Note: Descriptions are shown in the official language in which they were submitted.
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LOW-VOLTAGE ALKALINE PRODUCTION USING HYDROGEN AND ELECTROCATLYTIC
ELECTRODES
CROSS-REFERENCE
[0001] This application claims priority to US Provisional Application No.
61/151,472 filed
February 10, 2009 and titled "Low Voltage Electrochemical Hydroxide with
Circulating Hydrogen
Gas"; US Provisional Application No. 61/151,484 filed February 10, 2009 and
titled
"Electrocatalyst Electrodes for Low-voltage Electrochemical Hydroxide System";
and US
Provisional Application No.61/151,481 filed February 10, 2009 and titled "Low-
voltage
Electrochemical Hydroxide Cell Stacking System", all herein fully incorporated
by reference in
their entirety.
BACKGROUND
[0002] In many chemical processes an alkaline solution comprising, e.g.,
hydroxide ions
and/or carbonate ions and/or bicarbonate ions is utilized to achieve a
chemical reaction, e.g., to
neutralize an acid, or buffer the pH of a solution, or precipitate an
insoluble hydroxide and/or
carbonate and/or bicarbonate from a solution. An alkaline solutions can be
produced by an
electrochemical system that converts an aqueous salt solution to the alkaline
solution and an
acid as described in the above-referenced US Provisional patent applications,
herein
incorporated by reference in their entirety. However, as the process is very
energy intensive, a
large amount of electrical energy is consumed; also, large amounts of salt and
water are
consumed. Consequently, lowering the energy and material consumed are very
desirable.
SUMMARY
[0003] This invention provides for a low-voltage, low-energy electrochemical
system and
method of producing an alkaline solution, comprising reducing water at the
cathode to hydroxide
ions and hydrogen gas, and allowing the hydroxide ions to migrate into the
cathode electrolyte
to produce the alkaline solution. By the system and method, at the anode,
hydrogen gas is
oxidized to protons without producing a gas at the anode, and the protons are
allowed to
migrate into the anode electrolyte or an adjacent electrolyte to produce an
acid. In some
embodiments, the alkaline solution further comprises bicarbonate ions and/or
carbonate ions
produced by adding carbon dioxide to the cathode electrolyte. In some
embodiments, the
alkaline solution is produced by applying less than 3V across the anode and
cathode. In some
embodiments, hydrogen gas produced at the cathode is recovered and directed to
the anode
where it is reduced to protons.
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[0004] In some embodiments, an electrocatalyst is provided on the electrodes
to catalyze
the oxidation of hydrogen at the anode and catalyze the reduction of water to
hydroxide ions and
hydrogen gas at the cathode.
[0005] In some embodiments, a plurality of anodes and cathodes are arranged in
series, or
in parallel or in a cascading configuration to minimize the energy expended in
producing the
alkaline solution.
[0006] In one embodiment, the system comprises an electrochemical unit
comprising an
anode compartment comprising a hydrogen-oxidizing anode, a cathode compartment
comprising a cathode, and a hydrogen-delivery system configured to deliver
hydrogen gas to the
anode, wherein the unit is operable connected to a carbon sequestration system
configured to
sequester carbon dioxide with the cathode electrolyte.
[0007] In another embodiment, the system comprises a hydrogen-oxidizing anode
in
communication with a cathode electrolyte; and a carbon sequestration system
configured to
sequester carbon dioxide with the cathode electrolyte.
[0008] In some embodiments of the system, the hydrogen gas oxidized at the
anode is
obtained from hydrogen gas generated at a cathode. In some embodiments, the
cathode
electrolyte comprises added carbon dioxide; in some embodiments, the carbon
dioxide is
contained in an industrial waste gas. In some embodiments, the cathode
electrolyte comprises
hydroxide ions and/or carbonate ions and /or bicarbonate ions.
[0009] In some embodiments, the system is configured to produce bicarbonate
ions and/or
carbonate ions and/or hydroxide ions in the cathode electrolyte; produce
hydrogen gas at the
cathode; and produce protons at the anode by applying less than 3V across the
anode and
cathode, without producing a gas at the anode.
[0010] In some embodiments, the anode and/or cathode comprise an
electrocatalyst
selected from platinum, a single-crystal nickel, Raney nickel, platinized
nickel, a metal carbide
(W2C, Pt-W2C), a platinum group metal alloy (Pt-M, where M=Fe, Mn, Cr, Co,
Au), a transition
metal, a nickel alloy, sintered nickel, a platinum group metals (Pt, Pd, Ru,
Rh), gold, silver, a
precious or non-precious chalcogenides, a discrete macrocyclic complexe of
transition metals
and biological complexes. In some embodiments, the electrocatalyst is
configured on the anode
to catalyze the oxidation of hydrogen gas to protons; and on the cathode to
catalyze production
of hydrogen gas and hydroxide ions.
[00111 In some embodiments, the system includes a carbon sequestration system
configured to sequester carbon dioxide with the cathode electrolyte; in some
embodiments, the
carbon dioxide is contained in an industrial waste gas; in some embodiments,
the carbon
dioxide is sequestered as carbonates and/or bicarbonates comprising divalent
cations, e.g.,
calcium ions and/or magnesium ions.
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[0012] In some embodiments, the system is configured to produce an acid in the
anode
electrolyte; in some embodiments, the system includes an acid dissolution
system configured to
produce divalent cations with the acid; in some embodiments, the divalent
cations comprise
calcium ions and/or magnesium ions. In some embodiments, the divalent cations
are produced
by dissolving a mineral with the acid, and are provided to the carbon
sequestration system.
[0013] In some embodiments, the system comprises a plurality of pairs of
anodes and
cathodes configured in series to receive a series current through each anode-
cathode pair; in
some embodiments, the system comprises a plurality of pairs of anodes and
cathodes
configured in parallel to receive a parallel voltage across each pair anode-
cathode pair.
[0014] In some embodiments, the system comprises a plurality of pairs of anode
electrolytes
and cathode electrolytes wherein the cathode electrolyte of a first pair is
connected to the
cathode electrolyte of a second pair; and wherein the anode electrolyte of a
first pair is
connected to the anode electrolyte of a second pair. In some embodiments, the
pH of the
cathode electrolyte of the second pair is equal to or greater than the pH of
the cathode
electrolyte in the first pair; and the pH of the anode electrolyte of the
second pair is equal to or
less than the pH of the anode electrolyte of the first pair.
[0015] In some embodiments, the system comprises a plurality of pairs of anode
electrolytes
and cathode electrolytes wherein the cathode electrolyte and anode electrolyte
of a second pair
comprise cathode electrolyte from a first pair; and the cathode electrolyte
and anode electrolyte
of a third pair comprise anode electrolyte from the first pair. In some
embodiments, the pH of the
cathode electrolyte of the second pair is equal to or greater than pH of the
cathode electrolyte of
the first pair; and the pH of the anode electrolyte of the third pair is equal
to or less that pH of the
anode electrolyte of the first pair. In some embodiments, the cathode
electrolyte of the second
pair comprises diluted cathode electrolyte of the first pair; and the anode
electrolyte of the third
pair comprises diluted anode electrolyte of the first pair.
[0016] In another embodiment, the invention provides for a low-voltage, low
energy method
of producing an alkaline solution, comprising oxidizing hydrogen gas to
protons at an anode
without producing a gas at the anode; and producing bicarbonate ions in a
cathode electrolyte in
communication with the anode.
[0017] In another embodiment, the method comprises configuring an
electrochemical unit
comprising: an anode compartment comprising a hydrogen-oxidizing anode, a
cathode
compartment comprising a cathode, and a hydrogen-delivery system configured to
deliver
hydrogen gas to the anode, wherein the unit is operable connected to carbon
sequestration
system; and sequestering carbon dioxide with the cathode electrolyte.
[0018] In some embodiments of the method, the voltage applied across the anode
and a
cathode is less than 3V, and hydrogen gas oxidized at the anode is provided
from hydrogen
produced at the cathode.
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[0019] In some embodiments, the method comprises adding carbon dioxide to the
cathode
electrolyte and producing hydroxide ions and/or carbonate ions and/or
bicarbonate ions in the
cathode electrolyte; in some embodiments, the carbon dioxide is contained in
an industrial
waste gas.
[0020] In some embodiments of the method, the anode and/or cathode comprises
an
electrocatalyst selected from platinum, a single-crystal nickel, Raney nickel,
platinized nickel, a
metal carbide (W2C, Pt-W2C), a platinum group metal alloy (Pt-M, where M=Fe,
Mn, Cr, Co, Au),
a transition metal, a nickel alloy, sintered nickel, a platinum group metals
(Pt, Pd, Ru, Rh), gold,
silver, a precious or non-precious chalcogenides, a discrete macrocyclic
complexe of transition
metals and biological complexes. In some embodiments, the electrocatalyst is
configured to
catalyze oxidation of hydrogen gas to protons at the anode, and catalyze
production of hydrogen
gas and hydroxide ions at the cathode.
[00211 In some embodiments, the method comprises configuring a carbon
sequestration
system to sequester carbon dioxide with the cathode electrolyte, wherein the
carbon dioxide is
contained in an industrial waste gas; in some embodiments, the carbon dioxide
is sequestered
as carbonates and/or bicarbonates; in some embodiment, the carbonates and/or
bicarbonates
comprise divalent cations; in some embodiments, the divalent cations comprise
calcium ions
and/or magnesium ions.
[0022] In some embodiments, the method comprises producing an acid in the
anode
electrolyte; and configuring an acid dissolution system to produce divalent
cations with the acid.
In some embodiments, the divalent cations comprise calcium ions and/or
magnesium ions; and
in some embodiments, the method comprises producing divalent cations by
dissolving a mineral
with the acid. In some embodiments, the method comprises configuring the acid
dissolution
system to provide divalent cations to the carbon sequestration system.
[0023] In some embodiments, the method comprises configuring a plurality of
pairs of
anodes and cathodes in series to receive a series current through each pair of
anode and
cathode. In some embodiments, the method comprises configuring a plurality of
pairs of anodes
and cathodes in parallel to receive a parallel voltage across each anode-
cathode pair.
[0024] In some embodiments, the method comprises configuring a plurality of
pairs of the
anode electrolytes and cathode electrolytes whereby the cathode electrolyte of
a first pair is
connected to the cathode electrolyte of a second pair; and the anode
electrolyte of a first pair is
connected to the anode electrolyte of a second pair. In some embodiments, the
method
comprises adjusting the pH of the cathode electrolyte of the second pair to a
value equal to or
greater than the pH of the cathode electrolyte in the first pair; and
adjusting the pH of the anode
electrolyte of the second pair to a value equal to or less than the pH of the
anode electrolyte of
the first pair.
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[0025] In some embodiments, the method comprises configuring a plurality of
pairs of anode
electrolytes and cathode electrolytes whereby the cathode electrolyte and
anode electrolyte of a
second pair comprise cathode electrolyte from a first pair; and the cathode
electrolyte and
anode electrolyte of a third pair comprise anode electrolyte from the first
pair. In some
embodiments, the method comprises adjusting the pH of the cathode electrolyte
of the second
pair to a value equal to or greater than pH of the cathode electrolyte of the
first pair; and
adjusting the pH of the anode electrolyte of the third pair to a value equal
to or less that pH of
the anode electrolyte of the first pair.
[0026] In various embodiments, the products comprise sodium hydroxide and/or
sodium
bicarbonate and/or sodium carbonate, hydrochloric acid and an ion-depleted
brine from which
certain cation and anions have been removed. In some embodiments, the products
are utilized
to sequester carbon dioxide and other constituents of industrial waste gases,
e.g., sulfur gases,
nitrogen oxide gases and other combustion gases, by contacting the waste gas
with a solution
comprising divalent cations and the hydroxide and/or bicarbonate and/or
carbonate ions to
precipitate divalent cation carbonates and/or bicarbonates as described in
commonly assigned
U.S. Patent Application no. 12/344,019 filed on December 24, 2008, herein
incorporated by
reference in its entirety. The precipitates, comprising, e.g., calcium and/or
magnesium
carbonates and/or bicarbonates in various embodiments are utilized as building
materials, e.g.,
as cements and aggregates, as described in commonly assigned U.S. Patent
Application no.
