Zhong-Sheng Liu

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.

Nanophase segregation and water dynamics in hydrated Nafion: Molecular modeling and experimental validation

Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.

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.

Estimation of Local Relative Humidity in Cathode Catalyst Layers of PEFC

A simple method is presented to estimate the local relative humidity (RH) in cathode catalyst layers (CCLs) of polymer electrolyte fuel cells (PEFCs). Based on impedance measurements under different experimental conditions, this technique provides a means to estimate the average value for RH using a correlation with the catalyst layer effective proton resistance. At zero current, a fully humidified anode raises the RH inside the CCL from a nominal 30% to almost 70%. A current density of up to 0.4 A/cm 2 also humidifies the cathode, while drying is observed between 0.4 and 1.0 A/cm 2 .

Note: Determination of effective gas diffusion coefficients of stainless steel films with differently shaped holes using a Loschmidt diffusion cell

In this work, an in-house made Loschmidt diffusion cell is used to measure the effective O(2)-N(2) diffusion coefficients through four porous samples of different simple pore structures. One-dimensional through-plane mass diffusion theory is applied to process the experimental data. It is found that both bulk diffusion coefficient and the effective gas diffusion coefficients of the samples can then be precisely determined, and the measured bulk one is in good agreement with the literature value. Numerical computation of three-dimensional mass diffusion through the samples is performed to calculate the effective gas diffusion coefficients. The comparison between the measured and calculated coefficient values shows that if the gas diffusion through a sample is dominated by one-dimensional diffusion, which is determined by the pore structure of the sample, these two values are consistent, and the sample can be used as a standard sample to test a gas diffusion measurement system.

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.

Erratum: Estimation of Local Relative Humidity in Cathode Catalyst Layers of PEFC [ Electrochem. Solid-State Lett. , 13 , B58 (2010) ]

On page B58, the Experimental section should begin with the following additional sentence: The experimental data analyzed in this work has been presented in Ref. 1. On page B62, the Acknowledgment section should begin with the following additional sentence: The authors gratefully acknowledge Dr. Xinsheng Zhao for conducting the impedance measurements analyzed in this work. Also, an additional reference should be: