U.S. patent application number 10/994855 was filed with the patent office on 2005-04-28 for electrochemical synthesis of ammonia.
Invention is credited to Cisar, Alan, Denvir, Adrian, Murphy, Oliver J., Robertson, Priscilla, Uselton, Kyle.
Application Number | 20050087449 10/994855 |
Document ID | / |
Family ID | 34519556 |
Filed Date | 2005-04-28 |
United States Patent
Application |
20050087449 |
Kind Code |
A1 |
Denvir, Adrian ; et
al. |
April 28, 2005 |
Electrochemical synthesis of ammonia
Abstract
A method for electrochemical synthesis of ammonia gas comprising
providing an electrolyte between an anode and a cathode, providing
hydrogen gas to the anode, oxidizing negatively charged
nitrogen-containing species present in the electrolyte at the anode
to form an adsorbed nitrogen species, and reacting the hydrogen
with the adsorbed nitrogen species to form ammonia. Preferably, the
hydrogen gas is provided to the anode by passing the hydrogen gas
through a porous anode substrate. It is also preferred to produce
the negatively charged nitrogen-containing species in the
electrolyte by reducing nitrogen gas at the cathode. However, the
negatively charged nitrogen-containing species may also be provided
by supplying a nitrogen-containing salt, such as lithium nitride,
into the molten salt electrolyte mixture in a sufficient amount to
provide some or all of the nitrogen consumed in the production of
ammonia.
Inventors: |
Denvir, Adrian; (College
Station, TX) ; Murphy, Oliver J.; (Bryan, TX)
; Cisar, Alan; (Cypress, TX) ; Robertson,
Priscilla; (Bryan, TX) ; Uselton, Kyle;
(College Station, TX) |
Correspondence
Address: |
STREETS & STEELE
13831 NORTHWEST FREEWAY
SUITE 355
HOUSTON
TX
77040
US
|
Family ID: |
34519556 |
Appl. No.: |
10/994855 |
Filed: |
November 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10994855 |
Nov 22, 2004 |
|
|
|
10090444 |
Mar 4, 2002 |
|
|
|
Current U.S.
Class: |
205/552 |
Current CPC
Class: |
C25B 1/00 20130101 |
Class at
Publication: |
205/552 |
International
Class: |
C25B 001/00 |
Claims
1-36. (canceled)
37. An apparatus comprising: an anode substrate adapted for fluid
communication with a source of hydrogen gas; a porous cathode
substrate; and a liquid electrolyte disposed within a matrix,
wherein the matrix is disposed between the anode substrate and the
porous cathode substrate, wherein the liquid electrolyte comprises
an electroactive species containing nitrogen, and wherein the
liquid electrolyte is not an aqueous solution.
38. The apparatus of claim 37, further comprising: a catalyst
disposed on the anode substrate facing the electrolyte matrix.
39. The apparatus of claim 37, further comprising: a catalyst
disposed on the cathode substrate facing the electrolyte
matrix.
40. The apparatus of claim 37, wherein the anode substrate is
porous.
41. The apparatus of claim 40, wherein the porous anode substrate
has a porosity greater than 40 percent.
42. The apparatus of claim 40, wherein the porous anode substrate
has a porosity greater than 90 percent.
43. The apparatus of claim 37, further comprising: a metal membrane
having a thickness of between 1 and 200 microns disposed on the
anode substrate facing the electrolyte matrix.
44. The apparatus of claim 43, wherein the metal membrane is made
from a metal selected from palladium, a palladium alloy, iron,
tantalum, lanthanide metals, or combinations thereof.
45. The apparatus of claim 43, further comprising a matrix
supporting the metal membrane, wherein the matrix is formed from a
material selected from nickel or nickel-containing alloys.
46. The apparatus of claim 43, further comprising a matrix
supporting the metal membrane, wherein the matrix is formed from a
material selected from transition metals or transition
metal-containing alloys.
47. The apparatus of claim 43, further comprising a matrix
supporting the metal membrane, wherein the matrix is formed from an
electrically conducting inorganic ceramic material.
48. The apparatus of claim 37, further comprising: a metal membrane
having a thickness of between 1 and 200 microns disposed on both
sides of the porous anode substrate, wherein the porous anode
substrate is a non-noble metal and the metal membrane is palladium
or a palladium-containing alloy.
49. The apparatus of claim 48, wherein the non-noble metal is
selected from iron, tantalum, or lanthanide metals.
50. The apparatus of claim 43, wherein a catalyst is disposed on a
surface of the metal membrane facing the electrolyte.
51. The apparatus of claim 50, wherein the catalyst comprises a
metal selected from iron, ruthenium or combinations thereof.
52. The apparatus of claim 37, wherein the porous cathode substrate
is made from nickel, a nickel-containing compound, or a nickel
alloy.
53. The apparatus of claim 37, wherein the porous cathode substrate
is made from metal, metal alloy, ceramic or a combination
thereof.
54. The apparatus of claim 37, wherein the porous cathode substrate
has a pore size of about 0.2 microns.
55. The apparatus of claim 37, wherein the electroactive species is
selected from nitrides, azides or combinations thereof.
56. The apparatus of claim 37, wherein the electroactive species is
one or more azides.
57. The apparatus of claim 37, wherein the porous cathode substrate
is adapted for fluid communication with a source of nitrogen
gas.
58. The apparatus of claim 37, wherein the electrolyte supports
migration of negatively charged nitrogen-containing species between
the cathode substrate and the anode.
59. The apparatus of claim 37, wherein the electrolyte comprises a
molten salt.
60. The apparatus of claim 58, wherein the molten salt comprises
one or more metal chlorides.
61. The apparatus of claim 58, wherein the molten salt comprises
one or more metal salts selected from chlorides, iodides, bromides,
sulfides, phosphates, carbonates, or mixtures thereof.
