Method and apparatus for separating oxygen from a gaseous mixture

A process and apparatus is provided for separating oxygen from a mixture of gases such as air. The apparatus includes an electrochemical cell that includes a cathode, an anode and an electrolyte. Oxygen in the air is reduced to the superoxide ion (O.sub.2.sup.-) at the cathode; the superoxide ion is transported across the cell through the electrolyte; and the superoxide ion is then reoxidized to oxygen at the anode and collected.

FIELD OF THE INVENTION 
This invention relates to a process and apparatus useful for separating 
oxygen from a mixture of gases by reducing the oxygen to its superoxide 
ion (O.sub.2.sup.-), transporting the superoxide ion from the reducing 
environment to an oxidizing environment and reoxidizing the superoxide ion 
to oxygen. More particularly, this invention relates to a process for 
separating oxygen from air in an electrochemical cell, wherein oxygen is 
reduced to the superoxide ion at the cathode; the superoxide ion is 
transported across the cell through an electrolyte; and the superoxide ion 
is reoxidized to oxygen at the anode. 
BACKGROUND OF THE INVENTION 
Oxygen gas has many uses. For example, it can be used for treatment of 
patients in the medical field, for various industrial processes, and for 
breathing in an environment in which oxygen is deficient. As a result of 
the variety of uses for oxygen gas, there is currently a substantial 
demand for such gas and also for a process by which it can be produced 
economically, efficiently and safely. Preferably such a process can be 
carried out in relatively large units and also in relatively small units, 
e.g. portable units. 
One process that is presently used to produce oxygen gas is electrolysis of 
water. There are, however, several problems associated with electrolysis 
that make it unattractive. For example, electrolysis requires a large 
consumption of electrical energy and the oxygen gas produced can contain 
small amounts of hydrogen gas which must be removed before the oxygen can 
be used. Additionally the concomitant production of hydrogen along with 
oxygen during electrolysis presents serious safety hazards. 
In addition to electrolysis of water, processes are available in the art 
for producing pure oxygen gas by separating oxygen from a gaseous mixture 
such as air. 
The most widely used oxygen separation process is cryogenic liquifaction 
and distillation of air. Such cryogenic processes have several drawbacks, 
however; they are energy intensive with overall efficiencies of less than 
about 35-40 percent and they must be run in plants whose capacities exceed 
about 100 tons per day to take advantage of economy of scale. Because 
cryogenic units must be quite large to be economically feasible, smaller 
and/or portable units based on this technology are not available. 
Therefore, when a cryogenic process is used, the resulting oxygen usually 
must be shipped from a large central production facility to the end user. 
In this case the product oxygen is often transported as a liquid in 
expensive vehicles equipped with cryogenic dewars. The cost of the 
cryogenic process is further increased since the transport and storage of 
liquid oxygen is hazardous and thus, special precautions must be taken. 
In addition to the above described cryogenic processes, oxygen can be 
separated from air by means of known electrochemical processes which are 
based either on a two-electron reduction of oxygen or a four-electron 
reduction. For example, U.S. Pat. No. 3,888,749 to Chong, U.S. Pat. No. 
4,061,554 to Chillier-Duchatel et al and U.S. Pat. No. 4,300,987 to Tseung 
et al, disclose electrochemical processes for separating oxygen from air 
by means of a two-electron transfer. U.S. Pat. No. 3,410,783 to Tomter 
discloses electrochemical processes for separating oxygen from air by 
means of either two- or four-electron transfers. 
Since the electrical current which must be passed through an electrolyte in 
an electrolytic cell for separating a given amount of oxygen from a 
gaseous mixture is directly proportional to the number of electrons (n) 
that reduces each molecule of oxygen, a four-electron process requires 
twice the amount of current that is required by a two-electron process. 
