Process for formation of an electrode on an anion exchange membrane

The invention is a process for formation of an electrode on a solid polymer anion exchange membrane to increase rates of reaction at a reaction surface of the membrane. The process includes the steps of soaking a polymer anion exchange membrane in a solution containing an anionic entity wherein a desired metal catalyst is contained within the anionic entity so that anions containing the metal catalyst exchange into the membrane by electrostatic attraction, and exposing the membrane to a reducing agent so the metal catalyst is reduced to a metallic form to become physically secured at a reaction surface of the membrane to thereby form the electrode adjacent the reaction surface. In preparation of an electrode on an anion exchange membrane, the process is concluded by rinsing the membrane in distilled water and then the membrane is cycled through the soaking, exposing and rinsing steps until a desired level of catalyst loading is achieved. In a first preferred process, the polymer anion exchange membrane is a tetraflouroethelyene-flourinated ethylene propylene ("TFE-FEP") based membrane; the anionic entity is chloroplatinic acid; and the reducing agent is sodium borohydride. In a second preferred process, the polymer anion exchange membrane is a polyolefin based membrane; the anionic entity is potassium tetrachloroplatinate; and the reducing agent is sodium borohydride.

TECHNICAL FIELD 
The present invention relates to electrochemical cells that utilize solid 
polymer membrane electrolytes, and especially relates to a process for 
forming a catalyst electrode on a surface of a solid polymer membrane. 
BACKGROUND OF THE INVENTION 
Electrochemical cells are commonly used for producing product gases from a 
supply fluid such as water, and may be also used in "fuel cell" 
configurations to produce electrical energy from supply fuels. In a 
well-known application, electrochemical cells are utilized to electrolyze 
water into oxygen and hydrogen gases, and such cells may include planar 
shaped solid polymer electrolyte membranes between cathode and anode 
chambers to facilitate transport of hydrogen ions between the chambers. 
Upon application of an electrical potential to the electrolyte membrane, 
positively charged hydrogen ions (cations) are attracted to negatively 
charged electrons at a reaction surface of the solid polymer ion exchange 
membrane where the hydrogen cations combine with the electrons to form 
hydrogen gas. 
In such a common electrochemical cell, the solid polymer electrolyte 
membrane is characterized as a cation exchange membrane because it 
facilitates transport of positively charged cations across the membrane. 
To enhance electrical conductivity and increase rates of ion exchange 
across the membrane, metal catalysts are formed into an electrode and 
positioned at reaction surfaces of such ion exchange membranes. A variety 
of methods have been developed to bond such electrodes to surfaces of 
solid polymer electrolyte membranes. For example, U.S. Pat. No. 5,470,448 
to Molter et al., incorporated herein by reference and assigned to the 
assignee of the present invention, shows a process for forming a metal 
catalyst ionomer layer bonded to a reaction surface of a cation exchange 
membrane, wherein the membrane is a perfluoroionomer acid membrane that is 
well-known in the art, such as membranes sold by E.I. DuPont De Nemours, 
Inc. of Willmington, Del. under the trademark designation NAFION.RTM. 117 
membrane. The process disclosed in Molter et al. includes heating an 
aqueous solution of swollen ionomer solids and a metal catalyst until a 
dry powder remains; forming a solid paste of the dry powder and a fast 
drying agent or subliming agent; pressing the solid paste onto the 
reaction surface of the cation exchange membrane within a specific 
temperature and pressure range; and rehydrating the exchange membrane and 
bonded ionomer solids of the solid paste to form the metal catalyst 
ionomer layer (which layer is referred to herein as an electrode). That 
process utilizes a mixture of iridium and platinum metals as the catalyst, 
and can achieve catalyst loadings in such an electrode of at least 0.10 
milligrams (mg.)/square centimeter (cm.sup.2). 
Applications of polymeric ion exchange membranes in electrochemical cells 
other than water electrolysis cells however has generated new fabrication 
demands for formation of metal catalyst electrodes on such membranes. For 
example, in applications wherein electrochemical cells are utilized for 
electrodialytic salt-splitting to generate chlorine gas, or hydrochloric 
acid and caustic soda etc.; for treating waste water from gas scrubber 
treatment systems; or for revitalizing air in a closed-environment 
structure such as a submarine or space craft, the cells require both 
cation and anion exchange membranes. Known anion exchange membranes that 
contain quartenary ammonium functional groups are appropriate for such 
roles, but are limited to relatively low temperature applications of below 
130 degrees fahrenheit (.degree.F.). Consequently, known processes that 
include heating to bond a binder and catalyst to form an electrode at a 
reaction surface of known cation exchange membranes are inappropriate for 
formation of metal catalyst loaded electrode surfaces on a polymeric anion 
exchange membrane. 