62. The apparatus of claim 58, wherein the molten salt comprises
lithium chloride and potassium chloride.
63. The apparatus of claim 61, wherein the molten salt further
comprises a metal nitride salt.
64. The apparatus of claim 61, wherein the molten salt electrolyte
has a greater molar concentration of lithium chloride than
potassium chloride.
65. The apparatus of claim 61, wherein the molten salt electrolyte
further comprises rubidium chloride, cesium chloride, ruthenium
chloride, iron chloride, or a mixture thereof.
66. The apparatus of claim 37, wherein the electrolyte comprises a
salt dissolved in an organic solvent.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/090,444, filed Mar. 4, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This present invention relates to an electrochemical method
and apparatus for the synthesis of ammonia.
[0004] 2. Background to the Related Art
[0005] Ammonia (NH.sub.3) is a colorless alkaline gas that is
lighter than air and possesses a unique, penetrating odor. Since
nitrogen is an essential element to plant growth, the value of
nitrogen compounds as an ingredient of mineral fertilizers, was
recognized as early as 1840. Until the early 1900's, the nitrogen
source in farm soils was entirely derived from natural sources.
Haber and Bosch pioneered the synthesis of ammonia directly from
hydrogen and nitrogen on a commercial scale in 1913. Further
developments in large-scale ammonia production for fertilizers have
made a significant impact on increasing the world's food
supply.
[0006] Virtually every nitrogen atom of a nitrogen compound travels
from the atmosphere to its destined chemical combination by way of
ammonia. Industrial uses of ammonia as a nitrogen source have
recently consumed a greater share of the total ammonia production,
accounting for 20% of the world output. Up to 80% of the ammonia
produced is used for the production of nitrogen-based fertilizers,
accounting for about 3% of the world's energy consumption. In many
developing countries, the capability for ammonia synthesis is the
first sign of budding industrialization. In the United States last
year there was over 19 billion tons of ammonia produced.
[0007] Many methods of ammonia synthesis have been investigated.
These methods include the catalytic synthesis of ammonia from its
elements using large-scale pressures and temperatures, indirect
ammonia synthesis using the steam decomposition of nitrogen based
compounds, and the formation of ammonia with the aid of electrical
discharge. Only recently has the possibility of using
electrochemistry for ammonia synthesis been demonstrated. The
electrochemical process is operated at atmospheric pressure and
570.degree. C., which is a similar temperature to that used in the
Haber-Bosch process. The apparatus consists of a non-porous,
strontia-ceria-ytterbia (SCY) perovskite ceramic tube closed at one
end and then further enclosed in a ceramic tube. Electrodes, made
from polycrystalline palladium films, are deposited on the inner
and outer walls of the SCY tube.
[0008] Ammonia gas is passed through the system, where the amount
of decomposition due to heating can be measured. Subsequently,
gaseous hydrogen is passed through the quartz tube and over the
anode surface, where the hydrogen is converted to protons:
3H.sub.2.fwdarw.6H.sup.++6e.sup.- (1)
[0009] The protons then diffuse through the solid perovskite
electrolyte to the cathode surface, where they come in contact with
the nitrogen gas and the following reaction takes place:
N.sub.2+6H.sup.++6e.sup.=.fwdarw.2NH.sub.3 (2)
[0010] However, the efficiency of the reaction is reduced by the
high temperatures needed for the reaction to occur.
[0011] Therefore, there remains a need for an improved method of
producing ammonia. It would be desirable if the improved method
could produce ammonia at lower temperatures and lower pressures,
while achieving a greater conversion than existing methods. It
would be even further desirable if the improved method were
compatible with existing process units, such as being able to use
the same hydrogen and nitrogen sources as are used in the
Haber-Bosch process.
SUMMARY OF THE INVENTION
[0012] The present invention provides a method for synthesizing
ammonia gas, comprising the steps of providing an electrolyte
between an anode and a cathode, providing hydrogen gas to the
anode, oxidizing negatively charged nitrogen-containing species
present in the electrolyte at the anode to form adsorbed nitrogen
species, and reacting the hydrogen with the adsorbed nitrogen
species to form ammonia. The negatively charged nitrogen-containing
species is preferably a nitride ion, such as lithium nitride, or an
azide ion, such as sodium azide.
[0013] The reaction is preferably carried out at a temperature
between 0 and 1000. Celsius, such as a temperature between 25 and
800 Celsius or between 100 and 700 Celsius, or more preferably
between 300 and 600 Celsius, although a lower temperature of
between 25 and 150 Celsius may be desirable. The method includes
applying a voltage between the anode and the cathode, where the
voltage is preferably up to 2 Volts, up to 1 Volt, or up to 0.5
Volt. It is also preferred to apply a current density between the
anode and the cathode of up to 2 A/cm.sup.2, up to 1 A/cm.sup.2, or
up to 0.5 A/cm.sup.2. Furthermore, the reaction is typically
carried out at a pressure between 1 and 250 atmospheres, preferably
between 1 and 100 atmospheres, more preferably between 1 and 50
atmospheres, even more preferably between 1 and 20 atmospheres, and
most preferably up to 5 atmospheres, including atmospheric
pressure.
[0014] The hydrogen gas preferably has a purity of greater than 70
percent, more preferably greater than 70 percent. The hydrogen gas
is preferably provided to the anode by passing the hydrogen gas
through a porous anode substrate. Preferably, the hydrogen gas
passes from a first face of the porous anode substrate to a
parallel opposite face of the porous anode substrate, wherein the
parallel opposite face is in contact with the electrolyte.
[0015] The porous anode substrate preferably has porosity greater
than 40 percent, but may have porosity greater than 90 percent.
