Datong Song

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells I. Theoretical Modeling

The effect of Nafion loading on the electrode polarization characteristics of a proton exchange membrane fuel cell is studied with a macrohomogeneous model. The composition dependence of performance is rationalized by first relating mass fractions of the different components to their volume fractions and thereafter involving concepts of percolation theory to parameterize effective properties of the cathode catalyst layers. In particular, we explore systematically the effect of Nafion content on the performance. For a uniform layer, the best performance is obtained with a Nafion content of about 35 wt %, representing an optimum balance of proton transport, oxygen diffusion, and electrochemically active surface area. With the help of this modeling tool, we propose a nonuniform Nation catalyst layer and the modeling indicates that such a layer improves performance. Our preliminary experiments (to appear in Part II) confirm this claim. The two cases of nonuniform Nation distribution across the entire thickness include: a three-sublayer structure with equally thick layers, simulating a constant gradient, and a two-sublayer structure with variable thickness of the sublayers. Compared with the optimum Nafion content (35 wt %) in uniform distribution, the three-sublayer structure with higher Nation content on the membrane side exhibits significantly enhanced performance.

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells II. Experimental Study of the Effect of Nafion Distribution

Gas diffusion electrodes (GDEs) containing a graded distribution of Nafion were prepared and characterized, and their performance as fuel cell cathodes compared to GDEs possessing a uniform distribution of Nafion. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and porosimetry are used to characterize the variations in electrochemical properties, ionic conductivity, and microstructures. The cathodic performance was improved over uniform electrodes at intermediate and high levels of polarization when the Nafion content in the GDE was higher toward the catalyst layer/membrane interface and lower toward the catalyst layer/carbon paper interface since this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Fuel cell performance is much poorer when the gradient of Nafion content is reversed, i.e., highest at the catalyst layer/carbon paper interface since this distribution disfavors proton and gas transport in the regions where they need to be maximized.

Influence of Membrane Ion Exchange Capacity on the Catalyst Layer Performance in an Operating PEM Fuel Cell

The effect of ion exchange capacity (IEC) of polymer electrolyte membranes (PEMs) on cathode catalyst layer operation is investigated using a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC) and a series of tetrafluoroethylene-g-polystyrene sulfonic acid (ETFE-g-PSSA) membranes. The electrochemically active surface area (ESA) of the catalyst layer reveals a slight dependence on IEC. The steady-state beginning-of-life polarization curves show an increase in fuel cell performance with increased IEC. The membranes IEC and molecular structure controls the water content within, and regulates the water balance in the complete MEA. Comparing half-fuel-cell and fuel cell systems reveals that the ESA in the latter is lower as a result of reduced wetting of the catalyst layer but this is offset by an order of magnitude improvement of the effective O 2 diffusion. Consequently oxygen reduction reaction (ORR) performance is higher in the fuel cell system. The balance between electro-osmotic flux and hydraulic counterflux in the membrane is employed to explain the distinct effects of IEC in the half fuel cell and fuel cell systems. The two types of measurements thus provide a convenient tool for studying the interplay of different mechanisms of water flux in the membrane.

Transient Analysis of Hydrogen Sulfide Contamination on the Performance of a PEM Fuel Cell

A transient kinetic model for the anode catalyst layer of a proton exchange membrane (PEM) fuel cell is proposed to describe the performance loss introduced by the hydrogen sulfide contaminant. Reaction rates are considered as functions of cell current density and contamination level and are estimated based on the available experimental data. It is found that at a constant cell current density the surface coverage of Pt-S increases faster with time and the anode overpotential rises sharply when increasing the contamination level from 1 to 10 ppm, leading to a faster and more severe cell performance degradation. At a constant contamination level, the surface coverage of Pt-S also increases faster with time when increasing the cell current density from 0.1 to 1.0 A cm -2 , resulting in faster and more severe cell performance degradation. Simulation shows that the contaminant surface coverage at steady state is governed by current density. With the same contaminant concentration, to maintain a higher current density output, a significant decrease of steady-state contaminant coverage is required, while at a lower current density, steady-state contaminants coverage increases significantly.

