N-doped spherical activated carbon from dye adsorption: Bifunctional electrocatalyst for hydrazine oxidation and oxygen reduction

Considering that state-of-the-art on energy conversion devices is focused on the development of efficient and low-cost electrocatalysts, noble metal-free carbon materials have been studied as promising substitutes for commercial platinum-based catalysts. Given the importance of sustainability and minimization of the waste generation, we propose not to adopt an exclusive methodology for the new catalyst preparation, but to use a spherical activated carbon saturated with methylene blue molecules (SACsat) after the adsorption process. A brief thermal treatment of SACsat in N2 atmosphere at 635 °C for 30 min was performed to incorporate N atoms on carbon lattice, creating defects and active sites to improve the electron conductivity. The N-doped SAC (N-SAC) showed BET surface area of 661 m2 g−1 and 1.1% of nitrogen in its elemental composition, distributed in pyrrolic (51.0%), pyridinic (29.2%) and graphitic groups (19.8%). Electrocatalytic performances of bare SAC and N-SAC were tested over hydrazine oxidation and oxygen reduction reactions. The N-SAC promoted total oxidation of hydrazine and the total reduction of oxygen, since the calculated number of electrons transferred for such reactions was of approximately four. In addition, the N-SAC electrochemical responses as a bifunctional electrocatalysts were comparable or even better than other catalysts containing noble metals.