U.S. patent application number 12/143701 was filed with the patent office on 2009-12-24 for microporous carbon and method for making the same.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Qingyuan Hu, Gregory P. Meisner.
Application Number | 20090317613 12/143701 |
Document ID | / |
Family ID | 41431575 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090317613 |
Kind Code |
A1 |
Meisner; Gregory P. ; et
al. |
December 24, 2009 |
MICROPOROUS CARBON AND METHOD FOR MAKING THE SAME
Abstract
A carbon material includes a carbonized composite formed from a
substantially homogeneous composite including a carbon precursor
and an activation agent that is soluble in a solution including the
carbon precursor. Micropores having a substantially uniform size
distribution are formed throughout the carbonized composite. At
least 90% of a total pore volume of the carbon material is composed
of micropores, and 10% or less of the total pore volume is composed
of mesopores and macropores. A surface area of the carbon material
ranges from about 1400 m.sup.2/g to about 3000 m.sup.2/g.
Inventors: |
Meisner; Gregory P.; (Ann
Arbor, MI) ; Hu; Qingyuan; (Madison Heights,
MI) |
Correspondence
Address: |
Julia Church Dierker;Dierker & Associates, P.C.
3331 W. Big Beaver Road, Suite 109
Troy
MI
48084-2813
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
PURDUE UNIVERSITY
West Lafayette
IN
|
Family ID: |
41431575 |
Appl. No.: |
12/143701 |
Filed: |
June 20, 2008 |
Current U.S.
Class: |
428/219 ;
252/502; 423/414; 502/416; 502/439 |
Current CPC
Class: |
B01J 37/084 20130101;
B01J 20/3078 20130101; B01J 20/28066 20130101; F17C 11/005
20130101; C01B 32/348 20170801; B01J 20/2808 20130101; B82Y 30/00
20130101; B01J 21/18 20130101; B01J 20/30 20130101; Y02E 60/325
20130101; C01B 3/0021 20130101; B01J 20/20 20130101; C01P 2006/12
20130101; Y02E 60/32 20130101 |
Class at
Publication: |
428/219 ;
502/439; 502/416; 252/502; 423/414 |
International
Class: |
C01B 31/00 20060101
C01B031/00; B32B 5/18 20060101 B32B005/18; B01J 32/00 20060101
B01J032/00; B01J 20/20 20060101 B01J020/20; H01B 1/04 20060101
H01B001/04 |
Claims
1. A carbon material, comprising: a carbonized composite formed
from a substantially homogeneous composite including a carbon
precursor and an activation agent that is soluble in a solution
including the carbon precursor; and micropores having a
substantially uniform size distribution formed throughout the
carbonized composite; wherein at least 90% of a total pore volume
of the carbon material is composed of micropores, and 10% or less
of the total pore volume is composed of mesopores and macropores,
and wherein a surface area of the carbon material ranges from about
1400 m.sup.2/g to about 3000 m.sup.2/g.
2. The carbon material as defined in claim 1 wherein the carbon
precursor is a phenolic resin oligomer, resorcinol, or a
phloroglucinol based resin oligomer, and wherein the activation
agent is selected from potassium hydroxide, sodium hydroxide,
potassium carbonate, and sodium carbonate.
3. The carbon material as defined in claim 1 wherein hydrogen
absorption capacity of the carbon material ranges from about 2.8
wt.% to about 5.75 wt.% at pressures above 25 bar and at about 77
K.
4. The carbon material as defined in claim 1 wherein a weight ratio
of the activation agent to the carbon precursor ranges from 1 to
3.
5. A method for using the carbon material as defined in claim 1,
comprising incorporating the carbon material into a device as a
hydrogen storage medium, a catalyst support, an adsorbent, or an
electrode.
6. A method for making microporous carbon, comprising: preparing a
carbon precursor solution; mixing an activation agent in the carbon
precursor solution to form a substantially homogeneous mixture;
polymerizing the substantially homogeneous mixture, thereby forming
a substantially homogeneous composite; and substantially
simultaneously carbonizing and activating the substantially
homogeneous composite.
7. The method as defined in claim 6 wherein the carbon precursor
solution is prepared by reacting phenol with formaldehyde in a
liquid medium in the presence of a catalyst.
8. The method as defined in claim 7 wherein the liquid medium
includes water or a mixture of water and ethanol.
9. The method as defined in claim 8 wherein the activation agent is
selected from potassium hydroxide, sodium hydroxide, potassium
carbonate, and sodium carbonate.