Electrode formation on cation exchange perfluoroionomer acid membranes has 
also been achieved through electroless deposition of a positively charged 
ion from a dissolved precursor salt onto a reaction surface of a cation 
exchange membrane, followed by exposure of the surface to a reducing 
agent. Because known anion exchange membranes have different chemical 
properties than cation membranes, and because common cationic precursor 
salts and related compounds appropriate for electroless deposition of 
desired catalysts on cation exchange membranes are inappropriate for 
deposition of catalysts onto anion membranes, known electroless deposition 
processes for formation of electrodes on cation exchange membranes cannot 
be applied to anion exchange membranes. Therefore, an electroless process 
is needed for formation of a metal catalyst loaded electrode on an anion 
exchange membrane. 
Accordingly, it is the general object of the present invention to provide a 
process for formation of an electrode on an anion exchange membrane that 
overcomes the deficiencies of the prior art. 
It is a more specific object of the present invention to provide a process 
for formation of an electrode on an anion exchange membrane that can be 
implemented with known compounds and materials. 
It is yet another object of the present invention to provide a process for 
formation of an electrode on an anion exchange membrane that achieves 
desired levels of catalyst loading within the electrode. 
It is a further object of the present invention to provide a process for 
formation of an electrode on an anion exchange membrane that results in an 
electrode securely bonded to a reaction surface of the membrane. 
SUMMARY OF THE INVENTION 
A process for formation of an electrode on a polymer anion exchange 
membrane is disclosed to increase rates of reaction at reaction surfaces 
of the membrane. The process includes the steps of soaking a polymer anion 
exchange membrane in a solution containing an anionic entity wherein a 
desired metal catalyst is contained within the anionic entity so that 
anions containing the metal catalyst exchange into the membrane by 
electrostatic attraction, and exposing the membrane to a reducing agent so 
the metal catalyst is reduced to a metallic form to become physically 
secured at a reaction surface of the membrane to thereby form the 
electrode adjacent the reaction surface. In a first preferred process, the 
polymer anion exchange membrane is based on a 
tetraflouroethelyene-flourinated ethylene propylene ("TFE-FEP") backbone; 
the anionic entity is chloroplatinic acid; and the reducing agent is 
sodium borohydride. In a second preferred process, the polymer anion 
exchange membrane has a polyolefin membrane backbone; the anionic entity 
is potassium tetrachloroplatinate; and the reducing agent is sodium 
borohydride. In preparation of an electrode on an anion exchange membrane, 
the process is concluded by rinsing the membrane in distilled water and 
then the membrane is cycled through the soaking, exposing and rinsing 
steps until a desired level of catalyst loading is achieved. 
DETAILED DESCRIPTION TO THE INVENTION 
The process of the present invention may be applied to any planar shaped 
polymer membrane capable of transporting anions, many of which are 
well-known in the art. For example, one type of appropriate anion exchange 
membranes are made by radiation grafting ionic groups to a 
tetraflouroethelyene-flourinated ethylene propylene ("TFE-FEP") membranes 
such as those sold under the product designation "RAI R4030 membrane", 
manufactured by Pall RAI, Inc., of Hauppauge, N.Y., U.S.A. Other 
acceptable anion exchange membranes are well-known polyolefin membranes, 
such as those sold under the product designation "Tokuyama Soda AMH 
membrane", manufactured by Tokuyama Soda Co. Ltd., of Tokuyama City, 
Japan. 
Experiments have established that use of specific anionic entities of metal 
catalysts achieve optimal results under specific application procedures, 
but the invention includes application of anionic entities of any metals 
capable of increasing rates of reaction at a reaction surface of an anion 
transporting polymeric membrane. Those metals include platinum, iridium, 
ruthenium, palladium and any other metal that can be reduced in-situ by 
sodium borohydride or other reducing agents. Preferred anionic entities 
for the introduction of platinum metal into an anionic exchange membrane 
include chloroplatanic acid (H.sub.2 PtCl.sub.4 !) and potassium 
tetrachloroplatinate (K.sub.2 PtCl.sub.4 !). Appropriate anionic entities 
for introduction of other metals listed herein include salts well-know in 
the art that contain the metal in an anionic form. 
