Supporting Information
© Wiley-VCH 2007
69451 Weinheim, Germany
Methanol Behavior in Direct Methanol Fuel Cells
Younkee Paik, Seong-Soo Kim, and Oc Hee Han*
Experimental Section
Preparation of MEA: Standard MEA was prepared by hot pressing the sandwich of an anode, a Nafion 117 electrolyte membrane layer, and a cathode at 135 oC for 3 minutes under a pressure of 100 kg/cm2. Nafion 117 was used after being boiled for an hour each in the following solutions step by step: 3% H2O2, pure water, 0.5M H2SO4, and pure water again. Water was purified with a µ-Pure water system
(Pure Power, Korea; 12.5 MΩ) prior to use. The anode electrode was prepared by spraying a catalyst ink, consisting of PtRu/C catalyst (E-TEK, 1:1 a/o, 60 wt% on Vulcan XC-72) and Nafion solution (Aldrich, 5 wt%) dissolved in ethanol, onto carbon cloth (E-TEK, USA). The carbon cloth was prepared by spraying a homogeneous suspension of PTFE and acetylene black (4:1 w/w in EtOH), hot pressing at 10 kg/cm2 for 15 min, and heating at 375 oC for 30 min. The cathode electrode was made following the same procedure except that Pt/C (E-TEK, 60 wt% on Vulcan XC-72) was used instead of PtRu/C. Catalyst loadings, i.e., the amount of pure PtRu or Pt catalyst per unit area of the carbon cloth, were measured after drying the electrode at 70 oC. The loadings were adjusted to be 5.0 ± 0.2 mg/cm2 for an anode and 3.0 ± 0.2 mg/cm2
for an cathode by repeating carefully the spraying & drying cycles for 3-4 times. An MEA with a triple-layer PEM was prepared by hot pressing the sandwich of an anode, a triple layer of PEM, and a cathode using the same hot pressing condition as for the standard MEA.
Electrochemical tests of DMFCs: The experimental DMFC assembly consisted of an MEA in 4.5 cm × 1.8 cm and two rectangular blocks of graphite with serpentine flow fields machined on the inner surfaces to a depth and width of 1 mm. Each graphite block works as a current collector and electrically connected to an electronic analyzer, PRODIGIT 3351D (Prodigit Electronics, Taiwan), with a stainless steel screw holding one end of the cable onto the graphite block. The assembly was held together by two home-built stainless steel clamps. An aqueous solution of 2 M methanol was fed to the anode at a rate of 0.8 cm3/min using a peristaltic pump 323 S/D (Watson-Marlow, UK), and oxygen gas was fed to the cathode at a rate of 1000 cm3/min. Electronic measurements of the DMFC single cells were carried out with a DC electronic analyzer at a potential-controlled mode and ambient temperature. Data sets of the potential (E) and the current (j) were collected manually while the cell potential was lowered from its open-circuit voltage at an interval of 30 mV. Typically each cell was activated by being run on the electronic analyzer at a constant voltage (~350 mV) for 2-3 days, intermittently, before the experimental measurements.
Solid-state NMR analyses: From the DMFC operated with 2 M CD3OH (for 2D NMR) or 13CH3OH (for 13
C NMR) for 15-30 minutes, the middle layer of a triple-layer PEM was removed, freeze-milled into powders at liquid nitrogen temperature, and packed into a 4 mm rotor for MAS NMR experiments. The sample was exposed to air at room temperature during sample packing for less than 2 minutes. During
the exposure the evaporation of organic components or water in the PEM was confirmed to be less than 10%. All deuterated or 13C labelled compounds were purchased from Cambridge Isotope Laboratories, Inc.: methanol (D3, 99.8%), formaldehyde (D2, 98%; 20 wt% in D2O), formic acid (D2,
98%; ≤5% D2O), methanol (13C, 99%), formaldehyde (13C, 99%; 20 wt% in H2O), and formic acid(13C,
99%; ≤5% H2O). Reference solutions were prepared by dissolving 1g of each compound in 10 mL of
pure water. Pretreated and dried Nafion membranes were soaked in each of these reference solutions for 24 hours. The membranes were removed, after the water droplets on the surfaces were wiped out with Kimwipes, freeze-milled into powders at liquid nitrogen temperature, and packed into a 4 mm rotor for MAS NMR experiments. For comparisons, solution NMR spectra were also collected from each of the reference solutions. NMR experiments were carried out on a Bruker Avance II 400 MHz spectrometer using a double-resonance (1H-X) MAS probe equipped with a 4 mm rotor spinning module. Samples were spun at 5 kHz unless stated otherwise. The excitation pulse length for 90o flip was 3.0 µs for both 2D and 13C NMR and a pulse repetition delay of 3 sec was used. Proton decoupling was applied during 13C NMR experiments. No change in the sample mass occurred before and after the MAS NMR experiments.
