NMR methods for characterizing the pore structures and hydrogen storage properties of microporous carbons

¹H NMR spectroscopy is used to investigate a series of microporous activated carbons derived from a poly(ether ether ketone) (PEEK) precursor with varying amounts of burnoff (BO). In particular, properties relevant to hydrogen storage are evaluated such as pore structure, average pore size, uptake, and binding energy. High-pressure NMR with in situ H₂ loading is employed with H₂ pressure ranging from 100 Pa to 10 MPa. An N₂-cooled cryostat allows for NMR isotherm measurements at both room temperature (∼290 K) and 100 K. Two distinct ¹H NMR peaks appear in the spectra which represent the gaseous H₂ in intergranular pores and the H₂ residing in micropores. The chemical shift of the micropore peak is observed to evolve with changing pressure, the magnitude of this effect being correlated to the amount of BO and therefore the structure. This is attributed to the different pressure dependence of the amount of adsorbed and non-adsorbed molecules within micropores, which experience significantly different chemical shifts due to the strong distance dependence of the ring current effect. In pores with a critical diameter of 1.2 nm or less, no pressure dependence is observed because they are not wide enough to host non-adsorbed molecules; this is the case for samples with less than 35% BO. The largest estimated pore size that can contribute to the micropore peak is estimated to be around 2.4 nm. The total H₂ uptake associated with pores of this size or smaller is evaluated via a calibration of the isotherms, with the highest amount being observed at 59% BO. Two binding energies are present in the micropores, with the lower, more dominant one being on the order of 5 kJ mol^-1 and the higher one ranging from 7 to 9 kJ mol ^-1.