Optionally, the porous anode substrate has a thin nonporous,
hydrogen-permeable metal film or membrane facing the electrolyte to
produce adsorbed atomic hydrogen from hydrogen gas passing there
through. The metal membrane can be made from a metal selected from
palladium, a palladium alloy, iron, tantalum, and combinations
thereof. In addition, it is optional to provide a catalyst disposed
on a surface of the metal membrane facing the electrolyte,
preferably wherein the catalyst is disposed on at least part of the
second surface of the porous anode substrate facing the
electrolyte. The metal membrane can also be supported by a matrix
formed from a material selected from nickel and nickel-containing
alloys. Alternatively, the matrix can be formed from electrically
conducting inorganic ceramic materials or a material selected from
transition metals and transition metal-containing alloys.
Preferably, the metal membrane is a composite comprising a
non-noble metal, such as iron, tantalum and the lanthanide metals,
having palladium or a palladium-containing alloy on each side of
the non-noble metal. In operation, the hydrogen gas may be
delivered to the metal membrane from a process selected from steam
reformation, partial oxidation, autothermal reformation, and plasma
reformation. Alternatively, hydrogen gas may be provided to the
porous anode substrate by electrolyzing water. In any of these
embodiments, the hydrogen gas may be delivered to the porous anode
substrate along with a carrier gas.
[0016] It is preferred to produce the negatively charged
nitrogen-containing species in the electrolyte by reducing nitrogen
gas at the cathode. The nitrogen gas may be delivered through a
porous cathode substrate. The porous cathode substrate is
preferably made from a metal, metal alloy, ceramic or a combination
thereof, most preferably made from nickel, a nickel-containing
compound, or a nickel alloy. Alternatively, the porous cathode
substrate may be selected from metal carbides, metal borides and
metal nitrides. A preferred porous cathode substrate has a pore
size of about 0.2 microns. The porous cathode substrate may be
coated with a porous electrocatalyst, for example an
electrocatalyst selected from transition metals, noble metals, and
combinations thereof. The nitrogen gas preferably contains less
than 1000 ppm moisture, more preferably less than 100 ppm moisture,
and most preferably less than 10 ppm moisture. The moisture may be
controlled or reduced by passing the nitrogen gas through a water
sorbent material before delivery to the porous cathode. The
nitrogen gas should also contain less than 0.1 percent oxygen.
Preferably the process includes both providing the hydrogen to the
anode catalyst, and reducing nitrogen gas at the cathode to produce
negatively charged nitrogen-containing species in the electrolyte,
wherein the hydrogen gas and the nitrogen gas are provided at gas
pressures greater than the pressure of the reaction.
[0017] The electrolyte preferably comprises a molten salt
electrolyte that supports migration of the negatively charged
nitrogen-containing species between the cathode and the anode. A
preferred molten salt electrolyte comprises lithium chloride and
potassium chloride, most preferably wherein the molten salt has a
greater molar concentration of lithium chloride than potassium
chloride. An equally preferred molten salt is selected from the
alkali metal tetrachloroaluminates. Preferably, the molten salt
electrolyte is charged with a nitride compound, an azide compound,
or a combination thereof. The preferred nitride compounds are the
nitride salts, such a lithium nitride. Furthermore, the molten salt
may further comprise one or more metal salts selected from
chlorides, iodides, bromides, sulfides, phosphates, carbonates, and
mixtures thereof. Where the metal salt is a metal chloride, the
metal chloride may comprise rubidium chloride, cesium chloride,
ruthenium chloride, iron chloride, or a mixture thereof. The
electrolyte may optionally comprise a salt dissolved in an organic
solvent. The method should include maintaining an inert atmosphere
over the electrolyte.
[0018] The present invention also provides an apparatus comprising
a porous anode substrate in fluid communication with a source of
hydrogen gas, a porous cathode substrate in fluid communication
with a source of nitrogen gas, and an electrolyte disposed within a
matrix, wherein the matrix is disposed between the porous anode
substrate and the porous cathlode substrate. Optionally, a catalyst
may be disposed on the porous anode substrate and/or the porous
cathode substrate facing the electrolyte matrix. Alternatively, a
metal membrane may be disposed on the porous anode substrate facing
the electrolyte matrix, preferably including an ammonia generating
catalyst disposed on a surface of the metal membrane facing the
electrolyte. The preferred catalysts capable of generating ammonia
comprise a metal selected from iron, ruthenium and combinations
thereof. In particular, the catalyst may be a ruthenium catalyst
that is activated by cesium and barium and is supported on a
graphite bed, or a barium-activated ruthenium on a magnesium oxide
support.
[0019] Furthermore, the present invention provides an apparatus
comprising a plurality of electrolytic cells and a bipolar plate
separating each of the plurality of electrolytic cells.
Accordingly, each of the plurality of electrolytic cells comprises
a porous anode substrate in fluid communication with a source of
hydrogen gas, a porous cathode substrate in fluid communication
with a source of nitrogen gas, an electrolyte disposed within a
matrix placed between the porous anode substrate and the porous
cathode substrate, an anodic fluid flow field in electronic
communication with the porous anode substrate opposite the matrix,
and a cathodic fluid flow field in electronic communication with
the porous cathode substrate opposite the matrix. Preferably, the
anodic fluid flow field has a first face that is in electronic
communication with the porous anode substrate and a second face in
electronic communication with a first bipolar plate, and the
cathodic fluid flow field has a first face that is in electronic
communication with the porous cathode substrate and a second face
in electronic communication with a second bipolar plate. The
apparatus will typically further comprise hydrogen gas inlet and
outlet manifolds for providing the fluid communication between the
source of hydrogen gas and each of the porous anode substrates, and
nitrogen gas inlet and outlet manifolds for providing the fluid
communication between the source of nitrogen gas and each of the
porous cathode- substrates. The hydrogen gas manifolds and the
nitrogen gas manifolds are each selected from either an internal
manifold or an external manifold. In a preferred embodiment, anodic
cell frames and cathodic cell frames are disposed around the anode
flowfields and cathode flowfields, respectively. These cell frames
must be able to withstand the high temperatures, high pressures and
harsh chemical environment of the molten salts. Accordingly, the
cell frames may be made, for example, from graphite for process
temperatures up to 500 Celsius, Inconel or Monel.
