Polarisation curve

Cell voltage plotted against current density at controlled conditions.

The fundamental characterisation of an electrochemical cell. Its shape separates the loss mechanisms: activation losses dominate at low current density, ohmic losses grow roughly linearly with current, and mass transport limitations appear at the high end.

Captured at commissioning under controlled conditions, it is the reference against which all later performance is judged. Repeated periodically, changes in its shape indicate which mechanism has changed, which is considerably more informative than knowing that efficiency has fallen.

A commissioning polarisation curve is inexpensive to capture and impossible to recreate later. Its absence is one of the most common reasons a performance guarantee argument cannot be settled on the evidence.