Porous separatory member entirely made of polyphenylene sulfide for the electrolysis of water

A porous separator, to be used in electrolyzers for producing hydrogen and oxygen by water electrolysis, consists of a fabric or felt entirely made of polyphenylene sulfide (PPS). In order to reduce the voltage drop caused by the separator, the PPS polymeric chain can be made ionically active by the presence thereon of polar groups, such as sulfonic, carboxylic or phosphonic groups. The method for producing the separator comprises the preparation, according to conventional weaving or felt manufacturing techniques, of a fabric or felt entirely made of PPS and the subsequent functionalization for introducing polar groups in the polymeric chain. The functionalization can be carried out either on the starting material, such as PPS polymer powder or flakes, or in any other step of the production of fabric or felt.

DESCRIPTION 
The present invention concerns a porous separator to be used in 
electrolytic cells for producing hydrogen and oxygen by water 
electrolysis, the separator being disposed between anode and cathode of 
the cell. The invention also concerns the method for producing said 
separator. 
The conventional electrolyzers for water electrolysis generally comprise 
one or more electrolytic cells, every cell consisting of two electrodes 
(cathode and anode) immersed in a suitable electrolyte. An electrolyte 
permeable separatory member is positioned between cathode and anode. The 
separator divides the cell into anodic and cathodic compartments for 
avoiding the mutual contamination of hydrogen and oxygen evolved at 
cathode and anode respectively, which entails the danger of formation of 
explosive mixtures. The separatory members, obviously, will have to 
present as low as possible resistance to the electric current flowing 
between the electrodes. 
Conventional separators are made of asbestos, in form of fabric or 
cardboard. Such a material has a good resistance against the attack of 
strongly alkaline substances used as electrolyte, provided the inner 
temperature of the cell is not more than 90.degree.-100.degree. C. It is 
well-known that, beyond this temperature, a rapid degradation of this type 
of separators occurs. It is also known that asbestos separators cannot be 
used with acidic electrolytes. 
In order to be able to operate at higher temperature, involving a 
significant decrease of the working voltage of the cell and, therefore, a 
sharp improvement of the energy efficiency, a variety of separators have 
been proposed, consisting of organic and/or inorganic materials. In this 
connection, is must be pointed out that an increase of the cell 
temperature beyond certain limits has to be obligatorily coupled with a 
pressure increase, in order to prevent the electrolyte from boiling. The 
pressure increase is, however, advantageous in that it involves a decrease 
of the diameter of the gaseous bubbles in the electrolyte, which 
consequently decrease the voltage drop in the electrolyte itself. By 
operating under pressure an energy saving is also achieved, because the 
electrolyzer produces already compressed gases, whereby part of the 
subsequent compression work for their commercialization becomes 
unnecessary. 
For the purposes of the present invention, the ionically active separator 
described in the Italian Patent Application, open-laid, No. 24836 A/80, 
essentially consisting of a support and an ionically active part, presents 
a particular interest. The support is preferably in form of fabric or 
fibrils and is made of a variety of materials, preferably 
polytetrafluoroethylene (PTFE). The ionically active part, in the form of 
a support coating or impregnating substance, consists of a polymer, namely 
polyphenylene sulfide, functionalized according to various methods, 
sulfonated in the particular case. 
However, the preparative methods described in the above-mentioned Patent 
Application are very complex and complicated ones. Particularly the need 
of a high temperature molding for securing the PPS to the support limits 
the size of the product which can be obtained in the practice. In fact, 
during the molding, pressures of 150,000-180,000 N/m.sup.2 and 
temperatures of 300.degree.-450.degree. C. are required. Consequently, the 
molds for practical use have rather small sizes owing to the actual 
impossibility to realize a sufficiently precise coupling of the two shells 
of the mold, on account also of the thickness of the article being at most 
a few millimeters. Consequently, the article will be compulsorily 
subjected to non-uniform pressures, whereby a non-homogeneous article will 
be obtained. In other words, when using such a separator in a cell for 
water electrolysis, a non-uniform current density will result. This fact 
can involve serious consequences, well-known to those skilled in the art, 
particularly in the case of very high current densities and of electrode 
separation distances substantially equal to the separator thickness. 
