Cation exchange synthesis of NixCo(3−x)O4 (x = 1.25) nanoparticles on aminated carbon nanotubes with high catalytic bifunctionality for the oxygen reduction/evolution reaction toward efficient Zn–air batteries

Abstract

Proper control of the size and composition distribution and oxidation state of metal ions is of great importance to develop mixed transition metal oxides with advanced catalytic activities for oxygen reduction (ORR) and evolution reactions (OER), both of which are pivotal processes related to many renewable energy related technologies. Herein, a cation exchange reaction is employed for the synthesis of Ni_xCo_(3−x)O₄ nanoparticles (NPs) supported on aminated carbon nanotubes (Ni_xCo_(3−x)O₄/NH₂-CNTs). The obtained Ni_xCo_(3−x)O₄/NH₂-CNTs shows an enhanced catalytic bifunctionality for the ORR and OER. Specifically, the Ni_xCo_(3−x)O₄/NH₂-CNTs can deliver ORR onset (E_onset) and half-wave potentials (E_1/2) of 0.954 and 0.863 V vs. RHE, respectively, and only needs an overpotential of 386 mV to deliver a current density of 10 mA cm⁻². The ORR catalytic activity of the Ni_xCo_(3−x)O₄/NH₂-CNTs is comparable to that of the Pt/C (with an E_onset and E_1/2 of 0.964 and 0.858 V vs. RHE, respectively), while its OER activity is higher than that of RuO₂/C. Additionally, the Ni_xCo_(3−x)O₄/NH₂-CNTs exhibits a potential difference of 0.764 V in the OER current density at 10 mA cm⁻² and the ORR current density at 3 mA cm⁻², which is lower than those of most state-of-the-art catalysts. Most interestingly, the Zn–air battery with the Ni_xCo_(3−x)O₄/NH₂-CNTs shows higher durability and performance than the batteries with Pt–RuO₂/C (an integrated catalyst consisting of Pt/C and RuO₂/C). These results clearly demonstrate that the Ni_xCo_(3−x)O₄/NH₂-CNTs is a durable and efficient bifunctional catalyst with great potential for practical applications.