A 500W PEFC stack operating at 120°C

Despite an intensive research activity and interesting results on the development of materials and components for PEFCs able to work at high temperatures carried out in the last decade, there are few results on the real application of these components in a stack device. In this paper a 500We PEFC stack operating at 120°C was designed, realized and tested using MEA (MEA1-NZr) for high temperatures previously developed and characterized. A software developed in our laboratory for stack dimensioning and design was used. The active area for each cell is covered with two flow fields of 50 cm2 (100 cm2 each) with a parallel serpentine, symmetrically arranged around the longitudinal axis of the cell. This solution was adopted to improve the uniformity of the distribution of reagents, maintaining the pressure drop of a single cell to about 100mbar. The pressure drop was calculated on the base of the operating conditions of the single cell, considering reagents humidified to 100%, with flows equal to 2 times the stoichiometric for both hydrogen and air at a temperature of 120°C and a working pressure of 2 absolute bar. The dimensioning characteristics of the stack are: onominal power of 500W; osingle cell nominal potential 0.6V; ocurrent density 367 mAcm-2 ocells number 25; osingle cell active area 100 cm2 ostack nominal potential 15V onominal current 34A The flow field for the cooling side on the back of the bipolar plate, was designed assuming a precautionary power conversion efficiency of 30% (650W). Based on the equation of heat balance and by imposing a ?T of 5°C between the coolant input and output, the quantity of water required to maintain the average temperature of the stack was calculated. The obtained value is 1.86 l/min of water that gives rise to a pressure drop of about 50mbar. The bipolar plates were realized on graphite (stable at 160°C) by a CNC milling machine; composite MEAs based on nanocomposite Nafion Zirconia with a total Pt loading of 1 mg/cm2 were used. The realized stack was tested at 120°C in different conditions of relative humidity and by feeding the anode side with pure hydrogen or a mixture simulating gas from the reformer.