For perfectly reversible (ideal) electrochemical cells the voltage is 
fixed by the thermodynamic relationship: 
EQU .DELTA.G=-nFE (1) 
where .DELTA.G is the free energy change, n is the number of electrons 
transferred per mole of material passing through the cell, F is the Farady 
(1F=96,490 Coulombs) and E is the reversible, equilibrium cell voltage. 
For the separation of oxygen from air .DELTA.G is fixed and is independent 
of the method of separation. Since .DELTA.G is fixed in the ideal case, 
the energy efficiency is 100% regardless of the value of n because the 
voltage varies to compensate for n. Consider, for example, two ideal 
cells, A and B, with n equal to 4 and 2 respectively, where both cells 
operate with a .DELTA.G of 9.6 kilojoules/mole (kj/mole). The total amount 
of power required to electrolyze 1 millimole/second (mmol/s) of material 
in cells A and B is listed below in Table I. 
TABLE I 
______________________________________ 
Power-Watts 
Cell n Voltage Amps(I) 
(P = E .times. I) 
______________________________________ 
A 4 0.025 384 9.6 
B 2 0.05 192 9.6 
______________________________________ 
Although, as is shown above for ideal cells, while cell A must pass 384 
amps and cell B must pass 192 amps to electrolyze 1 mmol of material per 
second, the total power required is the same for both cells. 
Any real situation is somewhat different however because of unavoidable 
cell resistances and irreversibilities which prevent 100% efficiency. 
Thus, as is described below in greater detail, when oxygen is separated 
from air in an actual (non-ideal) electrochemical cell, the cell with a 
lower n value will be more efficient. 
For example, in the non-ideal cell the total power is defined by equation 
(2) below: 
EQU P=E(faradaic).times.I+I.sup.2 R(ohmic) (2) 
Since the four-electron process (n=4) must pass 2 times as much current as 
the two-electron process (n=2) to produce a given amount of product in a 
given amount of time, the ohmic term for the four-electron process will be 
four times as large as for the two-electron process. 
Thus, when equation (2) is applied to the example set forth above and an 
actual cell resistance of 0.001 ohms is assumed, the power requirement to 
produce 1 mmol/sec of material in cell A is 157 j/mmole while the power 
requirement is only 46.5 j/mmole in cell B. 
In the foregoing discussion it was assumed that equal amounts of product 
were produced during equal time periods in cells with the same resistance. 
In practice, it is possible to lower the cell resistance by increasing the 
area of the electrodes. For example, by allowing cell A to have four times 
the electrode area as cell B, and assuming the resistance of cell A is 
consequently lowered four-fold, the energy requirements for the two cells 
(cells A and B) will be the same. Thus, under conditions of equal energy 
requirements the cell with a lower n value has an advantage in cell size. 
Since the cost of electrochemical cells scales roughly with electrode 
surface area the advantage of providing a cell using a relatively lower n 
can be substantial. 
A person skilled in the art can design a cell in such a way that electrode 
area, current density, voltage, product output and rate are optimized to 
suit a particular need. In general the cell with smaller n value will have 
an advantage in one or more of these parameters. 
SUMMARY OF THE INVENTION 
This invention relates to a process in which an electrochemical cell 
provided in accordance with this invention is used for separating oxygen 
from air or other gaseous mixture. 
In one embodiment, the process of this invention comprises the steps of 
contacting the cathode of the electrochemical cell with a gaseous mixture 
comprising oxygen to thereby reduce oxygen by a single electron to its 
superoxide ion. Such superoxide ions are transported across the cell from 
the cathode to the anode through the electrolyte contained therein. The 
superoxide ions are oxidized to oxygen at the anode where the oxygen is 
discharged as oxygen gas. 
In a second embodiment, the process of this invention comprises the steps 
of adding a transition metal complex to the electrolyte contained in the 
electrochemical cell. Such a transition metal complex that is provided in 
accordance with this invention can be reduced at a potential more positive 
than oxygen reduction. A potential is then applied across the cell to 
reduce the transition metal complex by a single electron at the cathode to 
form a complex capable of reversibly binding oxygen. A gaseous mixture 
comprising oxygen is introduced into the electrochemical cell to contact 
the reduced transition metal complex so that oxygen is bound to the 
complex. The oxygen-containing complex is transported to the anode where 
the complex is reoxidized by a single electron at which time said complex 
releases the bound oxygen for recovery. 