12/126,776 filed on May 23, 2008, herein incorporated by reference in its
entirety.
[0027] In another application, the ion-depleted brine from which certain
cation and anions
have been removed, e.g., sodium and chloride ions, is used as feed water in a
desalination
system where the ion-depleted is further processed as described in commonly
assigned U.S.
Patent Application no. 12/163,205 filed on June 27, 2008, herein incorporated
by reference in its
entirety.
[0028] In another embodiment, the acid produced in the system and/or the
alkaline solution
produced in the cathode electrolyte are utilized to dissolve minerals and
waste materials
comprising divalent cations, e.g., Ca++ and Mg++ to produce divalent cation
solutions for use in
producing divalent metal ion carbonate precipitates using the cathode
electrolyte herein. In
various embodiments, the precipitates are used as building materials, e.g.,
cement and
aggregates as described in commonly assigned U.S. Patent application no.
12/126,776, herein
incorporated by reference in its entirety.
[0029] Advantageously, with the present system and method, since a relatively
low voltage
is utilized across the anode and cathode to produce the alkaline solution, and
since hydrogen
gas generated at the cathode is oxidized to protons at the anode without
producing a gas at the
anode, a relatively low energy is utilized to produce the alkaline solution.
Also, by the system
and method, since carbon dioxide from industrial waste gases is utilized to
produce the alkaline
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solution, the system and method is utilized to sequester large amounts of
carbon dioxide and
thus reduce carbon dioxide emissions into the atmosphere. Similarly, the acid
produced is
utilized in various ways including dissolving materials, e.g., minerals and
biomass to produce
cation for use in the system.
BRIEF DESCRIPTION OF THE DRAWINGS
The following are brief descriptions of drawings that illustrate embodiments
of the
invention:
[0030] Fig. 1 is an illustration of an embodiment of the system.
[0031] Fig. 2 is an illustration of an embodiment of the system.
[0032] Fig. 3 is an illustration of an embodiment of the method.
[0033] Fig. 4 is an illustration of an embodiment of the system.
[0034] Fig. 5 is an illustration of carbonate ion/bicarbonate ion speciation
in water in the
system.
[0035] Fig. 6 is an illustration of an effect of adding carbon dioxide to the
cathode electrolyte
of the system.
[0036] Fig. 7 is an illustration of an embodiment of the system.
[0037] Fig. 8 is a flow chart of an embodiment of the system.
[0038] Fig. 9 is an illustration of an embodiment of the hydrogen system.
[0039] Fig. 10 is an illustration of an embodiment of the electrodes.
[0040] Fig. 11 is an illustration of an embodiment of the electrodes.
[0041] Fig. 12 is an illustration of an embodiment of the electrodes.
[0042] Fig. 13 is an illustration of an embodiment of the cell configuration.
[0043] Fig. 14 is an illustration of an embodiment of the cell configuration.
[0044] Fig. 15 is an illustration of an embodiment of the cell configuration.
[0045] Fig. 16 is an illustration of an embodiment of the cell configuration.
[0046] Fig. 17 is an illustration of an embodiment of the cell configuration.
DETAILED DESCRIPTION
[0047] This invention provides for a low-voltage/low-energy system and method
of
producing an alkaline solution in an electrochemical system by oxidizing
hydrogen at the anode
to protons, and reducing water at the cathode to hydroxide ions and hydrogen
at the cathode. In
some embodiments, carbon dioxide is added to the cathode electrolyte to
produce carbonate
ions and/or bicarbonate ions in the cathode electrolyte; a gas is not produced
at the anode. In
some embodiments, an alkaline solution comprising, e.g., sodium hydroxide
and/or sodium
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carbonate and/or sodium bicarbonate is produced in the cathode electrolyte by
applying a
voltage across the anode and cathode. In some embodiments, the volts is less
than 3V.
[0048] In some embodiments, subterranean brine is utilized as a source of
salt/cations/anions used in producing the alkaline solution. In some
embodiments,
electrocatalysts are provided on the electrodes to catalyze the oxidation of
hydrogen at the
anode and catalyze the production hydroxide ions and hydrogen gas at the
cathode. In some
embodiments, a plurality of anodes and cathodes are arranged in series,
parallel and cascading
configurations to minimize the energy expended in producing the alkaline
solution. In some
embodiments, a salt solution comprising, e.g., sodium chloride, is used to
produce the alkaline
solution.
[0049] In some embodiments, an acid, e.g., hydrochloric acid, is produced in
the anode
electrolyte from hydrogen ions, produced at the anode and migrated from the
anode into the
anode, and cations in the cations in the electrolyte, e.g., chloride ions.
[0050] In some embodiments, the acid is utilized to dissolve a material, e.g.,
a mineral, e.g.,
serpentine or olivine, to provide divalent cation solution, e.g., calcium and
magnesium ions,
which may in some embodiments be used with the alkaline solution to
precipitate carbonates
and/ or bicarbonates derived from carbon dioxide in a waste gas stream, e.g.,
carbon dioxide in
the exhaust gases of a fossil fuelled power generating plant or a cement
producing plant. In
some embodiments, sodium chloride solution is used as the anode electrolyte.
[0051] In some embodiments, on applying a voltage across the anode and
cathode, cations,
e.g., sodium ions in the anode electrolyte, migrate from the salt solution
through a cation
exchange membrane into the cathode electrolyte to produce an alkaline solution
comprising,
e.g., sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in
the cathode
electrolyte; concurrently, anions in the salt solution, e.g., chloride ions,
migrate into the anode
electrolyte to produce an acid, e.g., hydrochloric acid, with in the protons
that form at the anode.
[0052] In some embodiments, hydrogen gas and hydroxide ions are produced at
the
cathode, and in some embodiments, some or all of the hydrogen gas produced at
the cathode is
directed to the anode where it is oxidized to produce hydrogen ions.
[0053] As can be appreciated by one ordinarily skilled in the art, since the
embodiments
herein can be configured with an alternative or equivalent salt, e.g., a
potassium sulfate solution,
to produce an equivalent alkaline solution, e.g., potassium hydroxide and/or
potassium
carbonate and/or potassium bicarbonate in the cathode electrolyte, and an
alternative acid, e.g.,
sulfuric acid in the anode electrolyte, by applying the voltage herein across
the anode and
cathode, the invention is not limited to the exemplarary embodiments described
herein, but is
useable with an equivalent salt, e.g., potassium sulfate, to produce an
alkaline solution in the
cathode electrolyte, e.g., potassium carbonate and/or potassium bicarbonate
and an acid, e.g.,
sulfuric acid in the anode electrolyte. Accordingly, to the extent that such
equivalents are based
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on or are suggested by the embodiment herein, these equivalents are within the
scope of the
appended claims.
[0054] In the following detailed description, embodiments of the system and
method are
described with reference to the one or more illustrated Figures. However, it
should be
understood that this description is illustrative and is not restrictive since
the invention is
adaptable for use with other cell configurations including a one-cell, a two-
cell, three-cell and
other multi-cell configurations, not described in detail herein but are
reasonably contemplated.
[0055] Similarly, it should be understood that although the invention is
described with
particularity with use of a brine comprising sodium ions and chloride ions,
this description also is
also illustrative and is not restrictive since the invention is adaptable for
use with equivalent
aqueous salt, e.g., sulfates and nitrates and like, e.g., potassium sulfate,
as can be appreciated
by those ordinarily skilled in the art.
[0056] With reference to Figs. 1 - 17, in some embodiments, carbon dioxide is
added to the
cathode electrolyte utilizing a gas mixer/gas absorber. In one embodiment, the
gas mixer/gas
absorber comprises a series of spray nozzles that produces a flat sheet or
curtain of liquid into
which the gas is absorbed; in another embodiment, the gas mixer/gas absorber
comprises a
spray absorber that creates a mist and into which the gas is absorbed; in
other embodiments,
other commercially available gas/liquid absorber, e.g., an absorber available
from Neumann
Systems, Colorado, USA is used.
[0057] The carbon dioxide added to the cathode electrolyte may be obtained
from various
industrial sources that releases carbon dioxide including carbon dioxide from
combustion gases
of fossil fuelled power plants, e.g., conventional coal, oil and gas power
plants, or IGCC
(Integrated Gasification Combined Cycle) power plants that generate power by
burning sygas;
cement manufacturing plants that convert limestone to lime; ore processing
plants; fermentation
plants; and the like. In some embodiments, the carbon dioxide may comprise
other gases, e.g.,
nitrogen, oxides of nitrogen (nitrous oxide, nitric oxide), sulfur and sulfur
gases (sulfur dioxide,
hydrogen sulfide), and vaporized materials.
[0058] In some embodiments, the system includes a gas treatment system that
removes
constituents in the carbon dioxide gas stream before the carbon dioxide is
utilized in the cathode
electrolyte. In some embodiments, a portion of, or the entire amount of,
cathode electrolyte
comprising bicarbonate ions and/or carbonate ions/ and or hydroxide ions is
withdrawn from the
system and is contacted with carbon dioxide gas in an exogenous carbon dioxide
gas/liquid
contactor to increase the absorbed carbon dioxide content in the solution. In
some
embodiments, the solution enriched with carbon dioxide is returned to the
cathode compartment;
in other embodiments, the solution enriched with carbon dioxide is reacted
with a solution
comprising divalent cations to produce divalent cation hydroxides, carbonates
and/or
bicarbonates.
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[0059] In some embodiments, the pH of the cathode electrolyte is adjusted
upwards by
hydroxide ions that migrate from the cathode, and/or downwards by dissolving
carbon dioxide
gas in the cathode electrolyte to produce carbonic acid and/or carbonate ions
and/or
bicarbonate ions that react with and remove hydroxide ions. Thus, it can be
appreciated that the
pH of the cathode electrolyte is determined, at least in part, by the balance
of these processes.
[0060] Referring to Fig.1, the system 100 in one embodiment comprises a gas
diffusion
anode 102 and a cathode 106 in contact with a cathode electrolyte 108, 108A,
108B comprising
dissolved carbon dioxide 107A. The system in some embodiments includes a gas
delivery
system 112 configured to deliver hydrogen gas to the anode 102; in some
embodiments, the
hydrogen gas is obtained from the cathode 106. In the system, the anode 102 is
configured to
produce protons, and the cathode 106 is configured to produce hydroxide ions
and hydrogen
gas when a low voltage 114, e.g., less than 2V is applied across the anode and
the cathode. In
the system, a gas is not produced at the anode 102.
[0061] As is illustrated in Fig. 1, first cation exchange membrane 116 is
positioned between
the cathode electrolyte 108, 108 A, 108B and a salt solution 118; and an anion
exchange
membrane 120 is positioned between the salt solution 118 and the anode
electrolyte 104 in a
configuration where the anode electrolyte 104 is separated from the anode 102
by second
cation exchange membrane 122. In the system, the second cation exchange
membrane 122 is
positioned between the anode 102 and the anode electrolyte 104 such that
anions may migrate
from the salt solution 118 to the anode electrolyte 104 through the anion
exchange membrane
120; however, anions are prevented from contacting the anode 102 by the second
cation
exchange membrane 122 adjacent to the anode 102.
[0062] In some embodiments, the system is configurable to migrate anions,
e.g., chloride
ions, from the salt solution 118 to the anode electrolyte 104 through the
anion exchange
membrane 120; migrate cations, e.g., sodium ions from the salt solution 118 to
the cathode
electrolyte 108, 108A, 108B through the first cation exchange membrane 116;
migrate protons
from the anode 102 to the anode electrolyte 104; and migrate hydroxide ions
from the cathode
106 to the cathode electrolyte 108, 108A, 108B. Thus, in some embodiments, the
system can be
configured to produce sodium hydroxide and/or sodium bicarbonate and/or sodium
carbonate in
the cathode electrolyte 108, 108A, 108B; and produce an acid e.g.,
hydrochloric acid 124 in the
anode electrolyte.
