In order to elucidate a porous structure consisting of carbon aggregate and ionomers, the agglomeration mechanism of carbon black in CL is examined based on the experimental results of particle diameter distribution of carbon black in CL ink with or without an ionomer and the transmission electron microscopy image of carbon black dispersion. A theoretical model of the attraction and repulsion between carbon particles is also discussed, and compared with experimental results. From this model and the measured particle distribution data, it is supposed that the structure with large isolated pores results from carbon black dispersion in CL slurry, and the interaction phenomenon of each carbon black particle depends on the weight ratio of ionomer and carbon. Furthermore, the carbon black aggregate and the agglomerate structure in CL are reproduced numerically, and the effects of the heterogeneous structure on gas diffusion performance are examined by simulations. These results suggest that gas diffusion performance depends on pore size and ionomer adhesion. In particular, ionomer migration near large pores strongly affects gas diffusion performance because of the existence of isolated pores. In addition, better gas diffusion needs a certain amount of pores of non-uniform sizes. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.