For the 3D simulations, the Darcy number was set to Da = 5 × 10 -4 . Although the Darcy number was chosen high in order to facilitate the convergence and help simulations converge in fewer Newton's iterations of the non-linear solver for the flow problem, obtaining two distinctive optimal designs shows the versatility of the developed topology optimization model. Choosing a higher Darcy number would lead to similar designs but with thinner flow pathways formed close to the membrane (Fig. ) or with channels around the electrochemically active area (Fig. ). This also needs adjusting the employed preconditioners and iterative solvers to deal with higher Darcy numbers in 3D while not affecting the parallel efficiency. The parallel efficiency and scaling behavior of the performed 3D simulations are presented in Appendix A. shape would affect the pressure drop and as a result, the hydraulic losses.