[0063] In some embodiments as illustrated in Fig.1, the system comprises a
partition 126
that partitions the cathode electrolyte 108 into a first cathode electrolyte
portion 108A and a
second cathode electrolyte portion 108B, wherein the second cathode
electrolyte portion 108B,
comprising dissolved carbon dioxide, contacts the cathode 106; and wherein the
first cathode
electrolyte portion 108A comprising dissolved carbon dioxide and gaseous
carbon dioxide is in
contact with the second cathode electrolyte portion 108B under the partition
126. In the system,
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the partition is positioned in the cathode electrolyte such that a gas, e.g.,
carbon dioxide in the
first cathode electrolyte portion 108A is isolated from cathode electrolyte in
the second cathode
electrolyte portion 108B. Thus, for example, where a gas, e.g., hydrogen, is
generated at the
cathode and it is desired to separate this cathode gas from a gas or vapor
that may evolve from
the cathode electrolyte, the partition may serve as a means to prevent mixing
of the gases form
the cathode and the gases and or vapor from the cathode electrolyte. While
this system is
illustrated in Fig. 1, it is applicable to any of the electrochemical system
described herein, e.g.,
the systems illustrated in Figs. 4, 7 and 8.
[0064] Thus, in some embodiments, on applying the present voltage across the
anode and
cathode, the system can be configured to produce hydroxide ions and hydrogen
gas at the
cathode 106; migrate hydroxide ions from the cathode into the cathode
electrolyte 108, 108B,
108A; migrate cations from the salt solution 118 to the cathode electrolyte
through the first
cation exchange membrane 116; migrate chloride ions from the salt solution 118
to the anode
electrolyte 104 through the anion exchange membrane 120; and migrate protons
from the anode
102 to the anode electrolyte 104. Hence, depending on the salt solution 118
used, the system
can be configured to produce an alkaline solution, e.g., sodium hydroxide in
the cathode
electrolyte.
[0065] In some embodiments, the system is operatively connected to a carbon
dioxide
gas/liquid contactor 128 configured to remove cathode electrolyte from the
system and dissolve
carbon dioxide in the cathode electrolyte in the gas/liquid contactor before
the cathode
electrolyte is returned to the system.
[0066] In other embodiments, the cathode electrolyte is operatively connected
to a system
(not shown) that is configured to precipitate divalent cation carbonates
and/or divalent cation
bicarbonates and/or divalent cation hydroxides from a solution comprising
carbon dioxide gas
and divalent cations.
[0067] Fig. 2 illustrates a schematic of a suitable gas diffusion anode that
can be used in
embodiments of the system described herein. In some embodiments, the gas
diffusion anode
comprises a conductive substrate 130 infused with a catalyst 136 that is
capable of catalyzing
the oxidation of hydrogen gas to protons when the present voltages are applied
across the
anode and cathode. In some embodiments, the anode comprises a first side 132
that interfaces
with hydrogen gas provided to the anode, and an opposed second side 134 that
interfaces with
the anode electrolyte 104. In some embodiments, the portion of the substrate
132 that interfaces
with the hydrogen gas is hydrophobic and is relatively dry; and the portion of
the substrate 134
that interfaces with the anode electrolyte 104 is hydrophilic and may be wet,
which facilitates
migration of protons from the anode to the anode electrolyte. In various
embodiments, the
substrate is porous to facilitate the movement of gas from the first side 132
to the catalyst 136
that may be located on second side 134 of the anode; in some embodiments, the
catalyst may
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also be located within the body of the substrate 130. The substrate 130 may be
selected for its
hydrophilic or hydrophobic characteristics as described herein, and also for
its low ohmic
resistance to facilitate electron conduction from the anode through a current
collector connected
to the voltage supply 114; the substrate may also be selected for it porosity
to ion migration,
e.g., proton migration, from the anode to the anode electrolyte 116.
[0068] In some embodiments, the catalyst may comprise platinum, ruthenium,
iridium,
rhodium, manganese, silver or alloys thereof. Suitable gas diffusion anodes
are available
commercially, e.g., from E-TEK (USA) and other suppliers.
[0069] In some embodiments of the anode as is illustrated in Fig. 8, the anode
comprises a
ion exchange membrane, e.g., a cation exchange membrane 122 that contacts the
second side
134 of the anode. In such embodiments, the ion exchange membrane can be used
to allow or
prevent migration of ions to or from the anode. Thus, for example, with
reference to Fig. 8, when
protons are generated at the anode, a cation exchange membrane may be used to
facilitate the
migration of the protons from the anode and/or block the migration of ions,
e.g., cations to the
substrate. In the some embodiments, the ion exchange membrane may be selected
to
preferentially allow passage of one type of cation, e.g., hydrogen ions, while
preventing the
passage of another type of ions, e.g., sodium ions.
[0070] As is illustrated in Fig. 1, the system includes a salt solution 118
located between the
anode electrolyte 104 and the cathode electrolyte 108, 108A, 108B. In some
embodiments, the
cathode electrolyte is separated from the salt solution by a first cation
exchange membrane 116
that is allows migration of cations, e.g., sodium ions, from the salt solution
to the cathode
electrolyte. The first cation exchange membrane 116 is also capable of
blocking the migration of
anions from the cathode electrolyte 108, 108A, 108B to the salt solution 118.
In some
embodiments, the anode electrolyte 104 is separated from the salt solution 118
by an anion
exchange membrane 108 that will allow migration of anions, e.g., chloride
ions, from the salt
solution 118 to the anode electrolyte 104. The anion exchange membrane,
however, will block
the migration of cations, e.g., protons from the anode electrolyte 104 to the
salt solution 118.
[0071] With reference to Figs. 1 and 2, the system includes a hydrogen gas
supply system
112 configured to provide hydrogen gas to the anode 102. The hydrogen may be
obtained from
the cathode 106 or may be obtained from external source, e.g., from a
commercial hydrogen
gas supplier, e.g., at start-up of the system when the hydrogen supply from
the cathode is
insufficient. In the system, the hydrogen gas is oxidized to protons and
electrons; un-reacted
hydrogen gas is recovered and circulated 140 at the anode.
[0072] Referring to Fig. 1, in operation, the cathode electrolyte 108, 108A,
108B is initially
charged with a alkaline electrolyte, e.g., sodium hydroxide solution, and the
anode electrolyte
104 is initially charged with an acidic electrolyte, e.g., dilute hydrochloric
acid. The cathode
electrolyte is also initially charged with carbon dioxide gas 107A, 128, and
hydrogen gas is
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provided to the anode. In the system, on applying a voltage across the anode
and cathode,
protons produced at the anode will enter into the anode electrolyte and
attempt to migrate from
the anode electrolyte 104 to the cathode 106 via the salt solution 118 between
the cathode and
anode. However, since the anion exchange membrane will block the migration of
protons to the
salt solution, the protons will accumulate in the anode electrolyte 104.
[0073] Simultaneously at the cathode 106, the voltage across the anode and
cathode will
produce hydroxide ions and hydrogen gas at the cathode. In some embodiments,
the hydrogen
produced at the cathode is recovered and directed to the anode 102 where it is
oxidized to
protons. In the system, hydroxide ions produced at the cathode 106 will enter
into the cathode
electrolyte 108, 108A, 108B from where they will attempt to migrate to the
anode 102 via the salt
solution 118 between the cathode and anode. However, since the cathode
electrolyte 108,
108A, 1088 is separated from the salt solution electrolyte by the first cation
exchange
membrane 116 which will block the passage of anions, the first cation exchange
membrane will
block the migration of hydroxide ions from the cathode electrolyte to the salt
solution;
consequently, the hydroxide ions will accumulate in the cathode electrolyte
108, 108A, 108B.
[0074] In the system as illustrated in Fig. 1, with the voltage across the
anode and cathode,
since the salt solution is separated from the cathode electrolyte by the first
cation exchange
membrane 116, cations in the salt solution, e.g., sodium ions, will migrate
through the first cation
exchange membrane 116 to the cathode electrolyte 108, 108A, 1086, and anions,
e.g., chloride
ions, will migrate to the anode electrolyte through the anion exchange
membrane 120.
Consequently, in the system, as illustrated in Fig. 1, an acid, e.g.,
hydrochloric acid 124 will be
produced in the anode electrolyte 104, and alkaline solution, e.g., sodium
hydroxide will be
produced in the cathode electrolyte. As can be appreciated, with the migration
of cations and
anions from the salt solution, the system in some embodiments can be
configured to produce a
partly de-ionized salt solution from the salt solution 118. In some
embodiments, this partially de-
ionized salt solution can be used as feed-water to a desalination facility
(not shown) where it can
be further processed to produce desalinated water as described in commonly
assigned U.S.
Patent Application no. 12/163,205 filed on June 27, 2008, herein incorporated
by reference in its
entirety; alternatively, the solution can be used in industrial and
agricultural applications where
its salinity is acceptable.
[0075] With reference to Fig. 1, the system in some embodiments includes a
second cation
exchange membrane 124, attached to the anode substrate 105, such that it
separates the
anode 102 from the anode electrolyte. In this configuration, as the second
cation exchange
membrane 122 is permeable to cations, protons formed at the anode will migrate
to the anode
electrolyte as described herein; however, as the second cation exchange
membrane 122 is
impermeable to anions, anions, e.g., chloride ions, in the anode electrolyte
will be blocked from
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migrating to the anode 102, thereby avoiding interaction between the anode and
the anions that
may interact with the anode, e.g., by corrosion.
[0076] With reference to Fig. 1, in some embodiments, the system includes a
partition 128
configured into J-shape structure and positioned in the cathode electrolyte
108, 108A, 108B to
define an upward-tapering channel 144 in the upper portion of the cathode
electrolyte
compartment. The partition also defines a downward-tapering channel 146 in
lower portion of
the cathode electrolyte. Thus, with the partition in the place, the cathode
electrolyte 108 is
partitioned into the first cathode electrolyte portion 108A and a second
cathode electrolyte
portion 108B. As is illustrated in Fig. 1, cathode electrolyte in the first
cathode electrolyte portion
108A is in contact with cathode electrolyte in the second cathode electrolyte
portion 108B;
however, a gas in the first electrolyte portion 108A, e.g., carbon dioxide, is
prevented from
mixing with cathode electrolyte in the second cathode electrolyte 108B.
Although this is
illustrated in for the system of Fig.1, such a configuration may be used in
any system where it is
desired to partition an electrolyte solution, e.g., a cathode electrolyte such
that a gas that is
introduced into one portion remains separate from another portion. For
example, such a
configuration may be used in any system as described herein as, e.g., in Figs.
7 and 8.
[0077] With reference to Fig.1, the system in some embodiments includes a
cathode
electrolyte circulating system 142 adapted for withdrawing and circulating
cathode electrolyte in
the system. In one embodiment, the cathode electrolyte circulating system
comprises a carbon
dioxide gas/liquid contactor 128 that is adapted for dissolving carbon dioxide
in the circulating
cathode electrolyte, and for circulating the electrolyte in the system. As can
be appreciated,
since the pH of the cathode electrolyte can be adjusted by withdrawing and/or
circulating
cathode electrolyte from the system, the pH of the cathode electrolyte
compartment can be by
regulated by regulating an amount of cathode electrolyte removed from the
system through the
carbon dioxide gas/liquid contactor 128.
[0078] In an alternative as illustrated in Fig. 4, the system comprises a
cathode 106 in
contact with a cathode electrolyte 108 and an anode 102 in contact with an
anode electrolyte
104. In this system, the cathode electrolyte comprises a salt solution that
functions as the
cathode electrolyte as well as a source of chloride and sodium ions for the
alkaline and acid
solution produced in the system. In this system, the cathode electrolyte is
separated from the
anode electrolyte by an anion exchange membrane 120 that allows migration of
anions, e.g.,
chloride ions, from the salt solution to the anode electrolyte. As is
illustrated in Fig. 4, the system
includes a hydrogen gas delivery system 112 configured to provide hydrogen gas
to the anode.
The hydrogen may be obtained from the cathode and/or obtained from an external
source, e.g.,
a commercial hydrogen gas supplier e.g., at start-up of operations when the
hydrogen supply
from the cathode is insufficient. In some embodiments, the hydrogen delivery
system is
configured to deliver gas to the anode where oxidation of the gas is catalyzed
to protons and
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electrons. In some embodiments, un-reacted hydrogen gas in the system is
recovered and re-
circulated to the anode.
[0079] Referring to Fig. 4, as with the system of Fig. 1, on applying a
voltage across the
anode and cathode, protons produced at the anode from oxidation of hydrogen
will enter into the
anode electrolyte from where they will attempt to migrate to the cathode
electrolyte across the
anion exchange membrane 120. However, since the anion exchange membranel20
will block
the passage of cations, the protons will accumulate in the anode electrolyte.
