Co and Co9S8 nanoparticles uniformly embedded in S, N-doped porous carbon as electrocatalysts for rechargeable zinc-air batteries

Co and Co9S8 nanoparticles uniformly embedded in S, N-doped porous carbons (Co/Co9S8@SNC) were fabricated by pyrolytic treatments of [Co(tdc)(bpy)]n2 (Co2+ is taken as the central ion, 2,5-thiophenedicarboxylic acid (tdc) and 4,4-bipyridine (bpy) are taken as the organic ligands) at 3 temperatures: 800 °C, 900 °C and 1000 °C. Among samples obtained, Co/Co9S8@SNC-900 from pyrolysis at 900 °C shows the highest proportion of Co and N atoms in form of Co nanoparticles and Pyridine-N. Due to the coordination between Pyridine-N and Co atoms, Co/Co9S8@SNC-900 has the best ORR/OER bifunctional electrocatalytic activity, conductivity and stability with the aid of the synergy of Co9S8 nanoparticles and C-S-C. Electrochemical test results show that the ORR limit current density of Co9S8@SNC-900 is 5.2 mA cm−2, which is close to commercial Pt/C (20 wt.%). In the OER tests, the overpotential of Co/Co9S8@SNC-900 at 10 mA cm−2 is 0.31 V, which is lower than that of RuO2. Co/Co9S8@SNC-900 also shows the best conductivity and pore volume. In application of Zn-air batteries (ZABs), Co/Co9S8@SNC-900 shows better maximum power density (106.6 mW cm−2) and stability than Pt/C + RuO2 catalyst.