Note: Descriptions are shown in the official language in which they were submitted.
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HIGH SURFACE AREA NITRIDE, CARBIDE AND BORIDE
ELECTRODES AND METHODS OF FABRICATION THEREOF
Background - Field of the Invention.
This invention relates generally to capacitors, batteries, fuel cells,
electrochemical synthesis reactors, sensors and other energy
storage%onversion devices, and more particularly to electrodes for use in such
devices.
Background - Discussion of Prior Art.
Electrodes are key elements in several classes of energy storage and
conversion devices, including for example, batteries, fuel cells, and
capacitors.
Technological advances in the electronics industry have created a substantial
and on-going need to reduce electrode volume and weight to attain increased
electrical and electrochemical energy and power densities. In general,
advances in miniaturization and weight reduction of energy storage devices
have not kept pace with the miniaturization and portability of other
electronic
components.
Electrical and electrochemical energy storage and peak power generally
scale with the available surface area of the electrode. Hence, a route to
increasing the ratio of stored energy and peak power to the weight and
volume of the electrodes is to increase the surface area of the electrodes.
Prior art teaches numerous ways to produce materials with high specific
areas (surface area divided by the mass or volume of the bulk material). U.S.
Pat. No. 4,515,763 and U.S. Pat. No. 4,851,206 teach the preparation of such
materials as metallic carbide and nitride powder catalysts. These patents,
however, do not teach conductivity or stability in electrolytic solutions or
the
application of these powder catalysts to electrodes.
Prior art identifies three basic types of high surface area electrodes.
One type consists of metallic bodies which are mechanically or chemically
etched to provide a roughened surface and high specific surface area. High
surface area electrodes based on etched or patterned metal surfaces are cited
in U.S. Pat. No. 5,062,025. A second type of high surface area electrode is
based on carbon powders or foams, as cited in U.S. Pat. No. 5,079,674 and
U.S. Pat. No. 4,327,400. The third class of high surface area electrodes is
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based on conductive metal oxides, e.g. ruthenium oxides, as taught by U.S.
Pat. No. 5,185,679. While each of these types of electrodes are the basis of
commercial electrical or electrochemical energy storage and conversion
devices, they are lacking in performance with respect to one or more of the
criteria listed in the Summary below.
Specifically, currently available high surface area metal electrodes are
limited by electrochemical stability. Metals are generally unstable in
oxidizing
environments, therefore their use is limited to the positive, reducing
electrode
or anode.
High surface area carbon electrodes are limited by their relatively low
electrical conductivity and difficulty in controlling the pore size
distribution
and surface area. Most high surface area carbon-based electrodes are formed
by dispersing and bonding the carbon materials onto more highly conductive
supports or substrates. These multi-step processes require the use of
dispersants, binders and conductivity enhancing additives.
High surface area ruthenium oxide based electrodes are also limited by
electrochemical stability and by the cost and availability of the electrode
materials. Prior art has shown that additives can be used to stabilize the
high
surface area of the ruthenium oxide when used as the negative, oxidizing
electrode or cathode. Unfortunately, ruthenium oxide, and metal oxides in
general cannot be stabilized for use as the positive, reducing electrode, or
anode. These materials are limited to positive electrical potentials of the
order
of 1.2 volts, beyond which electrochemical reactions occur, resulting in the
irreversible degradation of the electrode materials.
Summary Of The Invention.
For use as electrodes, additional chemical and physical properties are
desired. The electrode material must be substantially and highly electrically
conductive, with the possible exception of a thin dielectric or passivating
layer
on the exposed surfaces. The high surface area electrode material must also =
be chemically and physically stable under a range of processing and operating
environments. Specifically, the electrode material must retain its high
surface
area and pore size distribution in the presence of the ion-mobile electrolytes
typically used in electrical and electrochemical energy storage and conversion
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devices. Furthermore, it is desirable that the electrode material retain these
desirable properties under a range of positive and negative electrical
potentials
which may occur, by design or unintentionally, during the operation of the
energy storage and conversion devices. In addition to having high surface
area, high electrical conductivity, and physical/chemical stability, the
electrode
materials should be easily wetted by the ion-mobile electrolyte, should be
amenable to existing manufacturing processes and production equipment, and
should be assembled from inexpensive, widely available, and environmentally
acceptable materials.
