U.S. patent application number 12/375088 was filed with the patent office on 2009-12-03 for water electrolysis device.
This patent application is currently assigned to COMMISSARIAT A L'ENERGIE ATOMIQUE - CEA. Invention is credited to Regine Basseguy, Alain Bergel, Serge Da Silva, Leonardo De Silva Munoz, Damien Feron, Marc Roy.
Application Number | 20090294282 12/375088 |
Document ID | / |
Family ID | 37776533 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090294282 |
Kind Code |
A1 |
Basseguy; Regine ; et
al. |
December 3, 2009 |
WATER ELECTROLYSIS DEVICE
Abstract
Electrolysis device intended to produce hydrogen by the
reduction of water, comprising a cathode compartment, an anode
compartment, and an element connecting said compartments and
allowing ions to migrate between them, the device being
characterized in that the cathode compartment contains at least one
weak acid capable of catalyzing the reduction and an electrolytic
solution, the pH of which is in the range between 3 and 9.
Inventors: |
Basseguy; Regine;
(Vernerque, FR) ; Bergel; Alain; (Toulouse,
FR) ; Da Silva; Serge; (Perpignan, FR) ; De
Silva Munoz; Leonardo; (Toulouse, FR) ; Feron;
Damien; (Fontenay-aux-Roses, FR) ; Roy; Marc;
(Bures sur Yvette, FR) |
Correspondence
Address: |
BROWDY AND NEIMARK, P.L.L.C.;624 NINTH STREET, NW
SUITE 300
WASHINGTON
DC
20001-5303
US
|
Assignee: |
COMMISSARIAT A L'ENERGIE ATOMIQUE -
CEA
Paris
FR
INSTITUT NATIONAL POLY-TECHNIQUE DE TOULOUSE-INPT
Toulouse Cedex 4
FR
|
Family ID: |
37776533 |
Appl. No.: |
12/375088 |
Filed: |
June 11, 2007 |
PCT Filed: |
June 11, 2007 |
PCT NO: |
PCT/FR07/00949 |
371 Date: |
January 26, 2009 |
Current U.S.
Class: |
204/252 |
Current CPC
Class: |
C25B 9/73 20210101; Y02E
60/36 20130101; C25B 1/04 20130101; C25B 11/04 20130101 |
Class at
Publication: |
204/252 |
International
Class: |
C25B 9/00 20060101
C25B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2006 |
FR |
0606772 |
Claims
1-14. (canceled)
15. An electrolysis device intended to produce hydrogen by the
reduction of water, comprising a cathode compartment, an anode
compartment and an element connecting said compartments and
allowing ions to migrate between them, said device being
characterized in that said cathode compartment contains at least
one weak acid and an electrolytic solution, the pH of which is in
the range between 3 and 9, and in that it comprises an electrode in
contact with said electrolytic solution in the cathode compartment,
said electrode being partly or entirely made of at least one
material selected from the group consisting of conductive polymers,
oxidized or non-oxidized forms of Fe, Cr, Ni or Co.
16. An electrolysis device according to claim 15, characterized in
that said electrode is a stainless steel cathode.
17. An electrolysis device according to claim 16, characterized in
that said stainless steel is 316 L stainless steel.
18. An electrolysis device according to claim 15, characterized in
that the pH of the electrolytic solution contained in said cathode
compartment is in the range between 4 and 9.
19. An electrolysis device according to claim 18, characterized in
that the pH of the electrolytic solution contained in said cathode
compartment is in the range between 6 and 9.
20. An electrolysis device according to claim 19, characterized in
that the pH of the electrolytic solution contained in said cathode
compartment is equal to 8.
21. An electrolysis device according to claim 15, characterized in
that the pH of the electrolytic solution contained in said anode
compartment is substantially the same as that of the electrolytic
solution contained in said cathode compartment.
22. An electrolysis device according to claims 15, characterized in
that the pH of the electrolytic solution contained in said anode
compartment is basic.
23. An electrolysis device according to claim 22, characterized in
that the pH of the electrolytic solution contained in said anode
compartment is approximately 15.