Modeling of Ultrathin Two-Phase Catalyst Layers in PEFCs

We present a model of transport and reaction kinetics in ultrathin cathode catalyst layers for polymer electrolyte fuel cells (PEFCs). In contrast to conventional catalyst layers, ultrathin catalyst layers neither contain a separate electronic conductor, other than the catalyst itself, nor are they impregnated with perfluorinated sulfonic acid ionomer as an intrinsic proton conductor. They can thus be regarded as two-phase composites. The model utilizes Poisson-Nernst-Planck theory for proton transport in the layer. It relates calculated spatial distributions of oxygen and proton concentrations, electrode potential, and electrochemical reaction rates to catalyst utilization and current-voltage performance. By comparison with experimental data from literature for current-voltage relations at low and intermediate current densities the transfer coefficient α c of the cathodic reaction was estimated. Catalyst layer thickness, composition, and volume fraction of water-filled pore were systematically varied to determine the values that maximize Pt utilization and voltage efficiency of the layer. The significance of these results for the optimization of catalyst layers in view of operation conditions and synthesis methods is discussed.

The Effect of Relative Humidity on Binary Gas Diffusion

The dependence of diffusion coefficient of O2-N2 mixture in the presence of water vapor was experimentally determined as a function of relative humidity (RH) with different temperatures using an in-house made Loschmidt diffusion cell. The experimental results showed that O2-N2 diffusion coefficient increased more than 17% when RH increased from 0% to 80% at 79 degrees C. In the experiments, the RH in both top and bottom chambers of the diffusion cell were the same, and the pressure inside the diffusion cell was kept as ambient pressure (1 atm.). Maxwell-Stefan theory was employed to analyze the mass transport in the diffusion cell, and found that there was no effective water vapor diffusion taking place, indicating that the gas diffusion in this ternary (O2-N2-water vapor) system could be considered binary gas (O2-N2) diffusion. The Fuller, Schettler, and Giddings (FSG) empirical equation of the kinetic theory of gases was generalized to accommodate the dependence of the binary diffusion coefficient on the RH. The prediction of the generalized equation was found to be consistent with experimental results with the difference of less than 1.5%, showing that the generalized equation could be applied to calculate the diffusion coefficients of the binary gaseous mixture with different temperature and RH values. The effect of water vapor on the increase of O2-N2 diffusion coefficient was discussed using molecule theory.

Durability of PEMFC Cathodes Exposed to Toluene-contaminated Air

The effects of toluene contamination on the performance of polymer electrolyte membrane fuel cells (PEMFCs) were studied. The severity of the contamination effect increased with increases in the current density, the toluene concentration and air stoichiometry, but decreased with an increase in the back pressure. The effect of toluene contamination on current dynamic operation was also studied by cycling the current density from 0.0 to 0.2, and then to 1.0 A cm -2 at the presence of toluene. The degradation of the fuel cell performance during the current dynamic operation was accelerated by the presence of toluene.

Numerical Analysis of Water Transport Through the Membrane Electrolyte Assembly of a Polymer Exchange Membrane Fuel Cell

The effects of water transport through membrane electrolyte assembly of a polymer exchange membrane fuel cell on cell performance has been studied by a one-dimensional, nonisothermal, steady-state model. Three forms of water are considered in the model: dissolved water in the electrolyte or membrane, and liquid water and water vapor in the void space. Phase changes among these three forms of water are included based on the corresponding local equilibriums between the two involved forms. Water transport and its effect on cell performance have been discussed under different operating conditions by using the value and the sign of the net water transport coefficient, which is defined by the net flux of water transported from the anode side to the cathode side per proton flux. Optimal cell performance can be obtained by adjusting the liquid water saturation at the interface of the cathode gas diffusion layer and flow channels.