www.sciencemag.org/content/350/6267/1508/suppl/DC1

Supplementary Materials for

Nitrogen-doped mesoporous carbon of extraordinary capacitance for

electrochemical energy storage

Tianquan Lin, I-Wei Chen, Fengxin Liu, Chongyin Yang, Hui Bi, Fangfang Xu, Fuqiang Huang*

*Corresponding author. E-mail: huangfq@mail.sic.ac.cn

Published 18 December 2015, Science 350, 1508 (2015) DOI: 10.1126/science.aab3798

This PDF file includes: Materials and Methods

Figs. S1 to S14

Tables S1 and S2

References

Methods

Synthesis and N-doping of ordered mesoporous carbons and few-layer carbon

The materials below were fabricated either with a silica template or without. In both cases,

few-layer carbon was CVD grown with the aid of carbon source and nickel catalyst. If a template

was used, it was later removed by HF etching. In both cases, a self-supported ordered

mesoporous few-layer carbon (OMFLC) structure in Fig. 1A was obtained. Nitrogen doping was

implemented by including nitrogen source in the precursor and in the CVD gas of the above

process to obtain N-doped OMFLC (OMFLC-N) having N incorporated at various carbon

locations in Fig. 1B. The N content was adjustable by the precursor composition and by

post-CVD acid-oxidation. A more detailed description follows.

Ordered mesoporous carbon (OMC) was synthesized using ordered mesoscopic silica as the

template and polyfurfuryl alcohol (PFA) as the carbon source. The template (SBA-15) was

prepared following the procedure in the literature (11) using tetraethyl orthosilicate (TEOS) as

the silica source and PEO20PPO70PEO20 (Pluronic P123, Sigma-Aldrich), a triblock copolymer,

as the surfactant. The OMC was next prepared by adding 1.0 g of SBA-15 to a solution

containing 1.8 g PFA and 20 mL ethanol, then held at 150 °C before carbonization under a

nitrogen atmosphere at 800 °C. Finally, the template (SBA-15) was removed by HF (30%)

solution etching to obtain the remaining OMC. Nickel impregnated templates (SBA-15/Ni) were

also prepared from SBA-15 by adding 5 mL g−1 of Ni(NO3)2⋅6H2O solution (0.5 mol L−1). After

drying and calcination under a H2/Ar (10%) flow at 450 °C, a product (SBA-15/Ni) of ordered

mesoporous silica with nickel nanocrystals was obtained.

Nitrogen-doped ordered mesoporous few-layer carbon (OMFLC-N) using SBA-15/Ni

template was prepared by CVD. Filled with PFA and dicyandiamide (DCDA), the template after

drying was heated under a hydrogen and argon flow to 1000 °C. CVD was initiated by

introducing CH4 and NH3 into the gas flow with Ar:CH4:H2:NH3=300:10:20:100 sccm

(standard-state cubic centimeter per minute). After that the sample was cooled under hydrogen

flow and the template was similarly removed by HF etching to recover the product OMFLC-N.

The N content was adjusted by changing the amount of DCDA and further modified by a HNO3

treatment that partially oxidized N into N−O by immersing the samples into concentrated HNO3

for 12 h at room temperature. For example, the mass ratio of PFA:DCDA=1:1 enabled us to

2

obtain OMFLC-N S1 containing 8.2% N and, after HNO3 treatment, 7.5%; if the ratio increases

to 1:2, the content of nitrogen increases to 11.9% and, after HNO3 treatment, 10.6%. Undoped

OMFLC was similarly synthesized without using DCDA and NH3.

In addition to the above method that was used to obtain OMFLC-N S1, S2, S3 and S4, as

well as their mixture SM (at the ratio of S1:S2:S3=0.3:0.3:0.4), a simplified method was used to

obtain N-doped mesoporous few-layer carbon with properties comparable to OMFLC-N SM.

