An Improved Method for Obtaining Porous Metals by Electrolysis.
Year: 1896

page_text: Date of Application, 10th Aug., 1896
Complete Specification Left, 1st May, 1897-Accepted, 19th June, 1897
OTIONAL MULTICATION.
An Improved Method for Obtaining Porous Metals by Electrolysis.
I, LUDWIG HOEPFNER, of No. 6 Anhaltstrasse, Berlin, S.W. in the Empire
of Germany, Doctor of Philosophy, do hereby declare the nature of this invention
to be as follows:-
Many metals, such as lead and copper, when used for certain special purposes,
particularly in electrical storage batteries, prove more valuable if they can be
employed in a porous state without any sacrifice of their strength or cohesion.
The object of the method herein described is to precipitate metals by electrolysis,
either in the very process of their being obtained or refined by electrolysis, or if
preferred in a bath specially prepared for the purpose, in such a manner that they
combine the property of cohesiveness with great porosity.
The daily practice of electro-plating has shown that the structure of metals
separated by electrolysis depends upon many conditions, viz. the qualitative
composition of the bath, the concentration, tenperature and motion of the
electrolyte and, particularly, the density of the current. By a proper adjustment.
of these conditions it is always possible, for instance, to obtain a porous precipitate
of the metal, while in the case of non-compliance with those necessary conditions,
the metal must be precipitated in a compact form. By the term "porous state,'
I meau a state in which the several metallic particles adhere to each other at one
or more points in such a manner that between the particles in contact there remain.
hollow spaces or interstices. This term, therefore covers, besides precipitates in
the form of powder,-matter settling in the form of threads, leaves or moss, such
as is obtained in dealing with lend.
The main feature of this method for obtaining sufficiently cohesive porous metals
by electrolysis, is that the metal is precipitated alternately in a porous form and in
a compact state, in regular rotation. In the stage of precipitation of porous metal
thin-walled pores are produced, while in the compact precipitates the walls of the
pores become more solidified.
The means for securing such alternate precipitation consists in changing (in the
proper order of succession) one or the other of the conditions abovenamed, as will
be fully explained further on. As there are several such conditions, it might be
supposed that there are also several different methods derivable therefrom,
necessitating a different treatment in each case. But inasmuch as the conditions
referred to are inseparable from each other, since in determining the density of
the current, for example, the qualitative composition of the bath, and the
concentration and temperature of the electrolyte must always be taken into account,
this method, although the form in which it is carried out may vary, yet in its
general bearings presents one complete and connected whole.
A few specific examples may serve to illustrate the process.
To obtain porous copper the ordinary solution of sulphate of copper used in
electroplating may be employed. By starting with a dense current (exceeding
400 amp, per square metre at the ordinary temperature) copper in the form of
powder is first obtained and a current of less density is then allowed to follow until
the dark colour of the precipitate changes into the well-known light red hue of
copper obtained by electrolysis. The current is then again increased to obtain

page_text: copper in powder, which upon the current being once more reduced becomes a
porous mass, and this regular alternation is continued. If it be desired to work
with a smaller current, generally a less concentrated solution is used. In the case
of lead the method is similar. However it is preferable to precipitate lead in leaf
or mosslike crystals instead of in powder. The suitable current in this case is an
intermediate one between the current required to obtain spongy (powdered) lead
and that which should be used to produce compact metal. Such current is allowed
to act for some time; then its density is reduced in order that the leaves which at
first are very soft may become more firm.
An application of the process with a view to obtaining porous metal by a
regular alternation of porous and compact metallic precipitates consists in a regular
alternation of two equally different and distinct densities of current, one yielding
porous and the other compact or solid metal. The absolute density of the currents
to be applied from time to time is best determined for each bath of a certain
composition, concentration and temperature by experiment. It may be stated that
a single alternation of the current is already sufficient to obtain a stratum of porous
metal. The number of alternations is determined by the thickness of the porous
metal which may he required in each particular case.
