Densification of cellulose acetate-derived porous carbons for enhanced volumetric hydrogen adsorption performance

Abstract

There is a significant demand for hydrogen gas storage technology, particularly with high volumetric storage density, across various industries as an alternative to current high-pressure compression methods. In this study, cellulose acetate (CA) was used as a precursor to adjust the KOH activation conditions, synthesizing porous carbons with a high hydrogen adsorption capacity (approximately 2.8 wt% at −196 °C and 1 bar). By applying very high pressure to pelletize the porous carbon powder with a binder to a high density (0.8 g cm⁻³), a carbon pellet with a large volumetric hydrogen density was achieved (20.3 g-H₂ per L at −196 °C and 1 bar). For comparison, MOF-derived carbon with a hydrogen adsorption capacity equivalent to that of CA-derived carbon was synthesized and pelletized using the same method. The BET specific surface area was significantly reduced when the CA-derived carbon powder and the MOF-derived carbon powder were pelletized. As a result, the amount of hydrogen adsorption was considerably reduced in the MOF-derived carbon pellet, while the hydrogen adsorption level in the CA pellet remained high. Pore size distribution analysis revealed that in the case of the CA-derived carbon, the proportion of small pores (<0.8 nm) with high hydrogen adsorption efficiency increased more with pelletizing than that of the MOF-derived carbon pellet. Therefore, for CA-derived hydrochar, optimizing the KOH activation conditions through chemical treatment and applying physical compression densification at a high pressure of 800 MPa altered the pore size distribution within a narrow range of micropores. This resulted in constructing a pore structure that is favorable for hydrogen adsorption and achieved a high volumetric storage density. This research approach demonstrates that porous carbon pellets with a large volumetric hydrogen storage density can be produced by increasing the pore ratio favorable for hydrogen adsorption due to pelletizing porous materials with high gravimetric hydrogen storage capacity while effectively suppressing any loss of adsorption properties as much as possible.