Supporting Information

Engineered architecture of nitrogenous graphene encapsulating porous carbon

with nano-channel reactors enhancing the PEM fuel cell performance

Xiaogang Fu,a† Fathy M. Hassan,a† Pouyan Zamani,a Gaopeng Jiang,a Drew C.

Higgins,a,b Ja-Yeon Choi,a Xiaolei Wang,a Pan Xu,a Yanru Liua and Zhongwei Chena*

[†] The authors contributed equally to this work

a. Department of Chemical Engineering, University of Waterloo, 200 University Ave.

W, Waterloo, Ontario N2L 3G1, Canada

b. Department of Chemical Engineering, Stanford University, Shriram Center, 443

Via Ortega, Stanford, CA 94305, USA

*Corresponding author.

E-mail: zhwchen@uwaterloo.ca

1

2

Graphitic meso-porouscarbon sphere (MCS)Aerosol assisted spray drying technique

Fig. S1. An aerosol-assisted process used to synthesize meso-porous graphitized

carbon spheres. [1, 2]

3

-0.4 -0.3 -0.2 -0.1 0.0

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6C

urre

nt (m

A)

Potential (V vs. Ag/Agcl)

0.5 M H2SO4 Hydrogen oxidation

Hydrogen evolution

-0.216

Fig. S2. The calibration of Ag/AgCl (filled with 3M KCl) and the conversion

to RHE in hydrogen-saturated 0.5 M sulfuric acid electrolyte.

The conversion from Ag/AgCl to RHE scale was done in a typical three-electrode

system with polished platinum wires as the counter and working electrodes, and the

Ag/AgCl as the reference. The electrolyte (0.5 M H2SO4) was filled with pure

hydrogen. Linear scanning voltammetry was afterward performed at a scan rate of 0.1

mV s-1, and the potential at which the current is zero (-0.216 V vs. Ag/AgCl) was

considered to be the thermodynamic potential. Therefore:

ERHE=EAg /AgCl+0.216V

4

(a) (b)

Fig. S3. HR-TEM images of as-synthesized partially graphitized MCS. The graphitic

carbon exists not only on the surface of the carbon spheres, but also within the whole

spheres as the pore walls.

5

Fig. S4. TEM images of PANI-Fe-MCS. The green arrows designate the layered

graphene-like sheets, the yellow arrows designate the meso-porous carbon spheres.

6

Fig. S5. TEM images of PANI-Fe.

7

Fig. S6. TEM image of PANI-Fe-NCS. The green arrow designates the layered

graphene-like sheets, the yellow arrow designates the NCS.

8

(a) (b)

(c)

0.0 0.2 0.4 0.6 0.8 1.00

1000

2000

3000

Volim

e (S

TP) (

cm3 g

1)

P / Po

MCS0

50

100

150

NCS

1 10 100

NCS

dV/ d

log

(D) (

cm3 g

1 n

m1)

Pore size (nm)

MCS

0

500

1000

1500

11891240

51230

surfa

ce a

rea

(m2 g

1)

BET surface area Microporous surface area External surface area

NCS MCS

14189

Fig. S7. a) Nitrogen adsorption–desorption isotherms, b) the corresponding BJH

model pore size distribution, c) surface areas of NCS and MCS.

9

1200 1000 800 600 400 200 0

Inte

nsity

u (a

.u.)

Binding Energy (eV)

MCS PANI-Fe-MCS

S 2p

C 1s

N 1sO 1s

Fig. S8. XPS survey of MCS and PANI-Fe-MCS

10

0.2 0.4 0.6 0.8 1.0-5

-4

-3

-2

-1

0

Curre

nt d

ensi

ty (m

A cm

2)

Potential (V) vs. RHE

Pt/C

Fig. S9. Steady-state ORR polarization plots of commercial Pt/C catalysts (TKK,

28.2% Pt); RDE test conditions: 0.1 M HCLO4 saturated with O2, 900 RPM, 10 mV

s−1, catalyst loading 20 μgptcm−2.

