HU208596B - Rechargeable electrochemical cell - Google Patents

Abstract

In rechargeable, electrochemical cells, oxygen may evolve on charge, overcharge or any reversal of polarity. The invention concerns an auxiliary, electrochemical, transfer electrode to catalyze the recombination of such oxygen with the anode mass. The auxiliary electrode comprises porous carbon bonded with PTFE and is used in a cell having a zinc anode, a metal oxide cathode and an aqueous alkaline electrolyte contacting both anode and cathode.

Description

BACKGROUND OF THE INVENTION The present invention relates to a rechargeable electrochemical cell having a cylindrical metal oxide cathode, a cylindrical zinc anode, a separator separating the anode and a cathode, a current conductor extending inside the anode, an aqueous alkaline electrolyte in contact with the anode and cathode.

Recombination of oxygen reduces the risk of water loss and excessive pressure build-up. This danger is particularly true for rechargeable and therefore hermetically sealed cells, such as those containing zinc / manganese dioxide, where the electrolyte is potassium hydroxide. Therefore, it is important to reduce the amount of oxygen, for example by reducing the oxygen to water or hydroxide groups. Of course, the active material of the anode can also be oxidized.

Thus, in cells containing aqueous electrolyte, the two most important problems to be solved are water loss and gradually increasing internal gas pressure. Oxygen gas is produced in the cell when overcharged or when the cell polarity reverses.

One effective method of oxygen removal is recombination. Various arrangements are known for this, but in many cases the rate of recombination is below expectations. For example, oxygen recombination is relatively fast in cells containing lead and lead and nickel and cadmium. In contrast, in the case of alkaline cells made with zinc anode using nickel, manganese dioxide or mercury oxide in addition to zinc, the process is very slow. For example, in the case of lead and acidic cells for oxygen recombination, K. Kordesch, ed., "Batteries, Vol. 2, Lead-Acid Batteries and Electric Vehicles" (Marcel Dekker Inc., New York, 1977) 69-71. pages, while nickel-cadmium cells are referred to in A. Salkind's "Alkaline Storage Batteries" (John Wiley & Sons, Inc., New York, 1969). However, providing the conditions for recombination itself is not an easy task, and there are numerous reports of various problems. These include leakage problems, the gradual passivation of the negative cadmium electrode, the gradual degradation of the efficiency of the negative active substance, especially when the latter forms a reserve to minimize the amount of hydrogen produced during charging and overcharging, difficulties with recombination of hydrogen and hydrogen, etc.

Three basic approaches have become known to solve these problems. These are as follows:

An oxygen cycle is created by limiting the production of hydrogen and recombining the oxygen (which occurs during charging and overcharging) at the negative electrode that is always partially discharged (for example, see U.S. Patent No. 3,258,360, 1966). . These solutions have a low recombination rate and are therefore not suitable for solving the problems of oxygen evolution.

They create conditions for the catalytic recombination of hydrogen and oxygen inside or outside the cell, with the aim of developing a solution to return the water produced to the electrolyte (1. For example, 3637778, 3598653, 3 622398 and 1972). U.S. Patent No. 3,701,691, issued in 1972). These solutions are not applicable to hermetically sealed cells since recombination and water recirculation were not possible in a closed cell such as a cylindrical structure.

The insertion of a third auxiliary electrode that provides recombination during overcharging as described by P. Ruetschi and J. B. Ockerman (Electrochem. Technol. 4, 383, 1966). Such a solution is disclosed in U.S. Patent No. 4,246,326. U.S. Pat. Here, two auxiliary electrodes are used which are electrically and ionically conductive to the negative electrode of a button cell. The structure is sensitive to the physical position of the cell and therefore requires the use of two separate auxiliary electrodes. The auxiliary electrode is provided with a double-layer separator, one hydrophilic and the other hydrophobic. A separate ion-selective ion exchange membrane is used which provides selective permeability for OH 'ions.

This heading includes heading 3 438 812. U.S. Pat. No. 5,600,125, wherein platinum, cobalt or silver are used as active catalysts. On the one hand, such catalysts have a high price and, on the other hand, their oxides are at least partially soluble in the electrolyte and enhance the corrosion of zinc and thus the development of hydrogen.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a rechargeable electrochemical cell in which oxygen recombination can be achieved with increased efficiency over prior art solutions.

When examining the various materials, it was found that using a porous carbon disc of polytetrafluoroethylene as an electrochemical transfer electrode near the anode significantly accelerates the oxygen recombination process at the anode.

In order to solve this problem, we have developed a rechargeable electrochemical cell having a cylindrical metal oxide cathode, a cylindrical zinc anode, a separator separating the anode and cathode, a conductor extending into the anode, having an aqueous and alkaline electrolyte contacting the anode and cathode, that the third electrode is formed as an auxiliary electrode disposed at the anode, separated from it by a separator, but electrically and ionically coupled thereto, which is at least partially moistened by the electrolyte and comprises a porous carbon active substance bonded to polytetrafluoroethylene. The role of polytetrafluoroethylene is to bind fine carbon particles and therefore its amount can be varied within a very wide range. The lower limit is the minimum amount required for bonding, which is about 5% by weight of the auxiliary electrode active material, and the maximum amount is the amount of active agent.