24. An electrolysis device according to claim 22, characterized in
that the pH of the electrolytic solution contained in said cathode
compartment is approximately 4.
25. An electrolysis device according to claim 15, characterized in
that said weak acid has a pKa in the range between 3 and 9.
26. An electrolysis device according to claim 25, characterized in
that said pKa is in the range between 3 and 5.
27. An electrolysis device according to claim 15, characterized in
that said weak acid is selected so that its pKa is greater by at
least one unit than the pH of the electrolytic solution contained
in said cathode compartment.
28. An electrolysis device according to claim 15, characterized in
that said weak acid is selected from the group consisting of
orthophosphoric acid, dihydrogen phosphate, monohydrogen phosphate,
lactic acid, gluconic acid, acetic acid, monochloroacetic acid,
ascorbic acid, hydrogen sulfate, glycolic acid, and amino
acids.
29. An electrolysis device according to claim 28, characterized in
that said amino acid is leucine or lysine.
30. An electrolysis device according to claim 15, characterized in
that at least one additional weak acid is added to the electrolytic
solution contained in said cathode and/or anode compartment to
prevent or restrict the pH variation of said solution or solutions
during the reduction of water.
31. An electrolysis device according to claim 30, characterized in
that said additional weak acid has the same chemical structure as
the weak acid initially contained in said cathode compartment.
32. An electrolysis device according to claim 15, characterized in
that said cathode compartment is sealed.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to the field of water electrolysis
and more particularly, to a water electrolysis device for producing
hydrogen.
BACKGROUND OF THE INVENTION
[0002] In the field of energy production, taking into account the
increase in needs, costs, supply security and environmental risks,
calls for more extensive research work on the diversification and
optimal use of primary resources (whether they be fossil, nuclear,
renewable, etc.). In this regard, hydrogen, which allows energy to
be stored and distributed in a convenient manner while causing
little pollution, is a good candidate.
[0003] For the extensive use of hydrogen as a source of thermal and
electrical energy to be economically and ecologically viable, each
of the industrial processes involved, from its production to its
ultimate use, including its storage and distribution, must
nevertheless be developed.
[0004] Since hydrogen is not directly available in the environment,
it is particularly important to optimally fulfil these criteria
during its production, which must be kept competitive (by
maintaining relatively low production costs), clean (the process
should be non-polluting so as to preserve one of the major
advantages of hydrogen), and of optimal energy efficiency (energy
consumption should be limited).
[0005] One of the techniques for producing hydrogen is water
electrolysis, which is generally achieved using one of the
following two devices: [0006] a device for electrolyzing water in
an alkaline medium (essentially, potassium hydroxide at
concentrations from 25% to 40% by weight). Although it benefits
from considerable experience, its improvement requires the
development of new materials that fulfil several criteria,
including resistance to corrosion in an alkaline medium, and the
ability to catalyse the reactions taking place on the electrodes in
order to obtain a high current density and a small overpotential.
In particular, achieving a smaller cathode overpotential leads to
cathode activation by forming a catalytically active surface
deposit, typically by depositing nickel onto an iron base. As far
as the anode is concerned, the base, which should be of a more
noble material (nickel steel or bulk nickel), is often coated with
a catalyst, the deposition and stability of which are delicate
issues, which are the object of intensive research. As may be
noted, one drawback of water electrolysis in an alkaline medium is
the need to use a corrosive electrolytic solution, and electrodes
made with costly materials which degrade with time. [0007] a device
for electrolyzing water in an acidic medium (typically sulfuric
acid). This comprises lead as the conducting material for the
electrodes and manifolds, or noble metal-based catalysts (such as
platinum black) for the cathode and the anode, as well as a Nafion
(perfluoropolymer of sulfonic acid) cation-exchange membrane. The
problems of electrode corrosion caused by the strong acidity of the
medium (typically, negative pH values), of environmental
non-compliance due to the use of lead, and finally of the high cost
of catalysts, have long restricted the use of acidic medium
electrolysis to the production of small quantities of high purity
laboratory grade hydrogen.
SUMMARY OF THE INVENTION
[0008] It is accordingly an object of this invention to remedy the
problems and shortcomings of the prior art techniques by providing
a water electrolysis device which fully satisfies the above
mentioned technical, economical and environmental requirements for
producing hydrogen.