10. The method as defined in claim 6, further comprising
controlling a surface area of the microporous carbon by controlling
an amount of the activation agent mixed in the carbon precursor
solution.
11. The method as defined in claim 6 wherein mixing includes
stirring the activation agent in the carbon precursor solution, and
wherein the method further comprises heating the substantially
homogeneous composite prior to carbonizing.
12. The method as defined in claim 6 wherein carbonizing and
activating are accomplished by heating the substantially
homogeneous composite to a temperature ranging from about
500.degree. C. to about 900.degree. C. under an inert atmosphere
for a predetermined time period ranging from about 0.5 hours to
about 8 hours.
13. The method as defined in claim 6, further comprising washing
the substantially homogeneous carbonized composite.
14. Microporous carbon formed by the process as defined in claim 6.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to microporous
carbon and a method for making the same.
BACKGROUND
[0002] The use of hydrogen as an alternative to conventional fuels,
especially for automotive applications, is currently being
investigated. As such, the development of suitable hydrogen storage
media for commercial hydrogen powered vehicles is also of interest.
Current techniques for transporting and storing hydrogen include
liquid hydrogen, compressed hydrogen, chemisorption and
physisorption. Porous carbon materials (e.g., activated carbons,
mesoporous carbons, porous carbon fibers and carbide-derived
carbons) are widely used as adsorbents in industrial applications
because of the high surface area, low cost and relatively good
stability.
SUMMARY
[0003] A carbon material includes a carbonized composite formed
from a substantially homogeneous composite including a carbon
precursor and an activation agent that is soluble in a solution
including the carbon precursor. Micropores having a substantially
uniform size distribution are formed throughout the carbonized
composite. At least 90% of a total pore volume of the carbon
material is composed of micropores, and 10% or less of the total
pore volume is composed of mesopores and macropores. A surface area
of the carbon material ranges from about 1400 m.sup.2/g and 3000
m.sup.2/g. A method for forming the carbon material is also
disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Features and advantages of examples of the present
disclosure will become apparent by reference to the following
detailed description and drawings.
[0005] FIG. 1 is flow diagram illustrating an example of the
synthesis of the microporous carbon material;
[0006] FIG. 2 is a graph depicting nitrogen adsorption isotherms of
microporous carbons prepared with different ratios of activation
agent and carbon precursor;
[0007] FIG. 3 is a graph illustrating the effect of potassium
hydroxide (KOH) content on the surface area of microporous
carbons;
[0008] FIG. 4 is a graph illustrating the effect of
carbonization/activation temperature on the surface area of
microporous carbons;
[0009] FIG. 5 is a graph illustrating the effect of
carbonization/activation time on the surface area of microporous
carbons;
[0010] FIG. 6 is a graph depicting excess hydrogen absorption
isotherms of microporous carbons at 77K and room temperature;
[0011] FIG. 7 is a graph illustrating the effect of KOH content on
hydrogen uptake at 77K of the microporous carbons;
[0012] FIG. 8 is a graph illustrating the effect of
carbonization/activation time on the hydrogen uptake at 77K of the
microporous carbons;
[0013] FIG. 9 is a graph illustrating the effect of
carbonization/activation temperature on the hydrogen uptake at 77K
of the microporous carbons; and
[0014] FIG. 10 is a graph depicting the relationship of surface
area and hydrogen absorption capacity at 77K and 40 bar.
DETAILED DESCRIPTION
[0015] Embodiments of the method disclosed herein advantageously
enable control over the porosity of microporous carbon materials.
It is believed that homogeneous mixing of the carbon precursor and
activation agent contributes to relatively uniform pore size
distribution and high surface area of the formed porous carbon
materials. As used herein, the phrase "relatively uniform pore size
distribution" means that 90% or more of the pores in the material
have a diameter equal to or less than 2 nm, and as such, the
material does not have a broad distribution of pore sizes. In other
words, 90% or more of the total pore volume is made up of
micropores (.ltoreq.2nm), and 10% or less of the total pore volume
is made up of mesopores (2 nm-50 nm) and macropores (greater than
50 nm).
[0016] The microporous carbon materials disclosed herein may
advantageously be used in a variety of applications, including
catalysis, gas purification, fuel cell and battery electrodes, and
gas storage. More particularly, the microporous carbon materials
may be used, for example, as hydrogen storage media, catalyst
supports, adsorbents, or electrodes.