It has already been stated that if it be desired, for example, to work porous
copper with currents of great density, the ordinary bath of solution of sulphate of
copper is suitable, but that a less eoncentrated bath should generally be employed
in the case of currents of less density. Now this, for one thing, involves the
following alteration in the process. Instead of changing the density of the current
for a solution of the same degree of concentration, the density of current may be
left unaltered and the change, be effected in the concentration of the solution.
For it has been ascertained by special tests that the structure of the metal
precipitate is determined by the ratio of the density of the current to the density
of the electrolyte, the metal precipitated being the more porous or spongy, the
greater that ratio, and the more cohesive or dense (or the more nearly approaching
the solid state the smaller the ratio in question. A change in the structure or
texture of the metal obtained may, therefore, be effected by correspondingly
changing either the densities of current only, or solely the degree of concentration
of the solution, or both the density of current and degree of concentration at the
same time, the last-mentioned alternative has proved to be the most advantageous
in actual practice; in other words, the best course in obtaining porous or spongy
metal deposits, is to employ an increased density of current and a solution of a less
degree of concentration, and the best course in obtaining solid metal, to employ &
more highly concentrated solution and current of less density.
Besides, as has already been stated above, temperature also affects the question
of determining the proper densities of current. Experiments made to ascertain
this have, as a result, established the fact that the texture of the metal precipitate
is dependent upon the ratio of the density of current to the temperature, and that,
generally, the greater this ratio, the more spongy is the metal obtained, and the
smaller the said ratio the more dense will be the metal precipitated. A high
temperature therefore exercises a concentrating action upon the electrolyte. In
point of fact, when the temperature is high, a greater number of the molecules of
the electrolyte are found to approach the cathode in the same time, than when the
temperature is low. Apparently, this is the main cause of the influence which
temperature produces. Intensified dissociation, owing to an increase of temperature,
will also, however, bring about a greater concentration of the metal ions. Given,
therefore, equal densities of current and equal concentration, the required
alternation of the metal precipitates may also be obtained by a change of
temperature. In most cases, however, the operation will be conducted under the
most favourable circumstances, when, for spongy metal, a higher density of current
combined with a lower temperature and a reduced concentration of the fluid of the
bath, and for compact metal, a suitably reduced density of current iu conjunction
with the higher temperature and greater concentration, are employed.

page_text: In the same way as concentration and temperature influence the course of the
operation, so does the manner in which the cathodes move, or in which the fluid.
near the cathodes flows, effect the issue of the process. Where the cathode fluid
is in motion it promotes the separation of metal in a more solid form. Motion
therefore also has a concentrating action, more especially because it compensates
for, or equalizes, the rarefaction which is caused at the cathode by the various
movements of the ions. This effect of motion should be taken into account
particularly in those cases where, for the purpose of depolarizing the anodes,
moving electrodes are used.
Again, all other conditions being equal, a change in the form of the metal
precipitate may be obtained by suspending the cathodes, alternately, in baths
varying in qualitative composition, or by combining this change in the qualitative
composition (in order still further to enhance the effect) with a corresponding
modification of the density of current, temperature or conditions of motion, in
such a manner as to obtain spongy metal at one stage and dense or solid metal at
the next, alternatively.
The proper density of current, concentration, temperature or motion, in the case
of each individual bath of a given qualitative composition, may be best determined
by experiment, the appearance of the metal precipitate being the criterion.
The regular alternation of the conditions named may, generally speaking, be
secured by two different methods, viz., either by the employment of two separate
cells and separate fluids used in the baths in which the cathodes are suspended
alternately; or by always working in the same cell, in which case the values of,
say, the density of current, temperature and motion will have to change, in addition
to which it may be expedient to cause liquids varying in composition and
concentration to circulate at predetermined periods of time. It is suggested that
the two systems should be combined, the spongy precipitate being allowed to partly
solidify in its cell,-the density of current being somewhat reduced and the
temperature correspondingly raised for the purpose then it is transferred to
another cell wherein it may be further solidified.