In both of the above described embodiments of the process of this 
invention, oxygen is separated from air by means of a single electron 
transfer. 
The electrochemical cells provided in accordance with this invention 
comprise an anode, a cathode and an electrolyte. In one embodiment the 
electrolyte is an aqueous electrolyte and the cathode has a coating 
thereon and is provided to reduce oxygen to its superoxide ion. The 
coating is relatively impermeable to water while being relatively 
permeable to the superoxide ion. The aqueous electrolyte has a pH greater 
than about 7 and provides the means for transporting such superoxide ions 
across the cell from the cathode to the anode where the superoxide ions 
are reoxidized to oxygen and collected. 
In another embodiment of the electrochemical cell provided in accordance 
with this invention, the electrolyte comprises an aprotic solvent 
containing a dissolved salt. 
In yet another embodiment of the electrochemical cell provided in 
accordance with this invention, the electrolyte is a solid polymer 
electrolyte.

DETAILED DESCRIPTION 
Although the process and apparatus provided in accordance with practice of 
principles of this invention are both described below with reference to 
the schematic electrochemical cell 10 shown in the drawing, it should be 
understood that the components of the electrochemical cell comprising this 
invention can be provided in various configurations as are well known in 
the art of cell design. Furthermore, although this invention is discussed 
in terms of only a single electrochemical cell, the apparatus provided in 
accordance with this invention can include a plurality of such cells. 
The electrochemical cell 10 of this invention includes a cathode 12, an 
anode 14 and an electrolyte 16 extending between the cathode and anode. 
Briefly, in accordance with the process of this invention, the cell 10 is 
operated to separate oxygen from air (or from another gaseous mixture 
comprising oxygen) by impressing an appropriate potential across the anode 
and cathode and by introducing air into a chamber 18 in fluid 
communication with the cathode. The air contacts the cathode surface where 
oxygen in the air is reduced by one electron to its superoxide ion 
(O.sub.2.sup.-). (Excess air is vented from the chamber 18 via a vent 20 
or the like.) The superoxide ions produced at the cathode migrate into the 
electrolyte and travel through the electrolyte under the influence of 
diffusion, convection and electromigration to the anode where such ions 
are reoxidized (by one electron) to oxygen. Oxygen is liberated from the 
anode as oxygen gas and is collected in an oxygen chamber 22 from which it 
is withdrawn for use through a vent, such as the vent 24. 
The overall process of this invention is shown as the sum of two half cells 
by equations 3 and 4 below: 
EQU O.sub.2 +le.sup.- .fwdarw.O.sub.2.sup.- (cathode); and (3) 
EQU O.sub.2.sup.- .fwdarw.O.sub.2 .uparw.+le.sup.- (anode) (4) 
As is described below in greater detail, a key feature of the process and 
apparatus of this invention is the transport of the superoxide ions from 
the cathode, where they are formed, through the electrolyte to the anode 
for reoxidation to oxygen, without a significant number of such superoxide 
ions being reduced further to peroxide. Thus, the apparatus and process of 
this invention provide for oxygen to be separated from air by means of a 
single electron transfer instead of the two- or four-electron transfers 
known previously in the art. 
This invention is unique in that it can result in higher efficiencies than 
were previously achievable with electro separation cells i.e., 
electrochemical cells, based on two- or four-electron transfers. It is 
also unique in that no expensive electro catalysts are required for either 
the anode or cathode. Further, since the superoxide ion is not a strong 
oxidant, it does not oxidatively attack the electrodes or other cell 
components as does the peroxide ion used in prior art processes. 
