Rupturing Cotton Microfibers into Mesoporous Nitrogen-Doped Carbon Nanosheets as Metal-Free Catalysts for Efficient Oxygen Electroreduction

Mechanical grinding is exploited to effectively rupture biomass cotton microfibers into metal-free, nitrogen-doped carbon nanosheets with a large number of mesoporous textures. Experimentally, raw microfibers of absorbent cotton are presoaked with fuming sulfuric acid to generate plenty of hierarchical pores/cavities, which sufficiently expose the inner parts of cotton microfibers to nitrogen source for efficient incorporation of nitrogen dopants onto carbon skeletons in subsequent thermal annealing process. Mechanical grinding of these thermally annealed carbon microfibers leads to exfoliated nitrogen-doped thin carbon nanosheets with a high surface area of 912.1 m2/g as well as abundant mesopores and a considerable nitrogen content of 8.5 at. %. These characteristics contribute to an excellent electrocatalyst with marked catalytic activities toward oxygen reduction reaction in an alkaline electrolyte solution, including a more positive half-wave potential, much higher diffusion-limiting current, remarkably enhanced operation stability, and stronger immunity against fuel-crossover effects, as compared to commercial Pt/C catalysts. The present results provide a novel facile method to the scalable preparation of biomass-derived highly porous two-dimensional carbons for efficient electrochemical energy devices.