U.S. patent application number 12/706713 was filed with the patent office on 2011-08-18 for redox membrane-based flow fuel cell.
Invention is credited to Nikolai M. Kocherginsky.
Application Number | 20110200890 12/706713 |
Document ID | / |
Family ID | 44369863 |
Filed Date | 2011-08-18 |
United States Patent
Application |
20110200890 |
Kind Code |
A1 |
Kocherginsky; Nikolai M. |
August 18, 2011 |
Redox membrane-based flow fuel cell
Abstract
A flow fuel cell for use as a source of electrochemical energy
with the membrane separating two flowing aqueous solutions with
dissolved redox active components and electrodes of the second
kind, wherein the membrane is made of a redox active synthetic
metal polymer.
Inventors: |
Kocherginsky; Nikolai M.;
(Urbana, IL) |
Family ID: |
44369863 |
Appl. No.: |
12/706713 |
Filed: |
February 17, 2010 |
Current U.S.
Class: |
429/402 |
Current CPC
Class: |
H01M 8/1032 20130101;
H01M 8/20 20130101; H01M 2300/0082 20130101; H01M 2008/1095
20130101; H01M 8/103 20130101; Y02E 60/50 20130101; Y02E 60/528
20130101 |
Class at
Publication: |
429/402 |
International
Class: |
H01M 8/22 20060101
H01M008/22 |
Claims
1. Redox membrane-based flow fuel cell which can be used as a
source of electrochemical energy, said cell comprising: 1. A
chamber separated by a polymer membrane into two flow-through
compartments where each compartment has a current collecting
electrode of a second kind, the membrane is made of a synthetic
metal polymer, one compartment has a solution of reducing and
another compartment has a solution of oxidizing agent,
respectively. 2. A redox membrane-based flow fuel cell of claim 1
where the membrane is made of polyaniline, polypyrrole or
polythiophene or their derivatives so that the polymer is in
electroconductive state. 3. A redox membrane-based flow fuel cell
of claim 2 where polyaniline is acid-doped with inorganic or
organic acids or acidic polymers. 4. A redox membrane-based flow
fuel cell of claim 2 where solutions with reducing and oxidizing
agents are flowing through different and separated by the membrane
compartments and have concentrations of reducing and oxidizing
agents in the range from 1 mM to 5M. 5. A redox membrane-based flow
fuel cell of claim 2 where solutions with reducing and oxidizing
agents are flowing through different and separated by the membrane
compartments and these solutions have a mixture of reducing agents
and a mixture of oxidizing agents. 6. A redox membrane-based flow
fuel cell of claim 2 where the solution with reducing agent also
has inorganic salts of chlorides, bromides or iodides in
concentration from 1 mM to 5M. 7. A redox membrane-based flow fuel
cell of claim 2 where the solution with an oxidizing agent also has
more acidic pH than that of the solution with reducing agent. 8. A
battery of the redox membrane-based flow fuel cells of claim 1
where the solutions of the redox active chemicals are flowing from
one cell to another.
Description
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TECHNICAL FIELD
[0039] The present invention relates to a method to generate
electric energy using electrochemical membrane cell, in particular
using a dense membrane made of electroconductive polymers known as
synthetic metals, where the membrane separates two solutions with
different redox active substances.
BACKGROUND OF THE INVENTION
[0040] Systems for electrochemical energy storage and conversion
include batteries, fuel cells, and electrochemical capacitors.
Common features of these systems are that the energy-providing
electrochemical processes take place at the phase boundary of the
electrode/electrolyte interface and that the electron and ion
transport from one electrode to another are separated in space.
[0041] Batteries are closed systems, with the anode and cathode
being the charge-transfer medium taking an active role in the redox
reaction, so that the voltage is generated on the electrodes, while
a reducing or oxidizing reactants which provide chemical energy are
stored in the same compartment. Fuel cells are open systems where
the anode and cathode are just charge-transfer media and the active
chemicals undergoing the redox 110 reaction are delivered from
outside the cell. Usually these are oxygen from air, and fuels such
as hydrogen and hydrocarbons from a tank. Energy storage (in the
tank and air) and energy conversion (in the fuel cell) are thus
separated in space.
