Porous carbons with pore sizes in the ultramicropore range (0.6–0.7 nm) are of great interest for hydrogen storage, transportation, and application as a clean energy source. In particular, when they are nanomaterials, their high surface-to-volume ratio maximizes H2 adsorption capacity. Existing ultramicroporous carbon materials are often in powder form and obtained by environmentally hazardous processes. Here, two highly ultramicroporous, self-supporting, and nanostructured 2D carbonaceous materials with high H2 sorption capacity were developed from poly(vinyl alcohol) electrospun mats. The mats were stabilized by a heat treatment up to 195 °C, and subsequently, two strategies were followed: direct pyrolysis of the sample at 800 °C or application of an acidic activation treatment followed by pyrolysis. Both obtained materials showed high H2 adsorption capacity (6.48 and 11.47 mmol/g at 10 bar), similarly to or better than other materials reported in the literature, with the great advantage of being self-supporting. Moreover, both materials achieve complete desorption. Furthermore, the addition of the acidic activation enhances H2 adsorption capacity due to the greater ultramicroporous volume. This work will allow the development of a new range of ultramicroporous carbon materials obtained from green polymeric nanofibers for H2 storage.