The analysis of polarization curves was a topic of interest to Dr. H. R. Kunz throughout his long career in fuel-cell research. In fact, Dr. Kunz’s first publication in JECS was on a method of using polarization curves to determine the catalytic activity of carbon-supported catalysts in Teflon-bonded porous electrodes [1]. This work was one of many important contributions that Dr. Kunz made to the development of phosphoric-acid fuel cell systems at United Technologies Corporation (UTC). Thirty years later, Dr. Kunz published a paper on a method to analyze polarization curves to determine six major sources of overpotential in polymer-electrolyte fuel cells (PEFCs) [2]. This analysis was subsequently utilized to compare the initial overpotential breakdown with conventional PEFCs and UTC’s porous-plates PEFCs, as well the change in overpotentials after durability testing [3].

Analysis of polarization curves can be especially insightful since it can provide useful insights into the various sources of polarization over the entire operating range of cell under realistic conditions. Ideally, diagnostics should be as simple as possible in order to decrease the risk of errors and to minimize the time required to yield instructive results. One powerful simplification technique is to diagnose the changes in polarization curves under different operating conditions. This effectively eliminates the need to account for all polarization sources and focus on the overpotentials primarily responsible for changes in the performance. Polarization-change curve (PCC) analysis was first developed to help determine the root cause of performance losses due to degradation [4]. However, changes in cell performance that result from changes in operating conditions can readily utilize this same methodology, since whether the performance changes result from load hours or from operating conditions is irrelevant to PCC methodology.

A PCC is constructed by taking the difference between two polarization curves measured on the same cell. These curves can be measured under two different operating conditions (e.g., under two different humidity levels if one is investigating RH sensitivity). Alternatively, these two curves can be measured under identical conditions before and after conducting accelerated-stress test (AST) protocols (e.g., if one is investigating cell durability). In any case, the general shape of the resulting PCC can be instructive by simply comparing it to the limiting cases that result if one assumes the changes are due to a change in a single type of polarization.

In many cases, differentiating between ohmic and diffusive losses in the electrodes can be particularly challenging. In the case of the cathode catalyst layer, one can utilize the difference in the reaction order with respect to oxygen concentration within the double-Tafel region. For a cathode dominated by oxygen-diffusion limitations, first-order dependence is expected, while if ohmic limitations across the thickness of the cathode dominate, half-order dependence is expected [5]. However, transport losses may also be significant on length scales that are different from the thickness of the catalyst layer. Important length scales include the thickness of the ionomer film and agglomerate diameter, which may be especially important in PEFCs with ultra-low catalyst loadings with very thin catalyst layers.

 

This talk will focus on the possible combination of these two simple analysis tools taught in [4] and [5]. An example of this type of graphical PCC analysis is illustrated in Fig. 1. A numerical model of the PEFC [6] with detailed physics is utilized to investigate the soundness of this type of graphical analysis, as well as the potential limitations of this simple technique, especially with state-of-the-art thin catalyst layers. The numerical model is used to probe different limiting case behaviors to analyze PEFC performance.

Acknowledgements

Thanks to the organizers of this special symposia in the memory of Dr. Kunz, as well as for the invitation to present. This material is based upon work supported by U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under contract numbers DE-AC02-06CH11357 and DE-EE0007652.

References

1. H. R. Kunz & G.A. Gruver, JECS, 122, 1279 (1975).

2. M. V. Williams, H. R. Kunz, & J. M. Fenton, JECS, 152, A635 (2005).

3. X. Wang, X. Huang, L. J. Bonville, H. R. Kunz, M. L. Perry, & D. Condit, JECS, 161, F761 (2014).

4. M. L. Perry, R. Balliet, & R. Darling, “Experimental Diagnostics and Durability Testing Protocols,” in Modern Topics in Polymer Electrolyte Fuel Cell Degradation, M. Mench, et.al. (Eds.), Elsevier, Denmark (2011).

5. M. L. Perry, J. Newman, & E. J. Cairns, JECS, 145, 5 (1998).

6. I. V. Zenyuk, P. K. Das, & A. Z. Weber, JECS, 163, F691 (2016).