Supporting Information

Transcription

1 Supporting Information Wiley-VCH Weinheim, Germany

2 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 o C for 3 minutes under a pressure of 100 kg/cm 2. Nafion 117 was used after being boiled for an hour each in the following solutions step by step: 3% H 2 O 2, pure water, 0.5M H 2 SO 4, 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/cm 2 for 15 min, and heating at 375 o C 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 o C. The loadings were adjusted to be 5.0 ± 0.2 mg/cm 2 for an anode and 3.0 ± 0.2 mg/cm 2 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 cm 3 /min using a peristaltic pump 323 S/D (Watson-Marlow, UK), and oxygen gas was fed to the cathode at a rate of 1000 cm 3 /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 CD 3 OH (for 2 D NMR) or 13 CH 3 OH (for 13 C NMR) for 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

3 the exposure the evaporation of organic components or water in the PEM was confirmed to be less than 10%. All deuterated or 13 C labelled compounds were purchased from Cambridge Isotope Laboratories, Inc.: methanol (D 3, 99.8%), formaldehyde (D 2, 98%; 20 wt% in D 2 O), formic acid (D 2, 98%; 5% D 2 O), methanol ( 13 C, 99%), formaldehyde ( 13 C, 99%; 20 wt% in H 2 O), and formic acid( 13 C, 99%; 5% H 2 O). 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 ( 1 H-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 90 o flip was 3.0 µs for both 2 D and 13 C NMR and a pulse repetition delay of 3 sec was used. Proton decoupling was applied during 13 C NMR experiments. No change in the sample mass occurred before and after the MAS NMR experiments.

4 Table S1. 13 C peak assignment of MAS NMR spectra in Figure S1 Spectral number Reference compound Major chemical species in aqueous solution Chemical shift of each functional group (in ppm) CH 3 H 2 C(OH) HCOO Figure S1(a) methanol CH 3 OH Figure S1(b) formaldehyde CH 2 (OH) 2 82 Figure S1(c) formic acid HCOOH 166 Figure S1(d) 1:1 mixture of methanol and formaldehyde CH 3 OH CH 2 (OH) 2 CH 2 (OH)(OCH 3 ) CH 2 (OCH 3 ) Figure S1(e) 1:1 mixture of methanol and formic acid CH 3 OH HCOOH HCO(OCH 3 )

5 (a) CH 3 OH (b) CH 2 O 82 (c) HCOOH 166 (d) 1:1 mixture of CH 3 OH and CH 2 O (e) 1:1 mixture of CH 3 OH and HCOOH Figure S1. 13 C MAS NMR spectra of Nafion PEMs soaked in 2 M solutions of 13 C 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.

6 O O C CH 3 C H O 52 ppm H OH 163 ppm polymer backbone 166 ppm H OH C CH 3 OH ppm ssb H OCH 3 90 ppm 55 ppm Figure S2. The 13 C MAS NMR spectrum of the middle layer PEM extracted from the MEA with a triplelayer PEM in a DMFC in operation with 2 M 13 CH 3 OH. 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 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.

7 (a) 5.6 D 2 O (b) CD 3 OH (c) 5.6 CD 2 O (d) DCOOD 8.1 Figure S3. 2 D 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) D 2 O, (b) methanol (D 3, 99.8%), (c) formaldehyde (D 2, 98%; 20 wt% in D 2 O), and (d) formic acid (D 2, 98%; 5% D2O). In (a), the 2 D signals from deuterium oxide afforded a peak at 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 (CD 3 ) in both the solution and solid spectra. In (c), both the deuterium oxide and deuterated formaldehyde afforded a 2 D signal at ppm. The 2 D signal from deuterium oxide was shifted to 5.6 ppm but that from deuterated formaldehyde stayed at the same position ( ppm) when absorbed. In (d), the 2 D signal from deuterium oxide was shifted from ppm to 5.6 ppm while the signal from the deuterated formic group (DCOO) afforded a peak at 8.1 ppm in both solution and PEM.