[0009] A further object of this invention is to provide a water
electrolysis device that comprises neither a corrosive electrolytic
solution nor electrodes made of costly materials which degrade with
time.
[0010] The object of this invention is to provide an electrolysis
device intended to produce hydrogen by the reduction of water,
comprising a cathode compartment, an anode compartment, and an
element connecting said compartments and allowing ions to migrate
between them, the device being characterized in that the cathode
compartment contains at least one weak acid capable of catalyzing
the reduction, and an electrolytic solution the pH of which lies in
the range between 3 and 9.
[0011] Advantageously, said pH lies in the range between 4 and 9;
preferably, it lies between 6 and 9, and more preferably, it is
equal to 8.
[0012] The element connecting the compartments may be an
electrochemical bridge known in the art, such as a cation-exchange
membrane, a ceramic, and the like.
[0013] The electrolysis device of the present invention may
preferably be proposed in the form of two embodiments which differ
in the acid-base conditions of their cathode compartment, namely:
[0014] an electrolysis device wherein the pH of the electrolytic
solution contained in the anode compartment and the pH of the
electrolytic solution contained in the cathode compartment lie in
the range from 3 to 9. The compositions of the two electrolytes are
typically the same. Preferably, the pH of the electrolytic solution
contained in the anode compartment is substantially the same as
that of the electrolytic solution contained in the cathode
compartment, that is, it is in the same range or has the same value
as the pH of the electrolytic solution contained in the cathode
compartment. Such a device is typically used in the potentiostatic
mode. [0015] an electrolysis device wherein the pH of the
electrolytic solution contained in the anode compartment is basic
and is preferably approximately 15. This embodiment may optionally
have some of the features of an existing device for electrolyzing
water in an alkaline medium. Further, the pH of the electrolytic
solution contained in the cathode compartment is preferably
approximately 4. Such devices are typically used in the
galvanostatic mode.
[0016] Generally, when implementing the invention, the weak acid
intended to catalyze the reduction of water may be in the form of a
salt (partially or totally dissolved in the electrolytic solution)
and/or adsorbed onto the cathode. Of course, according to the pKa
of the weak acid and the pH conditions of the electrolytic
solution, the weak acid may be partially dissociated between its
acid form and its conjugate base, and each of these two species may
possibly contribute to the catalytic action.
[0017] However, advantageously, the weak acid is selected so that
its pKa is at least greater by one unit than the pH of the
electrolytic solution contained in the cathode compartment. Under
such conditions, it will undergo little or no dissociation.
Therefore, all or most of the weak acid molecules preserve their
acidic labile hydrogen atom. Since it is this atom which allows the
reduction of water to be catalyzed, the catalytic potential of the
weak acid is thus optimized.
[0018] Moreover, the weak acid preferably has a pKa in the range
between 3 and 9, and more preferably, between 3 and 5.
Consequently, the hydrogen atom responsible for the catalytic
effect of the weak acid is strongly labile and shows an increased
acidic character, thus allowing it to better catalyze the reduction
of water, which consequently requires less energy to occur.
[0019] The above two embodiments may advantageously be combined.
For example, glycolic acid, which has a pKa of 3.83 and a high
solubility of 11.6 M, may be added to the electrolytic solution in
the cathode compartment, which has a pH of 3.
[0020] During water electrolysis, OH and H.sup.+ ions are produced,
respectively, in the electrolytic solution contained in the cathode
compartment and in that contained in the anode compartment.
Preferably, in order for the water reduction to take place with
optimal energy efficiency, it is appropriate to prevent or restrict
the resulting pH variation. For that purpose, at least one
additional weak acid is added as a buffer to the electrolytic
solution contained in the cathode and/or anode compartment so as to
prevent or restrict pH variation of this solution or of these
solutions during the reduction of water. This additional acid,
selected as a function of the pH in the compartment to which it is
added, may furthermore function as a catalyst for the reduction of
water.
[0021] Advantageously, because of this additional weak acid, the pH
variation of the electrolytic solution contained in the anode
and/or cathode compartment does not vary during the reduction of
water by more than two pH units, preferably by one pH unit.