The present invention provides a new type of high surface area
electrode for use in electrical and electrochemical energy storage and
conversion devices. The electrode comprises conductive transition metal
nitrides, carbides, borides or combinations thereof, where the metal is
typically molybdenum or tungsten.
In another embodiment there is provided a method of manufacturing
the electrode of this invention, comprising forming or depositing a layer of
metal oxide where the metal is typically molybdenum or tungsten, then
exposingthe metal oxide layer at elevated temperature to a source of nitrogen,
carbon, or boron in a chemically reducing environment to form the desired
metal nitride, carbide or boride film. With careful control of the exposure
conditions, the nitridation, carburization, or boridation chemical reaction
can
lead to greatly enhanced specific surface areas relative to the film
precursors.
Nitrogen, carbon and boron sources are typically ammonia, methane, and
diborane, respectively.
In another embodiment there is provided an ultracapacitor device
comprised of the subject high surface area electrodes having a specific
capacitance of 100 mF/cm2 and an energy density of 100 mJ/cm3 with
improved conductivity and chemical stability when compared to currentiy
available electrodes.
Brief Description Of The Drawings.
FIG. 1 is a block flow diagram illustrating method steps for fabricating
the high surface area electrodes.
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FIG. 2 is an example of the temperature program used to convert an
oxide film to a high surface area electrode.
FIG. 3 is a block flow diagram illustrating other method steps for
fabricating the high surface area eiectrodes.
FIG. 4 is a schematic and enlarged illustration of a capacitor in
accordance with the invention.
FIG. 5 is a scanning electron micrograph of an oxide film prior to
nitridation, carburization and/or boridation at a magnification of 1,000X.
FIG. 6 is a scanning electron micrograph of a film after nitridation,
carburization and/or boridation at a magnification of 8,000X.
Description Of The Preferred Embodiments.
High specific surface area nitrides, carbides and borides are produced
by reacting a precursor with a source of nitrogen, carbon or boron at an
elevated temperature, or by interconverting the nitride, carbide or boride by
reaction with an appropriate source of nitrogen, carbon or boron.
Referring to FIG. 1, metal oxide or soluble precursors are mixed into
an appropriate solvent in the desired concentrations. Suitable compounds are
materials such as water soluble salts, organometallic complexes and alkoxides
of metals such as chromium, molybdenum, tungsten, vanadium, niobium,
tantalum, titanium and zirconium. These metals and others are selected from
Groups III, IV, V, VI and VII of the Periodic Table. The solution is applied
by
dip coating or spray deposition to a substrate. Other deposition methods such
as physical vapor deposition (evaporative coating) or plasma arc spraying may
be selected. Suitable substrates are materials like titanium, zirconium,
tantalum, molybdenum, tungsten and ruthenium oxide. These metals and
others are selected from Groups IV, V, VI, VII and VIII of the Periodic Table.
Once the precursor solution has been applied to the substrate, the material is
dried at an appropriate temperature.
The dried material is chemically converted to the oxides by reacting
with an oxidizing agent. Alternately, the surface of an appropriate substrate
material can be oxidized by reaction with a suitable oxidizing agent or an
oxide film can be deposited from the vapor phase onto the substrate. Suitable
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oxidizing agents are materials such as oxygen, water, nitrogen oxides, and
carbon oxides.
The oxide film is chemically converted to the nitride, carbide and/or
boride by reacting with a reductant as the temperature is increased in a
5 controlled manner. The rate of increase in the temperature may be linear (0
K/hr to 500 K/hr), for example as illustrated in FIG. 2, or nonlinear but
should
be uniform without sudden changes in rate. Suitable reductants include
ammonia, hydrazine, nitrogen, methyl amine, methane, ethane, borane and
diborane. The reaction should be rapidly quenched after completion or held
at the final reaction temperature (500 K to 1300 K) for a period of time then
quenched to room temperature. Hydrogen and/or inert gas may be added to
assist the conversion. The thickness of the nitride, carbide and/or boride
film
determines the final surface area or capacitance. Capacitance generally scales
with film thickness; however, with sufficient increase in thickness, the
capacitance eventually approaches an asymtotic limit.
Referring to FIG. 3, a high surface area nitride film produced using
methods illustrated in FIG. 1 may be converted into the carbide or boride.