11

0.97 0.98 0.99 1.00 1.01 1.02 1.03-0.4

-0.2

0.0

0.2

0.4

Curr

ent d

endi

ty (m

A cm

2)

Potential (V) vs. RHE

1 mV s1

5 mV s1

PANI-Fe-MCS

0.97 0.98 0.99 1.00 1.01 1.02 1.03

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3 PANI-Fe-NCS

Curr

ent d

endi

ty (m

A cm

2)

Potential (V) vs. RHE

1 mV s1

5 mV s1

0.97 0.98 0.99 1.00 1.01 1.02 1.03

-0.2

-0.1

0.0

0.1

0.2

Curre

nt d

endi

ty (m

A cm

2)

Potential (V) vs. RHE

1 mV s1

5 mV s1

PANI-Fe

1 2 3 4 5

0.1

0.2

0.3

Cdl = 69.14 mF cm2

R2 = 0.999

Cdl = 33.14 mF cm2

R2 = 0.999Curre

nt d

ensi

ty m

A cm

2

Scan rate mV s2

PANI-Fe PANI-Fe-NCS PANI-Fe-MCS

Cdl = 49.12 mF cm2

R2 = 0.999

(a) (b)

(c) (d)

Figure S10. The electrochemical double layer capacitance (Cdl) estimation. Cyclic

voltammetry (CV) data of (a) PANI-Fe, (b) PANI-Fe-NCS, and (c) PANI-Fe-MCS at

various scan rates (1 – 5 mV s-1). (d) Plots of current densities (taken at 1.00 V vs.

RHE) as a function of scan rates. All the CV curves were obtained in nitrogen-

saturated 0.5 M H2SO4.

12

ECSA can be properly estimated from the electrochemical double-layer capacitance

of the catalytic surface. [3, 4] To measure double-layer charging via CV, a potential

range in which no apparent Faradaic processes occur was determined from static CV.

Here, the potential range of 0.975-1.025 V was selected. All measured current in this

non-Faradaic potential region is assumed to be due to double-layer charging. The

double layer capacitance Cdl (F g−1) can be related to the current Eq (1)

cdl=i

v ×m

where i is the current, m is the electrode mass and v is the scan rate. The ECSA can be

estimated as the specific value from the Cdl by using Eq (2)[5]

ECSA=cdlcgc

where Cgc is the double layer capacitance (F m−2) of the glassy carbon electrode

surface, for which the typical value of 0.2 F m−2 was used. The improved ECSA of

PANI-Fe-MCS (576.16 m−2 g−1) compared to PANI-Fe-NCS (409.33 m−2 g−1) and the

PANI-Fe (276.16 m−2 g−1) suggests a higher ORR activity of PANI-Fe-MCS.

The estimated kinetic current at 0.85V in Fig. 4a are ca.0.42 mA cm−2 (PANI-Fe),

13

0.48 mA cm−2 (PANI-Fe-NCS), 0.54 mA cm−2 (PANI-Fe-MCS), respectively. Then,

the kinetic current is normalized by the EASA, and the specific activity at 0.85 V of

all three catalysts are about 0.25 μA cm−2 (PANI-Fe), 0.19 μA cm−2 (PANI-Fe-NCS)

and 0.16 μA cm−2 (PANI-Fe-MCS), respectively. These results are in line with

PEMFC performances in Fig. S11, where the ORR kinetic overpotentials are very

close in all three catalysts, while PANI-Fe has the lowest kinetic overpotential (ORR).

14

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Erev=1.2 V

ORR = 415 mV

iR-free and tx-free Ecell iR-free Ecell measured Ecell

PANI-Fe

E ce

ll

Current density (A cm2)

785 mV

437 mV

288 mV

tx= 348 mV

Eohmic= 149 mV

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

ORR

=437mV

E ce

ll

Current density (A cm2)

iR-free and tx-free Ecell iR-free Ecell measured Ecell

PANI-Fe-MCS

tx=49mV

Eohmic=101mV

763 mV

714 mV613 mV

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

ORR

=425mV

PANI-Fe-NCS

E ce

ll

Current density (A cm2)

iR-free and tx-free Ecell iR-free Ecell measured Ecell

775 mV

697 mV

590 mV

tx=78 mV

Eohmic=107mV

0.01 0.1 10.6

0.7

0.8

0.9

1.0

iR c

orre

cted

vol

tage

(V)

Log Current density (A cm2)

PANI-Fe-MCS

69 mV/decade

0.01 0.1 10.6

0.7

0.8

0.9

1.0

68 mV/decade

PANI-Fe-NCS

iR-fr

ee v

olta

ge (V

)

Log Current density (A cm2)

(a) (c) (e)

(b) (f)(d)

0.01 0.1 10.6

0.7

0.8

0.9

1.0

65 mV/decade

PANI-Fe

iR-fr

ee v

olta

ge (V

)

Log Current density (A cm2)

Fig. S11. a), c) and d) Tafel plots of the PEMFC iR-free polarization curves. b), d)

and f), Square symbols: measured performance of single-cells. Triangular symbols:

iR-corrected voltage by Fuel Cell instrument. Circular symbols: mass-transport-free

(tx-free) and ohmically corrected (iR-free) Ecell/Current density curve of measured

polarization plots.

