Co²⁺ ions migrate to the cathode and are reduced (Co²⁺ + 2e⁻ → Co) while water is oxidised at the anode (2H₂O → O₂ + 4H⁺ + 4e⁻). The deposited mass follows Faraday's law:
m = I·t·M·η / (n·F)
I = J·A (A = 1 m² cathode)
M = 58.93 g/mol, n = 2, F = 96 485 C/mol
Current efficiency η falls off away from an optimum current density (competing H₂ evolution), rises with Co²⁺ concentration (less mass-transport-limited plating) and peaks near 55 °C. Cell voltage is the reversible decomposition voltage plus a Tafel-type activation overpotential and an ohmic drop that falls as conductivity (concentration, temperature) improves. Specific energy is n·F·V / (M·η·3600) kWh per kg of cobalt. Process time is accelerated ×180 so cathode growth is visible within the session.
- Left plate — cathode; the metallic layer thickens as Co plates out.
- Right plate — anode; bubbles are the O₂ evolved.
- Drifting dots — Co²⁺ ions migrating cathode-ward; drift speed scales with current density.