U.S. patent application number 12/302769 was filed with the patent office on 2009-10-29 for porous electrically conductive carbon material and uses thereof.
This patent application is currently assigned to Max-Planck-Gesellschaft zur Foerderung der Wissenschaften e.V.. Invention is credited to Philipp Adelhelm, Markus Antonietti, Yu-Guo Guo, Sarmimala Hore, Yong-Sheng Hu, Joachim Maier, Bernd Smarsly.
Application Number | 20090269667 12/302769 |
Document ID | / |
Family ID | 38461768 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090269667 |
Kind Code |
A1 |
Antonietti; Markus ; et
al. |
October 29, 2009 |
Porous Electrically Conductive Carbon Material And Uses Thereof
Abstract
This disclosure relates to a porous electrically conductive
carbon material having interconnected pores in first and second
size ranges from 10 .mu.m to 100 nm and from less than 100 nm to 3
nm and a graphene structure and to diverse uses of the material
such as an electrode in a lithium-ion battery and a catalyst
support, e.g. for the oxidation of methanol in a fuel cell. The
carbon material has been heat treated to effect conversion to
non-graphitic carbon with the required degree of order at a
temperature in the range from 600.degree. C. to 1000.degree. C. A
lithium-ion battery and an electrode for a lithium-ion battery are
also claimed.
Inventors: |
Antonietti; Markus;
(Bergholz-Rehbruecke, DE) ; Smarsly; Bernd;
(Potsdam, DE) ; Adelhelm; Philipp; (Freiberg,
DE) ; Maier; Joachim; (Wiernsheim, DE) ; Hore;
Sarmimala; (Stuttgart, DE) ; Hu; Yong-Sheng;
(Stuttgart, DE) ; Guo; Yu-Guo; (Stuttgart,
DE) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
Max-Planck-Gesellschaft zur
Foerderung der Wissenschaften e.V.
Muenchen
DE
|
Family ID: |
38461768 |
Appl. No.: |
12/302769 |
Filed: |
May 25, 2007 |
PCT Filed: |
May 25, 2007 |
PCT NO: |
PCT/EP2007/004698 |
371 Date: |
July 8, 2009 |
Current U.S.
Class: |
429/231.4 ;
428/315.9; 428/316.6 |
Current CPC
Class: |
H01G 11/86 20130101;
H01M 4/583 20130101; Y02E 60/10 20130101; H01G 11/26 20130101; Y02E
60/13 20130101; C01B 32/05 20170801; Y10T 428/24998 20150401; H01G
11/32 20130101; H01M 4/04 20130101; H01M 4/587 20130101; H01M
10/0525 20130101; H01G 11/34 20130101; H01G 11/48 20130101; H01G
11/24 20130101; Y10T 428/249981 20150401 |
Class at
Publication: |
429/231.4 ;
428/315.9; 428/316.6 |
International
Class: |
H01M 4/58 20060101
H01M004/58; B32B 3/26 20060101 B32B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2006 |
EP |
06011198.6 |
Sep 8, 2006 |
EP |
06018886.9 |
Sep 15, 2006 |
EP |
06019348.9 |
Claims
1. A porous electrically conductive carbon material including
graphene stacks and having first and second pores in first and
second different pore size ranges respectively, wherein said first
pores are of irregular shape in three dimensions, are
interconnected to form transport passages through said carbon
material and have sizes in the size range from 10 .mu.m to 100 nm,
wherein said second pores are defined between neighbouring graphene
stacks, are of irregular shape in three dimensions, are
interconnected, communicate directly or indirectly via other second
pores with said first pores and have sizes in the size range from
less than 100 nm to 3 nm and wherein said graphene stacks defining
said second pores form wall material between said first pores.
2. A porous carbon material in accordance with claim 1 wherein a
majority of said second pores have sizes in the range from 50 nm to
3 nm, and particularly from 3 nm to 8 nm.
3. A porous carbon material in accordance with claim 1 wherein a
majority of said first pores have sizes in the range from 5 .mu.m
to 500 nm, and particularly in the range from 2 .mu.m to 500
nm.
4. A porous carbon material in accordance with claim 1 wherein the
total pore volume comprising micropores with a volume less than 3
nm, the second pores in the size range from less than 100 nm to 3
nm and the first pores in the size range from 10 .mu.m to 100 nm
lies in the range from 0.1 to 1.0 cc/g.
5. A porous carbon material in accordance with claim 4 wherein said
total pore volume lies in the range from 0.40 cc/g to 0.65 cc/g
with the volume of second pores lying in the range from 0.35 cc/g
to 0.55 cc/g and the volume of the first pores lying in the range
from 0.05 cc/g to 0.1 cc/g.
6. A porous carbon material in accordance with claim 1 wherein the
ratio of the total pore volume of the second pores to the total
pore volume of the first pores lies in the range from 2 to 12.
7. A porous carbon material in accordance with a claim 1 and having
a BET surface in the range from 50 m.sup.2/g to 800 m.sup.2/g,
especially from 250 m.sup.2/g to 350 m.sup.2/g and particularly of
around 350 m.sup.2/g.
8. A porous carbon material in accordance with claim 1 and having
an H/C atomic ratio in the range from 0.3 to 0.01 and preferably in
the range from 0.2 to 0.075 and especially of about 0.1.
9. A porous carbon material in accordance with claim 1 in which the
carbon material has been heat treated to effect conversion to
non-graphitic carbon with the required degree of order at a
temperature in the range from 600.degree. C. to 1000.degree. C.
10. A porous carbon material in accordance with claim 1 in which
the carbon is present in the form of non-graphitic carbon
comprising a plurality of randomly orientated graphene stacks
having stack heights in the range from 2 to 30 nm and lateral
extension values L.sub.A in the range from 2 to 8 nm, the graphene
stacks either contacting one another or being separated by
amorphous carbon usually distributed throughout the structure and
present between the graphene stacks in a total amount relative to
the graphene stacks material of less than 10% by weight.
11. A porous carbon material in accordance with claim 1, wherein
the carbon material is made by carbonizing a carbon monolith
precursor having a porosity generating fugitive phase dispersed
therein, said fugitive phase comprising particles in first and
second size ranges, said first size range being from 10 .mu.m to
100 nm and said second size range being from less than 100 nm to 3
nm, and subsequently removing said fugitive phase to form a porous
carbon monolith.
12. A porous carbon material in accordance with claim 11, wherein
said fugitive phase is SiO.sub.2 and is removed from said heat
treated carbon monolith precursor by chemical dissolution.
13. A porous carbon material in accordance with claim 11, wherein
said fugitive phase is polystyrene and is removed during
carbonizing of the carbon monolith precursor by vaporisation.
14. A porous carbon material in accordance with claim 1, wherein
the carbon material is made by manufacturing a mixture containing
at least one carbon precursor and an organic polymer in an organic
solvent, by vaporizing the solvent until a viscous or highly
viscous composition of a corresponding shaped body is obtained, by
shaping the viscous composition into a shaped body and by heating
the composition of the shaped body to a temperature between
600.degree. C. and 1000.degree. C.
15. A porous carbon material in accordance with claim 11, wherein
said carbon precursor is a naphthol solution.
16. A porous carbon material in accordance with claim 11, wherein
said carbon precursor is mesophase pitch.
17. A porous carbon material in accordance with claim 14, wherein
said organic polymer is polystyrene.
18. Use of a carbon material in accordance with claim 1 as an
electrode in a lithium-ion battery.
19. Use of a carbon material in accordance with claim 18, wherein
the carbon material is present in the form of a carbon
monolith.
20. Use of a carbon material in accordance with claim 10 as an
electrode, wherein the electrode is made by pasting a mixture of
the carbon material and a binder on a metal foil.
21. Use in accordance with claim 20, wherein the ratio of the
carbon material to the binder is approximately 9 to 1 by
weight.
22. Use in accordance with claim 19, wherein the binder is poly
(vinyl difluoride).
23. A lithium-ion battery having an electrode comprising a carbon
material as specified in claim 1.
24. An electrode for a lithium-ion battery comprising a carbon
material as specified in claim 1.
25. A porous carbon material in accordance with claim 1 when loaded
and/or coated with a catalyst.
26. A porous carbon material in accordance with claim 25 wherein
said catalyst is platinum (Pt).
27. A porous carbon material in accordance with claim 25 wherein it
is loaded and/or coated with ruthenium oxide before being loaded
and/or coated with platinum (Pt) as catalyst.
28. Use of a porous carbon material in accordance with claim 25 in
a fuel cell, in particular for the oxidation of methanol in a
direct methanol fuel cell.
29. Use of a porous carbon material in accordance with claim 26 in
a fuel cell in particular for the oxidation of methanol in a direct
methanol fuel cell.
30. Use of a porous carbon material in accordance with claim 27 in
a fuel cell, in particular for the oxidation of methanol in a
direct methanol fuel cell.
31. Use of a porous material in accordance with claim 1 as a
support in a chemical, electrochemical, biological or physical
device such as a detector, a reactor or a supercapacitor.
Description
[0001] The present invention relates to a porous electrically
conductive carbon material and to uses thereof e.g. in a
lithium-ion battery or in a fuel cell.
[0002] More specifically, the present invention relates to a porous
carbon material, which is particularly adapted for use as an
electrode material in a lithium-ion battery, especially for the
electrode referred to as an anode in the lithium-ion battery
community and for use in a fuel cell, especially a direct methanol
fuel cell, as well as for use as a general support medium, for
example for use in a chemical, electrochemical, biological or
physical device such as a detector, a reactor or a
supercapacitor.
[0003] Porous carbon materials are a well known class of
substances. They are widely used, mainly in the form of a
grain-like powder frequently known by the name of "activated
carbon", for example as sorbents for the cleaning and clarification
of aqueous media, for filters and gas masks, for air conditioning
units, etc., as well as for a catalyst support or as an electrode
material. More visionary fields of use are the storage of gas (e.g.
H.sub.2 or methane), the use as a conductive reactive membrane, for
example in a fuel cell, or in so-called "supercapacitors".
[0004] In practically all known systems the transport system formed
by the porous structure, which is essential for the
characteristics, and also the chemical and the physical
functionality, which is advantageous in use, is achieved purely
empirically by the selection of starting products, by calcination
and frequently also by a subsequent "activation". By way of
example, high performance absorbers, such as "Helsatech.RTM.", are
produced in the technical field by the pyrolysis of ion exchange
resins.
[0005] Attempts have been made to achieve carbon material
structures with a rationally designed porosity. A first route was
pioneered by Ryoo, see for example the article by Ryoo, R. S. H.