Owing to the fact that the surface of the cathode is constantly increased or
extended during the electrolytical process, the ratio of the density of current to
the cathode surface and also consequently, all other conditions remaining equal,-
the ratio of the density of current to the density of the electrolyte, must become
gradually smaller, that is to say the spongy metal separated becomes itself,
gradually converted into more solid metal, provided the initial density of current
has been so determined as but slightly to exceed the limit of current density
required for producing solid metal. Accordingly, the passage of the metal from
the spongy to the solid state may be allowed to take place automatically, by
causing the combination of conditions necessary for producing spongy metal to
approach as nearly as possible from the beginning the set of conditions required
for the formation of solid metal.
A particularly advantageous method for the automatic regulation of the alternate
changes in the structure of the metal precipitate,-in addition to the changing of
the density of current,-has been found to be the alternation of temperatures, the
temperature being capable of being readily and automatically controlled so as
uniformly to rise or fall as may be required.
In some cases, especially in connection with lead, it may be advisable to cause
a plate to exercise a slight pressure upon the porous metal, so as to produce a
uniform surface and not to allow too large hollow spaces to form.
In obtaining porous lead by electrolysis, all solutions are not equally well suited.
Where a nitric acid solution is used, for example, rigid plates or scales or needles
are obtained, which will break under pressure; whereas, in experimenting for the
present invention, it has been found that the small sheets, plates or scales obtained
from a solution of oxide of lead in caustic alkalies are very resistant to fracture.
Now where plates obtained in this manner are to be used in storage batteries, this
is a most important feature, as the oxidation of the lead, occurring in this

page_text: connection, is attended with considerable changes in the volume of the oxidized
particles, so that it is preferable that the several little leaves, of which the whole
consists, should be capable of performing lateral movements, without becoming
disconnected from each other.
In view of the fact that in electrolytical processes, that which takes place at the
cathode is not independent of that which takes place at the anode, and conversely,
the phenomena taking place at the anode should be taken into account and utilized
in reducing the cost of the "cathode" process. Thus, the present method may
with particular advantage be employed in connection with metal refining, say, for
obtaining porous lead, and at the same time refining the lead, in which case all the
constituent parts of the metal may be extracted from the anode "mud." Products
of oxidation, however, no matter of what kind-may be obtained as bye-products
by the "anode" process; and this is particularly the case where the density of
current is high and the temperature low. Experiments made within the temperature
limits, 100° and -9° have shown that the anodes become covered with undissolved
or insoluble oxides the more rapidly, as the temperature is lower. Accordingly, in
alternating temperatures and densities of current, a raised temperature and a
reduced density of current may be combined with metal refining, at the anode, and
a reduced temperature and an increased density of current may be combined with a
process of extraction of undissolved oxides and peroxides, as bye-products. The
process would however remain the same if the phenomena at the anode, the main
features, of the electrolytic process being identical, be, on the contrary, dealt with
as the main object of the operation, and if the porous metal obtained at the cathode
be regarded as a bye-product.
HASELTINE, LAKE & Co.,
45 Southampton Buildings, London, W.C., Agents for the Applicant.
I, Dr. LUDWIG HOEPFNER, of No. 6 Anhaltstrasse, Berlin S. W., in the Empire
of Germany, Doctor of Philosophy, do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
Many metals, such as lead and copper, when used for certain special purposes,
particularly in electrical storage batteries, prove more valuable if they can be
employed in a porous state without any sacrifice of their strength or cohesion.
The object of the method hereindescribed is to precipitate metals by electrolysis,
<ither in the very process of their being obtained or refined by electrolysis, or if
preferred in a bath specially prepared for the purpose, in such a manner that they
combine the property of cohesiveness with great porosity.
The daily practice of electro-plating has shown that the structure of metals
separated by electrolysis depends upon many conditions, viz. the qualitative
composition of the bath, the concentration, temperature and motion of the
electrolyte, and, particularly, the density of the current. By a proper adjustment
of these conditions it is always possible, for instance, to obtain a porous precipitate
of the metal, while in the case of non-compliance with those necessary conditions,
the metal must be precipitated in a compact form. By the term "porous state," I
mean a state in which the several metallio particles adhere to each other at one or
more points in such a manner that between the particles in contact there remain
hollow spaces or interstices. This term, therefore covers,-besides precipitates in

page_text: the form of powder,-matter settling in the form of threads, leaves or moss, such
as is obtained in dealing with lead.
