9818247193

where the term 181.6/(vj/-0.634) is the specific resistivity (fi.cm) at no current and at 30°C; the exponential term in the denominator is the temperaturę factor correction if the celi is not at 30°C.. The parameter v|/ is an adjustable parameter with a possible maximum value of 23. This parameter is influenced by the preparation procedurę of the membranę and it is a function of relative humidity and stoichiometry relation of the anodę gas. It may have a value order of 14 under the ideał condition of 100% of relative humidity. There are also reported values in the order of 22 and 23 under oversaturated conditions.

Using the value of (9) for the membranę resistance, the following expression determines the ohmic voltage drop:

(11)

Vohmic - hc    ⁺ ^c)

where Rc represents the resistance to the transfer of protons through the membranę, usually considered constant.

3.4. Concentration or mass transport voltage drop

The mass transport affects the concentrations of hydrogen and oxygen. This, by its tum, causes a decrease of the partial pressures of these gases. Reduction in the pressures of oxygen and hydrogen depend on the electrical current and on the physical characteristics of the system. To determine an equation for this voltage drop, it is defined a maximum current density, J_nmx, under which the fuel is being used at the same ratę of the maximum supply speed. The current density cannot surpass this limit because the fuel cannot be supplied at a larger ratę. Typical values for J„_mx are in the rangę of 500 to 1500 mA/cm².

Thus, the voltage drop due to the mass transport can be determined by:

(12)

where B (V) is a parametric coefficient, that depends on the celi and its operation State and J represents the actual current density of the celi (A/cm²).

3.5. Dynamics of the celi