U.S. patent application number 16/559551 was filed with the patent office on 2020-03-05 for aqueous route to nitrogen-doped mesoporous carbons.
The applicant listed for this patent is CARNEGIE MELLON UNIVERSITY. Invention is credited to Michael R. Bockstaller, Krzysztof Matyjaszewski, Jianan Zhang.
Application Number | 20200071168 16/559551 |
Document ID | / |
Family ID | 69638989 |
Filed Date | 2020-03-05 |
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United States Patent
Application |
20200071168 |
Kind Code |
A1 |
Matyjaszewski; Krzysztof ;
et al. |
March 5, 2020 |
AQUEOUS ROUTE TO NITROGEN-DOPED MESOPOROUS CARBONS
Abstract
A method for preparation of mesoporous nitrogen-doped carbon
includes forming a composition by solubilizing a
nitrogen-containing polymer in an aqueous solution of ZnCl.sub.2
and drying the aqueous solution, the method further includes
heating the composition after drying to a temperature sufficiently
high to carbonize the nitrogen-containing polymer to form the
mesoporous nitrogen-doped carbon.
Inventors: |
Matyjaszewski; Krzysztof;
(Pittsburgh, PA) ; Zhang; Jianan; (Hefei, CN)
; Bockstaller; Michael R.; (Pittsburgh, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CARNEGIE MELLON UNIVERSITY |
Pittsburgh |
PA |
US |
|
|
Family ID: |
69638989 |
Appl. No.: |
16/559551 |
Filed: |
September 3, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62765636 |
Sep 4, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 32/354 20170801;
C08L 33/20 20130101; C01B 32/00 20170801; C01B 32/205 20170801;
C01P 2002/86 20130101; C01P 2002/72 20130101; C01P 2002/82
20130101; C01P 2004/04 20130101; C01P 2006/17 20130101; C01B 32/05
20170801; C01P 2006/16 20130101; C01B 32/21 20170801; C01B 21/0605
20130101; C01P 2006/14 20130101; C01P 2006/12 20130101; C07D 401/00
20130101 |
International
Class: |
C01B 32/354 20060101
C01B032/354; C01B 32/21 20060101 C01B032/21; C08L 33/20 20060101
C08L033/20 |
Goverment Interests
GOVERNMENTAL INTEREST
[0002] This invention was made with government support under grant
no. DMR 1501324 and grant no. CMMI 1663305 awarded by the National
Science Foundation. The government has certain rights in this
invention.
Claims
1. A method for preparation of mesoporous nitrogen-doped carbon
comprising forming a composition by solubilizing a
nitrogen-containing polymer in an aqueous solution of ZnCl.sub.2
and drying the aqueous solution, the method further comprising
heating the composition after drying to a temperature to carbonize
the nitrogen-containing polymer to form the mesoporous
nitrogen-doped carbon.
2. The method of claim 1 further comprising dispersing a plurality
of porogenic fillers in the aqueous solution of the composition
prior to drying the composition.
3. The method of claim 2 wherein the porogenic fillers comprise at
least one of silica particles, cellulose-based nanocrystals or
filter paper.
4. The method of claim 2 wherein the composition is cast into a
desired form before heating.
5. The method of claim 2 wherein drying comprises
freeze-drying.
6. The method of claim 2 wherein the nitrogen-containing polymer is
polyacrylonitrile.
7. The method of claim 6 wherein the composition is stabilized by
heating at a temperature below 300.degree. C. after drying and
before heating the composition to carbonize the nitrogen-containing
polymer, wherein the temperature to carbonize the
nitrogen-containing polymer is less than 850.degree. C.
8. The method of claim 6 wherein the degree of polymerization of
the polyacrylonitrile is 100 or less.
9. The method of claim 6 wherein the degree of polymerization of
the polyacrylonitrile is 50 or less.
10. The method of claim 6 wherein the dispersity of the
polyacrylonitrile is less than 2.0.
11. The method of claim 6 wherein the dispersity of the
polyacrylonitrile is less than 1.5.
12. The method of claim 6 wherein the dispersity of the
polyacrylonitrile is less than 1.3.
13. The method of claim 6 wherein the mesoporous nitrogen-doped
carbon comprises interconnected pores.
14. The method of claim 6 wherein the mesoporous nitrogen-doped
carbon has a surface area greater than or equal to 750
m.sup.2/g.
15. The method of claim 6 wherein the mesoporous nitrogen-doped
carbon has a surface area greater than or equal to 1,000
m.sup.2/g.
16. The method of claim 14 wherein a percentage of the surface area
arising from mesopores is at least 84%.
17. The method of claim 6 wherein the mesoporous nitrogen-doped
carbon comprise both graphitic and disordered carbons.
18. The method of claim 17 wherein the graphitic carbons comprise
catalytically active edge on pyridine oxide-N, pyrrolic- or
pyridonic-N, and pyridinic-N (N--P) nitrogens.
19. The method of claim 6 wherein the ratio of fillers to PAN is
selected to provide sufficient PAN to fill the majority of the
interstitial volume between the fillers, thereby forming a coherent
structure.
20. A mesoporous nitrogen-doped carbon prepared by forming a
composition by solubilizing a nitrogen-containing polymer in an
aqueous solution of ZnCl.sub.2, drying the aqueous solution, and
heating the composition after drying the aqueous solution to
carbonize the nitrogen-containing polymer to form the mesoporous
nitrogen-doped carbon.
21. The mesoporous nitrogen-doped carbon of claim 20 wherein a
plurality of porogenic fillers are dispersed in the aqueous
solution of the composition prior to drying.
22. The mesoporous nitrogen-doped carbon of claim 21 wherein the
porogenic fillers comprise at least one of silica particles,
cellulose-based nanocrystals or filter paper.
23. The mesoporous nitrogen-doped carbon of claim 21 wherein the
composition is cast into a desired form before heating.
24. The mesoporous nitrogen-doped carbon of claim 21 wherein the
nitrogen-containing polymer is polyacrylonitrile.
25. The mesoporous nitrogen-doped carbon of claim 18 wherein the
composition is stabilized by heating at a temperature below
300.degree. C. after drying and before heating the composition to
carbonize the nitrogen containing polymer, wherein the temperature
to carbonize the nitrogen containing polymer is less than
850.degree. C.
26. The mesoporous nitrogen-doped carbon of claim 24 wherein the
degree of polymerization of the polyacrylonitrile is 100 or
less.
27. The mesoporous nitrogen-doped carbon of claim 24 wherein the
dispersity of the polyacrylonitrile is less than 1.5.
28. The mesoporous nitrogen-doped carbon of claim 24 wherein the
mesoporous nitrogen-doped carbon comprises interconnected
pores.
29. The mesoporous nitrogen-doped carbon of claim 24 having a
surface area greater than or equal to 750 m.sup.2/g.
30. The mesoporous nitrogen-doped carbon of claim 20 having a
surface area greater than 1,000 m.sup.2/g.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional Patent
Application Ser. No. 62/765,636, filed Sep. 4, 2018, the disclosure
of which is incorporated herein by reference.
BACKGROUND
[0003] The following information is provided to assist the reader
in understanding technologies disclosed below and the environment
in which such technologies may typically be used. The terms used
herein are not intended to be limited to any particular narrow
interpretation unless clearly stated otherwise in this document.
References set forth herein may facilitate understanding of the
technologies or the background thereof. The disclosure of all
references cited herein are incorporated by reference.
