Two-dimensional (2D) simulation of a positive streamer in air between a point anode and a plane cathode is performed within the scope of a diffusion-drift model. A fine structure of streamer head is obtained. Based on the simulation results, the head is treated as a moving region of an avalanche-to-streamer transition (AST). Using “empirical” relations, which follow from 2D simulations, and applying the Meek’s concept of an AST, two simple analytical models of streamer dynamics in a weak field are offered. The isolated head model gives results which correspond well to that of 2D simulations. The potential model qualitatively explains an effect recently detected in experiment of streamer expansion in a weak external field. Both models allow one to calculate the streamer velocity, number density of electrons, and radius of the plasma channel from a single input parameter.

Model of water transport in a membrane electrode assembly of a polymer electrolyte fuel cell (PEFC) is developed. The model takes into account nonlinear diffusion of liquid water in the membrane. It is shown that nonlinearity leads to the formation of closely located dry and wet regions in the membrane. The model is incorporated into our quasi-3D model of a PEFC and recent experiments of Büchi and Scherer are simulated. The model qualitatively reproduces measured membrane resistance and the mean profile of water content across the membrane. Calculated two-dimensional distribution of water in the cell cross section is discussed. © 2003 The Electrochemical Society. All rights reserved.

A two‐dimensional model of the cathode compartment of a polymer electrolyte fuel cell has been developed. The existence of gas channels in the current collector is taken into account. The model is based on continuity equations for concentrations of the gases and Poisson's equations for potentials of membrane and carbon phase, coupled by Tafel relation for reaction kinetics. Stefan‐Maxwell and Knudsen diffusion of gases are taken into account. The simulations were performed for high and low values of carbon phase conductivity. The results revealed (i) for a low value of carbon phase conductivity, a “dead zone” in the active layer in front of the gas channel is formed, where the reaction rate is small. One may remove catalyst from this zone without significant loss in cell performance. (ii) For a high carbon phase conductivity value, such a zone is absent, but removal of the catalyst from the same part of the active layer forces the reaction to proceed more rapidly in the remaining parts, with only marginal losses in performance. This conclusion is valid for high diffusivity of oxygen. For low diffusivity, dead zones are formed in front of the current collector, so that catalyst can be removed from these zones. The results, thus, show the possibilities for a considerable reduction of the amount of catalyst. © 1999 The Electrochemical Society. All rights reserved.