[0042] In electrochemical capacitors (or supercapacitors),
initially energy to charge the capacitor may be delivered both via
redox reactions and/or with the external source of AC or even DC
current. Redox processes result in the accumulation of energy-reach
redox state of the material inside the capacitor. The capacitor is
used for the charge separation and finally generates an electric
current during the discharge operation. In addition to this
process, orientation of electrolyte ions at the interface leads to
the initial charge and then discharge of electrical double layers,
which results in the energy-delivering process of electrons
movement in the external wire.
[0043] A galvanic or fuel cell often has a special separator,
usually an ion-exchange membrane or porous separator, so that pH
and composition near cathode and anode can be kept different.
Nevertheless, ions can easily and selectively penetrate through the
membrane, from one electrode to another, while electrons are
transported outside of the cell. Different redox potentials at the
anode and cathode lead to the redox interface reactions, charge
separation and generation of the open circuit voltage difference
between the anode and cathode. When electrons move in a wire
outside the battery from an anode to cathode, to keep total
electroneutrality negative ions should move in the cell in the
opposite direction or it may be that positive ions are moving
inside the cell from the anode to the cathode. Positive and small
H' ions have the highest diffusion coefficient and can present in
high concentrations, leading to low internal electric resistance of
the cell. This is why proton-selective ion-exchanging membranes are
used in many galvanic cells.
[0044] The problem with proton exchange membrane-based fuel cells,
including direct methanol fuel cells, is that usually used oxygen
reduction reaction, occurring at the cathode, is very slow and a
catalyst is needed. The electrodes in this case are usually made
with expensive alloys or Pt, which loose their catalytic activity
in the presence of different impurities, including CO.sub.2, CO and
H.sub.2S.
[0045] The problem becomes even more complicated because exchange
currents for anode and cathode reactions on Pt can be limited by
both ion transport in solutions and interface electron transfer,
thus leading to the current-overpotential, i.e. it is necessary to
use voltage higher than its thermodynamic equilibrium value to
start and conduct fast electrochemical reaction.
[0046] Application of fuel cells in underwater vehicles has another
problem. Unlike ground and air transportation, these vehicles must
carry both the fuel and the oxygen source because the oxygen
concentration in water is insufficient to meet the vehicle power
requirements. The oxygen source must possess a high oxygen content
to accommodate the weight and volume constraints of the vehicle
design, and to be amenable to safe handling and storage onboard
submarines and surface ships. Gaseous oxygen storage does not
provide adequate storage densities, while liquid oxygen storage
introduces challenges with handling and storage. Other liquid
sources, such as hydrogen peroxide (H.sub.2O.sub.2), require
compact, efficient, controllable conversion methods to produce
oxygen and handle reaction byproducts. Solid-state oxygen sources
such as sodium chlorate (NaClO.sub.3) and lithium perchlorate
(LiClO.sub.4) possess high oxygen contents and are stable under
ambient conditions; however, decomposition of these materials to
gaseous oxygen typically employs thermal methods that are often
difficult to start, stop, and control. All this clearly
demonstrates the necessity to find innovative approaches to use
chemical energy of these oxidants.
[0047] Fuel cells without the oxygen cathode are a promising
approach to resolve many issues with oxygen reduction reaction. One
of these approaches is to use redox flow batteries where another
electrode and oxidant are used instead of Pt/oxygen. These
batteries are based on reversible redox reactions and initially
were suggested in NASA. They can be used in submarine applications
to accumulate, to store and to use electric energy when necessary,
but have the major challenge, which is nonselective ionic and water
migration through the ion-exchange membrane.
[0048] Another principle of voltage generation can be found in
Nature. An electric organ of an eel Electrophorus electricus is a
battery of biological membranes, which during excitation can
generate voltages up to 1000V and currents up to 1 A. After
purification these membranes are not stable and can not be used as
a electrochemical power source in everyday life.
