CA1194925A - Oxygen gas permselective membrane - Google Patents

Abstract

Abstract:

There is disclosed an oxygen gas permselective membrane comprising a film of a water-containable or wettable metallic oxide.

The oxygen gas permselective membrane according to this invention, though being very thin, does not allow water vapor and carbon dioxide gas in air to permeate there-through and has a great function for allowing oxygen gas to selectively permeate therethrough.

Description

This invention relates -to an oxygen gas permselective mem-brane which can be effec-tively used in manufacturing an air electrode for a hydrogen/oxygen fuel cell, a metal/air cell or an oxygen sensor, more specifically to an oxygen gas permselective membrane wnich permits a heavy-load discharge for a long period of time, even if it is in a thin form, and which is excellent in storage properties.

Tnere have hitherto been used gas diffusion electrodes for air electrodes sucn as various fuel cells, air-metal cells typically including air/zinc cells, and Galvanic oxygen sensors. In the ini-tial period, a thick porous electrode in wnich distributed pores have a uniform diame-ter has been used as the gas diffusion electrode. In recen-t years, however, there have often been used an electrode having a two-layer struc-ture, w`nich comprises a porous elec-trode body having an electrochemical reduction function for oxy-gen gas (a function for ioni~ing oxygen) and simul-taneous-ly having a function as a current collec-tor and a thin wa-ter repellent layer deposited integrally on the gas-side surface of the elec-trode body.

In tnis case, the electrode body may be formed mainly by incorporating a conductive powder, such as an active car bon powder carry:ing a nickel tungstate havin~ a low reduc--t.ion overvoltage -to oY~ygen gas; a tungsten carbide coated wi-th palladium-cobalt; nickel; silver; platinum or palla-r dium, in-to a porous r,letallic body, a porous carbon body or a non-woven carbon fabric material, by the use of a binder sucn as polytetrafluoroe-thylene.

Further, the aforementioned wa-er repellen-t layer, which will be deposited integrally on the gas-side surface of -the electrode body, is a porous thin membrane tha-t com-prises afluorine-con-taining resin such as polytetrafluoro-ethylene, te-trafluoroethylene-hexafluoropropylene co~oly-mer, or etnylene-tetrafluoroe-thylene copolymer, or a resin such as polypropylene, in a form of a porous material including, for instance, a sintered powder material having a particle size of from 0.2 to 40 ~m; a paper-like non-woven fabric material prepared by neat treatment of fibers comprising the above resin; a similar woven fabric mate-rial; a powder material partially replaced tne above resin by a fluorinated graphite; a film material prepared by rolling fine powder toge-ther with a pore-increasing agent or a lubricant oil, followed by heat treatment, or a film material prepared by rolling without being followed by heat treatment (Japanese Patent Publication No. 44973/
1973).

In the air electrode having such a conventional structure as mentioned above, however, the water repellent layer deposited on the qas-side surface of the electrode body is impervious to a used electrolyte but is no-t impervious -to air and water vapor in air.

For -this reason, for e~ample, water vapor in air may per~-meate the electrode body through the water repellent layer in order to dilute the electrolyte; the water in -the electrode is otherwise given off through the water repel-lent layer in order -to concentrate the electrolyte. As a result, the concentra-tion of the electrolyte will fluc-tuate and i-t will thus be impossible to maintain a stable elec-tric discharge for a long time.

_ 3 In the case that carbon dioxide gas in air permea-tes the electrode oody t`nrough the ~ater repellent layer and is adsorbed by an active layer (a porous portion of the electrode body) therein, the electrochemical reducing function of the ac-tive layer -to oxygen gas will reduce at this position in order to exert a bad lnEluence upon a heavy-load discharge. Moreover, when an alkaline elec-trolyte is used, there will occur phenomena such as change in properties of the elctrolyte, reduc-tion in the concentration of the electrolyte and, when a used cathode is zinc, passivation of the zinc cathode. Furthermore, in such a case as mentioned above, a carbonate will be formed in the active layer -to close some pores and to thereby decrease the region ~nere an electrochemical reduction is carried out, whieh faet will lead to hin-dranee in the heavy-load diseharge.

A cell having such a strueture above will deteriorate in performance below a eertain design standard, when stored for a long period OL tlme or when used for a prolonged period.

In order to overeome sueh disadvantageous problems, there has been proposed a new-type eell in whieh a water repel-lent layer of an air eleetrode is provided, on the gas side (air side) thereof, with a layer eomprising a water~
absorbing agent such as ealeium ehloride or a earbon di-oxide gas-absorbing agent sueh as a hydroxide of an alka-line earth metal (Japanese Patent Publiea-tion No. 8411/
1973)~ This type of eell ean prevent the above-mentioned disadvantageous problems -to some extent, bu-t when the 3a absorbing agent has been saturated with water or earbon dioxide gas after a eertain period of time, its funetion will be lost and its effeet ean be expected no more.
After all, such a sugges-ted eell eannot solve the afore-mentioned problems basieally.

Further, it nas been attempted to laminate integrally, cn the above-mentioned wa-ter repellent layer, an oxygen perm-selective thin membrane such as a polysiloxane membrane (Japanese Pa-tent Publication No. 26896~1973). i-lowever, any sufficiently effective oxygen gas permselective membranes have not been developed yet at present.

An object of this invention is to provide an oxygen gas permselective membrane which is excellent in the function o. allowing oxygen gas to selectively permeate. There-fore, when applied to an air electrode, the oxygen gas permselective membrane according to this invention can prevent water vapor or carbon dioxide gas in air from permeating the air electrode body, thus permits a heavy-load discharge for a long time, and enables the manufac-ture of a thin air electrode having excellent storage properties.

