Activation overpotentials, due to the reaction kinetics at the surface of the electrodes are the dominant losses in low current densities in proton exchange membrane (PEM) fuel cells. Although the Butler-Volmer equation can be employed to model the reactions at the anode and cathode, there are still ambiguities regarding the estimation and modeling of the activation losses. In this paper, the Butler-Volmer equation for both the anode and cathode is simplified. It is shown that the anode activation overpotential can be modeled using the linearized Butler-Volmer equation. The cathode activation overpotential is determined using Tafel equation. The both equations are discussed to be very accurate in the entire range of fuel cell performance. The total activation overpotential is then determined.

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