Zn‐Based Oxides Anchored to Nitrogen‐Doped Carbon Nanotubes as Efficient Bifunctional Catalysts for Zn‐Air Batteries

Zn−Co oxide (ZnCoO x ), Zn−Mn oxide (ZnMnO x ), Zn−Mn−Co oxide (ZMCO), and Zn−Co−Fe oxide (ZCFO) nanoparticles were successfully synthesized on nitrogen‐doped carbon nanotubes in a one‐pot process. Porous carbon paper was simultaneously impregnated with the catalysts during synthesis and used as air electrodes for Zn‐air batteries. ZnMnO x /N‐CNT catalysts had the best ORR performance in half‐cell LSV experiments with a more positive onset potential than that of Pt−Ru/C (−0.067 V and −0.078 V vs Hg/HgO, respectively). ZCFO/N‐CNT catalysts had the best activity towards OER among the Zn‐based oxide catalysts in half‐cell linear sweep voltammetry (LSV) testing with an onset potential of 0.62 V vs Hg/HgO. Round‐trip efficiencies from battery rate tests at a current density of 20 mA cm −2 were 55.3 %, 57.5 %, 58.7 %, and 58.3 % for ZnCoO x /N‐CNT, ZnMnO x /N‐CNT, ZMCO/N‐CNT, and ZCFO/N‐CNT, respectively. Bifunctional cycling of the catalysts was done in a homemade Zn‐air battery at a current density of 10 mA cm −2 for 200 cycles. The final round trip efficiencies for ZnCoO x /N‐CNT, ZnMnO x /N‐CNT, ZMCO/N‐CNT, and ZCFO/N‐CNT were 55.8 %, 56.6 %, 54.2 %, and 55.0 %, respectively. All catalysts except ZCFO/N‐CNT compared favorably with Pt−Ru/C in terms of round‐trip efficiency after cycling (55.3 %). Incorporation of Zn into the metal oxide particles showed improved catalytic activity for ZnCoO x /N‐CNT and ZnMnO x /N‐CNT compared with MnO x /N‐CNT and CoO x /N‐CNT catalysts prepared via the same technique.