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 the cathode catalyst is a research priority in metal-air batteries. 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. On the basis of these strategies, the hybrid accelerates the reaction kinetics and avoids the change of structures. As expected, MnO₂/CNTs exhibit a 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 (Δ E): 0.85 V] and a 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 mA h g_Zn^-1, 918 mA h g_Al^-1, and 878 mA h g_Mg^-1). Importantly, the rechargeable ZABs reveal small charge-discharge voltage drop (0.81 V) and strong cycle durability (500 h), which are better than the noble-metal Pt/C + IrO₂ mixture catalyst (the voltage drop: 1.15 V at first and 2 V after 100 h). These superior performances together with the simple synthetic method of the hybrid reveal great promise in large-power energy storage and conversion equipment.