Fig. S11 was prepared to quantify the voltage loss contributions from: (i) the sluggish

O2 reduction kinetics, ORR, (ii) the ohmic losses, Eohmic and (iii) the mass-transport

15

losses, tx.[6, 7] Therefore, the cell voltage, Ecell, of a H2-O2 fuel cell can be described

as:

Eq.3

Ecell=Erev (P H 2 , PO2,T )−∆ Eohmic−ORR−tx

Eq.4

Erev (PH 2 , PO2 ,T )=EH 2 /O2o −E

H 2 /H +¿o+ RT2 F

ln( PH 2PO2

12

PH2O)¿

The first term on the right-hand side of Eq. (3) is the reversible H2-O2 cell voltage,

which depends on the partial pressures of the reactants and the cell temperature,

equating to 1.2 V under the operating conditions.

The ohmic voltage loss, Eohmic, can be measured directly via high-frequency

resistance (HFR) measurements. The HFR values obtained by the Fuel cell instructor

are ca.0.16, ca. 0.12 and ca. 0.11 Ω cm2 for PANI-Fe, PANI-Fe-NCS, and PANI-Fe-

MCS, respectively. Then the iR-free cell voltage can be determined. The iR-free cell

voltage of the MEAs at low current densities (< 0.1 A cm−2) is controlled solely by

the voltage loss due to the O2 reduction kinetics, and then Tafel-slope can be

16

determined in this current region (Fig S11 a, c, e). The resulted Tafel-slope values are

65 mV/decade, 68 mV/decade and 69 mV/decade for PANI-Fe, PANI-Fe-NCS and

PANI-Fe-MCS, respectively. Therefore, a conceptual polarization curve which would

be obtained in the absence of mass transport and ohmic resistances (tx = Eohmic = 0)

can be constructed by extrapolating the iR-free cell voltage obtained at low current

densities (< 0.1 A cm−2), resulting in a loss of 65 mV (68mV and 69 mV) for every

10-fold increase in current density. According to this Eq.3, after deducting kinetic and

ohmic voltage losses, the calculated tx of PANI-Fe, PANI-Fe-NCS and PANI-Fe-

MCS were 348, 78 and 49 mV at 1 A cm−2, respectively. This result indicates the

PANI-Fe-MCS catalyst has less mass transfer resistance than that of PANI-Fe, PANI-

Fe-NCS.

17

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

Current density (A cm2)

Cell

volta

ge (V

)

0.0

0.4

0.8

1.2

1.6 Pt/C

Pow

er d

ensi

ty (W

cm

2)

H2-O2

Fig. S12. Polarization and power density as the functions of the current density plots for

H2−O2 and PEMFC with Pt/C as both the cathode and anode catalysts. Nafion 211 was

used as the membrane. The gas humidifier and cell temperature were maintained at 80

°C. The partial pressures of H2 and O2 were both kept at 20 psig, and the gas flow rate

for H2 and O2 (air) is 300 and 400 sccm respectively.

18

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.2

0.4

0.6

0.8

1.0

Current density (A cm2)

Cell

volta

ge (V

) Pt/C 0.0

0.2

0.4

0.6

H2-air

Power density (W

cm2)

(a) (b)

0.0 0.5 1.0 1.5

0.2

0.4

0.6

0.8

1.0

Current density (mA cm2)

Cell

volta

ge (V

)

0.0

0.1

0.2

0.3

0.4

H2air

PANI-Fe PANI-Fe-NCS PANI-Fe-MCS

Pow

er d

ensi

ty (m

W c

m2)

Fig. S13. Polarization and power density as the functions of the current density plots for

H2−air PEMFCs with a) prepared catalysts and b) commercial Pt/C as both the cathode

and anode catalysts. Nafion 211 was used as the membrane. The gas humidifier and cell

temperature were maintained at 80 °C. The partial pressures of H2 and air were both kept

at 20 psig, and the gas flow rate for H2 and air is 300 and 400 sccm respectively.

As shown in Fig. S13a, the polarization plots of these prepared catalysts in H2−air

conditions follow similar trends to observations in H2–O2 fuel cells. The PANI-Fe-NCS

exhibits a Pmax value of 0.33 W cm−2, which exceeds the values measured with PANI-Fe-

19

NCS (0.28 W cm−2) and PANI-Fe (0.11 W cm−2) and reaches ≈ 55 % of the Pmax (0.60 W

cm−2) of Pt/C cathode under the identical operating conditions.