[0049] If an ion-selective membrane separates two solutions with
different activity of penetrating ions, this by itself should lead
to the charge separation and generation of transmembrane electric
potential. If the membrane has high permeability of these ions and,
as the result, low electric resistance, this could be used as an
electrochemical cell. These electrochemical cells are called
concentration elements, but they did not find practical
applications because of the low generated voltage. For an ideal
ion-selective membrane according the Nernst Law ten times
concentration difference at room temperature leads to less then 60
mV voltage. Even if pH difference is 14, an ideal voltage will be
only 840 mV, which is much less than the voltage, which is possible
to generate using redox reactions.
OBJECTS OF THE INVENTION
[0050] It is the object of the invention to provide an improved
electrochemical cell with the practically-important voltage
generated on a membrane.
[0051] Another object of the invention is to provide the
electrochemical flow cell, which will not be based on platinum
electrodes and will be able to react with many different reducing
and oxidizing agents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 shows the redox membrane-based flow fuel cell with
the membrane (1), current-collecting electrodes (2, 3) and inlets
and outlets (4-7).
[0053] FIGS. 2a and 2b show the behavior of transmembrane potential
with changing of Fe.sup.2+ and Fe.sup.3+ concentrations (pH
adjusted to 1.14 by HCl). For the experiments with addition of
FeCl.sub.2 the opposite solution had 0.01M FeCl.sub.3 and vice
versa. Membrane: PANI film doped with CSA.
[0054] FIGS. 3a and 3b show the behavior of transmembrane potential
with changing of K.sub.4Fe(CN).sub.6 and K.sub.3Fe(CN).sub.6
concentrations in a phosphate buffer with pH 6.4. For the
experiments with addition of K.sub.4Fe(CN).sub.6 the opposite
solution had 0.01M K.sub.3Fe(CN).sub.6 and vice versa. Membrane:
PANI film doped with CSA.
[0055] FIG. 4 shows changes of transmembrane potential and
difference of redox potentials between two liquid phases because of
transmembrane redox reaction. Reducing agent: 0.01M FeCl.sub.2+0.1M
HCl solution; oxidizing agent: 0.1M FeCl.sub.3+0.1M HCl
solution.
[0056] FIG. 5 illustrates transport of electrons and ions through
the PANI membrane. Electroneutral coupled chloride/electron counter
transport and proton/electron cotransport lead to the transmembrane
redox reactions, while fast and not compensated electron transport
leads to the charge separation and generation of transmembrane
voltage.
[0057] Scheme 1 shows continuous oxygen reduction and
H.sub.2O.sub.2 formation in redox polyaniline membrane-based flow
fuel cell with applied reducing potential (reducing agent)
BRIEF SUMMARY OF THE INVENTION
[0058] A flow fuel cell for use as a source of electrochemical
energy with the membrane separating two flowing aqueous solutions
with dissolved reducing and oxidizing components and electrodes of
the second kind, wherein the membrane is made of a redox active
synthetic metal polymer.
DISCLOSURE OF THE INVENTION
[0059] Derivatives of polyanilines, polythyophenes and polypyrroles
are typical of the so-called synthetic metals. As used herein
synthetic metals include not just polymers of the pure monomer but
rather the class of polymers derived from the base class of
substituted monomers. By way of illustration a polyaniline may
include not only aniline as a monomer but also copolymers of
aniline that can be selected from the aniline monomer substituted
with alkyl, alkoxy, aryl, hydroxyl, amino, sulfo and sulfonic
groups, substituted methyl anilines, substituted ethyl or propyl
anilines, substituted methoxy or ethoxy anilines, phenyl
substituted anilines, chloro-, fluoro-substituted anilines,
amino-substituted anilines, hydroxyl substituted anilines and the
like. Hydrogen atoms on the nitrogen of aniline can also be
substituted by methyl, propyl, butyl, and phenyl group, i.e.,
N-methyl, N-propyl, N-butyl, and N-phenyl substituted anilines.