Additionally, the potentials required at the electrodes of the instant 
invention are more reducing than the normal hydrogen electrode (NHE), 
whereas in the two- and four-electron processes the electrodes are set at 
more positive potentials. Because the required potentials are lower, the 
electrodes in the one-electron process of this invention are not exposed 
to as harsh an environment as in the two- and four-electron processes and 
thus, are less subject to oxidative degradation. 
Electrodes contemplated for use in accordance with practice of this 
invention can be carbon in the form of graphite, vitreous or glassy 
carbon, carbonblack, carbonized cloth, carbon fibers or other forms of 
carbon known in the art. Alternatively, the electrodes can be non-noble 
metals, e.g. mercury or lead, conducting inert borides, carbides, 
nitrides, silicides, phosphides, and sulfides or noble metals, e.g. 
platinum or gold. 
If desired, the cathode can be in the form of a gas diffusion electrode to 
increase the electrode surface available for contacting the air and thus, 
the oxygen contained in the air. The particulars of construction of such 
gas diffusion electrodes are well known to those skilled in the art and do 
not provide any part of the instant invention. 
As is described in greater detail below, the electrolytes contemplated for 
use in accordance with practice of this invention can be aqueous 
electrolytes, non-aqueous electrolytes or mixtures thereof. 
As was mentioned previously, key features of this invention are the 
reduction of oxygen in air to the superoxide ion at the cathode and 
inhibiting the superoxide ion from being reduced further to peroxide as it 
travels from the cathode to the anode through the electrolyte. 
In one embodiment of the cell 10 of this invention, an aqueous electrolyte 
is provided and the cathode includes a coating 26 (shown schematically in 
the drawing) that is relatively impermeable to water while being 
relatively permeable to the superoxide ion. 
If an aqueous electrolyte is used and no such coating is provided, peroxide 
is formed at the cathode, i.e., it is thought that the superoxide ion 
originally formed at the cathode is reduced further on the cathode surface 
to peroxide. A discussion of this reaction and of coating a mercury 
electrode with quinoline to prevent peroxide formation can be found in J. 
Chevalet et al, ELECTROGENERATION AND SOME PROPERTIES OF THE SUPEROXIDE 
ION IN AQUEOUS SOLUTIONS, J. Electroanal. Chem., 39(1972), which is 
incorporated herein by this reference. 
In accordance with practice of this invention such cathode coatings can be 
provided by adding a compound (hereinafter referred to as a surfactant) to 
the electrolyte which is capable of being adsorbed from the electrolyte 
onto the cathode surface. When such a surfactant is used it is desired, 
although not necessary, that the aqueous electrolyte is saturated with the 
surfactant. Alternatively, if desired, the coating can be a polymer 
applied directly to the cathode surface. Non-limiting examples of 
surfactants comtemplated for use in accordance with this invention include 
quinoline, triphenylphosphine oxide, pyridine and substituted pyridines, 
substituted quinolines, trialkyl amines, thiols and thioethers, 
cetyltrialkylammonium salts, benzyltrialkylammonium salts and other 
cationic surfactants, sodium lauryl sulfate, alkyl sulfates and 
sulfonates, alkyl phosphates and phosphonates and other anionic 
surfactants, polyethelyene glycols, polypropylene glycols, and other 
non-ionic surfactants. 
Non-limiting examples of polymers that are contemplated for use in coating 
the cathode are polyvinylpyridine, polyacrylonitrile, polyacrylamide, and 
their copolymers. 
