PEMFC analysis part 2

Pagina 1

Lesson 6 – Detailed analysis of PEMFC components 13rd March 2019 I m a g e s t a k e n f r o m E C C S c o u r s e P a g . 1 | 10 EIS @ high current density We consider Lumped model, we can observe at high current density an additional loss that is related to the transport of oxygen inside the gas diffusion layer. If you move close to the limiting current (so where the polarization curve is going down because of oxygen transport loss) you will find in your spectrum an additional feauture occurring at low frequency: this new feature is indicated with a blue dashed line and it is associated to oxygen transport resistance. To sum up, at high current density we have three major phenomena: - HFR, membrane ionic resistance; - Charge transfer resistance (RCT), how good is the kinetics in the ORR; - Mass transport resistance (RMT), it’s the diameter of the blue-dashed circle. You can find the value of this resistance performing again the derivative of the polarization curve with respect to current; remember that LFR is the derivative of polarization curve and LFR = HFR + RCT + RMT. Note: if you are far from limiting current (jlim – j) will be very large, so RMT is negligible, that’s why we don’t consider this kind of loss in the case of low current density, this feature is vanishing, no more present. This is consistent with the physics of the system because at low current density the loss due to mass transport inside the gas diffusion layer is negligible. Lumped model has a lot of limitations: we get some experimental data and we compare them with the Lumped model; at low current density you have a good consistency with your model in the polarization curve; if you consider the impedance spectrum at high frequency (see plot for 0.1 A/cm2) you have some differences between the…