Text Exercise 3

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PEM fuel cells. Exercise 1 - PEM fuel cell voltage analysis Exercise 1.a A PEM fuel cell operates at 70°C with high excess of pure H 2 and O2, supplying at cell voltage of 0.8 V a current density equal to 1 A/cm2. Assuming that: • HOR overpotential is negligible • chemical activity of oxygen and hydrogen are unity • membrane conductivity and thickness are respectively 0.05 S/cm and 50 µm • ORR transfer coefficient is 0.5 • mass transfer resistance is negligible Estimate the PEMFC voltage at 1.5 A/cm2. Exercise 1.b Given the results of the exercise 1, calculate the voltage at 1.5 A/cm2 considering that the oxygen activity in the cathode feed channel is lowered to 0.15 because of the use of ambient air. Additionally, the mass transfer resistance of the GDL is to be considered, given the diffusion coefficient and the thickness respectively equal to 0.03 cm2/s and 200 µm. Exercise 2 - PEM fuel cell stack A PEMFC stack supplies at full load a power of 100 kW at 195 V with an efficiency equal to 50% (LHV = 120 MJ/kg). Assuming that 5% of hydrogen is wasted and given the polarization curve for the average cell voltage in the figure, estimate the number of cells, cell area, current density and current. Compute the system efficiency considering the auxiliary power required by the compressor that processes air with a stoichiometry of 1.5 with a specific isentropic work of 90 kJ/kg and an isentropic efficiency of 0.85 (inlet air at 300K and 1 bar, molecular weight of air 29 kg/kmol ideal biatomic gas).