In this paper, electrochemical impedance spectroscopy ͑EIS͒ is used to resolve various sources of polarization loss in a pure hydrogen-fueled polymer electrolyte fuel cell ͑PEFC͒. EIS data are fitted to a fuel cell model in which the catalyst layer physics are accurately represented by a transmission line model. Extracted parameters include cell ohmic resistance, catalyst layer electrolyte resistance, and double-layer capacitance. The results showed that the catalyst layer electrolyte resistance for a stateof-the-art electrode ͑47 wt % Pt on Vulcan XC-72 carbon, 0.8 Nafion ͑1100EW͒-to-carbon weight ratio, 13 µm thick͒ at 80°C and fully humidified conditions was approximately 100 m⍀-cm 2 ; this translates to a dc voltage loss of about 33 mV at a current density of 1 A/cm 2 . Similar results were obtained for two experimental methods, one using H 2 ͑anode͒ and O 2 ͑cathode gas feed͒ and another with H 2 and N 2 supplies, and for two cell active areas, 5 and 50 cm 2 . The measured catalyst layer electrolyte resistance increased with decreasing ionomer concentration in the electrode, as expected. We also observed that the real impedance measured at 1 kHz, often interpreted as the ohmic resistance in the cell, can include contributions from the electrolyte in the catalyst layer.

Limiting current measurements in a polymer electrolyte membrane ͑PEM͒ fuel cell are used to separate the oxygen-transport resistance into individual component parts. By varying the thicknesses of the diffusion medium ͑DM͒ and the microporous layer in different cell builds, the total transport resistance is separated into contributions from flow channels, DM, microporous layer, and all other sources. By varying the pressure, the transport resistance is separated into a pressure-dependent component ͑inter-molecular gas diffusion͒ and a pressure-independent component ͑Knudsen diffusion or transport through ionomer/liquid water layers͒. In addition to oxygen diffusion in an anisotropic gas diffusion layer, the analysis accounts for coupled convective diffusion and reactant depletion in the flow channels. The present work is limited to conditions when no condensation occurs inside the cell. The analysis is applied to a large body of limiting current data collected on Toray diffusion media, both plain and treated with poly͑tetrafluoroethylene͒ ͑PTFE͒, with and without a microporous layer. Effective diffusion coefficients obtained from these methods for plain Toray papers compare reasonably well with independent ex situ measurements of water vapor diffusion through the same materials.Transport resistance affects voltage losses in polymer electrolyte membrane ͑PEM͒ fuel cells in two major ways. Oxygen-transport resistance impedes the flow of oxygen into the cell, creating voltage losses by reducing the concentration of oxygen at the cathode electrode. Water-vapor-transport resistance impedes the flow of product water out of the cell, keeping the membrane and electrodes humidified and lowering their proton resistance. It is generally desirable to maximize transport losses under dry conditions ͑usually associated with lower current densities͒ to keep the membrane humidified but to minimize them under wet conditions ͑usually associated with higher current densities͒ to avoid starving the cathode of oxygen. Gas-transport losses, whether for oxygen or water vapor, depend on the geometry of the cell, the porous structure of the cell materials, the temperature and pressure inside the cell, the presence of any liquid water partially blocking the diffusion path, and possibly, too, any obstruction created by the ionomer layer covering the catalyst sites in the electrode.Several authors have explored the use of limiting current measurements to characterize gas-transport resistance in PEM fuel cells. For example, Williams et al. 1 used limiting current to compare the performance of different types of gas diffusion layers ͑GDLs͒. StPierre et al. 2 combined limiting current data with a cell voltage model to determine an overall oxygen mass-transfer coefficient for the cell. Kocha 3 used limiting current to investigate different forms of oxygen-transport resistance in electrodes. He compared results with both oxygen-nitrogen and oxygen-helium cathode feeds to estimate the relative contributions of Knudsen and so-called thin-film ͑ionom...

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The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction (RnormalH+,cath) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The RnormalH+,cath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a normalH2/normalN2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting RnormalH+,cath , cathode capacitance (Ccath) , and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately <0.6/1 or an ionomer volume fraction of ∼13% in the electrode. The comparison to normalH2/normalO2 fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrode’s I/C ratio, which indicates that electrode proton resistivity ρnormalH+,cath (ratio of RnormalH+,cath over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied ( ⩾35% RH).

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