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Mass transport impedances, relating electrical and transport variables, are most useful to study kinetics of flow-based electrochemical systems like fuel cells. We have used the Current-modulated Hydrogen-flow rate spectroscopy (CH2S) to monitor mass transport in the anode of proton exchange membrane fuel cells (PEMFCs) by modulating current load, I, and measuring the perturbed hydrogen flow-rate, QH2, (H(jw)=nFAQH2/I). CH2S provides characteristic times of transport processes in the porous anode under working conditions (Fig.1), from which in-operando effective diffusivity of hydrogen can be obtained using the analytical model for transport in the gas-diffusion layer [1]. In this communication, CH2S is applied to a PEMFC operated under passive conditions, i.e. fed with static hydrogen and air atmospheres. This operation mode is very convenient for small and portable applications because it avoids the use of convective elements for gases inlet, allowing a decrease in weight and volume of hydrogen power systems. However, the slow water removal from passive cells leads to high water saturation in the porous electrodes and, with it, to important transport losses. Consequently, passive cells attain typically lower power densities per unit active area compared with conventional PEMFCs. CH2S may help to identify and palliate such losses. Fig.1 compares CH2S results for passive and conventional PEMFCs, including experimental (dots) and theoretical curves (line). The transport difficulties in passive operation are characterized by lower CH2S magnitude and higher t2 (lower H2 diffusivity). The diffusivity of hydrogen will be analyzed under different experimental conditions in a passive PEMFC (p-PEMFC) including the active area size, and current density. The hydrogen diffusivity will be compared with that of a conventional, convective, cell.