Yuxiu Liu

Proton Conduction and Oxygen Reduction Kinetics in PEM Fuel Cell Cathodes: Effects of Ionomer-to-Carbon Ratio and Relative Humidity

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 (R H+,each ) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The R H +, eath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a H 2 /N 2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting R H+,eath , cathode capacitance (C eath ), 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 H 2 /O 2 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 electrodes I/C ratio, which indicates that electrode proton resistivity ρ H+cath (ratio of R H+,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).

Proton Conduction in PEM Fuel Cell Cathodes: Effects of Electrode Thickness and Ionomer Equivalent Weight

The dependence of electrode proton resistivity on electrode thickness, Pt loading, ionomer loading, and ionomer equivalent weight (EW) in proton exchange membrane (PEM) fuel cell cathodes was investigated using a Pt/Vulcan catalyst. For uniform electrodes, the electrode proton resistivity is independent of the electrode thickness and Pt loading but depends on the ionomer loading and ionomer EW. There is a strong dependence on the ionomer EW when the ionomer/carbon weight (I/C) ratio is lower than 0.8. The electrode proton resistivity strongly depends on relative humidity (RH) and the density of ―SO 3 H groups in the electrode. The electrode proton resistivity becomes nearly independent of ionomer EW in electrodes when high I/C ratios are used. At low I/C ratios and low RH levels, electrodes with 850 EW ionomer exhibit better performance than those with 1050 EW. On the contrary, 850 EW electrodes give lower performance under overhumidified conditions due to electrode flooding.

Dependence of Electrode Proton Resistivity on Electrode Thickness and Ionomer Equivalent Weight in Cathode Catalyst Layer in PEM Fuel Cell

developed to predict electrode performance can be used in a wide range of electrodes and electrode thicknesses. Since the equivalent weight (EW) of the ionomer affects its proton conductivity, using ionomers with different EW and measuring their corresponding electrode’s proton resistance can give us a better understanding of the impact of the ionomer bulk proton resistivity on the electrode proton resistivity. While previous electrodes were made of 1050 EW ionomer, the present work employed electrodes made of an ionomer with lower EW. Lower electrode proton resistivities are expected for the electrodes made of low EW ionomers. H cath R ,

Thin Shell Model for Proton Conduction in PEM Fuel Cell Cathodes

A mathematical model is presented for proton conduction in the thin ionomer layer that covers the carbon supported catalyst in proton exchange membrane (PEM) fuel cells. The cathode proton resistance (R H+ , cath ) has been previously measured in a H 2 /N 2 cell using ac impedance as a function of ionomer-to-carbon mass ratio (I/C) m , ionomer equivalent weight, and relative humidities (RH). After considering the effect of roughness and ionomer absorption in the carbon support, the model favorably agrees with the experimental results. It shows that the cathode proton resistivity is indeed an intrinsic property, which for a given type of carbon support depends on the volumetric ionomer-to-carbon ratio, which accounts for the RH-induced ionomer swelling. Further analysis indicates that the roughness and porosity of the carbon particles play a significant role for the proton conduction. The amount of ionomer absorbed into the pores and the roughness factor of the carbon particles can be extracted from a comparison of the model and data.