Joo and S. Jun, "Synthesis of highly ordered carbon molecular
sieves via template-mediated structural transformations". Journal
of Physical Chemistry B, 1999, 103(37): p. 7743-7746 and Kruk, M.,
Ryoo, R., et al., Characterization of ordered mesoporous carbons
synthesized using MCM-48 silicas as templates. Journal of Physical
Chemistry B, 2000. 104(33): p. 7960-7968. this concept was
subsequently refined by Schuth, see for example Lu, A. H., Schuth
F. et al., Synthesis of ordered mesoporous carbon with bimodal pore
system and high pore volume. Advanced Materials, 2003. 15(19): p.
1602-+, and Lu, A. H., Schmidt, W. and Schuth, F., Simplified novel
synthesis of or dered mesoporous carbon with a bimodal pore system.
New Carbon Materials, 2003. 18(3): p. 181-185.
[0006] This first method is based on the "nanocasting" of silicate
structures. For this, grainy zeolite structures or mesoporous
silicate monoliths were used. It was found that large surfaces (up
to 2050 m.sup.2/g) can be combined with a readily accessible
mesoporous channel system. All these carbon materials are, however,
so designed that they are not electrically conductive or only
weakly electrically conductive. That is to say they consist in
large part of amorphous carbon. They are not therefore suitable for
use as conductive carbon material. So far as is known, electrically
conductive structures have not been manufactured in this way
because the graphitising temperatures that are required of greater
than 2000.degree. C. for the carbon precursors, which have hitherto
been used, result in the pore structure being broken down again in
large part.
[0007] At this stage, reference should be made to the published US
patent application 2005/0169829 A1, which is concerned with
providing a robust carbon monolith characterised by a skeleton size
of at least 100 nm with a hierarchical pore structure having
macropores and mesopores. This structure is proposed for use in a
chromatography column and the disclosure of the US reference is
heavily weighted towards such use of the carbon monolith. There is,
however, a brief mention to the effect that the monolithic carbon
column can be used as a porous electrode for any electrochemical
system. This is, however, not explained in any further detail. In
the methods that are described there a carbon monolith precursor,
which is defined as any material that can be carbonised to form a
carbon monolith that can be used for chromatographic separation,
has to include a particulate, porosity generating fugitive phase,
which serves as a template for the pores that characterise the
final product.
[0008] In one method, silica beads sized from 800 nm to 10 .mu.m
are dispersed in a solvent in a concentration range of 0.1 to 2 g/g
using an appropriate dispersing method such as ultrasonic mixing,
for example to form a colloid. FeCl.sub.3 is then dissolved into
the colloid in a concentration range of 0.01 to 0.5 g/g and
resorcinol is then dissolved into the colloid in a concentration of
0.1 to 2.5 g/g. The colloid is then agitated to facilitate a
reaction to form a resorcinol/Fe(III) complex. Formaldehyde is then
added and stirred to achieve homogeneity. The colloid is then
transferred into a mould of desired monolith shape and heated to a
temperature in the range from about 50.degree. C. to 95.degree. C.
for a period of between 0.5 h to 20 h to effect polymerisation of
the colloid into a solid monolith, shrinkage of the monolith from
the mould wall and curing of the monolith. The solid monolith is
then removed from the mould and the solvent evaporated to dryness.
The dry monolith is then cured at a temperature in the range of
about 40.degree. C. to 150.degree. C. for a time period of 3 h to
20 h to effect complete polymerisation of the monolith
material.
[0009] In an alternative method, polystyrene beads are used instead
of silica beads. In variants of the method, silica beads of two
discrete different particle sizes are used, for example larger
particles in the range from 800 nm to 10 .mu.m and smaller
particles in the range from 6 nm to 100 nm. The carbon monolith
precursors prepared by the above methods are subsequently
carbonised and graphitised through a programmed temperature cycle,
e.g. using a slow rise in temperature from room temperature to
750.degree. C. followed by a fast temperature ramp to a temperature
of up to 2400.degree. C. or higher to graphitise the carbonised
rods.
[0010] By varying the pore-forming agents, the sizes for macropores
and mesopores can be finely adjusted within the limits described
above. The result is a morphology of the carbon material involving
a skeleton having macropores of various sizes forming the primary
porosity of the carbon monolith. A secondary phase separation
results in the secondary porosity on the carbon skeleton.
[0011] The object of the present invention is to provide a novel
porous and electrically conductive carbon material having improved
properties for certain applications and novel uses of the carbon
material. It is a further object of the present invention to
provide an improved lithium-ion battery and an improved electrode
for a lithium-ion battery. It is a yet further object of the
present invention to provide an improved carbon-based
electrocatalyst material for use in a fuel cell such as a direct
methanol fuel cell.
[0012] In order to satisfy these objects, the present invention
provides a porous electrically conductive carbon material including
graphene stacks and having first and second pores in first and
second different pore size ranges respectively, wherein said first
pores are of irregular shape in three dimensions, are
interconnected to form transport passages through said carbon
material and have sizes in the size range from 10 .mu.m to 100 nm,
wherein said second pores are defined between neighbouring graphene
stacks, are of irregular shape in three dimensions, are
interconnected, communicate directly or indirectly via other second
pores with said first pores and have sizes in the size range from
less than 100 nm to 3 nm and wherein said graphene stacks defining
said second pores form wall material between said first pores.
[0013] In this material a majority of said second pores can have
sizes in the range from 50 nm to 3 nm, and preferably from 3 nm to
8 nm. Moreover, a majority of said first pores can have sizes in
the range from 5 .mu.m to 500 nm, and preferably in the range from
2 .mu.m to 500 nm. Here the term majority will generally be
understood to mean that over 50% of the pores in the first size
range have sizes in the preferred range (5 .mu.m to 500 nm or more
preferably 2 .mu.m to 500 nm) and that over 505 of the second pores
will have sizes in the preferred range (50 nm to 3 nm or more
preferably 3 to 8 nm).
[0014] The total pore volume comprising micropores with a volume
less than 3 nm, the second pores in the size range from less than
100 nm to 3 nm and the first pores in the size range from 10 .mu.m
to 100 nm can usefully lie in the range from 0.1 to 1.0 cc/g. A
total pore volume in the range from 0.40 cc/g to 0.65 cc/g is
particularly useful, with the volume of second pores lying in the
range from 0.35 cc/g to 0.55 cc/g and the volume of the first pores
lying in the range from 0.05 cc/g to 0.1 cc/g.
[0015] Such a porous carbon material has a ratio of the total pore
volume of the second pores to the total pore volume of the first
pores in the range from 3.5 to 12.
[0016] The porous carbon material usefully has a BET surface in the
range from 50 m.sup.2/g to 800 m.sup.2/g, especially from 250
m.sup.2/g to 350 m.sup.2/g and particularly of around 350
m.sup.2/g.
[0017] Moreover, an H/C atomic ratio for the porous carbon material
in the range from 0.3 to 0.01, especially in the range from 0.2 to
0.075 and particularly of about 0.1 or less is very useful when the
porous carbon material is used as an electrode in a lithium ion
battery.
[0018] The carbon material is expediently heat treated to effect
conversion to non-graphitic carbon with the required degree of
order at a temperature in the range from 600.degree. C. to
1000.degree. C.
[0019] An especially preferred structure for the porous carbon
material is such that the carbon is present in the form of
non-graphitic carbon comprising a plurality of randomly orientated
graphene stacks having stack heights in the range from 2 to 30 nm
and lateral extension values L.sub.A in the range from 2 to 8 nm,
the graphene stacks either contacting one another or being
separated by amorphous carbon usually distributed throughout the
structure and present between the graphene stacks in a total amount
relative to the graphene stacks material of less than 10% by
weight.
[0020] It has surprisingly been found that carbon materials can be
made using similar techniques to those described in US 2005/0169829
A1 and by other methods, but using comparatively low temperatures
which results in materials having a very different structure and
characteristics such that rationally designed porous non-graphitic
carbons are produced which have an excellent charge capacity and an
extremely high rate charge/discharge characteristic, dependent on
the carbon precursor that is used and on the heat treatment
temperature that is used.
[0021] Another approach to manufacturing monolithic porous carbon
materials lies in first manufacturing monolithic porous oxides, for
example monolithic silicate columns, available from the company
Merck under the designation "Chromolith" or round monolithic
silicate discs prepared in accordance with the teachings of
Minakuchi and Tanaka, see for example Minakuchi, H., Tanaka, et
al., Performance of an octadecylsilylated continuous porous silica
column in polypeptide separations. Journal Of Chromatography A,
1998. 828(1-2): p. 83-90.
[0022] The corresponding monolithic silicate components, for
example with a height of 0.5 cm and a diameter of 2.5 cm, were
taken as a starting point for the present invention. They were
first impregnated with a special carbon precursor and were then
carefully carbonized in the presence of the silicate. Thereafter,
the silicate was removed, for example using fluorides or NaOH, so
that a carbon monolith in the form of a negative replica of the
silicate structure resulted. By appropriate choice of the starting
product, for example in the form of "mesophase pitch", and also the
addition of further porogenes it was possible to generate a
monolithic carbon material which has pores being in first and
second different size ranges, in particular first pores having
sizes between 10 .mu.m and 100 nm and second pores having sizes in
the range from less than 100 nm to 1 nm, the material being in the
form of non-graphitic carbon.
[0023] Monolithic carbon materials have indeed already been
synthesized by this process as can be read in the following
articles:
Yang, H. F., et al., Synthesis of ordered mesoporous carbon
monoliths with bicontinuous cubic pore structure of Ia3d symmetry.
Chemical Communications, 2002(23): p. 2842-2843; Shi, Z. G., et
al., Synthesis of a carbon monolith with trimodal pores. Carbon,
2003. 41(13): p. 2677-2679, and Lu, A. H., J. H. Smatt, and M.
Linden, Combined surface and volume templating of highly porous
nanocast carbon monoliths. Advanced Functional Materials, 2005.
15(5): p. 865-871.
[0024] However, the monoliths manufactured in this way were often
less stable and consisted, as a result of other precursors, all of
amorphous carbon.
[0025] Mention should also be made here of GB-A-900,451 which
describes an improved fuel cell electrode comprising carbon having
a porosity in the range of 10% to 35%, an internal surface area of
from 100 to 300 sq. metres/gram a maximum pore diameter of 40 .mu.m
and a real density of 1.9 to 2.25 gram/cc., the carbon being
impregnated with a fuel cell anodic or cathodic catalyst. More
specifically, the document describes the material as having 20 to
80% of the pore volume of carbon formed by pores having diameters
in the range of about 0.2 .mu.m to 20 .mu.m and 30 to 70% formed by
pores having diameters in the range of about 5 nm to 40 nm.
