CA1195949A - Hydrogen chloride electrolysis in cell with polymeric membrane having catalytic electrodes bonbed thereto - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved anode for use in the electrolysis of hydrogen chloride for the generation of chlorine gas in an electrolytic cell having a solid polymer electro-lyte membrane with a cathode bonded to one side of the membrane and an anode bonded to the other side of the membrane, is described. The length of the diffusion path within the anode where the electrolytic oxidation takes place, is decreased or the porosity of the anode where the electrolytic oxidation takes place, is increased, to increase the rate of transport of the reactants (hydrogen chloride) and the reaction products (chlorine gas) within the anode. The diffusion path length is decreased by de-creasing the thickness of the anode catalyst material. A
preferred anode catalyst for the oxidation of an aqueous hydrogen chloride solution has a thickness of about 6.0 microns to about 50.0 microns.

Description

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HYDROGEN CHLORIDE ELECTROLYSIS IN CELL WITH POLYMERIC
MEMBRANE HAVING CATALYTIC ELECTRODES BONDED THERETO
This invention relates to the electrolysis of hydrogen chloride, and more particularly, to improved anodes for electrolytic cells which generate chlorine from hydrogen chloride.
Hydrogen chloride is a reaction by-product of many manufacturiny processes which use chlorine gas. For examplel chlorine ls used to manufacture polyvinylchloride and isocyanates, and hydrogen chloride is a by-produc-t of these processes. In certain instances, there is no use for the hydrogen chloride resulting from th0se processes, and oxidation of the waste hydrogen chloride to produce chlorine is often used to generate chlorine so tha~ the waste hydrogen chloride can be converted to a useful pro~
duct and reused or recycled.
The recovery of chlorine from hydrogen chloride is possible by both electrochemical and thermochemical processes. The electrochemical, iOe., electrolytic, systems are generally more advantageous when smaller quantities o~ hydrogen chloride are involved such as those installations which have an annual production rate of lèss than 160,000 tons of hydrogen chloride.
Solid polymer electrolyte electrolysis sytems have been used for the generation of chlorine from aqueous ~s~

hydrogen chloride as well as from sodium chloride (brine) solu~ions. The solid polymer electrolyte membra~e system used for hydrogen chloride elec~rolysis consists of a pair of catalytic electrodes in electrical contact with the surface~ of an ion exchange membrane, (also referred to herein as a solid polymer electrolyte membrane~. Other conventional components o the electrolysis cell include means for delivering curren~ and a c~rrent sourc~ as well as means for the delivery of reactants to ~he chambers and electrodes and means to remo~e the reaction products ~rom the chamb~rs. The electroly~ic eells are divided into an anode chamber and a ca~hode chamber by the solid pol~mer electrolyte membrane which has the anode and the cathode physic~lly attaohed, as by bonding or the like, to the sur-faces of the membrane; the anode chamber being th~ chamber adJacent to the ~node which is bonded to the solid polymer electrolyte membrane, and the ca~hode chamber being ad-jacent ~o the cathode which is bonded to tlle solid pol~mer electrolyte mem~ra~e surface.
In operation, aqueous hydrogen chloride is supplied to the anode ehamber of the elec~rolytic cell.
Hydrogen chloride diffu~es into the anode from the aqueous ~ydrogen chloride medium, and chloride ion is discharged at or very near ~he anode/membrane interface. The proton ~5 (H~) migrates across the membrane and is discharged a~ the cathode where it diffuse~ into th cathode chamber and is remo~ed ~herefrom as molecular hydrogen. Some water is electro-osmotically transferred acxoss the mPmbrane by the pro~on flux, and a quantity of hydrogen chloride ~150 difuses through the membrane ~o the rathode chamber to form dilute hydrogen chloride in the ca~hode chEmber. The chloride ion discharged at or near the anode/solid polymer electrolyte membrane interface converts to molecular ~hlorine and diffuses ~hrough ~he anode intc ~he anode chamber and is removed from the anode chamber by suitable removal means. Depleted hydrogen chloride is removed ~rom the anode chamber, and dilute hydrogen chloride is removed from the cathode c~amber by suitable means.
Generally, the deple~ed hydrogen chloride and a dilute hydrogen chloride are in aqueous form and are sufficiently low in hydrogen chloride content so that they can be discharged as waste or recycled for resa~uration with HC1 gas.
One of the disadvantages of the prior art electrolytic devices using a solid polymer electrolyte membrane with electrodes forming a part of the membrane has been the generation or evolution of oxygen which leads to the corrosion of the electrode components and current collector elements and generally contributes to the inefficiency of the electrolytic process. The oxygen evolution occurs when there is chloride starvation in the anode, and the cell curren~ is sustained by the electrolysis of water derived from the aqueous medium in the aqueous hydrogen chloride and/or from water within the hydrated membrane according to the following equation:

2 ~3~ 2 + 4H + 4e The oxygen evolution reaction is suppressed by acidic pH which increases the reversible potential o~ the process and by high chloride ion concentration which facilitates the desired reaction. Thus, a high rate of transfer of hydrogen chloride to the reaction site (in the anode or at the anode/membrane interface~ is beneficial to system operation.
Accordingly, it is the primary object of this invention to provide a method and device for improving the electrolysis of hydrogen chloride.

It is another object of this invention to pro vide a method and d~vice for substantially redueing or eliminating oxygen evolution in an electrolysis cell Q~
tne type usin~ a solid polymer electrolyte membrane with - 5 electrodes bonded to and forming a part of the surfaces of the membrane when chlorine is generated from aqueous hy drogen chloride.
It is another obj ect of this invention ~o pro-vide an apparatus and method which improve~ the rate of transfer of hydrogen ch~oride in an aqueous medium in ~che anode chambcr of an electrolysis cell to the rea~tion sîte in the anode or at the anode/membrane interface.
Still another obj ect of this invention is to provide a method and apparatus which permits the use of lS feed hydrogen chloride solutions of lower concentrations into the anode of an electroly~ic device in which chlorine gas is generated from the hydrogen chlo~ide.
Another object of this invention is to provide an pparatus and device whieh permits electrolysis of hydrogen chloride in an aqueous medium at higher curren~
densities.
Other objects and advantages of the invention will become apparent as ~he description thereof proceeds.
It has beerl di~covered that elec~roly~9is of hydrogen chloride in an electrolytic cell having a solid pol~mer electrolyte membrane, ~n anode into which hydrogen chloride dif:fuses and oxidizes, the anode bein~s bonded to one surface of the solid polymer electrolyte membrane, and a cathode bonded to the other surface of the solid polymer elec~roly~e membran~ 9 is improved by de~reasing the diffu-sion path length withi~ the anode. The decrease in the .. diffusion pa~h l~ngth in the anode increases the rate of transport of hydrogen chloride into the a~ode. It also in-creases the rate o transport of the reaction products out of the anode. Length of the diffusion path may be decreased s~