[0004] High specific surface area (SSA), high porosity, chemical
inertness and thermal stability make porous carbons ubiquitous
materials in numerous applications such as catalysis, water/gas
purification, or energy conversion/storage. Nitrogen (N)-doped
carbons have attracted particular interest, as a result of their
potential as metal-free electrocatalysts (for example, for use in
oxygen reduction reactions (ORR), a key process in fuel cell
technology and metal-air batteries). It has been demonstrated that
N-doped carbons can facilitate ORR via the four-electron pathway
and provide better performance than state-of-the-art Pt/C
catalysts. Such discoveries spurred extensive research to establish
N-doped carbons for other applications, such as hydrogen evolution
reaction (HER) and supercapacitors. The presence of high nitrogen
content and interconnected mesoporosity are important
characteristics for effective ORR electrocatalysis, since carbon
atoms adjacent to pyridinic and graphitic quaternary nitrogen
dopants enhance mass transport and provide access to active
sites.
[0005] Porous carbons are usually synthesized by direct
carbonization of organic precursors such as polymers or biomass
using a variety of chemical or physical activation methods.
Polyacrylonitrile (PAN) is an attractive precursor for N-doped
carbons because of its high nitrogen content and well-established
carbonization chemistry. Mesoporous carbons have been synthesized
from PAN via hard-templating or soft-templating procedures.
However, such an approach requires use of polar organic solvents
and surface functionalization or block copolymerization which raise
the costs and limit its technological impact. In that regard, high
boiling point organic solvents are required to dissolve/disperse
PAN and porogenic fillers, such as particles or nanofibers (for
example, silica nanoparticles or nanocellulose, to enable the
casting of uniform composite structures for the subsequent
pyrolysis.
SUMMARY
[0006] In one aspect, a method for preparation of mesoporous
nitrogen-doped carbon includes forming a composition by
solubilizing a nitrogen-containing polymer in an aqueous solution
of ZnCl.sub.2 and drying the aqueous solution, the method further
comprising heating the composition after drying to a temperature
sufficiently high to carbonize the nitrogen-containing polymer to
form the mesoporous nitrogen-doped carbon. Any ZnCl.sub.2 remaining
after drying may also be volatized upon heating the composition
after drying to carbonize the nitrogen-containing polymer. In a
number of embodiments, the method further includes dispersing a
plurality of porogenic (solid) fillers in the aqueous solution of
the composition prior to drying the composition. The porogenic
fillers may, for example, include at least one of silica particles,
cellulose-based nanocrystals or filter paper. In general, solid
fillers such as particles used herein are removable during
processing as known in the art. The composition may, for example,
be formed or cast into a desired form or conformation before
heating. In a number of embodiments, drying includes freeze-drying.
In a number of embodiments, the nitrogen-containing polymer is
polyacrylonitrile or PAN.
[0007] The composition may, for example, be stabilized by heating
at a temperature below 300.degree. C. after drying and before
heating the composition to carbonize the nitrogen-containing
polymer. The temperature to carbonize the nitrogen-containing
polymer may, for example, be less than 850.degree. C.
[0008] In a number of embodiments in which the nitrogen-containing
polymer is polyacrylonitrile, the degree of polymerization of the
polyacrylonitrile may be 100 or less. The degree of polymerization
of the polyacrylonitrile may, for example, be 50 or less. The
polydispersity or dispersity (mass average molar mass (or molecular
weight) divided by number-average molar mass (or molecular weight);
M.sub.w/M.sub.n) of the polyacrylonitrile may, for example, be less
than 2.0, less than 1.5 or less than 1.3.
[0009] In a number of embodiments, the mesoporous nitrogen-doped
carbon includes interconnected pores. The mesoporous nitrogen-doped
carbon may, for example, have a surface area greater than or equal
to 750 m.sup.2/g or greater than or equal to 1,000 m.sup.2/g. A
percentage of the specific surface area arising from mesopores may,
for example, be at least 80% or at least 84%.
[0010] The mesoporous nitrogen-doped carbon may, for example,
include both graphitic and disordered carbons. In a number of
embodiments, the graphitic carbons include a catalytically active
edge on pyridine oxide-N, pyrrolic- or pyridonic-N, and pyridinic-N
(N--P) nitrogens.
[0011] In general, the ratio of (solid) fillers to
nitrogen-containing polymer(s) (for example, PAN) may be selected
to provide sufficient nitrogen-containing polymer to fill the
majority of the interstitial volume between the fillers, thereby
forming a coherent structure.
[0012] In another aspect, a mesoporous nitrogen-doped carbon is
prepared by forming a composition by solubilizing a
nitrogen-containing polymer in an aqueous solution of ZnCl.sub.2,
drying the aqueous solution, and heating the composition after
drying the aqueous solution to carbonize the nitrogen-containing
polymer to form the mesoporous nitrogen-doped carbon. A plurality
of porogenic (solid) fillers may, for example, dispersed in the
aqueous solution of the composition prior to drying. In a number of
embodiments, the porogenic fillers includes at least one of silica
particles, cellulose-based nanocrystals or filter paper. The
composition may, for example, be formed cast into a desired form or
conformation before heating.
[0013] As described above, the composition may, for example, be
stabilized by heating at a temperature below 300.degree. C. after
drying and before heating the composition to carbonize the
nitrogen-containing polymer. The temperature to carbonize the
nitrogen-containing polymer may, for example, be less than
850.degree. C.
[0014] As also described above, in a number of embodiments in which
the nitrogen-containing polymer is polyacrylonitrile, the degree of
polymerization of the polyacrylonitrile may be 100 or less. The
degree of polymerization of the polyacrylonitrile may, for example,
be 50 or less. The polydispersity or dispersity (mass average molar
mass (or molecular weight) divided by number-average molar mass (or
molecular weight) M.sub.w/M.sub.n) of the polyacrylonitrile is less
than 2.0, less than 1.5 or less than 1.3.
[0015] In a number of embodiments, the mesoporous nitrogen-doped
carbon comprises interconnected pores. The mesoporous
nitrogen-doped carbon may, for example, have a surface area greater
than or equal to 750 m.sup.2/g or greater than or equal to 1,000
m.sup.2/g. A percentage of the specific surface area arising from
mesopores may, for example, be at least 80% or at least 84%.
[0016] The mesoporous nitrogen-doped carbon may, for example,
include both graphitic and disordered carbons. In a number of
embodiments, the graphitic carbons include catalytically active
edge on pyridine oxide-N, pyrrolic- or pyridonic-N, and pyridinic-N
(N--P) nitrogens.
[0017] In a further aspect, a mesoporous nitrogen-doped carbon
includes a surface area greater than or equal to 750 m.sup.2/g (or
greater than or equal to 1,000 m.sup.2/g) and a percentage of the
specific surface area arising from mesopores of at least 84%. In a
number of embodiments, the mesoporous nitrogen-doped carbon is
formed via carbonization of a nitrogen-containing polymer. In a
number of embodiments, the nitrogen-containing polymer is
polyacrylonitrile. The mesoporous nitrogen-doped carbon may, for
example, include both graphitic and disordered carbons. In a number
of embodiments, the graphitic carbons include a catalytically
active edge on pyridine oxide-N, pyrrolic- or pyridonic-N, and
pyridinic-N (N--P) nitrogens.
[0018] The methods, systems and compositions hereof, along with the
attributes and attendant advantages thereof, will best be
appreciated and understood in view of the following detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 illustrates an embodiment of an aqueous based
procedure for the preparation of mesoporous carbons.
