Stoichiometric flow rates of anode (s = 2) and cathode (s = 9.5 for O2 and s = 2 for air) reactants were used at current densities >0.2 A cm -2 and constant flows (corresponding to 0.2 A cm -2 flows) at <0.2 A cm -2 . Reactant humidification was achieved by water-bubblers, the temperatures of which were calibrated to yield the desired relative humidity (RH) values.