by decreasing the thickness of the anode. True diffusion path length is related to tortuosity and electrode thickness.
It has also been discovered that electrolysis of hydrogen chloride in an electrolytic cell haviny a solid polymer electrolyte membrane, an anode into which hydrogen chl.oride diffuses and oxidizes, the anode being bonded to one surface of the solid polymer electrolyte membrane, and a ca-thode bonded to -the other surface of the solid polymer electrolyte membrane, is improved by increasing the porosity of the anode catalyst material. The increase of porosity in the anode -to a void volume greater than 60 percent also increases the rate of transport of hydrogen chloride into the anode.
In particular, the present invention provides a method for the electrolysis of hydrogen chloride in an electrolytic cell having a cation transporting solid polymer electrolyte membrane, a porous gas and liquid permeable catalytic anode having tortuous pores extending therethrough being bonded to one surface of the solid polymer electrolyte membrane whereby hydrogen chloride and chloride ions diffuse through the pores -toward the surface o:E the cation transporting membrane to be oxidized and form reaction products, and a cathode catalyst bonded to the other surface of the solid polymer electrolyte membrane comprising -the step o:E
maximizing the transport rate of hydrogen chloride and chloride ions into said porous anode by maintaining a minimum diffusing path within the anodes as a function of the -thickness, which is less than 100 microns, and porosity of the anode, which is represented by a void volume greater than 60 percent, and the tortuosity of the pores whereby the rate of transport of the chloride ions to the electrode is sufEicient to sustain the cell current essentially by discharge of the chloride ions -to produce chlorine thereby minimizing co-evolution of oxygen.
In another aspect, the present invention provides a method for reducing the amount of oxygen generated in -the electrol-ysis of an aqueous chloride in an elec-trolytic cell having a hydrated cation transporting polymeric membrane, a ca-thode bonded to one surface of the membrane and a gas and liquid permeable anode bonded to the other surface of the polymeric membrane wherein aqueous chloride and chloride ions diffuse into the anode and are oxidized therein to produce chlorine, maximizing the transport rate of aqueous chloride and chloride ions into the porous anode by maintaining a minimum diffusion path within the anode as a function both of the porosity of the anode and the anode thickness by maintaining the thickness of the anode between 6.0 microns to 50.0 microns and by providing porosity such that the void volume of the anode is greater than 6~ percent whereby the rate of transfer of the chloride ions to -the anode is sufficient to sustain cell curren-t by discharge of the chloride ions while minimzing co-evolution of other electrolysis products.
The invention further provides in an apparatus for the electrolysis of hydrogen chloride in an electrolytic cell having a cation transporting solid polymer electroLyte membrane, a porous gas and liquid permeable catalytic anode having tortuous pores extending therethrough said anode being bonded to one surface of a solid polymer electrolyte mernbrane, whereby chloride ions diffuse through -the pores from one surface of the anode towards -the cation transporting membrane t.o which the anode is adapted to be bonded, to allow the chloride ions to be oxidized thereto form chlorine gas, the improvement in which said catalytic anode comprises a s-tructure with a -thickness of less than 100 microns -to maximize the -transpor-t ra-te of hydrogen chloride and chloride ions into and within said pores and in which the diffu-sion path length of the pores is a function of the -thickness and porosi-ty of the anode, which has a void volume greater than 60 1.0 percent, and of the tortuosity of the pores whereby the ra-te of transfer of the chloride ions is sufficient to sustain cell current by discharge of the chloride ion while co-evolution of other electrolysis products is minimized.
The invention further provides in an apparatus for -the genera-tion of chlorine from hydrogen chloride by electrolysis, an electrolytic cell having a cation transporting solid polymer electrolyte membrane a porous~ gas and liquid permeable catalytic anode bonded to one surface and a cathode bonded to the other surface of -the membrane, the cation transporting membrane dividing -the electroly-tic cell into an anode chamher on the side of the membrane having -the anode and into a cathode chamber on -the side of the membrane having -the cathode, means for providing elec-trical current at the anode and the ca-thode, feed means for feeding an a~ueous hydrogen chloride anolyte into the anode chamber, means :Eor removing chlorine and depleted hydrogen chloride anolyte from the anode chamber, and means for removing hydrogen from -the cathode chamber, the improvemen-t comprising an anode of - 6a -a thickness of about 6.0 microns to 50.0 microns and a void volume greater than 60 percent to minimize the dif-Eusion path length to provide an increase in the rate of transport of hydrogen chloride and chloride ions -towards the surface of -the membrane.
Thus, the invention concerns a method for improving the electrolysis of hydrogen chloride in an electrolytic cell having a solid polymer electrolyte membrane, an anode ca-talyst into which hydrogen chloride diffuses and oxidizes to form reaction products, the anode being bonded to one surface of the solid polymer elec-trolyte and a cathode catalyst bonded to the other surface of -the solid polymer electrolyte membrane, comprising decreasi.ng the diffusion path length within the anode catalyst and increasing the rate of transport of hydrogen chloride in the anode catalyst.
The rate of hydrogen chloride transport to the chlorine evolution sites in the anode or at or near the anode/membrane interface is increased and optimized by decreasing the diffusion path in the anode or increasing -the porosity of the anode or both.
In accordance with the present invention, it has also been discovered that there is a decrease in oxygen generation when the ~0 di:Efusi.on path length is decreased in the anode or the porosity of the anode is increased or both.
The irnproved electrode for the electrolysis of hydrogen chloride in an electrolytic cell having a solid polymer electrolyte membrane, is a porous anode into which hydrogen chloride diffuses and oxidizes, said anode being bonded to one surface of the solid polymer electrolyte membrane, and a cathode bonded to the other surface of the solid polymer electrolyte membrane, wherein -the - 6b -'' ``

