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When Equations (2-33) and (2-34) are substituted in Equation (2-35) Vcell = Ecathode – ⏐ηcathode⏐ – (Eanode + ⏐ηanode⏐) – iR (2-36) or Vcell = ∆Ee – ⏐ηcathode⏐ – ⏐ηanode⏐ – iR (2-37) where ∆Ee = Ecathode – Eanode. Equation (2-37) shows that current flow in a fuel cell results in a decrease in cell voltage because of losses by electrode and ohmic polarizations. The goal of fuel cell developers is to minimize the polarization so that Vcell approaches ∆Ee. This goal is approached by modifications to fuel cell design (improvement in electrode structures, better electro-catalysts, more conductive electrolyte, thinner cell components, etc.). For a given cell design, it is possible to improve the cell performance by modifying the operating conditions (e.g., higher gas pressure, higher temperature, change in gas composition to lower the gas impurity concentration). However, for any fuel cell, compromises exist between achieving higher performance by operating at higher temperature or pressure and the problems associated with the stability/durability of cell components encountered at the more severe conditions. 2.6 Fuel Cell Performance Variables The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilization, current density), cell design and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. The equations describing performance variables, which will be developed in Chapters 3 through 7, address changes in cell performance as a function of major operating conditions to allow the reader to perform quantitative parametric analysis. The following discussion provides basic insight into the effects of some operating parameters. Current Density: The effects on performance of increasing current density were addressed in the previous section that described how activation, ohmic, and concentration losses occur as the current is changed. Figure 2-7 is a simplified depiction of how these losses affect the shape of the cell voltage-current characteristic. As current is initially drawn, sluggish kinetics (activation losses) cause a decrease in cell voltage. At high current densities, there is an inability to diffuse enough reactants to the reaction sites (concentration losses) so the cell experiences a sharp performance decrease through reactant starvation. There also may be an associated problem of diffusing the reaction products from the cell. Ohmic losses predominate in normal fuel cell operation. These losses can be expressed as iR losses where i is the current and R is the summation of internal resistances within the cell, Equation (2-22). As is readily evident from the equation, the ohmic loss and hence voltage change is a direct function of current (current density multiplied by cell area). 2-18PDF Image | Fuel Cell Handbook (Seventh Edition)
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