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formed which drives the mass transport process. In a fuel cell with purely gas-phase reactants and products (such as an SOFC), gas diffusion processes control mass transfer. In other cells, multi-phase flow in the porous electrodes can have a significant impact (e.g. in PEFC). In hydrogen fuel cells, the evacuation of product is often more limiting than the supply of fuel, given the difference between the diffusivities of hydrogen and water (vapor). While at low current densities and high bulk reactant concentrations mass-transport losses are not significant, under practical conditions (high current densities, low fuel and air concentrations), they often contribute significantly to loss of cell potential. For gas-phase fuel cells, the rate of mass transport to an electrode surface in many cases can be described by Fick's first law of diffusion: i = nFD (CB − CS) (2-23) δ where D is the diffusion coefficient of the reacting species, CB is its bulk concentration, CS is its surface concentration, and δ is the thickness of the diffusion layer. The limiting current (iL) is a measure of the maximum rate at which a reactant can be supplied to an electrode, and it occurs when CS = 0, i.e., iL = nFDCB δ By appropriate manipulation of Equations (2-23) and (2-24), CS = 1 − ii BL (2-24) (2-25) The Nernst equation for the reactant species at equilibrium conditions, or when no current is flowing, is Ei=0 = E° + RT ln CB (2-26) nF When current is flowing, the surface concentration becomes less than the bulk concentration, and the Nernst equation becomes 2-15PDF Image | Fuel Cell Handbook (Seventh Edition)
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