Once the superoxide ion is formed on the cathode and migrates through the 
cathode coating (which is permeable to the superoxide ion) into the 
aqueous electrolyte it is inhibited from being further reduced to peroxide 
in accordance with practice of this invention by providing that the 
electrolyte has a pH greater than 10 and preferably greater than 12. For 
example, having a relatively high pH reduces the probability that the 
superoxide ions will react with protons and undergo disproportionation 
according to the following reaction: 
EQU 2O.sub.2.sup.- +2H.sup.+ .revreaction.O.sub.2 +H.sub.2 O.sub.2 ; or (5) 
EQU 2O.sub.2.sup.- +H.sub.2 O.revreaction.O.sub.2 +HO.sub.2.sup.- +OH.sup.-(6) 
Thus, by providing an aqueous electrolyte solution with a pH greater than 
10 and preferably greater than 12, the superoxide ions which are produced 
at the coated cathode travel through the electrolyte to the anode where 
they are reoxidized by a single electron transfer to oxygen. The oxygen 
gas liberated at the anode is then collected for use. 
In addition to the above described technique of maintaining the aqueous 
electrolyte at a relatively high pH to stabilize the superoxide ion, 
further stabilization can be provided in accordance with this invention by 
adding one or more nitriles to the electrolyte. Non-limiting examples of 
nitriles contemplated for use in accordance with practice of this 
invention include benzonitrile, propionitrile, butyronitrile, 
malononitrile, succinonitrile, adiponitrile, cyanoacetate, 2-cyanoethyl 
ether, the cyanopyridines, polyacrylonitrile and acrylonitrile 
co-polymers, polycyanoacrylate and cyanoacrylate co-polymers. 
Preferably such nitriles are added to the electrolyte in an amount to 
provide at least about 1% by weight of the nitrile to the total weight of 
the electrolyte solution. 
In addition to adding a nitrile to the aqueous electrolyte to stabilize the 
superoxide ion, or as an alternative to such nitrile addition, it is 
thought that Lewis acids can be added to such an aqueous electrolyte for 
stabilizing the superoxide ion. For example, it is thought that the 
superoxide ion will associate with cations, especially multivalent cations 
such as Ca++, Ba++, Zn++ and Al+++, making the superoxide ion less 
susceptible to disproportionation. Preferably such Lewis acids are added 
to the electrolyte in an amount sufficient to provide at least about a 
0.01 molar (M) solution. 
In addition to the foregoing techniques for stabilizing the superoxide ion 
in aqueous electrolytes or, as an alternative thereto, the superoxide ion 
can be stabilized by adding organic cations such as tetraalkylammonium, 
alkylpyridinium, phosphonium, cetyltrialkylammonium, alkyltriethanolamine, 
and quaternized polyvinylpyridines or polyethyleneimines to the 
electrolyte. Preferably, such organic cations are added to the electrolyte 
in an amount sufficient to provide at least about a 0.1 molar solution. 
Alternatively or in addition to the foregoing, the superoxide ion can be 
stabilized in an aqueous electrolyte in accordance with this invention by 
adding to the electrolyte certain macromolecules such as, for example, the 
crowns and cryptands. Preferably, such crowns and cryptands are anion 
binding crowns and cryptands and are added to the electrolyte in an amount 
sufficient to provide at least about a 0.01 molar solution. 
Some transition metals, e.g. iron and copper, are known to catalyze the 
disproportionation of the superoxide ion in aqueous electrolytes. Since 
however, it is also known that certain ligands will act to suppress 
superoxide ion disproportionation by such transition metal ions, it is 
contemplated, in accordance with this invention, that when an aqueous 
electrolyte is used, various ligands can be added to the electrolyte to 
increase the stability of the superoxide ion. Ligands contemplated for 
such use include, for example, ethylenediaminetetraacetate, 
nitrilotriacetate, triphosphate and ethylenediamine and the like. 
The amount of such ligands desired to be added to the electrolyte can be 
determined by one skilled in the art based on the amount of contaminants, 
such as iron and/or copper, present in the electrolyte. 
Non-limiting examples of aqueous electrolytes contemplated for use in 
accordance with this invention include alkali hydroxides (LiOH, NaOH, KOH, 
RbOH, CsOH), alkaline earth hydroxides (Mg(OH).sub.2, Ca(OH).sub.2, 
Ba(OH).sub.2), alkali silicates, alkali borates, alkali and alkaline earth 
phosphates, alkali sulfates, alkali and alkaline earth halides or 
combinations and mixtures of the above. 
