Record the experiment that confirms the PEM Faraday form - #164
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Yodwong et al. 2020 tested a 50 cm2 PEM cell at 1, 5 and 10 bar and fitted eta_F = a (i/A)^b + c with all three parameters free. The fit returned b = -1 and c = 1 at every pressure. That is 1 - i_x/i exactly, which is the form used here for PEM. The exponent was free to go anywhere and went to -1, so constant crossover is what their data says rather than an assumption we imposed. Their magnitude also shows this file sits at the pessimistic end. Their fitted crossover current rises linearly with pressure and reaches about 21 A/m2 at the 30 bar this case runs at, against the 280 A/m2 assumed here. The assumption stays as it is: their cell is a bench unit, 30 bar is past the pressures they tested, and lowering it would shrink the low-load penalty on the technology under test. Comment only. No numbers change.
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Comment only. No numbers change.
What this records
The PEM half of the derived electrolyser curve uses
eta_F = 1 - i_x/ifor Faraday efficiency, on the reasoning that a solid membrane loses hydrogen by permeation at a rate set by the pressure difference and largely independent of current. Alkaline usesf2 i^2/(f1 + i^2)instead, because a porous diaphragm loses hydrogen in proportion to the gas produced.That was an argument from physics. It now has an experiment behind it.
Yodwong et al. 2020 (Energies 13:4792) tested a 50 cm2 PEM cell at 1, 5 and 10 bar and fitted
with a, b and c all free. The regression returned b = -1 and c = 1 at every pressure, which is
1 - i_x/iexactly, and reproduces their own table to 1e-4. The exponent could have come back at anything and came back at -1 three times out of three.The magnitude says this file is on the pessimistic side
Their fitted crossover rises linearly with pressure:
So PEM is charged roughly thirteen times the crossover loss they measured, even after extrapolating to the 30 bar the case runs at.
Why it is left at 1 per cent
Their cell is a bench unit, 30 bar is past the pressures they tested, and crossover scales inversely with membrane thickness, which they do not report. Lowering
i_xwould shrink the low-load penalty on PEM, and PEM is the technology under test. The conservative direction is to leave it and say so.