A new cathodal diffusion layer (CDL) that utilizes mesoporous carbons (MCs) is fabricated and evaluated for the passive direct methanol fuel cell (DMFC). Experimental results reveal that the CDL with MCs (MC-CDL) can accelerate oxygen transport and promote the back diffusion of water from cathode to anode, leading to an increasing power density and enhanced discharge stability. The passive DMFC utilizing such an MC-CDL yields a specific energy of ca. 946 Wh kg−1 at 25°C, which is 2.3 times higher than that of the theoretical specific energy of lithium-ion batteries (∼410 Wh kg−1, LiCoO2/graphite).