[0026] The method of manufacture involves mixing fine particles of
graphite and lamp black or calcined coke with a suitable binding
material such as pitch or cellulosic materials. The mixture is
subsequently molded into the desired electrode shape under elevated
pressures and by heating to a temperature in the range from
1000.degree. F. to 2500.degree. F. and by subsequently oxidising
the material using CO.sub.2 at temperatures in the range from
1600.degree. F. to 2000.degree. F. or in oxygen air at 600.degree.
F. to 900.degree. F. These values lie generally outside of the
temperature range used for the present invention to generate the
desired graphene structure which is in any event not present in the
material of GB-A-900,451 because of the use of powdered graphite
for the starting material.
[0027] Mention should also be made of the document "Easy and
flexible preparation of nanocasted carbon monoliths exhibiting a
multimodal hierarchical porosity" by An-Hui Lu et al in Microporous
and Mesoporous Materials 72 (2004) 59-65. That document describes a
porous carbon material having first and second pore sizes of the
same general size as the porous material of the present teaching
but uses a different carbon precursor and does not therefore
achieve the special ordered graphene structure required for the
porous material of the present teaching. Instead the reference
describes that the mesopore size of the mode of porosity is too
small for close to graphitic carbon structures to form. The
reference to non-graphitic carbon structures is an ill-defined term
for what is correctly termed a graphene structure.
[0028] A preferred use of the porous carbon material of the present
invention is as an electrode in a lithium-ion battery. Moreover,
the present invention provides a lithium-ion battery having an
electrode formed with the carbon material having interconnected
pores in first and second different size ranges and also an
electrode of a carbon material having interconnected pores in first
and second different size ranges for use in a lithium-ion
battery.
[0029] The amorphous carbon material present between some of the
graphene stacks of the presently proposed carbon material consists
of sp3 carbon and can be thought of as mortar between randomly
orientated and irregularly shaped bricks, either in form of thin
layers (less than 2 nm on average) or also isolated species such as
CH or CH2 containing units.
[0030] The carbon material of the invention seems to be such that
the first pores are on the micrometer scale, separated by
micrometer sized walls (typically 1-10 micrometer). The second
pores are located within these thick walls, and their walls are
defined by graphene stacks.
[0031] Starting from a so-called non-graphitic carbon such as
mesophase pitch it has been found that heat treatment at a
temperature in the range from 600.degree. C. to 1000.degree. C. and
preferably of not more than 950.degree. C. results in graphene
stacks being formed with the desired parameters of height L.sub.c
and lateral extension L.sub.A as quoted above.
[0032] The term "stack height" or "stacking height" for L.sub.c as
used above means the height of the layers of graphene stacked on
top of each other. The term "lateral extension L.sub.A" has the
usual meaning attributed to it in the carbon community. The more
ordered the non-graphitic carbon is, the higher are L.sub.c and
L.sub.A. The values for L.sub.c and L.sub.A of the present
materials are significantly higher than for other precursors
treated to temperatures in the same range. For instance, when
furfuryl alcohol or sugar are used and are heated to a temperature
in the same range, then they produce lower stacking heights below 1
nm and L.sub.A<1 nm. Graphitization, which is undesirable for
the purpose of the present invention, can only be achieved at heat
treatment temperatures above 2000.degree. K.
[0033] In a first variant of the invention the carbon material is
directly used in the form of a carbon monolith, i.e. the carbon
monolith is directly used as an electrode in a lithium-ion
battery.
[0034] In a second variant of the invention, the electrode is made
by pasting a mixture of the carbon material and a binder on a metal
foil.
[0035] In this case, the ratio of the carbon material to the binder
is preferably approximately 9 to 1 by weight.
[0036] The binder can expediently be poly (vinyl difluoride) and
the metal foil can be a copper foil.
[0037] The carbon material can be made by carbonizing a carbon
monolith precursor having a porosity generating fugitive phase
dispersed therein, said fugitive phase comprising particles in
first and second size ranges, said first size range being from 10
.mu.m to 100 nm and said second size range being from less than 100
nm to 1 nm, and by subsequently removing said fugitive phase to
form a porous carbon monolith having first and second pores in the
same size ranges.
[0038] In an alternative embodiment a carbon material is used which
is made by manufacturing a mixture containing at least one carbon
containing material and an organic polymer in an organic solvent,
by vaporizing the solvent until a viscous or highly viscous
composition of a corresponding shaped body is obtained and by
heating the composition of the shaped body to a temperature between
600.degree. C. and 1000.degree. C. Said carbon containing material
or carbon precursor is preferably mesophase pitch and the organic
polymer is preferably polystyrene.
[0039] It has been found, in accordance with the invention, that
the structure of the non-graphitic carbon and the battery
performance have a direct relationship to one another. The ability
to charge a carbon anode with lithium, necessary for the operation
of a lithium-ion battery using a carbon anode of a predetermined
geometry, sinks with increasing heat treatment temperature and the
associated structural change of the non-graphitic carbon, however,
its stability and reversibility increases. Accordingly, it is a
characteristic of the present invention that the porous transport
system and the non-graphitic carbon structure can be selected such
that a very high capacity and a very high stability can be realised
at the same time as the ability to charge the battery very rapidly.
This is of particular importance for a variety of applications,
including the use of a lithium-ion battery as an accumulator in a
battery-driven vehicle which is charged, for example,
regeneratively or from an internal combustion engine or from a fuel
cell system. It is also of particular importance for purely
electrically driven vehicles, or power sources including mobile
phone batteries, which need to be recharged quickly at
intervals.
[0040] Preferred embodiments of the invention are also described in
the subordinate claims and in the following description.
[0041] The invention will now be described in more detail with
reference to examples and to the accompanying drawings in which are
shown:
[0042] FIG. 1 Galvanostatic Li-insertion/extraction curves of
carbon having pores in first and second size ranges as specified
above which has been carbonized (heat treated) at 700.degree. C.
and cycled at C/5 between voltage limits of 0.01 and 3 V,
[0043] FIG. 2 Cycling performance of the carbon material of FIG.
1,
[0044] FIG. 3 Rate performance of the carbon material of FIG. 1
heat treated at 700.degree. C. and cycled at different current
densities between voltage limits of 0.01 and 3 V,
[0045] FIG. 4 Rate performance of non-porous mesophase pitch carbon
heat treated at 700.degree. C. and cycled at different current
densities between voltage limits of 0.01 and 3 V,
[0046] FIG. 5 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 heat treated at 700.degree. C. and cycled
at 1C between voltage limits of 0.01 and 3 V,
[0047] FIG. 6 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 but heat treated at 850.degree. C. and
cycled at 1C between voltage limits of 0.01 and 3 V,
[0048] FIG. 7 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 but heat treated at 1000.degree. C. and
cycled at 1C between voltage limits of 0.01 and 3 V,
[0049] FIG. 8 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 but heat treated at 1500.degree. C. and
cycled at 1C between voltage limits of 0.01 and 3 V,
[0050] FIG. 9 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 but heat treated at 2500.degree. C. and
cycled at 1C between voltage limits of 0.01 and 3 V,
[0051] FIG. 10 Galvanostatic Li-insertion/extraction curves of the
carbon material of FIG. 1 in the form of a membrane and carbonized
(heat treated) at 700.degree. C. and cycled at 1C between voltage
limits of 0.01 and 3 V,
[0052] FIG. 11 Rate performance of the carbon material of FIG. 1
formed as a monolith of 3.0 cm diameter and 1 mm thickness and
carbonized (heat treated) at 700.degree. C. and cycled at different
current densities between voltage limits of 0.01 and 3 V,
[0053] FIG. 12A A diagram illustrating the measurement of lithium
insertion and extraction from a carbon monolith using a so-called
Swagelock cell,
[0054] FIG. 12B A table showing parameters of an electrode material
in accordance with present invention in the form of the carbon
material of FIG. 11,
[0055] FIG. 12C An X-ray diffraction diagram showing x-ray
diffraction patterns from three different carbon materials formed
from three different carbon precursors, including the carbon
material made from mesophase pitch on which the test of FIG. 11 was
conducted and illustrating a comparatively high level of order of
the non-graphitic carbon made from mesophase pitch (but not from
the other precursors),
[0056] FIG. 13 A schematic illustration of a lithium-ion battery in
accordance with the invention with a carbon electrode on a carrier
foil,
[0057] FIG. 14 A lithium-ion battery in accordance with the
invention similar to FIG. 1, however with a monolithic carbon
electrode,
[0058] FIGS. 15A to 15F A series of sketches explaining the method
in accordance with the invention,
[0059] FIGS. 16A and 16B SEM micrographs of porous carbon material
as used for an electrode in accordance with the invention,
[0060] FIGS. 17A and 17B Diagrams illustrating the concept of
non-graphitic carbon and the non-graphitic carbon structure useful
for the present invention,
[0061] FIGS. 18A and 18B The reversible storage capacity of carbon
monoliths made from mesophase pitch as a function of the H/C atomic
ratio and parameters of the same monoliths as a function of
different pyrolysis temperatures, which shows how the temperature
range of 600.degree. C. to 1000.degree. C. leads to beneficial
results,
[0062] FIGS. 19A and 19B An X-ray diffraction curves for mesophase
pitch, with FIG. 19A showing the curve for the raw material with a
pronounced 20 peak at 26.5.degree. for a CuK.alpha. source and FIG.
19B showing similar diffraction curves for the same material after
pyrolysis at different temperatures,
[0063] FIG. 20 A typical SEM image of a sample of a highly porous
carbon monolith referred to as HPCM-1,
[0064] FIG. 21 A typical SEM image of HPCM-1 loaded with Pt and
referred to as HPCM-Pt,
[0065] FIG. 22 A typical SEM image of HPCM-1 loaded with RuO.sub.2
and Pt and referred to as HPCM-RuO.sub.2--Pt,
[0066] FIG. 23 cyclic voltammograms of methanol on HPCM-Pt in IM
CH.sub.3OH/0.5 M H.sub.2SO.sub.4 electrolyte at 20 mVs-1 at room
temperature and
[0067] FIG. 24 cyclic voltammograms of methanol on
HPCM-RuO.sub.2'-Pt in IM CH.sub.3OH/0.5 M H.sub.2SO.sub.4
electrolyte at 20 mVs-1 at room temperature.
[0068] Turning first of all to FIGS. 17A and 17B the non-graphitic
carbon material underlying the present invention will first be
described. FIG. 17A shows, in the diagram at the top left, how the
structure of non graphitic carbon changes as it is subjected to
heat treatment (pyrolysis) at different temperatures. It can be
seen from this diagram that in the temperature range of 600 to
1000.degree. C. to which the present invention relates, i.e. 873 to
1273.degree. K., the structure comprises a plurality of randomly
orientated small graphene stacks 2. The stacks themselves comprise
sheets 4 of carbon atoms arranged in a generally hexagonal
arrangement. Each sheet 4 shown as a rectangle in FIG. 17B is
termed a graphene. The carbon atoms in each graphene have no
positional correlation with the position of carbon atoms in the
graphenes below and above.