The main feature of this method for obtaining sufficiently cohesive porous
metala by electrolysis, is that the metal is precipitated alternately in a porous form
and in a compact state, in regular rotation. In the stage of precipitation of porous
metal thin-walled pores are produced, while in the compact precipitates the walls
of the pores become more solidified.
The means for securing such alternate precipitation consists in changing (in the
proper order of succession) one or the other of the conditions above named, as will
be fully explained further on. As there are several such conditious, it might be sup-
posed that there are also several different methods derivable therefrom, necessitating
a different treatment in each case. But inasmuch as the conditions referred to are
inseparable from each other, since in determining the density of the current, for
example, the qualitative composition of the bath, and the concentration and
temperature of the electrolyte must always be taken into account, this method,―
although the form in which it is carried out may vary, yet in its general bearings
presents one complete and connected whole.
A few specific examples may serve to illustrate the process.
To obtain porous copper the ordinary solution of sulphate of copper used in
electroplating may be employed. By starting with a dense current (exceeding
400 amp. per square meter at the ordinary temperature) copper in the form of
powder is first obtained and a current of less density is then allowed to follow until
the dark colour of the precipitate changes into the well-known light red hue of
copper obtained by electrolysis. The current is then again increased to obtain
copper in powder, which upon the current being once more reduced becomes a
porous mass, and thus regular alternation is continued. If it be desired to work
with a smaller current, generally a less concentrated solution is used. In the case
of lead the method is similar. However it is preferable to precipitate lead in leaf
or mosslike crystals instead of in powder. The suitable current in this case is an
intermediate one between the current required to obtain spongy (powdered) lead
and that which should be used to produce compact metal. Such current is allowed
to act for some time; then its density is reduced in order that the leaves which at
first are very soft may become more firm.
An application of the process with a view to obtaining porous metal by a regular
alternation of porous and compact metallic precipitates consists in a regular
alternation of two equally different and distinct densities of current, one yielding
porous and the other compact or solid metal. The absolute density of the currents
to be applied from time to time is best determined for each bath of a certain
composition, concentration and temperature by experiment. It may be stated that
a single alternation of the current is already sufficient to obtain a stratum of porous
metal. The number of alternations is determined by the thickness of the porous
metal which may be required in each particular case.
It has already been stated that if it be desired, for example, to work porous
copper with currents of great density, the ordinary bath of solution of sulphate of
copper is suitable, but that a less concentrated bath should generally be employed
in the case of currents of less density. Now this, for one thing, involves the
following alteration in the process: Instead of changing the density of the current
for a solution of the same degree of concentration, the density of current may be
left unaltered and the change he effected in the concentration of the solution. For
it has been ascertained by special tests that the structure of the metal precipitate
is determined by the ratio of the density of the current to the density of the
electrolyte, the metal precipitated being the more porous or spongy, the greater
that ratio and the more cohesive or dense (or the more nearly approaching the
solid state) the smaller the ratio in question. A change in the structure or texture
of the metal obtained may, therefore, be effected by correspondingly changing
either the densities of current only, or solely the degree of concentration of the
solution, or both the density of current and degree of concentration at the same

page_text: time, the last-mentioned alternative has proved to be the most advantageous in
actual practice; in other words, the best course in obtaining porous or spongy
metal deposits, is to employ an increased density of current and a solution of a
less degree of concentration, and the best course in obtaining solid, metal, to employ
a more highly concentrated solution and current of less density.
Besides, as has already been stated above, temperature also affects the question
of determining the proper densities of current. Experiments made to ascertain
this have, as a result, established the fact that the texture of the metal precipitate
is dependent upon the ratio of the density of current to the temperature, and that,
generally, the greater this ratio, the more spongy is the metal obtained, and the
smaller the said ratio, the more dense will be the metal precipitated. A high
temperature therefore exercises a concentrating action upon the electrolyte. In
point of fact, where the temperature is high, a greater number of the molecules of
the electrolyte are found to approach the cathode in the same time, than when the
temperature is low. Apparently, this is the main cause of the influence which
temperature produces. Intensified dissociation, owing to an increase of temperature,
will also, however, bring about a greater concentration of the metal ions. Given,
therefore, equal densities of current and equal concentration, the required alter-
mation of the metal precipitates may also be obtained by a change of temperature.