[0020] In one embodiment, the porous anode substrate and the porous
cathode substrate are each selected from metal foams, metal grids,
sintered metal particles, sintered metal fibers, and combinations
thereof. Preferably, two or more of the metal components of the
cell are metallurgically bonded together, such as by a process
selected from welding, brazing, soldering, sintering, fusion
bonding, vacuum bonding, and combinations thereof. For example, the
anodic fluid flow field may be metallurgically bonded to the
bipolar plate, the cathodic fluid flow field may be metallurgically
bonded to the bipolar plate, the anodic fluid flow field may be
metallurgically bonded to the porous anode substrate, the cathodic
fluid flow field may be metallurgically bonded to the porous
cathode substrate, and combinations thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] So that the above recited features and advantages of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, is provided
in reference to the embodiments thereof, which are illustrated in
the appended drawings. It is to be noted, however, that the
appended drawings illustrate only typical embodiments of this
invention and are therefore not to be considered limiting of its
scope, for the invention may admit to other equally effective
embodiments.
[0022] FIG. 1 is a schematic flow diagram of an ammonia synthesis
cell of the present invention.
[0023] FIG. 2 is a schematic flow diagram of a second ammonia
synthesis cell of the present invention.
[0024] FIG. 3 is a schematic diagram of a composite metal membrane
for hydrogen diffusion.
[0025] FIG. 4 is a schematic structural diagram of an ammonia
synthesis cell stack.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The present invention provides a method for electrochemical
synthesis of ammonia gas. The method comprises providing an
electrolyte between an anode and a cathode, providing hydrogen gas
to the anode, oxidizing negatively charged nitrogen-containing
species present in the electrolyte at the anode to form an adsorbed
nitrogen species, and reacting the hydrogen with the adsorbed
nitrogen species to form ammonia. Preferably, the hydrogen gas is
provided to the anode by passing the hydrogen gas through a porous
anode substrate. It is also preferred to produce the negatively
charged nitrogen-containing species in the electrolyte by reducing
nitrogen gas at the cathode. However, the negatively charged
nitrogen-containing species may also be provided by supplying a
nitrogen-containing salt, such as lithium nitride, into the molten
salt electrolyte mixture in a sufficient amount to provide some or
all of the nitrogen consumed in the production of ammonia.
[0027] FIG. 1 is a schematic flow diagram of an ammonia synthesis
cell of the present invention. The electrochemical cell or reactor
10 is provided with a molten salt electrolyte 12. The cell is
heated to keep the electrolyte in a molten state and may be
pressurized. Nitrogen gas (N.sub.2) 14 is introduced into the cell
10 from an endplate 16 and through a porous cathode 18. The
molecular nitrogen gas 14 is reduced by electrons 20 to give two
nitride ions (N.sup.3-) 22 at the cathode 18 in a six-electron
reduction process. The nitride ions 22, which are stable in the
molten salt electrolyte, migrate through the electrolyte 12 towards
the anode 24.
[0028] The cathode is a porous, electronically conducting member
where nitrogen or nitrogen containing compounds are reduced to a
negatively charged nitrogen species. The cathode may be made from a
metal, metal alloy or ceramic material. Preferably, the cathode is
made from porous nickel or a nickel-containing compound, such as a
nickel alloy (Inconel, Monel, Stainless steel and their families of
alloys).
[0029] The anode is a porous, electronically conducting member 24
allowing the introduction of hydrogen gas 26 from endplate 28. At
the anode, the hydrogen 26 diffuses through the anode to the
surface 36 in contact with the molten salt 12 where the hydrogen is
adsorbed, perhaps in the form of adsorbed atomic hydrogen 32. The
nitride ions 22 reach the porous anode 24 where the electron
transfer reaction occurs and the nitride ion is oxidized to
adsorbed atomic nitrogen (N) 30 by giving up electrons 25. The
oxidation potential for the nitride ion to atomic nitrogen occurs
at a more negative potential than hydrogen oxidation and thus it
will occur in preference to the hydrogen reaction. The atomic
nitrogen 30 adsorbed on the anode surface 36 then reacts with
neighboring hydrogen atoms 32 to produce ammonia gas 34 that is
evolved and collected. Preferably, the porous anode substrate 24
includes a catalyst-coating, such as iron, ruthenium, or a mixture
thereof disposed on the surface 36 facing the electrolyte.
[0030] The kinetics of the ammonia production reaction can be
controlled by regulating the electrode potentials. For example, by
controlling the anode and cathode potentials, the current
efficiency for the conversion of nitrogen gas to nitride ion in a
molten salt electrolyte with a nickel cathode is greater than 93%.
The conversion of nitrogen and hydrogen to ammonia is an exothermic
reaction and hence the conversion increases with decreasing
temperature. The present method for electrochemical ammonia
generation will operate at considerably lower temperatures than
those used in the Haber-Bosch process, thereby benefiting the
equilibrium process represented by equation (3). It is believed
that by combining potentiometric control, low operating
temperatures, and pressure regulation, the present method will
produce ammonia in higher yields than that produced by current
methods.