[0022] Preferably, said additional weak acid has the same chemical
structure as the weak acid intended to catalyze the reduction.
[0023] Additional objects, features and advantages of the invention
will become apparent from the following description, which is given
by way of illustration only.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Reference will now be made to the three accompanying
drawings, which are explained in examples 1 and 2 below.
[0025] FIG. 1 shows the variation of current as a function of time
during constant-potential electrolysis.
[0026] FIG. 2 shows the variation as a function of time of the
volume of hydrogen produced by electrolyzing an electrolytic
solution of "KCl+dihydrogen phosphate".
[0027] FIG. 3 shows the variation as a function of time of the
potential across an electrolysis device according to this
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The following examples were conducted using dihydrogen
phosphate in solution as the catalyst for water reduction.
[0029] The weak acid may be mineral (such as orthophosphoric acid,
dihydrogen phosphate, monohydrogen phosphate, and the like) or
organic (such as lactic acid, gluconic acid, acetic acid,
monochloroacetic acid, ascorbic acid, hydrogen sulfate, glycolic
acid, amino acids, preferably leucine or lysine).
1) ELECTROLYSIS "UNDER NEAR-NEUTRAL CONDITIONS"
1.1 Operating Procedure
[0030] An electrolysis device according to the invention has the
following features: [0031] two compartments (anode and cathode
compartments) each with a volume of 125 cm.sup.3, containing the
same electrolytic solution, made of Plexiglas, and separated by a
Nafion 1135 membrane after prior cleaning by immersion into boiling
distilled water; [0032] a working electrode: 316 L stainless steal
cathode with a geometrical surface of 20 cm.sup.2; [0033] an
auxiliary electrode: anode made of a platinum grid, with a
geometrical surface of 20 cm.sup.2; [0034] a reference electrode:
Saturated Calomel Electrode (SCE).
[0035] Using said device, electrolysis of water at a constant
potential of -1.1 V/SCE and at a temperature in the range from 20
to 25.degree. C. was carried out for 100 minutes on two distinct
electrolytes, namely: [0036] 1) a reference electrolytic solution
comprising a solution of KCl (100 mM) at pH=8.0; [0037] 2) an
electrolytic solution according to the invention comprising KCl
(100 mM)+KH.sub.2PO.sub.4 (500 mM) at pH=8.0 (pH adjustment by
addition of KOH).
[0038] To recover the hydrogen thus formed, the cathode compartment
was sealed by a plug provided with a Teflon gasket and traversed by
a pipe opening into a graduated test tube filled with water and
turned upside down in a vessel which also contained water. It
should be noted that the device according to the invention might
also be used for producing oxygen, which would be generated within
the anode compartment also sealed in a similar fashion.
[0039] FIG. 1 shows the variation of current as a function of time
during constant-potential electrolysis of a reference electrolytic
solution (denoted "KCl alone") and of an electrolytic solution
having a pH of 8.0 and containing dihydrogen phosphate (denoted
"KCl+dihydrogen phosphate").
[0040] The results for the two electrolyses are summarized in Table
1. These data illustrate: [0041] the small cathode current and the
reduced or even non-existent reduction of the water for the
reference electrolytic solution (1), since no hydrogen evolution
was observed. This is to be compared with the significant cathode
current obtained with electrolytic solution (2), which in this case
is reflected by a noticeable evolution of hydrogen gas; [0042] the
stability of the cathode current when only the electrolytic
solution (2) is used.
