Hole Defect-Rich Nitrogen Doped Carbon Nanotubes for Efficient Oxygen Reduction and Evolution Reactions in Rechargeable Zinc-Air Battery

Abstract

Environmental concerns arising from the use of conventional fuel sources have driven the demand and development of eco-friendly vehicles. However, determining the right battery to use in them remains challenging. As an alternative to Li-ion batteries, Zn–air batteries are promising, but limited by their charge-discharge cycling efficiency and safety issues. Hence, in this study, nitrogen-doped holey carbon nanotubes (N-HCNTs) were fabricated for use as a cathode material of Zn–air batteries. Holes were formed in the CNT by reacting it with hydrogen peroxide, following which the oxidized CNT was pyrolyzed with urea to form N-HCNT. Nitrogen-doped CNTs (N-CNTs) were also produced by pyrolyzing acid-treated CNTs with urea to compare their electrocatalytic activities in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with those of N-HCNT. N-HCNT exhibited much better electrocatalytic activity toward the ORR and OER than N-CNT. The charge transfer resistance and density functional theory calculation indicated that the high electrocatalytic activity of the N-HCNT is due to its high electrical conductivity. The electrocatalytic activity of N-HCNT was compared with those of Pt/C and IrO2; N-HCNT exhibited excellent bifunctional electrocatalytic activity in both, the ORR and OER, while Pt/C and IrO2 exhibited excellent electrocatalytic activity only in the ORR and OER, respectively. Further, the maximum power density of a Zn–air rechargeable battery using the N-HCNT electrode was 17.6% higher than that of a battery using a mixture of Pt/C and IrO2 (1:1) as the electrode. Moreover, after 500 cycles, the change in the charge-discharge voltage gap of the battery using the N-HCNT electrode (18.3%) was much smaller than that of the battery using the mixture of Pt/C and IrO2 (118.1%). The excellent electrocatalytic activity and cycle life of the N-HCNT electrode in Zn–air battery demonstrate its promise in the development of future energy devices.