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improvement comprises anode material bonded to the solid polymer electrolyte membrane in an amount which decreases the di~fusion path length within the anode and thereby increases the rate of transport of hydrogen chloride into the anode. The amount of anode material on the membrane is decreased by providing an anode of lesser thickness as will be described in more detail herein-after. There is also an improved gas and li~uid permeable electrode for the electrolysis of hydrogen chloride when the anode material has an increased porosity.
The anode of the invention having decreased diffusion path length or increased porosity or bo-th to provide an increase in the rate of transport of hydrogen chloride into the anode, may be disposed in an apparatus for the generation of chlorine from hydrogen chloride by electrolysis wherein the electrolysis is carried out in an electrolytic cell having a - 6c -~ ~ 9 ~ ~ ~ 9 52-EE-0 319 solid polymer eleetrolyte membrane with the anode bonded to one surface ~nd a ca~hode bonded to the other surface of ~he solid polymer elec~xolyte membrane, the solid poly-mer electrolyte membrane dividing the elec~rolytic cell into an anode ch~mber on ~he slde o the membrane having the anode and into a cathode chamber on the side o the membrane having the eathode, means for providing electri-cal curre~t at ~he anode and the cathode, eed me~ns for feeding hydrogen chloride into the anode chamber, means for removing chlorine and depleted hydrogen chloride from the anode chamber and means for removing dilute hydrogen chloride and hydrogen from the cathode cnamber.
In aecordance with the presPnt invention, it has been found that by reducing the thickness of the ~node lS and thereby decreasi~g the length of ~he diffusion path through whieh hydrogen chloride and the oxidation products of hydrogen chloride mus~ pass, or by increasing the porosity of the a~ode, parasitic oxygen evolution i5 SUp-pressed, subs~antially redueed or elimlnated. It has been found that this penmits the usP of feed hydrogen chloride of lower concen~ratlons and also permi~ elee~ro~
lysis of hydro~en chloride at higher current densities.
It has been found ~hat ~he incr~ased rate of tran~port per-mits ~he use of ~he hydrogen chloride at low~r concen~ration ,' 25 in an aqueous or other medium and the operatiorl o tlle elec-trolytic cell at a higher curren~ density, and ~ha~ the levels of oxygen in the evolving chlorine gas in el~ctro-lytic cells h~ving the improved anodes of the present invention are lower than ~he levels of oxygen in ~he chlo-rine ~as of the prisr art sys~ems having thicker, less porous anodes.
These and various other objects, features and advantages of ~he inven~ion can best be understood from the following detailed descriptions taken in conJunction with the accompanying drawings in which:

~ ~ 9 ~ ~ 49 52-EE 0-319 , FIGURE 1 is a diagrammatic illustra~ion o~ a typical elec~rolysis cell for the generation o~ chlorine from ~qu~ous hydrQgen cllloride.
FIGURE 2 is a schematic illustration of ~he electrodes and the solid polymer elertrolyte membrane as well as ~he major reactions which take place in ~his por~
tion of the elee~rolytic cell.
FIGURE 3 is a graph illustrating the improved curren~ densi~y in an electrolytic cell which utilizes an anode of reduced thiekness in the preparation of chlorine from aqueous hydrogen chloride.
FIGU~E 4 is a graph illustra~ing ~he effec~ of anode thickness reduction on oxygen content in chl~rine gas prepared by the oxidation of hydrogen chloride in - lS an electrolytic cell.
FIGURE 5 is a graph illus~rating tne vol~me per cent of oxygen in effluent chlorine gas a~ various current densities rela~ivc to the conoentration in moles of effluent aqueous hydrogen chloride.
In FIGURE 1, a ty~ical eleo~rolysis cell is shown generally at 10 to illustrate the genera~lon of chlorine from aqueous hydrogen chloride in accordance with the present invention. Electrolysis cell 10 co~sists of a cathode compartment or chamber ll, an anode compar~ment or chamber 20 and a solid poly~er electrolyte membrane 13 which is p~eferably a hydrated perm~elective catlon PY~h~n~e membrane and separates cathode ~hamber 11 from - anode chanber 20. The gas and liqllid permeable ç~lec~rodes are bonded to, and physically form a part o~, the surfaces ~ 30 of solid polymer electrolyte membrane 13. Ca~hode 14 is ~ i95 ~ ~ 9 52-EE-0-319 bonded to one sidP of the solid polymer electrolyte mem-brane 13 and a catalytic anode (not shown) is bonded to _ the other side of solid polymer electrolyte membrane 13.
Each of the respective elec~rodes physically forms a part of membrane 13 and is -.in electrical con~act with a surface of the solid polymer electrolyte membrane 13. Cathode compartment 11 is located on the side of the solid polymer electrolyte membrane having the cathode ~hereon. Lîke-wise, anode compartment 20 is located on ~hat ~ide of solid polymer electrolyte membrane 13 which bears the anode.
- Typical of the composition of the anode material upon the surface of solid polymer electrolyte membrane 13 -- i5 an anode material having par~icles of a fluorocarbonr sucn as the fluorocarbon sold by E.I. Dupont de Nemours, - & Co. under its trademark "TEFLON" bonded to stabilized reduced oxides of ru~henium or iridium, stabilized re-_ duced oxides of rutheniumtiridium, rutheniu~/titanium, ruthenium/titanium/iridium, ruthenium/~antal~m/iridium, rutnenium/graphite and the like. ~he anode composi~ion is not critical in the practice of the present invention.
However, the anode ma~erial ~ust be deposited upon, bonded to or otherwise physically made ~ part of th~ surface of the solid polymer elec~rolyte membrane. The porosi~y of 2S ~he anode must be sufficlent to permit the diffusion o hydrogen chloride into the anode and the diffusi2n of chlorine Ollt of the anode~ In accordance wi~h the present invention, the porosity of the anode ma~rial must be increased, or the thickness of the layer of anode material bonded to the solid pol~mer electrolyte membranc mu5t be decreased, or bo~h to obtain the increased hydrogen chlo ride difusion rate and the decreased oxygen generation.

