  Both hydrogen and natural gas are challenging to economically store onboard
vehicles as fuels, due to their low volumetric energy density at ambient
conditions. One strategy to densify these gases is to pack the fuel tank with a
porous adsorbent material. The US Department of Energy (DOE) has set volumetric
deliverable capacity targets which, if met, would help enable commercial
adoption of hydrogen/natural gas as transportation fuels. Here, we present a
theoretical upper bound on the deliverable capacity of a gas in a rigid porous
material via an isothermal pressure swing. To provide an extremum, we consider
a substrate that provides a spatially uniform potential energy field for the
gas. Our bound relies directly on experimentally measured properties of the
pure gas. We conclude that the deliverable capacity targets set by the DOE for
room-temperature natural gas and hydrogen storage are just barely theoretically
possible. The targets are likely to be impossible for any real, rigid porous
material because of steric repulsion, which reduces the deliverable capacity
below our upper bound. Limitations to the scope of applicability of our upper
bound may guide fuel tank design and future material development. Firstly, one
could avoid using an isothermal pressure swing by heating the adsorbent to
drive off trapped, residual gas. Secondly, our upper bound assumes the material
does not change its structure in response to adsorbed gas, suggesting that
flexible materials could still satisfy the DOE targets.