Any reducing agent that is both capable of converting an anion of a metal 
catalyst to its metal form and that also compatible with a polymer anion 
exchange membrane is appropriate. Experiments have established that sodium 
borohydride (NaBH.sub.4) achieves optimal results. However, other 
acceptable reducing agents include hydrogen gas and various well-know 
organic reducing agents such as formaldehyde. The strength of a reducing 
agent being used depends upon the specific metal being used, in a 
well-known manner.

WORKING EXAMPLES 
In the following description of working examples of the present invention, 
well-known, commonly available polymer anion exchange membranes, anionic 
entities and reducing agents were utilized. 
Example A 
A RAI R4030 tetraflouroethelyene-flourinated ethylene propylene membrane 
cut to a size of approximately a 15 centimeter ("cm") by 15 cm square was 
soaked in 0.1 molar solution of (hereafter "M") sodium hydroxide (NaOH) at 
room temperature for one hour to exchange the membrane to a hydroxide ion 
form. It was then rinsed in distilled water and soaked in a 0.02M 
chloroplatinic acid (H.sub.2 PtC1.sub.4 !) for one hour at room 
temperature. A reaction surface of the membrane turned orange as it 
exchanged platinum ions. The membrane was then rinsed in distilled water 
and soaked for one hour in 1.0M sodium borohydride (NaBH.sub.4) at room 
temperature and evolved hydrogen and turned black and silver as the 
platinum was reduced. The membrane was then cycled two times sequentially 
through the steps of rinsing in distilled water; soaking in the 0.02M 
chloroplatanic acid (H.sub.2 PtCl.sub.4 !) for one hour at room 
temperature; rinsing with distilled water; and soaking in a 0.02M 
chloroplatanic acid (H.sub.2 PtC1.sub.4 !) for one hour at room 
temperature to increase the platinum loading of the electrode to a desired 
level. The resulting membrane exhibited a platinum loading of about 1.6 
milligrams of platinum per square centimeter (Pt/cm.sup.2). Electron 
microscope examination of the resulting membrane established that the 
almost all of the platinum remained adjacent the reaction surface of the 
membrane and did not penetrate into the membrane. 
Example B 
A Tokuyama Soda AMH polyolefin membrane was secured within a rectangular 
box shaped frame of a LEXAN.RTM. plastic wherein the frame had a window 
that defined an area of the reaction surface of the membrane for formation 
of an electrode, and the frame defined bath inlets adjacent a second 
surface of the membrane opposed to the reaction surface. (A membrane 
secured as described above will hereafter be referred to for convenience 
as a "framed membrane".) The framed membrane is appropriate for formation 
of an electrode having specific dimensions at a reaction surface of the 
membrane, as opposed to formation of an electrode over an entire membrane. 
The window of the frame defines dimensions of the electrode. 
The framed membrane was placed in a bath of 1.0M sodium borohydride 
(NaBH.sub.4) at room temperature wherein only the second surface of the 
membrane was exposed to the bath of sodium borohydride through the bath 
inlets and the reaction surface of the membrane within the window was not 
directly exposed to the bath, and the reaction surface of the membrane 
defined within the window was covered with 0.02M potassium 
tetrachloroplatinate (K.sub.2 PtCl.sub.4 !) for one hour at room 
temperature. Then the framed membrane was rinsed with distilled water. The 
membrane defined within the window turned black and silver as platinum 
deposited out of solution. The borohydride was absorbed into the membrane 
and migrated to the reaction surface within the window to reduce the 
platinum ion to platinum metal. 
The framed membrane was then cycled six times through the steps of exposing 
the second surface of the membrane to the sodium borohydride and covering 
the reaction surface of the membrane defined within the window with 
potassium tetrachloroplatinite for one hour at room temperature and then 
rinsing with distilled water in order to increase the platinum loading of 
the electrode to a desired level. The resulting electrode defined on the 
reaction surface of the membrane within the window appeared as a silver 
layer and exhibited a platinum loading of 3.9 mg/cm.sup.2. 
Use of solid polymer membrane electrolytes prepared according to the 
processes of both Example A and Example B has demonstrated very 
satisfactory performance in electrodialytic-type electrochemical cells 
used for salt splitting. In splitting an alkaline salt with a divalent 
anion, a cell using an electrode produced according to the process of 
Example A performed at approximately twenty per cent lower voltage than a 
cell using an electrode produced according to the process of Example B 
when operated at 100 amperes per square foot, while at higher current 
levels, performance of the cells using Example A and Example B electrodes 
was nearly equal. 
While the present invention has been described with respect to particular 
working examples, it will be understood by those skilled in the art that 
the present invention is not limited to those examples. Accordingly, 
reference should be made primarily to the attached claims rather that the 
foregoing specification to determine the scope of the invention.