_9_ 10 - 52-EE-0~319 The cathode, shown at 1~, may be a Te~lon*-bonded cathode and is similar to the anode catalyst.
Suitable cathode catalyst materials include finely-di~ided platinum, palladium, gold, silver, spinels, manyanese, cobalt, nickel, reduced platinum-group metal oxides, reduced platinum/ruthenium metal oxides, graphite and the like and suitable combinations thereof. The graphite or other catalyst materials deposited upon -the surface of the solid polymer electrolyte membrane are not critical in the practice of the present invention and many well-known cathode materials may be used as the cathode in the present invention just as many well-known anode materials may be used as the anode of the present inventionO
In one preferred embodiment, a graphite sheet (not shown in FIGURE 1 but illustrated in FIGURE 2 as numeral 36) may be used between cathode 14 and cathode current collector 16.
Current collectors in the form of metallic screens 15 and 16 are pressed against the electrodes.
The entire membrane/electrode assembly is firmly supported between the housing elemen-ts 12 and 26 by means of gaskets 17 and 18 which are made of any material resistant to or inert to the cell environment, namely, chlorine, oxygen, hydrogen chloride or aqueous hydrogen chloride and the like. One form of such a gasket is a filled organic rubber gasket of ethylene propylene terpolymer sold by the Irving Moore Company of Cambridge, Massachusetts and commonly known as EPDM rubber. Another preEerred gasket material is lead oxide cured VITON. VITON is a trademark of E.I. duPont de Nemours and Co. Gaske-ts 17 and 18 may be any suitable sealing means including cement to secure the elements together or O-rings to seal the respective chambers. In certain embodiments gaskets or cement ]7 and 18 may be omitted.
*~rademark The a~ueous hydxogen chloride solution, gener~
ally a waste product from a chemical processing plant, is introduced through elec~rolyte inlet 19 which commllnicates with anode chamber 20. Spent electroly~e (hydrogen chlo~
ride) and chlorine gas are removed through outlet conduit 21 which also passes through housing 12.
An optional cathode inlet conduit (not shown) may c~ u~licate with cathode chamb~r 11, that is, the chamber ~ormed by housing element 26, gasket 17 and cathode 14~ to permit the introduction of catholyte, water or any other suitable aqueous medium into the cathode chamber. The cathode inlet conduit i~ option~l, and generally there is no adva~tage in circulati~g catho~
lyte through cathode chambex 11 in the electrolysis of hydroyen chloride. Cathode outlet conduit 22 communica~es with cathode chamber ll to remove the dilute aqueous hydrogen chloride which migrates through membrane 13, hydrogen discharged at the cathode, and any excess water or other catholyte. A power cable or lead 24 is brought into the cathode ohambex and a comparable cable or lead (not shown) is brought into the anode chamber. The cables connect the cuxxent conductin~ screens 15 and 16 to a source of electrical power tnot shown).
In operation, aqueous hydrogen chloride is supplied to anode cham~er 20 in the cell of FIGURE 1.
Hydrogen chloride difuses into the anode (not shown~
from the bulk feed aqueous hydrogen chloride. Chloride ion is discharged in the anode at or very near the anode/
solid polymer electxolyte membrane interface, and protons 3Q (~+) migrate across m~mbrane 13 and are discharged as hydrogen at cathode 14. Some water is elec-tro-osomotically transferred acxoss mem~rane 13 by the proton flux, and a ~uantity of hydrogen chloride diffuses thxough solid polymer elec~rolyte membrane 13 to cathode chamber ll~

S~

- 1.2 - 52-EE-0-31~

The membrane potential which is established by the difference in acid activity across the membrane is exactly compensated for by the change in cathode poten-tial due to the lower portion acti~ity, and the electrolytic cell operates as if both electrodes were immersed in acid of the anolyte concentration. Thus, a separate cathode ~eed (cathode inlet condu.it) is optional, and there is generally no advantage to the separate cathode feed.
In FIGURE 2, there is illustrated a cross-section of a portion of -the electrodes, solid polymer electrolyte membrane, and currenk collectors in a preferred electro-lytic cell configuration showing the improved anode of -the invention. The major reactants and reaction products and their migration through the electrodes and solid polymer electrolyte membrane are schematically represented in FIGURE 2. Porous anode 39 is bonded to one sur~ace of solid polymer electrolyte membrane 33, and porous ca-thode 34 is bonded to the other surface of solid polymer elec-trolyte membrane 33. Anode current collector 32 is a metallic point contact col.lector and is in electrical contact with porous anode 39. Current collec-tor 38 is a metallic point contact collector and is in electrical contact with graphite sheet 36 which in turn contacts ~athode 34. Point contact collectors~ corrugated metal contact devices~ metal screens and various other conduc-tive current collectors may be used in electrical contact with the electrodes. Porous anode 39 and porous cathode 34 are bonded to solid polymer electrolyte 33 in any well.-known manner to establish electrical contac-t between the electrode and the respectlve surEace of solid polymer electrolyte membrane 33. The decreased thickness o~
anode 39 relative to cathode 34 is evident ~rom the illustration in FIGURE 2, however, -the embodiment shown in FIGURE 2 is no-t necessarily drawn to scale. I-t can

3~ be seen in FIGURE 2 -that -the dif~usion path in porous ~195i9~9 anode 39 is relatively shoxt or substantially decreased over the length of the diffusion path in cathode 34~ In accordance with the present invention, ~he length of the diffusion pa~h in porous anode 39 can be decreased hy de creasing the thickness of anode 39. ~he diffusion rate of hydrogen chloride can al~o be increased by increasin~
the porosity of anode 39.
In FIGURE 2, i~ can be seen that hydrogen chlo-ride generally in aqueous solution, diffuses into porous anode 39. In porous anode 39 th2 hydrogen chloride is oxidized to hydrogen ion (H~) and chloride ion (Cl~], and ~he chloride ion (Cl+) i~ urther oxidized to chlorine gas (C12~. Protons (X~) and water are tr nsported throug~
solid polymer electrolyte membrane 33 which i5 preferably a permselective cation exchange membrane well known in t~e art, along with small amounts of hydrogen chloride. The hydrogen chloride and water form a dilute hydrogen chloride in the cathode chamber, and hydrogen ion (H+) is convert~d to hydrogen gas (H2).
In a parasitic side reaction, oxygen gas is formed at the anode ~nd becomes mixed with the ~hlorine gas. As described above, this parasitic reaction is very undesirable in the hydrogen chloride electrolysis system because the evolution of oxygen decreases cell efficiency and leads to rapid corxosion of graphite and other elec ~rode components and current collector elements in the cell. This parasitic side reaction resulting in oxygen evolution sustains the cell cuxrent when there is chloride starvation at the anode, that is, when there i5 insu~fi-cient chloride diffusing into the anode for oxidation at oxidatiQn sites within the anode or at the anod~/solid polymer electrolyte membrane interface. The parasi~ic oxygen evolution reaction may be illustrated as follows:

