Julia Mainka

A modified form of Terzaghi's effective stress principle of unsaturated expansive clays derived from micromechanical analysis

A novel form of Terzaghis effective stress principle is proposed to describe electro-chemo-mechanical couplings in unsaturated swelling clays characterized by two porosity levels (nanopores saturated by an aqueous electrolyte solution and micropores partially saturated with water and air). Under mechanical equilibrium, the three-scale model is derived using the homogenization procedure. Macroscopic constitutive laws are derived within the framework of work-conjugated stress-strain variables leading to a set of three pairs of state variables. In addition to the contact stress between particles and a Bishop component-type related to capillarity effects along with the skeleton strain and water saturation, the novel form includes salinity as an additional strain variable and a three-scale swelling stress directly correlated with the electrochemical phenomena at the nanoscale. Numerical results illustrate the potential of the multiscale approach in capturing the complex features of unsaturated expansive clays.

A Simple Alternative to the Expression of Finite Warburg Diffusion Impedance in Porous Electrodes by Considering Oxygen Consumption Along the Air Channel

The classical expression of the finite Warburg element used for analysing electrochemical impedance spectroscopy (EIS) spectra is obtained assuming that the reaction occurs at the electrode/membrane interface and that the oxygen concentration at the gas channel/gas diffusion layer interface is constant. An alternative to this expression is proposed: it takes into account the variation in oxygen concentration along the gas channel due to the oxygen-reduction reaction (ORR). This pseudo-2D approach is well suited for investigating mass transfer limitations in the fuel cell membrane-electrode assemblies (MEA). Simple numerical applications starting from data already published by different authors show that with this new expression, conclusions about the mass transfer limiting layer and its main characteristics can be significantly modified. This improvement to the usual Warburg impedance could contribute to a better understanding of mass transfer limitation in fuel cells and eventually, to the design of MEAs in which diffusion losses are minimal.