Polarization curves for the porous lithiated NiO cathode are very often reported with a linear slope over a wide potential range. However, the MCFC behaviour at higher oxidant utilization, when the mass transfer becomes dominant, is mostly overlooked. Therefore, in this study, the two most common cathode mechanisms are utilized to compare their prediction capabilities at higher current densities. This is performed by means of a three-dimensional, non-isothermal mathematical model which is developed by employing volume-averaged equations. As an extension to previous studies, the presented model also considers the potential and current density variation in both solid electrode and liquid electrolyte phases. In essence, this model is a set of partial differential equations including conservation of mass, momentum, gaseous species, energy, electronic potential and ionic potential that are solved using a finite volume method. In brief, both peroxide and superoxide mechanisms predict the linear region of the polarization curve accurately. However, none of these mechanisms showed a downward bent in the polarization curve. A positive exponent for the oxygen mole fraction is essential in obtaining the downward bent knee at high current densities which is in contrast to what has been reported in the literature to-date. (C) 2012 Elsevier B.V. All rights reserved.