The nitride is chemically converted to the carbide and/or boride by reacting
with a suitable reactant such as methyl amine, methane, ethane, borane and/or
diborane. The reaction may be carried out isothermally or in a temperature
programmed manner. After completion, the reaction should be rapidly
quenched or held at the final reaction temperature for a period of time then
quenched to room temperature. Hydrogen and/or inert gas may be added to
assist the conversion. The thickness of the carbide and/or boride film
determines the final surface area or capacitance.
The nitride intermediate step followed by conversion to carbide or
boride appears to provide better properties for an ultracapacitor; however,
direct conversion from oxide to carbide or boride can be accomplished with
a suitable reductant as noted above.
The high surface area nitride, carbide and/or boride film may be
passivated by exposing the materials to a dilute mixture of an oxidizing agent
for a short period of time.
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In order to better understand the structure of the capacitor, we refer
now to an enlarged view of a portion of the device in FIG. 4. A separator xx
is applied to a high surface area electrode zz. A second electrode yy is used
to sandwich the separator xx. The first and second electrodes do not have to
be made of the same materials. For example, one electrode may be a high
surface area nitride, carbide and/or boride while the other is a ruthenium
oxide-based material. The oxide-based electrode material may be an oxide
formed on a base metal surface or foil. The sandwich is impregnated with an
ion-mobile electrolyte solution. Suitable electrolytes include aqueous
sulfuric
acid, a soiution of lithium perchlorate in propylene carbonate or a solution
of
tetrabutyl ammonium fluoride in acetonitrile. Special care must be taken to
dry the non-aqueous electrolytes. Residual water can be removed from the
impregnated sandwich by electrolysis (applying a positive potential of 2-4 V).
An alternative capacitor can be fabricated by using a solid electrolyte
in place of the liquid electrolyte and separator. The solid electrolyte must
be
infiltrated or diffused into the pore structure of the high surface area
electrodes.
Variations in the construction of the electrode and devices described
herein, while not described in detail, will be obvious to those with ordinary
skill in the art, and would not be construed as being beyond the scope of the
invention. For example, one practiced in the state of the art for
electrochemistry will also perceive that variations of the electrode materials
described herein may have advantageous applications in fuel cells,
electrochemical synthesis reactors, catalysts, and sensors.
EXAMPLES
The following test examples are offered by way of example and not by
way of limitation.
The molybdenum oxide films were deposited onto high purity Ti
(99.7% 0.0127 mm, Aldrich) Mo (99.9% 0.025 mm, Aldrich) foils. The foils
were cleaned prior to depositi.ng the oxide in order to remove any organic
residue or surface oxides. The Ti foils were cleaned at room temperature
using a 2:1 mixture of 12 M nitric acid and 50% hydrofluoric acid. The foil
substrates were immersed in the acid solution until red fumes evolved at
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which time the substrate was removed and rinsed with copious amounts of
distilled water. The Mo foils were cleaned by immersion in an aqueous 18
M sulfuric acid bath at - 75 C. The foil cleaning procedures were adapted
from methods described in the Metal Finishing Guidebook and Directory
(1993). After a period of 15 minutes or until a water-break-free surface was
obtained, the foils were removed from the acid bath and then rinsed with
distilled water. After cleaning, the substrates were placed immediately in the
coating solution in order to minimize oxidation prior to deposition.
Aqueous solutions of ammonium paramolybdate, (NH4)6Mo,O2404HZO
(99.999%), Johnson Matthey), were used to deposit the molybdenum oxide
coatings. After an appropriate amount of the salt was dissolved in distilled
water, the solutions were acidified with 10% nitric acid. The coating
solutions
were initially stirred using the substrate to ensure that the solution
concentrations were uniform. The substrates were suspended at least one
centimeter below the liquid surface for 5 minutes. The substrates were then
drawn out of the solution at a draw rate of 1 s/cm. The coated substrates
were dried on a hot plate (the temperature was less than 90 C) prior to
calcination.
The molybdate coatings were converted into MoO3 by calcination in
stagnant air for 30 minutes at temperatures less than 550 C.
Temperature programmed nitridation of the MoO3 films was carried out
in a specially designed reactor. This reactor was constructed from a one inch
diameter quartz tube and fitted with a water jacket to cool the effluent
gases.
