Note: Descriptions are shown in the official language in which they were submitted.
`~ ~13Z0~7
The present invention relates to a membrane, for an
electrochemical cell, inhibiting gas diffusion but allowing
ionic conduction.
More ~articularly, the present invention relates to a
membrane consisting of an organic binding agent and an ion con-
ducting inorganic powder, embedded in the binding agent in which
the inorganic powder maintains its ionic conductivity.
Membranes of this type are known from, e.g., French
patent No. 1,417,585 and Belgian patent No. 649,390. However,
these known membranes are only found in units exclusively provi-
ded for fuel cells and they comprise inorganic ionic conductors
which are not stable in a basic medium.
Accordingly, an object of the present invention is to
provide a membrane whic~ obviates or mitigates the above noted
disadvantages of the prior art.
According to an aspect of the present invention there
is provided a membrane for an electrochemical cell, inhibiting
the diffusion of gases and allowing ionic conduction, comprising
from about 2.5 to about 30 weight percent of an organic binding
agent and from about 97.5 to about 70 weight percent of polyanti-
monic acid powder, the grain size of the polyantimonic acid
powder being less than 38 ~m; the membrane being selectively
permeable to cations in a basic medium and being selectively
permeable to anions in an acidic medium.
According to a further aspec-t of the present invention
there is provided an electrochemical cell comprising the above
defined membrane.
The membrane disclosed herein is stable in a basic
medium and it can be adapted for use in an electrolytic cell. It
may, particularly but not exclusively, be used in an electrolytic
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cell for producing hydrogen.
The ionic conducting powder for the membrane disclosed
herein is polyantimonic acid. The fact that polyantimonic acid,
with the general formula {(H3Sb3O5(OH)8)3 (H5 5 6 18
ion exchanger is known, e.g., from L.H. Baetslé and D. Huys,
J. Inorg. Nucl. Chem., 30r 639 to 649, (1968).
However, a membrane comprising polyantimonic acid as
an ion conducting inorganic powder has a number of unexpected
and advantageous properties: the high capacity of polyantimonic
acid for exchanging cations is maintained in a membrane contain-
ing an organic binding agent; such a membrane has a pronounced
selectivity for ions; the resistivity of the membrane is low at
normal operating temperatures, the more so, if the amount of
organic binding agent is about 20~; the resistance of the mem-
brane remains constant in terms of current density; the membrane
keeps its physico-chemical and electrochemical properties up to
a temperature of approximately 150C in a highly concentrated
alkaline medium; the gas separating characteristic of the
membrane is good; and the concentration of electrolyte with which
the membrane is in contact is not critical for the conductivity
of the membrane.
A membrane in association with an electrolytic cell for
producing hydrogen is known, e.g., from the paper by L.J. Nuttall
and W.A. Titterington - Conference on the Electrolytic Production
of Hydrogen - City University London - February 25-26, 1975.
In this known cell, the membrane is made of sulphonated
and polymerized tetrafluoroethene and it is located between one
electrode, which acts as a catalyst, and functions as the anode
of the cell and which is made of a special alloy, and a second
electrode, which also acts as a catalyst, but functions as the
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cathode of the cell and is made of platinum black. This known
unit is preferably utilized in an acid medium as a consequence
of which the choice and price of the materials used are adversely
affected. However, when the electrolytic cell is utilized in an
alkaline medium, the membrane causes a significant drop in
- potential, due to its comparatively high resistivity and thick-
ness, the latter of which is necessary for inhibiting the dif-
fusion of a gas.
The membrane described herein overcomes these drawbacks
and has a less significant drop of potential than the above
described membranes when the electrolytic cell is working.
Ion conducting membranes comprising an oxide of anti-
mony are known, e.g., from U.S. patents No. 3,346,422 and No.
; 3,437,580. The membranes described herein differ from these
known membranes, inter alia as follows:
On the one hand, the antimonyoxidesof the prior art
membranes do not have a polymeric character, contrary to poly-
antimonic acid used in the membranes described herein; polyanti-
monic acid has a crystalline structure with a recur_ent "unitary
cell". On the other hand, the above known membranes have a
sintered structure of a compressed inorganic material, whereas
the membranes described herein comprise both an organic binding
agent and polyantimonic acid, the latter being an inorganic
powder.
