Kug-Seung Lee

Catalytic Reactions in Direct Ethanol Fuel Cells

For fuel-cell applications, ethanol is becoming a more attractive fuel than methanol or hydrogen because it has higher mass energy density and can be produced in great quantities from biomass. Additionally, ethanol is less toxic than methanol and easier to handle than hydrogen. 3] However, the C C bond in ethanol leads to more complicated reaction intermediates and products during oxidation, and catalysts must be able to activate C C bond scission for complete oxidation to CO2. Consequently, much effort has been made to investigate the reaction mechanisms of direct ethanol fuel cells (DEFCs) with various analytical methods. Especially the intermediates and products that are generated during the electrochemical reaction at different ethanol concentrations and potentials have been investigated and quantified by chromatographic techniques, infrared reflectance spectroscopy (IRS), and differential electrochemical mass spectrometry (DEMS). These studies revealed that most of the ethanol was oxidized to acetic acid (AA) or acetaldehyde (AAL) on Pt, but not much to CO2. Additionally, investigations of ethanol oxidation on various catalysts showed that alloying Pt with other transition elements improves the catalytic activity. 10, 12,13] However, DEMS is limited to the detection of volatile chemicals, and IRS requires smooth electrodes with sufficient reflectivity. On the other hand, liquid-state nuclear magnetic resonance (NMR) spectroscopy is a straightforward analytical method which can be applied to an operating fuel cell without any modification. In liquid-state NMR spectroscopy, peak areas are linearly proportional to the abundance of chemical species that are identifiable by their chemical shifts. The DEFC anode exhaust has been shown to give well-resolved C peaks that can unambiguously identify chemical species. We have used C liquid-state NMR spectroscopy to identify and quantify the reaction products present in the liquid anode exhaust of DEFCs that were operated with three different anode catalysts at various potentials. The results were used to explain the effect of elements such as Ru and Sn on the Pt/C anode catalyst and to propose reaction mechanisms of ethanol on Pt-based catalysts. The C liquid-state NMR experiments were performed on DEFCs containing 40 wt% Pt/C, PtRu/C, or Pt3Sn/C anode catalysts prepared by a polyol method. Full experimental details are described in the Supporting Information. Figure 1 shows the C NMR spectra of the anode exhaust from the DEFCs with Pt3Sn/C anode catalysts. The spectra were expanded in the y scale while maintaining the relative peak heights. The chemical species were assigned to the peaks in the spectrum according to literature data, and C atoms that are responsible for C NMR signals are underlined. In the exhaust, the dominant reaction products were AAL (d = 207 ppm), AA (d = 177 ppm), and ethane-1,1-diol (ED, d = 88 ppm) at various potentials. Ethyl acetate (d = 62, 175 ppm) and ethoxyhydroxyethane (d = 63, 95 ppm) also appeared, but only in trace amounts and hence were ignored. The coupling constants of 2.8 and 1.6 Hz between the C-labeled sites were used to distinguish CH2 groups in ethyl acetate and ethoxyhydroxyethane, respectively. For comparison purposes, the NMR spectra were also obtained for the DEFCs containing Pt/C and PtRu/C anode catalysts, and AA, AAL, and ED were major products detected for all three catalysts. Figure 2 shows the relative quantities of the major organic chemicals in the anode exhaust of the DEFCs with different anode catalysts at different potentials. For the DEFC with Pt/C anode catalyst, the NMR peak areas of the reaction products were monotonically depleted with increasing operating potential above 0.1 V versus the standard hydrogen electrode. Thus, more oxidation products were produced from the fuel when the DEFC was operated at a higher current and a lower potential. However, the addition of Ru or Sn to Pt caused variations in the NMR spectral patterns. Production of AA dramatically increased. Subtracting the product populations for Pt/C from those for PtRu/C and Pt3Sn/C (dotted lines in Figure 2) separates the contributions of Ru or Sn from those due to Pt/C. For example, the enhanced AAL and ED production on PtRu/C and Pt3Sn/C compared to on Pt/C was almost zero at 0.1 V and slightly increased above 0.2 V. In contrast, AA production was greatly enhanced and different production behaviors were observed depending on the anode catalysts. On the PtRu/C anode catalysts, AA production [*] Dr. I. Kim, Dr. O. H. Han, Dr. S. A. Chae, Dr. Y. Paik, S.-H. Kwon Analysis Research Division, Daegu Center Korea Basic Science Institute, Daegu, 702-701 (Korea) Fax: (+ 82)53-959-3405 E-mail: ohhan@kbsi.re.kr

Selective deposition of Pt onto supported metal clusters for fuel cell electrocatalysts

We report a new method for deposition of Pt on a metal core to develop real electrocatalysts with significantly reduced amounts of expensive Pt as well as enhanced activity for oxygen reduction reaction. Ru and Pd have different crystal structures and modify the electronic structure of Pt to a different extent (shifts in d-band center). They were chosen as core materials to examine whether hydroquinone dissolved in ethanol can be used to deposit additional Pt atoms onto preformed core nanoparticles, and whether the modified d-character of Pt on different host metals can result in the enhanced ORR activity. The physicochemical characteristics of Pd-Pt and Ru-Pt core-shell nanoparticles are investigated. The core-shell structure was identified through a combination of experimental methods, employing electron microscopy, electrochemical measurements, and synchrotron X-ray measurements such as powder X-ray diffraction, X-ray absorption fine structure, and X-ray photoelectron spectroscopy. The hydroquinone reduction method proved to be an excellent route for the epitaxial growth of a Pt shell on the metal cores, leading to enhanced ORR activities.

PtRu-Modified Au Nanoparticles as Electrocatalysts for Direct Methanol Fuel Cells

PtRu-modified Au nanoparticles on a carbon support were prepared using a polyol reduction process with various Pt/Ru ratios. The prepared nanoparticles were characterized using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammetry, and chronoamperometry. The surfaces of most Au nanoparticles were covered with bimetallic PtRu overlayers with a thickness of 1-2 monolayers. The thin PtRu overlayers represented distinct CO stripping characteristics, which may be attributable to the unique surface structures of the PtRu overlayers on the Au nanoparticles. PtRu utilization was enhanced by as much as two times compared to that of PtRu/C, which can be attributed to the PtRu overlayers that were deposited only on the surface of the Au nanoparticles. The distinct CO stripping characteristics and the enhanced PtRu utilization affected the electrocatalytic activities of methanol oxidation. Pt 2 Ru 1 overlayers exhibited the highest CO tolerance and the highest methanol oxidation activity. The unique electrocatalytic characteristics of the PtRu overlayer structures on the Au nanoparticles are expected to provide methods for reducing the use of active elements.