With respect to this invention, a firs-t aspect is directed to an oxygen gas permselective membrane comprising a tnin layer made of a water-containable or wettable metallic oxide, a second aspect is directed to a composite membrane having a two-layer struc-ture in which a -thin layer made of a water-containable or wettable metallic oxide is integrally deposited on ei-ther surface of a porous mem-brane of 0.1 ~m or less in pore size, and a third aspect is directed to a composite membrane having a three-layer structure in which a water repellent layer is integrally in-terposed between the porous membrane and the thin layer of metallic oxide.

Now, this invention will be further described in detail as follows:

The wa-ter-containable or wettable metallic oxide used in this invention means a material having the ability to adsorb water and having properties for permi-tting the water adsorbed thereon to exist orienting hydroxyl groups -thereof -to the surface of the oxide as chemically and 3 ~

pnyslcally adsorbecl water. In tn:is specification, the water-containable (wettable) properties mean tne phenome-non that a metallic oxide exists in combina-tion wi-th water molecules, or in a st~te having an interaction with water molecules. Such metallic oxides are stannic oxide (Sn~2), zinc oxide (ZnO), aluminum oxide (~23)' magnesium oxide (~gO), calcium oxide (CaO), s.rontium oxide (SrO), barium oxide (BaO), titanium di-oxide (TiO2) and silicon dioxide (SiO2), and they may be used alone or in the form of a composi-te cornprising an optional combination of two or more kinds thereof.

In this connection, it is preferred -tha-t the film has a thickness of 0.01 to 1.0 ~m. If the thickness of the film is less than 0.01 ~m, pin-holes will -tend to often appear in the formed film, the effect of preventing water vapor or carbon dioxide gas from permeating the electrode will be lost, and simultaneously the mechanical s-trength of the film will be deteriorated and it will be :Liable to break. In contrast, if the -thickness of the fi]m is more than 1.0 ~m, the amount of oxygen gas to be allowed to permeate -therethrough will be reduced, which fact will de-teriorate a heavy-load discharge function of -the pre-pared elec-trode.

In the composite membrane according -to this invention, any porous rnaterial may be employed for the porous mem-brane so long as it has fine pores as small as 0.1 ~m or less in pore size. In view of the fact -tha-t the porous membrane will be deposited on the electrode body, it is preferably rich in flexibility. Further, the preferred porous membrane has its fine pores distribu-ted in a uni-form state, and it is also preferred that the propor-tion of the s2ace volume of -the fine pores to the total volume of the membrane is wi-thin the range of 0.1 to 90 %.

Examples of such porous membrane include a porous fluoro-resin memorane (Fluoropore ~ (trademark) made by Surn:i-tomo ~lectrl.c Ind., Ltd.), a porous polycarbonate membrane (Nuclepc)re (~.aderlark) made by Nuclepore Corp.), a porous cellulose e~ter membrane (~lillipore l~embrane Fil-ter (lrn~dcmarlc) made by Millipore Corp.) and a porous poly-propylene membrane (Celgard(trademark) made by CelanesePlas~ics Company). ~hen the pore size of the porous mem-brane e~ceeds a level of 0.1 ~Im, a pin-holes will very ofterl occur in a film made of a metallic oxide or a water repellent layer, which will be described hereinaf-ter, deposited on the porous membrane. As a result, the effect of preventing water vapor or carbon dioxide gas from permeating the electrode will be lost, and the layer will be reduced in mechanical strength and will be liable to brea'-.

~ext, a material constitutin~ the water repellent layer should have water repellent proper-ties and electrolyte-resistant properties, and examples of such practiciable materials include polytetrafluoroethylene (PTFE), fluoro-ethylene-prop~lene (FEP), polypnenylene oxide (PPO), poly-phenylene sulfide (PPS), polyethylene (PE), polypropylene(PP), copolymers thereof, and mixtures thereof.

In a material to be thermally fused and bonded sucn as fluoroethylene-propylene (FEP), polyethylene (PE) or ethylene-tetrafluoroethylene copolymer is used for tne water repellent layer, the mechanical stren~th of the prepared composite membrane can be increased wit;~ -the aid of a suitable thermal trea-tment.

Examples of materials for the water repellent layer used in -this inven-tion include, in addition to the above-30 mentioned ones, a variety of organic compounds which is forrned on -the porous memkrane as a thin film by means of a plasma polymerization, for example, fluorinated organic compounds such as benzotrifluoride, m-cnlorobenzotri-fluoride, hexafluorobenzene, pentafluorobenzene, penta-fluorostyrene, and mixtures thereof; and hydrocarbon series compounds such as Cl to C12 saturated hydrocarbon compounds, Cl to C12 unsaturated hydrocarbon compounds, Cl to C14 alkylbenzene com?ounds,styrene, ~-methylstyrene and mixtures thereof. The layers all comprising these recited materials do not allow pin-holes to appear and are excellent in selective permeability to oxygen gas.
Particularly, the aforementioned fluorinated organic com-pounds are more useful, because their water repellent layers prepared by -the use of the plasma polymerization of tneir monomolecules are excellent in the effect of pre-venting water vapor or carbon dioxide gas from permeating -tne electrode. The thickness of the practicable water repellent layer is preferably within the range of 0.01 to 1.0 ~m, and when tne thickness is less than Q.01 ~m, the wa~er repellent layer will be formed in a mottling state and thus cannot cover uniformly the surface of the porous membrane, which fact will lead to the decrease in tne effect of innibiting the permeation of water ~apor or carbon dioxide gas through t~e electrode, and accord-ingly the mechanical strength of the whole layer wilideteriorate. Conversely, when the thickness of the water repellent layer is in excess of 1.0 ~m, the amount of oxy-gen gas to be fed to the electrode will be insufficient with the result that the electric discharge properties of the prepared electrode will deteriorate (i.e., the heavy-load discharge will become difficult).

Further, the water repellent layer may be formed in the style of a single layer, but on this layer a thin layer comprising an organic compound other than tne material of the former layer may be superincumbently formed.