Moreover, the presence of the PTFE separator itself, in form of either 
fibrils or fabric, represents a cause of non-homogeneity, intrinsic to 
this type of separator. Mixing spots can, therefore, result for the gases 
(hydrogen and oxygen) evolved at the electrodes, owing to the known, 
strongly hydrophobic nature of PTFE, which for this reason shows a higher 
affinity for the gases than for the electrolytes. 
As a further drawback, PPS shows a very poor adhesion for the PTFE support, 
which possesses well-known, excellent anti-adhesion properties. 
Supports made of different materials from PTFE were in fact foreseen in the 
above-mentioned Application, but their use is problematic in the practice, 
because very few materials are known which withstand the attack of 
concentrated caustic potash at temperatures higher than 100.degree. C. In 
the practice, the only other utilizable material is nickel, which, 
however, is an electric conductor. This can impair the requisible 
insulating properties of a separator, even if coated with PPS, in the case 
of a fortuitous contact with energized cell parts. This can easily occur 
owing to defects present, for various reasons, in the PPS coating and 
brings about serious, well-known consequences. The only conclusion, 
therefore, is that PTFE represents the only material practically 
utilizable. 
At least, serious drawback of such a known separator consists in PTFE and 
PPS having very different thermal expansion coefficients, which brings 
about a peeling effect between the two materials. 
The object of the present invention is to obviate the above-mentioned 
drawbacks of the separators according to the known art, by providing a 
separator, which, even in industrial sizes of a few square meters, 
assures, as far as it is concerned a uniform current density between the 
electrodes, owing to its physical, structural and chemical homogeneity. 
Another object of the present invention is to provide said type of 
separator which can be produced easily and in industrial sizes, yet 
assuring the complete separation of hydrogen and oxygen evolving at the 
respective electrodes. 
It is to be emphasized, at this point, that the requisite above-mentioned 
characteristics of the separator according to the present invention cannot 
be renounced, if a porous separator has to be produced for use in water 
electrolyzers operating at current densities of about 1 A/cm.sup.2 and 
electrode separation distances substantially equal to the separator 
thickness. In this case, in fact, the slightest non-uniformity present in 
the separator causes a localized, preferential way of current flow 
bringing about a possible piercing of the electrodes and/or the separator 
itself and the mixing of the gases evolving at the electrodes, whereby 
explosive mixtures are formed. 
The above objects are achieved by a porous separator according to the 
invention, characterized in that it consists of fabric or felt entirely 
made of polyphenylene sulfide. 
The term "felt" herein is to be meant in the broadest sense as comprising 
also the so-called non-woven materials and the needle-felts, with or 
without a fabric of support. In the presence of a fabric of support, both 
the felt and the fabric of support are obviously made of the same 
material, i.e. PPS. 
This complete elimination of the support from the separator of the Patent 
Application No. 24836 A/80 allows a separator to be obtained, having the 
required size and homogeneity level, in a surprisingly easy manner, 
according to the conventional techniques for producing fabrics and felts. 
Preferably the fabric or the felt forming the separator are obtained from 
commercially available polyphenylene sulfide fibers. Suitably, the 
particular type of fibers known as flake is used. 
The substance (weight per square meter) of fabric or felt is preferably in 
the range of 300 to 800 g/m.sup.2. 
Because the separator according to the invention solves the problem of the 
size limitations inherent to the separators of the Patent Application No. 
24836 A/80, its employment is allowed also in the usual electrolyzers 
operating at low temperatures (about 80.degree. C.) and low current 
density (about 0.2 A/cm.sup.2), but having large-sized electrodes, such as 
about a few square meters. In these equipment, it will be possible to 
replace the classic asbestos separator with the separator according to the 
invention, with consequent remarkable benefits for the environment (in 
view of the dangerousness of asbestos), the life of the separator itself 
and the saving of energy, owing to the lower cell voltage. 
In order to reduce the voltage drop due to the separator, this member is, 
furthermore, characterized in that the polymeric chain of polyphenylene 
sulfide carries polar groups making the material ionically active. These 
polar groups can, for example, be sulfonic, carboxylic or phosphonic 
groups. 
The method for producing a separator of polyphenylene sulfide having on the 
polymeric chain polar groups according to the invention comprises the 
preparation, using conventional techniques for producing fabrics or felts, 
of a fabric or felt entirely made of polyphenylene sulfide, this polymer 
being then subjected to a functionalization procedure for introducing 
polar groups in the polymeric chain, characterized in that the 
functionalization is carried out indifferently either on the starting 
material or in any other step of the production of fabric or felt. 
