Compare catalyst claims at the current density you actually run
Papers report at 10 mA/cm². Commercial PEM runs near 2 A/cm². This converts between them with a Tafel slope — and tells you where the conversion stops being trustworthy.
Reported
Kinetics
Typical acidic OER on iridium oxide: 40–60 mV/dec.
Target
Commercial PEM runs near 2000 mA/cm² (2 A/cm²).
Tafel extrapolation across two decades assumes the mechanism does not change, ignores mass-transport limitation, and ignores degradation. Papers that report activity at 10 mA/cm² are not describing behaviour at commercial current density. Use this to make claims comparable, not to predict a real cell.
Questions
Why convert overpotential at all?
Because published catalyst results are usually reported at 10 mA/cm² while commercial PEM electrolysers run near 2 A/cm² — two hundred times higher. Comparing a paper to a product without converting is comparing different questions.
What Tafel slope should I use?
If the paper reports one, use it. Acidic OER on iridium oxide is commonly in the 40–60 mV/dec range. The slope matters a great deal here: across two decades, 40 versus 60 mV/dec is a 40 mV difference in the answer.
Is Tafel extrapolation reliable?
Over a modest range and a single mechanism, reasonably. Across two decades it assumes no mechanism change and no mass-transport limitation, neither of which is guaranteed. It is a comparison aid, not a prediction of a real cell.
What should I do with the converted number?
Put it into the LCOH calculator and see what it costs. A claim that looks strong at 10 mA/cm² can look ordinary once it is extrapolated and priced.
Now price it
Take the converted overpotential into the LCOH calculator and see what it does to cost per kilogram.