Plasma tailored reactive nitrogen species in MOF derived carbon materials for hybrid sodium–air batteries

The rational design of efficient and durable electrocatalysts to accelerate sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics is highly desirable for enhancing the efficiency of fuel cells and metal-air batteries. Here, we demonstrated a low-temperature plasma strategy at atmospheric pressure for enhancing the catalytic activity of metal-organic framework derived N-doped carbon nanotubes (MOF-NCNTs) by changing the relative contents of Co-Nx sites, Co-Co bonds and pyridinic-N. The increase of pyridinic-N/pyrrolic-N ratio improves the ORR performance, while unsaturated Co-Nx sites and strong Co-Co bonds promote the OER performance. The relative contents of pyridinic-N, Co-Nx sites, and Co-Co bonds in MOF-NCNTs can be readily tailored by varying the plasma treatment time. The MOF-NCNTs treated with N2 plasma for 4 min (MOF-NCNTs-N2-4) exhibited improved ORR (ηonset: 0.91 V) and OER (η10: 0.44 V) activities compared to MOF-NCNTs because of the higher ratio of pyridinic-N to pyrrolic-N and higher relative contents of Co-Nx sites and Co-Co bonds. The hybrid sodium-air batteries (HSABs) assembled with MOF-NCNTs-N2-4 catalyst display a low overpotential of 0.35 V and excellent round trip efficiency of 88.9% at 0.1 mA cm-2. Besides, they also exhibited great cycling stability with an average discharge voltage of 2.75 V and an outstanding round trip efficiency of 84% after 150 cycles.