Others include electrically conducting polymers like polythiophen
and poly(3-methyl)thiophen and poly(3,4-ethylenedioxythiophen),
known as PEDOT, and the like known to those skilled in the art.
Provided that the polymer retains the basic electrical conductivity
and redox properties it is suitable for use in the invention. The
foregoing materials are collectively referred to as synthetic
metals in this application.
[0060] The process can operate with many synthetic metals; as
described more fully in the Detailed Description of the Invention
below, illustrated with examples of preferred materials.
[0061] Dopants such as CSA are preferred because its molecular size
is large and does not leave the polymer when it is in contact with
water. In this way PANI stays electroconductive even at neutral pH.
Sulfo- or sulfonic groups on polyaniline are important
substituents, because in this case PANI is self-doped, and, for
example, additional doping agents like camphor sulfonic acid (CSA)
are then not necessary. As the result of doping, electric
conductivity of PANI may increase by almost ten orders of
magnitude.
[0062] Preferred salt solutions should have an anion, which is able
to enter the polymer in exchange for an electron. The preferred
anion is chloride, but others are also possible, such as iodide,
fluoride, CNS.sup.-, SCN.sup.-, nitrophenolate.
[0063] Nonporous polymer membrane, made with some electrocoductive
polymers, can be redox active. These membranes can be reduced by a
reducing agent dissolved in an aqueous solution on one side, and
simultaneously they can be re-oxidized by an oxidant dissolved in a
solution on another side of the membrane. This process is possible
because the membranes are permeable not only for electrons but also
for ions. Transport of these ions makes the whole transmembrane
reaction electrically neutral. Simultaneously, if the transport of
electrons is much faster that ion transport, it leads to the charge
separation and generation of transmembrane voltage. The value of
the voltage is determined by the difference of redox potentials in
both separated by the membrane solutions and can be high enough for
practical applications. To use this voltage it is necessary to add
two current collecting electrodes of the second kind, one in each
solution. These electrodes, for example Ag/AgCl, are the electrodes
where a metal is in contact with a solution containing anions that
form a sparingly soluble salt with the metal cation. To avoid side
reactions with redox active components of the solution, such as
ferricianide, these Ag/AgCl electrodes are separated from the
solution and interact with it through an immobilized liquid
junction, which finally allows conversion of ionic current in a
solution into the current of electrons in a metal. In this case
electrons are transferred from a reducing agent to an oxidant
through the membrane inside the cell, then the current is carried
by ions in a solution and, finally, again by electrons in the
wires, while in a common electrochemical cell the reducing agent
reacts with an electrode and then electrons are transported outside
the electrochemical cell into an opposite direction.
[0064] The dense membrane does not have large pores and is made of
the polymers, which are known as "synthetic metals". The polymer
has a system of polyconjugated double bonds, serving as a path for
electron conductivity. When properly made, the polymer has high
electric conductivity determined by the electron transport through
the polymer. Further, at the interface this electron current can be
converted into ionic current of chloride ions in a solution,
similar to what happens at the interface of Ag/AgCl electrodes. As
the result, it is possible to generate the transmembrane voltage of
practically important magnitude and to use it in an electrochemical
system with low electric resistance as an efficient electric power
source.
[0065] In the suggested here redox fuel cell electric voltage is
generated directly on a membrane and Pt electrodes are not
necessary. Instead of oxygen or air it is possible to use different
oxidants in flowing aqueous solutions without their preliminary
conversion to oxygen. Reducing agents on another side of the
membrane also can be delivered as an aqueous solution. This will
give the ability to mechanically replenish both the oxidant and the
"fuel" source and to meet power and energy requirements. The
membrane is stable enough, so that it is possible to use an oxidant
and reducing agent in amounts equivalent to much more than usual
amount in a closed battery. Utilization of the reactants can be
further increased if the solutions are flowing in a battery from
one membrane-based flow fuel cell to another.