On the thus formed water repellent layer, the film of the water-containable or wettable metallic oxide is to he further superimposed. The thickness of the oxide film is preferably within the range of 0.01 to 1.0 ~m for the same reason as in the case of the water repellent layer.

The oxyg~n gas permselective membrane according to thLs invention may be prepared as follows:

Yirst, in the case o the oxygen gas permselective mem-brane co~prising the -thin layer made of the water-con-tainable or wettable metallic oxide, the deposition of thefilm may be carried out preferably by a deposition process or sputtering process which is prevalent as a film-forming process. When the deposition process is employed, the film may be formed, for example, the material which is -to be formed a film is set on the vacuum depositing equip-ment, the temperature therein is maintain at 150 C and the partial pressure of oxygen in the equipment is adjust to 5 x 10 Torr using a metal as a deposition source which is forma~le an aforementioned metallic oxide. And in the case of the sputtering process, forming the film may be accomplished, for example, by use of the water-containable or wettable metallic oxide as a sputtexing source in a mixed gas of argon and oxygen (ArO 90 vol%,

2 10 vol%) having a pressure of 2 x 10 3 Torr and at a high-frequency power of 100 ~.

Second, in the case of the composite membrane having a two-layer structure, a film of the water-containable or wettable metallic oxide may be deposited directly on either surface of the aforementioned porous membrane in the same procedures as mentioned above.

Third, in the case of a composite membrane having a three~
layer structure, the water repellent layer is formed on either surface of the porous membrane, and the film of the water-containable or wettable metallic oxide is then 3a deposited on the just prepared water repellent layer by applying such a cleposition process or sputtering process as in the case of the oxygen gas permselective membrane of the single layer structure described above.

In the respective cases of the above-mentioned three structures, the metallic oxide itself can be applied as a deposition source of sputtering in forming the film of the water-containable or wettable me-tallic oxide. ~IOw-ever, it is preferred that a metallic simple subs-tance for proaucing a metallic oxiae by a reaction with oxygen is used as the deposition soruce or sputtering source and a used atmos~here contains oxygen, because under such con-ditions, the rate of forming the film of the metallic oxide will be accelerated and the operation of forming the film will become easy.

AII air electrode in which the oxygen gas permselec-tive me~rane according to this invention is used may -take, for example, the follo~ing constitution:

The air electrode including the oxygen gas permselective membrane according to this invention comprises a porous electrode body having an electrochemical reducing func-tion to oxysen gas and simultaneously having a current collecting function, and the film of the water-containable or wettable metallic oxide which is, integrally and di-rectly or via a porous membrane, deposited on the gas-side surface of the electrode body. Manufacturing the air electrode can be carried out by depositing the film of the water-containable or wettable metallic oxide on the gas-side surface of the porous electrode body having the electrochemical reducing function to oxygen gas and simultaneously having- the current collec-ting fuction by means of a deposition process or sputtering process.
Alternatively, the air electrode can be otherwise manu-factured by depositing the film of the water-containable or wettable metallic oxide on one surface of the porous membrane of 0.1 ~m or less in pore diameter by means of the deposition process or sputtering process, and by com-pressedly bonding integrally another surface of the po-rous membrane to the gas-side surface of the electrode body having the electrochemical reducing function to oxy-gen gas and simultaneously having the current collecting function.

The elec-trode body used in the air electrode with respect to this invention has an active function for reducing electrochemically oxygen gas (for ionizing oxygen gas), and the body is further conductive as well as porous.
Materials for the electrode body include, for example, in addition to the aforesaid materials, a silver filter, Raney nickel, a sintered body of silver or nic]~el, a variety of foamed metals, a nickel-pla-ted and pressed stainless steel thin wire, and a metallic porous material obtained by plating the thus treated stainless steel with gold, palladium or silver. For the purposes of removing promptly the reduced ionic products of oxygen gas, which have been produced by the electrode reaction in the pores of the electrode body, ~rom these pores (reaction range~, and of permitting a heavy-load discharge of, for example, 50 mA/cm or more to smoothly continue, it is preferred that the pores distributed in the electrode body have a pore si~e of 0.1 to 10 ~m or so.

The air electrode just described has the structure that the film made of the water-containable or wettable metal-lic oxide is integrally deposited, directly or via a porous membrane, on the gas-side surface of such an elec-trode body as mentioned above.

In order to deposit integrally the film of the water con-tainable or wettable metallic oxide on the gas-side sur face of the electrode body, the following procedures may be applied:

A first procedure comprises depositing directly the water-containable or wettable me-tallic oxide on the gas-side surface of the electrode body in an ordinary film-forming manner such as a vacuum deposition process or sputtering process in order to form the film having a desired thick-ness on the electrode body.

A second procedure comprises depositiny directly the film of the water-contair.able or wettable me-tallic oxide on one surface of the porous membrane of 0.1 ~m or less in pore size by means of the deposi-tion process or sputter-ing process in order to prepare a composite membrane ofa two-layer structure, and bonding compressedly and inte-grally another surface of the porous membrane, i.e. the surface, opposite to the surface having the film, of -the composite membrane, to the gas-side surface of the elec--trode body under a predetermined pressure.

In the respec-tive cases of the firs-t and second proce~
dures mentioned above, the water-containable or wettable metallic oxide itself can be applied as a deposition souxce or sputtering source in forming the film of the water-containable or wettable metallic oxide. However, it is preferred that a metallic simple substance for producing a metallic oxide by a reaction with oxygen is used as the deposition source or sputtering source and a used atmosphere contains oxygen, because under such con-ditions, the rate of forming the film of the metallicoxide will be accelerated a nd the operation of forming it will become easy.

Further, it is preferred that the film of the water~
containable or wettable metallic oxide is adjusted -to the range of 0.01 to 1.0 ~m in thickness. If the thickness of the film is less than 0.01 ~m, pin-holes will increase and the effect of preventing water vapor or carbon dioxide gas from permeating the electrode will be reduced, and the mechanical strength of tne layer will be de-terioxated, so that it will be liable to break. In contrast, if the thickness of the thin membrane is more than 1.0 ~m, the amount of oxygen gas to be allowed to permea-te there-through will be reduced, which fact will render difficult the heavy-load discharge of the electrode.

