Measurement of Oxygen Transport Resistance in PEM Fuel Cells by Limiting Current Methods

Abstract

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...

Improved modeling and understanding of diffusion-media wettability on polymer-electrolyte-fuel-cell performance

“…One method to examine a performance signature for a GDL is to analyze the resulting limiting current density on air [30,40]. To keep the analysis simple, the highest possible mass-transfer limiting current is used, thus, the concentration of oxygen at the right or liquid-inlet side of the GDL (see Figure 7) is set equal to zero.…”

“…Often, a Bruggeman-type relation is used where n = 1.5; however, recent microscopic simulations and experimental studies have suggested that the value is closer to between 2 and 5 for typical GDLs [5,14,19,20,24,39,40,64]. Although, it should be noted that some of these studies change the volume fraction by compression, which alters the pore network and is different than examining a change in tortuosity and volume fraction due to pore filling.…”

“…Other studies have tried to measure the tortuosity of the DM for gas-phase diffusion with some success [14,39,40]. Finally, Ziegler and Gerteisen [41] have shown that gas and not liquid is the wetting fluid in treated GDLs, which is expected due to their hydrophobicity and intermediate wettability.…”

Polarization loss correction derived from hydrogen local-resistance measurement in low Pt-loaded polymer-electrolyte fuel cells

“…[1] Regrettably already for loadings below 0.10 mg Pt /cm² at the cathode side, the performance suffers quite drastically due to a poorly understood local resistance, which is thought to be related to an increased local mass-transport resistance (MTR), although other causes have been proposed as well. [2][3][4][5][6][7][8][9][10][11][12][13] In terms of physical interpretation, the MTR represents the local resistance of a reactant molecule towards reaching an active reaction site, which becomes more significant as the number of reaction sites decrease but the desired overall reaction rate remains the same.…”

“…[8] Recently, a limiting-current based on a hydrogen-pump setup has been tried, which alleviates many of the issues with respect to oxygen limiting-current, but the direct applicability of the measurement towards PEFCs under operation is unknown. [20] In this study, we examine whether the MTR by hydrogen limiting-current experiments can be correlated to the overall performance loss in H 2 /air polarization curves, thereby affirming that diffusion or mass-dependent processes govern the observed performance losses with low Ptloadings.…”