Current density

Operating current divided by active electrode area, usually in A/cm².

The normalised measure of how hard an electrolyser is being driven, and the parameter that makes cells of different sizes comparable.

Higher current density produces more hydrogen from the same physical footprint, which reduces capital cost per unit of output. It also raises cell voltage, through both activation and ohmic effects, which raises the energy required per kilogram. Electrolyser design is largely a negotiation between those two consequences.

PEM systems typically operate at higher current densities than conventional alkaline, which is a large part of why they are physically smaller for the same output. Because cell voltage depends strongly on current density, any comparison of voltages or degradation rates has to state the current density it was measured at, or it means very little.