Furthermore, for the porous membrane used in the second procedure described above, any material may be eMployed so long as the pore diameter is as small as 0.1 ~m or less. Examples of such porous membranes include afore-mentioned porous men~branes of a porous fluororesin mem-brane (Fluoropore (trade name) made by Sumitomo Elec-tric Ind., Ltd.), a porous polycarbonate membrane (Nuclepore (trade name) made by ~'uclepore Corp.), a porous cellulose ester membrane (Millipore Membrane Filter (trade name) made by Millipore Corp.) and a porous polypropylene mem-brane (Celgard (trade name) made by Celanese PlasticsCompany). When the film of water-containable or wettable metallic oxide is deposited on the porous membrane which embraces pores having a diameter more than 0.1 ~m, pin-holes will very often occur in the Eilm, so that the function of the film will be lost and its mechanical st-ength will decrease, which fact will lead to the dis-advantage that the film will be liable to break.

The thus preparea air electrode may be incorporated into a cell according to an ordinary manner. In this case, in order to permit the supply of momentary large current by the electrocnemical reduction of an electrode-consti-tuting element itself in addition to the electrochemical reduction of oxygen gas, it is preferable to deposit integrally, on the electrolyte side of the electrode body, a porous layer containins at least one of a metal, an oxide or a hydroxide in which oxidation state can vary by a more ignoble potential in the range of 0.4 V than the oxidation-reduction balanced potential of oxygen.
This porous layer can be oxidized wi-th oxygen gas by a local cell action during discharge at a light-load or a-t the time of open-circuit to reiurn to the oriyinal oxida-tion sta-te. Examples of materials constituting such po-rous layer include Ag2O, MnO2, Co2O3, PbO2, a variety of perovskite type oxides and spinel type oxides.

The air electrode may be incorporated into a cell not only in a plate form but also in a cylindrical form. In the latter case, the pla-te air elec-trode may be bent -to a cylinder-shape. For the pur?ose of imparting mechani-cal stability to the air electrode so that it may be guarded from breakage during the above bending operation, tne film of the water-containable or wettable metallic oxide is preferably further deposited, on the gas-side surface thereof, integrally with a porous thin membrane such as a porous fluororesin membrane, a porous polycar-bonate membrane, a porous cellulose ester membrane or a porous polypropylene membrane.

Now, this invention will be described in detail in accord-ance with the following Examples.

Examples 1 to 9 Each porous polycarbonate membrane in which the fine pores naving an average pore si~e of O.Q3 ~m are uniform-ly distributed and the pores of which have as much a space volume as 0.42 % (Nuclepore (trade name) made of Nuclepore Corp. and having a thic~ness of S ~m~ was sub-jected to a sputterins treatment by use of Sn, Zn, AQ, Mg, Ca, Sr, Ba, Ti or Si as a sputtering source in a mixed gas of argon and oxygen (consistins of 90 % by volume of Ar and 10 % by volume of 2) naving a pressure of 2 x 10 3 Torr and at a high-frequency power of 100 ~, in order to deposit each film of various water-containable or wettable metallic oxides on either sice of the poly-carbonate membrane, with the thickness of the ob-tained film being 0.2 ~m.

Examples 10 -to 18 Sputtering of fluoroethylene propylene (FEP) was carried ou-t for the same type of polycarbonate membranes as in Examples 1 to 9 in an argon aas having a pressure of 1 x 10 Torr and at a high-frequency power of 200 W in order to deposit a water repellent layer of 0.2 ~m in thickness on either surface Oc each membrane. Ihe same procedure as in E~amples 1 to 9 was then repeatea to further deposit each film having a thickness of G.2 ~m of various water--con-tainable or wettable metallic oxides on the water repellent layer already prepared on the polycar-bonate membrane.

Examples 19 to 27 The same type of polycarbonate membranes as in Examples 1 -to 9 was placed in a plasma reaction tank, a high-frequency power of 13.56 MHz was applied to the tank from outside, an argon gas and a monomer gas of penta-fluorostyrene were introduced into the tank at a flow rate of 600 mQ/min, and a plasma polymerization reaction was then carried out therein under the condition of a radio-frequency output of 0.4 W/cm2 in order to deposit a 0.2-~m-thick layer of the pentafluorostyrene polymer on either surface o~ each polycarbonate rmembrane.

Further, each film (0.2 ~m) of various water-containable or wettable metallic oxides was then deposited on the already prepared layer in the same manner as in Examples 1 to 9.

The thus obtained 27 kinds of composite membrane were measured for oxygen-permeation rates ~J2 cc/sec-cm2~
cmHg) and carbon dioxiae gas-permeation rates (JCO2: cc/
sec-cm2-cmHg) in accordance with an equable pressure methoa in which a gas chromatograph is employed as a detecting means, and for water vapor-permeation rates ( H2O: cc/sec-cm cmHg) in a manner corresponding to JIS Z 0208 (a cup method). Afterward, ratios (JO2/Jrl2O
30 and 2/ CO2) of H2O and JC2 to J2 were calculated out, which ratios can be taken as gas permeation ratios.

For comparison, the measurements of 2~ H2O and CO2 were similarly carried out for a polysiloxane membrane (Comparative Example 1) of 50 ~m in thic]cness, an inter-mediate density polyethylene membrane of 20 ~m in thick-ness (Comparative Example 2), a biaxially orientated poly-propylene membrane (Comparative Example 3) of 20 ~m in thickness, a polytetrafluoroethylene membrane (Compara-tive Example 4) of 20 ~m in thic~ness, a commercially available FEP membrane (Comparative Example 5) of 20 ~m in thickness, and an FEP membrane (Compara-tive Example 6) of 0.2 ~m in thickness which was deposited by -the same sputtering process as in Examples 10 to 18. And the ratios of J2/JH2 an~ Jo2~Jco2 were likewise calcula-ted out.