[0020] FIG. 2A illustrated N.sub.2 adsorption and desorption
isotherms, and the inset illustrates the pore size distribution of
hierarchically nanoporous carbon prepared with different ratios of
SiO.sub.2/PAN.
[0021] FIG. 2B illustrates contribution of different pore sizes to
the total S.sub.BET of the prepared nanoporous carbons.
[0022] FIG. 3. Schematic illustration of the formation of
nanoporous carbons with varied silica/PAN ratios (wherein gray
spheres represent silica nanoparticles, lines in upper panels a, b
and c represent PAN chains, and lines in lower panels a*, b* and c*
represent carbon after carbonization.
[0023] FIG. 4A illustrates N.sub.2 adsorption and desorption
isotherms
[0024] FIG. 4B illustrates pore size distributions of nanoporous
carbons obtained from pure PAN (NPC-PAN) and ZnCl.sub.2 activated
PAN (NPC-ZPAN), wherein an enlarged portion of pore size
distributions is shown in the inset to provide further detailed
information.
[0025] FIG. 5A illustrates the result of thermogravometric analysis
or TGA of ZnCl.sub.2, a mixture of PAN with ZnCl.sub.2, and PAN
measured under N.sub.2 atmosphere at a heating rate of 10.degree.
C./min.
[0026] FIG. 5B illustrates derivative thermogravimetry or DTG
analysis of ZnCl.sub.2, a mixture of PAN with ZnCl.sub.2, and PAN
measured under N.sub.2 atmosphere at a heating rate of 10.degree.
C./min.
[0027] FIG. 6 illustrates cumulative pore area versus average width
of nanoporous carbons prepared at different silica/PAN ratios.
[0028] FIG. 7 illustrates N.sub.2 adsorption isotherms (panels a,
c) and the corresponding pore size distribution or PSD (panels b,
d) of porous carbons prepared from nanocellulose (panels a, b) and
cellulose filter paper (panels c, d), demonstrating a significant
increase of specific surface area or SSA associated with the
formation of mesopores with a diameter of approximately 2.3 nm by
the volatilization of ZnCl.sub.2.
[0029] FIG. 8 Illustrates representative transmission electron
microscopy or TEM images of porous carbons prepared at
SiO.sub.2/PAN ratios of (panel a) 3.4, (panel b) 1.2, (panel c)
0.5, and (panel d) without the addition of colloidal SiO.sub.2 NPs,
templated from cellulose nanocrystals (panel e) and filter paper
(panel f) with filtration of PAN/ZnCl.sub.2 solution, wherein the
inset in panel e shows the optical picture of the corresponding
porous carbon film and scale bars are 200 nm in main figures and 20
nm in insets, and the inset in panel f shows that the infiltration
of PAN/ZnCl.sub.2 solutions in filter paper facilitated the
fabrication of monolithic NPC films.
[0030] FIG. 9A illustrates representative TEM image of
CNC/PAN/ZnCl.sub.2 composites before carbonization process showing
the well dispersed cellulose nanocrystals or CNC.
[0031] FIG. 9B illustrates representative TEM image of porous
carbon templated from cellulose nanocrystals after filtration of
PAN/ZnCl.sub.2 solution illustrating the existence of porous CNC
nanofibers.
[0032] FIG. 10A illustrates representative TEM image of porous
carbon templated from pristine filter paper without filtration of
PAN/ZnCl.sub.2 solution.
[0033] FIG. 10B illustrates representative TEM image of porous
carbons templated from filter paper after infiltration of
ZnCl.sub.2 solution showing the existence of porous fibers.
[0034] FIG. 11A illustrates X-ray diffraction or XRD profiles of an
NPC-S1.2 sample.
[0035] FIG. 11B illustrates Raman scattering spectrum of the
NPC-S1.2 sample.
[0036] FIG. 12 illustrates an X-ray photoelectron spectroscopy or
XPS survey spectrum of the NPC-S 1.2 sample.
[0037] FIG. 13 illustrates: (panel a) an XPS N 1 s spectrum of the
NPC-S1.2 sample and electrochemical characterization of NPC-S2.2 as
an electrocatalyst for ORR: (panel b) cyclic voltammetry CV curves
recorded N.sub.2-saturated and O.sub.2-saturated 0.1 M KOH
electrolyte at a scan rate 100 mV/s; (panel c) a rotating disk
electrode study in O.sub.2-saturated 0.1 M KOH electrolyte at a
scan rate 10 mV/s; and (panel d) Koutecky-Levich analysis of the
linear sweep voltammetry or LSV curves presented in panel c.
DETAILED DESCRIPTION
[0038] It will be readily understood that the components of the
embodiments, as generally described and illustrated in the figures
herein, may be arranged and designed in a wide variety of different
configurations in addition to the described representative
embodiments. Thus, the following more detailed description of the
representative embodiments, as illustrated in the figures, is not
intended to limit the scope of the embodiments, as claimed, but is
merely illustrative of representative embodiments.
[0039] Reference throughout this specification to "one embodiment"
or "an embodiment" (or the like) means that a particular feature,
structure, or characteristic described in connection with the
embodiment is included in at least one embodiment. Thus, the
appearance of the phrases "in one embodiment" or "in an embodiment"
or the like in various places throughout this specification are not
necessarily all referring to the same embodiment.
[0040] Furthermore, described features, structures, or
characteristics may be combined in any suitable manner in one or
more embodiments. In the following description, numerous specific
details are provided to give a thorough understanding of
embodiments. One skilled in the relevant art will recognize,
however, that the various embodiments can be practiced without one
or more of the specific details, or with other methods, components,
materials, et cetera. In other instances, well known structures,
materials, or operations are not shown or described in detail to
avoid obfuscation.
[0041] As used herein and in the appended claims, the singular
forms "a," "an", and "the" include plural references unless the
context clearly dictates otherwise. Thus, for example, reference to
"nitrogen-containing polymer" includes a plurality of such
nitrogen-containing polymers and equivalents thereof known to those
skilled in the art, and so forth, and reference to "the
nitrogen-containing polymer" is a reference to one or more such
nitrogen-containing polymers and equivalents thereof known to those
skilled in the art, and so forth. Recitation of ranges of values
herein are merely intended to serve as a shorthand method of
referring individually to each separate value falling within the
range. Unless otherwise indicated herein, and each separate value,
as well as intermediate ranges, are incorporated into the
specification as if individually recited herein. All methods
described herein can be performed in any suitable order unless
otherwise indicated herein or otherwise clearly contraindicated by
the text.
[0042] As user herein, the term "polymer" refers to a chemical
compound that is made of a plurality of small molecules or monomer
that are arranged in a repeating structure to form a larger
molecule. Polymers may occur naturally or be formed synthetically.
The use of the term "polymer" encompasses homopolymers as well as
copolymers. The term "copolymer" is used herein to include any
polymer having two or more different monomers. Copolymers may, for
example, include alternating copolymers, periodic copolymers,
statistical copolymers, random copolymers, block copolymers, graft
copolymers etc.
