Ultralight biomass-based carbon aerogel with hierarchical pore structure fabricated using unidirectional freeze casting and potassium hydroxide activation

•The scheme provides a new idea for high-quality transformation of straw waste.•The BCA-H has a good hierarchical pore structure.•Freeze casting is combined with KOH activation method to enrich pore structure.•Ultra-low cost of the BCA-H is good for commercial development.•The BCA-H has excellent cyclic adsorption stability.