[0069] As the heat treatment temperature increases, the
non-graphitic carbon becomes progressively more ordered and, at a
temperature above 2273.degree. K., i.e. 2000.degree. C., is
converted into graphite, i.e. a crystalline form of carbon.
[0070] The graphene stacks in the temperature range of interest and
for a material having first and second pore sizes in the ranges 10
.mu.m to 100 nm and less than 100 nm to 3 nm (preferably not less
than 3 nm) are shown to an enlarged scale in FIG. 17B. The graphene
stacks have stack heights in the range from 2 to 30 nm and lateral
extension values L.sub.A in the range from 2 to 8 nm. The graphene
stacks either contact one another or are separated by amorphous
carbon usually distributed throughout the structure between the
graphene stacks and present in a total amount relative to the
graphene stacks of less than 10% by weight.
[0071] As can be seen in FIG. 17B the first pores 6 are separated
by micrometer-thick walls, which themselves contain the second
pores 8. The second pores 8 are defined between neighbouring
graphene stacks 2. This structure has the particular advantage that
lithium atoms can readily move into and out of the structure and
this contributes to the outstanding properties of the carbon
material as an electrode in a lithium-ion battery. More
specifically, it has been found that in such non-graphitic carbon
material made from mesophase pitch the second pores 8 have an
almost ideal size in the range 3 to 8 nm and that relatively few
pores with a size less than 3 nm are present and this is extremely
favourable in a lithium-ion battery.
[0072] This range for the preferred size of the second pores 8,
i.e. 3 nm to 8 nm, also applies to uses of the carbon material in
applications other than lithium batteries. E.g., it is also a
beneficial size range for a carbon material used as a support for
Pt or a RuO.sub.2/Pt catalytic loading and/or coating. Because the
carbon material is porous the coating with, for example, a catalyst
means that the material involved, in this example the catalyst is
present on the carbon surfaces within the pores. This can be
considered as "internal coating" or as "loading" because the
material involved is distributed in the porous carbon in three
dimensions. Generally speaking, the range for the sizes of the
second pores can extend from 3 nm to under 100 nm, with the range
from 3 nm to 50 nm being preferred and the range from 3 nm to 8 nm
being particularly preferred. What these size ranges mean is that
the majority of the pores, which are of generally irregular shape,
have an average size generally at the middle of the range. Thus
there are finer and coarser pores within the size range on either
side of the average size and indeed there can be finer and coarser
pores outside of the size range. For the size ranges quoted above
for the second pores finer pores lying outside of the size range
are not normally desirable but frequently unavoidable. Larger pores
outside the size range for the second pores fall within the size
range given for the first pores and can have utility within this
range even if they are not preferred for the transport mechanisms
associated with the pores in the first, larger, size range. The
distribution of the pore sizes of the pores in the second size
range cannot be specified more closely with reference to a
particular shape of the pore size distribution because this
distribution does not necessarily correspond to a standard
distribution, such as a normal or Gaussian distribution. Generally
speaking, about 80% of the pores will have sizes in the preferred
range, with the remaining 20% having sizes outside of the preferred
range, but within the total range quoted. Thus, for the preferred
size range of 3 nm to 8 nm, 80% of the second pores will have sizes
in this range and will have an average size of 5.5 nm. Some pores
will be smaller down to the minimum useful size of about 3 nm and
some will be larger up to the maximum useful size of just under 100
nm. As noted above, this definition does not mean that pore sizes
below 3 nm are precluded. Indeed there can easily be a relatively
large number of pores with pores sizes below 3 nm, they are just
not important for the applications envisaged. Generally speaking,
pores with a pore size under 3 nm can have a total volume which
amounts to about 10% of the total pore volume of pores in the
second size range. The smaller this percentage the better, since it
means there is a higher proportion of the good second pores with a
size in the range 3 nm to just under 100 nm.
[0073] The question also arises as to what is understood by an
average pore size for pores that have an irregular shape and are
indeed largely interconnected so that they form irregular channels
in three dimensions.
[0074] One way of looking at this is to consider a section through
the material, e.g. as shown in FIGS. 16A and 16B. The large black
areas shown in FIGS. 16A and 16B are the pores in the first size
range, the first pores, defined here as being the range from 10
.mu.m to 100 nm. For each black area a maximum length dimension L
and a maximum width dimension W in the plane of the section can be
estimated and the value of (L+W)/2 taken as an average size.
[0075] Alternatively, the area of each black area can be estimated
and equated to the surface area of a circle having a surface area
of .pi.r.sup.2, with the resulting value of 2r being taken as the
average size. Both methods tend to lead to comparable results for
the average size of the pore. Precisely the same concept can be
used for the second pores which are also irregular and largely
interconnected in three dimensions.
[0076] Also it can easily be seen from FIGS. 16A and 16B that the
walls between the pores in the first size range, i.e. the material
of the walls of the structure bounding the interconnected first
pores, which has the pores in the second size range, generally has
thicknesses comparable to the average sizes of the pores in the
first size range. As a result of the presence of the interconnected
second pores in this material and the large number of these second
pores in the second size range, the average thickness of solid
material between the interconnected second pores tends to be
comparable with the average size of the pores in the second size
range.
[0077] Such structures typically have a ratio of the total volume
of the pores in the first size range to that of the pores in the
second size range as herein defined in the range from 2 to 12. Such
structures have been found to have extremely beneficial properties.
The amount of carbon material to pore volume provides for maximum
exploitation of the carbon material, e.g. in the sense that a
maximum amount of Li can be reversibly absorbed in an Li-ion
battery or in the sense that a large exploitable catalytic surface
is available in a fuel cell or other catalytic converter. The size
range of the first pores provides for good transport of the active
fluid, e.g. an electrolyte in a lithium-ion battery, or methanol in
a fuel cell, or gas flow in a catalyst supporting gas reactions,
through the porous material thereby providing access of the liquid
or gas to the active material. The pores in the second size range
make the active material readily accessible to the material being
transported, e.g. enabling Li ions to attach to the carbon material
and be detached from it again in an Li-ion battery or enabling
methanol or a suitable gas to reach a catalyst supported on the
solid material of the porous structure.
[0078] Again it is not necessary for the pores in the first size
range to have sizes throughout the size range although inpractise
this tends to be the case although the majority of the first pores
have a size in the range from 500 nm to 5 .mu.m and an average
size, e.g. as estimated by one of the methods given above, of about
1 .mu.m. Pores with sizes greater than 10 .mu.m are not considered
desirable because such sizes tend to reduce the amount of active
material present in a unit volume, thus reducing the performance of
the material. For this reason smaller first pore sizes are
preferred e.g. 1 .mu.m providing this allows adequate access of the
relevant fluid to the active material. Smaller pore sizes for the
first pores than 500 nm tend to increase the amount of active
material available per unit volume and also tend to make the wall
thicknesses between adjacent first pores smaller facilitating
access to the finer second pores. they can thus be beneficial down
to the bottom size of the range given of 100 nm providing they do
not undesirably impede the transport of the fluid involved to the
smaller pores.
[0079] The use of a material with graphene stacks is particularly
beneficial, not just because of the favourable poor structure but
also because the graphene stacks result in a degree of electronic
conductivity of the interconnected porous structure which is
essential for reversible Li-ion storage in an Li-ion battery and
for the generation and transport of electricity in a fuel cell.
[0080] Naturally, graphite material per se has good electrical
conductivity, however, in the absence of a suitable pore structure
it is not useful for any of the applications envisaged. For
example, it would not provide sufficient Li storage capacity per
gram. More specifically, pure graphite has a Li storage capacity of
372 mAh/gm for the first cycle but reducing for subsequent cycles,
whereas the carbon material of the present invention approaches
storage capacities of 2000 mA hour/gm for the first cycle and, even
allowing for losses in subsequent cycles, the reversible storage
capacity remains higher than 500 mAh/gm.
[0081] One other point that is important for the material of the
present invention is the so-called H/C atomic ratio. This ratio
depends on the precursor that is used and also on the temperature
at which the material is pyrolized. For temperatures in the
preferred range of 600.degree. C. to 1000.degree. C. the H/C atomic
ratio lies in the range from about 30% to about 1%. Ratios of
around 20% have been found to be particularly useful.
[0082] Preferred parameters for porous carbon monolith prepared
from mesophase pitch can be seen in the table of FIG. 18B. The
graph of FIG. 18A shows how the reversible storage capacity in
mAh/g is related to the H/C atomic ratio for the same material with
reference to different pyrolisis temperatures. FIG. 18A also
compares the reversible storage capacity to the theoretical
reversible storage capacity.
[0083] It should also be noted that values of the H/C atomic ratio
around 0.2 to 0.075 may be beneficial because, e.g. in a Li-ion
battery, the lithium ion can bond to the hydrogen sites giving rise
to rather large capacities.
[0084] The graphene stack structure described for the material of
the present invention with a certain amount of amorphous carbon
present between the individual graphene plates or sheets 4 of the
graphene stacks 2 and between the graphene stacks is also
advantageous in a lithium ion battery because it facilitates the
use of propylene carbonate in an electrolyte, e.g. 1M LiPF.sub.6 in
propylene carbonate as a solvent, for reasons which are not fully
understood. However, this is most beneficial because propylene
carbonate remains fluid at low temperatures and this makes it
possible to enhance the low temperature performance of lithium ion
batteries and indeed to enable them to work at all at temperatures
below minus 40 degrees C. Other organic anhydrous solvents than
propylene carbonate which remain fluid at low temperatures can also
be utilised to advantage in an electrolyte in a lithium ion battery
incorporating the carbon material of the present invention.
[0085] Mesophase pitch is a particularly beneficial starting
material, i.e. precursor for the formation of the porous carbon
material presently proposed. One characteristic of mesophase pitch,
which is chemically derived from naphthalene, is that it has a
pronounced diffraction peak in the 20 diffraction plane at
26.5.degree. which shows there is some degree of ordering present
similar to that of graphite in the [002] plane. Generally it is
found that precursors suitable for making the carbon material of
the present invention can be any organic material or organometallic
material having a pronounced diffraction peak at 26.5.degree..
[0086] FIGS. 19A and B shows such a diffraction curve for mesophase
pitch (mesophase pitch (AR) from Mitsubishi) Should an organic
material or an organometallic material or an improved form of
mesophase pitch be developed with a sharper 2.theta. peak than
mesophase pitch AR, then this should also be beneficial as a
precursor for the carbon material of the present invention.
