Sustainable Route for Molecularly Thin Cellulose Nanoribbons and Derived Nitrogen-Doped Carbon Electrocatalysts

Ultrathin cellulose nanoribbons were extracted from earth-abundant biomass using 2,2,6,6-tetramethylpiperidine-1-oxylcatalyzed (TEMPO-catalyzed) oxidation and sonication processes. By two TEMPO-oxide systems with different processing times, TEM and AFM observations indicate the obtained cellulose nanoribbons (Cel-NRs) with dimensions of 400-800 nm in length, 1.72-2.54 nm in width, and 0.78-2.67 nm in thickness. The dimension data indicate that the Cel-NRs from the TEMPO/NaBr/NaC1O system are much shorter but contain more cellulose chains than those from the TEMPO/NaClO/NaClO2 system. Moreover, these abundant biomass nanoribbons were fabricated from direct pyrolysis with NH3 activation. The obtained highly active nitrogen-doped carbon nanoribbons (N-CNRs) and metal-free oxygen reduction reaction (ORR) electrocatalysts show superb ORR activity (half-wave potential of 0.71 and 0.73 V versus reversible hydrogen electrode) and high selectivity (electron-transfer number of 3.26 and 3.74 at 0.8 V), comparable current density and onset potential (0.906 and 0.926 V), excellent electrochemical stability (higher than 89.5% and 91.6% after 20 000 potential cycles) in alkaline media, and better resistance to crossover effects in the ORR. More importantly, when used as a cathode catalyst for constructing the air electrode of the Zn air battery, the N-CNRs exhibit super. long-term stability and a capacity of 587 and 583 mAh g(-1) at the discharge current densities of 5 and 20 mA cm(-2), respectively, which are highly comparable with those of the state-of-the-art Pt/C catalyst (20 wt % Pt, Hispec 3000). This indicates that our present work is the first example of using atomically thin carbon nanoribbons as the metal-free electrocatalyst substitution to Pt for developing high-performance metal air batteries from earth abundant terrestrial plants.