Channel design affects ohmic losses, pressure drop and transport losses associated with PEM fuel cells. The objective of this work is to optimally design flow-fields using a combination of modeling tools and limited experiments to improve performance. More specifically, we focus on optimizing rib-channel dimensions and for this purpose we have developed a detailed and distributed steady-state model of a small section of a PEM fuel cell. Two-Dimensional gas and charge transport equations are solved simultaneously, while water transport is modeled using a response-surface obtained from higher fidelity simulations. Limited experimental data are used to tune the model. The model is then employed to determine optimal-rib channel dimensions subject to machining constraints. We present results for different gas-diffusion layer thicknesses and properties. We also demonstrate that it is possible to obtain improvements in power density by optimizing rib-channel dimensions.