[0087] Referring now to FIGS. 15A-15F, a first method of making a
carbon monolith will be described. FIG. 14A shows a glass beaker 10
with an open top 12 and a rod-like SiO.sub.2 monolith 14 resting on
the bottom of the beaker. The SiO.sub.2 monolith is porous having
pores in two different size ranges and is fabricated in the manner
known from the prior art, for example in the papers by K. Nakanishi
and N. Soga in Am. Cerm. Soc., 1991, 10, 2518; N. Tanaka, H.
Kobayashi, N. Ishizuka, H. Minakuchi, K. Nakanishi, K. Hosoya and
T. Ikegami in J. Chrom. A 2002, 965, 35 and M. Motokawa, H.
Kobayashi, N. Ishizuka, H. Minakuchi, K. Nakanishi, H. Jinnai, K.
Hosoya, T. Ikegami and N. Tanaka in J. Chrom. A 2002, 961, 53.
[0088] As indicated in FIG. 15B, the SiO.sub.2 monolith 14 is then
impregnated by adding mesophase pitch dissolved in THF
(tetrahydrofurane) into the beaker 10 so that the dissolved
mesophase pitch can gradually fill all the pores which is favoured
by the capillary action of the SiO.sub.2. In order to dissolve the
mesophase pitch (Mitsubishi AR) in THF, the mixture is subjected
for 20 minutes to ultrasonic agitation (100%) and shaking on a
horizontal shaker at low intensity. Alternatively, any other shaker
or magnetic stirrer can be used. After shaking for about 3 days,
the resultant mixture is centrifuged, for example for 10 minutes at
6500 rpm. The solution is used for the infiltration/impregnation
process. The not yet dissolved mesophase pitch can be reused. The
ratio by mass of the mesophase pitch solution (the carbon
precursor) to the SiO.sub.2 is preferably 80:1.
[0089] Thereafter, in accordance with FIG. 15C, the glass beaker
containing the SiO.sub.2 monolith and the mesophase pitch solution
are placed on a shaker 18 and the solution is concentrated by
evaporating the THF while subjecting the beaker and the SiO.sub.2
monolith to horizontal shaking (double arrow 20) at room
temperature. This ensures the pores are completely infiltrated with
mesophase pitch.
[0090] Thereafter, after evaporation of the THF, in accordance with
FIG. 15D, the SiO.sub.2/mesophase pitch hybrid body 14', i.e. the
infiltrated monolith, is placed in a quartz tube (22, FIG. 15E) and
left to dry for about one day.
[0091] The quartz tube should be slightly larger than the monolith
and should surround it. Without the quartz tube or other adequate
support, such as a metal container, there is a danger that the
monolith will deform or crack. The infiltrated monolith is then
carbonized in an oven (24, FIG. 15F) the quartz tube in an N.sub.2
or other inert gas atmosphere at a temperature between 600 and
1000.degree. which results in the desired structure of the
non-graphitic carbon material supplied in the form of the mesophase
pitch.
[0092] The heating rate should be 1.5.degree. K./min with a 6 h
plateau at a carbonisation temperature in the range
600-1000.degree. C.
[0093] Thereafter, the monolith is dipped in a bath of a solvent
for the silicate template. The solvent can be 4M ammonium hydrogen
difluoride in water in an amount of 100 ml solvent for 0.1 g
SiO.sub.2. Alternatively, the solvent can be a sodium
hydroxide-EtOH--H.sub.2O solution at a 100.degree. C. After being
horizontally shaken for about 3 days (gentle to and from movement),
the solution is poured away and the monolith is rinsed once with
water. For complete removal of the ammonium hydrogen difluoride,
the monolith is shaken to and from on the horizontal shaker in
demineralised water. Thereafter, all the liquid is poured away, the
monolith is rinsed again and dried for 2 hours in an oven at
100.degree. C. After removal of the silicate template, the carbon
monolith can be dried and is ready for use.
[0094] The FIGS. 16A and 16B show two SEM images of a carbon
monolith made in accordance with FIGS. 15A-15F using mesophase
pitch as the carbon precursor. FIG. 16B is drawn to a larger scale
than FIG. 16A and is in fact magnified by a factor of approximately
5 relative to FIG. 16A. The network-like support system, i.e. the
interconnected passage system shown in black in FIGS. 16A and 16B,
can be clearly recognised and this system is a 1:1 copy of the
skeleton of the corresponding silicate structure, i.e. the shape of
the silicate skeleton has been preserved in the form of the
passages of the carbon monolith from which FIGS. 16A and 16B were
prepared. The pore transport system present between the webs, the
lighter areas in the images, has a well-defined mesh width here of
ca. 2-5 .mu.m. The fine pores of the carbon system are not readily
visible in the images of FIGS. 16A and 16B, however, they are
indicated by the grainy nature of the lighter areas in FIG. 16B,
i.e. of the carbon material. Depending on the dimensions of the
silicate structure, pore volumes can readily be achieved in the
range from 0.4 to 0.8 cm.sup.3/g when using mesophase pitch as the
starting product. Moreover, the conditions described in relation to
FIGS. 15A-15F lead to a specific surface of ca. 300 m.sup.2/g.
After removal of the silicate template, the carbon monolith can be
dried and is ready for use. The monolith typically has first pores
in the size range 10 .mu.m to 100 nm, second pores in the size
range from less than 100 nm to 3 nm, a specific surface of
approximately 300 m.sup.2/g and a total pore volume of
approximately 0.4 cc/g.
[0095] It seems that the carbon precursor has to be carefully
selected in order to achieve an appropriate non-graphitic carbon
structure. This is certainly possibly using "mesophase pitch" with
preconjugated carbon units as the precursor. The manufacturing
process for the new carbon material is moreover simple (one step
infiltration/impregnation at room temperature). The starting
materials are favourably priced and the carbon monoliths that are
produced are crack-free. They can readily be scaled up to any size
that is desired.
[0096] The carbon monolith produced in the above manner can be used
as it stands as a carbon electrode in a lithium-ion battery, for
example as is illustrated at 30 in FIG. 14. The carbon electrode
shown here has a diameter of 4.3 cm and a thickness of 1 to 2 mm.
Here, in the manner usual in lithium batteries, the carbon
electrode 30 and a lithium electrode 32 are placed spaced apart
within a housing 34 and a suitable electrolyte 36 is present
between the electrodes. The solution can be a solution of 1 M
LiPF.sub.6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) in
the ratio 1:1 by volume obtained from Ube Industries Ltd., which is
accommodated in a glass fibre separator 38 (GF/D from Whatman). The
lithium electrode 32 is for example a pure lithium foil available
from Aldrich.
[0097] The experimental set up for measuring lithium insertion into
and extraction out of a monolithic carbon test body such as 30 is
shown in FIG. 12A and will be described using the same reference
numerals as in FIG. 13. In FIG. 12A the housing 34 (left hand
illustration) takes the form of a so-called Swagelock cell in which
the monolithic carbon disc 30 is sandwiched between a stainless
steel plate 41 and a glass fibre separator 38 (GF/D from Whatmann).
Reference numeral 32 is again a lithium electrode covered with a
further stainless steel plate 43. The assembly at the right hand
side of FIG. 12A is mounted in the cell at the left hand side with
a suitable electrolyte as described above in connection with FIG.
13. The connections to the electrodes 30 and 32 are made via the
stainless steel plates 41 and 43 respectively, which are of neutral
behaviour so far as the lithium-ion cell is concerned. The external
connections to the electrodes are made via the respective terminal
screws shown at the top and the bottom of the cell at the left side
of FIG. 12A.
[0098] The table of FIG. 12B shows characteristic values measured
at different charge densities and after a given number of
charging/discharging cycles for the initial capacity and the final
capacity of a porous monolithic carbon electrode in accordance with
the present teaching. These values were measured using the
experimental set up of FIG. 12A with a porous monolithic carbon
electrode carbonized at 700.degree. and made, as herein described
below, from mesophase pitch by infiltration of a porous SiO.sub.2
monolith and subsequent removal of the SiO.sub.2 to leave a carbon
structure having first pores in the size range from 1 .mu.m to 500
nm and second pores in the size range from less than 20 nm to 3 nm,
a specific surface area of around 300 m.sup.2/g and a pore volume
of 0.6 cm.sup.3/g. The local structure of the carbon was also
measured by X-ray diffraction. The resulting diffractogram shown in
FIG. 12C indicates a comparatively high level of order of the
non-graphitic carbon, which is typical of the preorganized
mesophase pitch based materials and exceeds other typical carbon
precursors. FIG. 12C also depicts for comparison purposes
corresponding samples made from sugar and furfuryl alcohol
precursors under identical conditions which do not lead to the
ordered graphene structure and thus also not to the beneficial
results achieved with mesophase pitch as a precursor.
[0099] Other electrolytes can be used in the cells of FIGS. 13 and
14, for example an electrolyte in accordance with EP-A-1505680 of
the present applicants.
[0100] As an alternative, the carbon monolith can be ground to a
powder, for example a powder with an average particle size of 20
.mu.m and can be mixed with a binder, such as polyvinyl difluoride
at a weight ratio of 9 parts of carbon material to 1 parts of PVDF
and then pasted onto a metal foil, for example a copper foil of
99.6% purity available from Goodfellow. A lithium-ion battery with
a carbon electrode 30' prepared in this manner is schematically
illustrated in FIG. 13, the copper foil has the reference numeral
40, otherwise the same reference numerals are used as in FIG. 14
and have the same significance.
[0101] The results of electrochemical experiments performed using
two-electrode Swagelok-Type.TM. cells such as that shown in FIG.
12A, in this case using a monolithic carbon electrode in accordance
with FIG. 14 prepared as described above. The cell of FIG. 14 was
assembled in an argon-filled glove box and investigations of the
electrochemical performance at different current densities were
carried out on an Arbin MSTAT system. The results will now be
discussed with reference to FIGS. 1 to 11.