Obtaineci results are set forth all together in Table 1 below:

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Examples 28 to 36 There were used, as electrode bodies, Raney nickel paltes (200 ~m in thickness) where the average pore size of each plate was 5 ~m and its porosity was 80 %. Each plate was set on a vacuum depositing equipment, the temperature therein was maintained at 150C, and the partial pressure of oxygen in the equipment was then adjusted to 5 x 10 3 Torr. As deposition sources, 9 metals of Sn, Zn, AQ, Mg, Ca, Sr, Ba, Ti and Si were each selected.

An ordinary deposition process has been employed to di-rectly deposit each aforementioned metal on either surface of the Raney nickel plate. In every case, a metallic oxide of 0.2 ~m in thickness was deposited on the surface of the-Raney nickel.

The thus obtained metallic oxide-deposited Raney nickels were dipped into a 2 % palladium chloride solution and were subjected to cathodic polarization in order to deposit thereon a palladiurn layer having a thickness of about 0.5 ~m inclusive of palladium in pores on the ~aney nickel plate. The thus obtained products are air elec-trodes.

Examples 37 to 45 The same manner as in Examples 28 to 36 was repeated except that the deposition process was replaced with a sputtering process. The sputtering treatment was carried out in a mixed gas of argon and oxygen (consisting of 90 % by volume of Ar and 10 % by volume of 2) a-t a pressure of 2 x 10 Torr and under a high-frequency power of 100 W. Every film of the metallic oxide had a thickness of 0.2 ~m.

Examples 46 to 5~

3'~

Porous polycarbonate membranes (Nuclepore (trade name) mace by Nuclepore Corp.) in which pores of 0.03 ~m in average diameter were uniformly distributed were each set on a vacuum deposi-ting equipment and an ambient temperature was maintained at 100C. The partial pres-sure of oxygen in the equipment was adjusted to a level of 5 x 10 3 Torr, and the same deposition sources as used in Exampl.es 28 -to 36 were applied in order to deposi.t each 0.2-~m~thic~ film of the metallic oxides on either surface of the membrane. Afterward, each porous membrane was compressedly bonded, on another surface thereof, to ei-ther surface of the Raney nickel plate (200 ~m in thick-ness) having an average pore size of 5 ~m and a porosity of 80 %~

The thus treated Raney nickel plates were each dipped into a 2 % palladiurn chloride solution and were each sub-jected to cathodic polarization in order to deposit there-on a palladium layer having a thickness of about 0.5 ~m inclusive of palladium in pores on the Raney nickel plate.
The thus ob-tained proaucts are air electrodes~

Examples 55 to 63 The sarne manner as in Examples 46 to 54 was repeated except that the deposition process was replaced witi~ the sputtering process under the sarne spu-ttering conditions as used in Examples 37 to 45 in order to prepare air elec-trodes.

Comparative Example 7 After active carbon powder was suspended in an a~ueous palladium chloride solution, reduction treatmen-t was carried out by the use of formalin in order to obtain the active carbon powder wearing palladium. Then, the obtained powder was subjected to a waterproof -treatment with a 10 to 15 % polytetrafluoroethylene dispersion, was mixed wi-th a PTFE powder as a binding a~ent, and was -then rolled to a sheet. The thus prepared sheet was com-pressedly bonded to a nickel net in order to obtain an electrode body of 0.6 mm in thickness. On the other hand, an artificial graphite powder was mixed with a PTFE
dispersion, followed by a heat treatmen-t to prepare a waterproof graphite powder. A PTFE powder which was a binding agent was mixed with this graphite powder, and the resultant mixture was then rolled to a sheet~ The thus obtained sheet was compressedly bonded to the already obtained electrode body in order to prepare an air elec-trode of 1.6 mm in thickness.

Comparative Example 8 A polysiloxane membrane (50 ~m in thickness~ which was a membrane for allowing oxygen gas to selectively permeate therethrough was compressedly bonded to either surface of a Raney nickel plate (200 ~m in thickness~ having an aver-age pore diameter of 5 ~m and a porosity of 80 %, and the whole membrane was subjected to catnodic polarization in a 2 ~ palladium chloride solution in order to deposit thereon a palladium layer having a thickness of 0.5 ~m inclusive of palladium in pores on the Raney nickel plate.
The thus obtained product is the air electrode.

Comparative Example 9 A wa-ter vapor-absorbing layer comprising calcium ch]oride was deposited on the air-side surface of the air electrode prepared in Comparative Example 7.

Comparative Example 10 A thin layer of SnO2 having a thickness of 0.2 ~m was deposited on one surface of a 5-~m-thick porous polycar-bonate membrane (Muclepore (trade name) made by Nuclepore Corp.) in which pores of 0.15 ~m in average pore size 2~ 5 - ~3 -were distributed, and another surface of the membrane above was compressedly bonded to either surface of a ~aney nickel plate having an average pore diameter of 5 ~m and a porosity of 80 ~. The whole plate was dipped into a 2 ~ palladium chloride solution and was subjected to cathodic polarization in or~er to deposit thereon a palladium layer of about 0.5 ~m in thickness inclusive of palladium in pores on the Raney nickel plate. The thus obtained product is an air electrode.

Comparative Example 11 The same manner as in Comparative Example lQ was repeated with the exception that there was employed a porous poly-carbonate membrane having an average pore size of 0.03 ~m and was deposited a film of SnO2 having a thick-ness of 0.005 ~m in order to prepare a desired air elec-trode.

Comparative Example 12 The procedure described in Example 11 was repeated except that a film of SnO2 having a thickness of 2.0 ~m was de-posited in order to prepare an air electrode.

