Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-03-15
2001-10-09
Nguyen, Nam (Department: 1753)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S068000, C429S069000
Reexamination Certificate
active
06299998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to metal-air batteries and more particularly to a movable anode design for a metal-air fuel cell battery for improved charging and discharging of the anode.
2. Description of the Related Art
Some of the problems in the past with recharging metal air systems were due to shape change of the anode, densification of the anode and dendrite formation on the anode. These anode related problems limited the life of the rechargeable system. Solutions to these problems typically involved decreasing current density (for both discharging and recharging) and depth of discharge. Both of these side-effects severely cripple the metal-air system's chance of having good energy and power densities.
Metal-air batteries have been limited in the past because there has been a trade off between high energy/power densities and good charging characteristics.
Another limiting factor in the past has been finding a bifunctional air electrode which is efficient for both recharging and discharging.
Shape change is related to the lifetime of the system. When the shape changes during each recycling the capacity of the system decreases significantly and also will cause some shorting problems.
One attempt to solve the shape change problem used a reticulated sponge-like zinc anode which increased the surface area of the zinc (decreasing current density). The lowered current density decreased the energy density of the system. Further, the reticulated sponge-like zinc anode did not prevent dendrite growth.
Dendrites grow from the anode, reach through the separator, and touch the air electrode which shorts out the cell.
Attempts to limit dendrite growth on the reticulated zinc anode included using a chemically inert coating on the exterior of the anode. This reduced the dendrite growth but the loss of anode area lowered the capacity of the cell.
Anode shape change was combated using a pump to circulate the electrolyte. By continually stirring the electrolyte a more uniform distribution of zinc ions in solution will result. A uniform mixture of zinc ions in the electrolyte will greatly reduce shape change to the anode over repeated cycling.
Another attempt to limit shape change and dendrite growth was by L. R. McCoy and L. A. Heredy in 1972 (U.S. Pat. No. 3,663,298) whereby zinc pellets and electrolyte were used to fill about ⅔ of the volume of a circular rotating drum. One of the walls of this drum was the air electrode. The drum would rotate during discharging and recharging, and the zinc particle bed would continually mix within the cell. Because the particles could move freely, fresh zinc would continually and evenly be exposed to the air electrode. This provided a longer discharge life at higher current densities by providing even depositing of zinc during recharging.
The rotatable electrode had improved rechargeability characteristics. It was found possible to recharge and discharge the rotating electrode repeatedly at rates up to 100 mA/cm
2
. Conventional zinc electrodes do not ordinarily withstand recharge rates in excess of 20 mA/cm
2
on repeated cycling without rapid failure by dendritic shorting. The high recharging rates were possible because the continual movement of the particle bed provided for a smooth, dendrite free, zinc coating on the pellets.
The rotatable electrode improved on conventional zinc/air technology, but required the use of an inefficient bifunctional air electrode.
Bifunctional air electrodes have very low cycle numbers because the electrode has to be used both for charging and discharging. It is very difficult to optimize such an electrode to function efficiently for both actions. In the past people have tried using many different catalysts and different electrode structures for bifunctional air electrodes, but the lives of the rechargeable zinc air systems are still severely limited.
Another attempt at solving the problems associated with recharging metal/air systems was in 1971 (see Fuel Cell and their Applications published in 1996, pg. 160). Sony corporation constructed a zinc/air cell containing a third electrode. The cell comprised a zinc anode sandwiched between one recharging air electrode and one discharging air electrode. The idea was to eliminate the need for a bifunctional air electrode. The zinc anode would be discharged from one side and recharged from the opposite side which optimized each electrode independently.
Sony's zinc/air cell was an improvement on the bifunctional air electrode. However, the zinc anode could only be discharged from one side, which cuts in half the power capabilities of the cell. Further, the zinc anode is charged from the side where it was discharged the least; which decreases the efficiency of the system.
Another problem with the Sony design is that the anode has to be a porous structure so that the electrolyte can flow from the discharge side to the recharge side to provide ions in solution from discharging in order to recharge again.
A patent to Faris U.S. Pat. No. 5,250,370 in
FIGS. 8 and 9
shows a rotating anode with one air electrode on one side of the anode. This is another bifunctional air electrode and it only discharges and recharges on one side of the anode disk.
SUMMARY OF THE INVENTION
The invention uses a movable anode sandwiched between two stationary air electrodes. The air electrodes are divided into a recharge air electrode portion, to maximize recharging of the anode, and a discharge air electrode portion for maximizing the discharging of the anode. The anode is moved either rotationally or linearly with respect to the air electrodes; exposing portions of the anode alternately to the recharging and discharging portions of the air electrode. Electrolyte fills the space between the air electrodes and the movable anode.
Previous solutions to the metal/air rechargeability problem could only increase cycle life at the expense of decreasing energy and power densities. The Movable Anode Fuel Cell Battery increases cycle life and discharge performance simultaneously. Recyclability will be increased because:
1) The recharging electrode is meant solely for recharging. No bifunctional air electrodes are necessary. An air electrode meant solely for recharging will not limit the lifetime of the cell. The cycle life of the cell will be limited by the lifetime of the anode.
2) The electrolyte in each cell will be continually stirred during recharging. The stirring creates an even distribution of zinc ions in solution. This results in an even plating on the zinc anode, which greatly reduces the shape change of the anode.
3) The anode will be continually moving during recharging, which greatly reduces dendrite growth and shape change. These phenomena occur because of an uneven electric field distribution on the zinc surface. If one spot has a slightly higher electric field than another, this spot. will continually attract zinc ions. However, with the anode moving, the point of peak electric field will be changing positions and moving in and out of the recharging area; reducing the chances of localized buildup. If the movement alone does not stop dendrite growth, they can be removed mechanically by a stationary wiper attached to the air electrode holder. As the anode moves past this wiper, the dendrites will be smoothed out or scraped off.
4) The recharging air electrode of the cell can be several times larger than the discharging air electrode. This will allow for fast recharging while still using a low current density. With fixed anode systems the only way to decrease charging time is to increase charging current density. High charging current density significantly decreases cycle life and turnaround efficiency. Turnaround efficiency is a ratio of the power output of a cell and the power required to charge it. A decreased turnaround efficiency means less power is required to charge the cell.
High energy density is obtained because:
1) The design of the Movable Anode cell allows the cell's weight to be dominated by the metal anode. Conseque
Morris William F.
Tsai Tsepin
Crispino Esq. Ralph J.
Nguyen Nam
Perkowski Esq. Thomas J.
Reveo Inc.
Ver Steeg Steven H.
LandOfFree
Movable anode fuel cell battery does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Movable anode fuel cell battery, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Movable anode fuel cell battery will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2602474