It follows that the membranes described herein are
composed of a heterogeneous material consisting of two differen-t
materials which are not sintered and can, for instance, be agglo-
merated by dry rolling according to a preferred embodiment.
The characterlstics of the membranes described herein
are basically different from those of known membranes comprising
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antimony oxides. For example, the ion exchange properties, the
resistivity and the resistance of the membranes described herein,
during electrolysis, are significantly better relative to the
prior art membranes. On the other hand, with membranes contain-
ing Sb2O3xH2O undesirable secondary electrochemical reactions are
; possible in an electrolytic cell.
Preferably at least one surface of the membrane is
coated with an electrode.
More preferably a surface oE the membrane is coated
with an active electrode (anode) which acts as a catalyst and
the other surface of the membrane is coated with an active elec-
trode (cathode) which also acts as a catalyst.
Embodiments of the invention will now be described by
way of example with reference to the accompanying drawings in
which:
;~ Figure 1 is a schematic sectional view of an electroly-
tic cell comprising a membrane as described herein; and
Figure 2 is a larger-scale sectional view through a
part of a unit of the electrolytic cell of Fig. 1.
The electrolytic cell of Fig. 1 and the unit of Fig. 2
comprise a membrane indicated by the reference number 5.
Composition Of The Membrane
The membrane 5 is made of grains of polyantimonic acid
which are ernbedded in an organic binding agent, preferably poly-
tetrafluoroethene.
The polyantirnonic acid particles have, for instance, a
maximum dimension of about 500 Angstr~m. However, many particles
can coalesce to form a conglomerate which can have a maximum
dimension of some tens of microns. This maximum dimension should,
preferably, not be greater than 30 microns.
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The membrane, preferably, comprises a polymeric mate-
rial as a binding agent. The polymeric material is, preferably,
the aforesaid polytetrafluoroethene, although,as a rule, a hydro-
philic polymer, for instance, polyethene can also be used.
The amount of binding agent relative to the amount of
ion conducting powder should be sufficient to form a membrane.
If polytetrafluoroethene is used as the binding agent, the mini-
mum amount relative to the complete membrane can be 2.5% by
weight. A membrane containing at least 5% by weight of the
binding agent is preferred. However, the amount of binding agent
relative to the amount of polyantimonic acid powder must not be
so high as to result in the powder loosing its ionic conductivity
within the membrane. If polytetrafluoroethene is used as the
binding agent, the maximum quantity relative to the complete
membrane can be 30% by weight. A membrane comprising at the most
20% by weight of the binding agent is preferred.
Characteristics Of The Membrane
. . ~
The membrane 5, particularly, as used herein manifests
advantageous properties generally not available with prior art
membranes.
a. Ion Exchange Characteristics
Due to the amphoteric character of polyantimonic acid, the mem-
brane is an anion exchanger in an acid medium and a cation
exchanger in a basic medium. The active OH groups give H ions
only in a basic medium; the exchange capacity for cations is a
measure of the amount of active groups within the membrane which
are capable of exchanging H ions for other cations such as K
and Na .
Table I shows that the exchange capacity, for cations, of poly-
- ~0 antimonic powder is maintained after the powder has been bound
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with a binding agent (polytetrafluoroethene) to form a membrane.
TABLE I
Exchange capacity for cations at pH 7 expressed in milliequiva-
lents per gram of polyantimonic acid.
..~
Exchange Capacity For Cations (meq.g 1)
Powder Membrane
_
NaOH 2.8 2.3
KOH 2.0 1.8
b. Selectivity For Ions
Apart from its high cation exchange capacity, the membrane has a
pronounced selectivity for ions.
This selectivity for ions is characterized by the transport
numbers for cations and anions, t+ and t , respectively.
The transport number ti represents the fraction of the charge
transported through the membrane by the i-ions. A dynamic
measurement of the selectivity at 30C for the polyantimonic acid
membrane gives a transport number tK+ amounting to 0.75.
c. Resistivity
:
In Te_ms Of Temperature
Table II shows the resistance of the membrane in terms of tempe-
rature for a lN NaOH and a lN KOH electrolyte. The values were
measured by means of a Hewlett-Packard AC bridge at 1,000 Hz.
TABLE II
Resistance of the membrane as a function of temperature at 1,000
Hz 2
Temperature Resistance of M~ mbrane (Qcm )
C lN NaOH lN KOH
1.25 1.75
3 50 0.91 1.16
: __ . __ . . _ . __.
