Experimental and theoretical insights into enhanced hydrogen uptake by H2-activated BNOC nanomaterials

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Advanced hydrogen storage systems are needed to develop the next generation of portable batteries and vehicles. In this work, effect of oxygen and carbon vacancies in oxygen- and carbon-substituted hexagonal boron nitride (h-BN) nanocrystallites on hydrogen adsorption (Had) is investigated for the first time. For this purpose, BNOC nanomaterials containing 8–13 at% O and 3–6 at% C were synthesized from melamine-boric acid precursors in an ammonia-hydrogen atmosphere at moderate temperatures. The hydrogen-binding properties of h-BN-based nanomaterials strongly depend on the type of defects and the dopant present; therefore, hydrogen was used in the synthesis as an activator, creating active centers by removing oxygen and carbon. The resulting BNOC nanomaterials are characterized by low crystallinity, with crystallite sizes less than 2 nm. The best sample with a specific surface area of 1405.1 m2×g−1, a specific pore volume of 0.779 cm3×g−1, and a micropore volume of 0.517 cm3×g−1 showed Had capacity of 5.2 wt% at T = 77.35 K and a pressure of 100 kPa. The pressed BNOC nanomaterials also exhibited decent volumetric H2 uptake in the range of 16.7–18.7 g × L−1 for pellet densities in the range of 0.59–0.74 g × cm−3. Density functional theory (DFT) calculations showed that Had up to 2.3 wt% at T = 77 K is only possible on pure BN, “armchair” carbon defects, and oxygen defects. Had up to 4.0 wt% is achievable on “zigzag” carbon defects at T < 70 K; above this temperature, hydrogen desorption occurs. Had from ∼3 to 5.3 wt% at T = 77 K is thermodynamically favorable only for oxygen defects, above 5.3 wt%, the adsorption energy becomes positive, indicating the limit of hydrogen sorption. X-ray photoelectron spectroscopy (XPS) analysis suggests that the superior H2 uptake of BNOC nanomaterials may be due to the hydrogen treatment-induced formation of active sites, such as oxygen and carbon vacancies. The obtained results not only demonstrate the promise of BNOC nanomaterials for hydrogen storage but also show the possibility of controlling structural defects to increase hydrogen uptake. Creating active sites through restructuring different types of vacancies may be a new strategy to improving hydrogen uptake.