In secondary metal-air batteries, it is indispensable to develop novel bifunctional catalysts for the low overpotential oxygen reduction/oxygen evolution reaction to allow on the one hand very high energy density combined with high capacity and low price.^1,2 Therefore, the widespread use of rechargeable metal-air batteries strongly depends on the substitution of precious metal-based bifunctional electrocatalysts towards the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) with catalysts from high-abundant materials.³ This seems to be possible since the electrolyte in typical Ni- or Zn-metal air batteries is highly alkaline.

Here, we report a novel Co/Co-Fe nanoparticle/N-doped carbon composite (Co/Co_xFe_y/NC) obtained by thermal treatment of a composite of Co-Fe layered double hydroxide (Co-Fe LDH) and polybenzoxaxine (pBO) under ammonia atmosphere. The latter undergoes thermal degradation to form a conductive nitrogen-doped carbon matrix encapsulating Co/Co-Fe nanoparticles thereby circumventing the use of any binder material to immobilize the catalyst particles on the electrode surface.⁴ Within this work, the influence of the temperature, duration of treatment and the nature of gas atmosphere employed during the thermal treatment, as well as the ratio Co-Fe LDH:pBO on the electrocatalytic activity performance were investigated, leading to identification of an optimal Co/Co_xFe_y/NC catalyst exhibiting remarkable bifunctional activity towards ORR and OER in 0.1 M KOH with an overpotential difference between these two reaction as low as 0.76 V at -1 mA cm^-2 for the ORR and at 10 mA cm^-2 for the OER, respectively. Moreover, the stability of Co/Co_xFe_y/NC catalyst was evaluated in an air-breathing set-up, alternating between the OER and the ORR at a current density of 10 mA cm^-2 in 6 M KOH to simulate the charge discharge cycles in rechargeable metal-air batteries for a total of 120 charge-discharge cycles each lasting 10 minutes.

References:

[1] Y. Li, H. Dai, Chem. Soc. Rev. 43 (2014) 5257-5275.

[2] J. Fu, Z.P. Cano, M.G. Park, A. Yu, M. Fowler, Z. Chen, Adv. Mater. 29 (2017) 1-34.

[3] J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grützke, P. Weide, M. Muhler, W. Schuhmann, Angew. Chem. Int. Ed. 53 (2014) 8508-8512.

[4] C. Andronescu, S. Barwe, E. Ventosa, J. Masa, E. Vasile, B. Konkena, S. Möller, W. Schuhmann, Angew. Chem. Int. Ed. 56 (2017) 11258-11262.