A model of the air diffusion electrode used in fuel cells was developed which accounts for the diffusion of oxygen in the gas‐filled pores as well as diffusion into the liquid‐filled pores, electrochemical reaction, and electrical conduction. The model was applied to a PTFE‐bonded, platinum‐on‐carbon cathode in acid electrolyte to evaluate the relative contribution of these effects. Gas diffusion contributed 38% and ohmic loss in the electrolyte contributed 48% to the voltage loss other than activation. Knudsen diffusion was found to be as important as molecular diffusion. Diffusion of dissolved oxygen and ohmic conduction produced small voltage losses. Diffusion contributes to the utilization loss, especially at high oxygen utilizations, but ohmic effects do not.
Equations were formulated for the electrolyte film mechanism, in which H2 diffuses through a film of electrolyte covering the surface of the pores in the electrode. The polarization characteristics at high and low currents were obtained analytically. The equations were solved numerically on a computer using data for H2 on Ni with an NaOH electrolyte for various values of the surface roughness, pore radius and length, temperature, concentration of electrolyte, and film thickness. A similar computer solution was made of Justi's equations for the surface diffusion mechanism, and the results of the two calculations were compared. The results show that the electrolyte film mechanism has a maximum current 300 times larger, and a slope of the polarization curve at low currents, which is 45 times less than that for the surface diffusion mechanism.
The polarization characteristics of an electrode are obtained by combining the previously derived behavior of a single pore, with data on the pore size distribution and internal surface area of the electrode. Application is made to a cell in which the electrodes are sandwiched against a central porous electrolyte vehicle. The hydrogen electrode is made of nickel and NaOH is the electrolyte. The electrode pores are approximated by an idealized system of parallel cylinders. The total electrode current is considered to be a sum of contributions from each of the pores at the same polarization. Theoretically calculated curves are in moderately good agreement with experimentally determined curves except that they predict to<) much polarization at low currents and too little polarization at high currents. The dependence of the polarization curves on temperature and surface area is in general agreement with experiment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.