The Darcy number (Da) indicates the resistance that the electrolyte faces to pass through the solid phase by affecting the Brinkman penalty term (Eq. 37). While the Darcy number directly affects the dissipation energy (equivalent to pumping power loss) according to Eq. 43, it influences the electrical ohmic losses indirectly via the change of the material distribution. The Darcy number shows a large impact on the improvement of the loss functions among the investigated parameters since it represents the permeability of the porous structure, affecting the flow regime drastically. Fig. shows the results for Da = 10 -5 , which was calculated based on the permeability of the Freudenberg H23 carbon paper. The results of other flow regimes for Darcy numbers of 10 -4 and 5 × 10 -4 are presented in the supplementary materials (Figs. and), which can be seen as the optimization results for porous structures with a higher permeability. The Darcy number can be defined more accurately to take into account the effect of the fabrication process in future studies.