ABSTRACT Liquid water distribution in the operational PEMFC was visualized by soft X-ray radiography. To achieve a maximum sensitivity towards liquid water, low electron beam energy in soft X-ray range was chosen. As a result, the spatial resolution of 1 µm and the temporal resolution of 0.5 sec were obtained. In addition, a correlation between the accumulation of water and the reduction of performance in higher current density range was observed. At low current densities, liquid water accumulation was observed under the ribs, because the longer diffusion length for the rib area conduced to a less effective water removal into the vapor phase. With increase of current density, liquid water was also observed under the channels and the reduction of performance with time was observed. These results suggested that the current density under the rib is much less than under the channel and the anisotropy of GDL and the geometry of flow field had a heavy impact on the liquid water distribution and the cell performance. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are regarded as a promising alternative clean power source for automobile applications. For successful implementation of PEMFCs for automobile application, cost reduction and downsizing of the PEMFC stack are needed. In order to meet these demands, further improvements of cell performance under severe operating conditions such as low relative humidity and high current density is required. Under high current density operation, excess water is generated and condenses, filling the pores of electrodes with liquid water and hence limiting the reactant transport to catalyst sites. This phenomenon known as "flooding" is an important limiting factor of PEMFC performance. A fundamental understanding of two-phas
