A theoretical upper bound on gas deliverable capacity via pressure-swing adsorption in nanoporous materials

Due to their low volumetric energy density at ambient conditions, both hydrogen and natural gas are challenging to commercially store onboard environmentally sustainable vehicles. One strategy to densify these gases is to pack the fuel tank with a porous material. Metal-organic frameworks are tunable, nanoporous materials with large internal surface areas and show considerable promise for densifying gases. The US Department of Energy (DOE) has set volumetric deliverable capacity targets which, if met, would help to enable commercial adoption of hydrogen/natural gas as transportation fuels. Here, we present a theoretical upper bound on the deliverable capacity via an isothermal pressure-swing storage. The goals set by DOE for natural gas and hydrogen storage are theoretically possible, but sufficiently close to the upper bound as to be impractical for any real porous material. However, this upper bound directly leads to important realizations which should guide future development. Firstly, one could extract the gas at a higher temperature than that used while filling. Secondly, the fundamental physics of our upper bound do not rule out any material that changes its structure due to the presence of gas, suggesting that flexible materials could still satisfy the DOE target.