Table S1. 13C peak assignment of MAS NMR spectra in Figure S1
Chemical shift of each functional group (in ppm) Spectral number Reference compound Major chemical species in aqueous
solution −CH3 H2C(OH)− HCOO−
Figure S1(a) methanol CH3OH 49 − −
Figure S1(b) formaldehyde CH2(OH)2 − 82 −
Figure S1(c) formic acid HCOOH − − 166
Figure S1(d) 1:1 mixture of methanol and formaldehyde CH3OH CH2(OH)2 CH2(OH)(OCH3) CH2(OCH3)2 49 − 55 55 − 82 90 97 − − − − Figure S1(e) 1:1 mixture of methanol and formic acid CH3OH HCOOH HCO(OCH3) 49 − 52 − − − − 166 163
Figure S1. 13C MAS NMR spectra of Nafion PEMs soaked in 2 M solutions of 13C labelled reference compounds indicated on the spectra: (a) methanol, (b) formaldehyde, (c) formic acid, (d) 1:1 mixture of methanol and formaldehyde, and (e) 1:1 mixture of methanol and formic acid. The chemical shifts of the
13
C signals are marked on the spectra and the peak assignments are summarized in Table S1. The PEMs were powdered by freeze-mill and packed into 4 mm rotors for MAS NMR experiments. The spectra were acquired at a sample spinning frequency of 5 kHz.
(e) 1:1 mixture of CH3OH and HCOOH
180 160 140 120 100 80 60 40 (d) 1:1 mixture of CH3OH and CH2O (c) HCOOH (b) CH2O (a) CH3OH 49 82 166 166 163 52 55 49 90 82 97 49
δ
/ ppmpolymer backbone 166 ppm 90 ppm 49 ppm 163 ppm 200 150 100 50 0 52 ppm 55 ppm ssb H OH O C H O O C CH3 OH H H OCH3 C CH3OH δ/ ppm polymer backbone 166 ppm 90 ppm 49 ppm 163 ppm 200 150 100 50 0 52 ppm 55 ppm ssb H OH O C H OH O C H O O C CH3 H O O C CHCH33 OH H H OCH3 C OH H H OCH3 C CH3OH δ/ ppm
Figure S2. The 13C MAS NMR spectrum of the middle layer PEM extracted from the MEA with a triple-layer PEM in a DMFC in operation with 2 M 13CH3OH. Pt/C catalysts were used for both the anode and
cathode. Peak assignments are indicated on the spectrum (see text for more description). Only the bottom 10% of the methanol peak at 49 ppm is shown for the appropriate display of all peaks. The sample in a 4 mm rotor was spun at 5 kHz and the spinning sideband (ssb) is marked on the spectrum.
Figure S3. 2D MAS NMR spectra of reference solutions absorbed in polymer electrolyte membranes (PEMs) (top) and of the solutions themselves (bottom). The reference solutions were prepared by dissolving 1g of each of the following reference compounds in 10 mL of pure water: (a) D2O, (b)
methanol (D3, 99.8%), (c) formaldehyde (D2, 98%; 20 wt% in D2O), and (d) formic acid (D2, 98%; ≤5%
D2O). In (a), the 2D signals from deuterium oxide afforded a peak at 4.8 ppm in the solution spectrum (bottom) which was shifted to 5.6 ppm when absorbed into the PEM (top, see text for the explanation). In (b), a chemical shift of 3.3 ppm was observed from the deuterated methyl group (−CD3) in both the
solution and solid spectra. In (c), both the deuterium oxide and deuterated formaldehyde afforded a 2D signal at 4.8 ppm in solution. The 2D signal from deuterium oxide was shifted to 5.6 ppm but that from deuterated formaldehyde stayed at the same position (4.8 ppm) when absorbed in PEM. In (d), the 2D signal from deuterium oxide was shifted from 4.8 ppm in solution to 5.6 ppm in PEM while the signal from the deuterated formic group (DCOO−) afforded a peak at 8.1 ppm in both solution and PEM.
10 5 0 (c) 10 5 0 (d) 10 5 0 (a) 5.6 4.8 10 5 0 (b) 3.3 3.3 5.6 4.8 4.8 4.8 5.6 8.1 8.1 in PEM in solution D2O in PEM in solution CD2O in PEM in solution DCOOD in PEM in solution CD3OH
