AU621580B2 - Catalytic recombination of evolved oxygen in galvanic cells - Google Patents

Description

1f PC] ANNOUNCEMENTCF THE LATER PUBLICATION OFAMENDED

(AND, WHERE APPLICABLE, STATEMENT UNDER ARTICLE 19 TY CT INTERNATIONAL .I o ,u1.iJ- uL- nv r A l urcLr uI u.cATY (PCT) (51) International Patent Classification 4 T nte io blication Number: WO 89/ 04070 HO1M 10/52 A (43 International Publication Date: 5 May 1989 (05.05.89) (21) International Application Number: PCT/US88/03810 (81) Designated States: AT (European patent), AU, BE (European patent), BR, CH (European patent), DE (Eu- (22) International Filing Date: 25 October 1988 (25.10.88) ropean patent), Fl, FR (European patent), GB (European patent), HU, IT (European patent), JP, KR. LU (European patent), NL (European patent), RO, SE (31) Priority Application Numbers: 550,335 (European patent), SU.

234,922 (32) Priority Dates: 27 October 1987 (27.10.87) 22 August 1988 (22.08.88) ublished (33) Priority Countries: CA With international search report US With amended claims o r r ,L laJims: (71 I 3 1 June 1989 (01.06.89) SECION 34 DIRECTION SEE FOLIO NAME DIRECTED a t\ &L4%O qbunvwin urivt I 74 0\Sl'^to-sea l 1 -vr i (54) Title: CATALYTIC RECOMBINATION OF EVOLVED OXYGEN IN GALVANIC CELLS 2 22 16 24 18 I Jo I i 1 2 nxg (57) 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.

ki i WO 89/04070' 1PCT/US88/03810 CATALYTIC RECOMBINATION OF EVOLVED OXYGEN IN GALVANIC CELLS FIELD OF THE INVENTION: This invention relates to galvanic cells having means to recombine evolved oxygen with the negative electrode active mass. Recombination of oxygen avoids the loss of water and reduces the risk of pressure build up in the cell. It is of particular importance in relation to sealed cells such as rechargeable alkaline, cells e.g., zinc/manganese dioxide cells with a potassium hydroxide electrolyte. The present invention may provide economic means of accelerating the consumption of oxygen by the anode. Oxygen gas build up is mitigated by oxygen reduction to OH' or water, and oxidation of the respective amount of anode active material. In all cases, the catalytically active material maintains electronic and ionic contact with the respective active battery electrode.

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WO 89/04070 PCT/US88/03810 2 BACKGROUND OF THE INVENTION: The prior art has concerned itself, for many years, with the problem of reducing or eliminating the loss of water in galvanic cells using aqueous electrolyte and avoiding build up of excessive gas pressure in sealed cells. Oxygen gas is evolved during overcharge or cell reversal.

Several galvanic couples are known with a capability of oxygen recombination cycles; however, the recombination races are not always as fast as might be desired. Pb02-Pb and Ni-Cd, for example, show high rates of oxygen recombination, while batteries employing Zn anodes (Ni-Zn, Mn02-Zn, HgO-Zn) exhibit low rates.

Reference to the theory of this lead-acid battery oxygen recombination for instance is made in "Batteries, Vol. 2, Lead-Acid Batteries and Electric Vehicles" pp. 69-61 by K.

Kordesch; and to the Ni-Cd couples in "Alkaline Storage Batteries", by A. Salkind. The reduction to practice, though, has not been easy and a variety of difficulties has been reported: such as sealing problems, negative "fade" passivation of the negative cadmium electrode with time), gradual decrease in effectiveness of the negative active material reserve which is provided in some designs in order to minimize if not entirely suppress hydrogen evolution during charge and overcharge, the general problem of coping with the recombination of nonstoichiometrically evolving oxygen and hydrogen, etc.

Three approaches are often used in efforts to

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tCT/US 88/03810 Rc'd PCT/PTO 1 SEP 1989 3 solve these problems. These approaches are: 1) Operation of the "oxygen cycle". Hydrogen evolution is suppressed and the evolved oxygen (during charge and overcharge) is recombined at the always partially discharged negative electrode 3,258,360 (1966)].

2) Catalytic recombination of hydrogen and oxygen inside or outside the battery; in the latter case, provisions are made for the return of the product water to the electrolyte chamber 3,630,778 (1971), U.S.

3,598,653 (1971), U.S. 3,622,398 (1971), U.S. 3,701,691 (1972)].

3) Use of an auxiliary (third) electrode as overcharge recombination reactors, as described in "Electrochem.

Technol.," 4, 383 (1966) by P. Ruetschi and J.B. Ockerman.

It has now been unexpectedly discovered that the addition of a disc of carbon bonded with polytetrafluoroethylene (PTFE) as an electrochemical, transfer electrode for the anode mass, has the effect to enhance oxygen recombination at the anode.