2 2 ~~-~ O ~ 4H+ ~ 4e~

The decreased leng~h of the diffusion path within anode 39 o FIGURE 2 or the increased porosity within anode 39 of FIGURE 2 or both, in accordance ~ith the present invention~ s~bstantially reduces or elim;~tes this parasitic reaction by providing a gr ater amount of diffusion of ~he hydrogen chloride in~o ~h~ a~ode so that the hydrogen chloride can be oxidized at oxidi~ation 5ite5 within the anode or at the anode/solid polymer elec~rolyte membrane interface. The decreased length of the diffu~ion path or the increased porosi~y wi~hin the anode also per~-mits an increased rate of transpoxt of the chlorine gas from the reaction or oxidation sites wi~hin the anode or at the anode/solid polymer electrolyte membrane interface, into the anode chamber. It has besn found ~hat the rate lS o transport of the hydrogen chloride, the chlorine gas and other reaciants and produc-ts is substantially increased when the thickness of the anode is d creased ox the porosi~y of the anode is increased or both. The prior d~vices having anodes of at least 100 micro~s in thicknes~ result in a subs~antially greater volume perce~tage of oxygen in the chlorine gas produced ~y the electroly~is of hydrogen chloride in the anode compartment than ~he ~lectroly~is cells of the present invention wherein th~ anodes are less than 100 microns in thickness and praferably about 6.0 microns to about 50.0 micron~ in thicknessO The most preferred embodiment appears to be realized when ~he thickrless of the anode is about lOoO mi~rons to about 13.0 micro~. This Lmprovement is illustrated in ~h~ graph .in FIGURE 4 where the molarity of spent hydrogen chlorlde 30 in wat~r is plotted against the volume percent o oxygen contamination in chlorine gas effluent from th~ anode compar~ment of an electrolysis cell having a feed s~ream of a~ueous hydrogen chloride.
The graph in FI~U~E 4 ~hows the volume pereent of oxygen in ~he stream of chlorine gas ~or anodes which ~14-are 100 microns in thickness, 50 microns and 13 microns in thickness. Although the cell current differs between the 13 micron thick anode ma-teri.al and the 50 and 100 micron thick anode materials in the graph representation in FIGURE ~
showing the effect of anode thickness reduction on oxygen content in effluent chlorine gas, the resul.ts are even more sig:nificant because at 1,000 amps/ft.2, chloride ion is consumed at a rate 250% greater than at ~00 amps/ft.2, yet the embodiment havi.ng a 13 micron thick anode has a substantially lower oxygen level at acid concentrations greater than 9 moles. At ~00 amps/ft.2, the oxygen levels from the 13 micron thick anode are very low, as shown in FIGURE ~.
The best cell performance for the electrolytic hydrcgen chloride was demon-strated in an electrolytic ce].l having an anode (graphite) 6 microns thick. In that cell, the oxygen level was 0.1% by volume in the chlorine gas exiting from the anode chamber when the anolyte was a ~.5 molar aqueous hydrogen chloride, and the cell was operated at 600 amps/ft.2.
In the electrolysis of hydrogen chloride in accordance with the pre-sent i.nvention, the transport of the hydrogen chloride into the anode occurs primarily by diffusion. When the rate of hydrogen chloride consumption in the anode exceeds the rate at which it is supplied by the diffusive transpor-t, oxygen is concurrently evolved with the chlorine. As explained above, the oxy~en is an undesi.rable contaminant in the chlorine gas product because it leads to decreased cell efEiciency and corrosion of cell components. It is for tllis reason that the invention is directed to increasing the rate o-f hydrogen chlo:ride transport into the anode by decreasing the length of the dif:Eusion path. This is accompli.shed by decreasing the thickness of the anode. The ra-te of hydrogen chloride transport into the anode may also be increased by incxeasing the porosity of the anode or by both re-ducing the thickness of ~he anode and increasing the poros.ity of the anode. This increased rate of transport permits the use o~ hydrogen chloride of lower concentra-tions and permi~s the operation of ~he electrolysis cellat a higher current denslty with the resulting leYels of o~ygen in chlorine gas bei~g lower than that o~ ~he prior art systems. In accordance with the presen~ invention, it is the length of the diffusion pa~h, ~ha~ i~, the ~h;rkness of ~he anode material, and/or the porosity of the anode material which is critical. The thickne~s and porosi~
of the cathode is no~ ~ critical aspect in the pres~nt invention r and standard thicknesses generally apply to the cathode material.
lS In accordance with the present i~vention, control of the tortuosity of the diffusion path relates to the length of the diffusion path, and generally the tortuosity remains constant in the anode. The relationship between dif~usion path length, tortuosity and electrode thickness is as follows:
DIFFUSION PATH LENGTH ~ TORTUOSIT~ X ELECTRODE THICKNE5S
where tortuosity i5 a constant.
By tortuosity, as used herein, is mean~ repeated ~wists, bends, turns, windings, and the general circuitous-25 ness of channels or pores wi thin the anode material ~, Thus,an increase in pore size can result in increased ~
cation between pores and ch~nnels within the anode material and thereby result in an increase in the diffu~ion rate at which hydrogen chloride and the oxidation products o hydro-gen chloride diffusively pass into~ through and out of theanode material~
In the anode material in electxical contact with the solid polymer electrolyte membrane in the electrolytic cells o the present invention, hydrcgen chloride is tran~
ported to the interface of the anode material and the ~s~

membrane by both diffusion and convective motion of the pore liquid caused by the transfer of solvents ~water~ across the membrane. Ilydrogen chloride leaves the pore liquid by two mechanisms. One of the mechanisms is by consumption in the electrode reaction and the o-ther by diffusion across the membrane. Diffu-sion of hydrogen chloride within the electrode occurs only within the pore liquid. Since the pores are -formed by a bed oE random:Ly o:riented particles, the true diffusion path is greater than the anode thickness. Thus, tortuosity and porosity become important factors in the oxidation reaction which takes place within the anode material or at the interface between the anode and the solid polymer electrolyte membrane. Lack of porosity can be particularly troublesome when the pores in the anode become partially obstruc-ted with gas. The present invention reduces or eliminates this troublesome problem. It decreases the length of the diffusion path by decreasing the thickness of the anode, and/or i-t increases the porosity of the anode to overcome these disadvantages and difficulties.
Additional information relating to the construction and operation of electrolysis cells having catalytic electrodes bonded to the surface of a solid po:lymer electrolyte membrane for the production o~ halogens can be found in Canadian Application Serial No. 315,519 filed October 13, 1978 by Dempsey et al under the -title "Production of Halogens by Electrolysis of Alkali Metal Halides in an electrolysis Cell Having Catalytic Electrodes Bonded to the Sur~ace of a SoLid Polymer Electrolyte Membrane." Other similar electrolysis cells and com-pollents of electrolysis cells are described in the prior art including U.S.
Patent No. 3,992,271 issued November 16, 1976 to Danzig et al, relating to a method for gas generation.
The catalytic electrodes may be constructed by any of the techniques well-known in the art. Anode and cathode ma-terials may be prepared by the ~ 5~

