“…In the last decade, enormous efforts have been made for developing high-performance and cost-effective noble metalfree catalysts for ORR (Dai et al, 2015;Chung et al, 2017;Gewirth et al, 2018;Li et al, 2018;Xue et al, 2018). These electrocatalysts were based on various kinds of heteroatoms, transition metals, nitrogen and carbon precursors, such as carbon-supported nitrogen and transition metals-doped catalytic materials (Bezerra et al, 2008;Jaouen et al, 2011;Higgins and Chen, 2013;Rauf et al, 2016), dual heteroatom-doped carbon nanotubes or graphene Nyoni et al, 2015;Rauf et al, 2017), metals-N 4 macrocycles (Jasinski, 1964;Seo et al, 2014), heteroatoms doped carbon nanomaterials Liu et al, 2019;Wu et al, 2019), etc., which showed high catalytic activity, stability, and high tolerance to small alcohol molecules or CO poisoning (Qu et al, 2010). Among them, carbon-supported nanomaterial with heteroatoms doping (e.g., Co, Fe, N, S, P, and halogens) were widely developed, and they are promising non-precious electrocatalysts to substitute Pt group metal (PGM)-free electrocatalysts for ORR (Gong et al, 2009;Yang et al, 2012;Yao et al, 2012;Zhang and Dai, 2012;Jeon et al, 2013;Chen et al, 2017;You et al, 2018).…”