Polarization Curve

In plain terms

Hydrogen output is proportional to current. Energy use is current times voltage. A polarization curve links the two: push more current through each square centimetre of cell, and the voltage needed climbs.

The starting point is thermodynamics: about 1.23 V is the minimum (reversible) voltage to split water at standard conditions, and about 1.48 V is the thermoneutral voltage, where the cell neither gains nor loses heat. Three losses, or overpotentials, add to it:

• Activation: the energy needed to start the reactions at each electrode. It dominates at low current.
• Ohmic: resistance of the electrolyte, membrane or diaphragm, and contacts. It grows in a straight line with current.
• Mass transport: gas bubbles and water supply limiting the reaction. It appears at the highest currents.

Choosing where to run on the curve is choosing between a smaller stack and a lower power bill.

Typical range

The US DOE publishes one operating point per technology (2022 status):

PEM reaches four times the current density of alkaline at the same voltage, which is why PEM stacks are more compact. DOE targets for 2026 move alkaline to 1.0 A/cm² at 1.8 V and PEM to 3.0 A/cm² at 1.8 V.

Why it moves LCOH

Running at higher current density makes more hydrogen from the same stack, cutting CAPEX per kilogram. It also raises voltage and therefore specific energy consumption, raising electricity cost per kilogram. Cheap power favours high current density; expensive power favours a lower operating point and a larger stack.

Common mistake