[0043] As demonstrated in a number of representative embodiments
herein the addition of ZnCl.sub.2, a relatively volatile
electrolyte, enables the effective co-solubilization of a
nitrogen-containing polymer such a polyacrylonitrile or PAN within
aqueous dispersions of porogenic fillers, such as commercial
LUDOX.RTM. SiO.sub.2 colloids (available from Grace of Columbia,
Md.) or nanocellulose, thereby allowing subsequent fabrication of
highly porous carbons. In a number studies hereof, PAN is used as a
representative example of a nitrogen-containing polymer exhibiting
solubility in an aqueous ZnCl.sub.2 solution. Other
nitrogen-containing polymer exhibiting solubility in in an aqueous
ZnCl.sub.2 solution may also be used in the methods hereof.
[0044] The Lewis acid ZnCl.sub.2 serves a dual role of a solubility
enhancer and porogen (that is, a material used to create pores).
The ZnCl.sub.2 further operates with porogenic fillers to create a
dual pore formation mechanism (that is, including concurrent hard
templating and electrolyte evaporation) that results in the
simultaneous formation of nano- and mesoporous-microstructures with
significantly increased SSA as compared to regular PAN and
additionally providing a N-content of 10%. As known in the art,
mesoporous materials include pores with diameters between 2 and 50
nm, according to IUPAC nomenclature. Nanoporous materials generally
include pores with diameters of 100 nm or less (and typically
between 1-100 nm). Infiltration of cellulose filters or filter
papers with aqueous PAN/ZnCl.sub.2 solutions yields bulk monolithic
N-doped nanocarbon films without binder components. The process is
illustrated in FIG. 1.
[0045] The versatility of the aqueous route to nitrogen-doped
mesoporous carbons from templated PAN solution was initially
demonstrated by using a commercial aqueous suspension of SiO.sub.2
nanoparticles or NPs (diameter 12.5.+-.1.5 nm) as an exemplary
model system. The PAN was dissolved in aqueous ZnCl.sub.2 (60 wt %)
containing SiO.sub.2 NPs. Although the amount of ZnCl.sub.2 in the
aqueous solution may vary, in a number of representative
embodiments, the mixed suspension of ZnCl.sub.2, solid filler, and
PAN was prepared by adding varying amounts of solid filler and PAN
into aqueous 60 wt % ZnCl.sub.2. For 100 g of aqueous 60 wt %
ZnCl.sub.2, the amount of added PAN may, for example, be in the
range of 50 g to 0.5 g, 20 g to 2 g, or, in a number of
embodiments, in the range of 10 g to 5 g, In a number of
embodiments, the amount of solid filler added in forming a mixed
suspension may, for example, be in the range 20 g to 0.2 g, 10 g to
0.5 g, or 5 g to 1 g. To ensure complete dissolution of the polymer
and an operable viscosity of the SiO.sub.2/ZnCl.sub.2/PAN
suspension, PAN with a degree of polymerization (DP) of 50
(M.sub.n=2700) and narrow molecular weight distribution
(M.sub.w/M.sub.n <1.20) was used. PAN with a DP of 100 was less
soluble and gave inferior results. Whatever nitrogen-containing
polymer is used, it is desirable to control the DP to provide
complete solubility and a desired viscosity for processing
according to a particular used. Ranges of desirable concentration
for ZnCl.sub.2, nitrogen-containing polymer and filler are readily
determined for a particular uses/pore characteristics as described
herein.
[0046] The PAN was synthesized by initiators for continuous
activator regeneration atom transfer radical polymerization (ICAR
ATRP) with ppm amounts of added copper catalyst (see, for example,
J. Am. Chem. Soc. 2014, 136, 6513; J. Polym. Sci., Part A: Polym.
Chem. 2016, 54, 1961; Macromolecules 1997, 30, 6398; Chem. Commun.
2012, 48, 11516; Chem. Rev. 2001, 101, 2921; Macromolecules 2012,
45, 4015; Macromolecules 2014, 47, 6316, the disclosures of which
are incorporated herein by reference) or directly in aqueous
ZnCl.sub.2 solution (see, for example, Macromolecules 2016, 49,
5877, the disclosure of which is incorporated herein by reference).
Upon drying (for example, freeze drying) to remove bulk solvent
from the SiO.sub.2/ZnCl.sub.2/PAN suspensions, hybrid scaffolds
were obtained. Other drying processes such as spray drying, drying
under elevated temperature and drying under elevated temperature
and/or vacuum may be used. Subsequent stabilization at 280.degree.
C. under air followed by carbonization at, for example, 800.degree.
C. in a N.sub.2 flow and etching of the SiO.sub.2 template with HF
yielded mesoporous carbons. Stabilization typically includes
heating at a temperature below the carbonization temperature to,
for example, render the polymer thermally stable and reduce/prevent
melting during the subsequent carbonization process. For example,
PAN may be transformed from linear PAN to a latter structure during
heating at temperature that are, for example, no greater than
300.degree. C. for thermal stability.
[0047] FIG. 2A illustrates Brunauer-Emmet-Teller (BET) N.sub.2
adsorption isotherms for carbons prepared from samples with
systematically varied compositions with SiO.sub.2:PAN ratios of
4.7, 3.4, 2.2, 1.2, and 0.5 (that is, with progressively higher PAN
concentration; compare FIG. 3) along with a pristine sample
prepared without the addition of colloidal SiO.sub.2 NPs. These
materials are identified as NPC-S4.7, NPC-S3.4, NPC-S2.2, NPC-S1.2,
NPC-S0.5, and NPC-S0, respectively. All adsorption isotherms
obtained from SiO.sub.2/PAN of various ratios are type IV according
to IUPAC classification and exhibit distinctive hysteresis loops at
relative pressures of 0.6-0.9, indicative of filling and emptying
of the mesopores by capillary condensation/evaporation. The BET
surface areas (S.sub.BET) and total pore volumes of the nanoporous
carbons are listed in Table 1.
TABLE-US-00001 TABLE 1 SSAs and total pore volumes of the
synthesized N-doped carbons. Ratio of silica/ Specific surface Pore
Pore PAN area (m.sup.2/g) volume size Samples (wt:wt) Micropore
Mesopore Total (cm.sup.3/g) (nm) YS-PAN Pure 128 63 191 0.11 3.6
PAN JZ-3-33CF PAN + 67 549 616 0.46 2.9 ZnCl.sub.2 JZ-3-29CF 4.7 63
697 760 0.75 3.6 JZ-3-28CF 3.4 154 1065 1219 2.3 6.9 JZ-3-30CF 2.2
243 1262 1505 2.2 5.6 JZ-3-32CF 1.2 190 1586 1776 2.7 5.8 JZ-3-37CF
0.5 158 1062 1220 2.3 6.8
[0048] As can be seen from FIG. 2A, even the sample carbonized
without the addition of SiO.sub.2 exhibited significant S.sub.BET
of 616 m.sup.2/g with large contribution from mesopores
(S.sub.meso=549 m.sup.2/g, FIG. 2b and Table 1. The corresponding
pore size distribution (PSD, inset in FIG. 2A) displayed a small
mesopore peak centered at 2.3 nm tailing up to .about.8 nm. The
high S.sub.meso originated from activation by ZnCl.sub.2, since PAN
(DP 50) carbonized without the addition of ZnCl.sub.2 had a much
lower S.sub.BET=191 m.sup.2/g and no pronounced hysteresis loop in
the adsorption isotherm, indicating a predominately microporous
material (see FIG. 4). Interestingly, ZnCl.sub.2 activation did not
increase the microporosity, but selectively enhanced the S.sub.meso
of PAN-derived carbons (see FIG. 4B and Table 1). Without
limitation to any mechanism. the selective formation of mesopores,
rather than micropores, that was observed in the above studies may,
for example, be a result of the low molecular weight PAN matrix
that is more conducive to the transport of gaseous electrolyte.
