Doping Effect of Boron and Phosphorus on Nitrogen

advertisement
Supplementary Material
Doping Effect of Boron and Phosphorus on Nitrogen-based Mesoporous
Carbons as Electrocatalysts for Oxygen Reduction Reaction in Acid Media
Ulziidelger Byambasuren1, Dorjgotov Altansukh1, Yukwon Jeon1,
and Yong-Gun Shul1,*
1
Department of Chemical Engineering, Yonsei University, Yonsei-ro 50, Seodaemun-gu,
Seoul 120-749, Republic of Korea
*Corresponding
author
Tel: +82 2 2123 2758. Fax: +82 2 312 6507
E-mail: [email protected] (Prof. Yong-gun Shul)
Page S1
The specific surface areas of the synthesized carbon (NC and NBC) catalysts were
determined by a multipoint Brunauer Emmett Teller analysis of the nitrogen
adsorption/desorption isotherms recorded on the BELSORP (Bel, mini II) surface area
analyzer. The pore size distribution was calculated using the Barrett Joyner Halenda (BJH)
method for mesoporous carbons. N2 adsorption-desorption isotherms are measured to
evaluate the textural properties of NC and NBC. The main characteristic parameters are
shown in Fig. S1 and Table 1. Table S1 shows the specific surface area, pore volume, average
pore diameter and boron contents of NC and NBC N2 adsorption-desorption measurements. It
can be seen that BET surface area of NBC have increased than NC. The average pore size of
NBC is slight larger than that of NC. NBC has an obvious increase in pore diameter (5.36 nm)
as compared to NC (5.28 nm) from Table S1, which may be attributed to the restraint of
excess B to the contraction of the carbon skeleton during carbonization process. For Fig. S1
NC and NBC samples show typical type IV isotherms, with sharp capillary condensation
steps at higher relative pressure, which is typical character of mesoporous structure. The
distinct capillary condensation steps are occurring at relative pressures of 0.4–0.9, indicating
that narrow pore size distribution with uniform mesoporous. From Fig. S1 it is found that the
pore size distribution becomes wider to NBC. There is distinctly increased sorption in the
isotherm curve of NBC at low relative pressure.
Page S2
Figure S1. a) N2 adsorption-desorption isotherms b) pore size distributions of NC and
NBC catalysts.
Table S1. Structure parameters of NC and NBC
Sample
NC
NBC
SBET (m2/g)
468.91
552.85
Vtotal (cm3/g)
0.6201
0.7411
Vmes (cm3/g)
0.5948
0.6578
DP average (nm)
5.28
5.36
SBET -surface area, Vtotal -total pore volume, measured at P/P0=0.9, Vmes -mesopore volume,
obtained by BJH method; DP– average pore diameter, calculated by 4V/A from BET
Page S3
To demonstrate the dependence on catalyst loading of Fe content on the ORR activity, NC-X%
Fe and NBC-X% Fe catalysts with five different Fe contents were synthesized and the ORR
activities were compared. Fig. S2 shows the polarization curves for oxygen reduction on NCX% Fe and NBC-X% Fe catalysts in O2-saturated 0.5 M H2SO4 at room temperature. The
potential scan rate was 5 mVs-1 and the rotation rate was 1600 rpm. There is an optimum Fe
content in these non-precious metal catalysts, similarly to previous reports by others [4, 33].
The inset of Fig. S2 shows the polarization curves of the oxygen-reduction currents on NC-X%
Fe and NBC-X% Fe catalysts at low Fe contents (1.5 wt.% Fe) the ORR activity increases
with increasing Fe content. When the Fe content increases to 3 wt.%, the obtained catalysts
show a lower ORR activity than NC-1.5% Fe and NBC-1.5% Fe. The optimized NC-1.5% Fe
and NBC-1.5% Fe catalysts show an onset potentials of both of about 0.83 V with a limited
diffusion currents of about 4.35 and 4.94, mA/cm2. In subsequent studies, the1.5 wt.% Fe
content catalysts were selected as the target catalysts.
Figure S2. Polarization curves for oxygen reduction on a) NC-X% Fe catalysts b) NBCX% Fe catalysts.
Page S4
Figure S3. Fuel cell performance test results, (a) cyclic voltammetry (CV) curves (b)
electrochemical impedance spectroscopy of NC 1.5% Fe and NBC 1.5% Fe catalysts.
From the ORR test, current densities of NC 1.5% Fe, NBC 1.5% Fe catalyst and Pt/C 40%
were 2.39 mA/cm2, 3.62 and 4.7 mA/cm2 at the 0.6 V respectively. The kinetic currents
calculated below process. The kinetic current densities of NC 1.5% Fe, NBC 1.5% Fe catalyst
and Pt/C 40% were 4.19 mA/cm2, 10.37 and 30.38 mA/cm2 at the 0.6 V respectively.
Page S5
Figure S4. Koutecky Levich plots of NC 1.5% Fe and NBC 1.5% Fe catalysts.
The kinetic current determined the below equation.