This is by combining CVD with a sol-gel process containing a sol made of polyethylene glycol

(PEG, as an inexpensive pore-forming agent and C source), urea (as an inexpensive N source)

and nickel nitrate (as before). The gel precursor was first prepared by dissolving Ni(NO3)2⋅6H2O

(1.0 g), urea (2.0 g) and PEG (2.5 g) in 10 mL ethanol under vigorous stirring, followed by

gelation at 30 °C for 8 h and heating at 80 °C for 4 h. Few-layer graphene-like carbon was grown

on the resulting xerogel at 1,000 oC under a gas mixture (Ar:CH4:H2:NH3=300:10:20:100 sccm)

flow. The N content was also adjusted by changing the amount of urea, and further modified by a

HNO3 treatment that partially oxidized N into N−O by immersing the samples into concentrated

HNO3 for 12 h at room temperature.

Characterization of materials

Low angle X-ray diffraction (θ-2 θ scan) to determine the superstructure of OMC, OMFLC

and OMFLC-N was performed on a Brucker D8 powder X-ray diffractometer using Cu Kα

radiation. Nitrogen adsorption-desorption isotherms at 77 K were measured on a Micromeritics

Tristar 3000 system using vacuum-degassed samples (180 °C for at least 6 h). The isotherms

were used to calculate (a) the specific surface area by the Brunauer-Emmett-Teller (BET)

method and (b) the pore volume and pore size by the Barrett−Joyner−Halenda (BJH) method.

Scanning electron microscopy (SEM) images were obtained in a field emission Magellan 400

microscope (FEI Company). Transmission electron microscopy (TEM), high angle annular dark

field (HAADF), electron energy loss spectroscopy (EELS, Gatan) examinations were conducted

at 200 kV in a JEOL 2011 microscope operated. X-ray photoelectron spectroscopy (XPS) was

collected in a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation

(hν =1253.6 eV). Binding energies were calibrated using containment carbon (C 1s =284.6 eV).

Because of the thin thickness (<2 nm) of the few layer carbon OMFLC-N used in this work and

the long mean free path of photoelectron (about 3.2 nm), XPS N/O/C 1s signal is representative

3

of the bulk composition when normalized by the C signal.(29) Raman spectra were collected in a

Thermal Dispersive Spectrometer using a 10 mW laser with an excitation wavelength of 532 nm.

Zeta potential of colloids in suspension was measured by a Zeta Nanoseries (Nano ZS90,

Malvern Instrument Ltd.)

Electrochemical characterization

To fabricate electrodes, active material (OMC, OMFLC, or OMFLC-N) powders were mixed

in N-methyl-2-pyrrolidone (NMP) to form a 10 mg mL−1 of homogeneous slurry. In the above,

polyvinylidene fluoride (PVDF) of 5 wt.% was added as binder when more than 2.0 mg cm−2

active material was loaded in the electrode; at lower mass loading, no PVDF was used. The

slurry was coated onto a porous 3D-graphene foam (1.2 mm thick, as current collector), which is

highly compressible. The preform (electrode) structure was dried under vacuum at 100 °C for 10

h to remove NMP, before being compressed into a disk to form the electrode. The final electrode

was ~10× thinner than the original foam thickness, according to the SEM viewing of the

cross-section (Fig. S14 shows four samples with an average thickness of 102 µm), but despite

including the 3D graphene it still maintained a BET surface area of 1,280 m2 g-1 indicating nearly

all powder surfaces remained accessible. The electrode thickness increased with the mass

loading in the electrode. The measured specific capacitance of pristine 3D-graphene foam with

density of 1.1 mg cm−2 is about 30 F g−1.

