What Is Manganese Oxide?

Manganese oxide is the general name for the family of compounds formed between manganese and oxygen, spanning oxidation states from +2 through +7 and including MnO, Mn3O4, Mn2O3, and MnO2. The term is used loosely in industry for manganese dioxide, MnO2, which is the commercially dominant member and occurs naturally as the mineral pyrolusite. Manganese sits between chromium and iron in the first transition series, and its accessible multi-electron redox chemistry, low cost, and low toxicity are what make these oxides useful across batteries, catalysis, and water treatment. The USGS national minerals information program tracks manganese supply and notes that steelmaking, where the element acts as a deoxidizer and sulfur scavenger, consumes the large majority of global output, with chemical and battery uses drawing on higher purity oxide grades.

Manganese oxides are also geochemically important in their own right. Poorly crystalline manganese oxide coatings on soil and sediment particles are among the strongest natural oxidants and sorbents in surface environments, controlling the fate of trace metals and many organic contaminants.

Oxidation States and Crystal Structures

Structure follows directly from oxidation state. MnO adopts the rock salt structure with Mn in the +2 state. Mn3O4, the mineral hausmannite, is a spinel containing both Mn(II) and Mn(III). Mn2O3 is a sesquioxide with Mn(III), which is subject to Jahn-Teller distortion. MnO2 itself is polymorphic, and the polymorphs are best described by how MnO6 octahedra connect. Tunnel structures include pyrolusite, the beta phase with 1 by 1 tunnels, and the alpha and todorokite phases with wider 2 by 2 and 3 by 3 tunnels that can host cations and water. Birnessite, the delta phase, is instead layered, with sheets of edge-sharing octahedra separated by hydrated interlayer cations. That tunnel and layer chemistry is the reason manganese oxides accept guest ions so readily, and it is the structural basis for both ion exchange behavior and intercalation electrochemistry.

Electrochemical Energy Storage

Manganese oxides have been battery materials since the Leclanché cell, and electrolytic manganese dioxide remains the cathode in alkaline primary cells. In lithium systems the spinel LiMn2O4 and manganese-rich layered oxides offer high operating voltage with far less cobalt than competing chemistries, at the cost of capacity fade driven by manganese dissolution and structural transformation during cycling. A review of manganese oxides as electrode materials for lithium-ion batteries and supercapacitors surveys how morphology control, doping, and carbon compositing are used to address those limitations. Layered birnessite is also studied for aqueous zinc and magnesium cells and for pseudocapacitors, where fast surface and near-surface redox rather than bulk intercalation carries the charge. Work on copper-intercalated layered MnO2 cathodes has shown that stabilizing the interlayer with a guest cation can make a rechargeable aqueous manganese cathode cycle far more reliably than the unmodified oxide.

Catalysis and Environmental Chemistry

The same redox flexibility makes manganese oxides effective heterogeneous catalysts. They oxidize carbon monoxide and volatile organic compounds at modest temperatures, decompose ozone in cabin air and photocopier filters, and serve as oxygen evolution catalysts in water splitting, where they are studied as inexpensive analogs of the manganese-calcium cluster in photosystem II. In water and soil, biogenic birnessite formed by manganese-oxidizing bacteria oxidizes arsenite to the more readily removed arsenate, sequesters lead and cobalt, and degrades some pharmaceutical residues. Manganese oxide media are used commercially in drinking water treatment to remove dissolved iron and manganese.

Applications

Manganese oxide is used across fields including:

• Primary and rechargeable battery cathodes, including alkaline, lithium-ion, and aqueous zinc cells
• Supercapacitor and pseudocapacitor electrodes
• Ferrite and soft magnetic component manufacture
• Oxidation catalysis, ozone abatement, and emission control
• Drinking water and wastewater treatment media
• Pigments, glass decolorizing, and ceramic glazes