In most cases, however, the operation will be conducted under the most favourable
circumstances, when, for spongy metal, a higher density of current combined with
a lower temperature and a reduced concentration of the fluid of the bath, and for
compact metal, a suitably reduced density of current in conjunction with the higher
temperature and greater concentration, are employed.
In the same way as concentration and temperature influence the course of the
operation, so does the manner in which the cathodes move, or in which the fluid
near the cathodes flows, effect the issue of the process. Where the cathode fluid
is in motion, it promotes the separation of metal in a more solid form. Motion
therefore also has a concentrating action, more especially because it compensates
for, or equalizes, the rarefaction which is caused at the cathode by the various
movements of the ions. This effect of motion should be taken into account
particularly in those cases where, for the purpose of depolarising the anodes,
moving electrodes are used.
Again, all other conditions being equal, a change in the form of the metal
precipitate may be obtained by suspending the cathodes, alternately, in baths
varying in qualitative composition, or by combining this change in the qualitative
composition (in order still further to enhance the effect) with a corresponding
modification of the density of current, temperature or conditions of motion, in such
a manner as to obtain spongy metal at one stage and dense or solid metal at the
next, alternatively.
The proper density of current, concentration, temperature or motion, in the case
of each individual bath of a given qualitative composition, may be best determined
by experiment, the appearance of the metal precipitate being the criterion.
The regular alternation of the conditions named may, generally speaking, be
secured by two different methods, riz., either by the employment of two separate
cells and separate fluids used in the baths in which the cathodes are suspended
alternately; or by always working in the same cell, in which case the values of,
say, the density of current, temperature and motion will have to change, in addition
to which it may be expedient to cause liquids varying in composition and con-
centration to circulate at predetermined periods of time. It is suggested that the
two systems should be combined, the spongy precipitate being allowed to partly
solidify in its cell, the density of current being somewhat reduced and the
temperature correspondingly raised for the purpose, then it is transferred to
another cell wherein it may be further solidified.
Owing to the fact that the surface of the cathode is constantly increased or
extended during the electrolytical process, the ratio of the density of current to the
-cathode surface and also consequently, all other conditions remaining equal,-the

page_text: ratio of the density of current to the density of the electrolyte, must become
gradually smaller, that is to say the spongy metal separated becomes itself,
gradually converted into more solid metal, provided the initial density of current.
has been so determined as but slightly to exceed the limit of current density
required for producing solid metal. Accordingly, the passage of the metal from
the spongy to the solid state may be allowed to take place automatically, by
causing the combination of conditions necessary for producing spongy metal to
approach as nearly as possible from the beginning the set of conditions required
for the formation of solid metals.
A particularly advantageous method for the automatic regulation of the alternate
changes in the structure of the metal precipitate, in addition to the changing of
the density of current,-has been found to be the alternation of temperatures, the
temperature being capable of being readily and automatically controlled so as
uniformly to rise or fall as may be required.
In some cases, especially in connection with lead, it may be advisable to
cause a plate to exercise a slight pressure upon the proús metal, so as to produce
a uniform surface and not to allow too large hollow spaces to form.
In obtaining porous lead by electrolysis, all solutions are not equally well suited.
Where a nitric acid solution is used, for example, rigid plates or scales or needles
are obtained, which will break under pressure; whereas, in experimenting for the
present invention, it has been found that the small sheets, plates or scales obtained
from a solution of oxide of lead in caustic alkalies are very resistant to fracture..
Now where plates obtained in this manner are to be used in storage batteries, this
is a most important feature, as the oxidation of the lead, occurring in this connection,
is attended with considerable changes in the volume of the oxidized particles, so
that it is preferable that the several little leaves, of which the whole consists, should
be capable of performing lateral movements, without becoming disconnected from
each other.