Electrochemical experiments were performed in several configurations and electrolytes. For

aqueous electrolytes, 0.5 M H2SO4 (pH 0), 2 M Li2SO4 (pH 0.5, 1.8, 9.2), and 1 M KOH (pH 14)

solutions were used. In three-electrode cells, in addition to the working electrode of active

material, platinum as the counter electrode and Ag/AgCl as the reference electrode were used. In

symmetric cells, two identical (by weight, size and composition) active-material electrodes were

used as cathode and anode. Cyclic voltammetry (CV) tests and galvanostatic charge–discharge

(CC) tests were performed using an electrochemical analyzer, CHI 660E, under ambient

conditions. Electric impedance spectroscopy (EIS) was performed with an amplitude of 10 mV,

from 10 mHz to 100 kHz.

Symmetric devices with two identically configured electrodes were also packaged. Since the

electrodes were highly conductive, they were directly connected to the electrochemical analyzer

by alligators without any current collector other than the 3D graphene foam that was an integral

4

part of the electrode construction. After inserting an ion-porous separator (Celgard 3501)

between the two electrodes, the electrode/electrolyte assembly was wrapped within a Kapton

tape seal to complete the package.

Electrochemical Data

For a nonlinear Faradaic capacitor, the capacitance is obtained from the integrated form

(1)

In the above, i is the average current during the CV/CC charging or discharging cycle, ∆V is the

potential window, and ∆t is the charging or discharging time.

(a) CV test

Since dV/dt=v is a set constant (i.e., the scan rate), integration over the entire loop gives the

average capacitance

(2)

Here is the loop area. The specific capacitance is the capacitance divided by the

mass of the electrode.

(3)

The volumetric capacitance is likewise obtained by dividing capacitance by the volume of the

electrode.

The energy density is obtained from the integral . To average over the

charging/discharging cycle, absolute values are used in integration

d d=

d d

Q I t i tCVV V

∆= =

∆∫ ∫∫ ∫

2AreaSC

v V=

∆

dAreaS I V= ∫

2AreaSC

mv V=

∆

= dE V Q∫

5

(4)

The specific energy is the energy divided by the mass of the electrode

(5)

Similarly, the energy density is obtained by dividing the energy by the volume of the electrode.

(b) CC test

Since I is a set constant (i.e., the charging/discharging rate), i=I so Eq (1) reduces to

C=I∆t/∆V (6) (For our Faradaic capacitance, the charging/discharging curves are rather linear, so ∆V/∆t is essentially the slope.) The specific capacitance is the capacitance divided by the mass of the electrode

(7)

The volumetric capacitance is likewise obtained by dividing capacitance by the volume of the

electrode.

The energy density is obtained from the integration

d= d d d

2

I V tE V Q VI t I V t= = = ∫∫ ∫ ∫ (8)

In the above, the last integral over both half cycles obtains an average.

The device energy and power are calculated considering the entire device weight and volume,

including the electrodes, the electrolyte, the separator and the packing material. Power (Pwt) is

obtained from Pwt=V2/4RESR and divided by the device weight and volume to obtain specific

d d= =

2 2

V Q VI VE

v∫ ∫

d=

2

VI VE

mv∫

=( )

ICm V t∆ ∆

6

power and power density, respectively. Here, V is the operating voltage and RESR is the

equivalent serial resistance (ESR) of the device. ESR is obtained from the CC test by dividing

the voltage drop (Vdrop) upon current reversal by twice the value of I, i.e., RESR=Vdrop/2I.

The results obtained from Eq (2, 4) from the CV test and Eq (7, 8) from the CC test are

compared in Table S2. Here, the specific capacitance, specific energy and specific power

consider the weight of active material in the electrode only. To self-consistently compare the

capacitance of a symmetric EC with the capacitance measured in a three-electrode cell, we

follow Stoller and Ruoff (23) and multiply the nominal specific capacitance of a symmetric EC

by 4 to account for the number (2×) of electrodes and the serial connection of two identical

capacitors (having total capacitance =C/2).