[0102] FIG. 1 shows the discharge (Li-insertion)/charge
(Li-extraction) curves of porous carbon (heat treated at
700.degree. C.) having first pores in the size range from 1 .mu.m
to 500 nm and second pores in the size range from less than 20 nm
to 3 nm, a specific surface area of around 300 m.sup.2/g and a pore
volume of 0.6 cm.sup.3/g and cycled at a rate of C/5 between
voltage limits of 0.01 and 3 V. Charge rates such as C, C/5, C6 are
well understood in the art of lithium batteries. It is known that
in a fully discharged battery a carbon electrode can incorporate
one atom of lithium for every six atoms of carbon, i.e. a compound
LiC.sub.6 is formed and has a theoretical maximum capacity of 372
mAh/g. If the battery is discharged in five hours, then the
discharging rate is specified as C/5, if the discharging is
effected in one hour, then the discharging rate is 1C. If the
discharging rate is 60C, then this means the battery is fully
discharged in 1 minute, at 30C it is fully discharged in two
minutes etc. At a current density corresponding to C/5, the first
charge capacity after four charging and discharging cycles is about
900 mAh/g. However, it can be seen from FIG. 2 that the charge
capacity decreases during the first several cycles and stabilizes
at about 500 mAh/g after 40 charging cycles. This is already about
1.6 times higher than the current "good practice" in industry, that
is the anode side can have a 1.6 times higher storage density,
assuming the same battery weight. It should be noted that it is an
important finding of the present invention that it is possible to
find higher charge capacities compared to the theoretical maximum
when the structure of the carbon electrode is not fully ordered and
it does not consist of graphite, but is rather a form of
non-graphitic carbon. It has also been discovered that the charge
capacity increases as the amount of order of the non-graphitic
carbon (i.e. L.sub.A and L.sub.c increases but that the reversible
charge capacity with repeated discharge cycles is not as high.
[0103] Another excellent property of this non-graphitic carbon
having first pores in the size range from 10 .mu.m to 100 nm and
second pores in the size range from less than 100 nm to 3 nm, a
specific surface area in the range from 50 m.sup.2/g to 800
m.sup.2/g and a pore volume in the range from 0.1 to 1.0 cm.sup.3/g
is the high rate capability. Results are shown in FIG. 3 in which
rates of up to 60C (one lithium per six formula unit in 1 min.)
have been employed. The cell was first cycled at 1C and, after 4
cycles, the rate was increased in stages to 60C. A specific charge
capacity of around 543 mAh g.sup.-1 was obtained at a rate of 1C
after 4 cycles; this value is lowered to 260 mAh g.sup.-1 at 10C,
185 mAh g.sup.-1 at 20C, 143 mAh g.sup.-1 at 30C, 112 mAh g.sup.-1
at 40C, 87 mAh g.sup.-1 at 50C, and finally, 70 mAh g.sup.-1 at
60C. This rate performance is much better that that of non-porous
carbon heat treated from the same precursor (mesophase pitch, see
FIG. 4). Surprisingly, it can also be seen that the higher the
current density, the better is the cycling performance. As far as
carbon is concerned, it is the best rate performance ever measured
for a carbon electrode in a lithium-ion battery, especially at a
higher charging rate.
[0104] FIGS. 5, 6, 7, 8 and 9 show the discharge/charge curves of
the same carbon material as described in connection with FIG. 1
carbonized (heat treated) at 700.degree. C., 850.degree. C.,
1000.degree. C., 1500.degree. C. and 2500.degree. C. and cycled at
1C between voltage limits of 0.01 and 3 V. It can be seen that the
charge capacity decreases with increasing pyrolysis temperature. It
can also be seen that the higher the pyrolyzing temperature, the
better is the cycling performance. At 2500.degree. C., the
electrochemical behaviour of the carbon is more like that of
graphite.
[0105] FIG. 10 shows the discharge/charge curves of another type of
carbon having first pores in the size range from 10 .mu.m to 100 nm
and second pores in the size range from less than 100 nm to 3 nm, a
specific surface area in the range from 50 m.sup.2/g to 800
m.sup.2/g and a pore volume in the range from 0.1 to 1.0 cm.sup.3/g
which has also been carbonized (heat treated) at 700.degree. C. and
cycled at 1C between voltage limits of 0.01 and 3 V. This is used
as a repeat experiment to prove that the observed properties do not
depend on the specific preparation, but are general for this type
of hierarchical porous carbon. FIG. 11 shows the rate performance
of this type of porous carbon. It can be seen that it shows almost
the same performance with the first sample. An alternative method
of preparing suitable porous carbon materials having first pores in
the size range from 10 .mu.m to 100 nm and second pores in the size
range from less than 100 nm to 3 nm, a specific surface area in the
range from 50 m.sup.2/g to 800 m.sup.2/g and a pore volume in the
range from 0.1 to 1.0 cm.sup.3/g will now be described including a
specific embodiment used for preparing the sample used for the
investigations underlying FIGS. 10 and 11.
[0106] In an alternative embodiment (not shown) a porous SiO.sub.2
monolith is heated in an oven to 200.degree. C. A naphthol solution
is poured over it for a short period of time while the monolith is
at 200.degree. C. The naphthol solution, which forms the carbon
precursor, is manufactured from naphthol, ethanol and sulphuric
acid in a ratio by weight of naphthol:ethanol: H.sub.2SO.sub.4
equal to 1:3.5:0.15. To prepare this solution, the naphthol is
first dissolved in the ethanol and the H.sub.2SO.sub.4 is then
added and the components are then mixed for 30 minutes with a
magnetic stirrer. The naphthol solution infiltrates the monolith.
The whole monolith is covered completely with the precursor
solution and immediately thereafter, the solution is removed with a
pipette. In this way a situation is achieved in which only a small
proportion of the solution penetrates into the monolith. The so
infiltrated monolith is then left for 30 minutes in the oven at
200.degree. C. At 200.degree. C. the ethanol vaporizes and the
naphthol carbonizes. This process can be repeated 3 to 4 times to
increase the quantity of carbonized naphthol in the monolith. In
the end, a stable monolith is obtained. Less frequent infiltration
leads to higher porosity (surface area.about.2500 m.sup.2/g) but
more unstable monoliths. The hybrid body comprising the SiO.sub.2
matrix and the carbonized naphthol is then heated to a higher
temperature such as 800.degree. C. More specifically, it can be
heated at a heating rate of 1.5.degree. C./min and held at a
plateau of 600.degree. C. to 1000.degree. C. for six hours.
Thereafter, the SiO.sub.2 is removed using an NaOH or HF solution.
A macroporous/mesoporous carbon material remains with a surface
area of ca. 800 m.sup.2/g.
[0107] In a further possible and generally applicable method, a
porous carbon material having first pores in the size range from 10
.mu.m to 100 nm and second pores in the size range from less than
100 nm to 3 nm, a specific surface area in the range from 50
m.sup.2/g to 800 m.sup.2/g and a pore volume in the range from 0.1
to 1.0 cm.sup.3/g and suitable for the present invention can be
prepared using the following techniques.
[0108] This general method is based around the concept of
manufacturing a mixture which contains at least one carbon
precursor and an organic polymer in an organic solvent, vaporizing
the solvent until a viscous or highly viscous composition or a
corresponding shaped body is obtained, with subsequent pyrolysis of
the composition of the shaped body at temperatures between
600.degree. C. and 1000.degree. C. Pitch, in particular mesophase
pitch, is preferably used as the carbon precursor. The organic
polymer can be polystyrene. A Lewis acid can be added to the
mixture during its preparation. The heating of the shaped body
preferably takes place initially to temperatures between 200 and
400.degree. C. and then to temperature between 500.degree. C. to
1000.degree. C. Many variations of the method are possible. For
example, the mixture, which is initially prepared, can contain two
or more different organic polymers of different molecular mass or
one organic polymer with two or more different molecular masses.
One or more softeners can be added to the mixture. The mass or
composition which is formed can be shaped by extrusion.
[0109] Basically, all substances can be used as carbon precursors
which produce directly, or after the carbonisation or pyrolysis, a
three-dimensional structure which consists predominantly of carbon.
Examples for such carbon precursors are pitches, in particular
mesophase pitch, but also naphthen or other organic compounds or
organo-metallic compounds can be considered providing they show a
pronounced 20 diffraction peak at 26.5.degree. using an appropriate
source such as a CuK.alpha. source. The carbon precursors can be
used individually or as a mixture of two or more carbon
precursors.
[0110] The term "pitch" includes all viscous to solid tar-like or
bituminous fusible materials which remain, for example, after
pyrolysis or distillation of organic materials (natural
substances), or of coal tar or bituminous tar. In general, pitches
consist of high molecular cyclic hydrocarbons and heterocycles
which can have a molecular mass of up to 30,000 g/mol.
[0111] Mesophase pitch is a type of pitch was consists of various,
principally aromatic hydrocarbons and contains anisotropic liquid
crystalline regions. A review concerning the manufacture and
characteristics of mesophase pitch is provided by Mochida et al.,
The Chemical Record, Vol. 2, 81-101 (2002). Mesophase pitch can,
for example, be purchased from the Mitsubishi Gas Chemical
Company.
[0112] As organic polymers it is possible to use all organic
polymers with a solubility parameter in accordance with Hildebrandt
between 8 and 12. In the same way, the term organic polymer will be
understood to mean mixtures of two or more corresponding organic
polymers which can have different molecular masses or the same
molecular masses. Furthermore, mixtures can be used as the organic
polymer, which have an organic polymer in two or more different
molecular masses. The term organic polymer will also be understood
to include copolymers or block polymers, such as for example
polyoxyethylene glycolether ("Brij" tensides) or poly(ethylene
oxide)-.beta.-poly(propylene oxide). In a preferred embodiment
polystyrene is used as the organic polymer. The molecular mass of
the polymers used typically lies between 500 g/mol and 1,000,000
g/mol, preferably between 10,000 and 500,000 g/mol. In principal,
polymers with molecular masses above 500,000 to 1,000,000 g/mol can
be used. It was, however, found that polymers with larger molecular
masses easily precipitate during the removal of the solvent and can
thus disturb the phase separation inherent in the manufacturing
method under discussion. If mixtures of different polymers or
mixtures of a polymer with different molecular masses are used,
then a mixture is preferably selected of an organic polymer with a
molecular mass between 500 and 10,000 g/mol and an organic polymer
with a molecular mass between 50,000 and 500,000 g/mol. Through the
choice of the organic polymer and its molecular mass, or the mass
distribution using polymer mixtures, it is possible to exert an
influence on the later pore distribution in the shaped body. The
molecular mass and the molecular mass distribution determine the
demixed structure which arises on evaporation of the solvent and
thus the porosity. Smaller molecular masses lead to later demixing
and thus to smaller pore systems.
[0113] All organic solvents or solvent mixtures can be used as the
organic solvent, which are able to dissolve the carbon precursor
and the organic polymer to an adequate degree. Furthermore, it is
advantageous when the solvent can be evaporated as simply as
possible. Accordingly, solvents with a low boiling point and/or
high vapour pressure are preferred. Examples for suitable solvents
are THF or Xylol.
[0114] Evaporation signifies in this context the at least partial
removal of the organic solvent up to a formation of a shapeable
composition. The evaporation can take place by simply allowing the
mixture to stand, i.e. by vaporisation, or can be accelerated, for
example in that a surface which is as large as possible is
produced, for example in a shallow container. Alternatively, or
additionally, the temperature can be increased or a vacuum can be
generated. Melt extrusion signifies in this context the
introduction of a concentrated shapeable composition in the
described sense into a heatable extrusion plant. The phase
separation can be completed in the extrusion plant and/or the
burning out of the organic polymer can at least be started there.