[0017] Referring now to FIG. 1, a non-limiting example of the
method for making the microporous carbon material is depicted. Very
generally, the method includes preparing a carbon precursor
solution; mixing an activation agent in the carbon precursor
solution to form a substantially homogeneous mixture; polymerizing
the substantially homogeneous mixture to form a substantially
homogeneous composite; and carbonizing the substantially
homogeneous composite to form the microporous carbon material.
[0018] Suitable carbon precursors include phenolic resin oligomers,
resorcinol, or phloroglucinol based resin oligomers. It is to be
understood that such materials are used as a source or precursor of
carbon that makes up the final material. The carbon precursor
solution may be formed by reacting two or more starting materials
(which form the carbon precursor) in a liquid medium in the
presence of a catalyst. The starting materials will depend, at
least in part, on the desirable carbon precursor that is to be
formed. As shown in FIG. 1, a phenolic resin oligomer solution is
prepared by reacting phenol and formaldehyde (in the presence of
KOH or some other suitable catalyst) in a liquid medium (e.g.,
water or a mixture of water and alcohol, such as ethanol).
[0019] It is believed that the carbon precursor solutions disclosed
herein are better starting materials than fully polymerized
materials, at least in part because the activation agent can
readily dissolve into the carbon precursor solution to form a
homogeneous solution.
[0020] Once the carbon precursor solution is prepared, the
activation agent is added thereto. The carbon precursor/activation
agent mixture is stirred to achieve substantially uniform dispersal
of the carbon precursor molecules and activation agent molecules in
the solution. Such a homogeneous solution results, at least in part
because the activation agent is soluble in the carbon precursor
solution. It is believed that the homogeneity of the solution
contributes to the substantially uniform pore size distribution and
high surface area of the resulting microporous carbon material.
Furthermore, the activation agent may also act as a catalyst for
the synthesis and subsequent polymerization of the carbon
precursor.
[0021] In one non-limiting example, potassium hydroxide (KOH) is
used as the activation agent to form generally uniformly sized
micropores in the carbon material. Other non-limiting examples of
suitable activation agents include sodium hydroxide (NaOH),
potassium carbonate, or sodium carbonate.
[0022] It is to be understood that the amount of activation agent
used is sufficient to form a material having at least 90% of its
total pore volume composed of micropores, and 10% or less of its
total pore volume composed of mesopores and macropores. In such
materials, the macropore volume is at or near 0%. As a non-limiting
example, the weight ratio of activation agent to carbon precursor
ranges from about 1 to about 3.
[0023] Furthermore, the surface area of the microporous carbon
material may be adjusted by controlling the amount of the
activation agent mixed into the carbon precursor solution. A
desirable amount generally results in the surface area ranging from
about 1400 m.sup.2/g or 2000 m.sup.2/g to about 3000 m.sup.2/g (as
determined, for example, from Brunauer, Emmett and Teller (BET)
nitrogen sorption surface area measurements).
[0024] The substantially homogeneous carbon precursor/activation
agent solution is then exposed to heating at a predetermined
temperature for a predetermined time. In a non-limiting example,
heating is accomplished at about 160.degree. C. for about 12-24
hours. Heating evaporates any solvent (e.g., water, or water and
alcohol) and initiates cross-linking and polymerization, thereby
forming a homogenous polymer/activation agent solid composite. A
homogeneous phenolic resin oligomer/KOH solid composite is shown in
FIG. 1.
[0025] The solid composite is then carbonized and activated at the
same time by heating to a temperature ranging from about
500.degree. C. to about 900.degree. C. under an inert atmosphere
(e.g., nitrogen or argon) for a time ranging from about 0.5 hours
to about 8 hours. As indicated, the heating step causes the polymer
to be converted to carbon while the activation agent acts as a
chemical activation reagent on the carbon to create porosity. As
such, the heating process results in the formation of the final
microporous carbon material.
[0026] The final product may then be washed with dilute hydrogen
chloride one or more times and with hot water several times to
remove any residual activation agent and other impurities.
[0027] To further illustrate embodiment(s) of the present
disclosure, an example is given herein. It is to be understood that
this example is provided for illustrative purposes and is not to be
construed as limiting the scope of the disclosed embodiment(s).
