Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Catalyst

Abstract

We report a two-to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbon supported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrystals were comprised of Pt alloyed with Mo in the ratio 8.7:1.3 as shown by both X-ray diffraction and in situ extended X-ray absorption fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate the onset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox behavior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo K edge.

Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

“…Alloy catalysts help improve CO tolerance by altering the adsorption enthalpies of CO and H 2 , as well as their oxidation rates. Thus, a combination of increasing operating temperature of the PEMFC and modified electrocatalysts can reduce performance losses of a PEMFC in the presence of CO [23,24]. …”

CO-Tolerant PtMo/C Fuel Cell Catalyst for H2Oxidation

“…This pseudocapacitive feature is attributed to the formation of OH species on Mo atoms on the surface of alloy particles. 23 The nature of the oxidation states of Mo surface atoms, however, is still obscure and not clearly resolved, but the investigation by Mukerjee and coworkers suggested that the oxyhydroxide state of Mo, predominantly as MoO(OH) 2 , is responsible for the oxidative removal of CO. 25 Another noticeable change in the cyclic voltammograms of Pt 4 Mo 1 /C catalysts is that the double-layer area becomes thicker as the annealing time increases. This implies that more Mo atoms were diffused out to the surface of alloy particles as the annealing under oxygen atmosphere was prolonged.…”

“…For instance, spectroscopic analysis revealed that Mo surface atoms have active oxygen species at all potentials, which may act as a promoter for the oxidization of adsorbed CO on neighboring Pt atoms. 9,23,25 While the bifunctional mechanism sounds plausible as in the case of PtRu alloy catalyst, one cannot completely exclude another possible reason for the improved CO-tolerance: the adsorption strength of CO on Pt atoms. If CO molecules are less tightly bound to the surface of Pt, the removal of CO would be much easier and do not require high overpotentials for the oxidation.…”

“…Grgur and coworkers reported that the magnitude of H 2 oxidation current with the PtMo catalyst in low-overpotential region is nearly the same as that observed with the PtRu catalyst. 9,23,24 Most importantly, PtMo requires less overpotential for CO electro-oxidation bỹ 0.15 V. 24,25 Despite the disparate chemical properties of Ru and Mo, the characteristics of PtMo catalyst appear to be similar to those of PtRu catalyst, thus the enhancement induced by Mo was generally ascribed to the bifunctional mechanism. The role of Mo in CO electro-oxidation, however, remains elusive.…”

Advances in the application of nanotechnology in enabling a ‘hydrogen economy’

“…A thin layer of Ru deposited on the surface of the electrode can act as a filter for CO thus improving tolerance of the fuel cell. Also, nanocrystalline PtMo/C catalysts have been reported to show a two to three times improvement in CO tolerance in a PEMFC as compared to the conventional Pt-Ru/C electrocatalysts [229].…”