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Different forms of δ–MnO2 were constructed with their reduction sites anchored on non-support or other supports. Morphology, composition, and electrochemical properties of MnO2 and its hybrids were analyzed. Results revealed that MnO2/reduced graphene oxide (rGO) layered hybrid showed a higher BET surface area due to the synergistic effect on the introduction of rGO and the morphologic transformation of anchored MnO2. MnO2/rGO–2 exhibits a higher ratio of Mn3+: Mn4+ content, a better adsorption performance for O2, and a lower charge-transfer resistance when respect to pristine δ–MnO2. Compared with other MnO2/rGO hybrids, porous MnO2/rGO-2 layered hybrid exhibits the best electrochemical property: a closer four-electron process (3.95), a more positive E onset (0.92 V vs RHE), and superior long-term durability (98.5% after 25,000 s). Especially, a mechanically reusable Mg–air battery with MnO2/rGO–2 cathode catalyst only exhibits a 5% decay for 264 h.

We synthesized MnO2/CeO2 electrocatalysts by in situ decoration of α–MnO2 with CeO2 particles during a one-step hydrothermal process. The morphology, composition, and electrochemical properties were studied in the context of application to the oxygen reduction reaction (ORR) and Mg-air battery. According to the results, α–MnO2/CeO2 microfibers exhibited better ORR performance than α–MnO2 microfibers due to the synergistic result between the introduction of Ce3+ in CeO2 lattice and the enhancement of Mn3+ content in MnO2 lattice. α–MnO2/CeO2 microfibers provided a higher surface area and more catalytic active sites than α–MnO2 microfibers by controlling the molar ratio of Ce3+/Mn7+ for the precursor. When the mole ratio of Ce3+ and Mn7+ in the precursors was 10%, the four-electron transfer process of the MnO2/CeO2 microfibers (MC-140-12-10) was found to be similar to that of the 20 wt% Pt/C commercial catalysts. MC-140-12-10 microfibers also showed the excellent long-term stability after 25,000 s and superior Mg–air battery performances than α–MnO2. Hence, the work paves the way for developing Mg-air batteries through a simple synthesis and cost-effective ORR catalyst.

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