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The values of Table II relate to a membrane consisting of 80
weight percent polyantimonic acid and 20 weight percent polyte-
trafluoroethene.
In Terms Of Amount Of Binding Agent
Table III shows the resistivity of the membrane in terms of the
amount of binding agent (polytetrafluoroethene) contained there-
in. The values were measured by means ofa 1,000 Hz AC bridge at
room temperature in a 25 weight percent KOH solution.
TABLE III
Resistivity as a function of the amount of binding agent.
Weight of Resistivity
Binding Agent (Qcm)
(~)
160
` 20 280
' 20 25 ~,.
d. Resistance During Electrolysis
Table IV shows -the resistance of the membrane as a function of
temperature; this resistance is a measure of the drop in potent-
~tial through the membrane.
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TABLE IV
T ( C~ R (Qcm )
5.8xlO-
83 2.8xlO-
105 2.5xlO-l
Since the resistance of the membrane remains constant as a
function of current density, it is possible, for instance, to
allow an electrolytic cell to work at 10 to 15 kAm 2 at tempera-
tures higher than 100C with a drop in potential through the
membrane that is only 0.15 V to 0.25 V.
e. Stability
The membrane maintains its physico-chemical and electrochemical
characteristics up to 150C in a highly concentrated alkaline
medium, contrary to the organic ion exchange membranes in current
use.
f. Gas Separating Character
From the volume of the pores and the distribution thereof, as
determined by means of the N2-adsorption isotherm (BET method),
it was determined that, for the polyantimonic acid powder used in
the tests, the grains were free from internal porosity and had a
diameter less than 500 Angstr~m. However, these grains form
conglomerates which may have diameters of tens of microns. If
such macro-grains, the diameter of which ranges from 212 ~m to
38~m, are used as a starting material and they are mixed with
polytetrafluoroethene in a ratio of 80/20, 18~ of H2 will be
found in the oxygen gas flow during the electrolysis of water at
atmospheric pressure. If, however, grains with a dimension
smaller than 38 ~m are used as the startins material, 1 to 2% of
H2 will be found in -the oxygen gas flow, the working conditions
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being the same. For the membrane to act as a gas separator,
the size of the macro-grains must not exceed a certain upper
limit.
Use Of The Membrane In An Electrolytic Cell
The details given below are for the electrolysis of
water which is dissociated into hydrogen and oxygen, but the
application of the membrane is not limited to this use.
The electrolytic cell shown in Fig. 1 comprises a con-
tainer 1 which is divided into chambers 2 and 3 by a unit. This
unit comprises an electrode 4 acting as a cathode, the membrane
5, described above, and an electrode 6 acting as an anode. The
electrodes 4 and 6 are connected to a potential source 9 by the
conductors 7 and 8, respectively, which pass through the wall of
the container 1.
The chambers 2 and 3, which are filled with electro-
lyte, each form a part of two liquid circuits (no* shown). The
products resulting from the electrolysis are recovered in those
circuits outside the chambers 2 and 3.
ln chamber 2 the liquid flows from the inlet 10 to the
outlet 11. Outside the chamber, the outlet 11 is connected to
the inlet 10 through a pipe system, a pump and apparatus for re-
covering the products developed by the electrolysis; the pipe
system, pump and recovery apparatus are not shown.
In chamber 3 the liquid flows from the inlet 12 to the
outlet 13. Outside the chamber, the outlet 13 is connected to
the inlet 12 through a pipe system, a pump and apparatus for
recovering the products developed by the electrolysis; the pipe
system, pump and recovery apparatus are not shown.
If -the liquid is alkaline, the electrochemical re-
actions are as follows, according to -the generally accepted
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theory:
at the cathode : 4 H2O + 4 e ~ 4 OH + 2 H2
at the anode 2 2
thus, the resultant reaction is ; 2 H2O ~ 2 H2 + 2
The electrons (e ) are transported through the potent-
ial source 9 from the anode 6 towards the cathode 4. The elec-
tric circuit is closed by ionic conduction through the membrane
5.
Thus, the mernbrane 5 ensures ionic conduction from one
electrode to the other; furthermore, the membrane also separates
the gases resulting from the electrolysis.
