Application of alternating current impedance to fuel cell modeling

AC impedance has provided a useful diagnostic tool in the Los Alamos polymer electrolyte fuel cell (PEFC) program. The author reviews the techniques he has used in ac impedance modeling. These techniques include equation implementation, model simplification and verification, least squares fitting, application of two-dimensional Laplace equation solvers handling complex interfacial boundary conditions, and interpretation of impedance features. The separate features of the complete electrode model are explained by analytic examples

Modeling of polymer electrolyte fuel cell performance with reformate fuel feed streams

Carbon monoxide poisoning of polymer electrolyte fuel cell anodes is a key problem to be overcome when operating on reformed fuels. CO adsorbs on the precious metal surface leading to substantial performance losses. Much recent work has explored this problem, using various Pt alloys in attempts to overcome the performance loss. In their studies of hydrogen oxidation on Pt and Pt alloy (Pt/Sn, Pt/Ru) electrodes exposed to H2/CO mixtures, Gasteiger et al. (1, 2) showed that a small oxidation current is observed before the onset of CO oxidative stripping (ca. 0.4 V) on Pt/Ru. However, these workers concluded that the observed current wa

Hydrogen oxidation reaction on Pt in acidic media: Adsorption isotherm and activation free energies

We present a method for quantifying the kinetic current of the hydrogen oxidation reaction (HOR) on Pt in acidic media using the adsorption and activation free energies as the intrinsic kinetic parameters. Our approach involves deriving the HOR intermediate's adsorption isotherm based on the same dual-pathway reaction model as for the kinetic equation. Coupled with density functional theory calculations of the coverage- and site-dependent hydrogen dissociation free energies, we analyzed the adsorption isotherms for H on atop (HAtop) sites and in hollow/bridge (HH/B) sites obtained respectively from infrared spectroscopic and electrochemical measurements. Although the hydrogen dissociative adsorption energy is not site-sensitive on Pt, HAtop has an order of magnitude lower coverage and higher activity than HH/B. We attributed this distinction to their different entropies; HH/B is less accessible to water than HAtop. and the formation of a hydrogen bond is essential in the oxidative generation of H3O+. Using volcano plots, we illustrated possible ways that the exchange current may vary with the adsorption free energy and discussed the factors that contributed to the exceptionally high activity of Pt for the HOR. © 2007 American Chemical Society

Practical aspects of the oxygen reduction reaction (ORR)

The oxygen reduction reaction (ORR) is quite sensitive to the details of the three phase interface at which the reaction occurs. We describe here studies of the ORR at a well-defined recast Nafion/Pt microelectrode interface, emphasizing the effects of temperature and humidification on the reaction rate. We compare our results to those obtained in thin film composite electrodes used in polymer electrolyte fuel cells

Water transport properties of fuel cell ionomers

We will report transport parameters measured for several available perfluorosulfonate membranes. The water sorption characteristics, diffusion coefficient of water, electroosmotic drag, and conductivity will be compared for these materials. The intrinsic properties of the membranes will be the basis of our comparison. An objective look at transport parameters should enable us to compare membranes without the skewing effects of extensive features such as membrane thickness. 8 refs., 4 figs., 2 tabs

PEM fuel cells for transportation and stationary power generation applications

We describe recent activities at LANL devoted to polymer electrolyte fuel cells in the contexts of stationary power generation and transportation applications. A low cost/high performance hydrogen or reformate/air stack technology is being developed based on ultralow Pt loadings and on non-machined, inexpensive elements for flow-fields and bipolar plates. On board methanol reforming is compared to the option of direct methanol fuel cells because of recent significant power density increases demonstrated in the latter