Controllable Hortensia-like MnO2 Synergized with Carbon nanotubes as an Efficient Electrocatalyst for Long-term Metal-air Batteries.

The exploitation of a high-activity and low-cost bifunctional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as cathode catalyst is a research priority in metal-air batteries (MeABs). Herein, a novel bifunctional hybrid catalyst of hortensia-like MnO2 synergized with carbon nanotubes (CNTs) (MnO2/CNTs) is controllably synthesized by reasonably designing the crystal structure and morphology as well as electronic arrangement. Based on these strategies, the hybrid accelerates the reaction kinetics and avoids the change of structure. As expected, MnO2/CNTs exhibit remarkable ORR and OER activity (low ORR Tafel slope: 71 mV dec-1, low OER Tafel slope: 67 mV dec-1, and small potential difference (ΔE): 0.85 V) and long-term stability, which should be attributed to its unique morphology, K+ ions in the 2×2 tunnels, and synergistic effect between MnO2 and CNTs. Notably, in zinc-air batteries (ZABs), aluminum-air batteries (AABs) and magnesium-air batteries (MABs), the composite shows high power density (ZABs: 243 mW cm-2, AABs: 530 mW cm-2 and MABs: 614 mW cm-2) and large specific capacities (793 mAh gZn-1, 918 mAh gAl-1 and 878 mAh gMg-1). Importantly, the rechargeable ZABs reveal small charge/discharge voltage drop (0.81 V) and strong cycle durability (500 hours), which are better than the noble-metal Pt/C+IrO2 mixture catalyst (the voltage drop: 1.15 V at initial and 2 V after 100 hours). These superior performances together with the simple synthetic method of the hybrid reveal great promise in large-power energy storage and conversion equipment.