Pressure Dependence of the Oxygen Reduction Reaction at the Platinum Microelectrode/Nafion Interface: Electrode Kinetics and Mass Transport

Abstract

The investigation of oxygen reduction kinetics at the platinum/Nation b interface is of great importance in the advancement of proton-exchange-membrane (PEM) fuel-cell technology. Investigations of this type have been conducted in our laboratories using a microelectrode technique. In these studies, pseudosteady-state voltammetry and electrochemical impedance spectroscopy were used to determine the oxygen reduction kinetics at ambient conditions as well as at elevated temperatures. This study focuses on the dependence of the oxygen reduction kinetics on oxygen pressure. Conventional Tafel analysis of the data shows that the reaction order with respect to oxygen is unity at both high and low current densities. Chronoamperometric measurements of the transport parameters for oxygen in Nation show that oxygen dissolution follows Henry's isotherm. The diffusion coefficient of oxygen is invariant with pressure; however, the diffusion coefficient for oxygen is lower when air is used as the equilibrating gas as compared to when oxygen is used for equilibration. These results are of value in understanding the influence of O2 partial pressure on the performance of PEM fuel cells and also in elucidating the mechanism of oxygen reduction at the platinum/Nation interface.Research on proton-exchange-membrane (PEM) fuel cells is rapidly gaining momentum because of the prospects of attaining high energy efficiencies and high power densities. ~-5 Both of these factors are vitally necessary for space and terrestrial applications. 6 It is well known that the slow kinetics of the oxygen reduction reaction (ORR) limits the performance of hydrogen-oxygen fuel cells. ~ We have, therefore, been studying the kinetics of the ORR under conditions that closely mimic typical operating conditions of the PEM fuel cell. A novel solid-state electrochemical cell based on a platinum-disk microelectrode was used for these investigations. Electrode-kinetic parameters for the ORR and mass-transport parameters for oxygen in Nation were determined using this solid-state cell. I-3 Studies to date have focused on the effect of temperature on O2-reduetion kinetics and 02 transport in Nation. 2 In a similar microelectrode study conducted at the Pt/recast Nalion interface, the ORR was studied over a wide range of temperatures at atmospheric pressure. 8 These studies also showed that membrane drying effects were pronounced at temperatures above 50~This pressure-dependence study was, therefore, conducted at a constant temperature of 50~Oxygen pressure also affects the kinetics of 02 reduction in the PEM fuel cell. 4 We have, therefore, used the cell and methods described in our previous reports I-~ to investigate the effects of oxygen pressure on the kinetics of the ORR at the Pt/Nafion interface and on the rate of 02 transport in Nation. Experiments with both pressurized 02 and pressurized air were conducted. The results of these experiments are described here. The pressure dependence of the ORR at the Pt/Nation interface has been previously investigated at a...

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…0-D models are used to determine kinetic and net ohmic resistance parameters from a polarization performance curve. [15][16][17][18][19][20][21][22] A typical 0-D model equation for a polarization curve is V = U + b log (i 0 ) − b log (i) − R i + b log 1 − i i lim [7] and accounts for the major losses as shown in Figure 1. The first term on the right corresponds to the thermodynamic cell potential.…”

“…Additionally, the diffusion coefficient is often modified for the tortuosity or path length required for the diffusion to occur in a multiphase system, [17] where τ k is the tortuosity of phase k and is greater than 1. If the interactions among the various species are important, then equation 12 needs to be replaced with the multicomponent StefanMaxwell equations that account for binary interactions among the various species [18] where x i is the mole fraction of species i and D eff i, j is the effective binary diffusion coefficient between species i and j. The first term on the right side accounts for pressure diffusion (e.g., in centrifugation) which often can be ignored, but, on the anode side, the differences between the molar masses of hydrogen and water vapor means that it can become important in certain circumstances.…”

“…131 As an example, the kinetic current for the RD step, which represents the desorption of OH, is [69] where the activation energies are potential dependent. 131 In this framework, the adjustable kinetic parameters are four activation and two adsorption free energies, which are fit to kinetic polarization data and cyclic-voltammetry data for surface oxide coverage, [137][138][139] or they can be estimated computationally by ab-initio modeling. 124,139,140 A double-trap modeling framework offers a versatile way to model the PEFC's reaction kinetics (provided good data for the fitting parameters is available), and it predicts the observed doubling of the Tafel slope due to reaction-pathway changes and captures the trends of the experimental oxide coverage.…”

Modeling Water Management in Polymer-Electrolyte Fuel Cells

“…30,32,[34][35][36][37][38][39][40][41] As with the case of the HOR, the dependence of the reaction rate on the hydrogen ion activity is not shown explicitly. While this is typically reasonable, as discussed in section 7.2, under low humidity conditions, the change in the proton concentration and especially its activity coefficient necessitate accounted explicitly for the proton activity.…”

“…Equation 37 can be either used as a separate momentum equation to determine the pressure, or it can be thought of as an additive term to the Stefan-Maxwell equations a la the dusty-gas…”