Representative carbon-based non-noble metal single-atom materials through different pyrolysis routes. (A) Preparation of Fe-N₄ SAC through support-free carbonization route. Reproduced with permission from ref. [69]. Copyright©2018, John Wiley and Sons. (B) Preparation of Ni-N-CNT catalyst through carbon supports connection route. Reproduced with permission from ref. [73]. Copyright©2021, Royal Society of Chemistry. (C) Preparation of Fe SAC-MIL 101-T through metal-organic frameworks (MOFs) construction route. Reproduced with permission from ref. [77]. Copyright©2021, John Wiley and Sons. (D) Preparation of Co, Fe-N-C mesoporous nanospheres SAC through sacrificial template-assisted route. Reproduced with permission from ref. [78]. Copyright©2018, John Wiley and Sons.