20

Rohm

Rct, cathode

CPE

Fig. S14. Equivalent circuit models used to fit the impedance spectra for PEM fuel

cell through Gamry software.

To interpret these spectra the assumption was made that the anode impedances are

negligible, so the charge transfer resistance (Rct) is only from cathode. A constant

phase element (CPE) is used to represent the double layer capacitance of cathode.[8]

Because these MEAs were fabricated by the same technique with the same

membrane, and the cell components were kept under the same conditions for all tests,

so the variation of the resistances could only from the differences of catalysts powder.

21

0 10 20 30 40 500.0

0.1

0.2

0.3

0.4

0.5Cu

rren

t den

sity

(A c

m2

)

Time (hours)

H2air

PANI-Fe-MCS

Fig. S15. Stability test of a PANI-Fe-MCS catalyst at a constant fuel cell voltage of

0.40 V in H2-air. Membrane: Nafion 115; cathode catalyst loading: 4 mg cm−2.

22

Table S1. Elemental composition of the samples obtained from XPS results.

Samples C (at.%) N (at.%) O (at.%) S (at.%) Fe (at.%)

NCS 90.07 --- 9.93 --- ---

MCS 90.25 --- 9.75 --- ---

PANI-Fe 88.34 3.95 6.32 0.96 0.43

PANI-Fe-NCS 87.12 4.16 7.28 1.11 0.33

PANI-Fe-MCS 86.48 4.53 7.43 1.17 0.39

Table S2. Atomic ratios of heterocyclic N components of PANI-Fe-MCS in the N 1s

binding energy region.

Samples Pyridinic N Fe-N Pyrrolic N Graphitic N Oxidized N

23

398.2 eV 399.0 eV 400.1 eV 401.1 eV 403 eV

PANI-Fe-MCS 21.40% 10.60% 11.00% 46.70% 0.34%

24

Table S3. Comparison of fuel cell performance of PANI-Fe-MCS materials

with published state-of-the-art M-N-C catalysts

CatalystT cell

(°C)

H2/O2

flow rate

(sccm)

Back

pressur

e

(gauge)

(psig)

MEA

area

(cm−2)

Pmax in H2−O2

fuel cell

(W cm−2)

Referenc

e

PANI-Fe-MCS 80 300/400 20 5 0.83This

work

F/N/C-SCN 80 300/300 15 1 0.94 [9]

Fe/Phen/Z8 80 300/300 15 5 0.91 [10]

Fe/N/CF 80 300/400 22 5 0.90 [11]

PFeTPP-1000 80 300/300 15 5 0.73 [12]

Zn (mIm)2TPIP 80 400/400 15 5 0.62 [13]

Fe/PI-1000-III-

NH3

80 300/300 29 1 0.60 [14]

FeCBDZ 80 100/100 25 5 0.70 [15]

PANI-FeCo-C 80 200/600 26 5 0.55 [16]

NMCC-C-SiO2 75 300/400 30 5 0.45 [17]

25

Table S4. Electrochemical characteristics of the cells prepared with different

catalysts.

Samples PANI-Fe

Ω cm2

PANI-Fe-NCS

Ω cm2

PANI-Fe-MCS

Ω cm2

@ 0.60 V Rohm 0.16 0.12 0.11

Rct 0.92 0.22 0.14

26

Table S5. Comparison of fuel cell durability of PANI-Fe-MCS materials with

published state-of-the-art M-N-C catalysts

Catalyst

Oxida

nt

Gas

flow

rate

(sccm)

Back

pressu

re

(gauge

)

(psig)

Test

time

(h)

Test

voltage

(V)

Initial

current

loss

(%)

Referen

ce

PANI-Fe-MCSH2/air

H2/O2

300/40

0

300/40

0

20

20

50

50

0.4

0.4 ca. 5 %

ca.23 %

This

work

F/N/C-SCN H2/O2

300/30

015 100 0.5 ca. 37% [9]

PANI-FeCo-C H2/air200/60

033 700

0.4ca. 3% [16]

Fe/Phen/Z8H2/air

H2/O2

300/30

0

300/30

0

29

29

100

100

0.5

0.5 ca. 15 %

ca.40 %[10]

Fe/N/CF H2/air

H2/O2

300/40

0

300/40

29

29

100

100

0.5

0.5

ca. 35 %

ca. 40 %

[11]

27

0

Zn

(mIm)2TPIPH2/O2

400/40

015 100

0.5ca. 75% [18]

NMCC-C-SiO2 H2/air300/40

030 100

0.4ca. 21% [17]

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