Note: Descriptions are shown in the official language in which they were submitted.
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ONE-STEP ELECTROSYNTHESIS OF BOROHYDRIDE
The present invention is directed to a method for one-step
electrosynthesis of borohydride.
Several electrolytic processes for production of borohydride have
been described in the literature, for example, in U.S. Pat. No. 3,734,842, to
Cooper. However, a study performed by E.L. Gyenge and C.W. Oloman,
and documented in Journal ofApplied Electrochemistry, vol. 28, pp. 1147-
51 (1998), demonstrated that the method of Cooper, as well as several
other published electrosyntheses of borohydride, actually does not produce
measurable amounts of borohydride.
The problem addressed by this invention is the need for an
electrochemical synthesis of borohydride.
STATEMENT OF THE INVENTION
The present invention is directed to a method of producing
borohydride. The method comprises causing current to flow in an
electrolytic cell between an anode and a cathode, wherein a solution of a
boron-containing compound is in contact with the cathode, and wherein
the cathode comprises a conductive material having activity as a high
hydrogen overpotential electrode.
In another aspect, the method comprises a method for producing
borohydride; said method comprising causing current to flow in an
electrolytic cell between an anode and a cathode, wherein a solution of a
boron-containing compound is in contact with the cathode, and wherein
the cathode comprises a synthetic polymer and at least one metal on the
surface of a high surface-area electrode; wherein the boron-containing
compound is a trialkyl borate or an acid or salt of a complex ion containing
only boron and oxygen; and wherein borohydride anions formed at the
cathode are prevented from migrating to the anode.
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DETAILED DESCRIPTION OF THE INVENTION
As used in this application, "borohydride" means the
tetrahydridoborate ion, BH4-.
In the electrolytic reaction of the present invention, borohydride
anions formed at the cathode are prevented from migrating to the anode.
In one embodiment of the invention, this is accomplished by providing a
cation-selective ion exchange membrane to separate the anode and
cathode compartments. The cation-selective membrane allows sodium, or
other cations, to cross into the cathode compartment to balance the charge
that would otherwise accumulate from production of hydroxide and
borohydride at the cathode. In another embodiment, the anolyte is acidic,
and protons cross the membrane into the cathode compartment and
maintain a relatively neutral pH therein. As an alternative to an ion-
exchange membrane, a microporous separator may be used
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to allow ions to cross in either direction; in this case, borohydride would
cross
over into the anode compartment to some extent and be oxidized.
In one embodiment of the invention, the electrolytic reaction occurs in a
non-aqueous solvent in which borohydride is soluble, e.g., C1-C4 aliphatic
alcohols, e.g., methanol, ethanol; ammonia; C1-C4 aliphatic amines; glycols;
glycol
ethers; and polar aprotic solvents, for example, dimethylformamide (DMF),
dimethylacetamide (DMAc), dimethyl sulfoxide, hexamethyl phosphoramide
(HMPA), and combinations thereof. Preferably, the non-aqueous solvent is
methanol, ethanol, DMF, HMPA, or combinations thereof. Preferably, the
amount of water present in non-aqueous solvents is less than 1%, more
preferably less than 0.1%, more preferably less than 100 ppm, and most
preferably the non-aqueous solvents are substantially free of water.
In another embodiment, the electrolytic reaction occurs in an aqueous
solvent or an aqueous/organic solvent mixture having more than 1% water.
Organic solvents used in an aqueous/organic solvent mixture are those having
sufficient solubility in water to form a solution.
Preferably, when protic solvents are used, especially water, methanol or
ethanol, alkali is present to stabilize the borohydride, preferably at least
0.1 N
alkali.
Preferably, the boron-containing compound of the present invention is a
salt or acid of a boron-containing ion, or a trialkyl borate, B(OR)s, wherein
R
preferably is methyl or ethyl. Preferably, the boron-containing ions used in
the
present invention are complex ions containing only boron and oxygen. More
preferably, the boron-containing ions are borate, tetraborate, or metaborate.
Most preferably, the boron-containing ion is metaborate or tetraborate.
A synthetic polymer used in the present invention includes, for example,
polyolefins, e.g., polymers made from monomers comprising ethylene, propylene,
other ethylenically unsaturated hydrocarbons or mixtures thereof polymers
made from monomers comprising halogenated olefins, e.g., halogenated
ethylenes; polystyrenes; polyethers; polyvinyl alcohols; polyamides; and
mixtures
thereof. In one embodiment of the invention, an addition polymer made from
ethylenically unsaturated monomers is used. In one embodiment of the
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invention, a hydrophobic synthetic polymer is used, e.g., an addition polymer
substantially free of atoms other than carbon, hydrogen and halogen atoms. In
one preferred embodiment, a hydrophobic synthetic polymer used in the present
invention is an addition polymer comprising at least 50% by weight of monomer
units derived from one or more fluorinated ethylene monomers, e.g.,
tetrafluoroethylene, 1, 1 -difluoroethylene, or trifluoroethylene. More
preferably,
the hydrophobic synthetic polymer comprises at least 75% of monomer units
derived from one or more fluorinated ethylene monomers. Most preferably, the
hydrophobic synthetic polymer is poly(tetrafluoroethylene) (PTFE).
