Effect of surface oxides on hydrogen storage of activated carbon

The influence of surface oxides of activated carbon (AC) was experimentally studied to ascertain the adsorption characteristics of hydrogen. Super activated carbon was prepared by chemical activation in nitrogen at 1073 K, using KOH as the activating agent and litchi wood as the precursor. The activated carbon was oxidized by nitric acid or hydrogen peroxide at various temperatures. Porous texture of all ACs was characterized by nitrogen adsorption at 77 K using an automatic adsorption system. The amounts of acidic surface oxides of oxidized ACs were determined by the Boehm's titration method and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77 K up to 0.1 MPa and 303 K up to 6 MPa, respectively. Experimental results revealed that specific surface area and micropore volume of oxidized ACs slightly changed after oxidation at lower temperatures (below 323 K), however, drastically decreased after oxidation at 373 K. The total surface acidity greatly increases due to the wet oxidation. The total acidity of the AC oxidized by 3N HNO3 at 373 K is about 29 times as large as that of the original AC. The hydrogen capacity of oxidized ACs is significantly suppressed when the acidic group amounts are larger than 0.8 mmol/g.