When an aqueous electrolyte is included in the electrochemical cell 
provided in accordance with this invention it is preferred that the 
process of this invention includes the step of removing carbon dioxide 
from the inlet air. Such carbon dioxide removal prevents precipitation of 
carbonates and is particularly important when the nature of the 
electrolyte would result in precipitation of insoluble carbonates. 
As was mentioned above, in another embodiment of this invention the 
electrolytic cell 10 includes non-aqueous electrolyte. Such non-aqueous 
electrolytes contemplated for use in accordance with practice of this 
invention are high-boiling (b.p. greater than about 100.degree. F.), 
aprotic polar solvents that contain an inert salt which dissolves to form 
at least a 0.1 molar solution. Preferably such solvents are selected from 
those that have little or no toxicity, especially when the product oxygen 
gas is intended for medical use. 
Aprotic high-boiling solvents contemplated for use in accordance with this 
invention include, but are not limited to: pyridine, 
N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile, benzonitrile, 
quinoline, substituted pyridines, non-limiting examples of which are 
methylpyridine, t-butylpyridine, di-t-butylpyridine, tri-t-butylpyridine, 
N-methylpyridinium salts, N-ethylpyridinium salts, and 
pyridinecarboxamides, also N-methylpyrrolidinone, dipyridylether, 
butyronitrile, propionitrile, adiponitrile, chlorocarbons, fluorocarbons 
and chlorofluorocarbons, perfluorinated amines and perfluorinated ethers. 
Salts that are contemplated for use in combination with the aprotic 
solvents to provide the electrolyte of this invention include, but are not 
limited to: tetraalkylammonium halides, where the alkyl groups are 
preferably hydrocarbons having 1 to 16 carbon atoms, preferably 
tetramethyl ammonium chloride, methylpyridinium halides, ethylpyridinium 
halides, tetraalkylammonium sulfates, perchlorates, acetates and 
trifluoroacetates, where the alkyl group is a straight chain hydrocarbon 
preferably having a length of 1 to 4 carbon atoms, alkali metal acetates 
and trifluoroacetates. 
When an aprotic solvent electrolyte system is included in the 
electrochemical cell of this invention, no coating is required on the 
cathode because no protons are available to catalyze further reduction of 
the superoxide ion formed on the cathode. However, to maintain the aprotic 
solvent free of protons, i.e., to maintain the solvent free of water, the 
inlet air is preferably dried in a drying step before it is introduced 
into the cell. Alternatively, or in addition to drying the air before it 
is introduced into the cell, a drying agent such as molecular sieves or 
activated silica can be added directly to the aprotic solvent electrolyte. 
Another feature of the present invention is based on the reactions of 
certain transition metal complexes, especially of cobalt, which can bind 
superoxide ions reversibly in accordance with the following one-electron 
transfer reactions: 
EQU O.sub.2.sup.- +L.sub.x Co(III).sup.n+ .revreaction.L.sub.x 
Co(III)O.sub.2.sup.(n-1)+ (7) 
and; 
EQU O.sub.2.sup.- +L.sub.x Co(II).sup.n+ .revreaction.L.sub.x Co(II)O.sub.2 
(n-1)+ (8) 
where L designates a ligand, x designates the number of such ligands 
associated with a cobalt ion, n is the total charge of the complex, and II 
and III represent the cobalt ion valance. 
In accordance with practice of this invention, if desired, such transition 
metal complexes can be added either to the above-described aqueous or 
non-aqueous electrolytes comprising the electrochemical cell of this 
invention to further stabilize the superoxide ions. Because the binding of 
such a superoxide ion by the transition metal complex is reversible, the 
complex can act to increase the "effective concentration" of the 
superoxide ion in solution. Said another way, the total concentration of 
bound and unbound superoxide ion in the electrolyte can be greater than 
the concentration of superoxide ion that can be obtained with only unbound 
superoxide when such a complex is not used. Having a higher superoxide ion 
concentration can result in higher current densities and smaller electrode 
surface areas thereby increasing the efficiency of the process. 