For the purposes of this invention, a high hydrogen overpotential electrode
is one where the reduction potential for electrolysis of water to form
hydrogen
under the reaction conditions is approximately equal to, or more negative than
the reduction potential for borate reduction. The theoretical reduction
potential
for borate reduction is -1.24 volts vs. a standard hydrogen electrode ("SHE").
In
one embodiment of the invention, the high hydrogen overpotential electrode
comprises a metal inherently having such activity, for example, lead, zinc,
cadmium, mercury and indium. In another embodiment, the electrode comprises
a high-surface-area electrode, preferably a carbon high-surface-area
electrode.
Examples of suitable carbons are carbon cloths and felts, vitreous carbon, and
reticulated vitreous carbon. The term "high-surface-area" means having a
surface area of at least 0.005 m2/g. Reticulated vitreous carbon foam having
approximately 10 pores per inch typically has a surface area of about 0.01
m2/g.
Carbon felt or cloth typically has a surface area of about 0.5 m2/g. Carbon
black
and gas diffusion electrodes fabricated with carbon black typically have a
surface
area of about 200 mz/g or more.
A "nickel screen electrode" is an expanded nickel mesh. An example is the
DelkerT"I 416 nickel mesh, having diamond-shaped openings, 0.416 inches x
0.170 inches, with a strand thickness of 0.005 inches, and approximately 75%
open space.
Preferably a cathode that comprises a synthetic polymer and a conductive
material having activity as a high hydrogen overpotential electrode comprises
a
mixture of the synthetic polymer and the conductive material supported on a
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metal or graphite base electrode. Preferably the conductive material is a
metal.
Preferably, the cathode is formed by plating from a mixture of polymer
particles
suspended in water and a solution containing a salt of the metal. The base
electrode onto which the mixture is plated preferably comprises the same metal
as the metal salt which is plated.
When the cathode comprises a synthetic polymer and a conductive
material having activity as a high hydrogen overpotential electrode,
preferably
the current density is no greater than 100 mA/cm2, more preferably no greater
than 75 mAJcm2, and most preferably no greater than 50 mA/cm2.
In one embodiment of the invention, the cathode comprises a synthetic
polymer and at least one metal on the surface of a high surface=area
electrode.
In this embodiment, the metal has activity as a high hydrogen overpotential
electrode due to the presence of the synthetic polymer. The metal preferably
is
nickel, an alloy comprising two metals, or a metal inherently having activity
as a
high hydrogen overpotential electrode. An alloy comprising two metals, A and
B,
preferably is of the form AB5, AB, A2B or AB2. Preferably at least one of the
metals is a transition metal. In one embodiment, at least one of the metals is
a
rare earth metal. In one embodiment, A and B are both transition metals.
Preferably, one of the metals is La, Ni, Ti or Zr. In one embodiment, AB5 is
LaNi5, optionally with additional metals, e.g., Sn, Ge, Al or Cu. In one
embodiment in which the alloy is of form AB2, the metals are Ti and Zr,
optionally with additional metals, e.g., Mn, Cr, Fe, V or Ni. In one
embodiment
in which the alloy is of form A2B, the alloy is Mg2Ni. In one embodiment in
which the alloy is of form AB, it is FeTi.
A gas diffusion electrode (GDE) is one that enables direct electronic
transfer from a gas phase to or from a solid phase. The GDE also provides a
path for ionic transfer. A GDE typically comprises a conductive porous
support,
e.g., carbon cloth, carbon paper or metal mesh. The GDE often has a wet-
proofing layer of carbon black, and optionally additional layers of wet-
proofing.
Finally, an electrocatalyst layer typically is applied to the surface, or is
applied
to carbon black prior to electrode assembly. The electrocatalyst facilitates
reduction of boron compounds over reduction of water. The wet-proofing
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material may be a synthetic polymer, as described above, e.g., PTFE, which may
be applied as an emulsion in water. A heat treatment often is applied as a
final
step to soften the polymer and embed the materials into a single substrate.
Optionally the GDE comprises a highly dispersed metal electrocatalyst which
5 can act as a very high surface area cathode. Hydrogen generated at the
cathode,
or alternatively, fed to the back of the electrode, may provide an activated
catalyst which allows in situ hydride formation.