7

Fig. S1. TEM and SEM images of OMFLC and OMFLC-N. (A) TEM image of OMFLC. (B,

C) TEM images of OMFLC-N at two magnifications, showing ordered channels of pores and

graphene-like structure. Few-layer graphene-like structure seen in (C). (D) SEM and

corresponding (E) dark field and (F) bright field STEM images of OMFLC-N.

8

Fig. S2. Properties of OMC, OMFLC and OMFLC-N (S1). (A) N2 adsorption−desorption

isotherms. All exhibit typical Langmuir hysteresis indicating presence of well-defined

mesopores. (B) Raman spectra. Characteristic 2D band of few-layer graphene-like structure

found in OMFLC and OMFLC-N but not OMC. Few-layer graphene-like structure in OMFLC is

estimated to have fewer than five layers according to peak position (2,681 cm-1) and

full-width-at-half-maximum (55 cm−1) of 2D band and intensity ratio (~0.65) of 2D band to G

band (30). This is consistent with HRTEM observation. Blue shifts of G-band (5 cm−1) and the

2D band (13 cm−1) of OMFLC-N indicate higher Fermi level according to the literature (31). (C)

Resistance of OMC, OMFLC and OMFLC-N (S1), indicating the improvement of structural

order of carbon (12).

9

Fig. S3. EELS and XPS. (A) Electron energy loss spectroscopy (EELS) spectra from OMC,

OMFLC and OMFLC-N (S1). (B) EELS features at 410 eV characteristic of N bonding. (C−F)

Bonding of OMFLC-N. High-resolution XPS spectra for N 1s of OMFLC-N samples (S1 and

S3: without HNO3 treatment; S2 and S4: with HNO3 treatment). (C) OMFLC-N S1; inset:

locations of N-dopant in carbon sheet, showing “pyridinic” N (N-6), “pyrrolic” N (N-5), and

“graphitic” N (N-Q). (D) Sample S2. (E) Sample S3. (F) Sample S4. Here, pyridinic N refers to

any N with one p-electron on π system, and pyrrolic N refers to any N with two p-electrons on π

system; the latter is not limited to five-member ring environment as in pyrrole (14, 32).

10

Fig. S4. Performance of OMFLC-N (S1 and SM) in acidic electrolyte. Electrochemical

performance of the OMFLC-N with 8.2 at.% N (S1) and mixed OMFLC-N (SM) in 0.5 M

H2SO4 electrolyte. (A) CV curves of S1 at voltage scan rates of 2−100 mV s−1. Here, current is

normalized by voltage scan rate ν. (B) Specific capacitance calculated from CV curves vs.

voltage scan rate for S1. CC curves at current densities of 1−40 A g−1 of (C) S1 and (E) SM.

Specific capacitance calculated from CC curves vs. current density for (D) S1 and (F) SM.

11

Fig. S5. YP-50 vs OMFLC-N (S1). Electrochemical performance of YP-50 and our OMFLC-N

(S1) in 0.5 M H2SO4 (pH 0) and 2.0 M Li2SO4 (pH 1.8) electrolytes. CC curves of YP-50 and S1

at sweep rates of 1 A g−1 in (A) 0.5 M H2SO4 and (C) 2.0 M Li2SO4 electrolyte. CV curves at

current densities of in 2 mV s−1 (B) 0.5 M H2SO4 and (D) 2.0 M Li2SO4 electrolyte. (E)

Percentage capacitance retantion of YP-50 in 0.5 M H2SO4 () and 2.0 M Li2SO4 () in cycling

is less than that of OMFLC-N in 0.5 M H2SO4 (▇) and 2.0 M Li2SO4 (□). (F) Complex-plane

plots of AC impedance, inset: enlarged view.

12

Fig. S6. Comparison of impedance and kinetics. (A) Enlarged view of Cole-Cole plots of AC

impedance (Fig. 2C) of OMC, OMFLC, OMFLC-N (S1 and SM). (B) Specific capacitance of

OMC, OMFLC and OMFLC-N (S1 and SM) electrodes vs. root-inverse sweep rate, v−1/2, with v

from 2 to 500 mV s−1, in 0.5 M H2SO4.