0.5 N.sub.2+1.5 H.sub.2.rarw. - - -
.fwdarw.NH.sub.3H.sub.298=-45.72 kJ/mol (3)
[0031] Nitrogen gas is the preferred source of the negatively
charged nitrogen containing species. Preferably, the nitrogen gas
used for the electrolysis is high purity and contains less than 2
ppm moisture. This can be achieved by using a high-purity nitrogen
source that passes though a water sorbent material before it enters
the reactor. The nitrogen may be supplied from the same nitrogen
source currently used in ammonia manufacturing. Alternatively, the
nitrogen gas can be provided by a liquid nitrogen source, air, or
the decomposition of nitrogen containing compounds. Nitrogen can
also be introduced to the cell in combination with a carrier gas
such as argon, or other inert gaseous materials, carbon dioxide or
other gaseous species or a combination thereof. Preferably, the
nitrogen is introduced to the system via a series of flow fields or
flow field/gas diffusion electrode arrangements. The flow of the
nitrogen can be controlled via series of pumps, valves, pressurized
vessels, suction devices or a combination thereof.
[0032] The hydrogen may be obtained from the same sources as it is
presently obtained for use in conventional processes for ammonia
production, including coke oven gas and coal, natural gas, naptha,
and other petroleum products converted via steam reformation or
partial oxidation. Alternatively, the hydrogen can be supplied by
the electrolysis of water or the decomposition of other hydrogen
containing compounds including metal hydrides. The hydrogen can
also be introduced to the cell along with a carrier gas, such as
argon or other inert gaseous materials, carbon dioxide or other
gaseous species, or a combination thereof. Preferably, the hydrogen
gas is introduced to the system via a series of flow fields or flow
field/gas diffusion electrode arrangements. The flow of the
hydrogen can be controlled via a series of pumps, valves,
pressurized vessels, suction devices or a combination thereof.
[0033] The electrolyte must be capable of forming, stabilizing and
permitting migration of the negatively charged nitrogen-containing
species between the cathode and anode. Also, the electrolyte must
be chemically and electrochemically stable and inert under the
conditions required for the electrochemical synthesis of ammonia.
The anion of the molten salt must not undergo an electrochemical
oxidation process at the anode and the cation of the molten salt
must not undergo an electrochemical reduction process at the
cathode. The preferred electrolyte comprises one or more molten
salts selected from metal chlorides, iodides, bromides, carbonates,
sulfides, phosphates, and mixtures thereof. It is also preferred to
add other salts, such as rubidium chloride, cesium chloride,
ruthenium chloride, iron chloride, or a mixture thereof, in small
portions, such as 0.1 to 5 percent, to lower the melting
temperature of the eutectic. One particularly preferred molten salt
mixture includes 59% LiCl/41%/KCl/0.1% Li.sub.3N. However, in
addition to the foregoing electrolytes, it is believed that the
present invention will also operate using low temperature molten
salts, described in more detail below.
[0034] The electrolyte will preferably contain an electroactive
species, such as nitride ions or azide ions, that are present not
as a result of a reduction or oxidation reaction of a nitrogen
containing species at the electrodes, but are present as species
that have been added to the electrolyte. For example, it is
preferred to provide the electrolyte with small quantities of
Li.sub.3N to allow the ammonia production reaction to start.
Finally, it is preferred to charge the electrochemical cell with
the mixed, dried electrolyte salts and heat them into a molten
state, but it is also possible to melt the electrolyte before
charging the electrolyte into the cell. Prior to melting, the salts
should be dried and mixed together in an inert atmosphere,
preferably with less than 2 ppm moisture.
[0035] The reaction is preferably carried out at a temperature
between 0 and 1000 Celsius, such as a temperature between 25 and
800 Celsius or between 100 and 700 Celsius, or more preferably
between 300 and 600 Celsius, although a lower temperature of
between 25 and 150 Celsius may be desirable from an energy
consumption standpoint.
[0036] Furthermore, the reaction is typically carried out at a
pressure between 1 and 250 atmospheres, preferably between 1 and
100 atmospheres, more preferably between 1 and 50 atmospheres, even
more preferably between 1 and 20 atmospheres, and most preferably
up to 5 atmospheres, including atmospheric pressure. The cell can
be pressurized using the reactant gases, but the internal pressure
of the cell must be prevented from exceeding the reactant gas
pressure within the anode or cathode in order to prevent backflow
of molten salts into the porous electrodes or failure of the
electrodes or metal membranes within the cell.
[0037] Once the cell has been assembled, charged with electrolyte,
and heated to the operational temperature and pressure, then a
voltage is applied between the anode and cathode. The preferred
voltage is up to 2 Volts, up to 1 Volt, or up to 0.5 Volts. It is
also preferred to apply a current density between the anode and the
cathode of up to 2 A/cm.sup.2, up to 1 A/cm.sup.2, or up to 0.5
A/cm.sup.2. In the preferred embodiment, nitrogen gas is introduced
at the cathode and hydrogen gas is introduced at the anode. While
the voltage may be regulated using a reference electrode, such as a
lithium/lithium ion reference electrode as used in the examples
below, it should be apparent that the no reference electrode is
required.
[0038] FIG. 2 is a schematic flow diagram of a second ammonia
synthesis cell of the present invention. The cell 40 operates in
almost identical fashion to cell 10 of FIG. 1, except that it
includes a gas diffusion electrode 42 in combination with the
porous cathode 18 and a metal membrane 46 in combination with the
porous anode 24.
[0039] The metal membrane 46 separates hydrogen gas from other
gaseous components or contaminants and splits the molecular
hydrogen 26 into atomic hydrogen 32. The atomic hydrogen 32
diffuses through the membrane 46 to the outer surface 48 where the
atomic hydrogen is adsorbed. The membrane structure is preferably
supported on a matrix that imparts greater mechanical strength to
the metal membrane. Most preferably, the support matrix is provided
by the porous anode 24 and includes the necessary flow field or
flow field/gas diffusion electrode arrangements to allow hydrogen
to be distributed evenly across the face of the anode.
[0040] The support matrix can be made from a nickel-containing
compound such as a nickel alloy (Inconel, Monel, Stainless steel
and their families of alloys), transition metals and their
corresponding families of alloys, or combinations thereof.