In an embodiment of the invention in which a gas diffusion electrode
comprising a metal electrocatalyst is used, preferably the current density is
greater than 120 mA/cm2. Preferably the amount of metal which is present on
the surface of the GDE is less than 2 mg/cm2.
In aqueous systems, the predominant anode reaction is the electrolysis of
water to form oxygen and protons. If the anolyte is acidic, protons will
transport
across the separator and neutralize the hydroxide that is generated at the
cathode along with borohydride. If the anolyte is basic, the protons will
neutralize the hydroxide in the anode compartment and sodium will transport
across the separator to make byproduct sodium hydroxide. In one embodiment,
the anode is a non-corroding material, for example, platinized titanium or
iridium oxide on titanium. If the anolyte is basic, a lower-cost material
would be
quite stable, e.g., nickel. In a non-aqueous system, the anode could be a
corrosion-resistant metal, e.g., platinum.
In one embodiment, the anolyte is an aqueous sodium salt, e.g., sodium
hydroxide, sodium carbonate or sodium bicarbonate. Protons generated would
then form stable species like water or carbon dioxide. Alternatively, any
aqueous
mineral acid would be suitable. In the case of a non-aqueous solvent, an
organic-
soluble conductive sodium salt would be suitable, e.g., a sodium alkoxide, or
a
lithium salt soluble in the non-aqueous solvent.
Other components may be used in the method of this invention to improve
yield of borohydride, including additives that would improve solvation in non-
aqueous systems; lithium or ammonium salts to raise hydrogen overpotential;
and redox species, e.g., naphthalene or anthracene.
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EXAMPLES
Example 1
Analytical Method for Borohydride Determination: The method of M.V. Mirkin
and A.J. Bard, Analptical Chem., vol. 63, pp. 532-33 (1991) was modified such
that borohydride was oxidized at a gold rotating disk electrode (800 rpm) at
approximately -0.150 V vs. a saturated calomel electrode (SCE). The height of
the wave is dependent on the square root of the rotation rate and independent
of
the scan rate. The voltammetric sweep was performed at 100 mV/sec. The
sensitivity of this method allows borohydride to be detected at levels below 1
ppm.
Experiments were performed in small divided glass H-cells when utilizing
non-porous cathodes, and in the ASTRIS QUICKCELL 200 test cell, which has
two acrylic compartments which individually feed to opposite sides of a
membrane separator, when utilizing gas diffusion electrodes. Catholyte volumes
were from 75 to 125 mL, and anolyte volumes from 35-55 mL. Some of the
experiments were performed using PTFE-nickel composite electrodes.
Preparation of these electrodes is similar to that described in Y. Kunugi et
al., J.
Electroanal. Chem., vol. 313, pp. 215-25 (1991). To a nickel sulfamate bath
(225
g Ni(NH2SO3)2 and 20 g HaBOs in 0.5 L H20) was added 80 mL of PTFE solution
(TEFLON 30b solution - 30% TEFLON powder in H20). The composite cathode
was prepared by plating the PTFE-nickel material from the bath onto a nickel
plate (5 cm2) at 20 mAlcm2 for 1400 coulombs of charge. Borate reduction was
then performed in a glass H-cell (1.0 M tetramethylammonium hydroxide
(TMAH), 0.5 M HaBOa catholyte, NAFION 324 cation exchange membrane
(available from DuPont Co.), 1.0 M NaOH anolyte, room temperature, platinum
anode).
The electrode made with Misch metal (LaNi5) was prepared by grinding
Misch metal and sieving to 100 mesh, thus providing a maximum particle size of
150 micron. An electrode was prepared by adding polyvinyl alcohol powder to 5%
by weight and compressing onto a nickel screen and heat treating to provide a
homogeneous electrode. The Misch metal concentration of the electrode was 425
mg/cm2.
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All experiments are described in Table 1. All experiments utilized a
NAFION 324 cation exchange membrane, with a 1.0 M NaOH anolyte and a Pt
or Pt/Nb anode, under current control with a DC power supply. The initial
experiment, utilizing TMAH, showed the presence of borohydride with the cyclic
voltammetric analysis method. The peak for borohydride was shifted from -0.15
V to -0.10 V, which is believed to be due to the higher concentration of
hydroxide
present in the actual sample from the electrolysis.