At the same time,
however, the anion exchange membrane 120 being pervious to anions will allow
the migration of
anions, e.g., chloride ions from the cathode electrolyte to the anode, thus in
this embodiment,
chloride ions will migrate to the anode electrolyte to produce hydrochloric
acid in the anode
electrolyte. In this system, the voltage across the anode and cathode is
adjusted to a level such
that hydroxide ions and hydrogen gas are produced at the cathode without
producing a gas,
e.g., chlorine or oxygen, at the anode. In this system, since cations will not
migrate from the
cathode electrolyte across the anion exchange membrane 116, sodium ions will
accumulate in
the cathode electrolyte 108 to produce an alkaline solution with hydroxide
ions produced at the
cathode. In embodiments where carbon dioxide gas is dissolved in the cathode
electrolyte,
sodium ions may also produce sodium bicarbonate and/or sodium carbonate in the
cathode
electrolyte as described herein with reference to Fig.1.
[0080] With reference to Fig. 1, depending on the pH of the cathode
electrolyte, carbon
dioxide gas introduced into the first cathode electrolyte portion 108A will
dissolve in the cathode
electrolyte and reversibly dissociate and equilibrate to produce carbonic
acid, protons,
carbonate and/or bicarbonate ions in the first cathode electrolyte compartment
as follows:
CO2 + H2O <__> H2CO3 <==> H+ + HCO3 <==> H++ CO32-
In the system, as cathode electrolyte in the first cathode electrolyte portion
108A may mix with
second cathode electrolyte portion 1086, the carbonic acid, bicarbonate and
carbonate ions
formed in the first cathode electrolyte portion 108A by absorption of carbon
dioxide in the
cathode electrolyte may migrate and equilibrate with cathode electrolyte in
the second cathode
electrolyte portion 108B. Thus, in some embodiments, first cathode electrolyte
portion 108A may
comprise dissolved and un-dissolved carbon dioxide gas, and/or carbonic acid,
and/ or
bicarbonate ions and/or carbonate ions; while second cathode electrolyte
portion 108B may
comprise dissolved carbon dioxide, and/or carbonic acid, and/ or bicarbonate
ions and/or
carbonate ions.
[00811 With reference to Fig. 1, on applying a voltage across anode 102 and
cathode 108,
the system 100 may produce hydroxide ions and hydrogen gas at the cathode from
water, as
follows:
2H20 +2e = H2+20H-
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As cathode electrolyte in first cathode electrolyte portion 108A can intermix
with cathode
electolyte in second cathode electrolyte portion 108B, hydroxide ions formed
in the second
cathode electrolyte portion may migrate and equilibrate with carbonate and
bicarbonate ions in
the first cathode electrolyte portion 108A. Thus, in some embodiments, the
cathode electrolyte
in the system may comprise hydroxide ions and dissolved and/or un-dissolved
carbon dioxide
gas, and/or carbonic acid, and/ or bicarbonate ions and/or carbonate ions. In
the system, as the
solubility of carbon dioxide and the concentration of bicarbonate and
carbonate ions in the
cathode electrolyte are dependent on the pH of the electrolyte, the overall
reaction in the
cathode electrolyte 104 is either:
Scenario 1: 2H20 + 2CO2 + 2e" = H2 + 2HC03 ; or
Scenario 2: H2O + CO2 + 2e- = H2 + C032-
or a combination of both, depending on the pH of the cathode electrolyte. This
is illustrated in as
a arbonate speciation diagram in Fig. 5.
[0082] For either scenario, the overall cell potential of the system can be
determined
through the Gibbs energy change of the reaction by the formula:
Ecell = -AG/nF
Or, at standard temperature and pressure conditions:
E Ce,l = -AG /nF
where, ECell is the cell voltage, AG is the Gibbs energy of reaction, n is the
number of electrons
transferred, and F is the Faraday constant (96485 JNmol). The ECe11 of each of
these reactions is
pH dependent based on the Nernst equestion as illustrated in Fig.6.
[0083] Also, for either scenario, the overall cell potential can be determined
through the
combination of Nernst equations for each half cell reaction:
E=E -RTIn(Q)/nF
where, E is the standard reduction potential, R is the universal gas
constant, (8.314 J/mol K) T
is the absolute temperature, n is the number of electrons involved in the half
cell reaction, F is
Faraday's constant (96485 JN mol), and Q is the reaction quotient such that:
Etotal = Ecathode + Eanode=
When hydrogen is oxidized to protons at the anode as follows:
H2=2H++2e,
E is 0.00 V, n is 2, and Q is the square of the activity of H+ so that:
Eanode = +0.059 pHa,
where pHa is the pH of the anode electrolyte.
When water is reduced to hydroxide ions and hydrogen gas at the cathode as
follows:
2H20 + 2e = H2 + 20H-,
E is -0.83 V, n is 2, and Q is the square of the activity of OH' so that:
Ecathode = -0.059 pHc,
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where pH, is the pH of the cathode electrolyte.
[0084] For either Scenario, the E for the cathode and anode reactions varies
with the pH of
the anode and cathode electrolytes. Thus, for Scenario 1 if the anode
reaction, which is
occurring in an acidic environment, is at a pH of 0, then the E of the
reaction is OV for the half
cell reaction. For the cathode reaction, if the generation of bicarbonate ions
occur at a pH of 7,
then the theoretical E is 7 x (-0.059 V) = -0.413V for the half cell reaction
where a negative E
means energy is needed to be input into the half cell or full cell for the
reaction to proceed. Thus,
if the anode pH is 0 and the cathode pH is 7 then the overall cell potential
would be -0.413V,
where:
Et0 1= -0.059 (pH, - pH,) = -0.059 ApH.
[0085] For Scenario 2 in which carbonate ions are produced, if the anode pH is
0 and the
cathode pH is 10, this would represent an E of 0.59 V.
[0086] Thus, in some embodiments, directing CO2 gas into the cathode
electrolyte may
lower the pH of the cathode electrolyte by producing bicarbonate ions and/or
carbonate ions in
the cathode electrolyte, which consequently may lower the voltage across the
anode and
cathode in producing hydroxide, carbonate and/or bicarbonate in the cathode
electrolyte.
[0087] Thus, if the cathode electrolyte is allowed to increase to a pH of 14
or greater, the
difference between the anode half-cell potential (represented as the thin
dashed horizontal line,
Scenario 1, above) and the cathode half cell potential (represented as the
thick solid sloping line
in Scenario 1, above) will increase to 0.83V. With increased duration of cell
operation without
CO2 addition or other intervention, e.g., diluting with water, the required
cell potential will
continue to increase. The cell potential may also increase due to ohmic
resistance loses across
the membranes in the electrolyte and the cell's overvoltage potential.
[0088] Herein, an overvoltage potential refers to the voltage difference
between a
thermodynamically determined half-cell reduction potential, and the
experimentally observed
potential at which the redox reaction occurs. The term is related to a cell
voltage efficiency as
the overvoltage potential requires more energy than is thermodynamically
required to drive a
reaction. In each case, the extra energy is lost as heat. Overvoltage
potential is specific to each
cell design and will vary between cells and operational conditions even for
the same reaction.
[0089] In embodiments wherein it is desired to produce bicarbonate and/or
carbonate ions in
the cathode electrolyte, the system as illustrated in Figs. 1-2, and as
described above with
reference to production of hydroxide ions in the cathode electrolyte, can be
configured to
produce bicarbonate ions and/or carbonate ions in the first cathode
electrolyte by dissolving
carbon dioxide in the first cathode electrolyte and applying a voltage of less
than 3V, or less
than 2.5 V, or less than 2V, or less than 1.5V such as less than 1.OV, or even
less than 0.8 V or
0.6V across the cathode and anode.
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[0090] In some embodiments, hydroxide ions and/or carbonate ions and/or
bicarbonate ions
produced in the cathode electrolyte, and hydrochloric acid produced in the
anode electrolyte are
removed from the system, while sodium chloride in the salt solution
electrolyte is replenished to
maintain continuous operation of the system. In some embodiments, the system
can be
configured to operate in various production modes including batch mode, semi-
batch mode,
continuous flow mode, with or without the option to withdraw portions of the
hydroxide solution
produced in the cathode electrolyte, or withdraw all or a portions of the acid
produced in the
anode electrolyte, or direct the hydrogen gas produced at the cathode to the
anode where it may
be oxidized.
[00911 In some embodiments, hydroxide ions and/or bicarbonate ions and/or
carbonate ion
solutions are produced in the cathode electrolyte when the voltage applied
across the anode
and cathode is less than 3V, 2.9V or less, 2.8V or less, 2.7V or less, 2.6V or
less, 2.5V or less,
2.4V or less, 2.3V or less, 2.2V or less, 2.1V or less, 2.OV or less, 1.9V or
less, 1.8V or less,
1.7V or less, 1.6V, or less 1.5V or less, 1.4V or less, 1.3V or less, 1.2V or
less, 1.1V or less,
1.OV or less, 0.9V or less or less, 0.8V or less, 0.7V or less, 0.6V or less,
0.5V or less, 0.4V or
less, 0.3V or less, 0.2V or less, or 0.1 V or less.
[0092] In another embodiment, the voltage across the anode and cathode can be
adjusted
such that gas will form at the anode, e.g., oxygen or chlorine, while
hydroxide ions, carbonate
ions and bicarbonate ions are produced in the cathode electrolyte and hydrogen
gas is
generated at the cathode. However, in this embodiment, hydrogen gas is not
supplied to the
anode. As can be appreciated by one ordinarily skilled in the art, in this
embodiment, the voltage
across the anode and cathode will be generally higher compared to the
embodiment when a gas
does not form at the anode.
[0093] With reference to Figs.1 - 2, in some embodiments, the invention
provides for a
system comprising one or more anion exchange membrane 120, and cation exchange
membranes 116, 122 located between the gas diffusion anode 102 and the cathode
106. In
some embodiments, the membranes should be selected such that they can function
in an acidic
and/or basic electrolytic solution as appropriate. Other desirable
characteristics of the
membranes include high ion selectivity, low ionic resistance, high burst
strength, and high
stability in an acidic electrolytic solution in a temperature range of 0 C to
100 C or higher, or a
alkaline solution in similar temperature range may be used. In some
embodiments, a membrane
that is stable in the range of 0 C to 80 C, or 0 C to 90 C, but not stable
above these ranges
may be used. For other embodiments, it may be useful to utilize an ion-
specific ion exchange
membranes that allows migration of one type of cation but not another; or
migration of one type
of anion and not another, to achieve a desired product or products in an
electrolyte. In some
embodiments, the membrane should be stable and functional for a desirable
length of time in
the system, e.g., several days, weeks or months or years at temperatures in
the range of 0 C to
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80 C, or 0 C to 90 C and higher and/or lower. In some embodiments, for
example, the
membranes should be stable and functional for at least 5 days, 10 days, 15
days, 20 days, 100
days, 1000 days or more in electrolyte temperatures at 80 C, 70 C, 60 C, 50
C, 40 C, 30 C,
20 C, 10 C, 5 C and more or less.
[0094] The ohmic resistance of the membranes will affect the voltage drop
across the anode
and cathode, e.g., as the ohmic resistance of the membranes increase, the
voltage drop across
the anode and cathode will increase, and vice versa. Membranes currently
available can be
used and they include membranes with relatively low ohmic resistance and
relatively high ionic
mobility; similarly, membranes currently available with relatively high
hydration characteristics
that increase with temperatures, and thus decreasing the ohmic resistance can
be used.
Consequently, as can be appreciated, by selecting currently available
membranes with lower
ohmic resistance, the voltage drop across the anode and cathode at a specified
temperature
can be lowered.
[0095] Scattered through currently available membrane are ionic channels
consisting of acid
groups. These ionic channels may extend from the internal surface of the
matrix to the external
surface and the acid groups may readily bind water in a reversible reaction as
water-of-
hydration. This binding of water as water-of-hydration follows first order
reaction kinetics, such
that the rate of reaction is proportional to temperature. Consequently,
currently available
membranes can be selected to provide a relatively low ohmic and ionic
resistance while
providing for improved strength and resistance in the system for a range of
operating
temperatures. Suitable membranes are commercially available from Asahi Kasei
of Tokyo,
Japan; or from Membrane International of Glen Rock, NJ, and USA.
