Tuning NiCo2O4 bifunctionality with nitrogen-doped graphene nanoribbons in oxygen electrocatalysis for zinc-air battery application

One of the major barriers to the widespread use and commercialization of zinc-air batteries (ZAB) is the use of inefficient and expensive bifunctional materials for oxygen reduction reaction (ORR) and oxygen evolution reac-tion (OER) catalysis on the air electrode. Herein, we propose a highly durable ORR/OER bifunctional catalyst, which was thermally prepared using NiCo2O4 blended with nitrogen-doped graphene nanoribbons (GNRN). We evaluated the activity and electrochemical stability of catalysts with different ratios of NiCo2O4 and GNRN as well as the role of each component in the electrocatalytic process to find the most efficient catalyst for both ORR and OER. The ORR results revealed that the NiCo2O4(80):GNRN(20) and NiCo2O4(90):GNRN(10) novel catalysts exhibited a half-wave potential of ca. 0.79 VRHE in a 4-electron pathway and were practically unaf-fected by the presence of methanol. With regard to OER, the unblended NiCo2O4 catalyst exhibited the lowest potential required to reach 10 mA cm-2 (1.60 VRHE), followed by NiCo2O4(90):GNRN(10) (1.62 VRHE), and NiCo2O4(80):GNRN(20) (1.68 VRHE); however, the presence of tuned amount of N-doped graphene proved to be crucial for enhancing charge transfer, stability, and the ORR/OER bifunctionality of catalysts. After applying NiCo2O4/GNRN materials and commercial Pt-Ru/C as catalysts in ZAB, the results showed that NiCo2O4(80): GNRN(20) and NiCo2O4(90):GNRN(10) exhibited maximum power densities of 78 and 69 mW cm-2, respec-tively; these values were quite similar to that obtained for Pt-Ru/C (81 mW cm-2). The specific capacity for Pt-Ru/C-based ZAB was 707 mAh gZn-1, while both NiCo2O4/GNRN-based ZABs showed 702 mAh gZn-1 (86 % of the theoretical capacity). Moreover, long-term operation tests involving high current densities and charge-dis-charge ZAB cycling revealed that NiCo2O4/GNRN catalysts are clearly more suitable for use at ZAB air electrode than Pt-Ru/C and are as efficient as other non-precious metal catalysts of similar kind which have been reported in the literature.