Mass transport and conductivity properties of the carbon fiber papers (CFPs) used in gas diffusion layers are among the critical parameters that affect a proton exchange membrane fuel cell (PEMFC) performance. Here, instead of carbonization/graphitization in the commercial CFPs manufacturing steps, the expanded graphite (EG) is numerically added to the CFP substrate to compensate and enhance the conductivity. The microstructures of different polymer binder/carbon fiber (CF) composites with and without EG are reconstructed using a digital 3D reconstruction technique. The binder phase is discriminated as an individual phase using image processing, and the effects of different binder percentages are demonstrated. In comparing the properties, the reconstructed CFP exhibited satisfactory properties against the values of the commercial CFPs in the literature and our conducted experimental tests. For the binder/CF/EG CFPs, 1.9 W/mK of effective thermal conductivity in the through-plane direction and also 30.2 m Omega.cm of the in-plane electrical resistivity are reached, compared to 0.48-1.8 W/mK (for Sigracet and Toray papers) and 12.9 m Omega.cm (for untreated Toray TGP-H-60), in the conventional CFPs. Overall, adding EG to the CFPs substrate provides valuable properties and can be considered as a proper replacement in future design and manufacturing of CFPs. (C) The Author(s) 2019. Published by ECS.