According to the present invention, there is provided a rechargeable electrochemical cell having a metal oxide cathode, a zinc anode, and an aqueous alkaline electrolyte contacting the anode and the cathode, in which cell oxygen may evolve on charge, overcharge, or any reversal of cell polarity. The cell includes an SUBSTITUTE

SHEET

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I I i; I WO 89/04070 PCT/US88/03810 4 auxiliary, electrochemical, transfer electrode for the anode. The auxiliary electrode is physically separated from the anode but is in electronic and ionic contact with it, and is at least partially wetted by the electrolyte. The auxiliary electrode comprises porous carbon bonded with polytetrafluoroethylene.

The metal oxide of the cathode may be manganese dioxide, which may be mixed with graphite in an amo-nt of from 5% to 20% by weight, or may be mixed with nickel oxide in an amount of from 10% to 20% by weight.

Alternatively the metal oxide may be nickel oxide, silver oxide or iron oxide and may be mixed with graphite in an amount of from 5% to 20% by weight.

The preisent invention may provide economic and effective means of reabsorbing oxgen gas in galvanic cells.

Embodiments of the invention will now be desc.ribed by way of illustration with reference to the .pa droawings in conjunction with the Example, describing an electr)de of the invention, and its operating characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 is a vertical cross section of a typical embodiment of the invention; Figure 2 is a graph comparing the operating characteris'tics of prior art and an inventive cell, as described in the Example.

PCT/US 3 3/ 8 Rec'd PCT/PTO 1 .EP 1989 DESCRIPTION OF THE PREFERRED EMBODIMENTS: Figure 1 of the drawings shows a typical embodiment of a cell according to the present invention.

The cell comprises a steel can 10 housing a conventional metal oxide cathode 12. The base of can 10 has a boss 11 forming the cathode contact, formed cylindrically around anode 14. The cathode 12 may comprise finely divided manganese dioxide and graphite, and is separated from anode 14 which may comprise zinc powder, by an electrolyte permeable separator 16. The electrolyte, which may be aqueous potassium hydroxide, permeates the zinc powder of anode 14, and cathode 12, through separator 16. The cathode 12 may also be provided with auxiliary cathode material to catalyse the reabsorbtion of hydrogen as described in copending application No. PCT/US88/03811 in the names of KORDESCH and

TOMANTSCHGER.

As shown, the anode is confined by a basket 18, made for example, of Chicopee" Rayon/polyvinyl acetate. The 2 0 basket 18 also carries an auxiliary, electrochemical, transfer electrode 19 comprising a disk of porous carbon bonded with PTFE. The auxiliary electrode 19 is wetted by the electrolyte. The basket 18 is provided with an end cap for example of brass, insulated from the base of can insulating disc 15. The cathode 12 is confined into cylindrical shape by screen 22 and an.iular plastic cap 23.

A current collector nail 24 projects into the anode 14 through a casing cover 25, with its head 26 being SUBSTITUTE

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iRrMS /NP u pTUS 88/03 810 Rec'd PCT/PTO 14 SEP1989 6 outside of the cover 25 to form the anode contact. The cover 25 seals the can 10 by crimping formed around its edge.

Example A conventional gelled or immobilized zinc anode as used in primary alkaline or rechargeable alkaline Mn02-Zn cells was formed, extruded into a separator basket (Chicopee Rayon/PVA), and placed in the center of a C cell can employing a polyethylene spacer, as shown in Figure 1.

Suitable Anode Compositions are:- 61.4% 3% or 6% Hg New Jersey t 1205 Zn ZnO MgO 0.8% 70/30 CMC/940 (or Carbopol") 34.8% 9 N KOH with 8% Zn0 Total weight: 8.7g 50.0% amalgamated Zn 20.0% Ca(OH)2 30.0% 9 N KOH with 8% ZnO Total weight: 8.7g An oxygen reduction electrode was prepared by forming a 400 micron layer comprising a mixure of carbon available commercially as "SHAWINIGAN BLACK"" and PTFE. A separator sheet (Dexter" C1235) was pressed in one side and 25 a Ni screen into the other side of the carbon/PTFE layer, comprising 62.5% carbon and 37.5% PTFE. A disc with a diameter of 11 mm was punched out of the foil and the carbon disc placed on the top of an anode (which was formed, for example, from either of the above compositions with the separator side facing the zinc. Thereafter, the brass nail current collector was driven through the carbon r SUBSTITUTE SHEET _0 15)k

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~s;c~ I i- i i L WO 8.9/04070 PCT/US88/03810 7 disc into the gelled anode.

The function of the separator disc is to soak up electrolyte assisting in partial wetting of the carbon disc by the electrolyte. The brass nail penetrates both electrodes assuring electronic contact of the two electrodes, and thereby establishing a "zinc-oxygen short circuit element".

