Skip to main content

Cobalt(II,III) Oxide

Co3O4 oxide
Molar Mass
240.795 g/mol
Calculate in Molar Mass Calculator →

Properties

StateSolid
ColorBlack
SolubilityInsoluble in water; slowly dissolves in concentrated acids
Melting Point895 °C (decomposes to CoO above 900 °C)

About Cobalt(II,III) Oxide

Co3O4 is the cobalt analog of magnetite Fe3O4 — a normal-spinel, mixed-valence oxide where Co²⁺ sits in tetrahedral A-sites and Co³⁺ fills octahedral B-sites in a 1:2 ratio, formally [Co²⁺]A[Co³⁺]2BO4. Unlike magnetite (which is an inverse spinel and conducts via fast Fe²⁺/Fe³⁺ electron hopping), Co3O4 is a normal spinel: the Co³⁺ in octahedral coordination is locked in a low-spin t2g⁶ ground state with no unpaired electrons, while the tetrahedral Co²⁺ stays high-spin. That separation makes Co3O4 a p-type semiconductor with a direct band gap around 2.0 eV and a smaller indirect gap near 0.8 eV, suitable for photocatalysis under visible light. The mixed-valence character is what makes it electrochemically interesting: Co²⁺ ↔ Co³⁺ ↔ Co⁴⁺ transitions in alkaline solution power the oxygen-evolution reaction (OER) at overpotentials competitive with state-of-the-art IrO2, which is why Co3O4 nanoparticles dispersed on nickel foam are one of the leading non-precious-metal water-splitting catalysts under industrial development. As a Li-ion anode, Co3O4 stores 8 Li⁺ per formula unit through a conversion reaction (Co3O4 + 8 Li⁺ + 8 e⁻ → 3 Co⁰ + 4 Li2O), giving 890 mAh/g theoretical — about 2.4× graphite — though the volume change on cycling and the ~1 V voltage hysteresis between charge and discharge keeps it confined to research cells. Synthesis is straightforward: heat Co(NO3)2·6H2O or CoCO3 in air at 600–800 °C; below 900 °C the spinel is the stable phase, and above 900 °C it decomposes back to CoO + 1/2 O2.

Where you'll encounter it

If you've ever run a CV (cyclic voltammogram) on a cobalt-containing electrocatalyst film in 1 M KOH, the redox couple at +1.45 V vs RHE just before oxygen evolves is the Co³⁺/Co⁴⁺ transition in surface Co3O4 — that's the active site doing the OER work. In materials labs, Co3O4 nanocubes are a benchtop reference for benchmarking new OER catalysts, and the powder shows up as a black pigment in heat-resistant exhaust-manifold paints (rated to 800 °C) where it doubles as an emissivity enhancer for radiative cooling.

Common Uses

  • Oxygen-evolution-reaction electrocatalyst on nickel-foam supports for alkaline water electrolysis
  • Conversion-type lithium-ion anode storing 8 Li⁺ per formula unit at 890 mAh/g theoretical capacity
  • Heterogeneous catalyst for low-temperature CO oxidation in fuel-cell air-purification cartridges
  • Methane combustion catalyst at lean conditions in natural-gas vehicle exhaust aftertreatment systems
  • Precursor to LiCoO2 cathode synthesis through solid-state reaction with Li2CO3 at 700–900 °C
  • Black pigment in high-temperature ceramic glazes and exhaust-manifold paint rated to 800 °C
  • Visible-light photocatalyst for water oxidation with 2.0 eV direct band gap
  • N2O decomposition catalyst for nitric-acid-plant tail-gas emission control under EU IED limits

Safety Information

GHS H332 (harmful if inhaled), H317 (skin sensitization), H334 (respiratory sensitization), H341 (suspected mutagen), H350i (carcinogenic by inhalation, IARC Group 2B for cobalt and cobalt compounds), H360F (reproductive toxicity), H410 (very toxic to aquatic life). OSHA PEL for cobalt is 0.1 mg Co/m³ as an 8-hour TWA; ACGIH TLV is the more restrictive 0.02 mg/m³. The dust is the main hazard — chronic inhalation causes hard-metal lung disease (giant-cell interstitial pneumonia) seen historically in tungsten-carbide tool sharpeners. Handle in a fume hood; full-face respirator with P100 cartridges for any operation that aerosolizes powder. Skin sensitization is permanent once acquired and cross-reacts with all other cobalt compounds.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

Co — Cobalt O — Oxygen

Frequently Asked Questions

What is the molar mass of Co3O4?
Co3O4 is 240.795 g/mol: three cobalts at 58.933 each (176.799) plus four oxygens at 15.999 each (63.996). For battery-electrode preparation, that means 100 mg of Co3O4 active material represents 4.15 × 10⁻⁴ mol of formula units, capable of storing up to 3.32 mmol of Li (8 × 4.15 × 10⁻⁴), which gives the 890 mAh/g theoretical capacity by Faraday's law.
Why is Co3O4 called a mixed-valence oxide?
The 3:4 stoichiometry means the average oxidation state must be +8/3, which can't be achieved with a single integer state. Crystallographically Co3O4 is a normal spinel with one Co²⁺ in a tetrahedral A-site and two Co³⁺ in octahedral B-sites per formula unit — formally [Co²⁺](A)[Co³⁺]2(B)O4. The mixed valence is what gives it the rich redox chemistry used in OER catalysis and Li-ion conversion anodes. Magnetite Fe3O4 has the same 2:3 stoichiometry but as an inverse spinel where electron hopping between Fe²⁺ and Fe³⁺ in octahedral sites makes it a metallic conductor — Co3O4 lacks that fast hopping pathway and is a p-type semiconductor instead.
Why does Co3O4 underperform expectations as a Li-ion anode?
Theoretical 890 mAh/g, experimentally seen capacities of ~700 mAh/g in research cells, but commercial cells use graphite at ~370 mAh/g instead because of three killer issues: (1) the conversion reaction has a ~1 V voltage hysteresis between lithiation and delithiation, which destroys round-trip energy efficiency. (2) The 250%+ volume swing between Co3O4 and metallic Co + Li2O cracks the electrode after a few hundred cycles. (3) The lithiation plateau is at 1.0 V vs Li/Li⁺, much higher than graphite's 0.1 V, which reduces full-cell voltage. Researchers continue working on porous nanostructures and carbon composites to mitigate these, but graphite still wins for now.