Next, an air-zinc cell was assembled by the use of each of 42 air electrodes thus prepared above, an opposite electrode of a geled zinc which was amalgamated with 3 ~
by weight of mercury, an electrolyte of potassium hydro-xide, and a separator of a polyamide non-woven fabric material.

These assembled 42 cells were then allowed to stand in air at 25C for 16 hours. Afterward, measurement was carried out for the current density of each cell at the time when a terminal voltage dropped below 1.0 V after 5 minutes' discharge under a variety of currents.
Further, each cell was stored in an atmosphere at 45C

- ~4 -and at a relative humidity of 90 %, and the leakage state o a used elec-trolyte was observed.

Moreover, the same discharge test as mentioned above was carried out for each cell which had undergone the above storage step, and the proportion (%) of a curren-t value at this test time to an initial current value was calcu-lated out. The thus calculated values each represent a degradation level of the air electrode in the cell and can be taken as a mainte-nance proportion of its discharge properties. In other words, it can be meant that the greater this value, the smaller the deterioration in the air electrode is.

Further, the film deposited on each electrode was measured for a permeability rate to oxygen gas in accordance with an equable pressure method in which a gas chromatograph is employed as a detecting means, and for a permeability rate to water-vapor in a manner corresponding to JIS z 0208 (a cup method). Afterward, ratios of both the rates were calculated out.

The obtained results are set forth in Table 2 below:

Table 2 h'ater-containable Porous Deposition Current Mainte- Gas per-E;lectrode or wettable membrane manner of density nance meation body metallic oxide film propor- ratio of (pore size: ( A/ 2) tion of filrn Type Thick-~rype Thickness 7,l!n) properties (llm) (%) (02/H20) Example 28 ~liC'~el SnO2 .2 Absent process 9 7 1.9 " ~3 " " ZnO " " " 57 74 1.6 " " AQ203 " " " 58 73 1.8 e~
" 31 " " MgO " " " 56 74 1.5 ~"
" 32 " " CaO " " " 56 74 1.6 " 33 " " SrO " " " 55 73 1.5 34 " " BaO " " " 55 73 1.5 " 35 " " TiO2 " " " 58 73 1.7 " 36 " " SiO2 " " " 57 74 1.8 Table 2 (contd) E:l t d Water-containable Porous Deposition Current Mainte- Gas per-ec ro e or wettable membrane manner of densi'y nance meation body metallic oxide film propor- ratio or (pore si~e: tion of film Thick- Thickness ~ 2 discharge \ Type nes5Type (~Im) l~m) ~ / ) properties (o /H O) (llm) P nickel Srl02 0.2 Absent process 2.0 " 38 " " ZnO " " " 57 74 1.7 " 39 " " A~203 " " " 58 73 1.9 " 40 " " MgO " " " 56 74 1.6 " 41 " " CaO " " " 56 74 1.7 " 42 " " SrO " " " 55 73 1.6 " 43 " " BaO " " " 55 73 1.6 44 ~ ~ TiO2 ~ 58 73 1.8 " 45 " " SiO2 " " " 57 74 1.9 Table 2 (contd) \ -1 tnode ~1ater-containable Porous Deposition Current Mainte- Gas per-ec _ or wettable membrane manner of density nance meation body metallic oxide film propor- ratio of \ 2 tion of film \ Type Thick- Tyve Tnickne5s (pore size:(mA/cm ) discharge - ness ~ m) ~m) properties 2 2 present:
Example 46 R~-kYl 200 SnO2 0.2 ~ bonding 59 76 1.9 after diposition " 47 " " ZnO " " " 57 75 1.6 " 48 A~ O " " " 58 74 1.8 " 49 " " MgO " " " 56 75 1.5 ~
" 50 " " CaO " " " 56 75 1.6 f~-" 51 " " SrO " " " 55 74 1.5 " 52 " " BaO " " " 55 74 1.5 53 " "TiO2 " " " 58 74 1.7 " 54 " " ~i2 " " " 57 75 1 8 Table 2 (contd) \ ~-ater-containable Porous Deposition Current Mainte- Gas per-\ Electrode or wettable membrane manner of density nance meation \ body metallic oxide film propor- ratio of \ 2 tion of film \ l'ype Thick- Type Tnickness (pore size: (~A/cm ) iS~erties (O2/H2O) \ (~m) ~m) (%) Example 55 Raney 200 SnO 0.2 Poeo3e ~m Compressive 59 77 2.0 nlckel 2 ~onding after sputtering " 56 " " ~nO " " " 57 76 1.7 57 " " A~2O3 " " " 58 75 1.9 58 " " MgO " " " 56 76 1.6 " 59 " " CaO " " " 56 76 1.
" 6G " " SrO " " " 55 75 1.6 " 61 " " BaO " " " 55 75 1.6 62 " ~ Ti2 " " " 58 75 1.8 " 63 " " SiO2 " " " 57 76 1.9 f ri~

to~

Table 2 (contd) Water-containable Porous Deposition Current Mainte- Gas per-Electrode or wettable membrane manner of density nance meation body metallic oxide film propor- ratio of \ (pore size: tion of film ThlCk- ThlCkneS5 ( IA / ) ciisc arge Type ness Type (~m)~m) ~ ,cm properties (2/~2) ~ (~m) (~) Compara- Active Graphite + Compressive tive carbon 600 - - PTFE bonding 25 40 ~mpl ~ 7 with Palla- ~d dium ~
e~
8 Raney 200 Polysiloxane membrane " 50 G0 0.032 nickel ,~
Active ~_ carbon " 9 with 600 Graphite powder + PTF`E " 20 50 Palla-di~m present:
" 10 Raney 200 SnO2 .2 0.15 -~m bonding 59 42 after deposition present:
" 11 " " ' 0-005 0.03 ~m " 58 41 " 12 " " " 2.0 " " 12 ~0 Examples 64 to 72 There were used, as electrode bodies, Raney nickel pla-tes (200 ~m in thic]cness) where the average pore size of each plate was 5 ~m and its porosity was 80 %. Sputtering of fluoroethylene propylene (FEP) was carried out for the cne side of the Raney nickel plate in an argon gas having a pressure of 1 x 10 Torr and at a high-frequency power of 200 W in order to deposit a water repellent layer of 0.2 ~m in thickness on either surface of each membrane.

