Anion effect on oxygen reduction reaction activity of nitrogen doped carbon nanotube encapsulated cobalt nanoparticles

In this work, we report the diverse cobalt precursors for the synthesis of nitrogen doped carbon nanotubes encapsulated Co nanoparticles (Co@NCNT) as oxygen reduction reaction (ORR) electrocatalyst to verify the anion effect. The X-ray photoelectron spectroscopy (XPS) test indicates that more pyridinic and pyrrolic N species coordinate with Co atoms for cobalt chloride (CoCl2) due to more defective sites on NCNT etched by chlorine compared to the counterparts of cobalt nitrate (Co(NO3)2), cobalt acetate (Co(Ac)2) and cobalt triacetoacetate (Co(acac)2). With higher content of Co-Npyridinic+pyrrolic structures, Co@NCNT-Cl exhibits a robust ORR performance highlighted by half-wave potential of 880 mV vs. RHE, corresponding to a 4-fold better kinetic current density than commercial Pt/C. The superior catalytic activity toward ORR originates from more Co-Npyridinic+pyrrolic structures efficiently hydrogenated the adsorbed O2 to *OOH intermediate; additionally, its low binding strength with *OOH allows the mobility to metallic Co for splitting to generate O* species. Thus, a relay ORR catalysis is noticed for Co@NCNT-Cl. The assembled rechargeable zinc-air battery (ZAB) reaches a power density of 152 mW cm-2 sustained for 500 h, superior to Pt/C-IrO2. Moreover, the all-solid-state ZAB delivers a maximum power density of 54 mW cm-2 and maintains for 30 h.