A metal-organic framework-derived bifunctional catalyst for hybrid sodium-air batteries

Metal-organic framework (MOF)-derived carbon nanomaterials are investigated as promising non-noble metal-based oxygen electrocatalysts for metal-air batteries. Herein, metal-organic framework-derived N-doped carbon nanotubes (MOF-NCNTs) were first employed as electrocatalysts for hybrid sodium-air batteries (SABs), which exhibited higher electrocatalytic activity and stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) compared to commercial Pt/C. The battery using MOF-NCNTs displayed the voltage gap of 0.30 V at a current density of 0.1 mA.cm(-2), which is the lowest among all the tested catalysts including commercial Pt/C (0.50 V), RuO2 (0.50 V), Co-CNTs (0.67 V), NCNTs (0.77 V), MWNTs (0.90 V), and carbon paper (1.18 V). In addition, the average discharge plateau and round trip efficiency of the battery was 2.81 V and 87% during 35 cycles at a current density of 0.1 mA.cm(-2), respectively. The remarkable electrocatalytic activity is mainly ascribed to the synergistic effect between the N dopants and confined Co nanoparticles in the CNTs, the hollow structure of NCNTs, and the robust porous cage structure. The N dopants and confined Co nanoparticles in the CNTs induce more catalytic active sites and promote electron transfer for the ORR and OER. The hollow framework structure of NCNTs not only offer structural defect sites for O-2 adsorption, but also improves mass transport and electronic conductivity, resulting in enhanced catalytic activity. The robust porous cage structure contributes to the stability of the catalysts. The highly efficient and inexpensive metal-organic framework-derived NCNT is a promising bifunctional oxygen electrocatalyst for practical applications in hybrid SABs and other metal-air batteries.