[0066] Initial oxidant and reducing agents can be stored in a
stable and save solid form, thus making the system attractable
based both on a total system weight and volume. Synthetic metals
can be reduced by very many different reducing chemicals, thus
eliminating the need to use hydrocarbons as a fuel. Additional
opportunity is to use reversible redox reactants, so that the
electrochemical system becomes rechargeable. In this case an
external source of electrochemical energy for example, from power
stations during night time, can be used to recover the initial
reactants, and then the chemicals can be reused in a daytime when
the energy demand is higher.
[0067] Suggested fuel cell system is very volume- and
energy-efficient. The ancillary equipment to operate the device is
a simple pump with two channels to supply solutions of reducing and
oxidizing agent. Energy necessary for these steps is much less than
the chemical energy generated by the cell, which leads to high
energy conversion efficiency. The total energy delivered by a flow
fuel cell system is limited only by the amount of redox agent and
oxidant available.
[0068] One of the best-known electroconductive polymers is
polyaniline (PANI). Among many other applications, it was suggested
to use it as an electrode for oxygen reduction in fuel cells and
also as a supercapactor, which can withstand many
charging/discharging 325 cycles. By 2010 there were more than 230
papers published only in the J. of Power Sources, mentioning or
describing different applications of polyaniline in electrodes for
batteries and fuel cells, and also as a material for
supercapacitors. PANI is a redox-active polymer and usually when
synthesized it has approximately 50% of monomer fragments reduced
and 50% oxidized. This redox form is called emeraldine, while 100%
reduced 330 form and 100% oxidized forms are leucoemeraldine and
pernigraniline, respectively. PANI behavior in contact with aqueous
solutions was described in our papers. In many of these experiments
PANI was used as a membrane, separating two aqueous solutions. To
convert PANI into an active form it necessary to use doping by HCl
or other acids. For the cells with aqueous solutions it is
important to have a membrane active at neutral pH. It is possible
to achieve this using the membrane doped with d,l-camphor sulfonic
acid (CSA). This acid is able to protonate the nitrogens in the
polymer, making it electroconductive. After acid doping
transmembrane resistance decreases by at least six orders of
magnitude, and the membrane becomes Cl.sup.-/H.sup.+-selective.
EXAMPLES 1, 2
[0069] The films were made by evaporation of 1:2 solution of PANI
and CSA in m-cresol and had thickness near 100 micron. The surface
area was 5 cm.sup.2. The polymer was in the 345 protonated
emeraldine form (EB). Polyaniline membrane used in the examples
below is not porous and the transmembrane leakage of ions, which is
the major challenge of traditional redox fuel cells, is not
essential. Two reference Ag/AgCl electrodes were located in two
membrane-separated aqueous solutions. It is possible to use as
current collectors other electrodes of the second kind, i.e. the
electrodes which consist of a metal whose surface is coated with a
thin layer of an insoluble salt of the metal and its anion. We have
discovered that an addition of an oxidant on one side of the
membrane and a reducing agent on another side leads to the
spontaneous formation of transmembrane difference of electric
potentials. Transmembrane electrical potential for the redox
couples of Fe.sup.2+/Fe.sup.3+ (example 1) and
Fe(CN).sub.6.sup.4-/Fe(CN).sub.6.sup.3- (example 2) increased with
the increase of concentration in one solution, when concentration
in another was kept constant (FIG. 2a, 2b and FIGS. 3a and 3b).
[0070] It was possible to observe the transmembrane potentials
above 110 mV with ferri- and ferrocianide concentrations 0.01M.
Ferricianide was added into one solution, and the opposite solution
was 0.01M ferrocianide in the same 0.01M phosphate buffer, pH 6.4.
In another experiment ferrocianide was added into the opposite
side, while ferricianide concentration was constant and equal to
0.01M. The slope was near ideal Nernst slope 56 mV per ten times
concentration change. This demonstrates that the transmembrane
potential was formed not because of membrane sensitivity to the
polyvalent ions but because of one-electron transfer mechanism of
transmembrane redox reaction. As the result, further increase of
concentration from 0.01M to 1M leads to additional voltage increase
by 120 mV.
