The search for low-cost, highly active, and stable catalysts to replace the Pt-based catalysts for oxygen reduction reaction (ORR) has recently become a topic of interest. Herein, we report a new strategy to design a nitrogen-doped carbon nanomaterial for use as a metal-free ORR catalyst based on facile pyrolysis of protein-rich enoki mushroom (Flammulina velutipes) biomass at 900 °C with carbon nanotubes as a conductive agent and inserting matrix. We found that various forms of nitrogen (nitrile, pyrrolic and graphitic) were incorporated into the carbon molecular skeleton of the product, which exhibited more excellent ORR electrocatalytic activity and better durability in alkaline medium than those in acidic medium. Remarkably, the ORR half-wave potential measured on our material was around 0.81 V in alkaline medium, slightly lower than that on the commercial 20 wt% Pt/C catalyst (0.86 V). Meanwhile, the ORR followed the desired 4-electron transfer mechanism involving the direct reduction pathway. The ORR performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon-based catalysts. Besides, we significantly proposed that the graphitic-nitrogen species that is most responsible for the ORR activity can function as the electrocatalytically active center for ORR, and the pyrrolic-nitrogen species can act as an effective promoter for ORR only. The results suggested a promising route based on economical and sustainable fungi biomass towards the large-scale production of valuable carbon nanomaterials as highly active and stable metal-free catalysts for ORR under alkaline conditions.

The improvement of total nitrogen content and nitrogen-doping efficiency in carbon-based electrocatalysts is greatly significant to boost the electrocatalytic activity for the oxygen reduction reaction (ORR). Here, we report a new strategy for the synthesis of a highly mesoporous cobalt and nitrogen co-doped carbon electrocatalyst (3D-Co–N–C) with a three-dimensional network structure and a high BET surface area (∼638 m² g⁻¹) via using a novel cobalt-2,4,6-tri(2-pyridyl)-1,3,5-triazine complex with a strong molecular-level coordination effect as a single-source precursor and self-assembled sodium chloride aggregates as a space-confined nanoreactor for effective control of a high-temperature calcination process to reduce the thermal loss of nitrogen atoms and promote the nitrogen-doping efficiency, facilitating boosting of the ORR electrocatalytic activity in alkaline medium. The prepared 3D-Co–N–C catalyst exhibits unexpectedly excellent ORR activity with an onset potential of ∼1.0 V and a half-wave potential of ∼0.83 V, which is comparable to that of the commercial 20 wt% Pt/C catalyst. Additionally, the H₂O₂ yield (<17.0%) and the average electron transfer number of ∼3.8 for 3D-Co–N–C indicate a quasi four-electron pathway for ORR catalysis, suggesting that 3D-Co–N–C is a promising carbon-based electrocatalyst. It is proposed that the formation of the Co–N_x active structure can effectively enhance the electrocatalytic activity, but high contents of pyridinic- and graphitic-N can be mainly responsible for the ORR activity, which may be the electrocatalytically active site centers for the ORR. Besides, high BET surface area, highly mesoporous characteristics and outstanding electronic conductivity are also significant for the improvement of ORR activity. This study can provide a new, facile and green method for building high-performance carbon-based ORR electrocatalysts derived from easily available and innoxious transition metal–organic complexes, which can also help us to better understand the origin of the activity, active sites and their catalysis mechanism.