Thermochemical method for controlling pore structure to enhance hydrogen storage capacity of biochar

Developing new carbon-based hydrogen storage materials can significantly promote solid-state hydrogen storage technology. Biochar with high hydrogen storage capacity can be prepared by KOH melt activation, which has a high proportion of micropores (96.56%) compared with the porous carbon in the existing literature. Its specific surface area and pore volume are 2801.88 m2/g and 1.44 cm3/g, respectively. The size of the nanopores is affected by the activation ratio, but the temperature has little effect at the low activation ratio. SEM results show that the KOH activation process gradually shifts from the biochar's inside to the outside. A low KOH/char ratio (less than 2:1) can promote the formation of small aromatic rings. Due to its high specific surface area and microporosity, the absolute adsorption capacity of hydrogen in biochar is 2.53 wt% at −196 °C and 1 bar, rising to 5.32 wt% at 50 bar. The hydrogen adsorption process conforms to the Langmuir model. Microporous, mesoporous, and macroporous exhibit different hydrogen adsorption characteristics in various pressure ranges. However, ultramicroporous (<0.7 nm) always plays a decisive role in the hydrogen storage of biochar.