HU 208 596 Β material limit. The upper limit is about 75% by weight, but with an even greater amount of binder an effective auxiliary electrode can be made. For example, 37.5% by weight of polytetrafluoroethylene is preferred.

It is preferred that the metal oxide in the cathode is 5-20% by weight of graphite manganese dioxide, or silver oxide, nickel oxide, iron oxide, and pure manganese dioxide or 10-20% by weight of nickel oxide manganese dioxide.

The anode is preferably formed as a gelled zinc anode.

The present invention provides an inexpensive means of effectively binding oxygen generated in rechargeable electrochemical cells.

DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in more detail with reference to the accompanying drawings. In the drawing it is

Figure 1 is a longitudinal sectional view of the rechargeable electrochemical cell according to the present invention;

Figure 2 is a comparative graph of the operating parameters of an electrochemical cell according to the invention and an electrochemical cell of known construction.

A preferred embodiment of the rechargeable electrochemical cell of the present invention is shown in Figure 1. This cell has a steel cylindrical cup (10) containing a metal oxide (12) cathode. A cylindrical tip (11) protrudes from the center of the lower end face (28) of the cup (10) and forms the positive (cathode) outlet of the cell. The anode (14) of the steel bar (14) is located in a cylindrical space defined by the cathode (12) and its lower end is near the apex (11).

For example, cathode 12 is a mixture of manganese dioxide and graphite powder, and anode 14 is a gel containing zinc powder. The cathode (12) is separated from the anode (14) by a separator (16) permeable to the aqueous potassium hydroxide electrolyte used. The cathode (12) may also contain additional auxiliary components for trapping hydrogen evolved in the cell.

The anode (14) is surrounded by a basket (18) made of, for example, polyvinyl acetate. The upper part (19) of the basket (18) holds an auxiliary electrode. The auxiliary electrode (19) is a porous structure and consists of carbon powder bonded to polytetrafluoroethylene. The porous electrode body is wetted by the electrolyte. The bottom of the basket (18) is sealed, for example, by a closure (20) made of brass, which is insulated with an insulating disk (15) from the cup (10). The mechanical connection of the cylindrical cathode (12) is facilitated by a cylindrical lattice (22) and an annular plastic cover (23).

Through the lid (25) which closes the cell at the top, a current conducting pin (24) extends into the inside of the anode (14), the head (26) of which protrudes from the lid (25) and forms the negative (anode) contact of the cell. The lid (25) hermetically seals the interior of the cell by folding the rim of the cup (10).

The invention will now be further illustrated by the following exemplary embodiment.

Example

In the case of primary or rechargeable manganese zinc cells, a standard immobilized or gelled zinc (14) anode was prepared which was inserted into the cathode space by inserting a spacer made of polyethylene as shown in FIG. The anode gel was extruded into the interior of the separator (consisting of poly5 vinyl acetate manufactured by Chicopee Rayon) (16). The cell was a standard "C" size. Anode (14) was prepared in the following exemplary compositions:

(A) 61.4% by weight

2.0% by weight

1.0% by weight

New Jersey 1205 zinc containing 0.8% or 6% Hg, manufactured by Zinc Corporation of Zinc Corporation of America of Monaca, Pennsylvania,

MgO,

70/30 CMC / 940 (or Carbopol brand name), which are listed under Β. E Goodrich Chemical Group of Cleveland, Ohio Manufactures 9n KOH 8% ZnO

34.8% by weight

Total weight of the mixture: 8.7 g.

(B) 50.0% by weight of amalgamated Zn,

20.0% Ca (OH) ₂ ,

9 n KOH containing 30.0 wt% 8 wt% ZnO. Total weight of the mixture: 8.7 g.

The oxygen-reducing auxiliary electrode (19) was prepared as follows: 62.5% by weight of commercially available carbon black powder and 37.5% by weight of polytetrafluoroethylene powder were made and extruded into a 400 micrometer plate. The plate was sandwiched on one side with a separator plate (for example, a C 1235 plate from Dexter Corporation of Windsor Locks, Connecticut) and on the other side with a thin nickel mesh. The resulting three-layer plate was cut into discs of 11 mm diameter and one of these disks was placed as auxiliary electrode (19) on top of the anode (14) as shown in Figure 1, with the separator layer facing the anode (14). The anode (14) is made according to the compositions given in the example. The current conducting mandrel (24) extends deeply into the anode (14) through the auxiliary electrode (19).

In the three-layer structure, the absorbent separator layer is impregnated with the electrolyte, thereby keeping the porous carbon layer above it moist. The current-conducting mandrel (24) contacts metallically the auxiliary electrode (19) and the anode (14) to form a short-circuited "zinc-oxygen" cell.

