A Br-regulated transition metal active-site anchoring and exposure strategy in biomass-derived carbon nanosheets for obtaining robust ORR/HER electrocatalysts at all pH values

Abstract

A novel Br-regulated “Transition Metal Active-site Anchoring and Exposure” (TMAAE) strategy is reported, for the first time, to fabricate a hierarchical porous Co/N-doped bio-carbon bifunctional electrocatalyst (CoTBrPP@bio-C) for an all-pH ORR and HER by using a simple one-pot co-pyrolysis over a mushroom (MR) template with adsorbed Br-substituted porphyrinato cobalt (CoTBrPP). Introduction of CoTBrPP onto the MR template not only leads to a larger specific area of 604 m² g⁻¹ in CoTBrPP@bio-C relative to that of CoTPP@bio-C (CoTPP = Br-free CoTBrPP), 98 m² g⁻¹, but more importantly also regulates the oriented distribution of pores precisely which promotes effective exposure of multiple active sites depending on the departure of Br-species attached to the Co–N₄ macrocycle periphery. Impressively, CoTBrPP@bio-C with a Co loading down to 0.77 at% exhibits higher all-pH ORR and HER performance compared to CoTPP@bio-C with a Co loading of 3.5 at%. An efficient TMAAE makes CoTBrPP@bio-C possess higher onset and half-wave potentials of 0.93 and 0.85 V for the ORR, and a smaller Tafel slope of 80 mV dec⁻¹ for the HER in an alkaline medium. CoTBrPP@bio-C also had a superior all-pH long-term stability and outstanding methanol tolerance, surpassing commercial Pt/C and most non-precious-metal catalysts reported to date. Furthermore, Zn–air batteries assembled with CoTBrPP@bio-C exhibited a higher peak power density of 100 mW cm⁻² and excellent durability than Pt/C.