Supporting Information belonging to the paper:
Biologically Inspired Oxygen Reduction Reaction Iron Phthalocyanine Catalysts for PEMFCs Cathode Wenmu Li†, Aiping Yu†, Drew C Higgins†, Bernard G Llanos†, Zhongwei Chen*,† †
Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo,200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1. RECEIVED DATE (automatically inserted by publisher); E-mail Address:
[email protected] - SUPPORTING INFORMATION Reference: 16
a) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K. I.; Iwashita, N. Chem. Rev. 2007, 107, 3904.
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1.0 Experimental procedures 1.1 Materials 2,6-diphenylthiophenol, 4-nitrophthalonitrile, anhydrous ferrous acetate, Cs2CO3, KCN and pyridine were purchased from Sigma-Aldrich Ltd. Commercial grade 1,2dichloroethane, quinoline, toluene, NMP,
and pyridine were dried overnight over
calcium hydride and distilled prior to use. Other commercially available materials and solvents were used without further purification. 1.2 Synthesis of Ferrous 2, 9,16,23-tetranitro phthalocyanine 1.73 g (10 mmol) of 4-nitrophthalonitrile and 0.43 g (2.5 mmol) of Fe(OAc)2 were dissolved in 100 ml of quinoline in a three necked, round bottom flask. The mixture was stirred at 210 °C for 24 h under a N2 atmosphere. After cooling, 200 mL of MeOH was added, followed by filtration to collect solid precipitate. The crude product was purified by column chromatography on silica gel with pyridine as the eluent to obtain a green powder. The resulting yield was 0.75 g (40%). FT-IR(KBr): 1372.1(isoindole C–N stretch vibration), 1148.0(pyrrole C=C bending), 1082.9(Pc C–H stretch vibration), 940.8(Fe–Pc), 741.8(ring breathing, C–H wagging) cm-1. 1.3
Synthesis
of
Ferrous
2,9,16,23-tetra(2’,6’-diphenylphenthio
ether)
phthalocyanine (Fe-SPc) 2, 6-diphenylthiophenol (0.72 g, 2.75 mmol), Cs2CO3 (1.00 g, 3.0 mmol), NMP(10 mL), and toluene (10 mL) were charged into a 50 mL three-necked roundbottom flask equipped with a magnetic stirrer, an N2 inlet, and a Dean-Stark trap with a condenser. The mixture was heated and stirred at 145 ºC for 1 h under N2 atmosphere to remove any water. Heating was continued until all of the toluene was removed, then the
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reaction mixture was cooled and Ferrous 2,9,16,23-tetranitro phthalocyanine (0.50 g, 0.68 mmol) was added to the same flask under nitrogen protection. This mixture was heated and stirred at 180 ºC for 12 h. After cooling, the mixture was poured into methanol to precipitate a green power. The crude product was purified by column chromatography on silica gel with dichloromethane as the eluent to obtain the resulting green solid. Yield was 1.29 g (60%). FT-IR: 1394.3(isoindole C–N stretch vibration), 1141.4(pyrrole C=C bending), 1072.2(Pc C–H stretch vibration), 931.5(Fe–Pc) and 753.5(ring breathing, C–H wagging) cm-1. 1H NMR (CD2Cl2, 300 MHz) 8.72-8.76 (d, 1H), 8.45 (s, 1H), 7.51-7.56 (m, 7H), 7.19-7.26 (m, 7H). HRMS (ESI) m/z calcd for C104H64FeN8S4 [M]+: 1609.36. Found: 1609.45. [M]2+ m/2z calcd for: 804.68. Found: 804.71. 1.3 Material Characterization Hydrogen nuclear magnetic resonance (1H NMR) spectra for the Fe-SPc compounds were obtained in CD2Cl2 at 300 MHz. The Fourier transfer infrared (FT-IR) spectroscopy was obtained on a Bruker FT-IR spectrometer. High resolution electrospray ionization mass spectroscopy (ESI-MS) was run on a Kratos MA890. 1.4 Catalyst Sample Preparation 20 mg of Fe-SPc was dissolved in 5 mL of 1,2-dichloroethane at the same time that 20 mg of KJ300 was sonicated for 1 h in 5 mL of 1,2-dichloroethane. The carbon suspensions and Fe-SPc solution were then combined and the resulting mixture was sonicated for another 30 min. The sample was then refluxed for 2 h, after which the solvent was subsequently distilled off and the Fe-SPc/KJ300 was collected from the flask. 1.5 Electrode Preparation
