Bifunctionality on Pt Alloy Nanocluster Electrocatalysts for Enhanced Methanol Oxidation and CO Tolerance in PEM Fuel Cells: Electrochemical and In situ Synchrotron Spectroscopy
S. Mukerjee* and R. C. Urian Electrochimica Acta.,47 (19), 3219 (2002)
Abstract
Electrocatalysis of CO tolerance and direct methanol oxidation on PtMo/C (3:1 a/o) has been investigated in a PEM fuel cell environment. While a 3-fold enhancement is observed for CO tolerance when compared with PtRu/C (1:1), no such enhancement occurred for methanol oxidation. In situ XAS at the Pt L and alloying element K edges for Pt/C, PtRu/C and PtMo/C showed that in contrast to PtRu/C, both Mo and Pt surfaces play a distinct role for CO oxidation. While on the Ru surface there is a competition between oxide formation (from activation of water) and CO adsorption, Mo oxide surface showed no affinity for CO. This provided for efficient CO oxidation at low overpotentials on PtMo/C. However, the corresponding behavior for methanol oxidation showed that Mo oxy-hydroxides were inhibited from efficient removal of CO and CHO species in contrast to Ru oxides. The Mo surface oxides also showed a redox couple involving (V to VI) oxidation states in the presence of both CO and methanol.
The CO Poisoning Mechanism of the Hydrogen Oxidation Reaction in Proton Exchange Membrane Fuel Cells
Abstract
The CO tolerance mechanism of the hydrogen oxidation reaction was investigated on several highly dispersed carbon-supported nanocrystalline Pt and binary Pt alloys. For this purpose, current/potential behavior was derived from half-cells under actual proton exchange membrane fuel cell operating conditions and correlated with expressions derived from kinetic models. Kinetic analyses have shown that the CO poisoning effect on Pt/C, PtRu/C, and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. For all catalysts, the onset of CO oxidation occurs via the bridge-bonded species, but for PtRu/C and PtSn/C, the reaction starts at smaller potentials. Under this condition, the hydrogen oxidation currents are generated on the vacancies of a carbon monoxide adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. The linearly adsorbed CO is oxidized at higher overpotentials, leading to an increase of the holes on the CO layer and thus of the rate of the hydrogen oxidation process.
Block Copolymer-Templated Nanocomposite Electrodes for Rechargeable Lithium Batteries
Abstract
A self-organizing, nanocomposite electrode ~SONE! system was developed as a model lithium alloy-based anode for rechargeable lithium batteries. In situ X-ray adsorption spectroscopy, galvanostatic testing, cyclic voltammetry, X-ray diffraction, and transmission electron microscopy were used to analyze the electrode, which was fabricated from a polyethylene oxide-based block copolymer, single-walled carbon nanotubes, and gold salt. Processing involved a single mixing step without need of a reducing agent. It was found that thermodynamic self-assembly of the block copolymer could provide a template for incorporation of both the gold salt and nanotubes. Electrochemical testing and subsequent analysis showed that owing to the small particle size and the surrounding block copolymer matrix, the SONE system could cycle over 600 cycles with rates varying between C/1.8 and 8.8C with little evidence of decrepitation or coarsening.