After each plate was set on a vacuum depositing equipment, the temperature of -the side of FEP water repellent layer was maintained at 100C, and the partial pressure of oxy-gen in the equipment was adjusted to 5 x 10 3 Torr. As deposition sources, 9 metals of Sn, Zn, AQ, Mg, Ca, Sr, Ba, Ti and Si were each selected. An ordinary deposition process has been employed to directly deposit each afore-mentioned metal on either surface of the FEP water repel-lent layer. In every case, a metallic oxide of 0.2 ~m in thickness was deposited on tne surface of the FEP water repellen-t layer.

The thus obtained metallic oxide-deposited plates were dipped into a 2 ~ palladium chloride solution and were subjected to cathodic polarization in order to deposit thereon a palladium layer having a thickness of about 0.5 ~m inclusive of palladium in pores on the Raney nickel plate. The thus obtained products are air electrode.

Example 73 to ~1 The same manner as in Examples 64 to 72 was repeated except tha-t the deposition process was replaced with a sputtering process when the wa-ter-containable or wettable metallic thin layer was formed on the surface of the FEP
water repellen-t layer. The sputtering treatment was carried out in a mixed gas of argon and oxygen (consisting of 90 % by volume of Ar and 10 ~ by volume of 2) at a pressure of 2 x 10 Torr and under a high-frequency power of 100 W. Every film of the metallic oxide had a thick-ness of 0.1 ~m.

Examples 82 to 90 To one side of porous polycarbonate membrane (~uclepore (trade name) made by Nuclepore Corp.) in which pores of 0.03 ~m in averase diameter were uniformly distributed, sputtering of fluoroe-.hylene propylene (FEP) was carried out in an argon gas having a pressure of 1 x 10 Torr and at a high-frequency power of 200 W in order to deposit a water repellent layer of 0.2 ~m in thic]cness on either surface of each membrane.

After each plate was set on a vacuum depositing equip-ment, the temperature of the water repellent layer was maintained at 100C. And the partial pressure of oxygen in the equipment was adjusted to 5 x 10 3 Torr in order to form a thin layer of water-containable or wettable metallic oxide on the surface of the water repellent layer using the same deposition source as in Examples 64 to 72. In every case, a thin layer of 0.1 ~m in thick-ness was deposited on the surface of the FEP water repel-lent layer.

The thus obtained porous polycarbonate membrane of the composite thin membrane was compressedly bonded -to either surface of the Raney nickel plate (200 ~m in thickness) having an average pore size of 5 ~m and a porosity of 80 %.

The thus obtained metallic oxide-deposited plates were dipped into a 2 % palladium chloride solution and were subjected to cathodic polarization in order to deposit thereon a palladium layer having a thickness of about 0~5 ~m inclusive of palladium in pores on the Raney nickel plate. The thus obtained products are air electrodes.

Example 91 -to 99 The same manner as in Examples 82 to 90 was repeated except that the deposition process was replaced with a sputtering process when -the water-containable or wettable metallic thin layer was formed on the surface of the FEP
water repellent layer. In this case, the sputtering treatmen-t was carried out ~n a mixed gas of argon and oxygen (consis-ting of 90 ~ by volume of Ar and 10 % by volume of 2) at a pressure of 2 x 10 3 Torr and under a high-fre~uency power of 100 W. Every thin layer of the metallic oxide had a thickness of 0.1 ~m.

These assembled 36 cells were then allowed to stand in air at 25C for 16 hours. Afterward, measurement was carried out for the current density, tne maintenance proportion of their discharge properties and the gas permeation ratio of the film of each cell as the same procedures in Examples 28 to 63.

The obtained results are set forth in Table 3 below:

Table 3 \

Water-containable Electro~e hlater repel or wettable Porous Deposition Current Gas per-~cdy lent layer m~tallic o,Yide membrar~ manner of density nar oe meatiGn \ thin layer film propor- ratio of \ (present tion of flLm \ Thi~c- ~hic}c- c~ Pore size~ 2 discharge \ ~pe ness Type ness Iyp (~m) ~n~cm ) properties (O2/i~2O) \ (~m) (~Im) Fx~m~l~ 64 Raney 200 ~ p 0,z SnO 0.2 Absent Sputteriny 60 g2 7.4 nl~cel 2 Deposltlon " 65 " " " " ZnO " " 58 91 7.1 66 " " " " AQ2O3 " " " 59 90 7.4 " 67 " " " " ~lgO " " " 57 92 7.0 68 " " " " CaO " " " 57 3~ 7.1 " 69 " " " " SrO " " " 56 91 7 0 " 70 " " " " BaO " " " 56 91 7.0 71 " " " " TiO2 " " " 5g 90 7.3 72 " " " " SiO2 " " " ~8 31 7.4 Table 3 (contd~

