Mei Wang

Traditional noble metal platinum (Pt) is regarded as a bifunctional oxygen catalyst due to its highly catalytic efficiency, but its commercial availability and application is often restricted by high cost. Herein, a cheap and effective catalyst mixed with α-MnO₂ and nitrogen-doped Ketjenblack (N-KB) (denoted as MnO₂-SM150-0.5) is examined as a potential electrocatalyst in oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). This α-MnO₂ is prepared by redox reaction between K₂S₂O₈ and MnSO₄ in acid conditions with a facile hydrothermal process (named the SM method). As a result, MnO₂-SM150-0.5 exhibits a good catalytic performance for ORR in alkaline solution, and this result is comparable to a Pt/C catalyst. Moreover, this catalyst also shows superior durability and methanol tolerance compared with a Pt/C catalyst. It also displays a discharge voltage (~1.28 V) at a discharge density of 50 mA cm⁻² in homemade Al–air batteries that is higher than commercial 20% Pt/C (~1.19 V). The superior electrocatalytic performance of MnO₂-SM150-0.5 could be attributed to its higher Mn³⁺/Mn⁴⁺ ratio and the synergistic effect between MnO₂ and the nitrogen-doped KB. This study provides a novel strategy for the preparation of an MnO₂-based composite electrocatalyst.

Scalable, low-cost and highly efficient catalysis of oxygen electrocatalytic reactions (ORR/OER) are required for the rapid development of clean and renewable energy conversion/storage technologies. Herein, two types of α-MnO₂ nanorods were prepared under hydrothermal treatment at 150 °C for 0.5 h (MnO₂-150-0.5) or 120 °C for 12 h (MnO₂-120-12), then supported on N-doped ketjenblack carbon (N-KB) as bi-functional ORR/OER catalysts. Their electrocatalytic activities toward ORR and OER were investigated systematically. As a result, MnO₂-150-0.5/N-KB displays superior ORR catalytic activity, with much more positive half-wave potential and much larger limiting current density (0.76 V and 6.0 mA cm⁻²), comparable to those of 20 wt. % Pt/C (0.82 V and 5.10 mA cm⁻²). MnO₂-150-0.5/N-KB also shows high electron transfer number (3.86~3.97) and low yield of peroxides (1–7%) during ORR process in the whole potential range of 0–1.0 V (vs. RHE). Meanwhile, the MnO₂-150-0.5/N-KB also exhibits better OER activity with low overpotential, comparable to IrO₂/N-KB. The excellent electrocatalytic activity of MnO₂-150-0.5/N-KB can be attributed to the synergistic effect, relatively smaller size, higher amount of Mn³⁺, and low charge transfer resistance. This work offers a new strategy for scalable preparation of more efficient and cost-effective α-MnO₂ bi-functional oxygen catalysts.