Combination of graphene oxide with flax-derived cellulose dissolved in NaOH/urea medium to generate hierarchically structured composite carbon aerogels

•Flax-derived cellulose was firstly used as the raw material of carbon.•Carbon aerogels were gotten from mixtures of cellulosic solution and graphene oxide.•Synergistic activation between sodium hydroxide and urea in a carbonization process.•N-doped porous carbon aerogel has a high efficiency in the adsorption of Cu (II).