Stability of negative ionization fronts: regularization by electric screening?

We recently have proposed that a reduced interfacial model for streamer propagation is able to explain spontaneous branching. Such models require regularization. In the present paper we investigate how transversal Fourier modes of a planar ionization front are regularized by the electric screening length. For a fixed value of the electric field ahead of the front we calculate the dispersion relation numerically. These results guide the derivation of analytical asymptotes for arbitrary fields: for small wave-vector k, the growth rate s(k) grows linearly with k, for large k, it saturates at some positive plateau value. We give a physical interpretation of these results.Comment: 11 pages, 2 figure

A model for optimal catalyst layer in a fuel cell

A model for catalyst layer (CL) performance is used to determine optimal shapes of electrolyte content and catalyst loading through the CL thickness. The effect of optimization strongly depends on the dimensionless working current density (j) over tilde (0). At small (j) over tilde (0) characteristic to a hydrogen cell, the effect of cathode optimization is marginal, while at large (j) over tilde (0) typical for a direct methanol fuel cell, optimization of the anode doubles the CL performance. (C) 2012 Elsevier Ltd. All rights reserved

A model for carbon and Ru corrosion due to methanol depletion in DMFC

A model for carbon and Ru corrosion in a DMFC anode under strong methanol depletion is developed. The model is based on current conservation equation in the membrane. In the methanol-depleted domain, methanol oxidation reaction is substituted by the carbon oxidation (corrosion). This dramatically lowers the membrane potential and creates an environment for electrochemical oxidation of Ru. Calculations show that 50-100 mV loss in the cell potential manifests quite a significant (above 50%) methanol-depleted fraction of the cell active area. (C) 2011 Elsevier Ltd. All rights reserved

The voltage–current curve of a polymer electrolyte fuel cell: “exact” and fitting equations

An analytical solution to the problem of the performance of the cathode electrocatalyst layer in the polymer electrolyte fuel cell is obtained. This solution describes the effect of limiting current density due to imperfect oxygen transport in backing layer. Simple formulas for electrocatalyst layer polarization voltage are derived in the limiting cases of small and large currents. Based on exact formulas, the fitting equation for the polarization voltage is obtained. All fitting parameters are expressed in terms of transport and kinetic parameters. Comparison of theoretical and experimental voltage–current curves of a fuel cell shows reasonable agreement. The fitting equation furnishes an opportunity to characterize fuel cells using experimental voltage–current curves. Keywords: Catalyst layer, PEFC, Polarization voltage, voltage–current curve, Fitting equatio

Thermal stability of the catalyst layer operation in a fuel cell

Stability of the catalyst layer (CL) operation with respect to small temperature disturbance is analyzed. The model includes the non-stationary heat transport equation coupled to the steady-state CL performance model. Dispersion relation of the system shows that for typical operating conditions, the CL operation is stable. Thermal instability caused by the growth of the exchange current density with the temperature can only occur in the case of dramatic reduction of the CL thermal conductivity. This instability should, therefore, be excluded from the list of "suspects" in provoking CL and membrane local degradation. (C) 2010 Elsevier B.V. An rights reserved

A model for mixed potential in the DMFC cathode and a novel cell design

We report a model for performance of the cathode catalyst layer (CCL) in a DMFC. Analytical solutions for the CCL polarization voltage and for the specific voltage loss due to methanol crossover (mixed potential) are obtained. The methanol-oxygen reaction in the CCL decreases the local oxygen concentration in the electrode, thereby raising the local overpotential and increasing the oxygen flux in the CCL. For typical parameters, the mixed potential peak of 150-200 mV is reached at the current density of 10-50 mA cm(-2). A key parameter of crossover, which determines the value of mixed potential can be measured using the half-cell impedance spectroscopy. (C) 2011 Elsevier Ltd. All rights reserved

Polarization curve of a PEM fuel cell with poor oxygen or proton transport in the cathode catalyst layer

Simple analytical expressions for the polarization voltage of the cathode side of a PEM fuel cell for the cases of poor oxygen or proton transport in the cathode catalyst layer (CCL) are derived. In addition, the shapes of local current density and oxygen concentration along the channel are obtained. All the expressions take into account finite oxygen stoichiometry of the oxygen flow. The results can be used for in-situ analysis of the nature of transport problems in the catalyst layer. (C) 2011 Elsevier B.V. All rights reserved