When one or more transition metal complexes are used, oxygen is reduced to 
its superoxide ion at the cathode and both free and bound superoxide ions, 
the relative amounts of which are determined by the binding constant of 
the complex, are transported across the cell to the anode where the free 
superoxide is oxidized to oxygen. Additionally the superoxide bound to the 
complex is released at the anode and is oxidized to oxygen. The complex, 
free of the superoxide ion, then returns to the cathode to pick up 
superoxide ions being newly formed. 
Preferably, such transition metal complexes are added to the electrolyte in 
an amount sufficient to provide at least a 0.01 molar solution. More 
preferably at least a 0.1 molar solution is provided. 
In another exemplary embodiment of practice of principles of this 
invention, a redox active transition metal complex can be added to the 
aqueous or non-aqueous electrolyte 16 of the cell 10 of this invention. 
Although such use of redox active transition metal complexes results in a 
different mechanism for separating oxygen from air than was described 
previously, both mechanisms accomplish oxygen separation by means of a 
one-electron transfer. For example, when cobalt transition metal complexes 
are used, if the characteristics of the ligand are such that Co(III) is 
reduced to Co(II) at potentials more positive than oxygen reduction, the 
reaction at the cathode will be the production of Co(II). The Co(II) 
complex will then bind the oxygen from the air and the bound oxygen will 
be transported across the cell to the anode on the Co(II) complex. Because 
the binding is reversible the bound and unbound oxygen can equilibrate. At 
the anode the free Co(II) complex is oxidized to Co(III) in which state it 
can no longer bind oxygen and thus, the oxygen is released at the anode. 
The cobalt(III) complex then returns to the cathode to complete the cycle. 
The above described process of this invention is shown by the equations 8, 
9 and 10 below: 
##STR1## 
When redox active transition metal complexes are used, as described above, 
preferably such transition metal complexes are added to the electrolyte in 
an amount sufficient to provide at least a 0.01 molar solution. More 
preferably, at least a 0.1 molar solution is provided. It is thought that 
if less than a 0.01 molar solution is provided the process will not be as 
economical as desired. 
In yet another embodiment, the electrochemical cell of this invention can 
include mixtures of aqueous and non-aqueous electrolytes. For example, the 
superoxide ion is thought to have an appreciable lifetime in mixtures of 
acetonitrile and water. Such mixed solvents can provide stability to the 
superoxide ion that is comparable to the stability provided by non-aqueous 
solvents while eliminating the necessity to dry the inlet air. Preferably 
the mixture comprises at least about 1% acetonitrile by weight compared to 
the total weight of the electrolyte. 
When mixtures of aqueous and non-aqueous electrolytes are used, such 
mixtures can be treated as described above for stabilizing the superoxide 
ion, for example by adding nitriles, Lewis acids, organic cations, certain 
macromolecules such as crowns and cryptands and/or ligands such as 
ethylenediaminetetraacetate, nitrilotriacetate, triphosphate and 
ethylenediamine. Further, if desired, the transition metal complexes 
described above can be added to the mixed electrolyte. 
In addition to the above described non-aqueous electrolytes the electrolyte 
provided in yet another exemplary embodiment of practice of principles of 
this invention can be a solid polymer electrolyte. Solid polymer 
electrolytes useful in practice of this invention must be resistant to 
nucleophilic attack and oxidation, must be stable in the presence of the 
superoxide ion and have a low resistance to superoxide migration. Such 
solid polymer electrolytes contemplated for use in accordance with this 
invention include but are not limited to: polyvinylpyridine, 
polyvinylpyridine-vinylpyridinium salts, polyethyleneimine and alkylated 
polyethyleneimine, copolymers whose components are chosen from 
vinylpyridine, vinylpyridinium salts, ethyleneimine, ethylene oxide, 
propylene oxide, acrylonitrile, cyanoacrylates, methylmethacrylate, methyl 
acrylate, styrene, divinylbenzene, divinylpyridine, cumene, 
pyridylisopropylene and maleic anhydride. 