"
In view of the fact that in electrolytical processes, that which takes place at the
cathode is not independent of that which takes place at the anode, and conversely,
the phenomena taking place at the anode should be taken into account and utilized
in reducing the cost of the "cathode process. Thus, the present method may
with particular advantage be employed in connection with metal refining, say, for
obtaining porous lead, and at the same time refining the lead, in which case all the
constituent parts of the metal may be extracted from the anode "mud." Products.
of oxidation, however, no matter of what kind,-may be obtained as bye-products.
by the "anode" process; and this is particularly the case where the density of
current is high and the temperature low. Experiments made within the temperature
limits, of 100° and -9° have shown that the anodes become covered with undissolved.
or insoluble oxides the more rapidly, as the temperature is lower. Accordingly, in
alternating temperatures and densities of current, a raised temperature and a.
reduced density of current may be combined with metal refining, at the anode, and
a reduced temperature and an increased density of current may be combined with
a process of extraction of uudissolved oxides and peroxides, as bye-products. The
process would however remain the same if the phenomena at the anode, the main
features, as stated in Claims 1 and 2, of the electrolytic process being identical-
be, on the contrary, dealt with as the main object of the operation, and if the-
porous metal obtained at the cathode be regarded as a bye-product.
Having now particularly described and ascertained the nature of my said inven-.
tion and in what manner the same is to be performed, I declare that what I
claim is :-
(1.) A process for obtaining porous metals by electrolysis, mainly consisting in
producing, alternately, spongy or porous, and dense metal precipitates, one-
operation serving to form the pores, and the next, to solidify or strengthen the
walls of such pores.

page_text: (2.) In the performance of the alternate process, covered by the first claim, for
taining metal precipitates, such as are clearly specified in that claim;
a. The alternate employment of two different densities of current, characterized
by the effects they respectively produce, as stated;
b. The alternate employment of two different degrees of concentration of the
electrolyte, characterized by the effects they respectively produce, as
stated;
c. The alternate employment of two different temperatures of the electrolyte,
characterized by their respectivo effects, as stated;
d. The alternate rest and motion of either the electrolyte or the cathole,
characterized by their respective effects, as stated;
e. The alternate employment of two baths differing in composition and
characterized by their respective effects, as stated;
f. The alternate employment of two different combinations of the changes
enumerated above, from a to e, in the electrolytic process, the respective
effects of the said combinations being substantially such that one of them
causes spongy metal to be precipitated, and the other produces a precipitate
of dense (cohesive or solid) metal.
(3.) In the performance of the process covered by the first and second claims,
e adoption of the following two methods of operation, either separately or in
mbination;
a. Alternately suspending the cathodes in separate cells, the operation being
EO conducted, in accordance with Claim 2, that there separates spongy
metal in one cell and dense metal in the other cell.
b. Leaving the cathodes in the same cell, wherein the density of current, and
the temperature and motion of the fluid in the bath-which is made to
circulate, alternate in regular rotation; different fluids being, moreover,
alternately used if desired.
(4.) In the electrolytic process covered by the first claim, the automatic regu-
tion of the process by which the spongy metal separated becomes converted into
ense metal, such regulation being brought about by properly utilizing the
xtension of the cathode-surface,which extension takes place of its own accord,
nd the consequent reduction in the density of current.
(5.) In the process referred to in Claims 1 to 3, the provision of devices for the
atomatic regulation or control of temperatures and densities of current.
(6.) In the process covered by the Claims 1, 2 and 4, utilizing a slight pressure,
capable of affecting the porosity of metal,-alternately with the action of the
rrent.
(7.) In the process protected by Claims 1 to 6, the employment, as an electro-
te, of a solution of oxide of-lead in caustic alkalies.
(8.) In the electrolysis of dissolved metal compounds by the process characterized
y Claims 1 and 2, the employment, as anodes, of impure metals with a view to
eir refinement, and the utilization of the oxides forming at such anodes in
btaining bye-products.
(9.) The employment of porous metals obtained by the process covered by the
rst claim in the preparation of plates for electrical storage batteries.
(10.) Electrodes for use in secondary batteries, consisting of metal which has
een rendered porous according to the process above referred to; substantially as
escribed.
HASELTINE, LAKE & Co.,
45 Southampton Buildings, London, W.C., Agents for the Applicant.