The calcined substrates were placed on a firebrick inside the reactor which
was placed in a Lindberg SB tube furnace. The temperature was controlled
using an Omega CN2010 programmable temperature controller with a
= chromel-alumel thermocouple. High purity NH3 (99.99%, Matheson) was
used for nitridation. Referring to FIG. 2, the reaction temperature was
quickly
increased from room temperature to 350 C over 30 minutes. Two linear
heating segments were employed in nitriding the oxide films. The
temperature was increased from 350 to 450 C at rate 91 then from 450 to
700 C at a rate S2. Subsequently, the temperature was held constant at
700 C for one hour. After the nitridation program was completed, the
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materials were cooled to room temperature in flowing NH31 then passivated
in a fiowing mixture of 1.06% 02 in He for one hour in order to prevent bulk
oxidation. The gas flow rates were monitored using a calibrated rotameter and
controlled by needle valves.
The MoO3 films supported on Mo substrates possessed porous
microstructures and consisted of plate-like particles averaging 2Nm in
thickness
and 10Nm in diameter (see FIG. 5). The MoO3 supported on Ti substrates was
also porous but consisted of very fine grains approximately 21im in diameter.
The gross morphologies of the nitrided films were similar to those of the
oxides, however, the nitride particles contained very fine cracks (see FIG.
6).
The development of cracks would lead to the exposure of internal surface area
and production of high surface area materials. Finally, it was observed that
there was much less surface charging during scanning electron micrography
(SEM) for the nitride films than for the oxide films, which is consistent with
the
nitride films being electrically conductive.
The weights and BET (Brunauer, Emmett and Teller) surface areas of the
Mo nitride films increased with each dip. The materials listed in Table 1 were
prepared via the temperature programmed nitridation of MoO3 films in flowing
NH3 (100 cm3/min) using first and second heating rates of 40 and 200 C/h,
respectively. The choice of substrate had a marked effect on the weight of y-
MoZN generated. The weights and surface areas of the nitride films supported
on Mo substrates were generally higher than those of the films supported on
Ti substrates. The surface area increased nearly linearly with film weight for
both substrates indicating that the specific surface area was not a strong
function of the substrate employed. There was no evidence of delamination,
occlusion or consolidation. These observations suggested that the films were
porous and that the nitride surface area per unit substrate area can be
increased by increasing the coating mass and thickness.
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Table 1. _ Weights and Surface Areas of the Mo Nitride Films
Sample Weight Surface Area Specific
(mg) (cm2/cm2) Surface Area
(m2/gr)
Mo-5.0-0.2-1-1 1.3 735 57
Mo-5.0-0.2-1-2 2.7 1469 54
Mo-5.0-0.2-1-4 4.7 3131 67
Ti-5.0-0.2-1-1 0.5 88 18
Ti-5.0-0.2-1-2 0.5 346 59
Ti-5.0-0.2-1-4 2.8 1037 37
The surface areas were functions of the heating rates and flow rate
employed during nitridation. Effects of the nitridation conditions on the
surface areas of the Mo nitride films can be deduced from the results given in
Table 2. When the nitridation program with 9, and R2 equal to 100 and
200 C/h, respectively, was employed, the surface area decreased as the flow
rate was increased. A similar behavior was observed when both of the
heating rates were 100 C/hr. The opposite effect was observed when the
heating schedule with 91 and R2 equal 40 and 200 Gh, respectively, was
used.
Effects of similar magnitude were observed on varying the first heating
rate 91. When the nitridation was carried out using the low flow rate (100
cm3/min), increasing 9, caused a decrease in the surface area. For films
prepared using the high flow rate (1000 cm3/min), increasing 9, caused an
increase in the surface area. The most significant changes were observed
when the second heating rate was varied. Increasing SZ caused a marked
decrease in the surface area. Furthermore, surface areas in excess of 70 mZ/gr
were achieved using the low value for 92.
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Table 2. Effects of the Nitridation Conditions
on the Mo Nitride Surface Areas
Sample B, (?Z QNH3 Surface Area Specific
( CJhr) ( Uhr) (cm3/min) (cmZ/cmZ) Surface Area
(mZ/gr)
Mo-5.0-0.2-1-2A 100 200 1000 3574 44
5 Mo-5.0-0.2-1-2B 100 200 100 1868 37
Mo-5.0-0.2-1-2C 100 100 1000 2213 105
Mo-5.0-0.2-1-2D 100 100 100 1314 73
Mo-5.0-0.2-1-2E 40 200 1000 1970 35
Mo-5.0-0.2-1-2F 40 200 100 1469 54
10 The capacitor test cell consisted of a 100 ml round-bottom flask with
a tapered ground-glass joint. A rubber stopper with holes for the leads was
used to seal off the flask. Either 2.39 M LiCIO4 (99.99%, Aldrich) in
propylene carbonate (99% anhydrous, )ohnson Matthey) or 4.18 M HZSO4 in
distilled water was used as the electrolyte. The cell containing the
perchlorate
solution was assembled in a N2-filled glovebox to avoid exposure to moisture.