Thus, the functionalization can be carried either directly on the PPS 
polymer powder or PPS fibers employed for producing the fabric or the felt 
or on the fabric or felt obtained from the PPS fibers. The 
functionalization can, however, be carried out also in intermediate steps 
of the production of the fabric or felt. 
The method according to the invention is further characterized in that the 
polar groups are sulfonic, carboxylic or phosphonic groups. The 
functionalization is indifferently carried out either on the polyphenylene 
sulfide polymer powder or fibers, or after the production of fabric or 
felt. 
The separator according to the invention, being of the polar type, allows a 
significant energy saving to be obtained, owing to the very low voltage 
drop (about 0.1-0.2 ohm cm.sup.2) caused by the electric current flow: 
this characteristic remains unchanged in the long run. 
The separator according to the invention allows a temperature up to 
180.degree. C., preferably between 120.degree. and 160.degree. C., to be 
maintained in the interior of the electrolyser, with all the consequent, 
above-mentioned advantages inherent to a higher operating temperature of 
the cell. Moreover, the remarkable physical, structural and chemical 
homogeneity of the separator assures, as far as this latter is concerned, 
a uniform current density between the electrodes even with electrodes of 
large size. 
The production of the separator, even of large size, is very easy, because 
the thoroughly tested weaving and felt manufacturing techniques are 
employed for the purpose. 
The functionalization of PPS is also very easy. 
The fabric or felt forming the separator has a substance in the range 
between 250 and 1200 g/m.sup.2, a thickness between 0.5 mm and 6.0 mm and 
a thread or fiber diameter corresponding to between 1 and 6 deniers. 
These properties allow a separator to be obtained which is extremely 
reliable and ecological and has a very long life and interesting 
characteristics of energy saving.

The invention will be better understood from the following examples, which 
obviously are not to be meant as limitative of the invention itself. 
EXAMPLE 1 
A sample of needle-felt obtained from 100% PPS fibers of 3 deniers and 
having the following characteristics: 
substance: 500 g/m.sup.2 
thickness: 2.5 mm 
air permeability: 350 dm.sup.3 /dm.sup.2 min at 200 Pascals 
bubble point: 4 cm H.sub.2 O at 20.degree. C. 
water uptake: 75% 
voltage drop: 0.324 ohm cm.sup.2 in 30% KOH at 100.degree. C., was 
sulfonated with liquid SO.sub.3 in 1,2-dichloroethane at 0.degree. C. 
When the reaction was completed, the product, stabilized with water and 
washed to neutrality, showed a water uptake of 76.5%, based on the total 
weight of the wet sample. 
The sample was subjected to life tests in 50% KOH, at the boiling 
temperature of the solution (140.degree.-145.degree. C.) for 500 hours. 
At the end of the test, the sample was washed, weighed again and evaluated 
for possible structural changes. 
The sample passed successfully the examination showing the same 
characteristics of the starting material. The sample was then employed in 
a nickel electrolytic cell, with nickel electrodes leaning upon the 
separator. 
The current density was 1 A/cm.sup.2, the test temperature 120.degree. C. 
at atmospheric pressure and the electrolyte a solution of 30% KOH. The 
test was carried out for 5000 hours. 
The purity of gases and the voltage drop caused by the separator was 
controlled throughout the test. 
The purity of hydrogen was at most 99.998%, the purity of oxygen at most 
99.99% and the voltage drop caused by the separator was 0.19-0.21 ohm 
cm.sup.2, throughout the test. At the end, the separator was recovered and 
again evaluated. The sample showed the same characteristics of samples 
never used in electrolytic cells. 
EXAMPLE 2 
A PPS felt sample having the same characteristics of the sample described 
in Example 1 was sulfonated in conc. Sulfuric acid (96%) at 
100.degree.-110.degree. C. for two hours. After stabilization the water, 
the sample was evaluated as described in Example 1 with the following 
results: 
Voltage drop less than 0.250 ohm cm.sup.2 in 30% KOH at 100.degree. C., 
hydrogen purity more than 99.99%, 
oxygen purity more than 99.96%, 
bubble point: 9 cm H.sub.2 O at 20.degree. C. 
EXAMPLE 3 
A sulfonated sample as described in Example 1 was tested in a suitable 
electrolytic cell at the autogenous pressure of 30 bars for 5000 hours. 
The gas purity throughout the test was at most 99.99% for hydrogen and 
99.97% for oxygen; the voltage drop was about 0.20 ohm cm.sup.2. 
