273110 Incorporating a Size Distribution and Interfacial Oxygen Resistance Into a PEFC Agglomerate Model
In the past, the popular agglomerate model for polymer electrolyte fuel cells (PEFCs) has typically simplified the electrode by assuming a single, representative agglomerate diameter. In reality, the electrode contains a distribution of solid agglomerate sizes. In this work, we examine the effect of incorporating an agglomerate size distribution (ASD) into the agglomerate model. The ASD is obtained from our own prior nanoscale X-ray CT measurements. To further study oxygen transport through the ionomer binder that surrounds agglomerates, we additionally incorporate an interfacial oxygen mass transport resistance at the gas/ionomer interface. This resistance accounts for the rate-dependent disparity between the equilibrium concentration just inside the ionomer (as expected from Henry’s law) and the actual concentration there when oxygen flux is underway.
We find a substantial difference (or “error”) between the agglomerate model with an ASD and the model with a single agglomerate size. This error is a strong function of overpotential, and of the choice of agglomerate size employed in the single-size model. We find the single agglomerate diameter that gives the lowest maximum error across all overpotentials for our particular electrode geometry, and when considering the interfacial mass transport resistance). This diameter is 105 nm, in which case the single-agglomerate version of the model still errs by as much as +/- 20% versus the ASD case. We also present the ideal (no error) values of single-size agglomerate choices, or the “effective diameter.” This value is a function of overpotential and of whether or not the interfacial resistance is used. The effective agglomerate diameter becomes lower at higher overpotentials. This is because at higher current, transport is a greater issue, forcing the smaller (and less transport-hindered) agglomerates to contribute disproportionately more to the reaction rate.