Ternary (N, B, F)-Doped Biocarbon Derived from Bean Residues as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

Abstract

Development of efficient metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of great significance for Zn-air batteries. Herein, a porous biocarbon bifunctional catalyst (C-NBF-G) was directly synthesized via simple alkali activation and carbonization from bean residues. C-NBF-G exhibited hierarchical porous structures, ternary heteroatom (N, B, and F) doping, a large specific surface area, and a relatively high degree of graphitization. The synergistic action of these characteristics contributed to the outstanding catalytic properties of C-NBF-G for ORR and OER. The catalyst demonstrated an onset potential of 0.94 V, half-wave potential of 0.824 V, and a limiting current density of 5.92 mA cm-2, comparable to those of the commercial 20 wt% Pt/C catalysts. C-NBF-G also exhibited an OER overpotential of 333 mV at 10 mA cm-2 and a Tafel slope of 114 mV dec-1, lower than those of the commercial Pt/C and RuO2 catalysts. These results proved the promising performance of C-NBF-G as a bifunctional catalyst for the ORR and OER.

A brief review on solar charging of Zn–air batteries

“…Even though, graphene and carbon nanotubes remain always popular, [36][37][38] in recent years, there is a strong interest expressed towards carbon electrocatalysts obtained by pyrolysis of carbon-rich substrates including biomass-derived wastes. [39][40][41][42][43][44][45][46][47][48] The three principal properties of any electrocatalyst are electric conductivity, high active surface and presence of active sites which facilitate electron transfer from the solid to the liquid phase. Carbon based materials are conductive, except of course in rare crystalline forms like diamond, and when appropriately treated they are porous and have high active surface.…”

“…Usual dopants of metal-free electrocatalysts are N, P, F, B and S. For this reason, most recent works focus on dopping porous carbon. [36][37][38][39][40][41]43,45,48 These metal free bifunctional electrocatalysts sometimes outperformed noble metal electrocatalysts and provided long time stability under recyclyning of Zn-air batteries and a small DV gap between charging and discharging. 39 Biochars obtained by pyrolysis of biomass and further treated with KOH at high temperature obtain a hierarchical porous structure of very high active surface.…”

“…39 Biochars obtained by pyrolysis of biomass and further treated with KOH at high temperature obtain a hierarchical porous structure of very high active surface. 49 Such materials have so far received small attention as OER/ORR electrocatalysts 40,42,50,51 but it is expected that they will become much more popular in the near future. Their hierarchical micro-meso-macro-porous structure (cf.…”