How the activation process modifies the hydrogen storage behavior of biomass-derived activated carbons

Microporous activated carbons (ACs) derived from biomass residues, by virtue of their low-cost, good thermo-mechanical stability and easy adsorbent regeneration, are widely considered as hydrogen storage materials for near-term applications. The hydrogen uptake performance of activated carbons is known to depend on the pore-textural and surface characteristics, such as size and distribution of micropores and specific surface area. Here, we present a detailed investigation on how the activation processes using KOH, CO2, K2CO3, and H3PO4 modify the microstructure of olive stones-derived ACs and how they affect the ACs' hydrogen storage behavior. The KOH-activation results in the formation of exfoliated graphene sheets, which are not common in lignocellulose-derived ACs. In addition, the KOH-activation forms supermicropores (1-2 nm) that enhance the hydrogen uptake capacity at high pressures (200 bar). The absolute hydrogen adsorption of KOH-activated sample at 200 bar and 77 K is 6.11 wt%, which is among the highest reported for activated carbon samples. The best hydrogen uptake density per surface area of the carbon we obtained is 2.1 x 10(-3) wt% m(-2) g which is very close to the theoretical maximum hydrogen uptake density on a single graphene sheet. CO2 and H3PO4 activations are more effective on the creation of ultramicropores (d <= 0.7 nm) in the carbon matrix. This order of pore size is useful when hydrogen adsorption is performed at sub-atmospheric pressures. Our study suggests that activated carbons with a homogenous pore size distribution centered at narrow range are not as efficient H-2 adsorbents as the ACs with a bimodal PSD.