An Isothermal Model for Predicting Performance Loss in PEMFCs Due to Contamination Mechanisms

In the development of proton exchange membrane fuel cells (PEMFCs), contamination can affect the performance on PEMFCs at a slow rate and thereby influence the apparent durability. The extent to which contamination is reversible may improve the potential for commercialization.^1-3Mechanistic models may allow for understanding the differences between contamination, durability, and reversibility of performance loss.

To understand contamination mechanisms and their impact on PEMFCs, experimental studies were conducted for relatively low levels of contamination. These studies have served as the basis for the development of a simple isothermal model for initial performance losses⁷ and a technique for correlating ex-situ and in-situ measurement.⁸These experiments and model highlighted three mechanisms of contamination: adsorption on the Pt catalyst, absorption into the membrane, and ion-exchange with ionomeric components.

The new model presented here allows for prediction of the distribution of performance loss (i.e., voltage change) for the long-term ion-exchange which was superficially treated in the previous model.⁷ That is, the simple model could only predict the initial slope and a final slope of voltage losses (solid lines) in Figure 1.  The simplification could not predict, a-priori, the time when the rate limiting mechanisms changed and the change in slopes of voltage loss.  For example, Figure 1 shows a discrete peak point at 20 h, rather than a gradual transition between curves. This point represents the change of the governing mechanism from contamination of the ionomer in the catalyst layer to contamination of the ionomer in the membrane.

The long-term ion exchange in the new model is a result of the membrane acting as a reservoir for the cation and this reservoir decreases the effect of the ionomer contamination on voltage (i.e., performance) loss. Further, the new model allows more precise predictions for the contamination by functional group (e.g. cationic, aromatic, aliphatic etc.).  The new model also shows that under some circumstances, the reaction plane approximation of the simple model is valid; however, since the membrane is attached to the catalyst ionomer the two entities interact through diffusion of the cation and ion-exchange. The model predictions are shown for three sources of voltage loss and the model accounts for time dependent two-dimensional contamination along the channel and into the membrane. The ability of the new model to predict the reversibility shown in Figure 1 after 105 h, is also discussed.

Figure 1. Comparison of model^7,8 predictions (lines) and in-situ voltage loss (open circles) for 125 ppm of 2,6-diaminotoluene (2,6-DAT) at an infusion rate = 0.03ccm in a 50 cm² cell. (T = 80°C, I = 0.2A/cm², stoic=2.0/2.0, back pressure =150 /150 kPa.)

 

Acknowledgements

The authors gratefully acknowledge support for this work by the DOE EERE Fuel Cell Technologies Office (DE-AC36- 08GO28308) under a subcontract from NREL (ZGB-0-99180-1) to the University of South Carolina

 

 

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