Conducting inorganic materials including ceramics in combination
with the metal species mentioned above can also be used. The metal
membrane may also be used as an anode without the use of a support
matrix.
[0041] Preferably, the metal membrane is made from palladium
alloys, where the palladium concentration varies from 100 wt % to 5
wt % and the alloying metal is a transition metal, main group metal
(sp), or a combination thereof. The most preferred metal membrane
is made from a palladium silver alloy 75:25 wt % Pd:Ag.
Furthermore, body-centered cubic refractory metals, such as
zirconium, niobium, tantalum, and vanadium, having significantly
higher bulk hydrogen permeability than palladium, can be used as a
direct replacement for palladium.
[0042] FIG. 3 is a schematic diagram of a composite metal membrane
for hydrogen diffusion. The composite structure 60 includes a
palladium-containing layer 62 deposited on both sides of a
refractory metal 64. This construction allows the dissociation of
the molecular hydrogen 26 into atomic hydrogen 32 upon passing
through the palladium surface layer 62, followed by rapid transport
of the atomic hydrogen 32 through the refractory metal 64, so that
the atomic hydrogen is adsorbed on the opposite palladium surface
facing the electrolyte 12. The refractory metal is chosen for its
ability to transport hydrogen and to offer structural integrity for
the composite membrane. Such a structure has several advantages.
First, greater overall atomic hydrogen fluxes are possible because
the diffusion is not limited by the face centered cubic (f.c.c)
structure of the palladium. Because of this, the membrane can be
thicker, providing improved mechanical or structural properties
while still providing acceptable, and even improved, hydrogen
fluxes. Second, since the refractory metals are significantly less
expensive than palladium, these membranes are more economical
because only two thin layers of palladium are needed. Further,
while the Group V metals are subject to hydrogen embrittlement,
this regime is only a problem well below room temperature. Should
the palladium layer develop defects, such as those caused by the
palladium phase transformation, the membrane would still be
functional because the defect would expose only a minute area of
the refractory metal.
[0043] Optionally, the metal membrane system will incorporate an
ammonia generating catalyst to act as the electrode on the outer
surface 48 of the membrane 46 facing the electrolyte 12 (See FIG.
2). The hydrogen atoms diffuse through the metal membrane layer
onto the ammonia catalyst surface where they react with the
adsorbed nitrogen atoms.
[0044] The metal membrane may have any reasonable thickness, but it
does not need to be any thicker than 1 to 200 .mu.m. However, the
thickness of the membrane can be increased to improve the
mechanical strength or decreased to provide for more increased
hydrogen transport. For example, a particularly preferred composite
metal membrane may be prepared from a tantalum foil. The tantalum
foil is placed into a vacuum chamber that is pumped down to
10.sup.-6 torr. An argon gun may then be used to remove the native
surface oxides, followed by a sputtering of palladium onto the
tantalum surface.
[0045] The present invention also provides an apparatus for
generating ammonia gas. The apparatus comprises a porous anode
substrate in fluid communication with a source of hydrogen gas, a
porous cathode substrate in fluid communication with a source of
nitrogen gas, and an electrolyte disposed between the porous anode
substrate and the porous cathode, where the electrolyte is
preferably a molten salt disposed within a matrix. The anode
substrate and/or the cathode substrate may have a catalyst disposed
on the surface of the substrate facing the electrolyte. The
apparatus may include a stack of electrochemical cells, including a
bipolar separator plate disposed between each of the cells in the
stack. The apparatus is compatible with either internal manifolding
or external manifolding for the supply of the hydrogen and nitrogen
gases, as well as the removal of the ammonia gas produced. In a
particularly preferred embodiment, two or more adjacent cell
components are metallurgically bonded to form an integrated
subassembly in order to reduce the electrical resistance of the
cell and reduce the number of separate components that must be
assembled.
[0046] FIG. 4 is a schematic structural diagram of an ammonia
synthesis cell or reactor 70 similar to a molten carbonate fuel
cell. The reactor 70 includes an anode endplate 72 and cathode
endplate 74 that secure the cell components together and are
coupled to the positive terminal 76 and negative terminal 78 of a
power supply 80, respectively. An anode flow field 86 is provided
to distribute hydrogen gas over the porous anode 88. Similarly, a
cathode flow field 82 is provided to distribute nitrogen gas over
the porous cathode 84. An electrolyte 90 is disposed between the
porous anode and the porous cathode. Many important factors, such
as materials compatibility, electrolyte loss, and operating
conditions have been well developed for working in the aggressive
environment associated with molten salts. Also, the design of the
electrochemical cell allows for minimum ohmic losses in the system
that leads to a reduction of power consumption.
[0047] The electrolyte matrix may be a tile fabricated by
hot-pressing alkali-chlorides and LiAlO.sub.2 or tape-casting
LiAlO.sub.2 matrices. Tape casting can continuously manufacture
matrices as thin as 0.03 to 0.07 centimeters and 45-55% porous with
a mean pore size of 0.5 micrometers.
Low Temperature Molten Salts
[0048] Lewis acids are covalently bonded compounds capable of
accepting a pair of electrons to complete a shell. Aluminum
chloride (AlCl.sub.3) is the preeminent example of a Lewis acid.
This molecule, which occurs as the dimer (Al.sub.2Cl.sub.6), will
readily combine with almost any free chloride to form a tetrahedral
aluminum tetrachloride anion (AlCl.sub.4.sup.-). This covalently
bonded ion acts as a large monovalent ion, with the negative charge
dispersed over a large volume.
[0049] All of the alkali metal tetrachloroaluminates are known, and
all have a key feature in common: the complex salt, with the
negative charge dispersed over a large volume, has a far lower
melting point than the corresponding simple chloride. These complex
salts are well known and have been used as moderately high
temperature (150-300.degree. C.) solvents for a variety of
purposes, including electrochemistry, spectroscopy, and crystal
growth. A variety of unusual species have been found to be stable
in acidic tetrachloroaluminate melts that cannot be synthesized in
other ways.