[0049] Because ZnCl.sub.2 can act as a dehydrating agent, the
effect of ZnCl.sub.2 on thermal degradation of PAN was investigated
by thermogravimetric analysis (TGA). After addition of ZnCl.sub.2,
the cyclization of PAN occurred at 218.degree. C., compared to
276.degree. C. observed in pure PAN (see FIG. 5). Thus, ZnCl.sub.2
likely promoted dehydration of PAN and aromatization of nitrile
groups. Furthermore, a mixture of ZnCl.sub.2 and PAN decomposed at
a temperature lower than pure ZnCl.sub.2 and completely volatilized
below 550.degree. C., thus allowing for complete removal of the
salt during pyrolysis at temperatures >600.degree. C.
[0050] After the addition of SiO.sub.2 particles to the
PAN/ZnCl.sub.2 solution, the S.sub.BET of carbons obtained
utilizing the process described in FIG. 1 increased to 1220, 1776
and 1505 m.sup.2/g for samples NPC-S0.5, NPC-S1.2 and NPC-S2.2,
respectively. ZnCl.sub.2-induced mesopores accounted for as much as
84-90% of the total SSA (see Table 1). Without limitation to any
mechanism, the remarkably high S.sub.meso is believed to have
originated from a synergistic effect of ZnCl.sub.2 activation and
SiO.sub.2 templating. Indeed, the mesopore peak centered at
approximately 2.3 nm, similar to one observed for NPC-S0, was still
visible in the PSD of all SiO.sub.2-templated samples. However, the
evolution of two new peaks, at 10.2 and 12.5 nm, corresponding to
the size of the SiO.sub.2 NPs (12.5.+-.1.5 nm), is clearly visible,
indicating efficient templating. Since a broad distribution of
mesopores was observed rather than a narrow peak corresponding to
12.5 nm SiO.sub.2 NPs, partial aggregation of NPs during the freeze
drying and carbonization processes is possible. Further increase of
the SiO.sub.2/PAN ratio resulted in a decrease of S.sub.BET of
corresponding carbons to 1219 and 760 m.sup.2/g for NPC-S3.4 and
NPC-S4.7, respectively. The pores originating from SiO.sub.2 NPs
were no longer visible in the PSD of NPC-S4.7, very similar to that
of NPC-S0 (see the inset in FIG. 2A). Without limitation to any
mechanism, that observation is likely a result of an insufficient
amount of PAN to efficiently encapsulate the SiO.sub.2 NPs. Indeed,
this hypothesis was confirmed by plotting the cumulative pore area
versus average width of nanoporous carbon prepared at different
SiO.sub.2/PAN ratios and pure PAN (see FIG. 6). With increasing
SiO.sub.2/PAN ratio, the contribution of pores below 5 nm to the
pore areas decreased accordingly.
[0051] To evaluate the more general applicability of the
co-solubilization approach to form a benign all-organic material
systems, commercial cellulose nanocrystals as well as cellulose
filter paper were used as templates to prepare nanoporous carbons.
FIG. 7 compares the BET N.sub.2 adsorption isotherms and the
corresponding PSD of porous carbons prepared from commercial
cellulose nanocrystals (NPC-C) and filter paper (NPC-P) with
materials obtained after infiltration with ZnCl.sub.2 (NPC-CZ,
NPC-PZ) and PAN/ZnCl.sub.2 (NPC-PZ, NPC-PAZ), respectively.
[0052] ZnCl.sub.2 activation enabled the formation of highly porous
carbon with S.sub.BET of 1366 and 1501 m.sup.2/g for nanocellulose
and filter paper templated systems, respectively. In both cases the
PSD reveals the formation of mesopores with a diameter or
approximately 2.3 nm that can be attributed to the volatilization
of ZnCl.sub.2. The size of mesopores was approximately equal for
all studied template systems and the size distribution was narrower
for PAN/ZnCl.sub.2 infiltrated systems. Thus, the cumulative effect
of ZnCl.sub.2 volatilization and PAN carbonization determines the
final size of mesopores. Type H3 loop characteristics was observed
for both NPC-CAZ and NPC-PAZ (see FIG. 7, panels a and c). Without
limitation to any mechanism, this result may, for example, be
attributed to the fibrous morphology of the template that favors
the formation of slit-shaped pores.
[0053] The highly porous structure of NPCs prepared through the
distinct routes discussed above are depicted in FIG. 8. The figure
reveals the increase of the density of micropores with silica
particle content (see panels a-d of FIG. 8), as well as the more
anisotropic pore structure of cellulose derived NPCs (see panels e
and f of FIG. 8).
[0054] A comprehensive comparison of the microstructures observed
before and after carbonization is further shown in FIGS. 9 and 10.
The infiltration of PAN/ZnCl.sub.2 solutions in filter paper
facilitated the fabrication of monolithic NPC films as shown in the
inset of panel f FIG. 8, which benefits both the processing and
integration of NPCs and enhances the advantages of NPC materials
hereof.
[0055] XRD pattern and Raman spectrum for nanoporous carbon
demonstrate the co-existence of graphitic and disordered carbons as
illustrated in FIGS. 11A and 11B. Elemental composition of the
prepared mesoporous carbon (NPC-S1.2) was determined by elemental
analysis (combustion method) with the nitrogen content of 10 wt %,
consistent with typical values for PAN-derived carbons prepared at
this temperature (800.degree. C.).
[0056] X-ray photoelectron spectroscopy (XPS) analysis was carried
out to evaluate the chemical identities of the heteroatoms in the
carbon network. For survey spectrum of NPC-S1.2 as illustrated in
FIG. 12, three main peaks were shown including the C 1 s peak at
.about.282-296 eV, the N 1 s peak at .about.395-408 eV, and the O 1
s peak at .about.527-540 eV, suggesting the coexistence of carbon,
nitrogen and oxygen, with the respective atomic ratio of 84.0%,
9.6% and 4.9%. Similar nitrogen contents obtained from elemental
analysis and XPS indicate the uniformity of nitrogen distribution
in the entire carbon.
[0057] The high resolution N 1 s spectrum illustrated in FIG. 13
was further deconvoluted to three peaks with the binding energies
of 403.3, 399.9, and 398.2 eV, attributed to pyridine oxide-N
(N--O), pyrrolic- or pyridonic-N (N--X), and pyridinic-N (N--P),
respectively. The ratios of different nitrogen types are 21.7%
(pyridinic-N), 56.7 (pyridonic- or pyrrolic-N) and 21.6% (pyridine
oxide-N). The chemical environments of these nitrogen atoms is
consistent with their location along the outer edges of
nanographitic domains, which could explain their electrochemical
availability. Furthermore, the full width at the half-maximum
(fwhm) of the N--P peak observed was only 1.4 eV. This is
significantly less than previously reported values for pyridinic
nitrogen in pyrolytic carbons derived from PAN, demonstrating the
high degree of uniformity of the NPCs prepared by the disclosed
processes. See, for example, Carbon 1995, 33, 1641.
[0058] The electrocatalytic activity of the NPC-S2.2 sample,
representative of prepared carbons, was evaluated for an ORR in a
standard three-electrode setup at room temperature in 0.1 M KOH as
the electrolyte. The active material was deposited on a glassy
carbon disk and used as the working electrode with a Ag/AgCl
reference electrode and graphite counter electrode. Cyclic
voltammetry (CV) scans recorded at 100 mV/s showed no redox peak
when the electrolyte was continuously purged with argon. In
contrast, when the solution was saturated with O.sub.2, a
pronounced cathodic peak appeared in the CV scan, FIG. 13, panel b.
