Electrooxidation polyols

Different electron-conducting polymers (polyaniline, polypyrrole, polythiophene) are considered as convenient substrates for the electrodeposition of highly dispersed metal electrocatalysts. The preparation and the characterization of electronconducting polymers modified by noble metal nanoparticles are first discussed. Then, their catalytic activities are presented for many important electrochemical reactions related to fuel cells oxygen reduction, hydrogen oxidation, oxidation of Cl molecules (formic acid, formaldehyde, methanol, carbon monoxide), and electrooxidation of alcohols and polyols. 

Recently, we [63] synthesized various sulfonated multiwalled carbon nanotube (SF-MWCNT)-supported Pd-M (M=Ni and Sn) using microwave-assisted polyol synthesis (Fig. 6.6) and compared their activity towards ethanol electrooxidation. We showed that the bimetallic catalyst (i.e., obtained by simple ultrasonicatimi of the metallic mixtures) gave enhanced electrocatalysis towards alcohol than the alloy counterparts (i.e., obtained by co-reduction of the metals). 

Liu Z, Guo B, Hong L, Lim TH (2006) Microwave heated polyol synthesis of carbcm supported PtSn nanoparticles for methanol electrooxidation. Electrochem Commun 8 83-90... 

Figure 4.10. DMFC polarization curves using PtSn anode catalysts produced by a microwave-assisted polyol method. Anode catalyst load 4 mg cm , 2 M CH3OH with 2 ml min cathode Pt/C 3 mg cm , O2 pressure not specified in the original source, O2 flow rate 500 cm min 353 K [85]. (Reproduced from Electrochemistry Communications, 8(1), Liu Z, Guo B, Hong L, Lim TH, Microwave heated polyol synthesis of carbon-supported PtSn nanoparticles for methanol electrooxidation, 83-90, 2006, with permission from Elsevier.)...

Figure 14.15. TEM images of microwave-synthesized Pt/CNTs from the ethylene glycol solutions of H2PtCl5 with different pH in the presence of CNTs (a) pH = 3.6 (h) pH = 5.8 (c) pH = 7.4 and (d) pH = 9.2 [112]. (Reprinted from Carhon, 43(10), Li X, Chen W-X, Zhao J, Xing W, Xu Z-D, Microwave polyol synthesis of Pt/CNTs catalysts Effects of pH on particle size and electrocatal5nic activity for methanol electrooxidization, 2168-74, 2005, with permission from Elsevier.)...

Chen W, Zhao J, Lee JY, Liu Z. Microwave heated polyol synthesis of carbon nanotubes supported Pt nanoparticles for methanol electrooxidation. Mater Chem Phys 2005 91 124-9. 

Li X, Chen W-X, Zhao J, Xing W, Xu Z-D. Microwave polyol synthesis of Pt/CNTs catalysts Effects of pH on particle size and electrocatalytic activity for methanol electrooxidization. Carbon 2005 43 2168-74. 

Z. Liu, B. Guo, L. Hong, T.H. lim. Microwave heated polyol s)mthesis of carbon-supported PtSn nanoparticles for methanol electrooxidation, Electrochem. Commun. 8 (2006) 83-90. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

In contrast, the variation of AnQ -Zne- values and of peak oxidation currents significantly depends on both the initial concentrations of OH" ions or sorbitol (Figures 7 and 8). Peak oxidation currents, reach a maximum for Cqh- 3.10" M and suddenly decrease at Cqh- 10-3 m ith the concentration of OH- ions in the bulk of solution (Figure 7). In contrast, AnoH-Z e- s log Cqh- Plots display a minimum value close to 1.2 for Cog- 5.10- but increase sharply for solutions containing more than 0.01 H NaOH. At relatively high Cqh- concentrations (more than 0.01 M) in the bulk, the accumulation of OH ions near the charged surface favors various chemical transformations of polyols or of the intermediate species issued from the electrooxidation process. The consumption of OH" ions is thus increased with respect to the quantity of electricity measured. 

The general trends described above for the influence of various factors on the electrooxidation of sorbitol are more or less valid for the other polyols and, to some extent, even for glucose. So, when the peak oxidation currents, are plotted against the peak potentials, Em,... 

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