[0096] In some embodiments, the cathode electrolyte 108, 108A, 108B is
operatively
connected to a waste gas treatment system (not illustrated) where the alkaline
solution
produced in the cathode electrolyte is utilized, e.g., to sequester carbon
dioxide contained in the
waste gas by contacting the waste gas and the cathode electrolyte with a
solution of divalent
cations to precipitate hydroxides and/or carbonates and/or bicarbonates as
described in
commonly assigned U.S. Patent Application no. 12/344,019 filed on December 24,
2008, herein
incorporated by reference in its entirety. The precipitates, comprising, e.g.,
calcium and
magnesium hydroxides, carbonates and bicarbonates in some embodiments may be
utilized as
building materials, e.g., as cements and aggregates, as described in commonly
assigned U.S.
Patent Application no. 12/126,776 filed on May 23, 2008, supra, herein
incorporated by
reference in its entirety. In some embodiments, some or all of the carbonates
and/or
bicarbonates are allowed to remain in an aqueous medium, e.g., a slurry or a
suspension, and
are disposed of in an aqueous medium, e.g., in the ocean depths or a
subterranean site.
[0097] In some embodiments, the cathode and anode are also operatively
connected to an
off-peak electrical power-supply system 114 that supplies off-peak voltage to
the electrodes.
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Since the cost of off-peak power is lower than the cost of power supplied
during peak power-
supply times, the system can utilize off-peak power to produce an alkaline
solution in the
cathode electrolyte at a relatively lower cost.
[0098] In another embodiment, the system produces an acid, e.g., hydrochloric
acid 124 in
the anode electrolyte 104. In some embodiments, the anode compartment is
operably
connected to a system for dissolving minerals and/or waste materials
comprising divalent
cations to produce a solution of divalent cations, e.g., Ca++ and Mg++. In
some embodiments,
the divalent cation solution is utilized to precipitate hydroxides, carbonates
and/or bicarbonates
by contacting the divalent cation solution with the present alkaline solution
and a source of
carbon dioxide gas as described in U.S. Patent Application no. 12/344,019
filed on December
24, 2008, supra, herein incorporated by reference in its entirety. In some
embodiments, the
precipitates are used as building materials e.g., cement and aggregates as
described in
commonly assigned U.S. Patent application no. 12/126,776, supra, herein
incorporated by
reference in its entirety.
[0099] With reference to Fig. 1, on applying a voltage across the anode 102
and cathode
106, protons will form at the anode from oxidation of hydrogen gas supplied to
the anode, while
hydroxide ions and hydrogen gas will form at the cathode electrolyte from the
reduction of water,
as follows:
H2 = 2H` + 2e" (anode, oxidation reaction)
2H20 + 2e- = H2 + 201-1- (cathode, reduction reaction)
[00100] Since protons are formed at the anode from hydrogen gas provided to
the anode;
and since a gas such as oxygen does not form at the anode; and since water in
the cathode
electrolyte forms hydroxide ions and hydrogen gas at the cathode, the system
will produce
hydroxide ions in the cathode electrolyte and protons in the anode electrolyte
when a voltage is
applied across the anode and cathode. Further, as can be appreciated, in the
present system
since a gas does not form at the anode, the system will produce hydroxide ions
in the cathode
electrolyte and hydrogen gas at the cathode and hydrogen ions at the anode
when less than 2V
is applied across the anode and cathode, in contrast to the higher voltage
that is required when
a gas is generated at the anode, e.g., chlorine or oxygen. For example, in
some embodiments,
hydroxide ions are produced when less than 2.OV, 1.5V, 1.4V, 1.3V, 1.2V, 1.1V,
1.OV, 0.9V,
0.8V, 0.7V, 0.6V, 0.5V, 0.4V, 0.3V, 0.2V, 0.1 V or less is applied across the
anode and cathode.
[00101] In the system, on applying a voltage across the anode 102 and cathode
106, the
positively charged protons formed at the anode will attempt to migrate to the
cathode through
the anode electrolyte 104, while the negatively charged hydroxide ions formed
at the cathode
will attempt to migrate to the anode through the cathode electrolyte 108,
108A, 108B. As is
illustrated in Fig. 1 and with reference to hydroxide ions in the cathode
electrolyte 108, 108A,
1088, since the first cation exchange membrane 116 will restrict the migration
of anions from the
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cathode electrolyte 108, 108A, 108B, and since the anion exchange membrane 120
will prevent
the migration of anions from the anode electrolyte 104 to the salt solution
118, the hydroxide
ions generated in the cathode electrolyte will be prevented from migrating out
of the cathode
electrolyte through the cation exchange membrane. Consequently, on applying
the voltage
across the anode and cathode, the hydroxide ions produced at the cathode will
be contained in
the cathode electrolyte. Thus, depending on the flow rate of fluids into and
out of the cathode
electrolyte and the rate of carbon dioxide dissolution in the cathode
electrolyte, the pH of the
cathode electrolyte will adjust, e.g., the pH may increase, decrease or remain
the same.
[00102] In some embodiments, depending on the ionic species desired in cathode
electroyte
108, 108A, 108B and/or the anode electolyte 104 and/or the salt solution 118,
alternative
reactants can be utilized. Thus, for example, if a potassium salt such as
potassium hydroxide or
potassium carbonate is desired in the cathode elelctolyte 1108, 108A, 108B,
then a potassium
salt such as potassium chloride can be utilized in the salt solution 118.
Similarly, if sulfuric acid
is desired in the anode electrolyte, then a sulfate such as sodium sulfate can
be utilized in the
salt solution 118. As described in some embodiments herein, carbon dioxide gas
is absorbed in
the cathode electrolyte; however, it will be appreciated that other gases,
including volatile
vapors, can be absorbed in the electrolyte, e.g., sulfur dioxide, or organic
vapors to produce a
desired result. The gas can be added to the electrolyte in various ways, e.g.,
by bubbling it
directly into the electrolyte, or dissolving the gas in a separate compartment
connected to the
cathode compartment and then directed to the cathode electrolyte as described
herein.
[00103] With reference to Figs 1 and 3, method 300 in some embodiments
comprises a step
302 of applying a voltage across a cathode 106 and a gas diffusion anode 102
in an
electrochemical system 100, wherein the cathode contacts a cathode electrolyte
comprising
dissolved carbon dioxide. In some embodiments, the method includes a step of
providing
hydrogen to the gas diffusion anode 102; a step of contacting the cathode 106
with a cathode
electrolyte 108, 108A, 108B comprising dissolved carbon dioxide gas 107A; and
a step of
applying a voltage 114 across the anode and cathode; a step whereby protons
are produced at
the anode and hydroxide ions and hydrogen gas produced at the cathode; a step
whereby a gas
is not produced at the anode when the voltage is applied across the anode and
cathode; a step
wherein the voltage applied across the anode and cathode is less than 2V; a
step comprising
directing hydrogen gas from the cathode to the anode; a step comprising
whereby protons are
migrated from the anode to an anode electrolyte; a step comprising interposing
an anion
exchange membrane between the anode electrolyte and the salt solution; a step
comprising
interposing a first cation exchange membrane between the cathode electrolyte
and the salt
solution, wherein the salt solution is contained between the anion exchange
membrane and the
first cation exchange membrane; a step comprising whereby anions migrate from
the salt
solution to the anode electrolyte through the anion exchange membrane, and
cations migrate
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from the salt solution to the cathode electrolyte through the first cation
exchange membrane; a
step comprising producing hydroxide ions and/or carbonate ions and/or
bicarbonate ions in the
cathode electrolyte; a step comprising producing an acid in the anode
electrolyte; a step
comprising producing sodium hydroxide and/or sodium carbonate and/or sodium
bicarbonate in
the cathode electrolyte; a step whereby hydrochloric acid is produced in the
anode electrolyte; a
step comprising contacting the cathode electrolyte with a divalent cation
solution, wherein the
divalent cations comprise calcium and magnesium ions; a step comprising
producing partially
desalinated water from the salt solution; a step comprising withdrawing a
first portion of the
cathode electrolyte and contacting the first portion of cathode electrolyte
with carbon dioxide;
and a step comprising contacting the first portion of cathode electrolyte with
a divalent cation
solution.
[00104] In some embodiments, hydroxide ions are formed at the cathode 106 and
in the
cathode electrolyte 108, 108A, 108B by applying a voltage of less than 2V
across the anode and
cathode without forming a gas at the anode, while providing hydrogen gas at
the anode for
oxidation at the anode. In some embodiments, method 300 does not form a gas at
the anode
when the voltage applied across the anode and cathode is less than 3V or less,
2.9V or less,
2.8V or less, 2.7V or less, 2.6V or less, 2.5V or less, 2.4V or less, 2.3V or
less, 2.2V or less,
2.1V or less, 2.OV or less, 1.9V or less, 1.8V or less, 1.7V or less, 1.6V or
less, 1.5V or less,
1.4V or less, 1.3V or less, 1.2V or less, 1.1 V or less, 1.OV or less, 0.9V or
less, 0.8V or less,
0.7V or less, 0.6V or less, 0.5V or less, 0.4V or less, 0.3V or less, 0.2V or
less, or 0.1 V or less,
while hydrogen gas is provided to the anode where it is oxidized to protons.
As will be
appreciated by one ordinarily skilled in the art, by not forming a gas at the
anode and by
providing hydrogen gas to the anode for oxidation at the anode, and by
otherwise controlling the
resistance in the system for example by decreasing the electrolyte path
lengths and by selecting
ionic membranes with low resistance and any other method know in the art,
hydroxide ions can
be produced in the cathode electrolyte with the present lower voltages.
[00105] In some embodiments, hydroxide ions, bicarbonate ions and carbonate
ions are
produced in the cathode electrolyte where the voltage applied across the anode
and cathode is
less than 3.OV, 2.9V, 2.8V, 2.7V, 2.6V, 2.5V, 2.4V, 2.3V, 2.2V, 2.1V, 2.OV,
1.9V, 1.8V, 1.7V,
1.6V, 1.5V, 1.4V, 1.3V, 1.2V, 1.1 V, 1.OV, 0.9V, 0.8V, 0.7V, 0.6V, 0.5V, 0.4V,
0.3V, 0.2V, 0.1 V or
less without forming a gas at the anode. In some embodiments, the method is
adapted to
withdraw and replenish at least a portion of the cathode electrolyte and the
acid in the anode
electrolyte back into the system in either a batch, semi-batch or continuous
mode of operation.
[00106] With reference to Fig. 7, in one embodiment, the system 700 comprises
a cathode
106 in contact with a cathode electrolyte 108 comprising added carbon dioxide
107, wherein the
cathode electrolyte is separated from an anode electrolyte 104 by first cation
exchange
membrane 116. In an embodiment as is illustrated in Fig. 8, the system 800
comprises an anode
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102 that is separated from the anode electrolyte by a second cation exchange
membrane 122
that is in contact with the anode 102.
[00107] In systems as illustrated in Figs. 1 - 9, the first cation exchange
membrane 116 is
located between the cathode 106 and anode 102 such it separates the cathode
electrolyte 108
from the anode electrolyte 104. Thus as is illustrated in Figs. 7 and 8, on
applying a relatively
low voltage, e.g., less than 2V or less than 1V, across the anode 102 and
cathode 106,
hydroxide ions (OH-) and hydrogen gas (H2) are produced at the cathode 106,
and hydrogen
gas is oxidized at the anode 102 to produce hydrogen ions at the anode 102,
without producing
a gas at the anode. In certain embodiments, the hydrogen gas produced at the
cathode is
directed to the anode through a hydrogen gas delivery system 112, and is
oxidized to hydrogen
ions at the anode. In various embodiments, utilizing hydrogen gas at the anode
from hydrogen
generated at the cathode, eliminates the need for an external supply of
hydrogen; consequently,
the utilization of energy by the system to produce the alkaline solution is
reduced.