13

Fig. S7. Performance of OMFLC-N (S1) in basic electrolyte. (A−D) Electrochemical

performance of the OMFLC-N (S1) in 1.0 M KOH electrolyte. (A) CV curves at voltage scan

rates of 2−50 mV s−1. Rectangular CV shape observed at all rates indicating efficient

double-layer formation. Here, current is normalized by voltage scan rate ν. (B) Specific

capacitance calculated from CV curves vs. voltage scan rate. (C) CC curves at current density of

1−20 A g−1. (D) Specific capacitance calculated from CC curves vs. current density. (E) CV

curves at 2 mV s−1 of OMFLC in 1.0 M KOH (alkaline, pH 14) and 0.5 M H2SO4 (acidic, pH 0)

electrolytes, showing nearly identical capacitance.

14

Fig. S8. Threshold voltage of water splitting. Determined by H2 and O2 accumulation

(measured by gas chromatography) in sealed symmetric cell with two OMFLC-N (SM)

electrodes of same capacitance in (A) 0.5 M H2SO4 (pH 0) and (B) 2 M Li2SO4 (pH 1.8) over 24

h under CV sweeping at 2 mV s−1.

Fig. S9. Behavior in Li2SO4 electrolytes. (A) CV curves of OMFLC-N (S1) in Li2SO4

electrolytes with different pH at sweep rates of 2 mV s−1. Li2SO4 (pH 1.8) electrolyte contains

sufficient H+ for the redox process, as evidenced by the redox peak at a potential similar to that

0.5 M H2SO4. The same is true at pH 0.5 although it is so acidic that it also generates H2. In basic

electrolyte (pH 9.2), the redox peak is lost. (B) CV curves at 2 mV s−1 of OMFLC-N (S1) in 1.0

M KOH and Li2SO4 (pH 9.2) electrolytes, showing approximate capacitance.

15

Fig. S10. Microstructure of sol-gel/CVD N-doped mesoporous few-layer carbon. (A) TEM

image showing ordered channels of pores, (B) High resolution TEM image showing

few-layer-graphene like structure.

Fig. S11. Properties of as-prepared sol-gel/CVD N-doped mesoporous few-layer carbon. (A)

N2 adsorption−desorption isotherm exhibits a typical Langmuir hysteresis indicating presence of

well-defined mesopores. (B) Pore-size distributions. (C) Low-angle X-ray diffraction patterns

showing characteristic (100), (110) and (200) peaks of hexagonal packing. (D) Raman spectrum

showing characteristic 2D band of few-layer-graphene like structure, estimated to have fewer

than five layers according to peak position (2,691 cm-1) and full-width-at-half-maximum (65

cm−1) of 2D band and intensity ratio (~0.52) of 2D band to G band. This is consistent with high

resolution TEM observation. (E) High-resolution XPS spectrum for N 1s corresponding to (F)

locations of N-dopant in few-layer carbon, showing “pyridinic” N (N-6), “pyrrolic” N (N-5), and

“graphitic” N (N-Q). The N content is 8.9 at.%.

16

Fig. S12. Electrochemical performances of the sol-gel/CVD N-doped mesoporous few-layer

carbon in 0.5 M H2SO4 and 2 M Li2SO4 (pH 1.8) electrolytes obtained by three-electrode

cells. (A) CV curves at voltage scan rate of 2 mV s−1. (B) Specific capacitance calculated from

CV curves vs. voltage scan rate. (C) CC curves at a current density of 1 A g−1. (D) Specific

capacitance calculated from CC curves vs. current density.