The films doped with CSA or some other organic acids or even
polymers with acidic groups stay active at neutral pH, which is
different from depressed electroactivity of PANI-HCl films under
these conditions.
EXAMPLE 3
[0071] This example demonstrates that it is possible to use not
only inorganic but also organic reducing agents. Ascorbic acid,
NADH and NADPH, redox active dyes Neutral red, Nile blue and
N-phenylanthranilic acid can be used as reducing agents reacting
with PANI. The membrane had a good response to redox active
substances, which normally do not have satisfactory response on Pt
electrode, i.e. ascorbic acid. The slope for ascorbic acid was near
26 mV for ten times concentration changes, corresponding to the
redox mechanism with two electrons transferred through the membrane
from one molecule of ascorbic acid. The lower concentration limit
when these substances start changing transmembrane potential is
near 0.2 mM in 0.01M phosphate buffer, pH 6.4.
EXAMPLE 4
[0072] Combination of the membrane together with many different
redox agents, which can easily react with PANI and have different
redox potentials can be used as a source of electric energy. This
example demonstrates that using strong oxidizing agents at higher
concentrations leads to higher and practically important
transmembrane voltage. PANI can electro-catalyze the reduction of
O.sub.2 in sulfuric acid in fuel cell operations with
H.sub.2O.sub.2 as the product:
LEB+2A.sup.-+2H.sup.++O.sub.2.fwdarw.ES+H.sub.2O.sub.2 (1)
Here LEB and ES refer to leucoemeraldine base and emeraldine salt,
respectively; A.sup.- is the anion from solution, which is then
incorporated into ES as the counter ion. Freshly made polyaniline
in acidic solutions in contacts with air can easily change its
color from green to dark blue and then to black even without
electrodes because of redox processes. Mechanism of possible
continuous H.sub.2O.sub.2 formation and PANI regeneration using
reducing potential is presented in the Scheme 1.
##STR00001##
[0073] Thermodynamic electrode potential for two-electron oxygen
reduction to hydrogen peroxide in acidic media is 0.7V. The
standard electrode potentials for reactions of several very common
water-soluble chlorine-based oxidants are:
ClO.sup.-+H.sub.2O+2e.sup.-.fwdarw.Cl.sup.-+2OH.sup.-+0.89V
ClO.sub.4.sup.-+2H.sup.++2e.sup.-.fwdarw.ClO.sub.3.sup.-+H.sub.2O+1.19V
ClO.sub.3.sup.-+3H.sup.++2e.sup.-.fwdarw.HClO.sub.2+H.sub.2O+1.21V
HClO+H.sup.++e.sup.-.fwdarw.0.5Cl.sub.2+H.sub.2O+1.63V
HClO.sub.2+2H.sup.++2e.sup.-.fwdarw.HClO+H.sub.2O+1.64V
These species react with doped emeraldine and oxidize it. It is
also possible to use different reducing agents, including different
sulfides or H.sub.2S dissolved in anoxic water on the opposite side
of the membrane. Thermodynamic electrode potential for
S+2e.sup.-.fwdarw.S.sup.2- is near -0.447V. If the membrane has an
ideal redox-selectivity, the transmembrane voltage will be in the
range from -1.337 V to -2.087 V, which is even better than the
difference of thermodynamic electrode potentials for fuel cells
with air and hydrogen as fuel.
[0074] It is also possible to use different pares of reducing and
oxidizing agents which have found applications in redox flow cells,
including bromide/polysulfide, vanadium(V)/vanadium(IV),
vanadium(IV)/bromide, chromium(III)/iron(II), etc.
EXAMPLE 5
[0075] This example demonstrates that transmembrane potential can
be additionally influenced 425 by changes of chloride anion
concentration. Addition of potassium chlorides into the reducing
solution makes increases the transmembrane voltage and makes it
even more negative. Addition of KCl into the opposite solution
makes decreases the value of transmembrane voltage.
