Chunxin Ji

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).

Effects of Catalyst Carbon Support on Proton Conduction and Cathode Performance in PEM Fuel Cells

The effects of the porosity and pore size distribution of catalyst carbon support on the proton conduction and electrode performance have been studied by comparing two carbon supports [Vulcan XC 72 (V) and Ketjenblack (KB)]. Differences in the H 2 /O 2 cell performance of the two catalysts are shown to be consistent with differences in the electrodes proton resistivity and the area of the Pt catalysts available for the oxygen reduction reaction (ORR) at a similar area specific activity. For the same ionomer-to-carbon weight ratio (I/C) and relative humidity (RH), the electrode with KB-supported catalyst exhibits higher proton resistivity than the one with Vulcan-supported catalyst. The cumulative surface area and cumulative volume distributions of the two catalysts were obtained from N 2 BET measurements. It is concluded that the difference in surface area and pore volume for these two catalysts comes from pores smaller than 4 nm. The extent of ionomer captured within these small pores is substantial for Pt/KB and minor for Pt/V. It is shown that the ionomer captured within these small pores of the primary carbon particles does not contribute to the proton conductivity across the electrode. If the total electrode I/C-ratio is corrected for the amount of ionomer captured within the primary carbon particles, the dependence of the proton resistivity of Pt/KB and Pt/V on the effective I/C-ratio becomes identical. Since Pt/KB contains more pores smaller than 4 nm, higher total I/C-ratio is needed for Pt/KB to achieve the same proton resistivity as Pt/V based electrodes. Pt particles are more uniformly dispersed on higher surface-area carbon supports, resulting in a higher Pt area available to ORR. The selection of the proper carbon support depends on the porosity and its effect on cathode proton conduction, ORR kinetics and electrode durability.

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 ,