Fig. S13. Electrochemical performance of symmetric cells. With sol-gel/CVD N-doped

mesoporous few-layer carbon cathode and anode in 0.5 M H2SO4 electrolyte and 2 M Li2SO4

(pH 1.8) electrolytes. (A) Cyclic voltammetry at 2 mV s−1 scan rate. (B) Galvanostatic

charge/discharge curves at 1.0 A g−1.

17

Fig. S14. SEM images of OMFLC-N electrode. (A) Top-view and (B−F) cross-section images

of OMFLC-N electrode after compressing the powder/3D-graphene assembly from the original

thickness 1.2 mm.

18

Table S1. N and O content correlated to redox potential in OMFLC-N samples.

Sample N (at.%)

O (at.%)

%N-5† %N-6† %N-Q† %N-O† %N-5 + %N-6

∆%N-5* φ (V) #

S1 8.2 0.6 1.4 6.1 0.7 0 7.5 − 0.40

S2 7.5 6.2 0.9 5.1 0.6 0.9 6.0 0.5 0.50

S3 11.9 0.8 2.2 5.8 3.9 0 8.0 − 0.29

S4 10.6 7.7 1.2 4.0 3.7 1.7 5.2 1.0 0.53

† %N-6, %N-5, and %N-Q are the percentage of “pyridinic” N (N-6), “pyrrolic” N (N-5), and “graphitic” N (N-Q)

in few-layer carbon, respectively; see a schematic of these nitrogen locations in Fig. 1B, Fig. S3C, and Fig. S11F.

Likewise, %N-O is the percentage of N-O, i.e., N associated with one O. * ∆%N-5 is the decrement of %N-5 after

HNO3 treatment; # Redox potential at peak charging current in the CV curve. Oxidative HNO3 treatment caused the

least stable N-5 to substantially convert to N−O without affecting the most stable N-Q, as suggested by the

correlation between %N-O and ∆%N-5. Note that %N-5+%N-6 represents the total non-N-Q fraction of nitrogen,

and it decreases in the order of Samples S3, S1, S2 and S4 whose redox potentials (φ) increase in the same order.

Thus, non-N-Q nitrogen is more responsible for redox reactions than N-Q nitrogen.

19

Table S2. Properties of YP-50, OMC, OMFLC and OMFLC-N in aqueous electrochemical cells.*

Samples Electrolyte#

CC test at 1 A g−1 CV test at 2 mV s−1

3-electrode capacitance

(F g−1)

Symmetric-cell capacitance

(F g−1)

Specific energy

(Wh kg−1)

Specific power

(kW kg−1)

3-electrode capacitance

(F g−1)

Symmetric-cell capacitance

(F g−1)

Specific energy

(Wh kg−1)

YP-50 H2SO4 175 155 6.0 17.5 180 165 8.0

Li2SO4 160 150 12.5 25.5 170 160 14.0

OMC H2SO4 135 130 5.5 8.5 165 155 7.5

Li2SO4 115 105 8.0 10.0 145 135 11.5

OMFLC H2SO4 325 315 14.0 19.5 330 320 15.5

Li2SO4 300 290 22.5 30.5 300 285 25.0

OMFLC-N

(S1)

H2SO4 715 625 25.5 34.5 665 575 28.5

Li2SO4 690 590 40.0 40.5 630 540 47.5

OMFLC-N

(S2)

H2SO4 730 640 27.0 38.0 675 590 29.0

Li2SO4 690 590 40.5 45.0 635 550 48.0

OMFLC-N

(S3)

H2SO4 665 595 24.5 32.5 615 545 27.0

Li2SO4 600 530 38.0 38.0 565 495 43.0

OMFLC-N

(SM)

H2SO4 855 840 36.5 42.5 820 790 39.5

Li2SO4 780 740 54.5 44.0 725 715 63.0

*Mass loading of active material per electrode is 0.5 mg cm−2 in three-electrode-cell and symmetric-cell. The gravimetric basis is that of active material only. #0.5 M H2SO4 (pH 0) and 2 M Li2SO4 (pH 1.8).

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