Through the melt extrusion a shaped body is formed. This is,
however, not the only possibility of forming a shaped body; the
composition can simply be cast into an appropriately shaped
mold.
[0115] Pyrolysis signifies in this connection a tempering or
temperature treatment, i.e. heating of the composition. As a rule,
the organic polymer is at least partly burned out by pyrolysis,
i.e. removed or converted into non-graphitic carbon or graphite.
Carbonisation is also a type of pyrolysis.
[0116] Carbonisation signifies here the conversion of a carbon
precursor into non-graphitic carbon or graphite or both.
[0117] In carrying out the process, a mixture is first formed which
contains a carbon precursor and an organic polymer in an organic
solvent. A quantity of the solvent is not critical in this respect,
since it is later removed by evaporation. Suitable mixing ratios
(carbon precursor plus organic polymer: organic solvent) typically
have weight ratios between 1:100 and 3:1, depending on the
solubility of the carbon precursor and of the organic polymer in
the organic solvent.
[0118] The mixture, which contains a carbon precursor and an
organic polymer in an organic solvent, is preferably a solution.
The mixture can, however, also include small proportions of
undissolved carbon precursor and/or organic polymer without this
disturbing the further conduction of the process. Furthermore,
non-soluble substances, such as inorganic pigments, particles or
the like, can be added to the mixture.
[0119] The mixture in accordance with the invention can also be an
emulsion. Here, the terms "dissolve" or "dissolving" are used in
conjunction with the manufacture of the mixture which contains at
least one carbon precursor and at least one organic polymer in an
organic solvent. However, these terms do not mean that 100% of the
substances are dissolved, but rather that a part of the substances
has been dissolved, for example preferably 70 to 95%. If only a
smaller proportion of the components is dissolved, then the total
or predominantly remaining part of the non-dissolved material can
be separated by filtration or centrifuging or by decanting. The
carbon precursor and the organic polymer are, however, preferably
fully dissolved. The carbon precursor and the organic polymer can
first be separately dissolved in the organic solvent and
subsequently mixed, or can be directly dissolved at the same time
or after one another in the organic solvent. Generally, it is more
expedient to dissolve the carbon precursor and the organic polymers
separately in the organic solvent and then to mix them, since in
this way the dissolution characteristics of the components can be
better taken into account. For example, when using pitches such as
mesophase pitch, it can transpire that these components cannot be
fully dissolved in the quantity of solvent provided. The operator
can then decide whether to increase the quantity of solvent or to
simply use the dissolved material by separating it from the
non-dissolved material. Dissolving can be assisted by technical
means, such as heating, stirring or ultrasonic treatment.
[0120] If initially separate solutions of the carbon precursor and
the organic polymer are produced in the organic solvent, then the
preferred concentrations for these solutions are 10-70% by weight,
in particular 40-70% by weight for a carbon precursor and 10-60% by
weight, preferably 30-60% by weight of the organic polymer. The
volume relationships between the carbon precursor and the organic
polymer are determined by the desired macroporosity. Typical volume
ratios between the carbon precursor and the organic polymer lie
between 1:0.1 to 1:10 and preferably between 1:0.5 and 1:4.
[0121] If the two solutions are formed separately, then they are
substantially united with vigorous stirring to ensure full mixing.
The carbon precursor and the organic polymer can also be dissolved
in different solvents if, after uniting the two solutions, the
final mixture is adequately homogenous and no precipitation of one
of the components is observed.
[0122] Further substances can be added to the mixture of the
organic solvent, carbon precursor and/or organic polymer. These
can, for example, be substances which influence the later demixing,
such as softeners, further solvents, tensides, substances which
influence the later carbonisation behaviour such as, for example,
Lewis acids like FeCl.sub.3, or Fe, Co, Ni or Mn, or substances
which influence the material characteristics of the later shaped
body. When Lewis acids are added, then these are preferably added
in a quantity which corresponds to 0.1 to 10% by weight of the
carbon precursor.
[0123] The at least partial phase separation which is aimed at for
the formation of the macroporous structures can take place both
during evaporation of the solvent and also during later mechanical
or thermal treatment, for example melt extrusion. As a rule, the
phase separation already starts during the evaporation of the
solvent and is continued during later mechanical and/or thermal
treatment.
[0124] In just the same way an extraction step can be carried out
prior to heating of the shaped body. This extraction step can serve
for the extraction of an organic solvent which is difficult to
remove completely by allowing the solvent to vaporize or, however,
for the removal of at least a part of the organic polymer. Thus,
the extraction step can fully or partly replace the pyrolysis of
the organic polymer. The extraction can take place with all aqueous
or typically organic solvents or solvent mixtures. Depending on the
purpose of the extraction, the person skilled in the art is able to
select suitable solvents.
[0125] During the heating or pyrolysis, the organic polymer
remaining in the composition is burned out or also carbonized and
in this way generates a pore structure. Depending on the organic
polymer the situation can be such that the organic polymer is
almost fully burned out or, however, that a certain proportion of
residues (principally carbon residues) from the organic polymer
remains in the carbon-containing material or shaped body following
pyrolysis.
[0126] Moreover, during the heating or pyrolysis, the structure of
the carbon precursor changes. For the pitch or mesophase pitch
which is preferably used as a carbon precursor a certain ordering
of the material takes place during the temperature treatment.
Through the temperature treatment, the graphenes grow laterally and
the graphene stacks grow vertically. Moreover, the degree of order
of the graphene stack increases.
[0127] It has been found that the higher the carbonisation
temperature and the more complete the carbonisation, the more the
total porosity reduces, with the porosity given by pores in the
second size range reducing more strongly. The heating can take
place while precluding oxygen, i.e. under an inert gas atmosphere,
such as one of the noble gases or nitrogen. In a preferred
embodiment the heating of the shaped body takes place in steps,
with it being heated initially to temperatures between 200 to
400.degree. C. and subsequently to temperatures between 500 and
1000.degree. C.
[0128] The first tempering to 200 to 400.degree. C. serves for the
preliminary cross-linking of the carbon precursor and thus the
generation or ripening of the demixing structure which is of
importance here. Typically this temperature is held for a period
from 1 hour to 48 hours.
[0129] In a second tempering step the shaped body is then heated to
temperatures between 500 and 1000.degree. C. Here, the duration of
the heating and the level of the temperatures determines how
completely the carbonisation is to be carried out. In particular,
the duration of the carbonisation and the temperature curve during
the carbonisation make it possible to exert an influence on the
material characteristics, such as the proportion of carbon and the
porosity.
[0130] During the at least partial evaporation of the organic
solvent and prior to, during or after the heating of the viscous
composition of the shaped body, the latter can be additionally
activated. Activation signifies here that the pore structure of the
shaped carbon monolith and/or its surface is modified relative to a
carbon monolith otherwise produced in the same manner. An
activation can, for example, take place by treating the green body
prior to heating with substances such as acids, H.sub.2O.sub.2, or
zinc chloride which attack the structure of the monolith and in
particular lead to a change of the pore structure during subsequent
heating or chemically change the surface of the shaped body. In
just the same way, such substances can also be used during the
heating, or heating can take place in an oxygen flux. Such forms of
activation in particular lead to the formation of micropores or
other chemical functionalization of the surface of the shaped body,
for example by the formation of OH or COOH groups by oxidation.
[0131] The activated or non-activated carbon monoliths obtained
after heating can be used directly or can be previously
mechanically or chemically processed. For example, they can be cut
by means of suitable saws or provided with specific chemical
functionalities by means of chemical derivatisation methods, i.e.
activated.
[0132] It is thus possible at almost every stage of the method to
influence the material characteristics of the later carbon monolith
by the addition of specific substances or to introduce specific
chemical functionalities. It is also possible to add stabilizers,
substances to assist in carbonisation, inorganic particles or
fibres to the solution.
[0133] The porous carbon monoliths produced in the above manner
have a porosity which can be intentionally set. Through the use of
a method in which at least a partial phase separation takes place a
bimodal or oligomodal pore structure can be produced. In a bimodal
pore structure in which the pores are, in particular, produced by
phase separation they can have a bimodal or oligomodal pore
structure. With a bimodal pore structure or oligomodal pore
structure, the carbon monolith has communicating pores in first and
second different size ranges, so that it is possible for liquids to
migrate through the interlinked pores in the shaped body in the
first size range and reach the pores in the second size range. The
size and number of the pores in each of the two size ranges can be
determined by the choice of the organic polymer, by its
concentration and by its molecular weight. An influence can be
effected on the pore size and the pore size distribution also by
the duration and temperature of the pyrolysis step. The size of the
pores in the second size range can typically be set between 3 and
100 nm, preferably between 5 nm and 30 nm, and the pores in the
first size range typically have a size between 100 nm and 5 .mu.m.
Total porosities of over 50%, preferably between 60 and 80% by
volume, can be produced without problems while preserving the
favourable mechanical characteristics.
[0134] Through the above described manufacturing methods, the
porosity of the carbon monoliths can be intentionally set over a
wide pore size range and a hierarchical pore size distribution can
be produced. The specific surface of the shaped bodies produced
typically lies above 50 m.sup.2/g, preferably above 300 m.sup.2/g,
with higher values also being obtainable.
[0135] A specific example of the above described method will now be
given.
[0136] First of all, mesophase pitch (Mitsubishi AR) is dissolved
in THF with a weight ratio mesophase pitch: THF of 1:3 which is
conducted in a closable vessel. In order to dissolve the mesophase
pitch, the mixture is subjected to 20 minutes of ultrasonic
excitation (100%) and shaking on a horizontal shaker at low
intensity. As an alternative, any other shaker or magnetic stirrer
can be used. After seven days the mixture is centrifuged (10 min at
6500 rpm). The solution then contains 10% by weight of mesophase
pitch. The non-dissolved mesophase pitch can be reused.
[0137] In order to introduce the carbonisation at low temperatures,
a Lewis acid such as FeCl.sub.3 is added to the mesophase pitch
solution (1-10% by weight FeCl.sub.3 related to the solid component
in the mesophase pitch solution). The solution is then stirred for
15 minutes.
[0138] The organic polymer, here polystyrene (molecular weight
250,000 g/mol) is then dissolved in THF (weight ratio
polystyrene:THF=1:20).
[0139] The polystyrene solution is then dropped into the mesophase
pitch solution while stirring vigorously. The relative quantity of
polystyrene to mesophase pitch determines the final absolute
porosity of the material. The finished solution is then stirred
vigorously for 30 minutes.