At the end of the test the sample showed the same characteristics of a 
sample never tested in electrolytic cells. 
EXAMPLE 4 
A PPS fabric sample obtained from 100% PPS fibers of 3 deniers and having 
the following characteristics: 
Substance: 400 g/m.sup.2 
Thickness: 1.5 mm 
Type of fabric: Twill 
Warp count 17/2 n (metric) threads/cm 18 
Weft count 17 n (metric) threads/cm 26 
Air permeability: 110 dm.sup.3 /dm.sup.2 min. at 200 Pascals 
Voltage drop: 0.25 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Bubble point in water: 17 cm H.sub.2 O at 20.degree. C. 
Water uptake: 28.33%, was sulfonated according to the same procedure 
described in Example 1. 
When subjected to the same tests of Example 1 and 3, the following results 
were obtained: 
Water uptake: 29.5% 
Bubble point: 20.5 cm H.sub.2 O at 20.degree. C. 
Gas purity: 
at atmospheric pressure 
hydrogen 99.99% 
oxygen 99.9% 
at 30 bars 
hydrogen 99.98% 
oxygen 99.90% 
EXAMPLE 5 
A fabric sample with the same characteristics of Example 4 was sulfonated 
in 96% sulfuric acid at 100.degree.-110.degree. C. for 2 hours. 
After stabilization with water, the sample showed the following 
characteristics: 
Water uptake: 33.6% 
Voltage drop 0.19 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Gas purity: similar to Example 4. 
EXAMPLE 6 
The same as the Example 4, except that the sulfonation was carried out with 
sulfuric acid/20% oleum mixture in a 2:1 (volume/volume) ratio. 
Reaction temperature: 20.degree. C. 
Time: 2 hours 
Characteristics of the resulting separator: 
Bubble point: 20.5 cm H.sub.2 O at 20.degree. C., 
Water uptake: 31.5% 
Voltage drop 0.18 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Gas purity: the same as Example 4. 
EXAMPLE 7 
A sample with the same characteristics of Example 1 was functionalized by 
introducing carboxylic groups. 
The reaction was carried out with phosgene as carboxylating agent, the 
sample being immersed in an inert solvent containing dissolved a Lewis 
acid as catalyst (aluminium chloride, iron chloride, zinc chloride, etc.). 
At the end of the reaction, the sample was treated with hot water for 
stabilization and evaluated as described in Examples 1 and 3, obtaining 
the following results: 
Voltage drop 0.27 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Water uptake: 76% 
Bubble point 8 cm H.sub.2 O at 20.degree. C., 
Gas purity: 
at atmospheric pressure 
hydrogen 99.99% 
oxygen 99.8% 
at 30 bars 
hydrogen 99.97% 
oxygen 99.6% 
EXAMPLE 8 
A sample with the same characteristics of Example 4 was functionalized by 
introducing carboxylic groups according to the method of Example 7. 
The following results were obtained: 
Voltage drop less than 0.25 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Bubble point: 18 cm H.sub.2 O at 20.degree. C., 
Gas purity 
at atmospheric pressure: 
hydrogen 99.99% 
oxygen 99.98% 
EXAMPLE 9 
Felt and fabric samples, similar to those described in Examples 1 and 4 
respectively, were functionalised with phosphonic groups by reaction with 
phosphorus oxychloride in inert solvents and in the presence of Lewis 
acids as catalyst. 
The characteristics of the resulting separators were, at atmospheric 
pressure: 
Felt: 
Bubble point: 8 cm H.sub.2 O at 20.degree. C. 
Voltage drop : 0.26 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Water uptake: 77% 
Gas purity: 
hydrogen 99.99% 
oxygen 99.89% 
Fabric: 
Bubble point: 21 cm H.sub.2 O at 20.degree. C. 
Voltage drop: 0.29 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Water uptake: 27% 
Gas purity: 
hydrogen 99.99% 
oxygen 99.90% 
EXAMPLE 10 
PPS flake samples were sulfonated as described in Example 2, carboxylated 
as described in Example 7 and phosphonated as described in Example 9. 
After product stabilization with potash for converting the acidic groups 
introduced in the polymeric chain into the corresponding potassium salts, 
the usual procedures of spinning, weaving and/or needling were carried out 
on the functionalized flakes in order to obtain yarns, fabrics, 
needle-felts and non-woven felts. 