[00108] In certain embodiments as illustrated in Figs. 1-9, under the applied
voltage 114
across the anode 102 and cathode 106, hydroxide ions are produced at the
cathode 106 and
migrate into the cathode electrolyte 108, and hydrogen gas is produced at the
cathode. In
certain embodiments, the hydrogen gas produced at the cathode 106 is collected
and directed
to the anode, e.g., by a hydrogen gas delivery system 122, where it is
oxidized to produce
hydrogen ions at the anode. Also, as illustrated in Figs. 7 -9, under the
applied voltage 114
across the anode 102 and cathode 106, hydrogen ions produced at the anode 102
migrate from
the anode 102 into the anode electrolyte 104 to produce an acid, e.g.,
hydrochloric acid.
[00109] In certain embodiments, the first cation exchange membrane 116 is
selected to allow
passage of cations therethrough while restricting passage of anions
therethrough. Thus, as is
illustrated in Figs. 1-9, on applying the low voltage across the anode 102 and
cathode 106,
cations in the anode electrolyte 104 , e.g., sodium ions in the anode
electrolyte comprising
sodium chloride, migrate into the cathode electrolyte through the first cation
exchange
membrane 116, while anions in the cathode electrolyte 108, e.g., hydroxide
ions, and/or
carbonate ions, and/or bicarbonate ions, are prevented from migrating from the
cathode
electrolyte through the first cation exchange membrane 116 and into the anode
electrolyte 104.
[00110] As is illustrated in Figs. 1-9, where the anode electrolyte 104
comprises an aqueous
salt solution such as sodium chloride in water, a solution, e.g., and alkaline
solution, is produced
in the cathode electrolyte 108 comprising cations, e.g., sodium ions, that
migrate from the anode
electrolyte 104, and anions, e.g., hydroxide ions produced at the cathode 106,
and/or carbonate
ions and or bicarbonate ions produced by dissolving carbon dioxide 107 in the
cathode
electrolyte.
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[001111 Concurrently, in the anode electrolyte 104, an acid, e.g.,
hydrochloric acid is
produced from hydrogen ions migrating from the anode 102 and anions, e.g.,
chloride ions,
present from the anode electrolyte.
[00112] As is illustrated in Fig. 8, an anode comprising a second cation
exchange membrane
122 is utilized to separate the anode 102 from the anode electrolyte 104 such
that on a first
surface, the cation exchange membrane 122 is in contact with the anode 102,
and an opposed
second surface it is in contact with the anode electrode electrolyte 104.
Thus, since the second
cation exchange membrane is permeable to cations, e.g., hydrogen ions, the
anode is in
electrical contact with the anode electrolyte through the second cation
exchange membrane. In
some embodiments, the anode as illustrated in Fig. 8 may comprise a gas
diffusion anode as
described below.
[00113] Thus, in the embodiment of Fig. 8, as with the embodiment illustrated
in Fig.7, on
applying the low voltage across the anode 102 and cathode 106, hydrogen ions
produced at the
anode 102 from oxidation of hydrogen gas at the anode migrate through the
second cation
exchange membrane 122 into the anode electrolyte 104. At the same time,
cations in the anode
electrolyte, e.g., sodium ions in the anode electrolyte comprising sodium
chloride, migrate from
the anode electrolyte 104 into the cathode electrolyte 108 through the first
cation exchange
membrane 116, while anions in the cathode electrolyte 108, e.g., hydroxide
ions, and/or
carbonate ions, and/or bicarbonate ions, are prevented from migrating from the
cathode
electrolyte 108 to the anode electrolyte 104 through the first cation exchange
membrane 116.
[00114] Also, in the embodiment as illustrated in Fig. 8, hydrogen ions
migrating from the
anode 102 through the second cation exchange membrane 122 into the anode
electrolyte 104
will produce an acid, e.g., hydrochloric acid with the cations, e.g., chloride
ions, present in the
anode electrolyte; and in the cathode electrolyte 108, an alkaline solution is
produce from
cations present in the cathode electrolyte and anions, e.g., sodium ions, that
migrate from the
anode to the cathode electrolyte through the first cation exchange membrane
116.
[00115] In some embodiments, cation exchange membranes 116 and 122 are
conventional
and are available from, for example, Asahi Kasei of Tokyo, Japan; or from
Membrane
International of Glen Rock, NJ, or DuPont, in the USA. However, it will be
appreciated that in
some embodiments, depending on the need to restrict or allow migration of a
specific cation or
an anion species between the electrolytes, a cation exchange membrane that is
more restrictive
and thus allows migration of one species of cations while restricting the
migration of another
species of cations may be used as, e.g., a cation exchange membrane that
allows migration of
sodium ions into the cathode electrolyte from the anode electrolyte while
restricting migration of
hydrogen ions from the anode electrolyte into the cathode electrolyte, may be
used. Such
restrictive cation exchange membranes are commercially available and can be
selected by one
ordinarily skilled in the art.
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[00116] As is illustrated in Fig. 8, the anode 102 comprises a second cation
exchange
membrane 112 that separates the anode 102 from the anode electrolyte 104 and
is attached to
the anode. Thus, in some embodiments, the anode and second cation exchange
membrane
may comprise an integral gas diffusion anode that is commercially available,
or can be
fabricated as described for example in co-pending and commonly assigned US
Provisional
Patent Application no. 61/151,484, titled "Electro-catalyst Electrodes for Low-
voltage
electrochemical Hydroxide System", filed February 10, 2009, herein fully
incorporated by
reference. However, notwithstanding that a gas diffusion anode is illustrated
and utilized in Figs.
7 and 8 and described herein, in the some embodiments, any conventional anode
that can be
configured to oxidize hydrogen gas to produce hydrogen ions as described
herein can be
utilized.
[00117] With reference to Figs. 1 - 17, in some embodiments, the cathode
electrolyte 108 is
operatively connected to a supply of carbon dioxide gas 107, contained, e.g.,
in an industrial
plant, e.g., a power generating plant, a cement plant, or an ore smelting
plant. If necessary, this
source of carbon dioxide comprises a gas wherein the concentration of carbon
dioxide is greater
than the concentration of carbon dioxide in the ambient atmosphere. This
source of carbon
dioxide may also contain other gaseous and non-gaseous components of a
combustion
process, e.g., nitrogen gas, SOx, NOx., as is described in co-pending and
commonly assigned
US Provisional Patent application no. 61/223,657, titled "Gas, Liquids, Solids
Contacting
Methods and Apparatus", filed July 7, 2009 herein fully incorporated by
reference. However, as
can be appreciated, this source of carbon dioxide can be cleaned and utilized
as the carbon
dioxide added to the cathode electrolyte 108.
[00118] Although carbon dioxide is present in ordinary ambient air, partly due
to the very low
concentration, ambient carbon dioxide may not provide sufficient carbon
dioxide to achieve the
results obtained with the present system and method that utilize carbon
dioxide taken from an
industrial waste gas steam, e.g., from the stack gases of a fossil fuelled
power generating plant
or a cement production plant. Also, in some embodiments of the system and
method, since the
cathode electrolyte is contained in closed system wherein the pressure of the
added carbon
dioxide gas within the system is greater than the ambient atmospheric
pressure, ambient air and
hence ambient carbon dioxide is typically prevented from infiltrating into the
cathode electrolyte.
[00119] In some embodiments, and with reference to Figs. 1- 17, carbon dioxide
is added to
the cathode electrolyte to dissolve and produce carbonic acid that dissociates
to hydrogen ions
and carbonate ions and/or bicarbonate ions, depending on the pH of the cathode
electrolyte.
Concurrently, as described above, hydroxide ions, produced from electrolyzing
water in the
cathode electrolyte, may react with the hydrogen ions to produce water in the
cathode
electrolyte. Thus, depending on the degree of alkalinity desired in the
cathode electrolyte, the
pH of the cathode electrolyte may be adjusted and in some embodiments is
maintained between
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and 7 and 14 or greater; or between 7 and 9; or between 8 and 11 as is well
understood in the
art, and as illustrated in carbonate speciation diagram of Fig. 5. In some
embodiments, the pH
of the cathode electrolyte may be adjusted to any value between 7 and 14 or
greater, including a
pH 7.0, 7.5, 8.0, 8.5. 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0,
13.5, 14.0 and greater.
[00120] Similarly, in some embodiments of the system, the pH of the anode
electrolyte is
adjusted and is maintained between less than 0 and up to 7 and/or between less
than 0 and up
to 4, by regulating the concentration of hydrogen ions that migrate into the
anode electrolyte
from oxidation of hydrogen gas at the anode, and/or the withdrawal and
replenishment of anode
electrolyte in the system. In this regard and as can be appreciated by one
ordinarily skilled in the
art and with reference to Fig. 6, since the voltage across the anode and
cathode is dependent
on several factors including the difference in pH between the anode
electrolyte and the cathode
electrolyte as can be determined by the Nerst equation, in some embodiments,
the pH of the
anode electrolyte is adjusted to a value between 0 and 7, including 0, 0.5,
1.0, 1.5, 2.0, 2.5, 3.0,
3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7, depending on the desired operating
voltage across the
anode and cathode. Thus, as can be appreciated, in equivalent systems, where
it is desired to
reduce the energy used and/ or the voltage across the anode and cathode, e.g.,
as in the
Chloralkali process, carbon dioxide can be added to the electrolyte as
disclosed herein to
achieve a desired pH difference between the anode electrolyte and cathode
electrolyte. Thus, to
the extent that such systems utilize carbon dioxide, these equivalent systems
are within the
scope of the present invention.
[00121] With reference to Fig. 1- 17, in some embodiments, the anode
electrolyte 102
comprises a salt solution that includes sodium ions and chloride ions; the
system is configured
to produce the alkaline solution in the cathode electrolyte 108 while also
producing hydrogen
ions at the anode 106, with less than 1V across the anode 122 and cathode 106,
without
producing a gas at the anode; the system is configured to migrate hydrogen
ions from the anode
into the anode electrolyte; the anode electrolyte comprises an acid; the
system is configured to
produce bicarbonate ions and/or carbonate ions in the cathode electrolyte 108;
migrate
hydroxide ions from the cathode 106 into the cathode electrolyte; migrate
cations, e.g., sodium
ions, from the anode electrolyte 104 into the cathode electrolyte through the
first cation
exchange membrane 116; hydrogen gas is provided to the anode; and a hydrogen
gas delivery
system 112 is configured to direct hydrogen gas from the cathode to the anode.
[00122] With reference to Figs. 1-17, in some embodiments the cathode
electrolyte 108 may
be operatively connected to a system for further processing of the cathode
electrolyte, e.g., a
carbonate and/or bicarbonate precipitating system comprising a precipitator
configured to
precipitate carbonates and/or bicarbonates from a solution, wherein in some
embodiments the
carbonates and/or bicarbonates comprise calcium and/or magnesium carbonate
and/or
bicarbonate. Also, in some embodiments, the anode electrolyte 104 comprising
an acid, e.g.,
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hydrochloric acid, and a depleted salt solution comprising low amount sodium
ions, is
operatively connected to a system for further processing of an acid, e.g., a
mineral dissolution
system that is configured to dissolve minerals and produce a mineral solution
comprising
calcium ions and/or magnesium ions, e.g., mafic minerals such as olivine and
serpentine. In
some embodiments, the acid may used for other purposes in addition to or
instead of mineral
dissolution. Such use includes use as a reactant in production of cellulosic
biofules, use the
production of polyvinyl chloride (PVC), and the like. System appropriate to
such uses may be
operatively connected to the electrochemistry unit, or the acid may be
transported to the
appropriate site for use.
[00123] In the some embodiments, the mineral dissolution system is operatively
connected to
nano-filtration system that is configured to separate sodium ions and chloride
ions from the
mineral solution comprising, e.g., calcium ions, magnesium ions, silica,
hydrochloric acid and/or
sodium hydroxide. In some embodiments, the nano-filtration system 910 is
configured with a
reverse osmosis system that is capable of concentrating sodium ions and
chloride ions into a
salt solution that is used as the anode electrolyte 104 .
[00124] With reference to Figs. 1 - 17, the method in some embodiments
comprises a step of
adding carbon dioxide into a cathode electrolyte 108 in contact with a cathode
106 wherein the
cathode electrolyte is separated from an anode electrolyte 104 by a first
cation exchange
membrane 116; and producing an alkaline solution in the cathode electrolyte by
applying a
voltage 114 of less that 1V across the cathode 106 and an anode 102 in contact
with the anode
electrolyte without producing a gas at the anode.
