Hydrogen storage potential of coals as a function of pressure, temperature, and rank

We conducted measurements of hydrogen adsorption on three coal samples of varying ranks at high pressure (0 to 102 bar) and elevated temperatures (303 K to 333 K) to assess their hydrogen storage potential. The excess adsorption capacity increased with increasing pressure but decreased with increasing temperature irrespective of coal rank. The highest hydrogen adsorption recorded was 0.721 mol/kg for the anthracite coal at 303 K and 102 bar. Furthermore, the hydrogen adsorption capacity correlated positively with coal vitrinite and fixed carbon contents (i.e. the high-rank coal exhibited greater adsorption), while all samples depicted predominantly type-I adsorption behavior for the entire pressure range. Micropore analysis and Fourier-transform infrared spectroscopy measurements were conducted to explore the microstructural and surface chemistry associated with these adsorption trends. The micropore content of the three samples followed the order: anthracite > sub-bituminous > bituminous, while H₂ adsorption followed the trend: anthracite > bituminous > sub-bituminous - i.e., no direct correlation between coal micropore content and its H₂ adsorption capacity - attributable to high clay content of bituminous coal which lowered its micropore content. Moreover, bituminous, and sub-bituminous samples exhibited an abundance of oxygen-containing functional groups, while anthracite coal depicted notable aromatic content - suggesting that the H₂ adsorption capacity is a complex function of coal surface chemistry and micropore content. Overall, high-rank coal seams at high pressure and temperature showed the largest hydrogen adsorption i.e., analogous to CO₂ adsorption potential albeitat lower absolute values. These results, therefore, provide preliminary data on the hydrogen storage potential of coal seams and the associated scientific understanding of the mechanisms causing hydrogen adsorption.