The resulting samples were again evaluated according to the procedure of 
Example 1 with the following results: 
__________________________________________________________________________ 
Voltage drop 
Flake Substance 
Thickness 
ohm cm.sup.2 in 30% 
Bubble point 
Water uptake 
functionalization 
Obtained product 
g/m.sup.2 
mm KOH at 100.degree. C. 
cmH.sub.2 O 
% % 
% 
__________________________________________________________________________ 
O.sub.2 
Sulfonation 
Fabric - Twill 
400 2.2 0.21 19 29 99.99 
99.98 
" Fabric - Cloth 
300 1.5 0.19 18 28 99.98 
99.97 
" Needle - felt 
500 2.0 0.20 9 77 99.998 
99.99 
" Needle - felt 
300 1.5 0.18 8 76 99.998 
99.99 
Carboxylation 
Fabric - Twill 
400 2.2 0.24 21 28 99.992 
99.98 
" Fabric - Cloth 
300 2.0 0.23 20 29 99.991 
99.97 
" Needle - felt 
500 2.0 0.26 9 75 99.996 
99.98 
" Needle - felt 
300 1.5 0.24 8 76 99.991 
99.96 
Phosphonation 
Fabric - Twill 
400 2.2 0.29 21 28 99.99 
99.92 
" Fabric - Cloth 
300 2.0 0.26 20 28 99.99 
99.90 
" Needle - felt 
500 2.0 0.27 8 75 99.99 
99.92 
" Needle - felt 
300 1.5 0.24 8 75 99.98 
99.87 
__________________________________________________________________________ 
EXAMPLE 11 
A needle-felt sample with the same characteristics of Example 1, but not 
functionalized, was tested in an electrolytic cell under the same 
conditions of Example 1, with the following results: 
Voltage drop: 0.324 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Gas purity: 
hydrogen 99.98% 
oxygen 99.96% 
EXAMPLE 12 
A fabric sample with the same characteristics of Example 4, but not 
functionalized, was tested in an electrolytic cell under the same 
conditions of Example 1, with the following results: 
Voltage drop: 0.25 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Gas purity: 
hydrogen 99.99% 
oxygen 99.96% 
EXAMPLE 13 
A felt sample obtained from 100% PPS fibers of 4 deniers and having the 
following characteristics: 
Substance: 620 g/m.sup.2 
Thickness: 1.45-1.55 mm 
Water uptake: 82% 
Voltage drop: 0.20 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Air permeability: 85-115 dm.sup.3 /dm.sup.3 min at 200 Pascals 
Bubble point in water: 25 cm H.sub.2 O at 20.degree. C. 
not functionalized was tested in an electrolytic cell under the same 
conditions of Example 1, with the following results: 
Voltage drop: 0.18-0.20 ohm cm.sup.2 
Gas purity: 
hydrogen 99.992% 
oxygen 99.983% 
EXAMPLE 14 
Felt sample like that of Example 13. 
Substance: 700 g/m.sup.2 
Thickness: 2 mm 
Water uptake: 81% 
Voltage drop: 0.25 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Air permeability: 90-120 dm.sup.3 /dm.sup.3 min at 200 Pascals 
Bubble point: 30 cm H.sub.2 O at 20.degree. C. 
Test as described in Example 1. 
Gas purity: 
hydrogen 99.993% 
oxygen 99.987% 
EXAMPLE 15 
Felt sample like that of Example 13. Fibers of 6 deniers. 
Substance: 800 g/m.sup.2 
Thickness: 2 mm 
Water uptake: 80% 
Voltage drop: 0.30 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Air permeability: 100-120 dm.sup.3 /dm.sup.3 min at 200 Pascals 
Bubble point: 22 cm H.sub.2 O at 20.degree. C. 
Gas purity: 
hydrogen 99.990% 
oxygen 99.970% 
EXAMPLE 16 
Sample as that of Example 13. Fibers of 3 deniers. 
Substance: 900 g/m.sup.2 
Thickness: 3 mm 
Water uptake: 88% 
Voltage drop: 0.29 ohm cm.sup.2 in 30% KOH at 100.degree. C. 
Air permeability: 80-90 dm.sup.3 /dm.sup.3 min at 200 Pascals 
Bubble point in water: 35 cm H.sub.2 O at 20.degree. C. 
Gas purity: 
hydrogen 99.998% 
oxygen 99.992%