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The glassy carbon working electrode (total area 0.196 cm2) was carefully polished with gamma alumina powder (0.05 micron, CH Instruments) until a mirror finish was obtained. Then the electrode was rinsed and sonicated with plenty of double distilled water to remove any alumina residues, followed by drying in a vacuum. A sample of FeSPc/KJ300 (2 mg) was dispersed in 1 mL solvent mixture of Nafion (5%) and EtOH (V: V ratio = 1:9) by sonication. For oxygen reduction reaction (ORR) readings and H2O2 detection, 60 μL of the 2 mg/mL Fe-SPc/KJ300 solution was pipetted onto the glassy carbon electrode surface and dried in air. The catalyst loaded glassy carbon disk was used as the working electrode, coupled with a platinum ring electrode for rotating ring disc electrode (RRDE) voltammetry. 1.7 RRDE voltammetry of Fe-SPc/KJ300 for ORR under steady-state conditions Electrochemical testing was carried out in a glass cell containing 0.1 M HClO4, 0.1M NaOH or 0.1M NaOH + 0.01M KCN electrolyte. A double junction Ag/AgCl reference electrode was utilized along with a platinum wire counter electrode ORR activity was investigated by sweeping the potential from −0.25 to 0.95 V vs. Ag/AgCl in the oxygen saturated electrolyte, at a sweep rate of 10 mV/s, while the ring potential was held constant at 1.0 V vs Ag/AgCl. RRDE voltammetry was performed at various rotation speeds with background correction currents obtained under nitrogen saturated conditions. 2.0 Supplementary results and discussion The method designed and carried out to synthesize ferrous 2,9,16,23-tetra (2’,6’diphenylphenthio ether) phthalocyanine (Fe-SPc) is summarized in Scheme S1. The sterically hindered Fe-SPc was synthesized by the direct reaction of iron
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tetranitrophthalocyanine with 2,6-diphenylthiophenol using potassium carbonate as a catalyst at 180 ºC. The NMR spectrum and FT-IR spectra of Fe-SPc is provided in Figure S1 and Figure S2. The characteristic absorptions at 1394.3 (isoindole C–N stretch vibration), 1141.4(pyrrole C=C bending), 1072.2(Pc C–H stretch vibration), 931.5(Fe–Pc) and 753.5(ring breathing, C–H wagging) cm-1 confirmed the formation of Fe-SPc. The presence of Fe(III)-SPc -oxo dimmers and Fe(III)-SPc-O2- Fe(III)-SPc (Figure S3) was confirmed by the appearance of a strong intensity band at 822.8 and 804.8 cm-1. This is consistent with a recent report by Ercolani et al, in which Fe-O-Fe and Fe-O2-Fe dioxo species demonstrate IR adsorptions in the 860-800cm-1 region 1. Introduction of electron-donating substituents surphur group to the Fe-Pc ring could results in a new redox couple observed at 0.81-1.01 V vs. SCE.2,3 This new peak was assigned to [FeIII-Pc(-1)]2+/[FeIII-Pc(-2)]+. This redox couple is not observed in unsubstituted FePc or FePc modified with electron-withdrawing substituents forming complexes such as FePc(Cl)16 since their [FeIII-Pc(-1)]2+/[FeIII-Pc(-2)]+ redox couple should be on much higher potentials (above ~1.2 V vs SCE), exceeding the range investigated in this work. In our case, the sulphur group was attached directly to the mterphenyl group which could stabilize [FeIII-Pc(-1)]2+ intermediates through a delocalization of the positive charge. One can then expect that there is a [FeIII-SPc(1)]2+/[FeIII-SPc(-2)]+ redox couple in our case as well. Indeed a peak at 0.81 V vs. RHE (0.60 V vs. SCE) was observed (Peak I in Figure S5). Another peak (Peak II in Figure S5) at 0.65 V vs. RHE (0.43 V vs. SCE) could be assigned to the [FeIII-SPc(-2)]+/[FeIISPc(-2)] redox couple. Therefore, there are two redox couples observed in our Fe-
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SPc/KJ300 based catalysts in the CV range of 0 to 1.2 V vs RHE; i) [FeIII-SPc(1)]2+/[Fe=-SPc(-2)]+ and ii) [FeIII-SPc(-2)]+/[FeII-SPc(-2)]. Conversely, there is only one redox couple [FeIII-Pc(-2)]+/[FeII-Pc(-2)] observed in commercially available FePc/KJ300 based catalysts. This can serve to explain the increase in magnitude of the CV curve obtained on Fe-SPc/KJ300 catalyst in comparison with commercial Fe-Pc/KJ300. The stability of Fe-SPc/KJ300 under nitrogen saturated electrolyte conditions is displayed in Figure S6, where after 1200 potential cycles, the ORR activity only displays a minimal loss, corresponding to an 8.3 % decrease in current density at an electrode potential of 0.5 V vs. RHE. Figure 65 also shows the typical CV curves of a FeSPc/KJ300 and Fe-Pc/KJ300 in N2-saturated 0.1M HClO4 solution at a scan rate of 10 mV/s. From these CV curves, Fe-SPc/KJ300 based catalysts display an increase in the magnitude of their CV curves after 1200 cycles in N2 saturated 0.1M HClO4 solution and 100 cycles in O2 saturated 0.1M HClO4 solution. On the other hand, Fe-Pc/KJ300 based catalysts show a significant decrease in the FeIII/FeII redox couples. Such evidence indicates that Fe-Pc was almost completely deactivated after 100 cyles in O2 saturated 0.1M HClO4 solution. Comparative ORR polarization curves are also provided in Figure S8 for FeSPc/KJ300 and commercial Fe-Pc/KJ300 in order to illustrate the differences in initial ORR performance of each material. Fe-Pc/KJ300 displays slightly higher activity, with a 20 mV increase in half-wave potential coupled with a higher diffusion limited current density. The Koutecky-Levich equation can be utilized in order to separate the observed current densities into diffusion limited and kinetically limited contributions according to
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equation (1) 4 (1)
B√
Where j is the observed current density, jk is the kinetically limited current density, ω is the electrode rotation speed and B is the Levich slope given by equation (2). B
0.62nFCO DO V
(2)
Where n is the number of electrons transferred, F is the Faraday constant, CO2 is the bulk oxygen concentration, DO2 is the diffusion coefficient of oxygen and ν is the viscosity of the solution. ORR polarization data obtained for Fe-SPc/KJ300 (Figure S7A) at various rotation speeds were used to generate a plot of 1/i versus 1/ω0.5, also known as a Koutecky-Levich plot as displayed in Figure S78. These plots display linear, parallel behavior, indicating first order reaction kinetics with respect to molecular oxygen 2. The total number of electrons transferred in the ORR can be calculated according to equation (3) 5.
n
ID I
ID NR
(3)
Where n is the number of electrons transferred, ID is the disc current, IR is the ring current and N is the ring collection efficiency (0.26). This parameter provides indication of the selectivity of ORR catalysts towards the more efficient 4 electron reduction forming water over the 2 electron reduction forming peroxide byproducts. The number of electrons transferred per oxygen molecule is displayed in Figure S7C, where over the potential range investigated, n is very close to 4. This indicates that Fe-SPc/KJ300
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catalysts have a high selectivity towards the complete 4 electron ORR. Figure S7D displays the ORR activity of Fe-SPc/KJ300 and commercial Pt/C in oxygen saturated 0.1 M HClO4, either in the presence or absence of methanol. Clearly, Fe-SPc/KJ300 displays limited methanol oxidation activity, thus performance decrease utilizing Fe-SPc based cathode catalysts in direct methanol fuel cells would be minimal due to methanol crossover.
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Scheme S1. Synthesis method of Fe-SPc. Reagents and conditions: a) Fe(OAc)2, Quinoline, 210 °C. b) K2CO3, N-Methylpyrrolidone,
180 °C.
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O2
CN-
O2
FeII O2
O
FeII
FeIII O
O FeIII
H2O
CN
+e-
CN-
O (2)
FeIII
HO2-
O2Superoxide OH-
(3)
(4)
CN-
FeIII
(1)
CN-
O2-
FeII
CN-
+e
FeII
(5)
FeII
: FeII-Pc
Scheme II
Scheme S2. The possible mechanism of CN- ions inhibit Fe-Pc based catalysts catalyze ORR.
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Ph S Ph
Ph
Ph
N
N
S
N Fe N
N
N N S N
Ph
Ph
Ph S Ph
8.8
8.4
8.0
7.6
7.2
5.6
5.2
4.8
4.4
4.0
Figure S1. NMR spectrum of Fe-SPc in CCl2D2-d2.
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1500
1200
S12
900
Wavenumber / cm 600
-1
Figure S2. Transmission FT-IR data of Fe-SPc film.
931.5 892.5 822.8 804.8
753.5 695.1
1141.4 1107.2 1072.2 1038.9
1312.0 1259.6
1511.0 1443.4 1394.3
1601.4
Intensity
Figure S3. Chemical structures of Fe-SPc and its possible oxygen binding complexes in the high resolution ESI-MS spectra.
