Pre-mixed precursors for modulating the porosity of carbons for enhanced hydrogen storage: towards predicting the activation behaviour of carbonaceous matter

Abstract

Highly porous carbons prepared from pre-mixtures of polypyrrole and raw sawdust or sawdust hydrochar achieve much higher surface area than is possible from single use of any one of the precursors. The pre-mixed precursors offer carbons with ultrahigh surface area (up to 3815 m² g⁻¹) and pore volume (up to ∼2.6 cm³ g⁻¹) comprising two pore systems in the micropore (6–12 Å) and mesopore (22–28 Å) range. The porosity can be tailored via choice of pre-mix precursor ratios such that it is possible, under identical activation conditions, to generate carbons that are either microporous or mesoporous. The elemental composition of the precursors, in particular the molar ratio of oxygen to carbon (i.e., O/C molar ratio), is a key variable in determining the development of mesopores, with a high ratio favouring greater mesoporosity. The resulting activated carbons are homogeneous regardless of the pre-mix precursor ratios, and exhibit excellent hydrogen storage capacity that is much higher than can be attained by single-precursor derived samples. The carbons have excess hydrogen uptake (at −196 °C) of up to 3.6 wt% (at 1 bar) and 6.7 wt% (at 20 bar). The total hydrogen uptake is up to 8.1 wt% (at 20 bar), and 10 wt% (at 40 bar), which is much higher than that of most currently available benchmark porous materials. Due to their lower mesoporosity, the pre-mix samples have improved packing density, which means that their volumetric hydrogen uptake (at 40 bar) is much greater (ca. 40 g L⁻¹) than that of single precursor samples (ca. 28 g L⁻¹). The carbons are comparable to or outperform many benchmark materials such as MOFs in terms of their hydrogen uptake, including gravimetric uptake, volumetric uptake and deliverable hydrogen capacity (100 to 5 bar at 77 K). The carbons also have attractive room temperature hydrogen storage capacity. Our findings provide a new method for modulating the porosity of carbons that goes beyond current practice. Furthermore, the new insights on the effect of the O/C ratio make it possible to predict the activation behaviour of precursors in a manner that allows optimising porosity of carbons to match specific applications as demonstrated here for hydrogen storage.