[00125] In some embodiments of the method, the anode 102 is in contact with a
second
cation exchange membrane 122 that separates the anode from the anode
electrolyte; the
alkaline solution 108 comprises hydroxide ions and/or bicarbonate ions and/or
carbonate ions;
the carbon dioxide 107 is contained in wastes gases of an industrial plant,
e.g., an electrical
power generating plant, a cement production plant, a fermentation process or
an ore processing
facility.
[00126] In some embodiments, ambient air is excluded the cathode electrolyte
108; a pH of
between and 7 and 14 or greater is maintained in the cathode electrolyte; a pH
of between 7
and 9 is maintained in the cathode electrolyte; a pH of between 8 and 11 is
maintained in the
cathode electrolyte; a pH of from less than 0 and up to 7 is maintained in the
anode electrolyte;
a pH of from less than 0 and up to 4 is maintained in the anode electrolyte;
hydrogen gas is
oxidized at the anode 102 to produce hydrogen ions and hydrogen ions are
migrated from the
anode through the second cation exchange membrane 122 into the anode
electrolyte; hydroxide
ions and hydrogen gas are produced at the cathode 106; hydroxide ions are
migrated from the
cathode 106 into the cathode electrolyte 108; hydrogen gas is directed from
the cathode 106 to
the anode 102; cations ions are migrated from the anode electrolyte 104
through the first cation
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exchange membrane 122 into the cathode electrolyte 108 wherein the cations
comprise sodium
ions.
[00127] In some embodiments, the method comprises producing sodium hydroxide
and/or
sodium carbonate ions and/or sodium bicarbonate ions in the cathode
electrolyte 108; producing
an acid and a depleted salt solution in the anode electrolyte 104 comprising
sodium ions and
chloride ions; utilizing the anode electrolyte to dissolve minerals and
produce a mineral solution
comprising calcium ions and/or magnesium ions, wherein the minerals comprises
mafic
minerals; filtering the mineral solution to produce a filtrate comprising
sodium ions and chloride
ions; concentrating the filtrate to produce the salt solution, wherein the
concentrator comprises a
reverse osmosis system; utilizing the salt solution as the anode electrolyte
104; precipitating a
carbonate and/or bicarbonate with the cathode electrolyte; wherein the
carbonate and/or
bicarbonate comprises calcium and/or magnesium carbonate and/or bicarbonate.
In some
embodiments, the method includes disposing of the acid in an underground
storage site where
the acid can be stored in an un-reactive salt or rock formation without
environmental
acidification.
[00128] With reference to Figs. 1 - 17, the method in another embodiment
comprises a step
of producing an acid 124 in an electrochemical system, e.g., system,
comprising added carbon
dioxide 106A, 107 in the cathode electrolyte 108; and contacting a mineral
with the acid 124. In
some embodiment the method further producing the acid in the anode electrolyte
104, without
generating a gas at the anode 102, and oxidizing hydrogen gas 112 at the
anode, wherein the
acid comprises hydrochloric acid 124; and wherein the hydrogen gas 112 is
produced at the
cathode 106; producing an alkaline solution in the cathode electrolyte 108;
migrating sodium
ions into the cathode electrolyte; wherein the alkaline solution comprises
sodium hydroxide,
sodium bicarbonate and/or sodium carbonate; wherein the voltage is less than 2
V or less than
1V; wherein the anode electrolyte 104 is separated from the cathode
electrolyte 108 by first
cation exchange membrane 116; wherein the anode 102 comprises a second cation
exchange
membrane 122 in contact with the anode electrolyte 102; wherein the anode
electrolyte
comprises a salt, e.g., sodium chloride; dissolving a mineral with the acid
124 to produce a
mineral solution; producing calcium ions and/or magnesium ions; wherein the
mineral comprises
a mafic mineral, e.g., olivine or serpentine; filtering the mineral solution
to produce a filtrate
comprising sodium ions and chloride ions solution; concentrating the filtrate
to produce a salt
solution; utilizing the salt solution as the anode electrolyte 104;
precipitating a carbonate and/or
bicarbonate with the cathode electrolyte; wherein the carbonate and/or
bicarbonate comprises
calcium and/or magnesium carbonate and/or bicarbonate. In some embodiments,
the method
includes disposing of the acid in an underground storage site where the acid
can be stored in an
un-reactive salt or rock formation and hence does not an environmental
acidification.
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[00129] With reference to Figs. 1-17, the system and method in various
embodiments are
operable to produce an alkaline solution 108 and an acid 124 by a redox
reaction that utilizes a
salt solution 130 and water 902, wherein on applying a voltage, e.g., less
than 3V across the
anode and cathode, hydroxide ions and hydrogen gas are produce at the cathode,
and
hydrogen is oxidized at the anode without generating a gas at the anode. In
some embodiments,
carbon dioxide is added to the cathode electrolyte in contact with the
cathode.
[00130] In some embodiments, the alkaline solution is produced in the cathode
electrolyte
and comprises hydroxide ions (from the cathode) and/or bicarbonate ions and/or
carbonate ions
from added carbon dioxide. In some embodiments, the hydrogen generated at the
cathode is
recovered and oxidized at the anode. In various embodiments, the redox
reaction at the anode
and cathode are catalyzed by electrocatalysts.
[001311 With reference to Figs. 1-17, the system in one embodiment 900
comprises a
hydrogen-oxidizing anode 102 in communication with a cathode electrolyte 108
comprising
bicarbonate ions; and a hydrogen delivery system 112, configured to deliver
hydrogen gas to the
anode 102. In another embodiment, the system 900 comprises an electrochemical
unit 100,
400, 700, 800, 900 comprising an anode compartment comprising a hydrogen-
oxidizing anode
122, a cathode compartment comprising a cathode 106, and a hydrogen-delivery
system 112
configured to deliver hydrogen gas to the anode, wherein the unit is operable
connected a
carbon sequestration system (not shown) configured to sequester carbon dioxide
with the
cathode electrolyte 108.
[00132] In some embodiments of the system, hydrogen gas to the anode is
provided from
hydrogen gas generated at a cathode in contact with the cathode electrolyte;
the cathode
electrolyte comprises added carbon dioxide; the cathode electrolyte comprises
hydroxide ions
and/or bicarbonate ions and/or carbonate ions; the bicarbonate ions and/or
carbonate ions
and/or hydroxide ions, hydrogen gas at the cathode, and the protons at the
anode are produced
by a voltage applied across the anode and cathode, without producing a gas at
the anode; and
the voltage is less than 3V.
[00133] In some embodiment of the system, and with reference to Figs. 2, 10,
11 and 12, the
anode 102 and/or cathode 106 may comprises an electrocatalyst 136 selected
from platinum, a
single-crystal nickel, Raney nickel, platinized nickel, a metal carbide (W2C,
Pt-W2C), a platinum
group metal alloy (Pt-M, where M=Fe, Mn, Cr, Co, Au), a transition metal, a
nickel alloy, sintered
nickel, a platinum group metals (Pt, Pd, Ru, Rh), gold, silver, a precious or
non-precious
chalcogenides, a discrete macrocyclic complex of transition metals and
biological complexes. In
various configurations, the electrocatalyst is provided on a solid electrode,
or is provided on a
mesh/gauze, a fiber, or as p[porous particles. In some embodiments, the
electrocatalyst is
configured to catalyzed oxidation of hydrogen gas to protons at the anode, and
catalyze
production of hydrogen gas and hydroxide ions at the cathode.
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[00134] In some embodiments, the system comprises a carbon sequestration
system (not
shown) configured to sequester carbon dioxide with the cathode electrolyte; in
some
embodiments, the carbon dioxide is contained in an industrial waste gas; in
some embodiments,
the carbon dioxide is sequestered as carbonates and/or bicarbonates comprising
divalent
cations e.g., calcium ions and/or magnesium ions.
[00135] In some embodiments, the system comprises an acid 124 in the anode
electrolyte
104; in some embodiments, the system comprises an acid dissolution system
configured to
produce divalent cations e.g., calcium ions and magnesium ions with the acid;
in some system
the divalent cations are produced by dissolving a mineral with the acid, and
the divalent cations
are provided to the carbon sequestration system.
[00136] With reference Figs.13, in some embodiments, the system comprises a
plurality of
pairs of the anodes and cathodes 1302, 1304, 1306 configured to receive a
series current
through each pair of anode and cathode. In another embodiment, an with
reference to Fig. 14,
the system comprises plurality of pairs of anodes and cathodes 1402, 1404,
1406 configured to
receive a parallel voltage across each pair of anode and cathode. With
reference to Fig. 15, in
another embodiment, the system comprises a plurality of pairs of the anode
electrolyte and
cathode electrolyte wherein the cathode electrolyte of a first pair 1502 is
connected to the
cathode electrolyte of a second pair 1504; and the anode electrolyte of a
first pair 1502 is
connected to the anode electrolyte of a second pair 1502.
[00137] In some embodiments of the system 1500 as illustrated in Fig. 15, the
pH of the
anode electrolyte and cathode electrolyte can be adjusted to optimize the
energy used in
producing the alkaline solution. Thus, for example, the pH of the cathode
electrolyte 1504 of the
second pair (pH = 11) can be adjusted such that it is equal to or greater than
the pH of the
cathode electrolyte 1502 in the first pair (pH = 9); similarly, the pH of the
anode electrolyte 1504
of the second pair (pH = 3) can be adjusted to be equal to or less than the pH
of the anode
electrolyte 1502 of the first pair (pH = 5). As can be appreciated, these pH
adjustments are
exemplarary and thus other pH configurations can be established, e.g.,
adjusting the flow of
fluids in the system, e.g., adding carbon dioxide to the cathode electrolyte
and/or adding water
to the electrolyte, and/or removing acid and alkaline solution from the
system.
[00138] In some embodiments of the system as illustrated in Fig. 16, the
system 1600
comprises a plurality of pairs of the anode electrolytes and cathode
electrolytes 1602, 1604,
1606 wherein the cathode electrolyte and anode electrolyte of a second pair
1606 comprise
cathode electrolyte from a first pair 1602; and the cathode electrolyte and
anode electrolyte of a
third pair 1604 comprise anode electrolyte from the first pair 1602 such that
in the system the pH
of the cathode electrolyte of the second pair is equal to or greater than pH
of the cathode
electrolyte of the first pair; and the pH of the anode electrolyte of the
third pair is equal to or less
that pH of the anode electrolyte of the first pair. As with the system of Fig.
15, these pH
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adjustments are exemplarary and thus other pH configurations can be
established, e.g.,
adjusting the flow of fluids in the system, e.g., adding carbon dioxide to the
cathode electrolyte
and/or adding water to the electrolyte, and/or removing acid and alkaline
solution from the
system.
[00139] With reference to the system 1700 as illustrated in Fig. 17, the
system is configured
such that in some embodiments the cathode electrolyte of the second pair of
anodes and
cathodes 1706 comprises diluted cathode electrolyte of the first pair 1702;
and the anode
electrolyte of the third pair 1704 comprises diluted anode electrolyte of the
first pair 1702. As
with the system of Figs. 15 and 16, these pH adjustments are exemplarary and
thus other pH
configurations can be established, e.g., adjusting the flow of fluids in the
system, e.g., adding
carbon dioxide to the cathode electrolyte and/or adding water to the
electrolyte, and/or removing
acid and alkaline solution from the system.
[00140] With reference to Figs. 1-17, the method in various embodiments
comprises a step of
oxidizing hydrogen gas to protons at an anode 102 without producing a gas at
the anode 102;
and producing bicarbonate ions in a cathode electrolyte 108 in communication
with the anode
102. In another embodiment, the method comprises a step of oxidizing hydrogen
gas at a
hydrogen-oxidizing anode 102 in communication with a cathode electrolyte 108 ;
and
sequestering carbon dioxide with the cathode electrolyte 108.
[00141] In some embodiments, the method comprises applying a voltage across
the anode
and a cathode in contact with the cathode electrolyte, e.g. a voltage of less
than 3V, and
producing hydrogen gas at the cathode. In some embodiments, the method
comprises
configuring a hydrogen delivery system to provide the hydrogen gas to the
anode from hydrogen
gas produced at the cathode, and adding carbon dioxide to the cathode
electrolyte. In some
embodiments, the method comprises comprising producing hydroxide ions and/or
carbonate
ions and/or bicarbonate ions in the cathode electrolyte.
