Promotion of the bifunctional electrocatalytic oxygen activity of manganese oxides with dual-affinity phosphate

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

Effect of Thermally Induced Oxygen Vacancy of α-MnO2 Nanorods toward Oxygen Reduction Reaction

MnO₂ has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO₂ crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO₂ has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO₂ nanorods were explored for the first time under thermal treatment with different temperatures (300, 400, and 500 °C). Under thermal treatment, oxygen vacancies were induced. The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO₂ materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO₂ nanorods were strengthened with the temperature increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO₂ nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO₂ nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.

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.

One-pot achievement of MnO2/Fe2O3 nanocomposites for the oxygen reduction reaction with enhanced catalytic activity

MnO₂/Fe₂O₃ nanocomposites were achieved in one-pot followed by high-temperature treatment, which presented excellent electrocatalytic activity for the oxygen reduction reaction.