To demonstrate the capability of the present invention in terms of oxygen recombination, two half cells of the C-cell size were fabricated, one with and one without the carbon disc. Both open cells were placed vertically in a tube, the cathode void was filled with 9 N KOH to the height of the polyethylene spacer and the cells were galvanostatically discharged at 50 mA for 20 hours removing 1 Ah of the negative electrodes (total .pa capacity appr. 4 Ah). Cell tops used to close the elements contained tube fittings attached to U tubes filled with water by means of flexible tubing. After crimping, the cells were gas tight and any pressure change was indicated by the manometers.

Both cells were galvanostatically charged with mA for three hours at room temperature. The negative electr6de reaction consisted of reduction of ZnO to metallic Zn. The counter reaction involved generation of oxygen on the surface of the can at a rate of 10 ml oxygen per hour (at 50 mA). Figure 2 shows the resulting pressure curves.

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~11__1_ AMD/0497a 8 Curve A represents pressure increase with time for the conventional cell without the carbon electrode. Curve B illustrates similar data for the cell containing the catalytically active disc. This cell developed a significant underpressure after cell closure, which is due to the reduction of the air oxygen present in the gas space of the cell. During the three hours of overcharge at 50 mA, the 1 square cm disc recombined 30 ml NPT of oxygen gas by maintaining layer than atmospheric pressure. An increase in the overcharge current at 100 mA (20 ml oxygen per hour) caused the pressure to stabilise at a somewhat higher value; however, still below atmospheric pressure. No significant pressure change was observed over a period .pa of two hours, indicating the recombination of additional 40 ml oxygen gas e• evolved during change. The maximum gas recombination rate was determined to be 25 ml oxygen per hour per cm 2 electrode area equivalent to an oxygen evolution current of 120 mA which, for the cell size used, is significantly o more than required under "realistic user conditions".

oe The present invention may provide economic and effective means for removing oxygen in galvanic cells. In alkaline electrolytes, carbon materials generally comprise sufficient catalytic activity for oxygen reduction Similar electrodes, using noble or non-noble metal oxide catalysts, are described in co-pending Application No. 27233/88 "Metal and Metal Oxide Catalyzed Electrodes for Electrochemical Cells, and Methods of Making WO 89/04070 PCT/US88/03810 9 Same" by K. Kordesch and K. Tomantschger. They may be employed if higher recombination current densities are desireable.

To determine the long term electrode performance a half cell containing a catalytically active disc as described was operated continously at 20 mA/cm2 for 365 hours, and thereafter the current density was increased to mA/cm2. The test was discontinued after consumption of in excess of 3.5 litres NPT oxygen. The following table demonstrates the performance obtained in 6 N KOH electrolyte at room temperature, for .pa air as reaction gas (use of oxygen led to an increase in potential by 40-50 my): time Oxygen Current IR Free Potential [hrs.] Consumption [mA/cm21 [mV vs Zn] [ml] 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 [The IR free potential is determined using laboratory procedures and standards, and is measured in millivolts as against the Reversible Hydrogen Electrode Reference].

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Claims (7)

1. A rechargeable electrochemical cell having a cylindrical metal oxide cathode, a cylindrical zinc anode, a current collector extending into said anode and an aqueous alkaline electrolyte contacting the anode and cathode, in which cell oxygen may evolve on charge, overcharge, or any reversal of cell polarity; the cell including an auxiliary transfer electrode provided for the anode and physically separated therefrom by a separator sheet but in electronic and ionic contact with said anode, said auxiliary transfer electrode being at least partially wetted by said electrolyte and thereby establishing said ionic contact between said auxiliary transfer electrode comprising porous carbon bonded with polytetrafluorcethylene; wherein said current collector pierces said auxiliary transfer electrode -and thereby establishes said electronic contact between said auxiliary transfer electrode and said anode; whereby oxygen evolved within said cell is recombined by said auxiliary transfer electrode.

2. The rechargeable cell of claim 1, wherein the metal oxide is manganese dioxide and is mixed with graphite in an amount of from 5% to 20% by weight.

3. The rechargeable cell of claim 1 wherein the metal oxide is selected from silver oxide, nickel oxide, iron oxide, and manganese dioxide.

4. The rechargeable cell of claim 3, where the metal oxide is manganese dioxide and is mixed with nickel oxide in an 9 amount of from 10% to 20% by weight.

5. The rechargeable cell of claim 1, wherein said auxiliary transfer electrode comprises a mixture of 62.5% "carbon and 37.5% PTFE.

6. The rechargeable cell of claim 2, wherein said anode is a gelled zinc anode. 0 i I AMD/0497a 11

7. A rechargeable electrochemical cell, substantially as herein described with reference to the accompanying drawings. DATED this 30th day of December, 1991. BATTERY TECHNOLOGIES INC By Its Patent Attorney DAVIES COLLISON CAVE o e fd 7 ~ro IC- III r