as an auxilliary source of power in an electric vehicle equipped for recharging of both batteries and capacitors during normal road travel operation, via either combustion engine-driven current generation, fuel cell power generation, or current-producing regenerative braking.
2. Description of Related Art
The leading edge in the art of devising superior supercapacitors has encountered very recently, a laboratory-verified difficulty with finding an effective balance between a highly desirable regularly interconnected pores structure for carbon electrodes procuring electrical double-layer capacitance, on the one hand, and the use of pseudocapacitance material to load the pores, to raise specific capacitance by adding pseudo-capacitance, on the other hand. To gain appreciation of this trade-off problem, one may first turn to a published comparison between a typical molecular-sieving carbon body and a new porous carbon body with a regularly interconnected network of somewhat larger pores than in the molecular-sieving carbon. If electrochemists of the past have sometimes tended to too blithely assume that larger surface area for electrodes is always better, the report next briefly reviewed may give pause for re-assessment of the suitability of porous materials containing myriad randomly distributed very small pores.
Seoul National University researchers S. Yoon et al, in their report, “Electrical Double-Layer Capacitor Performance of a New Mesoporous Carbon”, Journal of the Electrochemical Society, 147 (7), pages 2507-2512 (2000), examine the differences between their new mesoporous carbon body and a typical molecular sieving carbon body, wherein the latter possesses the larger surface area of the two bodies because of smaller pores, and yet is outperformed in terms of charging/discharging rate capability at higher current densities by the new carbon body having smaller surface area because of larger pores, but featuring a regularly interconnected pores network. Even though the mesoporous body calculates as being of lower specific capacitance, in fact it stores more charge at high current density than does the molecular sieving carbon with higher calculated specific capacitance. S. Yoon et al explain the differences largely in terms of how phenomena are dominated by electrolytic resistance effects within pores. An ionic motions problem is involved. To further understand implications highly pertinent to the present invention, a follow-up report to the foregoing report is brought into the picture, bearing in mind that both types of carbon electrodes in the first report are electrical double-layer capacitors only, neither of them having a pseudo-capacitance aspect.
A report presented at the Third Hawaii Battery Conference by Seoul National University researchers J. Jang et al, entitled “Electrochemical Capacitor Performance of Ruthenium Oxide/Mesoporous Carbon Electrodes”, is concerned with combining pseudo-capacitance procuring electrode material with electrical double-layer capacitance procuring electrode materials the latter being the same new mesoporous carbon as for the superior electrodes of the preceding report by S. Yoon et al, colleagues at Seoul with J. Jang et al. Ruthenium oxide loading of pores of the mesoporous carbon adds pseudo-capacitance related characteristics.
While success at raising the specific capacitance under relatively lower current densities was achieved by loading the carbon mesopores with ruthenium oxide, the researchers found evidence of pore-blocking that shifts the problem area back again to impaired ionic motions at higher current densities. Quoting J. Jang et al, bottom of page 8 and top of page 9: “In this work, we tried to enhance the specific capacitance of mesoporous carbon electrodes by loading ruthenium oxide that carries the pseudo capacitor characteristics. An enhanced specific capacitance is expected with these composite electrodes as two types of capacitors are combined. This beneficial effect may, however, be counterbalanced by a loss of rate capability because an excessive ruthenium oxide loading inside the mesopores may narrow down the pore size that eventually retards ionic motions.” (emphasis added)
Drawing on the published revelations of the Korean research, the present inventor formulates the following desideratum: that in order to optimize high current performance of supercapacitors intended as compact energy reser
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