[0050] Ambient temperature molten salts based on the same acid-base
interactions were first reported in 1951. Interest in this field
accelerated in the 1980's with the appearance of the widely studied
substituted imidazoles. Table I shows some of the compounds capable
of forming ambient temperature molten salts when combined with
aluminum chloride.
[0051] All of these materials are ionic chlorides. With the
exception of TMPAC, all have the positive charge delocalized to
some degree through a .pi.-conjugated system over a large portion
of the volume of the bulky cation. In all cases, the combination of
a large cation, with a low charge density and a large anion, with a
low charge density, leads to a low melting solid. The combination
is an ionic liquid that actually behaves in some respects more like
a molecular liquid. Unlike high temperature molten salts, which
tend to interact only through non-directional charge-charge
interactions, these molten salts are hydrogen-bonded liquids with
the cations forming a water-like network.
[0052] With Lewis acid systems, such as those formed by AlCl.sub.3
and amine chlorides, which are aprotic, acidity and basicity are
defined differently than in aqueous systems. A solution is acidic
when the AlCl.sub.3:aminie chloride mole ratio is >1.0, basic
when the ratio is <1.0, and neutral when the ratio is 1.0. Under
basic conditions there are free chloride ions present. Under acidic
conditions part of the aluminum chloride remains complexed to other
aluminum chloride molecules, with heptachlorodialuminate,
Al.sub.2Cl.sub.7.sup.-, being a primary aluminum species. Under
very acidic conditions, the trialuminate species,
Al.sub.3Cl.sub.10.sup.31 has been observed in (EMIM)AlCl.sub.4
melts as well.
1TABLE I Compounds that Form Room Temperature Tetrachloroaluminate
Salts Compound Formula Abbr. 1-ethyl-3-methylimidazolium chloride
C.sub.6H.sub.11N.sub.2Cl EMIM Trimethylphenylammonium chloride
C.sub.6H.sub.14NCl TMPAC 1-methyl-3-ethyl-imidazolium chloride
C.sub.6H.sub.11N.sub.2Cl MEIC 1,3-dimethyl-imidazolium chloride
C.sub.5H.sub.9N.sub.2Cl 1-methyl-3-propyl-imidazolium chloride
C.sub.7H.sub.13N.sub.2Cl 1-methyl-3-butyl-imidazolium chloride
C.sub.8H.sub.15N.sub.2Cl 1,3-dibutyl-imidazolium chloride
C.sub.11H.sub.21N.sub.2Cl 1,2-dimethyl-3-propyl-imidazolium
chloride C.sub.8H.sub.15N.sub.2Cl DMPrICl N-butylpyridinium
chloride C.sub.9H.sub.14NCl BPC N-propylpyridinium chloride
C.sub.8H.sub.12NCl N-ethylpyridinium chloride C.sub.7H.sub.10NCl
N-methylpyridinium chloride C.sub.6H.sub.8NCl
[0053] Impurities in the melt can alter its properties, or
interfere with the electrochemistry. Minimizing these difficulties
requires that all handling and use of these compounds be carried
out under exceedingly inert, dry conditions. Impurities in melts,
whether present in the starting materials or introduced later, can
be removed using a number of purification processes developed for
this purpose. Protons can be removed from melts by treatment with
ethylaluminum dichloride, which reacts to generate ethane and
AlCl.sub.3. As the protons are removed, the melt becomes more
acidic. Oxide and hydroxyl species can be removed from these
systems by purging with phosgene. The oxo species react with the
phosgene (COCl.sub.2) to form CO.sub.2 and free chloride ions,
reducing the acidity of the melt.
[0054] Other work has led to the identification of other modifiers
for specific properties of these melts. A variety of compounds,
including anisole, 1,2-dichlorobenzene, diphenylether,
chlorobenzene, fluorobenzene, and 1,4-difluorobenzene, have been
demonstrated as viscosity modifiers for these systems.
[0055] Salts of most of the transition metals have already been
demonstrated to dissolve in room temperature molten salts. Some of
these dissolve under basic conditions, and others under acidic
conditions. Of the eight transition elements not already reported
as solution species, five are considered likely to form
solutions.
[0056] The solution species formed by many of these elements have
been identified. The solution species present when NiCl.sub.2 and
CdCl.sub.2 are dissolved in the pure 1-ethyl-3-methyl-imidazolium
(EMIM) chloride base have been identified as tetrahedral
MCl.sub.4.sup.= ions by single crystal x-ray diffraction studies of
the (EMIM).sub.2MCl.sub.4 salts. While both salts have melting
points significantly above room temperature (100.degree. C. for the
Co salt and 92.degree. C. for the Ni salt), both are soluble in
(EMIM)AlCl.sub.4, especially in the presence of excess acid
(AlCl.sub.3). The Co, Ni and Mn species present in these solutions
have been identified as [M(AlCl.sub.4).sub.3].sup.- (M=Ni, Co, or
Mn). Other solution species have been identified as well. Au goes
into solution as the well-known tetrahedral AuCl.sub.4.sup.- ion.
Vanadium dissolves in (EMIM)AlCl.sub.4 as the square pyramidal
VOCl.sub.4.sup.= ion.
EXAMPLE 1
[0057] Anhydrous lithium chloride and potassium chloride (Sigma
Aldrich, St. Louis, Mo.) was vacuum dried for 48 hours at
140.degree. C. After drying, the powders were removed from the
vacuum oven and immediately placed into a vacuum dessicator before
being transferred to a Vacuum Atmosphere Company dry box. An argon
atmosphere was maintained at all times in the dry box, with oxygen
and moisture concentrations below the detection limit of the
sensors (1 ppm). A 59% LiCl/41% KCL/0.1% Li.sub.3N molar salt
mixture was prepared by grinding the salts together with mortar and
pestle, before transferring to a 100 ml high form alumina crucible
(Fisher Scientific, Pittsburgh, Pa.).
