Two main water transport phenomena occur in the electrolyte of PEMFC. Electro-osmotic drag results from proton movements that promote water transport from anode to cathode. Back diffusion is caused by concentration difference that promotes water diffusion from cathode to anode. Electro-osmotic drag is more dominant than back diffusion when the net water transfer coefficient (α) is over zero, and vice versa. Species mole fractions are determined from net water transfer as a result of water transport phenomena and reaction rates related to current density. The hydrogen mole fraction has an extreme point at α = 0.5, while the extremum of the oxygen mole fraction is located at α = −0.75. The change pattern of species mole fraction varies at these extreme points. Net water transfer through membrane influences membrane conductivity and activation loss which are main factors that influence the performance of PEMFC. The relationship between net water movement and species mole fraction is considered in this paper. It is suggested that criteria predict the species mole fraction.

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