Abstract
The synthesis of metal-free carbon-based electrocatalysts for oxygen reduction reactions (ORR) to replace conventional Pt-based catalysts has become a hot spot in current research. This work proposes an activation-assisted carbonization strategy, to manufacture N-doped ultra-thin carbon nanosheets (GWS180M800) with high catalytic activity, namely, melamine is used as an accelerator/nitrogen source, and walnut green peels biological waste as a carbon source. The melamine acts as a nitrogen donor in the hydrothermal process, effectively enhancing the nitrogen doping rate. The content of pyridine nitrogen groups accounts for up to 48.5% of the total nitrogen content. Electrochemical tests show that the GWS180M800 has excellent ORR electrocatalytic activity and stability, and makes a quasi-four-electron ORR pathway clear in the alkaline electrolyte. The initial potential and half slope potential are as high as 1.01 and 0.82 V vs. RHE, respectively. The GWS180M800 catalyst has a better ability to avoid methanol cross poisoning than Pt/C has. Compared with 20 wt% Pt/C, GWS180M800 has improved methanol tolerance and stability. It is a metal-free biochar ORR catalyst with great development potential and application prospects. This result provides a new space for the preparation of valuable porous nano-carbon materials based on carbonaceous solid waste and provides new ideas for catalyzing a wide range of electrochemical reactions in the future.
Highlights
The results show that after activation, a large number of organic pores are formed, a higher specific surface area, and the N doping rate (10.46%) and defects have been significantly improved, which is of great significance to the improvement of catalyst activity
What’s more, the linear sweep voltammetry (LSV) curve (Figure 2b) records the saturated KOH solution obtained by O2 at a speed of 1600 rpm to further understand the catalytic activity of GWS800, GWS180M700, GWS180M800, GWS180M900, and
GWS800 showed a large Tafel slope of 217 mV dec−1, indicating a poor oxygen reduction reactions (ORR) dynamic process. These results indicate that the optimal pyrolysis temperature is essential for enhancing electrochemical activity [39]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In the process of walnut kernel processing, a large amount of green walnut peel is wasted because it is not effectively used. Converting it into high value-added ORR catalyst products has huge environmental and economic benefits. This research will provide an easy-to-implement method to prepare a stable ORR catalyst from renewable waste green walnut hull biomass and provide an innovative strategy for the production of biomass. This strategy converting agricultural and forestry wastes into high-value-added products is simple, low-cost, and easy to promote. The mechanism can be described as follows: O2 +4H + +4e− → 2H2 O
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