Linear sweep voltammograms (LSV) were recorded using a rotating
disk electrode (RDE) at different rotation speeds. As can be seen
from the polarization curves in FIG. 13, panel c, the onset
potential of NPC-S2.2 based electrode was .about.0.9 V vs
reversible hydrogen electrode or RHE, comparable with that of
commercial Pt/C catalysts. The limiting current gradually increased
with rotation speed. Since ORR can proceed via either two- or
four-electron transfer mechanism, Koutecky-Levich analysis was
performed to determine the number of transferred electrons
(n.sub.e). The linear relationship between the current density
(j.sup.1), as a function of a square root of the rotation speed
(.omega..sup.-1/2) in the potential range of 0.3-0.6 V vs RHE can
be inferred from the Koutecky-Levich plots (see FIG. 13, panel d).
The number of electrons (n.sub.e) transferred in the process
determined by the Koutecky-Levich equations ranged between 3.90 and
4.19. This result indicates that the ORR process occurred via the
four-electron pathway, as expected based on the structural
characteristics of this N-doped mesoporous carbon. See, for
example, Chem. Rev. 2015, 115, 4823.
[0059] In a number of embodiments hereof, a facile, benign and
scalable aqueous-based method for synthesis of mesoporous N-doped
carbons is provided that that can be applied to both inorganic and
all-organic templating. Application of ZnCl.sub.2 as a
solubility-enhancing porogen enables the solubilization of a
nitrogen-containing polymer such as a relatively low molecular
weight PAN and dispersion of porogenic particle fillers in water
and significantly enhances the surface area as compared to regular
templated systems. The resulting materials exhibited a nitrogen
content of 10 wt % and showed excellent catalytic activity toward
ORR via the four-electron mechanism.
[0060] The methods, compositions and systems hereof open new
opportunities for tuning pore size distributions in NPCs under
facile and benign conditions that should promote the application of
NPCs in a range of applications including, for example, metal-air
batteries, fuel cells, and CO.sub.2 capture.
EXPERIMENTAL EXAMPLES
[0061] Materials. Acrylonitrile (99.9%, available from
Sigma-Aldrich of St. Louis Mo.) was passed over a column of basic
alumina directly before use to remove the inhibitor. Silica
particles (30 wt % silica in water, effective diameter 12.5.+-.1.5
nm, from LUDOX, available from Grace) was used as the template.
WHATMAN.TM. ashless filter paper (available from Sigma-Aldrich)
were used as received. Cellulose nanocrystals (CNC) were kindly
provided by CelluForce Company of Montreal, Canada.
.alpha.,.alpha.'-Azoisobutyronitrile (AIBN, available from
Sigma-Aldrich, 98%) was recrystallized from methanol. Zinc chloride
(>98%, available from Sigma-Aldrich), copper(II) bromide
(99.999%, available from Sigma-Aldrich), 2-bromopropionitrile (BPN,
available from Sigma-Aldrich, 97%), potassium hydroxide
(>99.97%, available from Fluka), dimethyl sulfoxide (DMSO,
available from Fisher Scientific of Waltham, Mass., 99.9%),
methanol (Fisher Scientific, 99.9%) N,N-dimethylformamide (DMF)
(Fisher Scientific), dimethyl sulfoxide (DMSO, Fisher Scientific,
99.9%) and hydrofluoric acid (50 vol % HF, Acros), were used as
received. Milli-Q water (available from Millipore Sigma of
Burlington Mass.) was used in all experiments.
Tris(2-pyridylmethyl)amine (TPMA) was synthesized according to
published procedures. See, for example, Inorganic Chemistry 2005,
44, 8125; Macromolecules 1999, 32, 2434, the disclosure of which is
incorporated herein by reference.
[0062] Analytical Procedures. Characterization of polymers. The
apparent molecular weights and molecular weight distributions
(M.sub.w/M.sub.n) of PAN were determined by gel permeation
chromatography (GPC). The GPC system used a Waters 515 HPLC pump
and a Waters 2414 refractive index detector using Waters columns
(STYRAGEL.RTM. 10.sup.2, 10.sup.3, 10.sup.5 .ANG.) with 10 mM
LiBr-containing DMF as the eluent at a flow rate of 1 mL/min at
50.degree. C. using linear poly(ethylene oxide) (PEO) standards.
Exact DP values were determined by .sup.1H NMR spectroscopy
measurements performed on a Bruker Avance 300 MHz spectrometer.
Thermogravimetric analysis (TGA) was performed on a TA Instruments
Q50 with 60 mL/min flow rate of air or nitrogen.
[0063] Characterization of the nanocarbons. Brunauer-Emmet-Taller
(BET) specific surface area measurements were carried out using a
Micromeritics Gemini VII 2390 Surface Area Analyzer with VacPrep
061 degasser. Carbon samples were degassed at 300.degree. C. and 20
mTorr vacuum for at least 8 hours prior to measurement. The
adsorption isotherms were fitted to the Barrett-Joyner-Halenda
(BJH) model with the Kruk-Jaroniec-Sayari (KJS) correction to yield
pore-size distributions. The surface area of micropores was
estimated using the t-plot method with the KJS thickness
correction. The micropore surface area (S.sub.micro) was obtained
from a t-plot method using the de Bore equation. The mesopore
surface area (S.sub.meso) is simply calculated from the value of
S.sub.BET-S.sub.micro. The mesopore size distribution was obtained
from Barett-Joyner-Halenda (BJH) method from the desorption branch.
Transmission electron microscopy (TEM) (HT-7700, Hitachi Ltd.
Tokyo, Japan) was conducted at an accelerating voltage of 120 kV.
X-ray diffraction (XRD) patterns were recorded on a Rigaku
Geigerflex equipped with a theta/theta goniometer. The Raman
spectra were collected on a Jobin Yvon T64000 triple Raman system
(ISA, Edison, N.J.) in a subtractive mode with microprobe sampling
optics. The excitation was at 514.5 nm (Art laser, Model 95, Lexel
Laser, Fremont, Calif.). XPS was performed using an ESCALAB 250Xi
X-ray Photoelectron Spectrometer Microprobe, with a 900 mm spot
size. Inductively coupled plasma mass spectrometry (ICP-MS) was
carried out using an Agilent 7700x ICP-MS under high energy helium
flow.
[0064] Electrochemistry. A glassy carbon (GC) electrode (5 mm, from
Gamry) was carefully polished with 3 .mu.m, 1 .mu.m, and 0.25 .mu.m
diamond successively to obtain a mirror-like surface. Then the
electrode was washed with double-distilled water and acetone and
finally dried in air. Five milligrams of carbon were dispersed in 1
mL of a solvent mixture of NAFION.RTM. (5%; an ionomeric polymer or
ionomer available from Chemours Company of Wilmington, Del.) and
ethanol (1/9, v/v) by sonication for 1 h. Twenty microliters of the
solution were drop cast on the GC electrode surface and dried in
air to obtain the catalyst loading of 0.5 mg/cm.sup.2.
Voltammograms were recorded at 25.degree. C. with a Gamry Reference
600 potentiostat. Measurements were carried out at a scan rate of
10 mV/s or 100 mV/s using the nanocarbon-modified GC disk as
working electrode and a graphite rod counter electrode in
Ar-saturated or O.sub.2-saturated 0.1 M aqueous KOH electrolyte.