EXAMPLE
[0028] Sample Preparation
[0029] Oligomeric phenol-formaldehyde was synthesized by reacting
13 mmol phenol, 26 mmol formaldehyde and 1.3 mmol potassium
hydroxide at 70.degree. C. for about 1 hour. A desired amount of
potassium hydroxide solution (5M) was added gradually into the
phenol-formaldehyde oligomer solution under stirring. Various
samples (C-1 through C-18 in Table 1 below) were made using
different ratios of KOH to oligomeric phenol-formaldehyde. The
oligomer-KOH solutions were heated in an oven at 160.degree. C.
overnight. During this heating process, the phenol-formaldehyde
oligomers continued to crosslink and polymerize to form a
thermoset, carbonizable polymer under the catalysis of potassium
hydroxide.
[0030] 311 The polymers were then carbonized/activated at high
temperature (ranging from about 500.degree. C. to about 900.degree.
C.) under an inert atmosphere. The final carbon materials were
rinsed several times with dilute hydrochloric acid and de-ionized
water, and dried in an oven at 150.degree. C. overnight.
[0031] Sample Characterization
[0032] The samples were characterized using nitrogen sorption
surface area measurements (Micromeritics ASAP 2010 operated at
77K). Cryogenic hydrogen sorption measurements at high pressures
were performed using a PCTPro 2000 (Hy-Energy Scientific
Instruments) at 77K and room temperature.
[0033] Results
[0034] High surface area microporous carbon materials were obtained
using the process disclosed herein. FIG. 2 illustrates nitrogen
adsorption isotherms of microporous carbons (samples C-1 and C-4)
prepared with different ratios of activation agent and carbon
precursor. Table 1 shows the detailed synthesis conditions and the
hydrogen absorption capacities of the obtained porous carbons at
77K and 40 bar.
TABLE-US-00001 TABLE 1 BET surface areas and hydrogen absorption
capacities (at 77K and 40 Bar) of the porous carbon materials
Weight ratio Carbonization/ Carbonization/ Excess Hydrogen Sample
of KOH to Activation Activation BET surface Absorption number
oligomer Temp. (.degree. C.) Time (hour) area (m.sup.2/g) (wt. %)
C-1 1 700 4 1424 2.81 C-2 1.5 700 4 1662 3.69 C-3 2 700 4 2711 5.25
C-4 2.2 700 4 2811 5.68 C-5 2.25 700 4 2943 5.75 C-6 2.5 700 4 2466
4.86 C-7 1 800 4 1587 3.30 C-8 1.5 800 4 2075 3.91 C-9 2 800 4 2698
5.02 C-10 2.25 800 4 2441 4.84 C-11 2.5 800 4 1992 3.33 C-12 2 700
0.5 1627 3.05 C-13 2 700 1 1752 3.32 C-14 2 700 2 2438 4.30 C-15 2
700 8 2625 4.51 C-16 2 500 4 1627 3.50 C-17 2 600 4 2228 4.36 C-18
2 900 4 2545 4.61
[0035] As shown in Table 1, the surface areas of the porous carbon
materials were high (a non-limiting example of which was 2943
m.sup.2/g). It is believed that the higher surface area and uniform
pore size distribution is a result of the homogeneous mixing of the
carbon precursor and activation agent.
[0036] In this example (as shown in Table 1), the porous carbons
were prepared at a weight ratio of activation agent (KOH) to carbon
precursor ranging from 1 to 2.5, carbonization/activation
temperature ranging from 500.degree. C. to 900.degree. C., and
carbonization/activation time up to 8 hours. The optimal activation
conditions were experimentally determined.
[0037] FIG. 3 shows the effect of KOH content on the surface area.
From these results, it can be seen that the ratio of activation
agent to carbon precursor affects the surface area and porosity of
the final products. At 700.degree. C., with the increase of KOH
content from 1 g to 2.25 g, the surface area was doubled. As shown
in FIG. 3, additional increases in the KOH content decreased the
surface area, which indicates that the microstructures of the
obtained porous carbons may partially collapse when the wall is
etched too much. The results are similar at 800.degree. C. FIG. 3
further illustrates that at low KOH content, a higher activation
temperature results in higher surface areas. It is believed that
these phenomena can be explained by the mechanism of the KOH
activation process disclosed herein. During the high temperature
activation process, the KOH reacts with carbon, and the pores are
created by etching carbon molecules from the bulk carbon materials.