[0140] For the demixing, the solution is then poured into a Petri
dish. After evaporation of the THF, a thin layer of
polystyrene/mesophase pitch solution remains. The sample is
subjected to preliminary cross-linking in the Petri dish for 48
hours at 340.degree. C. and under an N.sub.2 atmosphere. Further
carbonisation is then carried out at 500-750.degree. C. to preserve
the structure and to achieve the desired porosity.
[0141] The carbon material which is obtained contains first pores
in the size range from 10 .mu.m to 100 nm and second pores in the
size range from less than 100 nm to 3 nm, a specific surface area
in the range from 50 m.sup.2/g to 800 m.sup.2/g and a pore volume
in the range from 0.1 to 1.0 cm.sup.3/g, the foregoing values being
determined by means of Hg porosimetry and by use of a scanning
electron microscope.
[0142] In an alternative example, the manufacture of the carbon
monolith takes place analogously to the above described example,
but using the following precursor solutions:
[0143] Mesophase pitch in THF:
ca. 2 g mesophase pitch (Mitsubishi AR)+10 g THF+0.2 g
FeCl.sub.3.
[0144] Solution of the organic polymers:
1 g Brij 58+20 g THF.
[0145] To conclude, the new carbon material shows a combination of
energy storage density, stability, and positive
charging/discharging speed not previously accessible. Finally, it
should be noted that carbon material which has been carbonized at
2500.degree. C. leads to a porous graphite having just first pores
in the size range from 10 .mu.m to 100 nm and has an
electrochemical behaviour more like that of graphite. However, it
shows a much better rate performance than commercial graphite.
[0146] Considerable attention has been paid to direct methanol fuel
cells (DMFCs) because of their potential use in powering portable
electronic devices. An effective electrocatalyst is the key
component in DMFCs. Among various candidates, Pt and Pt-based
composites have shown the highest electrocatalytic activity towards
methanol oxidation. However, a critical problem with Pt-based
catalysts is their prohibitive cost. In order to lower the cost, in
recent years efforts have focused on the development of
high-surface-area porous carbon supports with controlled porosity
which can lower the platinum-based catalyst loading in DMFCs.
[0147] The experiments, which will be described in the following,
were carried out on a sample of highly porous carbon monolith
referred to in the following as HPCM-1. This sample is the sample
shown at the third entry in the table of FIG. 18B. More
specifically, the sample HPCM-1 carbonized from mesophase pitch at
1000.degree. C. has a surface area of 277 m.sup.2 g.sup.-1 and pore
volume of 0.47 cm.sup.3 g.sup.-1. The diameters of mesopores and
macropores in HPCM-1 are .about.7 nm and 1-4 .mu.m respectively.
The electrical conductivity of this HPCM-1 is about 0.1 S cm.sup.-1
because of a continuous electronic pathway provided by the
well-interconnected graphene structure within the walls of HPCM-1.
From FIG. 20 the network structure of HPCM can be clearly observed
with a fully interconnected macro- and mesoporosity.
[0148] Such hierarchical networks offer a very good compromise
between infiltration rate and surface area. The connecting carbon
bridges are nanoporous in themselves.
[0149] More specifically, rods of HPCM-1 were cut into small pieces
(HPCM: diameter 4 mm, thickness 1 mm) and loaded/coated with
Pt.
[0150] The electrochemical deposition and characterization of
platinum nanoparticles were carried out in a one-compartment cell
connected to a solartron 1255 impedance/gain-phase analyzer coupled
with a solartron 1287 electrochemical interface instrument. The
HPCM supports were tied on Ni mesh by Ni wire and the remaining
part of the Ni mesh was covered by teflon tape. The cell was
equipped with a Platinum foil as a counter electrode and a
saturated calomel electrode (SCE) as a reference electrode. All
potentials described here were measured versus SCE. An electrolyte
solution consisting of 0.5 M H.sub.2SO.sub.4 and 2 mM
H.sub.2PtCl.sub.6 was used for the electrodeposition of platinum.
The Pt nanoparticles were electrodeposited on the HPCM-1 sample at
a constant potential of -0.2 V for 60 s. The mass ratios of Pt
electrodeposited, calculated from the cathodic charge passed
corresponding to the reduction of Pt.sup.IV to Pt.sup.0, is about
3%.about.4%. An SEM image of the resultant Pt loaded/coated with
HPLM-1 sample is shown in FIG. 21.
[0151] A further sample was prepared with a loading/coating of
RuO.sub.2 being applied before the loading/coating with Pt. An SEM
image of this sample is shown in FIG. 22.
[0152] For this sample, RuO.sub.2 nanoparticles were first loaded
into the HPCM-1 sample by using a low-temperature decomposition of
ruthenium tetroxide (RuO.sub.4), a precursor that has been employed
to prepare RuO.sub.2 thin films at rather low temperature. Aqueous
RuO.sub.4 (0.5 wt-%) solution was received from Strem Chemicals.
RuO.sub.2 and the coating experiment was carried out using the
method described by Z. Yuan, R. J. Puddephatt, M. Sayer, in Chem.
Mater. 1993, 5, 908, and by J. V. Ryan, A. D. Berry, M. L.
Anderson, J. W. Long, R. M. Stroud, V. M. Cepak, V. M. Browning, D.
R. Rolison, C. I. Merzbacher, in Nature 2000, 406, 169. A piece of
HPCM-1 was placed in one side of a "H" shaped vessel. About 2 mL of
pentane was added to the other side of the same "H" shaped vessel
at -78.degree. C., then warmed to room temperature and allowed to
equilibrate with the HPCM for some time. By slowly cooling the "H"
shaped vessel, the pentane was condensed into the other side and
filled the HPCM. In order to operate at such low temperature, a
solution of RuO.sub.4 in pentane with a very low melting point and
low viscosity was used to minimize capillary forces on the HPCM
during the wetting process at such critical conditions. 10 mL of
pentane was employed to extract RuO.sub.4 from the aqueous
RuO.sub.4 solution at low temperature. A certain amount of the
solution of RuO.sub.4 in pentane was added to the side of the "H"
shaped vessel containing the HPCM which was pre-cooled to
-78.degree. C. under a dry ice/acetone bath. The "H" shaped vessel
was allowed to warm slowly to room temperature over a period of
several days. All the operations were carried out in a well-vented
hood until all the pentane was evaporated in the "H" shaped vessel,
the obtained dry sample was then put into a vacuum oven and heated
at 200.degree. C. for 1 h. The amount of RuO.sub.2 is about 4 wt-%
corresponding to a complete extraction and transformation of
RuO.sub.4.
[0153] The procedure described above for loading Pt into HPCM-1
without prior coating with RuO.sub.2 was then used for introducing
Pt into the sample of HPCM-1 loaded/coated with RuO.sub.2. From a
comparison of the SEM images shown in FIGS. 21 and 22, it can
clearly be seen that the Pt nanoparticles are smaller and much
better dispersed in the HPCM with RuO.sub.2 than the one without
RuO.sub.2.
[0154] The electrocatalytic performances of the two samples were
characterized with a three-electrode configuration, where a
platinum foil, saturated calomel electrode (SCE) and
HPCM-Pt/HPCM-RuO.sub.2--Pt electrode were used as counter,
reference and working electrodes, respectively. The used
electrolytes were 1 M methanol in 0.5 M H.sub.2SO.sub.4 solution.
The electrolyte was purged with nitrogen gas for 30 min prior to
electrochemical measurements. Cyclic voltammograms were carried out
on a Solartron SI 1287 electrochemical interface. The results are
shown in FIGS. 23 and 24 for HPCM-Pt and HPCM-RuO.sub.2--Pt
respectively.
[0155] Pt together with its alloys, have been widely used as
catalysts in various chemical reactions, especially in the direct
methanol fuel cells (DMFCs) owing to their excellent properties
regarding adsorption and dissociation. The electrocatalytic
activity of HPCM-Pt and HPCM-RuO.sub.2--Pt for the oxidation of
methanol was measured in an electrolyte of 1 M methanol in 0.5 M
H.sub.2SO.sub.4 by using cyclic voltammograms (CVs).
[0156] The peak potential for the oxidation of methanol on HPCM-Pt
is approximately 0.8 V (vs. SCE). The peak current density of the
first forward scan (I.sub.f) cycle for the HPCM-Pt with a Pt
loading of 0.25 mg is up to 87 mA (i.e. the mass current density
per unit mass of platinum is 348 mA mg.sup.-1). It is believed that
the HPCM-Pt sample described here shows the highest catalytic
activity observed for pure Pt with carbon as support.
[0157] In the reverse scan, an oxidation peak is observed, which is
primarily associated with the removal of the residual carbon
species formed in the forward scan. The residual carbon species are
oxidized according to the following reaction:
Pt.dbd.C.dbd.O+Pt--OH.sub.ad.fwdarw.2Pt+CO.sub.2+H.sup.++e.sup.-
[0158] Therefore, the ratio of the forward oxidation current peak
(I.sub.f) to the reverse current peak (I.sub.b), I.sub.f/I.sub.b,
is an index of the catalyst tolerance to the poisoning species,
Pt.dbd.C.dbd.O. A higher ratio indicates more effective removal of
the poisoning species on the catalyst surface. The I.sub.f/I.sub.b
ration of HPCM-RuO.sub.2--Pt (see FIG. 24) is 1.4, much higher than
that of the HPCM-Pt (see FIG. 23) ca. 0.8, showing much better
catalyst tolerance of the HPCM-RuO.sub.2-Pt composite.
[0159] The experimental results reported here highlight the
potential application of the HPCM as an efficient Pt
electrocatalyst support for methanol oxidation.
[0160] Thus, a novel porous carbon monolith (HPCM) with high
surface area, and hierarchically porous structure consisting of
both macropores and mesopores have been synthesized by a soft
colloidal template route and investigated as an efficient
electrocatalyst support for methanol oxidation. The high surface
area can enhance the catalyst dispersion, and the hierarchically
porous structure of the carbon monolith with the right sizes allows
a quick transport and easy accessibility of the reagent molecules
to the catalytic sites of dispersed catalysts, consequently
enhancing the utilization of catalysts and giving a good
performance of methanol anode. The carbon monolith loaded with Pt
catalysts exhibits outstanding mass activity (.about.350 mA/mg Pt)
for methanol oxidation. Better tolerance to poisoning species has
also been achieved by first introducing RuO.sub.2 nanoparticles to
the carbon monolith followed by Pt loading on them.
[0161] It has also been found that a coating of gold on the porous
carbon material of the present invention has particular utility as
a detector.
[0162] The term electrically conductive as use in relation to the
material of the present invention means a conductivity comparable
to that of a good semiconductor but less than that of a metal, e.g.
a conductivity in the range 10.sup.4 Sm.sup.-1 to 0.1
Sm.sup.-1.
* * * * *