Note: Descriptions are shown in the official language in which they were submitted.
~s~
- 1 - 52 EE-0-319
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~
52-EE-0-319
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
--2--
52-EE-0-319
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.
52-EE-0-313
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
--- 5
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
-- 6
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 -
'' ``
S~
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:
--7--
~ ~ 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
52-EE-0-319
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
52-EE-0-319
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~
-13-
52-EE-0-319
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
-15-
52-EE-0-319
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
-17-
~ 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~
- 18 - 52-EE 0-319
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
-19-
- 20 - 52-EE-0-319
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
52 EE-0-319
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
-21-
~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.
EXAMPLE 1
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
-22
52-EE-0-319
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.
EXAMPLE 2
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
-23-
52-EE-0-319
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.
EXAMPLE 4
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.
-24
S~
52-EE-0-319
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.
TABLE 1
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~
-25-
~ 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.