In operation of the electrolytic cell 10 of this invention, temperatures, 
which are generally in the range of from about 0.degree. to 100.degree. 
C., are limited by the choice of the electrolyte. 
Inlet and outlet pressures of the cell 10 may vary from a partial vacuum of 
about 20 torr to several atmospheres. Preferably the inlet pressure will 
be maintained at one atmosphere or ambient pressure and the outlet 
pressure will be maintained from about 5 to about 10 psi above the inlet 
pressure. It is understood that a higher pressure differential across the 
cell will normally require an increase in the applied potential. 
Appropriate pressure regulating systems for use on the electrochemical 
cell 10 of this invention are known in the art. 
In accordance with practice of the invention the rate at which oxygen is 
separated from air or other gasous mixture can be controlled by adjusting 
the flow of air into the cell, the inlet and outlet cell pressure, and 
either the voltage across the cell or the current density. If desired the 
oxygen given off at the anode can be cleaned of contaminants by methods 
known in the art. 
The following non-limiting Examples illustrate the separation of oxygen 
from air in accordance with the process of this invention. 
EXAMPLE 1 
A plexiglas electrolytic cell is divided into two compartments by means of 
a polyethylene frit and each such compartment is fitted with mercury 
electrodes. Both compartments are filled with 1 Normal (N) NaOH solution 
containing 1% (by weight) quinoline. A voltage of 0.5 volts is applied 
across the cell and a stream of air which has been depleted of CO.sub.2 by 
bubbling it through a gas washing bottle filled with 5 molar NaOH is 
directed into the cathode, or negative compartment. The oxygen in the air 
is reduced by one electron to its superoxide ion at the cathode. The 
superoxide ions formed at the cathode migrate through the quinoline, which 
has adsorbed onto the cathode to form a coating, and thence through the 
NaOH solution to the anode. The superoxide ions are oxidized to oxygen at 
the anode and released from the anode as oxygen gas. The anode or positive 
compartment is protected from air, and gas evolved from the anode is 
collected by displacement of electrolyte in an inverted chamber or by some 
other means. The gas evolved at the anode is found to be substantially 
enriched in oxygen compared with the inlet air. 
EXAMPLE 2 
A glass two compartment electrolytic cell is fitted with graphite 
electrodes and filled with dry pyridine containing 0.1 molar 
tetraethylammonium chloride. Air dried by passage over anhydrous calcium 
sulfate and activated molecular sieves is bubbled into a first electrode 
compartment. A potential of 0.5 volts is applied to the electrodes with 
the electrode in the first compartment held negative with respect to the 
electrode in the second compartment. The oxygen in the air is reduced by 
one electron on the electrode in the first compartment (the cathode) to 
its superoxide ion. The superoxide ions formed at the cathode travel 
through the electrolyte to the electrode, i.e., the anode in the second 
compartment. The superoxide ions are oxidized to oxygen at the anode and 
released from the anode as oxygen gas. The second or anode compartment is 
fitted with a suitable means for collecting any gas evolved at the anode 
in such a manner that it is not mixed or contaminated with air. The 
collected gas evolved at the anode is found to be substantially enriched 
in oxygen compared to the air introduced into the first cathode chamber. 
The evolved oxygen gas is cleaned of entrained pyridine by passage over 
activated charcoal. 
The above descriptions of preferred embodiments of an apparatus and process 
for separating oxygen from air are for illustrative purposes. Because of 
variations which will be apparent to those skilled in the art, the present 
invention is not intended to be limited to the particular embodiments 
described above. The scope of the invention is defined in the following 
claims.