Water management is a critical issue in the development of proton exchange membrane (PEM) fuel cells with robust operation Liquid water can accumulate and flood the gas delivery microchannels and the porous electrodes within PEM fuel cells and deteriorate performance Since the liquid distribution fluctuates in time for two-phase flow the rate of oxygen transport to the cathode catalyst layer also fluctuates resulting in unstable power density and efficiency This paper reports experimental data on the mean voltage loss and the voltage fluctuations during constant current operation as a function of both the number of parallel microchannels and the air flow rate stoichiometric ratio We define channel plurality as a flow field design parameter to describe the number of channels per unit of active area The voltage loss was found to scale proportionally to channel plurality divided by the air stoichiometric ratio The amplitude of the voltage fluctuations was found to be linearly proportional to channel plurality and inversely proportional to the air stoictuometric ratio squared By analyzing pressure drop data and power spectra we conclude that the voltage fluctuations are well-correlated with two-phase flow instabilities in the cathode s parallel microchannels Finally a scaling analysis is presented for generalizing the results for fuel cells having different active area and channel cross-section (c) 2010 Elsevier BV All rights reserved