Electrical energy devices

Details Electrical energy devices Electrical energy devices

1998-06-12

2002-09-17

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S223000, C429S228000, C429S235000, C429S236000, C429S231950, C501S095100, C252S519120, C252S519330

Reexamination Certificate

active

06451485

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrochemical devices such as batteries, fuel cells, capacitors and sensors which employ electrically conductive ceramic materials, fibers, powder, chips and substrates therein to improve the performance of the electrochemical device.
BACKGROUND OF THE INVENTION
There are numerous applications which involve the transfer of electrical current in environments which are highly corrosive or otherwise degrading to metallic conductors. Most notably are electrochemical devices operating under highly corrosive conditions and high temperatures. Examples of such applications are the use of electrodes for the chlor-alkali cell to make chlorine gas, electrodes for metal recovery, electrodes in hydrogen/oxygen fuel cells, electrodes for producing ozone, electrolysis of water and electrodes in high temperature solid oxide fuel cells. Most of these applications involve the contact of an electrode with an electrolyte under conditions which render the electrode ineffective during prolonged use. The loss of effectiveness can be gradual, such loss being manifested by reduced current-carrying capacity of the electrode. Exemplary types of conditions which render electrodes ineffective as they are used in current-carrying applications are described below.
One such condition involves chemical attack of the electrode by corrosive gas which is evolved from the electrolyte as it is decomposed during use. For example, the evolution of chlorine gas, a highly corrosive material, from an aqueous chloride-containing electrolyte such as, in the chlor-alkali cell is exemplary.
Another type of condition involves passivation of the electrode as it combines with the anions from the electrolyte to form an insoluble layer on its surface. This passivation condition occurs when the product from the electrochemical reaction can not diffuse from the electrode surface and this produces a blocking of the electrochemical sites and/or pores. The end result is a diminishing of the electrode current carrying capacity. An example of this passivation is the lead dioxide electrode in an aqueous sulfuric acid solution.
Another type of condition which renders electrodes ineffective involves the dissolution of the electrode by the electrolyte. The use of a zinc electrode in an aqueous potassium hydroxide solution is exemplary.
Various types of batteries such as secondary(rechargeable) batteries: lead-acid(Pb/PbO
2
), NaS, Ni/Cd, NiMH(metal hydride), Ni/Zn, Zn/AgO, Zn/MnO
2
, Zn/Br
2
; and primary(non-rechargeable) batteries: Zn/MnO
2
, AgCl/Mg, Zn/HgO, Al/Air(O
2
), Zn/Air(O
2
), Li/SO
2
, Li/Ag
2
CrO
4
and Li/MnO
2
exist.
Although a variety of batteries are available, the lead-acid battery remains favored for uses such as starting internal combustion engines, electric vehicle motive power, as well as portable and emergency power for industrial and military applications.
Lead-acid batteries include a cathode comprising a lead alloy grid (active material support structure and electrical network structure contact with the battery terminals) having PbO
2
active material thereon; and an anode comprising sponge lead on a grid. The active material on a grid is called the plate and electrically, the anode (Pb) plate is negative and the cathode (PbO
2
) plate is positive. A separator, either glass fibers or porous plastic, is used to separate the cathode and anode from direct contact when the plates are in sulfuric acid electrolyte. For the lead-acid battery, the rated capacity (ampere-hours) depends on the total amount of electrochemically active material in the battery plates, the concentration and amount of sulfuric acid electrolyte, the discharge rate and the percent utilization (conversion of active material into ampere-hours) for the active materials (the cathode or PbO
2
usually being the limiting factor).
During discharge of a lead-acid battery, the lead and lead dioxide active materials are converted to lead sulfate. The lead sulfate can form an undesirable, insulating layer or passivation around the cathode active material particles which reduces the active material utilization during discharge. This passivating layer can be the result of improper battery charging, low temperature operation, and/or excessive (high current) discharge rates. In order to increase the cathode active material utilization, which is desirable for battery performance, means to increase the cathode active material porosity which increases the amount of active material contact with the sulfuric acid and/or active material conductivity which minimizes resistance and electrical isolation of the active material particles are useful. However, raising the cathode active material porosity tends to increase the tendency for a loosening and possible loss of active material from the plate as well as electrical isolation of the active material from the grid structure. Wrapping the cathode plate with a glass mat holds the loosened active material tightly to the plate and minimizes the tendency for active material sediment (electrochemically lost cathode material) in the bottom of the battery container. The addition of conductive materials (carbon, petroleum coke, graphite) to increase the conductivity of the cathode active material is well-known, but these materials are degraded rapidly from the oxygen generated at the cathode during charging.
Since the lead-acid battery anode is very conductive, the additives for the sponge lead active material have concentrated on improving low temperature battery performance and cycle life. The fundamental additive to the anode is the expander which is comprised of lampblack, barium sulfate and lignosulfonic acid mixed with the lead oxide (PbO) carrier agent. The expander addition to the sponge lead inhibits densification or decrease in the sponge lead porosity. If the anode active material becomes too dense, it is unable to operate at low temperatures and can no longer sustain practical current discharges.
In the manufacture of lead-acid batteries, cathode electrodes are usually prepared from lead alloy grids which are filled with an active paste that contains sulfated lead oxide. This sulfated lead oxide is then later converted or formed into sponge lead for the anode and lead dioxide for the cathode. In an alternative construction, known as tubular cathode plates, the cathode active material is a sulfated lead oxide powder that is poured into a non-conductive tube (braided or woven glass or polyethylene) containing a protruding lead alloy rod or spine. Several of these tubes make up the grid structure and electrical connections are made to the terminals by the protruding lead alloy rods. The tubular cathodes and the usual plate anodes are then assembled into elements and these are then placed in a battery container. The cells are filled with electrolyte and the battery is subjected to the formation process. See details on lead-acid batteries, by Doe in Kirk-Othmer: Encyclopedia of Chemical Technology, Volume 3 (1978), page 640-663.
During lead-acid battery formation, active material particles in contact with the grid are formed first and particles further away from the grid are formed later. This tends to reduce the efficiency of formation. An apparent solution to this problem is addition of a conductive material to the active material paste. The additive should be electrochemically stable in the lead-acid system both with respect to oxidation and reduction at the potentials experienced during charge and discharge of the cell, as well as to chemical attack by the sulfuric acid solution. The use of barium metaplumbate and other ceramic perovskite powder and plating additives to the lead-acid battery anode and cathode are reported to enhance the formation of lead-acid batteries. See U.S. Pat. No. 5,045,170 by Bullock and Kao. However, these additives are limited to the lead-acid battery system and require up to a 50 weight percent addition to be effective.
For other battery systems, the cathode materials such as, MoO
3
, V
2
O
5
, Ag
2
CrO
4
and (CF
x
)
n
that are

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