[0058] All electrochemical measurements were performed versus a
lithium/lithium ion reference electrode. The electrochemical cell
was assembled in the glove box with the fuel cell type anode and
cathode electrodes positioned with the active sides facing each
other. The cathode was a sintered nickel gas diffusion electrode
and the anode was a palladium metal membrane hydrogen separator.
The cell was removed from the glove box and connected to the
appropriate gas stream. Nitrogen was used for the cathode and
hydrogen for the anode. Argon was used to provide an inert
substitute for the reactive gases for background measurements. The
current potential curves were recorded using an EG&G Parc Model
175 Universal programmer and an EG&G Model 371
Potentiostat-Galvanostat.
[0059] Ammonia was collected by bubbling gas from the exit line of
the electrochemical ammonia synthesis cell into dilute (pH 3)
hydrochloric acid solution. Ammonium ions (NH.sub.4.sup.+) are
soluble in dilute HCl solution. As desired, the solution was
sampled and ammonium concentration was determined using a Dionex
DX-100 ion chromatograph with a Dionex 4270 integrator. The
concentration of ammonia produced by the electrochemical cell could
then be calculated.
[0060] The cell was assembled as previously described in a dry box
under argon atmosphere. The nitrogen inlet tube on the cathode was
connected to an ultra dry source of nitrogen and the anode attached
to an ultra dry hydrogen source. The outlet of both the anode and
cathode were sealed to the external atmosphere.
[0061] The cell was heated to 550.degree. C. to melt the salts, and
then the temperature was lowered to 500.degree. C. for operation.
Synthesis gases were flowed into the electrodes and the exit gas
from the cell was collected in a dilute HCl solution (pH 3). The
anode and cathode were attached to an EG&G Princeton Applied
Research Model 371 Potentiostat/Galvanostst. The electrochemistry
was controlled using an EG&G Parc Model 175 Universal
Programmer. The cell was run under constant voltage, which was
fixed at 0.382 V vs. Li/Li.sup.+.
[0062] When the potential was applied a current of 16 Amps was
measured. There was a strong smell of ammonia in the headspace
above the collection solution. After 2 minutes of cell operation an
aliquot of the solution was removed and analyzed using a Dionex DX
100 ion chromatograph. A 1.07-ppm standard ammonium solution was
used to identify retention time for the ammonium ion. The standard
had a retention time of 3.85 minutes with a peak area of 9271489.
The 2-minute ammonia sample was run and it was found that the
signal at 3.85 minutes saturated out. The sample was diluted by a
factor of 250 and re-run. A signal was observed at 3.81 minutes
with a peak area of 9992438. Calculating the ammonia concentration
from the chromatograph showed the concentration of ammonium ion in
the collection solution was 288 ppm, which was equivalent to 29 mg
of ammonia produced in the first 2 minutes of cell operation, which
is 1.7.times.10.sup.-3 Moles of ammonia.
[0063] The total charge consumed by the reaction Q=mnF
[0064] Where: m=number of moles of product formed
[0065] n=number of electrons involved in the reaction
[0066] and F=Faraday constant (96455 coulombs mole.sup.-1)
[0067] Based on the results from the ammonia reactor the charged
consumed in the reaction was 987 As. The total charge passed in the
experiment was 1920 As. Therefore the current efficiency for the
ammonia production (charge consumed in reaction of interest/total
charge passed) was 51%.
[0068] It should be noted that the 51% current efficiency is based
on the amount of ammonia collected in the solution. As mentioned
previously there was a strong ammonia smell in the headspace above
the collection solution indicating that not all of the ammonia
being generated was being dissolved into the collection solution.
Therefore 51% current efficiency is a minimum current efficiency
based on the limited collection method.
EXAMPLE 2
[0069] An electrolyte salt mixture was prepared as described in
Example 1 in a high form crucible. A fuel cell type cathode having
a sintered nickel face was used along with an anode made from a
titanium sheet attached to a nickel wire. Both electrodes were
sealed into the reactor cap, placed into the powdered electrolyte
salt. The reactor cap was then secured in position and sealed and
an inert atmosphere was maintained. The reactor was then placed
into a heater unit. Both electrodes were attached to a Tenmax
Laboratory DC Power Supply Model No. 72-420. The nitrogen gas used
to generate the nitride ion was dried using molecular sieves before
it was flowed into the cell. Once a gas flow was established, the
system was back pressured using a valve on the outlet line to
prevent the molten salt from filling the internal cavity. Once the
eutectic salt mixture reached the melting temperature a constant
current of 0.1 Amp was applied for 45 minutes. As the reaction
proceeded, the cell voltage increased with time until reaching a
stable value of 1.0 V. When the reaction was completed the reactor
cap was removed and the electrodes recovered while the salt was
still in the molten state. Visual examination of the titanium
electrode showed that the surface had turned a golden yellow color
that is characteristic of titanium nitride.
[0070] The increase in the cell voltage with time is believed to be
related to changing resistance within the cell and is commonly
observed with film formation. The fact that the electrochemical
reaction was current limited shows that the formation of the
nitride ion is controlled by the mass transfer of the nitrogen gas
through the surface of the porous nickel. The formation of titanium
nitride on the surface of the titanium anode verifies the cell's
ability to generate the nitride ion.
[0071] The term "comprising" means that the recited elements or
steps may be only part of the device and does not exclude
additional unrecited elements or steps.
[0072] While the foregoing is directed to the preferred embodiment
of the present invention, other and further embodiments of the
invention may be devised without departing from the basic scope
thereof, and the scope thereof is determined by the claims that
follow.
* * * * *