[0076] The transmembrane electric potential difference across the
doped PANI membrane is a mixed potential due to both electron
transport in redox processes and simultaneous Cl.sup.- ion
transport. When the first reversible pare of redox agents is
present in one of the solutions and the second in another,
transmembrane voltage is described by the equation
V = .DELTA. E 0 - 2.3 RT nF Log red 1 + .alpha. Cl 1 - ox 1 -
.beta. Cl 1 - + 2.3 RT nF Log red 2 + .alpha. Cl 2 - ox 2 - .beta.
Cl 2 - ##EQU00001##
Here red, ox and Cl.sup.- are activity of corresponding species;
subscripts .sub.1 and .sub.2 correspond to the two solutions
separated by the membrane; and .alpha. and .beta. characterize
membrane selectivity for pares red/Cl.sup.- and ox/Cl.sup.-.
.DELTA.E.sub.0 is the difference of standard redox potentials of
the second and first redox pair, respectively. Without chloride
this equation is simplified to the Nernst equation:
V = .DELTA. E 0 - 2.3 RT nF Log red 1 ox 1 + 2.3 RT nF Log red 2 ox
2 ##EQU00002##
[0077] Transmembrane electric potential in this case is equal to
the difference of redox potentials in solutions, which was
confirmed by direct measurements of redox potential in both
solutions using Pt and reference electrodes (FIG. 4). Without
renewal of the solutions the transmembrane voltage slowly decreased
with time because of the simultaneous transmembrane redox
reactions, leading to the concentration changes of the redox active
components in the two solutions. This electrically neutral process
is much slower than charge separation through the membrane
(electron mobility is .about.four orders of magnitude higher than
that of ions in the polymer), which explains generation of
practically ideal transmembrane Nernst potential.
EXAMPLE 6
[0078] Electric resistance of a mechanically stable CSA-doped PANI
membrane with the surface area less than 5 cm.sup.2 and thickness
40 micron was less than 1 ohm, which means a possibility to produce
currents above 1 A.
EXAMPLE 7
[0079] One minute was enough for formation of transmembrane
potential, which was initially above 220 mV. This value was exactly
equal to the difference of redox potentials measured in separated
by the membrane solutions (FIG. 4). With time the transmembrane
potential slowly decreased because of the transmembrane redox
reaction (FIG. 5), but it was possible to keep it constant when
both reducing and oxidizing solutions were continuously flowing
through the separated by the membrane compartments. Utilization of
an oxidant will depend on the rate of reaction, and on flow rate
and recycling of the oxidant.
EXAMPLE 8
[0080] With time AgCl from the current collector electrodes
dissolves in one solution and it is formed in another. Possibility
to swap the flows in the chamber (initially the reducing and
oxidant solutions are flowing through compartments 1 and 2, and
then--through 2 and 1, respectively) allows further use of the flow
fuel cell.
[0081] Taking into account the continuous supply of fuel to the
system during long-term operation the energy density of the fuel
cell in terms of Wh/kg will be greater than that of advanced
battery systems. It is possible to do a simple estimate. Assume
that 1 kg of a solid oxidant has 10 mols of a salt, and each
molecule accepts 7 electrons, as it is for KClO.sub.3 reduced into
Cl.sup.-. In this case the total charge transferred is
7.times.10.sup.6 Q/kg or 1944 Ah per kg of potassium chlorate. If
the transmembrane voltage is 1 V, this should give energy
7.times.10.sup.6 J/kg or 1944 Wh per kg of solid oxidant phase. The
electric power supplied by 5 cm.sup.2 membrane is more than
1V.times.1 A=1 W. To have 50 W source we will need less than 17
cm.times.17 cm membrane.
[0082] The advantages of proposed system are low cost, modularity,
flexible operation, possibility to use sea water with dissolved
sodium chloride and to discharge solutions completely for
maintenance without damaging the electrochemical cells. Cost
effectiveness of suggested approach will be better than existing
technology because it will be based on easily made, simple and
cheap polymer materials instead of Pt electrodes.
* * * * *