Electrical measurements using the HZSO4 solution were carried out in ambient
air. Fisherbrand P8, coarse porosity filter paper was used to separate the
electrodes in the perchlorate solution while Fisherbrand glass fiber circles
(coarse porosity) were used for the acid solution. The capacitor was
fabricated
by placing a separator between two electrodes and clamping them together
with an alligator clip to secure the assembly. Additional shielded alligator
clips were used to connect the foil leads to the voltage source and the
coulometer. The cells were charged with a constant voltage source supplied
by 1.5 Vdc batteries used separately or in a series of four to obtain voltages
greater than 6 Vdc. The total charge capacity was measured while allowing
the capacitor cell to fully discharge. The capacitance was taken as the total
charge stored divided by the charging voltage. The total charge stored by the
capacitor was measured using an EG&G 2790A coulometer, which was
calibrated against two commercial capacitors.
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Tables 3 and 4 summarize effects of the film properties, electrolyte
composition, and charging voltage on the electrical performance of the test
capacitors. Charging times between 1 and 10 minutes were used and the
specific capacitance is based on the superficial area of the substrate (m 1
cm2).
The capacitances of the cells fabricated from the nitride films were much
greater than those of blank cells assembled using uncoated metal substrates.
This clearly indicated that the capacitance was due to the presence of the
nitride film. Furthermore, the electrical properties were reproducible through
several charge/discharge cycles suggesting that the films were stable even at
voltages greater than 6V.
In every case, the capacitors fabricated using the Mo substrates had
higher capacitances than those consisting of the Ti substrates. The choice of
electrolyte also played a major role in determining the charge storage
capacity.
The specific capacitances achieved using the HZS04 electrolyte approached 1
F/cm2 while those for the LiCIO4 electrolyte ranged from 0.02 to 0.14 F/cm2.
The capacitance increased linearly with the surface area of the Mo
nitride film for the cell using HZSO4 as the electrolyte. This result clearly
demonstrated four properties of the Mo nitride. (1) y-MoZN is an electrically
conducting phase. (2) The high surface area Mo nitrides prepared using the
methods outlined above formed contiguous films. (3) The capacitance for the
y-M02N based electrodes was likely due to a surface and not a bulk charge
storage process. (4) The capacitance of 200,uF/cmZ is superior to that
reported
for high surface area ruthenium oxide electrodes with HZSO4 electrolyte
(Raistrick and Sherman, 1987).
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Table 3. Properties of Mo Nitride Electrodes
with LiCIO4 Electrolyte (1.6 Vdc)
Sample Charging Total Charge Specific
Voltage Stored Capacitance
(V) (C) (mF/cm2)
Mo Foil (cleaned) 1.59 0.0004 0.2
Mo-5.0-0.2-1-2A 1.58 0.068 43
Mo-5.0-0.2-1-2 B 1.60 0.034 22
Mo-5.0-0.2-1-2C 1.59 0.072 45
Mo-5.0-0.2-1-2D 1.59 0.127 80
Mo-5.0-0.2-1-2 E 1.60 0.097 61
Mo-5.0-0.2-1-2F 1.57 0.119 76
Ti Foil (cleaned) 1.59 0.0004 0.2
Ti-5.0-0.2-1-2 1.59 0.027 17
Table 4. Properties of Dip-Coated Mo Nitride
Electrodes with HZSO4 Electrolyte
Sample Charging Total Charge Specific
Voltage Stored Capacitance
(V) (C) (mF/cm2)
Mo Foil (cleaned) 1.59 0.265 167
Mo-5.0-0.2-1-2A 1.58 1.39 880
Mo-5.0-0.2-1-2 B 1.60 0.643 405
Mo-5.0-0.2-1-2C 1.59 0.701 441
Mo-5.0-0.2-1-2D 1.57 0.790 454
Mo-5.0-0.2-1-2E 1.60 0.638 401
Mo-5.0-0.2-1-2F 1.57 0.791 504
Ti Foil (cleaned) 1.59 0.00007 0.04
Ti-5.0-0.2-1-2 1.59 0.059 37 25 Molybdenum nitride electrodes were also
prepared by spray deposition
of paramolybdate precursors. Solutions of the paramolybdate precursor were
prepared as described above for spray deposition of the oxide coatings. An
ultrasonic spray system or atomized spray deposition can be used to deposit
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the oxide precursor onto the substrates. The process of spray deposition and
calcination were alternated to build up a multi-layer of the oxide. The
substrates were heated to a temperature of about 150 C to evaporate the
solvent and enhance adhesion, and then calcined as described above. The
heating rates (B1 and .B2) and capacitance results for the spray coated
molybdenum nitride electrodes are shown in Table 5.