TABLE-US-00001 [0042] TABLE 1 Electrolyte (1) KCl (100 mM) (2) KCl
(100 mM) + KH.sub.2PO.sub.4 (500 mM) Cathode current From 4 to 13 A
m.sup.-2 1.7 A m.sup.-2 (very stable) Hydrogen volume No 10 mL
produced in observable 70 minutes production Hydrogen 0 mL/hr From
9 to 10 production mL/hr, or 4.5 to 5 rate L/hr/m.sup.2
1.2 Computation of Efficiency
[0043] During the production of hydrogen from the electrolyte (2),
the Faraday efficiency was computed from the data summarized in
Table 2. The "raw" Faraday efficiency obtained under these
conditions was nearly 72%. Since no production was detected when
the experiment was carried out using the reference electrolytic
solution (1), the current thus obtained was considered to be a
residual current, probably caused by the reduction of the
electrode's surface oxides. The decrease in current from 4 to 1.7
A/m.sup.2 in 70 minutes supported this hypothesis. Therefore, this
portion of the current was not used to transform a species in
solution, but rather to induce a change in the surface condition of
the electrode. For a long-duration process, this portion of the
current may be expected to tend to zero when all oxides are reduced
(after a few tens of hours). This quantity of electricity was
therefore subtracted in order to derive the "corrected" Faraday
efficiency that would be obtained after a few hours of
electrolysis. By subtracting the residual quantity of electricity,
the "corrected" Faraday efficiency was 92%, that is, 92% of the
additional electricity consumption induced by the presence of
dihydrogen phosphate was used for the production of hydrogen.
TABLE-US-00002 TABLE 2 Universal Gas constant R (J/K/mole) 8.314
Faraday constant (C/mole) 96500 Temperature (K) 298 Pressure (Pa)
1.013 10.sup.5 Electrode surface area (cm.sup.2) 20 Number of
electrons involved per 2 molecule of H.sub.2 produced Electrolysis
time considered (sec) 4186 Volume of hydrogen produced (mL) 10
Number of moles of H.sub.2 produced 4.09 10.sup.-4 Total quantity
of electricity (C/cm.sup.2) 5.516 "Raw" Faraday efficiency 71.6%
Residual quantity of electricity 1.247 (C/cm.sup.2) (KCl alone)
Quantity of additional electricity 4.269 induced by the presence of
dihydrogen phosphate (C/cm.sup.2) "Corrected" Faraday efficiency
92%
[0044] The presence of dihydrogen phosphate in solution at
near-neutral pH (pH=8.0) enables electrochemical production of
hydrogen (4 to 5 L/hr/m.sup.2) on stainless steel in the range of
potentials for which no production would be obtained without
dihydrogen phosphate. More than 92% of the quantity of electricity
consumed in the presence of dihydrogen phosphate ions is used for
producing hydrogen, which is excellent in terms of efficiency.
[0045] Various observations have demonstrated that the weak acid of
this invention indeed catalyzed the reduction of water.
[0046] For example, at pH=8.0, no pH variation occurred in the
cathode compartment during the electrolysis of water although
OH.sup.- ions were produced. This is because at pH=8.0, dihydrogen
phosphate and monohydrogen phosphate were the dominant phosphate
species (14% and 86% of this species, respectively) and acted as a
buffer (the H.sub.2PO.sub.4.sup.-/HPO.sub.4.sup.2- couple had a pKa
of 7.20). The pH thus being constant, the free proton concentration
at pH=8.0 was however consistently very small, at 10.sup.-8 M.
Therefore, this concentration could not be responsible for the high
cathode current of 13 A.m.sup.-2, which furthermore was much
greater than the cathode current of the reference electrolyte (1)
(KCl 100 mM), also at pH=8.0.
2) ELECTROLYSIS UNDER "BASIC CONDITIONS"
2.1 Operating Procedure
[0047] The following examples were carried out with the same
electrolysis device and according to the same operating protocol as
described in the preceding example, except that the electrolyses
were now conducted at a constant current of -13.5 A/m.sup.-2 on
three different electrolytes whose characteristics are summarized
in Table 3.
TABLE-US-00003 TABLE 3 Electrolyte in the Electrolyte in the
Electrolysis anode compartment cathode compartment (I) KOH KOH 25%
by weight KOH 25% by weight, pH (reference) 15.0 (II) KOH-PO4 KOH
25% by weight 0.5M KH.sub.2PO.sub.4, pH 8.0 (0.5M) (III) KOH-PO4
KOH 25% by weight 1M KH.sub.2PO.sub.4, pH 4.0 (1M)
2.2 Demonstration of the Electrolysis Device's Stability and
Computation of Efficiency
[0048] The electrolyses lasted 2 hours, with the temperature
ranging from 20.degree. C. to 25.degree. C. in the three
experiments. The production of hydrogen, measured as described
above, was on average of the order of 10 mL/hr, which corresponds
to a "raw" Faraday efficiency of approximately 80%.
