A novel reactor design is proposed for the soluble lead-acid flow battery (SLFB), in which a three-dimensional honeycomb-shaped positive PbO2-electrode is sandwiched between two planar negative electrodes. A two-dimensional stationary model is developed to predict the electrochemical behaviour of the cell, especially the current distribution over the positive structure and the cell voltage, as a function of the honeycomb dimensions and the electrolyte composition. The model includes several experimentally-based parameters measured over a wide range of electrolyte compositions. The results show that the positive current distribution is almost entirely determined by geometrical effects, with little influence from the hydrodynamic. It is also suggested that an increase in the electrolyte acidity diminishes the overvoltage during discharge but leads at the same time to a more heterogeneous reaction rate distribution on account of the faster kinetics of PbO2 dissolution. Finally, the cycling of experimental mono-cells is performed and the voltage response is in fairly good accordance with the model predictions.