Table 5. Properties of Spray Deposited Mo Nitride
Electrodes with HZS04 Electrolyte
0, ( C/hr) .82 ( C/hr) Specific Capacitance
(F/cm~
80 400 1.65 + 0.04
100 1.91 + 0.05
80 100 1.61 0.02
80 200 1.86 + 0.04
20 200 1.54 0.03
15 40 400 1.86 + 0.07
40 100 1.11 + 0.06
The electrode capacitances in Table 5 were obtained by coulometry as
described previousiy. The electrolyte used was 4.5 M. sulfuric acid. Each
capacitor was charged at 1.0 V with a current of 150 mA for a duration of 5
20 minutes to ensure charge saturation. These results demonstrate that spray
deposition can also be used to produce molybdenum nitride electrodes with
beneficial high surface areas.
Other new high surface area electrodes were also prepared as
demonstrations of the materials and methods of this invention.
Molybdenum carbide electrodes were prepared from molybdenum
nitride electrodes, prepared using the spray deposition, calcination, and
nitridation methods as previously described. The nitride was converted to
carbide by temperature programmed reaction with a flowing 1:3 mixture of
methane/hydrogen. The temperature programmed reaction heating profiles
and reactant flow rates are provided in Table 6 below. Cyclic voltammetry
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was performed to determine the capacitance and stability of the carbide films
in saturated aqueous KCI and LiCI electrolyte solutions. Capacitance, C, was
determined by dividing the voltammetry, i, by the potential stepping rate,
(dE/dt) over the plateau region of the voltammogram: C-i/(dE/dt). The
sample electrode served as the working electrode in a standard three electrode
cyclic voltammetry configuration. The capacitance data indicate that
molybdenum oxide precursors can be converted into carbide electrodes with
beneficial high surface areas.
Table 6. Capacitance of Carbide Films
Heating Rate Flow Rate Capacitance Capacitance
df3 ( C/Hr) (cc/min) (F/cm2) (F/cm2)
KCI electrolyte LiCI electrolyte
22 100 2.32 2.18
22 500 1.37 1.51
67 100 1.61 1.03
67 500 1.11 0.88
Tungsten/molybdenum nitride electrodes were prepared by spray
deposition of paratungstate/paramolybdate solutions. Solutions of the mixed
W/Mo oxide precursors were prepared by mixing equal volumes of saturated
solutions of (NH4)6Mo7O24.4H20 and (NH4)10W,2047.5H20. Spray deposition,
calcination, and nitridation were performed as described in previous examples.
Capacitance was evaluated by cyclic voltammetry. Capacitance for these films
is reported in Table 7. The capacitance data confirm that W/Mo solid oxide
solutions can also be converted to beneficial high surface area electrode
films.
Table 7 also shows the cell loading in mg/cm2. Since the specific capacitance
scales with loading, one may increase electrode capacitance by increasing the
loading.
Vanadium nitride electrodes were prepared by spray deposition of
saturated ammonium vanadate (NH4)VO3 solutions. Spray deposition,
calcination, and nitridation were performed using the procedures as described
for molybdenum nitride electrodes. Capacitance was evaluated by cyclic
voltammetry. The capacitance for these films as reported in Table 7 confirm
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that vanadium, a group Va element, can also be converted to beneficial high
surface area electrode films.
Table 7.
Electrode Material Loading (mg/cm2) Capacitance (F/cmZ)
5 molybdenum nitride 13 1.9
tungsten/ 5.5 0.3
molybdenum nitride
vanadium nitride 4.9 0.3