~7ater-containable Mainte- Gas per-Electrode Water repel- or wettable Porous Deposition Current nance rneation body lent layer metallic oxide rr~T~rane manner of density propor- ratio of thin layer film tion of fllm (Present- discharge k ~ick Pore size) (mA,/crn ) (%) 2 2 ( ~n) ( ~n) Example 73 Raney 200 FEP 0 . 2SnO2 .1 AbsentSputtering 60 92 7.5 nickel ~ Sputt2ring " 74 " " " " ZnO " " " 58 91 7. 2 ~b " 75 " " " " AQ2O3 " " " 59 90 7.5 76 " " " " MgO " " " 57 92 7.1 ,~
77 " " " " CaO " " " 57 92 7. 2 ~~
" 78 " " " " SrO " " " 56 91 7.1 " 79 " " " " BaO " " " 56 91 7.1 " " " " Ti2 " " " 59 90 7.4 81 2 58 91 7.5 Table 3 (contd) Water-corltainable Mainte- ~Gas per-Electrode Water repel- or wettable Porous Deposition Current nance meation body lent layer rnetallic oxide r~3mbrane manner of density propor- ratio of \ thin layer film tion of film \ Thick- Thick- Thickness Pore s'ze) (r~cm2) properties (O /H O) \ ~pe ness Type ness Type ( ~) ~ (%) \ (~m) (~m) Example 82 RankYL 200 FEP 0.2 SnO2 0.1 0.03 ~m tDepositiorl 60 93 7.4 bonding " 83 " " " " Zr10 " " " 58 92 7.1 84 ~ " " " AQ203 " " " 59 91 7.4 ~
" 85 " " " " MgO " " " 57 93 7.0 ~~
" 86 " " " " CaO " " " 57 93 7.1 87 " " " " SrO " " " 56 92 7.0 88 " " " " BaO " " " 56 92 7.0 " 89 " " " " ~2 " " " 59 91 7.3 " 90 " " " " SiO2 " " " 58 9Z 7.4 Table 3 (contd) Electrode Water repel- or ~ttable Porous Deposition CuLrrent nance Gas ~er-\ thln layer film tion of film \ Thick- Thic]c- Thickness Pore size) ( ~ 2) properties (O /H O) \ Type ness Type ness Type (~m) (%) ~ 2 ~m ~m Raney present: Sputtering Exa p e 91 nickel FEP 0.2 SnO2 0.1 0.03 ~m ~Spllttering 60 94 7.5 C~pressive bonding "92 " " " " ZnO " " " 58 93 7.2 "93 " " " " AQ203 " " " 59 92 7.5 "94 " " " " MgO " " " 57 94 7.1 ~v "95 " " " " CaO " " " 57 94 7.2 "96 " " " " SrO " " " 56 93 7.1 "` 97 " " " " BaO " " " 56 93 7.1 "98 " " " " TiO2 " " " 59 92 7.4 "99 " " " " SiO " " " 58 93 7.5 I`he performance assemssment of the air electrodes in E~amples above was determined using potassium hydroxide as an elec-trolyte. However, it ls definite that similar effects can also be obtained, needless to say, by use of another electrolyte, for example, ammonium chloride, so-dium hydroxide, or an electrolyte which is added rubidium hydroxide, li-thium hydroxide, cesium hydroxide or the like to the aforementioned electrolyte. Additionally, it has been found that -the air-electrode in which the com-posite membrane with respect to this invention is employedcan be applied in an air-iron cell~

As be apparent from the above description, the oxygen gas permselective membrane according to this invention, though being very thin, does not allow water vapor and carbon dioxide gas in air to permeate therethrough and has a great function for allowing oxygen gas to selec-tively permeate therethrougn. Therefore, the air elec-trode comprising a combination of this oxygen gas perm-selective membrane and the electrode body may be designed overall in a thin form and enables a heavy-load discharge for a long period of time. It is noteworthy that such an air electrode also improves in storage proper-ties and leakage resistance.

Therefore, it can be estimated that the oxygen gas perm-selective membrane according to ihis invention is indus-trially highly valuable and beneficial.

Further, the air electrode in which the aforementioned oxygen gas permselective membrane is employed may be designed overall in a thin structure and does no-t allow water vapor or carbon dioxide gas in air to permeate the electrode body. Therefore, such an electrode can be utili~ed for a prolonged heavy-load discharge and is excellent in storage properties. It can thus be concluded that such an air electrode is industrially valuable and beneficial.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An oxygen gas perselective membrane which compri-ses a film of at least one water-containable or wettable metallic oxide selected from the group consisting of stannic oxide, zinc oxide, aluminum oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, titanium dioxide and silicon dioxide.

2. An oxygen gas perselective membrane according to claim 1, wherein said film has a thickness of 0.01 to 1.0 µm.

3. An oxygen gas perselective membrane according to claim 1, wherein said film of the water-containable or wettable metallic oxide is deposited integrally on either surface of a porous membrane of 0.1 µm or less in pore size.

4. An oxygen gas perselective membrane according to claim 3, wherein said film has a thickness of 0.01 to 1.0 µm.

5. An oxygen gas perselective membrane according to claim 3, wherein a water repellent layer is interposed integrally and laminatedly between said film of the water-containable or wettable metallic oxide and said porous mem-brane.

6. An oxygen gas perselective membrane according to claim 5, wherein said water repellent layer is a thin layer obtained by plasma polymerization of a monomolecular fluorinated organic compound.

7. An oxygen gas perselective membrane according to claim 5, wherein said water repellent layer and said film of the metallic oxide each have a thickness within the range of 0.01 to 1.0 µm.

8. An air electrode having oxygen gas perselec-tive membrane according to claim 1, integrally deposited on the gas-side surface of a porous electrode body having an electrochemical reducing function to oxygen gas and simul-taneously a current collecting function.

9. An electrode according to claim 8, wherein said metallic oxide constituting said membrane is deposited on the gas-side surface of the electrode body of the air elec-trode by means of a deposition process or sputtering process.

10. An electrode according to claim 8, which is pre-pared by integrally and compressedly bonding, to the gas-side surface of the electrode body, a formation where the film of the water-containable or wettable metallic oxide is deposited on either surface of the porous membrane of 0.1 µm or less in pore size.

11. An electrode according to claim 10, wherein a water repellent layer is interposed integrally and laminatedly between said film of the water-containable or wettable metal-lic oxide and said porous membrane.

12. An electrode according to claim 11, wherein said water repellent layer is a thin layer obtained by plasma polymerization of a monomolecular fluorinated organic compound.

13. An electrode according to claim 11, wherein said water repellent layer and said film of the metallic oxide each have a thickness within the range of 0.01 to 1.0 µm.