In order to demonstrate that the present invention is suitable for oxygen recombination, two half-cells were constructed at standard cell size (C) 50, where only one half-cell contained the auxiliary electrode (19). Both open cells were placed vertically in a tube and 9 N potassium hydroxide was poured into the cathode space up to the height of the polyethylene (23) lid. The cells were galvanically discharged for 20 hours at a load of 55 to 50 mA, which was to remove 1 Ah of electrical energy from the negative electrode (total cell capacity can be set to about 4 Ah). The upper part of the cells was sealed with a lid which was filled with water-filled U-shaped tubes and allowed the internal imprinting of the sealed cells.

EN 208 596 Β. The evolution of pressure in the hermetically sealed interior after cell closure was monitored by a pressure gauge.

The cells were electroplated at 50 mA for 3 hours at room temperature. During the reaction at the negative electrode, the zinc oxide was reduced to zinc metal. During the counter-reaction, 10 ml of oxygen (50mA) were generated per hour along the surface of the cup (10). The development of the internal pressures of the cells is shown in the diagrams in Figure 2.

The curve (A) of Fig. 2 shows the increase in pressure of a conventionally constructed cell without auxiliary electrode (19). Curve (B) illustrates the evolution of the internal pressure of the cell containing the catalytically active disk, i.e. the auxiliary electrode (19). In this cell, the internal pressure after sealing was significantly reduced, which was caused by the reduction of oxygen present in the cell interior. When the cell was overcharged for 3 hours at 50 mA, each cm ^{2 of the} auxiliary electrode (19) recombined with 30 ml of oxygen gas, resulting in a lower pressure inside the cell. When the overcharge current was increased to 100 mA (at which time 20 ml of oxygen was released per hour), the pressure stabilized to a slightly higher value but still remained below atmospheric pressure. The pressure did not change substantially over an additional 2 hour period, when about 40 ml of oxygen gas was required to be recombined, since the cell produced so much oxygen under the given conditions. Measurements showed that the maximum oxygen recombination rate of the auxiliary electrode (19) was 25 ml per hour per cm ² of active surface, which corresponds to a current of 120 mA for the given cell sizes. However, this value is much higher than what would occur under normal user conditions.

The present invention is capable of absorbing oxygen produced in electrochemical cells economically and efficiently. In the case of alkaline electrolytes, carbon generally provides a sufficiently high catalytic activity to bind the evolving oxygen. Similar electrodes can be constructed of noble and noble metals which can be used at higher recombination current densities.

To illustrate the long-term properties of the electrode, the auxiliary electrode (19) containing the auxiliary electrode (19) was continuously operated at 20 mA / cm ² for 365 hours, and then the current density was increased to 50 mA / cm ² . The experiment was terminated when about 3.5 liters of normal oxygen had been consumed. Table 1 shows the values obtained at room temperature for the experiment using a 6 N KOH electrolyte when the reaction gas was normal air. If oxygen had been used, this potential would have increased by about 4050 mV.

Known laboratory methods were used to measure the internal resistance of the cell and this voltage was expressed in mV with respect to the reversible hydrogen reference electrode.

Table I

Time, clock Oxygen consumption, ml Current density, mA / cm ² Free potential, mV, against zinc 0 0 20 1126 24 100 20 1191 48 199 20 1183 96 398 20 1179 145 602 20 1174 194 805 20 1190 290 1203 20 1168 338 1406 20 1163 365 1519 50 1217 365 1519 50 1164 389 1768 50 1163 413 2015 50 1128 461 2515 50 1130 509 3012 50 1100 565 3593 50 1066

Table 1 shows that during a total of 565 hours, 3.5 liters of oxygen were consumed, although the cell voltage remained essentially unchanged throughout, meaning that the cell function was uninterrupted. The 3.5 L gas is an amount that cannot evolve over the life of any cell and therefore the auxiliary electrode (19) is operable throughout the life of the cell.

Claims (4)

A rechargeable electrochemical cell having a cylindrical metal oxide cathode, a cylindrical zinc anode, a separator separating the anode and cathode, a current conductor extending into the anode, an aqueous alkaline electrolyte and a third electrode contacting the anode and the cathode, formed as an auxiliary electrode (19) disposed at the anode (14) separated from it by a separator but connected therewith by electrical and ionic conductivity which is at least partially moistened by the electrolyte and porous carbon bonded with polytetrafluoroethylene. The rechargeable electrochemical cell of claim 1, wherein the metal oxide cathode contains 5 to 20% by weight of graphite containing manganese dioxide. The rechargeable electrochemical cell of claim 1, wherein the auxiliary electrode (19) comprises porous carbon containing 37.5% by weight of polytetrafluoroethylene. A rechargeable electrochemical cell according to claim 2, characterized in that it has a gelled zinc anode (14).