π‘–π‘˜ =
π‘–π‘š ∗ 𝑖𝑑
𝑖𝑑 − π‘–π‘š
If the id as below
1
𝑖𝑑 = π΅πœ”1/2 = 13.9 ∗ 10−2 ∗ 16002 = 5.56 π‘šπ΄/π‘π‘š2
Kinetic current was in the case of NC 1.5% Fe
π‘–π‘˜ =
5.56 ∗ 2.39
= 4.19 π‘šπ΄/π‘π‘š2
5.56 − 2.39
Page S6
In the case of NBC 1.5% Fe
π‘–π‘˜ =
5.56 ∗ 3.62
= 10.37 π‘šπ΄/π‘π‘š2
5.56 − 3.62
π‘–π‘˜ =
5.56 ∗ 4.7
= 30.38 π‘šπ΄/π‘π‘š2
5.56 − 4.7
In the case of Pt/C 40%
where im is measured current density, ik, kinetic current density, id, diffusion limited current
density,
Table S1. Kinetic currents densities of NC and NBC
Sample name
im, measured
current
density,
mA/cm2
ik, kinetic current
density, mA/cm2
NC 1.5% Fe
2.39
4.19
NBC 1.5% Fe
3.62
10.37
Pt/C 40%
4.7
30.38
The working electrode diameter was 3 mm, and the suspension ink amount was 4 µl that was
dropped on the working electrode. The suspension ink concentration was 10 mg catalyst in
the 1.08 ml solution. And from the N2 adsorption-desorption isotherms, surface areas (SBET)
of NC 1.5% Fe and NBC 1.5% Fe catalyst were 468.91 m2/g and 552.85 m2/g. The specific
activities were calculated as below.
Page S7
πΏπ‘œπ‘Žπ‘‘π‘–π‘›π‘” π‘Žπ‘šπ‘œπ‘’π‘›π‘‘ =
10 π‘šπ‘” ∗ 0.004 π‘šπ‘™
= 0.037 π‘šπ‘”
1.08 π‘šπ‘™
πΈπ‘™π‘’π‘π‘‘π‘Ÿπ‘œπ‘‘π‘’ π‘”π‘’π‘œπ‘šπ‘’π‘‘π‘’π‘Ÿ π‘Žπ‘Ÿπ‘’ = πœ‹π‘Ÿ 2 = 3.14 ∗ 1.5 π‘šπ‘š2 = 0.07068 π‘π‘š2
πΏπ‘œπ‘Žπ‘‘π‘–π‘›π‘” π‘Žπ‘šπ‘œπ‘’π‘›π‘‘ π‘Žπ‘‘ π‘‘β„Žπ‘’ π‘’π‘™π‘’π‘π‘‘π‘Ÿπ‘œπ‘‘π‘’ =
πΏπ‘œπ‘Žπ‘‘π‘–π‘›π‘” π‘Žπ‘šπ‘œπ‘’π‘›π‘‘
0.037 π‘šπ‘”
=
πΈπ‘™π‘’π‘π‘‘π‘Ÿπ‘œπ‘‘π‘’ π‘”π‘’π‘œπ‘šπ‘’π‘‘π‘’π‘Ÿ π‘Žπ‘Ÿπ‘’
0.07068 π‘π‘š2
= 0.52 π‘šπ‘”/π‘π‘š2
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 π‘Žπ‘π‘‘π‘–π‘£π‘–π‘‘π‘¦ =
πΆπ‘’π‘Ÿπ‘Ÿπ‘’π‘›π‘‘ 𝑑𝑒𝑛𝑠𝑖𝑑𝑦
πΏπ‘œπ‘Žπ‘‘π‘–π‘›π‘” π‘Žπ‘šπ‘œπ‘’π‘›π‘‘ π‘Žπ‘‘ π‘‘β„Žπ‘’ π‘’π‘™π‘’π‘π‘‘π‘Ÿπ‘œπ‘‘π‘’ ∗ π‘†π‘’π‘Ÿπ‘“π‘Žπ‘π‘’ π‘Žπ‘Ÿπ‘’π‘Ž
In the case of NC 1.5% Fe
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 π‘Žπ‘π‘‘π‘–π‘£π‘–π‘‘π‘¦ =
4.19 π‘šπ΄/π‘π‘š2
0.52π‘šπ‘”
π‘š2
∗ 10−3 ∗ 468.91 𝑔 ∗ 104
2
π‘π‘š
= 1.7 ∗ 10−3 π‘šπ΄/π‘π‘š2
In the case of NBC 1.5% Fe
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 π‘Žπ‘π‘‘π‘–π‘£π‘–π‘‘π‘¦ =
10.37 π‘šπ΄/π‘π‘š2
0.52π‘šπ‘”
π‘š2
∗ 10−3 ∗ 552.85 𝑔 ∗ 104
2
π‘π‘š
= 3.6 ∗ 10−3 π‘šπ΄/π‘π‘š2
In the case of Pt/C 40%
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 π‘Žπ‘π‘‘π‘–π‘£π‘–π‘‘π‘¦ =
30.38 π‘šπ΄/π‘π‘š2
= 9.7 ∗ 10−2 π‘šπ΄/π‘π‘š2
2
0.52π‘šπ‘”
π‘š
∗ 10−3 ∗ 60 𝑔 104
π‘π‘š2
In the case of only Pt
Page S8
9.7 ∗
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 π‘Žπ‘π‘‘π‘–π‘£π‘–π‘‘π‘¦ =
10−2 π‘šπ΄
∗ 40
π‘π‘š2
= 3.88 ∗ 10−2 π‘šπ΄/π‘π‘š2
100
A direct comparison of just Pt is not feasible as Pt/C was utilized for the baseline comparison.
While the surface area of Pt loaded on the carbon is calculable as a weight percent, the
specific surface area of individual Pt is unknown. Therefore, it is difficult to determine the
specific activity of Pt only. As such, a Pt/C comparison against the newly prepared catalysts
was added as a third line in Table 3. A comparison with only Pt calculated from the weight
percent of Pt/C is shown at the end of the calculations
Page S9
Download