Adams method or by modifying the Adams method or hy any other similar tech-niqu~s. Anodes of decreased thickness may be parpared as decals and suitably bonded to the surface of solid polymer electrolyte membranes, or they may be made by the -17a-~ ~5~

dry process technique which embraces abrading or roughen-ing the surface of the solid polymer electroly-te membrane, preferably to place a cross-hatched pattern in the surface of the membrane and fixing a low loading of anode catalyst particles upon the patterned surface, or they may be made by any well-known prior art process~ In the dry process technique described in U.S. patent No. 4,272,353 issued June 9, 1981 to Lawrance et al enti-tled "Method of ~aking Solid Polymer Electrolyte Catalytic Electrodes and Electrodes Made Thereby" and assigned to the instant assiynee, anode catalyst material is applied to the surface of a solid polymer electrolyte membrane by first roughening the surface of the solid polymer electrolyte membrane; clepositing anode catalyst particles upon the roughened surface, e.g., by heat and/or pressure. The membrane is preferably in a dried s-tate during the process and may be suitably hydrated after ~he fixing of the anode catalyst. ~ preferred cross-hatched pattern is placed in the membrane surface during the roughening step or steps by sanding the membrane with an abrasive in a first direction followed by sanding the membrane with the abrasive in a second direction, preferably at a 90 angle to the first direction.
Ion exchange resins and solid polymer electrolyte membranes are described in U.S. Patent NoO 3,297,484 issued January 10, 1967 to Niedrach where catalytically ac-tive electrodes are prepared from finely-divided metal powders mixed with a binder such as polytetrafluoroethylene resin, and the electrode comprises a bonded structure formed from a mixture of resin and catalyst bonded upon each of the two major surfaces of a solid polymer electrolyte solid matrix, shee-t, or membrane.
The resin and catalyst are formed into an electrode struc-ture by forming a film from an emulsion of the material;
or alternatively, the mixture of resin binder and catalyst material is mixed dry and shaped, pressed and sin-tered onto a sheet which can be shaped or cut to be used as the elec-trode, and bonded to the solid polymer electrolyte membrane.
The resin and catalyst powder mix may also be calendared, ;
.

~:L~

pressed, cast or ctherwise formed into a sheet or decal. Alternately a fibrous cloth or mat may be impregnated with the mixture o:E binder and catalyst mate-rial or surface coatecl with a mixture of binder and cakalyst material. In other prior art techniques, the elec~rode material may be spread upon the sur-face of an ion exchange membrane or on the press platens used to press the elec-trode material into the surface of the ion exchange membranel and the assembly of the ion exchange membrane and the electrode materials are placed between the platens and subjected to sufficient pressure preferably at an elevated tempera-ture sufficient to cause the resin in either the membrane or in the admixture with the electrode catalyst material either to complete the polymeri~ation i~
the resin is only partially polymeri7ed, or to flow if the resin contains a thermoplastic binder. The method of bonding the electrode or electrodes to the surface of the membrane so that they physically form a part of the membrane in accordance with the present invention is not critical, and any of the well-knol~l prior art techniques may be used as long as the gas and liquid permeable anode of reduced thickness and/or increased porosity results.
Porosity may be increased by any well-known prior art ~echniques.
One method of increasing the porosity is by incorporating solvent-soluble addi-tives or particles in the anode material prior to -the formation of the anode, thereafter forming the anode and treating the anode with solvents to remove the solvent-soluble material therefrom. For example, solid calcium carbonate of suitable si~e can be incorporated in the anode material before the anode is for3ned and dissolved by using mineral acid after the anode is formed. In-creased porosity can also be accomplished electrochemically by incorporating additives in the anode material which can be removed electrochemically after the formation of the anode in the desired form or after the anode material has been deposited upon the surface of a ~olid polymer electrolyte membrane.
It is also within the purview of one skilled in the art to include additives which vaporize by heating or sintering, into the anode material prlor to the formation of the anode and thereafter removing the vaporizable material by the application of heat. This step may occur simultaneously or concurrently with the sintering of the anode material.
The porosity in the anode may also be increased by increasing the particle size of the powder components, e.g., the particle size of the metal, metal oxide, metal alloy and/or binder material such as Teflon*, which are used to form the anode. For example, by increasing the size of the powder components from 2-5 microns ln diameter to 8-lO microns in diameter, the resulting anode will have a greater porosity, that is, the pores or channels in the anode material will be larger, and the diffusion rate of hydrogen chloride through the anode will be improved.
However, in accordance with the present in~ention, it was also discovered that the parasitic generation of oxygen is substantially reduced or eliminated when the porosity, that is, pore size, or number of pores or both is increased~
The porosity of the anode may also be increased by increasing the irregularities in the shape of the solid or powdery components, particles or elements of the anode material, or by increasing the si~e or number of irregulari-ties upon the surEaces of powder components in the ancde material. For example, a spheroidal-shaped particle will have little or no irregularity upon its surface, but if the surface is distorted or stressed, the irregularities upon the surface increase, and when such particles are used as components of the anode material, the anode porosi-ty will be greater~ Porosity is a function of *Trademark s~ructure. Therefoxe, packed flakes result in a less porous anode than packed spheres, and packed particles having irregular shapes xesult in a more porous anode than packed spheres. Accordingly, porosity can be increased by changing the geometry and surface irregularities in ~he particle~.
As used herein, an increase in porosity may be an increase in the size of ~h~ pores or channels w.ithin the anode or an increase in the number of pores or channels within the anode, or both, and such an increase will result in increa~ed hydrogen chlorid~. diffusion and decreased paxasitic oxygen generation.
Generally, porosity or void volume of the prior ar.t anodes i5 about 50~ or less (by ~olume). In accord-ance wi~h ~he present invention, the porosity or voidvolume is preferably increased at least 20% (by volume~
and most preferably by at least 50~. Thus, prefexred void volumes or porosity are at least about 60% a~d more preferably at least about 75~ The upper limit of poro~
sity is that void volume wherein the pore vol~e is so great that there is insuficient electrical continulty for the flow of current and/or an insufficient n~mber of catalytic reaction sites in the anode catalyst. Generally, the void volume is increased in accordance with th~ pr~sent ~5 invention to a void volume of 60~6 up to a void volume of 90%. As used herein, void volume or porosity is that volume in the anode catalyst which is free of catalyst material and is generally that part of the anode element which comprise pores, channels, conduits and th~ like t 30 through which gases and fluids pass and~or which gases and fluids occupy w.ithin the anode material.
Any of these foregoing techniques or similar techniques which increase the porosity of the anode material or decrease the difusio~ path length may be used to obtain the improved electrodes and methods in ~5~

52-EE-0~319 accordance with the present invention. These techniques may also be signiicant factors in decreasing the tortuo-sity of the channels and poxes within the anode material and may promote the intercommunica~ion of channels and pores within the anode material and thereby increase diffusion rate of reactants and reaction products therein.
The following examples are presented for purposes of illustration only, and the details therein should not be construed as limitations upon the true scope o the invention as se~ forth in the claims.