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A
B 20
0
Without 0.01M KCN With 0.01M KCN Current / A
Current / mA*cm
-2
16
-1 -
2e -2
Without 0.01M KCN With 0.01M KCN
12 8 4
-
-3 -1.0
4e -0.8
-0.6 -0.4 -0.2 Potential / V vs SCE
0 -1.0
0.0
-0.8
-0.6 -0.4 -0.2 0.0 Potential / V vs SCE
0.2
Figure S4. (a) Polarization curves for the ORR and (b) Ring current on Fe-SPc/KJ300 with and without 0.01M KCN in O2 saturated
0.1 M NaOH. The potential sweep rate was 10 mV/s and ORR readings were obtained at 400 rpm.
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400
10mV/s 25mV/s 50mV/s 100mV/s
Current /A
200
II
I
0.6
0.8
0
-200 -400
0.0
0.2
0.4
1.0
1.2
Potential / V vs. RHE Figure S5. CV on Fe-SPc/KJ300 in nitrogen saturated 0.1 M HClO4 at different scan rates.
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A
B
0
Initial After 1200 cycles in N2
-1
50 Current / A
Current /mA * cm
-2
100
-2 -3 Initial After 1200 cycles in N2
-4
0.2
0.4 0.6 0.8 Potential / V vs. RHE
0 -50 -100 -150 0.0
1.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs RHE
D
C
100 Initial After 1200 cycles in N2
0 -50
-100
Initial After 100 cycles in O2
50 Current / A
Current /A
50
100
0 -50
-100
0.0
0.2
0.4
0.6
0.8
1.0
-150 0.0
1.2
Potential / V vs. RHE
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs RHE
E Current / A
50
Initial After 100Cycles in acid
25 0 -25 -50 0.0
0.2
0.4 0.6 0.8 1.0 Potential / V vs RHE
1.2
Figure S6. (A) Polarization curves on Fe-SPc/KJ300 for the ORR before and after 1200 potential cycles in nitrogen saturated 0.1 M
HClO4.. (B) CV on Fe-SPc/KJ300 and (C) CV on Fe-Pc/KJ300 before and after 1200 potential cycles in nitrogen saturated 0.1 M HClO4. (D) CV on Fe-SPc/KJ300 and (E) Fe-Pc/KJ300 before and after 100 potential cycles in O2 saturated 0.1 M HClO4.The potential sweep rate was 10 mV/s and ORR readings were obtained at 400 rpm.
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Current /mA * cm
-2
0 -1
Fe-Pc/KJ300 Fe-SPc/KJ300
-2 -3 -4 0.2
0.4 0.6 0.8 Potential / V vs. RHE
1.0
Figure S7. Polarization curves for the ORR on Fe-SPc/KJ300 and Fe-Pc/KJ300 coated electrodes in oxygen saturated 0.1 M HClO4 at
a sweep rate of 10 mV/s and electrode rotation of 400rpm.
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A
B
0
1.5
-2 -3
1/j (cm /mA)
100rpm 400rpm 900rpm 1600rpm
-4 -5
2
Current /mA * cm
-2
-1
-6 0.2 C 4
0.4 0.6 0.8 Potential / V vs. RHE
1.2
0.53V 0.55V 0.57V 0.59V
0.9 0.6 0.3 0.02
1.0
0.04
D
0.06 0.08 -0.5 (rpm )
0.10
-0.5
-2
Current (mA*cm )
3
3 100rpm 400rpm 900rpm 1600rpm
nt
2 1
i ii iii iv
2 1 0 -1 -2 -3
0 0.1
0.2
0.3 0.4 0.5 0.6 Potential / V vs. RHE
0.7
-4 0.2
0.4 0.6 0.8 Voltage (V vs. RHE)
1.0
Figure S8. (a) Polarization curves for the O2 reduction reaction on Fe-SPc/KJ300 electrode in oxygen -saturated 0.1
M HClO4 electrolyte (m = 10 mV s-1). (b) Koutecky-Levich plots determined from (a). (c) Corresponding total numbers of electrons exchanged in the ORR as a function of potential. (d) Polarization curves for (i) Fe-SPc/KJ300 and (iii) Pt/C in oxygen saturdated 0.1 M HClO4, along with (ii) Fe-SPc/KJ300 and (iv) Pt/C in a 0.1M HClO4 and 1M methanol solution at 400rpm.
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Table S1. Performance of Fe-Pc/KJ300 and Fe-SPc/KJ300 before and after potential cycling.
Current at 0.5 V RHE (mA/cm2)
Half wave potential (mV)
Samples
Initial
10 cycles
100 cycles
Initial
10 cycles
100 cycles
Fe-Pc
631
408
351
3.40
0.67
0.37
Fe-SPc
611
608
591
3.18
3.05
2.72
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