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

Abstract

• Composites thermally prepared from NiCo 2 O 4 and nitrogen-doped graphene nanoribbons; • Tuning the NiCo 2 O 4 and N-doped graphene ratio tailors the bifunctional activity for ORR and OER; • NiCo 2 O 4 /GNRN-based catalysts as efficient bifunctional catalysts for air electrode of ZAB; • Presence of GNRN proved to be crucial for enhancing the catalysts bifunctionality. 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 reaction (OER) catalysis on the air electrode. Herein, we propose a highly durable ORR/OER bifunctional catalyst, which was thermally prepared using NiCo 2 O 4 blended with nitrogen-doped graphene nanoribbons (GNRN). We evaluated the activity and electrochemical stability of catalysts with different ratios of NiCo 2 O 4 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. ORR results revealed that the NiCo 2 O 4 (80):GNRN(20) and NiCo 2 O 4 (90):GNRN(10) novel catalysts exhibited a half-wave potential of ca. 0.79 V RHE in a 4-electron pathway and were practically unaffected by the presence of methanol. With regard to OER, the unblended NiCo 2 O 4 catalyst exhibited the lowest potential required to reach 10 mA cm −2 (1.60 V RHE ), followed by NiCo 2 O 4 (90):GNRN(10) (1.62 V RHE ), and NiCo 2 O 4 (80):GNRN(20) (1.68 V RHE ); 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 NiCo 2 O 4 /GNRN materials and commercial Pt-Ru/C as catalysts in ZAB, the results showed that NiCo 2 O 4 (80):GNRN(20) and NiCo 2 O 4 (90):GNRN(10) exhibited maximum power densities of 78 and 69 mW cm -2 , respectively; 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 g Zn -1 , while both NiCo 2 O 4 /GNRN-based ZABs showed 702 mAh g Zn -1 (86% of the theoretical capacity). Moreover, long-term operation tests involving high current densities and charge-discharge ZAB cycling revealed that NiCo 2 O 4 /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.