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Table 1
Cathode Catholyte Current Charge, BH4-
Density Coul. Current
(mA/cmz) Eff. (%)
PTFE-Ni LOM TMAH, 0.5M H3BO3 50 3154 2.9
PTFE-Ni 1.OM TMAH, 0.5M H3B03 120 3780 <0.05
PTFE-Ni 1.OM NaOH, 0.5M H3B03 50 2225 <0.05
PTFE=Ni 1.OM NaOH, 0.5M H3B03 150 2900 <0.05
Ni/C GDE, 1.OM NaOH, 0.5M H3B03 80 3200 <0.05
1.0 mg Ni/cm2
Au/C GDE, 1.OM NaOH, 0.5M H3B03 40 2501 <0.05
0.22 mg Au/cm2
Au/C GDE, 1.OM NaOH, 0.5M H3B03 160 2713 <0.05
0.22 mg Au/cm2
Raney Ni GDE, 1.OM NaOH, 0.5M H3B03 25 2100 <0.05
300 mg Ni/cm2
Raney Ni GDE, 1.OM NaOH, 0.5M H3B03 100 2400 <0.05
300 mg Ni/cm2
Rh/C GDE, 1.OM NaOH, 0.5M H3B03 25 2250 <0.05
mg Rh/cm2
Zn/Ni screen, 1.OM NaOH, 0.5M H3B03 40 2484 <0.05
300 m /cm2
Zn/Ni screen, 1.OM NaOH, 0.5M H3BO3 90 2429 <0.05
300 m /cm2
NiO-Co203/C 1.OM NaOH, 0.5M H3B03 30 2497 <0.05
GDE, 3 m /cm2
NiO-Co2O3/C 1.OM NaOH, 0.5M H3B03 90 2429 <0.05
GDE, 3 m /cm2
Ni/C Felt 10.OM NaOH, 0.5M 30 2536 <0.05
H3B03, 1% TMAH
LaNi5/Ni Screen 10.OM NaOH, 0.5M 50 2498 0.10
H3BO3, 1% TMAH
PTFE-Ni 10.OM NaOH, 0.5M 30 2250 <0.05
H3BO3, 1% TMAH
Ni/C GDE, 10.OM NaOH, 0.5M 150 3421 0.15
1.0 mg Nz/cm2 H3BO3, 1% TMAH
Ni/C GDE, 10.OM NaOH, 0.5M 30 2625 <0.05
2.0 mg Ni/cm2 H3B03, 1% TMAH
Ni/C GDE, 10.OM NaOH, 0.5M 120 3416 <0.05
2.0 mg Ni/cm2 H3B03, 1% TMAH
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Example 2
Electrode Preparation for Platinum/Palladium Alloy Plated Graphite Felt:
Graphite felt was washed with dilute hydrochloric acid and then water to
remove
any metal ion impurities present. The felt was then plated with a
platinum/palladium alloy. The plating was performed using a plating bath of
the
following composition:
(NH4)2Pd(N02)2 5 g/L
(NH4)9,Pt(N02)2 0.3 g/L
KHPO4 5 g/L
The bath was adjusted to pH 9 using ammonium hydroxide.
Plating was performed at 90 C using a constant current of 20 mA/cm2 and
a charge of 2000 coulombs passed. The plating was gray in appearance and
concentrated at the outer surface of the felt.
All other electrodes used in this study were not pretreated except for acid
washing to clean the surface.
General Electrolysis Procedure: A typical electrolysis was performed in a
two-compartment glass cell divided with a NAFION 417 membrane (available
from DuPont Co.). The anolyte consisted of 1 M sodium hydroxide (80 mL) and
the anode material was platinized titanium or nickel. Unless specified
otherwise, the catholyte was 25% by weight sodium metaborate adjusted to pH
11-12 with sodium hydroxide. The electrolysis was carried out at constant
current.
Analytical Procedure: The amount of borohydride in the catholyte was
determined indirectly by allowing it to react with cyclohexanone and
determining the amount of cyclohexanol formed by gas chromatography. A 5 mL
sample of the catholyte was reacted with 5 mL of a solution containing 2% by
weight cyclohexanone in methanol. After reaction with the large excess of
cyclohexanone, the mixture was injected directly into a gas chromatograph. The
cyclohexanol peak was compared to the cyclohexanol peaks determined by
reacting aqueous borate solution containing known amounts of borohydride.
The results of several experiments are presented in Table 2.
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Table 2
Cathode Catholyte Current BH4'
Density Current
(mA/cm2) Eff. (%)
Gr felt plated with Pt/Pd 25% NaBO2/pH 11-12 5 0.42
Pt flag 25% NaBO2/ H 11-12 5 0.65
Gr felt 25% NaB02/ H 11-12 5 0.42
Gr felt 25% NaB02/ H 11-12 1 1.3
Pb flag 25% NaBO2/pH 11-12 5 0.7
Zn flag 25% NaBO2/pH 11-12 5 0.44
Ni flag 25% NaBO2/pH 11-12/5% 5 0.5
TMAH
Gr=graphite
5
