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The oxygen reduction reaction (ORR) kinetics of a high-surface-area carbon-supported platinum catalyst (Pt∕C) were measured in an operating proton exchange membrane fuel cell (PEMFC). The ORR kinetics of Pt∕C can be described over a wide range of temperature, pressure, and current density using four catalyst-specific parameters: transfer coefficient, exchange current density, reaction order with respect to oxygen partial pressure, and activation energy. These parameters were extracted using a combined kinetic and thermodynamic model, either referenced to the reversible cell potential (i.e., using exchange current density as activity parameter) or referenced to a constant ohmic-resistance-corrected (i.e., iR-free) cell voltage. The latter has the advantage of using an activity parameter (activity at 0.9V iR-free cell voltage) which can be measured explicitly without extrapolation, in contrast to the exchange current density required in the former model. It was found that much of the variation in the published values for these catalyst-specific kinetic parameters derives from applying the same parameter name (e.g., activation energy) without specifying which of its many possible definitions is being used. The obviously significant numerical differences both for “oxygen reaction order” and for “activation energy” due to different definitions (often tacitly assumed and rarely explicitly stated in the literature) are illustrated by the kinetic ORR parameters which we determined for Pt∕C : (i) at zero overpotential, where reaction order and activation energy are ∼0.5 and 67kJ∕mol , respectively, and (ii) at 0.9V iR-free cell voltage, where reaction order and activation energy are ∼0.75 and 10kJ∕mol , respectively.

The exchange current density for the hydrogen oxidation/evolution reactions was determined in a proton exchange membrane fuel cell. Ultralow Pt-loaded electrodes ͑0.003 mg Pt /cm 2 ͒ were used to obtain measurable kinetic overpotential signals ͑50 mV at 2 A/cm 2 ͒. Using a simple Butler-Volmer equation, the exchange current density and transfer coefficient were determined to lie within the range of 235-600 mA/cm Pt 2 and 0.5-1, respectively. Due to the fast kinetics, no measurable voltage losses are predicted for pure-H 2 /air proton exchange membrane fuel cell applications when lowering the anode Pt loadings from its current value of 0.4 mg Pt /cm 2 to the automotive target of 0.05 mg Pt /cm 2 .

The following model/experiment comparisons aid in predicting the maximum performance of a normalH2∕oxygen proton exchange membrane fuel cell (PEMFC) for a cathode catalyst with known oxygen reduction reaction (ORR) kinetics in the absence of gas transport resistances. Specific focus was on modeling the voltage loss within the cathode catalyst layer, which results from a balance between slow ORR kinetics and resistance to proton transport. A unique plot of proton resistance correction vs the ratio of ohmic (expressed by the proton conduction sheet resistance, Rsheet ) to charge transfer resistance (expressed by the ratio of the Tafel slope of the ORR over the current density, b∕i ) in a PEMFC cathode (iRsheet∕b) was developed, based on an analytical solution to effective proton resistance in the porous cathode electrode of a proton exchange membrane fuel cell. Additionally, a plot of catalyst utilization (u) vs iRsheet∕b was developed to serve as a guideline of experimental design parameters such that catalyst utilization is kept above 90%, which is a prerequisite for measuring the ORR kinetics. The model is applicable as long as the ORR follow simple Tafel kinetics.

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).