[0049] The change in potential across the electrolysis device
(denoted Ecell) is shown in FIG. 3, which illustrates the change as
a function of time of the potential across an electrolysis device
in the course of an electrolysis carried out with a constant
current of -13.5 A.m.sup.-2, of a reference electrolytic solution
(denoted "KOH"), of an electrolytic solution containing dihydrogen
phosphate at pH=8.0 (denoted "KOH--PO4 (0.5M)"), and of an
electrolytic solution containing dihydrogen phosphate at pH=4.0
(denoted "KOH--PO4 (1M)"). As illustrated in this figure, the
presence of dihydrogen phosphate as a catalyst here again allowed
the energy efficiency to be improved, since an increase in
potential of 200 and 600 mV relative to the reference electrolytic
solution (I) was observed in the presence of 0.5 M and 1 M of
dihydrogen phosphate, respectively.
[0050] Moreover, the potential Ecell remained substantially
constant while the production of hydrogen obeyed a linear law as a
function of time. This demonstrates the stability of the stainless
steel electrode, which showed no change in its surface condition
(pollution, adsorption, corrosion, etc.).
[0051] The energy consumption during the production of hydrogen
from the three electrolytes was computed (Table 4), taking into
account the fact that when the energy consumption is expressed in
kWh/Nm.sup.3, 1 Nm.sup.3 corresponds to 1 m.sup.3 of gas measured
at 0.degree. C. and at atmospheric pressure.
TABLE-US-00004 TABLE 4 Average Energy Energy Energy Energy Ecell
spent in spent in consumption consumption Electrolyte (V) 2 hours
(kJ) 2 hours (kWh) kWh/m.sup.3 of H.sub.2 kWh/Nm.sup.3 of H.sub.2
(I) KOH 1.90 0.369 1.02E-04 4.6 4.9 (II) KOH--PO4 1.67 0.324
9.01E-05 4.0 4.3 (0.5M) (III) KOH--PO4 1.30 0.252 7.01E-05 3.1 3.3
(1M)
[0052] The presence of dihydrogen phosphate in the electrolytic
solution contained in the cathode compartment provides an energy
gain of 13% and 33% for a concentration of 0.5 M and 1 M of
dihydrogen phosphate, respectively.
[0053] It should be noted that the energy efficiency is roughly
proportional to the weak acid concentration. Therefore, this
concentration may advantageously be increased as long as the energy
efficiency increases, in particular up to the point where the weak
acid precipitates and/or becomes excessively adsorbed onto the
cathode.
3) CONCLUSIONS
[0054] As illustrated by the above examples, the electrolysis
device according to the invention in both of its main embodiments,
advantageously leads to excellent Faraday efficiency during the
production of hydrogen.
[0055] Furthermore, the stainless steel cathodes of the
electrolysis device according to the invention do not suffer any
observable degradation. The use of an electrolytic solution of
moderate pH in the cathode compartment, combined with the
catalyzing power of the weak acid it contains therefore permits the
manufacture of a high performance electrolysis device which
comprises at least one element in contact with the electrolytic
solution in the cathode compartment, this element being partially
or entirely made of at least one less noble material. A less noble
material appropriate in the implementation of the present invention
may be selected from the group consisting of the conductive
polymers, the oxidized or non-oxidized forms of Fe, Cr, Ni or Co.
This material may be included in the composition of parts of the
electrolysis device such as electrodes, compartment walls, circuits
for circulating the solutions, etc. The element may thus be a
stainless steel cathode, preferably made of 316 L stainless
steel.
[0056] The use, within the scope of the present invention, of at
least one less noble material offers the advantages of
substantially reducing the manufacturing costs since this type of
material is generally less costly than those conventionally used,
such as platinum, of optimally satisfying environmental
requirements, of increasing the lives of such devices, while
achieving excellent hydrogen production efficiency through the
electrolysis of water.
* * * * *