Two porous electrode members having an anode upon one sur~ace of a solid polymer electrolyte membr~ne and a cathode upon the other surface of the solid pol~mer electrolyte membrane, identical in construction except for the loading of the anode material, i.e., ~hickness of the anode material were prepared for testing. Both electrode elements had 75% ruthenium oxide/25~ iridium oxide suppor~ed upon graphite as electrode catalysts. One of the anodes was prepared at the prior art loading (thickness) of 4.0 mg. graphite per cm.2. This resulted in an anode thickness of 100 microns. The other anode was prepared at a loading of 2.Q mg. graphite/cm.2, and this produced an anode having a thickness of 50.0 microns. The anode surface area in both cases was 9 in2 (7.6 cm x 7.6 cm. or 58 cm2). These membrane/electrode combinations were then employed in an electrolytic cell similar to the one described above and illustrated in FIGU~E 1 and FIGURE 2 and used for the electrolysis of aqueous hydrogen chloride. The yraph in FIGURE 3 illustrates the amount of oxygen in volume per-cent in the chlorine gas produced in the electrolysis ofthe aqueous hydrogen chloride at the two different thick~
nesses of anode when the cell was operated at a constant concentration of hydrogen chloride (constant percent hydrogen chloride conversion of 3.5 percent) at va~yiny current densities and an 8.0 molar aqueous hydrogen chlo-ride feed stream. It can be seen fxom the graph that the anode Qf reduced ~hickness, the one designated by the triangles in the cur~e, was superior at all current den-sitie measured as amps/ft.2. The current collectors employed in this experiment were metallic distributor screens. The cathodes were 100 microns thic~ and were made of platinum bLack.

Another experiment was conducted to show the effect of anode thickness reduction on oxygen content in chlorine. Cell components and conditions, unless other wise specified, were the same as those set forth i~
Example 1. Three different anode ~hicknesses were com pared. One anode comprising the oxide o 75% ruthenium/25%
iridium upon graphite was 100 microns thick and the c~ll wa~ run at 400 amps/ft.2O Another anode made of the same material was 50 microns thick, and the electrolytic cell for th~ oxidation o spent aqueous hydrogen chloride was run ~t 400 ~mps/~t.2. A third anode made of the same anode material was 13 microns thick, and the elec~roly~ic cal:L was run at 1000 amps/ft.2O The results were reporte~
in concentration of the spent acid (molarity) versus the vol-~me percent Of evolved oxygen in evolved chlorine gas.
~5 The results are reported in the graph in FIGURE 4 and clearly demonstrate the influenc2 of the anode thickness, i.e., diffusion pa~h leng~h, un the amount of o~ygen in the Pffluent chlorine gas.
The results are even more striking when it .is ~0 noted that at 1000 amps/ft.2, chloride ion is being con-swm~d at a rate which is 250% greater than at 400 amp~/ft.2, even ~hough the anode which is 13 mlcrons thick has a sub-stantially lower oxygen level at acid concentrations greater than 8.0 moles. As shown in FIGURE 5, at 400 amps/ft.2 the oxygen levels (repor~ed in vol~me percent in the graph) in chlorine from the anode having a 13-micron thickness are exceedingly low.

EX~MPLE 3 S In another series of comparative expexLmen~s, electrolytic cells similar to tho~e described and illus-trated in FIGURE 1 above were used with anodes having a thickness of about 13.0 microns of the oxides of 75%
ru~henium/25% iridium upon graphite. The volume percent of parasitic oxygen in chlorine gas in the anode c~mpart~
ments was plotted agains~ ~he concentration of the aqueou~
hydrogen chloride (in moles) exi~ing from ~he anode chamber after the oxidation of the aqueous hydrogen chloride in the cell. The graph showing ~hese resul~s is illustrated in FIGUR~ 5 showing the effect of curren~ density upQn the volume percent of parasitic oxygen in the chlorine gas ormed in the anode or at the anode/membrane interface~
The current density in amps/ft.2 was 400, 600, and l,000, respectivelyD It can be seen from this data that e~en at ~3 400 amps/ft.2, the oxygen levels in the chloxine gas ~re ~ery low.

A series of electrodes having various anode thicknesses were tested in electrolytic cells in accsrdance with the conditions and components ~et orth in Example l.
Anodes of various thicknesses are reported in Table L below.
The cell temperature, the concentration (in mole~) of the exiting aqueous hydrogen chloride and the cell voltaye at a current density of 600 amps/ft.2 are also reported in Table l below. The membrane surface having the anode with a thickness of 25.0 microns was well~covered wi~h the anode material, and the elPc rode was clearly continuous.

S~

The anode having an anode material loadlng su~ficient for a 3.0 micron thickness did not cover the membrane ~ur~ace very well, and this electrode appeared highly discontinu-ous with very large areas of the membran~ exposed after the bonding of the anodP material thereto. The re~ul~s are reported in Table l ~elow.

ELECTROLYTIC CELL PERFORMANCE FOR OXIDATION
OF AQOE OUS HYDROGE~ CHLORIDE WITH VARIOUS THICK
NESSES OF ANODE MATERIAL
10 Anode Thickness Cell Voltage at Cell Temp. Exit HCl ~microns) 600 amps/ft.2(C) (moles) 1.92 47 7.7 1.76 53 8ql 23 1.79 54 ~.8 6 1.87 50 5.g 3 2.10 55 9~8 The composi~e electrode comprising the anod , the solid polymer electrolyte membrane and the cathode wherein the anode had a thi~kness of about 3.0 microns, per~o~med very poorly. There was 3 vol~me percent oxygen in the chlorine gas in the anode compartment at a~ aqueous hydrogen chloride concentration of 9.8 moles and a current density of 600 amp~/ft.~. The degradation in performance of th~ electrolytic cells for the electrolysis of aqueous hydrogen chloride with anodes ha~ing a thickness below about 6.0 microns, is clearly reflected in the ~ell volt-ages shown in Table l abo~e.
The lower limit of the electrode thickn~ss i~
determined by the particle size distribution of the material forming the electrodeO When the electrode thickness approaches the mean particle size, the elec-trode becomes discontinuous, as discussed above for the anode having a thickness of 3.0 microns, and high l~cal current densities resultO It can be seen in the Tabl~

~ 95 ~ 49 52-EE-0-319 above that for t~e 75% ruthenium/25% iridium oxide cata-lyst upon graphite used as an anode material to prepare the anodes of the present invention, the lower limit lies between about 3 microns and 5 microns.
It can be determined from Table I above and from the other experimental data reported herein that the mini-mum thickness of the anode material in accordance with the present invention is about 6.0 microns. It has also been determined that the op~imum thickness is about 10 microns to about 13 microns because ~hese are ~hicknesses which are easily reproducible in the manufac~ure of membranes.
Al~hough the 6.0 micron thick electrode is operable in accordance with ~he present invention, anodes of that thickness are dlfficult to manufacture commercially.
Unless otherwise specified, the foregoi~g el~c-trolysis cells or the electrolysis of hydrogen chloride had an anode surface of 9 in~ ~3" x 3"). The ~ells were operated at about 50 C unless otherwise specified. Direct current was applied to the electrodes. In all cases, tne solid polymer elec~rolyte membrane was a ca~ion exchange membrane sup~lied commercially by E.I. Dupont de Nemours & Co. under the trademark "NAFION". The ion P~ch2nge mem brane was a perfluorocarbon sulfonic acid cation membrane wherein the ion exchange groups are hydrated sulfonic acid ~5 groups which are a~ached to the perfluorocarbon polymer backbone by sulfonation.
It was also found that oxygen evolution was sup-pressed by high pH which increases the reversible potentlal of the process and by high chloride ion concentration which facilitates the desired reac~ion. Thus, a high rate of transf~r of hydrogen chloride is beneficial to system opera-tion.
In accordance witn the presen~ invention, elec-trolysis of hy~rogen chloride has been i~lproved. A me~l~od and device have been provided which substantlally reduce or eliminate oxy~en evolution in an electrolysis cell of the type Itsing a solid poly~er elec~rolyte membrane having gas ~_ ~s~