It is believed that higher temperatures result in a higher etching
rate. More carbon molecules will be removed at higher KOH content
or higher temperature, which increases the porosity and surface
area of the carbon material. However, if the etching process is too
fast and/or too violent, larger pores may be formed, and the carbon
framework may become too thin to support itself. In such instances,
the carbon framework partially collapses, and the surface area
decreases.
[0038] FIGS. 4 and 5 illustrate the effect of
carbonization/activation temperature and carbonization/activation
time on the surface area, respectively. As shown in the Figures,
the surface areas of obtained porous carbon materials increase with
increased carbonization/activation time up to 4 hours and
temperature up to 700.degree. C. through 800.degree. C., and then
decrease with further increased carbonization/activation time and
temperature. It is believed that this behavior can also be
explained by the mechanism of the KOH activation process disclosed
herein.
[0039] In this example, hydrogen sorption isotherms of the
microporous carbons were measured using a cryostat attached to a
PCTPro 2000 apparatus at different temperatures and hydrogen
pressure up to 40 bar. The detailed hydrogen absorption capacities
of the porous carbons are shown in Table 1 above.
[0040] FIG. 6 shows the hydrogen absorption isotherms of samples
C-1 and C-4 at 77K and room temperature. The hydrogen uptake at 77K
increased with increased pressure and reached saturation at about
25 bar. As shown in the Figure, the sample with the larger surface
area had a higher hydrogen uptake. Specifically, sample C-4 with
surface area of 2811 m.sup.2/g absorbed 5.7 wt % at 40 bar, and
sample C-1 with surface area of 1424 m.sup.2/g absorbed 2.8 wt % at
40 bar. At room temperature, the microporous carbons absorb very
limited hydrogen, for example, sample C-4 absorbed about 0.5 wt %
hydrogen at 40 bar. It is believed that at room temperature, the
hydrogen absorption on the high surface area carbon materials is
limited, at least in part, because of the weak interaction between
hydrogen molecules and the carbon surface. Cryogenic condition may
be applied to depress the thermal motion of the hydrogen
molecules.
[0041] FIG. 7 shows the effect of the ratio of KOH to carbon
precursor on the hydrogen uptake of microporous carbons at
different carbonization/activation temperatures. FIGS. 2 and 3
(discussed hereinabove) illustrate that the ratio of KOH to carbon
precursor may be used to control the surface area and porosity of
the obtained carbon materials. It is also believed that the KOH (or
other activation agent) content affects the hydrogen absorption of
the obtained microporous carbons. In fact, FIG. 7 is similar to
FIG. 3 because the change of the hydrogen uptake reflects the
change of the surface area. When carbonization/activation takes
place at 700.degree. C., the hydrogen uptake at 77K and 40 bar of
the samples increased from 2.8 wt % to 5.75 wt % with the KOH
content increased from 1 g to 2.25 g. As shown in FIG. 7, further
increases in the KOH content led to the decrease of the hydrogen
absorption capacities.
[0042] FIGS. 8 and 9 show the effect of carbonization/activation
time and temperature on the hydrogen absorption capacities of the
microporous carbons. Similar to the effect that time and
temperature had on the surface area (see FIGS. 4 and 5) the
hydrogen uptake increased with an increase in
carbonization/activation time and temperature, and then decreased
with additional increases in carbonization/activation time and
temperature. Since the surface areas and hydrogen uptake of the
porous carbons illustrate high correlativity, the relationship of
surface areas and hydrogen absorption capacity at 77K and 40 bar is
shown in FIG. 10. At cryogenic condition, a linear relationship is
obtained between the surface area and the amount of hydrogen
absorbed.
[0043] The chemical activation synthesis method disclosed herein
involves an activation agent and a carbon precursor that are mixed
substantially homogeneously. This process results in the formation
of microporous carbon materials with desirably high surface areas
and relatively uniform pore size distribution throughout the
material. It is believed that the ratio of activation agent to
carbon precursor and the temperature of the
carbonization/activation process affect the final surface area and
porosity, and thus enable control over the structure of the
resulting carbon material. The microporous carbons disclosed herein
advantageously have a substantially uniform pore size distribution,
a micropore volume greater than or equal to 90%, a high surface
area, and hydrogen absorption capacity up to 5.75 wt % at pressures
at or above 25 bar (where uptake reaches a saturation point) and
about 77K.
[0044] While several embodiments have been described in detail, it
will be apparent to those skilled in the art that the disclosed
embodiments may be modified. Therefore, the foregoing description
is to be considered exemplary rather than limiting.
* * * * *