N-doped sponge-like biochar: A promising CO2 sorbent for CO₂/CH₄ and CO2/N₂ gas separation

•Sponge-like biochar sorbents are prepared with various types and levels of N-species.•N-doped sponge-like biochar show high selectivity for CO2 gas over CH4 and N2 gases.•A higher amount of ultramicropores enhanced the adsorption and selectivity towards CO2.•Graphitic valley-N or pyridine-N oxide species’ presence enhanced the CO2 adsorption.•The best N-doped sponge-like biochar is water resistant and multicycle reusable.