[00142] In some embodiments of the method, the anode and/or cathode comprises
an
electrocatalyst selected from platinum, a single-crystal nickel, Raney nickel,
platinized nickel, a
metal carbide (W2C, Pt-W2C), a platinum group metal alloy (Pt-M, where M=Fe,
Mn, Cr, Co, Au),
a transition metal, a nickel alloy, sintered nickel, a platinum group metals
(Pt, Pd, Ru, Rh), gold,
silver, a precious or non-precious chalcogenides, a discrete macrocyclic
complex of transition
metals and biological complexes. In some embodiments, the electrocatalyst is
configured to
catalyze the oxidation of hydrogen gas to protons at the anode, and catalyze
production of
hydrogen gas and hydroxide ions at the cathode.
[00143] In some embodiments, the method comprises sequestering carbon dioxide
with the
cathode electrolyte, wherein the carbon dioxide is contained in an industrial
waste gas, and
wherein the carbon dioxide is sequestered as carbonates and/or bicarbonates,
e.g., as
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carbonates and/or bicarbonates comprising divalent cations such as calcium
ions and/or
magnesium ions.
[00144] In some embodiments, the method comprises producing an acid in the
anode
electrolyte, and configuring an acid dissolution system to produce divalent
cations with the acid,
e.g., produce calcium ions and/or magnesium ions by dissolving a mineral with
the acid. In some
embodiments, the acid dissolution system is configured to provide divalent
cations to the carbon
sequestration system.
[00145] In some embodiments of the method, and with reference to the system
1300 of Fig.
13, the method comprises configuring a plurality of pairs of the anodes and
cathodes 1302,
1304, 1306 in series to receive a common current through each pair of anode
and cathode.
[00146] In some embodiments is illustrated in the system of Fig 14, the method
comprises
configuring a plurality of pairs of anodes and cathodes 1402, 1404, 1406 in
parallel to receive a
common voltage 114 across each pair of anode and cathode.
[00147] In some embodiments of the method, and with reference to the system
1500 of Fig.
15, the method comprises configuring a plurality of pairs of the anode
electrolyte and cathode
electrolyte whereby the cathode electrolyte of a first pair 1502 is connected
to the cathode
electrolyte of a second pair 1504; and the anode electrolyte of a first pair
1502 is connected to
the anode electrolyte of a second pair 1504, and adjusting the pH of the
cathode electrolyte of
the second pair 1504 to a value equal to or greater than the pH of the cathode
electrolyte in the
first pair 1502; and adjusting the pH of the anode electrolyte of the second
pair 1504 to a value
equal to or less than the pH of the anode electrolyte of the first pair 1502.
As can be
appreciated, these pH adjustments are exemplarary and thus other pH
configurations can be
established, e.g., adjusting the flow of fluids in the system, e.g., adding
carbon dioxide to the
cathode electrolyte and/or adding water to the electrolyte, and/or removing
acid and alkaline
solution from the system.
[00148] In some embodiments of the method and with reference to the Fig. 16,
the method
comprises configuring a plurality of pairs of the anode electrolyte and
cathode electrolyte
whereby the cathode electrolyte and anode electrolyte of a second pair 1606
comprise cathode
electrolyte from a first pair 1602; and the cathode electrolyte and anode
electrolyte of a third pair
1604 comprise anode electrolyte from the first pair 1602. As can be
appreciated, these pH
adjustments are exemplarary and thus other pH configurations can be
established, e.g.,
adjusting the flow of fluids in the system, e.g., adding carbon dioxide to the
cathode electrolyte
and/or adding water to the electrolyte, and/or removing acid and alkaline
solution from the
system.
[00149] In another embodiment, and with reference to Fig. 17, the method
comprises
adjusting the pH of the cathode electrolyte of the second pain 706 to a value
equal to or greater
than pH of the cathode electrolyte of the first pair 1702; and adjusting the
pH of the anode
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electrolyte of the third pair 1704 to a value equal to or less that pH of the
anode electrolyte of
the first pair 1702. As can be appreciated, these pH adjustments are
exemplarary and thus other
pH configurations can be established, e.g., adjusting the flow of fluids in
the system, e.g., adding
carbon dioxide to the cathode electrolyte and/or adding water to the
electrolyte, and/or removing
acid and alkaline solution from the system.
[00150] In an exemplarary embodiment, a system configured substantially as
illustrated in
Figs. 1 - 17 was operated with a constant current density applied across the
electrodes at
steady state conditions while carbon dioxide gas was continuously dissolved
into the cathode
electrolyte, at various temperatures and voltages. In the system, a platinum
catalyst, gas
diffusion anode obtained from E-TEK Corporation, (USA) was used as the anode.
A Raney
nickel deposited onto a nickel gauze substrate was used as the cathode. In the
system, the
initial acid concentration in the anode electrolyte was 1 M; the initial
sodium chloride salt solution
was 5 M; and the initial concentration of the sodium hydroxide solution in the
cathode
compartment was 1 M. In the system, the pH of the cathode compartment was
maintained at
either 8 or 10 by regulating the amount of carbon dioxide dissolved in the
cathode electrolyte.
Table 1: Experimental Current Density, Temperature and Voltage Characteristics
of the
System
T ( C) Potential (V) pH Current density
(mA/cm)
0.8 10 8.6
8 11.2
1.2 10 28.3
8 29.2
1.6 10 50.2
8 50.6
75 0.8 10 13.3
8 17.8
1.2 10 45.3
8 49.8
1.6 10 80.8
8 84.7
[001511 As is illustrated in Table 1, a range of current densities was
achieved across the
20 electrode in the system. As can be appreciated, the current density that
can be achieved with
other configurations of the system may vary, depending on several factors
including the
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cumulative electrical resistance losses in the cell, environmental test
conditions, the over-
potential associated with the anodic and cathodic reactions, and other
factors.
[00152] The current densities achieved in the present configuration and as set
forth in Table
1 are correlated with the production of hydroxide ions at the cathode, and
thus correlates with
the production of sodium hydroxide and/or sodium carbonate and/or sodium
bicarbonate in the
cathode electrolyte, as follows. With reference to Table 1, at 75 C, 0.8 V
and a pH of 10, each
cm2 of electrode passed 13.3 mA of current, where current is a measure of
charge passed
(Coulomb) per time (second). Based on Faraday's Laws, the amount of product,
e.g., hydroxide
ions, produced at an electrode is proportional to the total electrical charge
passed through the
electrode as follows:
n=(I"t)/(F*z)
where n is moles of product, I is a current, t is time, F is Faraday's
constant, and z is the
electrons transferred per product ionic species (or reagent ionic species).
Thus, based on the
present example, 1.38x10 moles of hydroxide ions are produced per second per
cm2 of
electrode, which is correlated with the production of sodium hydroxide in the
cathode electrolyte.
In the system the production rate of NaOH dictates the production rate of
NaHCO3 and Na2CO3
through Le Chatelier's principle following the net chemical equilibria
equations of
H2CO3 + OH" = H2O + HCO3
and HCO3 + OH" = H2O + C032_,
where an increase in concentration of one species in equilibria will change
the concentration of
all species so that the equilibrium product maintains the equilibrium
constant. Thus, in the
system, the equilibrium concentrations of H2CO3, HCO3, and CO32- vs. pH in the
electrolyte will
follow the carbonate speciation diagram as discussed above.
[00153] In the system as illustrated in Figs. 1 - 9 and as discussed with
reference to the
carbonate speciation graph, supra, the solubility of carbon dioxide in the
cathode electrolyte is
dependent on the pH of the electrolyte. Also in the system, the voltage across
the cathode and
anode is dependent on several factors including the pH difference between the
anode
electrolyte and cathode electrolyte. Thus, in some embodiments the system can
be configured
to operate at a specified pH and voltage to absorb carbon dioxide and produce
carbonic acid,
carbonate ions and/or bicarbonate ions in the cathode electrolyte. In
embodiments where
carbon dioxide gas is dissolved in the cathode electrolyte, as protons are
removed from the
cathode electrolyte more carbon dioxide may be dissolved to form carbonic
acid, bicarbonate
ions and/or carbonate ions. Depending on the pH of the cathode electrolyte the
balance is
shifted toward bicarbonate ions or toward carbonate ions, as is well
understood in the art and as
is illustrated in the carbonate speciation diagram, above. In these
embodiments the pH of the
cathode electrolyte solution may decrease, remain the same, or increase,
depending on the
rate of removal of protons compared to rate of introduction of carbon dioxide.
It will be
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appreciated that no carbonic acid, hydroxide ions, carbonate ions or
bicarbonate ions are
formed in these embodiments, or that carbonic acid, hydroxide ions, carbonate
ions, bicarbonate
ions may not form during one period but form during another period.
[00154] In another embodiment, the system and method are integrated with a
carbonate
and/or bicarbonate precipitation system (not illustrated) wherein a solution
of divalent cations,
when added to the present cathode electrolyte, causes formation of
precipitates of divalent
carbonate and/or bicarbonate compounds, e.g., calcium carbonate or magnesium
carbonate
and/or their bicarbonates. In some embodiments, the precipitated divalent
carbonate and/or
bicarbonate compounds may be utilized as building materials, e.g., cements and
aggregates as
described for example in commonly assigned U.S. Patent Application no.
12/126,776 filed on
May 23, 2008, herein incorporated by reference in its entirety.
[00155] In an alternative embodiment, the system and method are integrated
with a mineral
and/or material dissolution and recovery system (not illustrated) wherein the
acidic anode
electrolyte solution 104 or the basic cathode electrolyte 108 is utilized to
dissolve calcium and/or
magnesium-rich minerals e.g., serpentine or olivine, or waste materials, e.g.,
fly ash, red mud
and the like, to form divalent cation solutions that may be utilized, e.g., to
precipitate carbonates
and/or bicarbonates as described herein. In some embodiments, the precipitated
divalent
carbonate and/or bicarbonate compounds may be utilized as building materials,
e.g., cements
and aggregates as described for example in commonly assigned U.S. Patent
Application no.
12/126,776 filed on May 23, 2008, herein incorporated by reference in its
entirety.
[00156] In an alternative embodiment, the system and method are integrated
with an
industrial waste gas treatment system (not illustrated) for sequestering
carbon dioxide and other
constituents of industrial waste gases, e.g., sulfur gases, nitrogen oxide
gases, metal and
particulates, wherein by contacting the flue gas with a solution comprising
divalent cations and
the present cathode electrolyte comprising hydroxide, bicarbonate and/or
carbonate ions,
divalent cation carbonates and/or bicarbonates are precipitated as described
in commonly
assigned U.S. Patent Application no. 12/344,019 filed on December 24, 2008,
herein
incorporated by reference in its entirety. The precipitates, comprising, e.g.,
calcium and/or
magnesium carbonates and bicarbonates in some embodiments may be utilized as
building
materials, e.g., as cements and aggregates, as described in commonly assigned
U.S. Patent
Application no. 12/126,776 filed on May 23, 2008, herein incorporated by
reference in its
entirety.
[00157] In another embodiment, the system and method are integrated with an
aqueous
desalination system (not illustrated) wherein the partially desalinated water
of the third
electrolyte of the present system is used as feed-water for the desalination
system, as described
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in commonly assigned U.S. Patent Application no. 12/163,205 filed on June 27,
2008, herein
incorporated by reference in its entirety.
[00158] In an alternative embodiment, the system and method are integrated
with a
carbonate and/or bicarbonate solution disposal system (not illustrated)
wherein, rather than
producing precipitates by contacting a solution of divalent cations with the
first electrolyte
solution to form precipitates, the system produces a solution, slurry or
suspension comprising
carbonates and/or bicarbonates. In some embodiments, the solution, slurry or
suspension is
disposed of in a location where it is held stable for an extended periods of
time, e.g., the
solution/slurry/suspension is disposed in an ocean at a depth where the
temperature and
pressure are sufficient to keep the slurry stable indefinitely, as described
in U.S. Patent
Application no. 12/344,019 filed on December 24, 2008, herein incorporated by
reference in its
entirety; or in a subterranean site.
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