cathode flow field through the elec

cathode flow field through the electrolyte to the anode side. As fuel is consumed inthe fuel cell, the concentration of nitrogen in the anode compartment increases, thus accumulating therein and negatively impacting the performance of the fuel cell [5]. Moreover, the humidification control (see Sect. 4.5) could create operative phases characterized by a dangerous stagnation of liquid droplets on surface of electrodes in cathode and anode sides, favoring flooding of the compartments and fuel starvation, and then interfering with the access of the hydrogen fuel during stack power requirement. Then the purge action, draining out the possible excess of nitrogen and water accumulated in anode compartment, can eliminate most of liquid molecules from the catalyst surface and refresh the feed. As a consequence the overall strategy for maintaining the well-hydrated cell membranes, assuring a reliable and efficient stack operation with minimization of nitrogen crossover and simultaneously avoiding the flooding phenomena, should involve the periodic acting of the purge valve. This is normally closed but, when necessary, the control strategy expects to open it by managing the opening frequency for a specifiedopening time, sufficient to drain the contaminants but not too long for undesiredleaking of useful fuel. In order to evaluate the effect of purge operation on FCSefficiency, it is possible to define a coefficient expressing the ratio of fuelconverted to fuel supplied (gutil in Sect. 6.2), which can reach values higher than90% in optimized realizations