Potentials were recorded versus a Ag/AgCl reference electrode. All
potentials were converted to reversible hydrogen electrode (RHE)
according to the equation: E (RHE)=E.sup.0 (Ag/AgCl)+E
(Ag/AgCl)+0.0059.times.pH.
[0065] Kinetics of the ORR process was followed by Koutecky-Levich
analysis of linear sweep voltammograms using Koutecky-Levich
equations:
1 j = 1 j L + 1 j K = 1 B .omega. 1 / 2 + 1 j K ##EQU00001## B =
0.62 n e FC 0 D 0 2 / 3 v - 1 / 6 ##EQU00001.2## j K = n e FkC 0
##EQU00001.3##
where j (mA/cm.sup.2) is the measured current density, j.sub.K and
j.sub.L (mA/cm.sup.2) are the kinetic- and diffusion-limiting
current densities, .omega. is the angular velocity of the rotating
disk (.omega.=2.pi.N, where N is the linear rotating speed in rpm),
n.sub.e is the overall number of electrons transferred in ORR, F is
the Faraday constant (96485 C/mol), C.sub.0 is the bulk
concentration of O.sub.2 (1.2.times.10.sup.-3 mol/L), D.sub.0 is
diffusion coefficient of O.sub.2 (1.9.times.10.sup.-5 cm.sup.2/s),
v is the kinematic viscosity of the electrolyte (0.01 cm.sup.2/s),
and k is the electron transfer rate constant, respectively. The
number of electrons transferred (n.sub.e) and the kinetic-limiting
current j.sub.K can be obtained from the slope and intercept of the
Koutecky-Levich plots (1/j versus .omega..sup.-1/2, FIG. 4d),
respectively.
[0066] Morphology of LUDOX silica nanoparticles TEM images of LUDOX
SM-30 silica nanoparticles at different magnitudes were examined
and showed a narrow size distribution and the diameter of about
12.5 nm. The average diameter of silica nanoparticles was
determined by ImageJ analysis software from the TEM images. The
size of LUDOX SM-30 was 12.5.+-.1.5 nm.
[0067] XRD pattern and Raman Spectrum characterization. XRD
patterns for the nanoporous carbon observed at 2.theta. of
25.degree., 45.degree., and 80.degree. can be identified as (002),
(100), and (110) reflections of partially nanographitic structures
(see FIGS. 11A and 11B). J. Am. Chem. Soc. 2014, 136, 7845; Angew.
Chem. Int. Ed. 2016; Nat. Nanotech. 2014, 9, 618, the disclosures
of which are incorporated herein by reference. The lateral size of
partially graphitic domains can be estimated based on the width of
the (100) peak (using Debye-Scherrer equation) and indicates pore
sizes .ltoreq.3 nm. The (002) diffraction peak centered at a
2.theta..about.25.degree. reveals that .pi.-stacking of
nanographitic platelets did not exceed more than two to three
.pi.-stacked nanographene sheets. The Raman spectrum shows peaks at
1360 and 1586 cm.sup.-1, correspond to the characteristic D and G
bands of graphitic carbons, respectively (see FIG. 11B). As the G
band is related to tangential vibrations of sp.sup.2 carbon atoms,
its presence in the spectra suggests the existence of graphitic
structures in the nanoporous carbon materials. On the other hand,
the D band corresponds to the defect band. By measuring the ratio
of intensities of the two bands (i.e., I.sub.D/I.sub.G), the
relative degree of order/disorder in the nanoporous carbon was
1.07, suggesting the co-existence of graphitic and disordered
carbons.
[0068] EXAMPLE 1. Synthesis of low molecular weight PAN. PAN
samples were synthesized by initiators for continuous activator
regeneration atom transfer radical polymerization (ICAR ATRP)
following a recently published procedure. J. Polym. Sci., Part A:
Polym. Chem. 2016, 54, 1961, the disclosure of which is
incorporated herein by reference. To prepare PAN with degree of
polymerization (DP)=50; 37.8 mg of AIBN (0.23 mmol, 0.1 equiv.),
18.75 mL of DMSO and 1.85 ml of DMF (as NMR internal standard) were
charged added to a Schlenk flask and degassed for 30 minutes. A
stock solution of CuBr.sub.2 and TPMA in DMF was prepared and
degassed for 30 min, so that 2.58 mg (0.011 mmol, 0.005 equiv.) of
CuBr.sub.2, 10.03 mg (0.034 mmol, 0.03 equiv.) of TPMA could be
added to the Schlenk flask. 12.22 g (0.23 mol, 100 equiv.) of
degassed AN was added to the flask, and finally, 308 mg (2.30 mmol,
1 equiv.) of BPN was added and the polymerization was started by
immersing the flask in an oil bath at 65.degree. C. and conducted
over 6.5 h. The reaction was stopped at a monomer conversion of
49.6% (determined by .sup.1H NMR). The PAN was isolated by
precipitation into methanol/water (4:1, v/v) and dried under vacuum
at room temperature overnight. The DP of the obtained polymer was
confirmed by .sup.1H NMR to be 49.
[0069] 1A) Effect of PAN molecular weight on the formation of a
stable composite solution. PANs with two different DPs were used to
fill the interstitial voids of silica nanoparticles. PAN/ZnCl.sub.2
solution of DP 100 and DP 50 were prepared and added to LUDOX SM-30
silica nanoparticles. The PAN sample with a DP 100 could not be
completely dissolved in the aqueous solvent and formed an opaque
solution with a relatively high viscosity. The PAN sample with a DP
50 formed a transparent solution, indicating that PAN with a lower
DP can be completely dissolved in the aqueous ZnCl.sub.2 solution.
In general, PANs having a DP of 100 or less are suitable for use
herein.
[0070] 1B) Effect of addition of ZnCl.sub.2 on S.sub.BET of carbons
formed from low-MW PAN. To elucidate the effect of ZnCl.sub.2
activation on the development of pores, pure PAN and ZnCl.sub.2
activated PAN with DP .about.50 were carbonized. The resulting
carbons were termed NPC-PAN and NPC-ZPAN, respectively. The N.sub.2
adsorption isotherms recorded for NPC-PAN and NPC-ZPAN are type I
and type IV, respectively. Both exhibit a steep increase at low
relative pressure indicating the presence of micropores, however,
the carbons exhibited different hysteresis loops at relative
pressures of 0.6-0.9 (see FIG. 4A). The carbon obtained from pure
PAN showed a typical characteristic of a microporous material while
the shape of the isotherms of ZnCl.sub.2 activated PAN indicated
the existence of both micropores and mesopores (FIG. 4A). This
conclusion is supported by the pore size distribution analysis
which showed a long tailing up to 9-10 nm in the case of NPC-ZPAN
and mostly micropores for NPC-PAN.