The ionic conduction takes place by cations and/or
anions, depending on the capacity of the membrane for exchanging
either anions or cations. The selectivity of the membrane for
ions determines whether the majority of the current is transport-
ed in the membrane by the cations or anions. Thus, with poly-
antimonic acid in a KOH medium, it is mainly the K ions that
will migrate from the anode 6 towards the cathode 4, whilst in a
NaOH medium it is mainly the Na ions which migrate.
In order to ensure the separation of gases, the mem-
brane has a finite thic~ness depending on its composition and on
the type and amount of binding agent it contains. The optimum
thickness of a membrane comprising 20% by weight of polytetra-
; fluoroethene, as a binding agent, is, for instance, 200 microns.
` The electrodes 4 and 6 between which the membrane is
- located in close contact therewith to form a unit can be prepared` in a number of different ways.
In the unit shown in Fig. 2, the electrode acting as
the cathode is made from a gauze 15 and an electrically conduct-
ing coating 14 which acts as a catalyst and covers the gauze 15.
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The electrode acting as the anode is made from a gauze 17 and an
electrically conducting coating 16 which acts as a catalyst and
covers the gauze 17. The gauzes 15 and 17 act as conducting
collectors for the input and output of current. The unit was
prepared as follows:
A 55 mesh nickel wire gauze with a wire thickness of
3701lmand an area of 100 cm2 was used for the collector 15.
Electrolytic platinum was deposited on this gauze, to form the
coating 14, using a solution containing 3% by weight of H2PtC16
; 10 salt. The electrolytic deposition was carried out at room tempe-
rature, the current density being kept at 2 kAm 2 for 30 minutes.
In this way, there was formed on the nickel gauze 15 a porous
coating of platinum, active as a catalyst, of about 1.5 mg per
cm .
The coating 16 was prepared by mixing nickel nitrate
and cobalt nitrate in l-butanol and at a stoichiometric ratio in
order to obtain NiCo2O4. The collector 17 was made from the
same nickel wire gause as was used for collector 15. This gauze
was dipped into the NiCo2O4 solution, after which it was dried
for 2 or 3 minutes at 250 C in an oven. This operation was
repeated ten times and the electrode was finally heated at 350C
and kept for about 10 hours at that temperature. In this way, a
coating of NiCo2O4, active as a catalyst, was deposited on the
porous nickel. In the unit, the nickel gauzes 15 and 17 will
` ensure the transport of the electrons.
The preparation of the coating 16 was similar to the
preparation of mixed oxides for oxygen-evolution-electrodes des-
cribed by G. Sing, M.H. Miles and S. Srinivasan, BNL - 20,984
(1975)-
- 30 For preparing the membrane 5, 4 g of polyantimonic acid
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powder with a yrain size less than 30 microns and a specific area
(BET) of 25 m /g, available under the trade mark POLYAN from
Applied Research, 1,080 Brussels, and 1 g of polytetrafluoro-
ethene (Du Pont type 6N) were mixed together. ~The preparatiOn
- of polyantimonic acid is known, e.g., from Belgian patent No.
649,746). This mixture was agglomerated by rolling to a thick-
ness of 300 llm, by about ten passages through a rolling-mill.
The membrane 5 thus formed was placed on top of the
gauze 15, precoated with the active catalyst coating 14. The
gauze 17 precoated with the active catalyst coating 16 was then
placed on top of the membrane 5; and these three layers were
formed into a unit of 800 ~m thickness by rolling.
The unit thus prepared was immersed in 6N KOH. By
connecting the gauze 15 and the gauze 17 to an external potential
source, the fol]owing results were achieved.
The total potential drop (E) through the unit, expres-
sed in volts, was measured for current strengths expressed in
kilo-ampere per square meter for temperatures of 22C, 50C and
85C
20I (kA/~I )E 22 C (V) E 50 C (V) E 85 C (V)
2 1.61 1.55 1.47
4 1.73 1.65 1.57
6 1.86 1.75 1.63
8 2.00 1.84 1.70
2.14 1.94 1.77
The gauze of the electrodes may be replaced by any type
of current collector, for instance, by a perforated plate.
Though the given example relates to the application of
a membrane, as described herein, in a unit for an electrolytic
cell intended to electrolyse water, the membrane can also be used
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in units for other electrolytic cells, for instance, for produ-
: cing chlorine, in fuel cells and for the removal of salt from
sea-water.
The membrane does not necessarily have to form a unit
with the electrodes, but may also be used in a cell when located
between the electrodes without being in close contact with them.
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