Investigation of Highly Active Carbon-, Cobalt-, and Noble Metal-Free MnO2/NiO/Ni-Based Bifunctional Air Electrodes for Metal-Air Batteries with an Alkaline Electrolyte

Compared to other battery technologies, metal-air batteries offer high specific capacities because the active material at the cathode side is supplied by ambient atmosphere. To secure and further extend this advantage, the development of highly active and stable bifunctional air electrodes is currently the main challenge that needs to be resolved. Herein, a highly active carbon-, cobalt-, and noble-metal-free MnO₂/NiO-based bifunctional air electrode is presented for metal-air batteries in alkaline electrolytes. Notably, while electrodes without MnO₂ reveal stable current densities over 100 cyclic voltammetry cycles, MnO₂ containing samples show a superior initial activity and an elevated open circuit potential. Along this line, the partial substitution of MnO₂ by NiO drastically increases the cycling stability of the electrode. X-ray diffractograms, scanning electron microscopy images, and energy-dispersive X-ray spectra are obtained before and after cycling to investigate structural changes of the hot-pressed electrodes. XRD results suggest that MnO₂ is dissolved or transformed into an amorphous phase during cycling. Furthermore, SEM micrographs show that the porous structure of a MnO₂ and NiO containing electrode is not maintained during cycling.

Enhancement in the Charge-Transfer Kinetics of Pseudocapacitive Iridium-Doped Layered Manganese Oxide

Birnessite manganese oxide is a promising candidate as an electrode material for aqueous supercapacitors owing to its pseudocapacitance associated with fast redox processes. While manganese oxides are semiconductive, the conductivity is much lower than that of typical materials used for capacitive electrodes such as activated carbon or ruthenium oxide. In an attempt to increase the electronic conductivity of birnessite, a new solid solution phase, K_y(Mn_1-xIr_x)O₂, was synthesized, and the electrochemical charge storage capability of Ir-doped birnessite was studied in aqueous Li₂SO₄. Structural characterization revealed that the single-phase K_y(Mn_1-xIr_x)O₂ could be synthesized up to x = 0.1. An increase in the pseudocapacitive charge was observed with the increase in Ir content. In addition to the increase in the pseudocapacitive charge, an unusual change in the peak potential was observed. The peak-to-peak difference for the Mn⁴⁺/Mn³⁺ redox decreased with increasing Ir content, indicating an increase in the reversibility of the pseudocapacitive process. The decrease in peak-to-peak difference was observed only by Ir substitution and was not observed for physical mixtures of K_0.28MnO₂ and IrO₂, suggesting a strong electronic interaction between the host Mn ion and the substituting Ir ion.

MOF-derived RuO2/Co3O4heterojunctions as highly efficient bifunctional electrocatalysts for HER and OER in alkaline solutions

A RuO₂/Co₃O₄heterojunction catalyst with a perfect OER and HER overpotential in 1 M KOH solution was synthesized. It contains only a small amount of precious metal oxides but demonstrates a better performance than most reported Co₃O₄-based electrocatalysts.