[0071] 1C) Effect of ZnCl.sub.2 addition on the thermal behavior of
PAN. The effect of the ZnCl.sub.2 addition on the thermal behavior
of PAN was studied by TGA using a heating rate of 10.degree. C./min
in N.sub.2. FIGS. 5A and B shows TGA results of ZnCl.sub.2, PAN,
and a mixture of PAN with ZnCl.sub.2 (a solution of 0.4 g of PAN
and 13.5 g of ZnCl.sub.2). The PAN and ZnCl.sub.2 blend samples
were freeze dried before TGA analysis. The weight loss of pure
ZnCl.sub.2 started at about 400.degree. C., had a maximum weight
loss rate at 607.degree. C. at a heating rate of 10.degree. C./min
in N.sub.2, and all the ZnCl.sub.2 decomposed before reaching
617.degree. C. A typical TGA curve for pure PAN displayed peaks at
228.degree. C. and 311.degree. C. corresponding to the cyclization
of nitrile groups. Further carbonization of the crosslinked PAN
occurred with slight weight loss between 400 and 800.degree. C. A
total mass loss of .about.50% was recorded, in line with the
well-known PAN carbonization mechanism. However, three main peaks
were observed at 218, 276, and 534.degree. C., for the mixture of
PAN and ZnCl.sub.2. The cyclization of PAN occurred at 218.degree.
C., which was lower than that observed in the pure PAN system.
ZnCl.sub.2 is generally believed to function as a dehydrating
agent, eliminating water and inducing the aromatization of carbon
which suggests that the incorporated zinc chloride accelerated the
oxidative stabilization reactions and promoted the dehydration
reactions of the PAN at lower temperatures, which results in
aromatization of the carbon skeleton with the concomitant
generation of a pore structure.
[0072] Furthermore, the ZnCl.sub.2 present inside the PAN
volatilized out of the composites at a lower temperature compared
with that measured for pure ZnCl.sub.2. The removal/volatilization
of ZnCl.sub.2 from the composite occurred at a lower temperature
and the maximum weight loss rate shifted to 534.degree. C., leading
to the nearly complete volatilization of ZnCl.sub.2. This suggested
that nearly zero amounts of ZnCl.sub.2 remain entrapped after
pyrolysis at the high carbonization temperature.
[0073] EXAMPLE 2. Preparation of N-doped mesoporous carbon from
colloidal silica-templated PAN. The aqueous suspension of colloidal
silica was dialyzed against water to about 10 wt %. In a typical
synthesis, different ratios of PAN and ZnCl.sub.2 were added to 10
mL of an aqueous colloidal silica, and the concentration of
ZnCl.sub.2 in the aqueous solution was adjusted to 60 wt % to
ensure complete solubility of the PAN. The mixture was stirred for
1 h at room temperature until a transparent solution was formed.
Then, the silica/PAN solution was subjected to freeze-drying to
yield a solid silica/PAN composite. The composite was then
stabilized under air at 280.degree. C. followed by carbonization at
800.degree. C. for 30 min under nitrogen to yield a silica-carbon
composite. The obtained carbon samples were then stirred in 50 wt %
aqueous HF solution for 12 h to remove the silica template.
[0074] 2B) Preparation of nanoporous carbons with varied silica/PAN
ratios (wt:wt). FIG. 3 shows a schematic illustration of the effect
of silica/PAN ratio on the formation of nanoporous carbons. After
the addition of silica to the PAN/ZnCl.sub.2 solution, the
S.sub.BET of nanoporous carbons increased greatly. When the
silica/PAN ratio was high, 4.7, incomplete encapsulation and/or
aggregation of silica NPs occurred during the freeze-drying and
carbonization processes (FIG. 3, panel a) and thus silica NPs
contributed little to the total SSA of the resulting nanoporous
carbon after the etching process (FIG. 3, panel a*). However, a
further decrease of the silica/PAN ratio (FIG. 3, panel b) part of
silica NPs were encapsulated by PAN chains leading to the increase
of S.sub.BET of corresponding carbons (FIG. 3, panel b*). The pores
originating from silica became visible in PSD of NPC-S3.4 as a
result of sufficient encapsulation of silica NPs by PAN. At a
higher volume fraction PAN (FIG. 3, panel c), more and more silica
NPs started to be encapsulated. Consequently, the silica NPs had a
higher contribution to the total SSA of nanoporous carbon
demonstrating the significant effect of efficient silica templating
(FIG. 3, panel c*).
[0075] 2C) Contributions of various pores to S.sub.BET. The
cumulative pore area versus average width of nanoporous carbon
prepared at different silica/PAN ratios and pure PAN are shown in
FIG. 6. With increasing silica/PAN ratios, the contribution of 4.5
nm pores to the pore areas decreased accordingly and the
contribution of mesopores in the range of 5-20 nm to the pore areas
increased gradually as the silica/PAN ratio decreased.
[0076] EXAMPLE 3. Preparation of N-doped porous carbon from
CNC-templated PAN. In a typical synthesis, a certain amount of CNC
was added to 10 mL of an aqueous ZnCl.sub.2/PAN solution. The
concentration of ZnCl.sub.2 in the aqueous solution was adjusted to
60 wt % to ensure complete solubility of the PAN. The mixture was
stirred overnight at room temperature and then, the CNC/PAN
dispersion was subjected to freeze-drying to yield a solid CNC/PAN
composite. The composite was then stabilized under air at
280.degree. C. followed by carbonization at 800.degree. C. for 30
min under nitrogen to yield porous carbon. For comparison, the
pristine CNC and CNC filtered from aqueous ZnCl.sub.2 solution were
also carbonized according to the above process. Images of the
porous carbons formed by this procedure are shown in FIGS. 9A and
9B. Images shown in FIG. 9A indicated that before conducting the
carbonization process, the cellulose nanocrystals were dispersed
uniformly within the PAN matrix. The images in FIG. 9B show the
nanostructure of porous carbon templated from cellulose
nanocrystals after filtration of PAN/ZnCl.sub.2 solution. During
the pyrolysis process, PAN was activated by ZnCl.sub.2 to generate
highly porous structure. Cellulose nanocrystals were also
infiltrated by ZnCl.sub.2 and promoted the formation of porous
nanofiber. This was confirmed by the morphology changes of
nanofibers after the carbonization process when comparing FIG. 9A
and FIG. 9B.
[0077] EXAMPLE 4. Preparation of N-doped mesoporous carbon from
filter paper-templated PAN. In a typical synthesis, 10 mL of an
aqueous ZnCl.sub.2/PAN (6 g/1.0 g) solution was added dropwise onto
three pieces of filter paper (3.2 g). The concentration of
ZnCl.sub.2 in the aqueous solution was adjusted to 60 wt % to
ensure complete solubility of the PAN. The ZnCl.sub.2/PAN filter
paper composites were dried under vacuum at 45.degree. C. The
composite was then stabilized under air at 280.degree. C. followed
by carbonization at 800.degree. C. for 30 min under nitrogen to
yield porous carbon film. For comparison, the pristine filter paper
and filter paper filtered from aqueous ZnCl.sub.2 solution were
also carbonized according to the above process. The ZnCl.sub.2
penetration and further activation of filter paper generated porous
carbon film with much higher surface areas. FIG. 10A shows the
representative TEM image of porous carbon templated from pristine
filter paper without filtration of PAN/ZnCl.sub.2 solution. With
infiltration of ZnCl.sub.2 solution, the typical TEM images of
porous carbon templated from filter paper are shown in FIG. 10B.
When comparing the TEM images with FIG. 10A, the formation of
porous structure can be clearly observed in FIG. 10B.
[0078] The foregoing description and accompanying drawings set
forth a number of representative embodiments at the present time.
Various modifications, additions and alternative designs will, of
course, become apparent to those skilled in the art in light of the
foregoing teachings without departing from the scope hereof, which
is indicated by the following claims rather than by the foregoing
description. All changes and variations that fall within the
meaning and range of equivalency of the claims are to be embraced
within their scope.
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