Understanding the species transport in polymer electrolyte membrane (PEM) fuel cells is critical for improving the cell performance. Poor performance stems from inefficient species transport. Particularly, excess accumulation of liquid water in the gas diffusion layer (GDL) substrate hinders transport of the reactant gases;​ the membrane dehydrates with increasing current density, leading to ohmic losses;​ the catalyst carbon support corrodes in the presence of water and high local potentials. In this thesis, these phenomena were investigated for improving the cell performance. In addition, imaging procedures for synchrotron radiography were improved to accurately visualize the water evolution in PEM fuel cells.

The roles of undesired secondary scattering and harmonic photons on imaging accuracy were numerically determined. Both the secondary scattering and harmonic components increased with increasing water thickness, leading to a decrease in the calibrated attenuation coefficient and subsequent decrease in imaging accuracy. Next, the GDL substrate land and channel region contributions to oxygen transport resistance were resolved. Both the substrate oxygen transport resistance and the mass transport resistance of the fuel cell were more sensitive to the local saturation in the substrate channel region than that in the substrate land region. In a related study, membrane dehydration under high relative humidities (above 70%) caused significant membrane shrinkage. Via experimental visualization and performance testing techniques, the dehydration was attributed to increases in the local temperature. Lastly, a novel diagnostic method was developed to measure the electrode performance for the real-time detection of carbon corrosion in dead-ended anode operation. The accumulation of nitrogen in the corroded cathode led to sudden increases in cathode polarization resistance, and this performance characteristic was identified as a new indicator for a corroded cathode. The main findings in this thesis advance the understanding of species transport as a basis for developing next-generation high-performance PEM fuel cells.