Degradation rate

The rate at which cell voltage rises over operating time at fixed conditions.

Usually expressed in microvolts per hour at a reference current density, or as a percentage performance loss per thousand operating hours. It is the parameter that determines when a stack reaches the end of its economic life, and therefore when the largest recurring expenditure in an electrolyser plant falls due.

Because voltage depends on current density and temperature, a degradation rate is only meaningful once measurements have been normalised back to reference conditions. Comparing raw voltages taken at different operating points can produce an apparent degradation rate of almost any value.

Degradation is slow by nature, a trend across thousands of hours rather than a step between two months. A sharp change in plant-level efficiency is far more likely to originate in the measurement boundary, the load profile or auxiliary consumption.