- 27 - 52-E~-0-319 and liquid permeable electrodes bonded to the surface and physically forming a part of the membrane when chlorine is generated from aqueous hydrogen chloride.
The rate of transfer of hydrogen chloride in an aqueous medium in an anode chamber of an electrolysis cell from the reaction sites in the anode or at the anode/
membrane interface has been improved by decreasing the diffusion path length within the anode catalyst and/or increasing porosity of the anode catalyst materialA This permits the use of ~eed hydrogen chloride solutions of lower concentrations in the anode compartment of an electrolytic cell in which chlorine gas is generated from the hydrogen chloride. It also permits the elec~rolysis of hydrogen chloride in an a~ueous medium at higher current densities.
While other modifications of the invention and variations thereof which may be employed within the scope of the invention, have not been described, ~he invention is intended to include such modifications as may be embraced within the following claims.

Claims (17)

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

1. A method for the electrolysis of hydrogen chloride in an electrolytic cell having a cation transporting solid polymer electrolyte membrane, a porous gas and liquid permeable catalytic anode having tortuous pores extending there-through being bonded to one surface of the solid polymer electrolyte membrane whereby hydrogen chloride and chloride ions diffuse through the pores toward the surface of the cation transporting membrane to be oxidized and form reac-tion products, and a cathode catalyst bonded to the other surface of the solid polymer electrolyte membrane comprising the step of maximizing the transport rate of hydrogen chloride and chloride ions into said porous anode by maintain-ing a minimum diffusing path within the anodes as a function of the thickness, which is less than 100 microns, and porosity of the anode, which is represented by a void volume greater than 60 percent, and the tortuosity of the pores where-by the rate of transport of the chloride ions to the electrode is sufficient to sustain the cell current essentially by discharge of the chloride ions to pro-duce chlorine thereby minimizing co-evolution of oxygen.

2. The method of claim 1, wherein the thickness of the anode catalyst is about 6.0 microns to about 50 microns.

3. The method of claim 1, wherein the thickness of the anode catalyst is about 10.0 microns to about 13.0 microns.

4. The method according to claim 1, wherein the liquid and gas permeable porous anode has a void volume ranging between 60 and 90 percent.

5. The method according to claim 3, wherein the liquid and gas permeable porous anode has a void volume ranging between 60 and 90 percent.

6. The method according to claim l wherein void volume of the porous anode is between 60 and 75 percent.

7. The method according to claim 1 wherein the void volume of the porous anode is substantially 75 percent.

8. A method for reducing the amount of oxygen generated in the electrolysis of an aqueous chloride in an electrolytic cell having a hydrated cation transporting polymeric membrane, a cathode bonded to one surface of the membrane and a gas and liquid permeable anode bonded to the other surface of the polymeric membrane wherein aqueous chloride and chloride ions diffuse into the anode and are oxidized therein to produce chlorine, maximizing the transport rate of aqueous chloride and chloride ion into the porous anode by maintaining a minimum diffusion path within the anode as a function both of the porosity of the anode and the anode thickness by maintaining the thickness of the anode between 6.0 microns to 50.0 microns and by providing porosity such that the void volume of the anode is greater than 60 percent whereby the rate of transfer of the chloride ions to the anode is sufficient to sustain cell current by discharge of the chloride ions while minimizing co-evolution of other electrolysis products.

9. The method of claim 8, wherein the thickness of the anode is about 10.0 microns to about 13.0 microns.

10. In an apparatus for the electrolysis of hydrogen chloride in an electrolytic cell having a cation transporting solid polymer electrolyte membrane, a porous gas and liquid permeable catalytic anode having tortuous pores extending therethrough said anode being bonded to one surface of a solid polymer electrolyte membrane, whereby chloride ions diffuse through the pores from one surface of the anode towards the cation transporting membrane to which the anode is adapted to be bonded, to allow the chloride ions to be oxidized there to form chlorine gas, the improvement in which said catalytic anode comprises a structure with a thickness of less than 100 microns to maximize the transport rate of hydrogen chloride and chloride ions into and within said pores and in which the diffusion path length of the pores is a function of the thickness and porosity of the anode, which has a void volume greater than 60 percent, and of the tortuosity of the pores whereby the rate of transfer of the chloride ions is sufficient to sustain cell current by discharge of the chloride ion while co-evolution of other electro-lysis products is minimized.

11. The apparatus of claim 10, in which the thickness of the anode is about 6.0 microns to about 50.0 microns.

12. The apparatus of claim 10, in which the thickness of the anode is about 10.0 microns to about 13.0 microns.

13. The apparatus according to claim 10 in which the porous anode has a void volume between 60 and 90 percent.

14. The apparatus according to claim 10 in which the porous anode has a void volume between 60 and 75 percent.

15. The apparatus according to claim 10 in which the porous anode has a void volume which is substantially at 75 percent.

16. In an apparatus for the generation of chlorine from hydrogen chloride by electrolysis, an electrolytic cell having a cation transporting solid polymer electrolyte membrane a porous, gas and liquid permeable catalytic anode bonded to one surface and a cathode bonded to the other surface of the membrane, the cation transporting membrane dividing the electrolytic cell into an anode chamber on the side of the membrane having the anode and into a cathode chamber on the side of the membrane having the cathode, means for providing electrical current at the anode and the cathode, feed means for feeding an aqueous hydrogen chloride anolyte into the anode chamber, means for removing chlorine and depleted hydrogen chloride anolyte from the anode chamber, and means for removing hydrogen from the cathode chamber, the improvement comprising an anode of a thickness of about 6.0 microns to 50.0 microns and a void volume greater than 60 percent to minimize the diffusion path length to provide an increase in the rate of transport of hydrogen chloride and chloride ions towards the surface of the membrane.

17. The apparatus of claim 16, wherein the thickness of the anode material is about 10.0 microns to about 13.0 microns.