(Invited) Diagnostics of Fuel-Cell Performance Utilizing Simple Graphical Methods Based on Theoretical Limiting Cases
Authors: Perry Mike L. 
Published: February 2020

A systematic methodology to determine what is limiting the performance of a fuel cell will be presented.  The basis of this methodology is to utilize in-situ cell data combined with simple analytical techniques.  The key analytical techniques are based on theoretically-predicted limiting cases for the different types of overpotential that can limit the performance of a polymer-electrolyte fuel cell (PEFC).  These limiting cases are compared to PEFC performance data utilizing simple graphical techniques in order to provide semi-quantitative insight into the mechanisms primarily responsible for the cell polarization observed operating under the operating conditions of interest.

 This diagnostic methodology is ideally suited to investigate changes in the performance of a fuel cell.  For example, the use of this methodology to investigate durability losses in PEFCs has been described [1].  However, the same techniques can be used to determine why the performance of one type of cell is different than a cell of a different configuration, or why the performance of a given cell varies with different operating conditions.  Examples of each of these types of cell-performance investigations will be used to illustrate the methodology. 

The diagnostic method recommended here is a systematic, step-by-step method.  The first step is to determine what major type(s) of overpotential are responsible for the changes in performance, namely: kinetic, ohmic, or mass transport.  Polarization-change curves, such as those depicted in Figure 1, are a simple and useful tool to assist in apportioning performance changes to the different types of overpotential.  The limiting cases depicted in Figure 1 were constructed with a relatively simple model of cell polarization [1].  These simple limiting cases, as well as some actual polarization-change curves examples, will be presented.

 Once the major types of overpotential have been identified, additional cell diagnostics can be selected to establish the sources (i.e., cell components and/or locations) of these changes.  In-cell diagnostics for each of the major types of polarization are also recommended.  For example, to determine whether mass-transport resistance is increasing within or external to the catalyst layer (or catalyst agglomerate) one can utilize an oxygen-gain analysis, which is based on a relatively simple model of the cathode potential and the two limiting cases predicted by this model [2].  To determine whether mass-transport losses are primarily due to oxygen transport and/or proton transport (i.e., ohmic losses in the catalyst layer), one can utilize an oxygen-dependence analysis, which is also based on limiting cases predicted by a simple model of the cathode [3].

 The examples used to illustrate these diagnostic techniques will be on state-of-the-art PEFCs, so that the actual performance limitations being investigated should also be of interest to PEFC research community, such as mass-transport losses in PEFCs with ultra-low catalyst loadings.

Acknowledgements

Thanks to the organizers of this Symposia for the invitation to present.  The author would also like to thank his many fuel-cell collaborators at United Technologies Corporation (both past and present).  Funding from U.S. Department of Energy, EERE’s Fuel Cell Technologies Office under contract numbers DE-AC02-05CH11231 and DE-AC02-06CH11357 has enabled much of the recent work at UTRC on PEFCs that will be used as examples here, and is also gratefully acknowledged.

References:

1.  M. Perry, R. Balliet, and R. Darling, “Experimental Diagnostics and Durability Testing Protocols,” in Modern Topics in Polymer Electrolyte Fuel Cell Degradation, M. Mench, E. Kumbur, and T. Veziroglu (Editors); Elsevier, Denmark, 2011.

2.  K. O’Neil, J. Meyers, R. Darling, and M. Perry, “Oxygen Gain Analysis in Proton Exchange Fuel Cells,” International Journal of Hydrogen Energy, 37 (2012) 1, 373.

3.  M. Perry, J. Newman, and E. Cairns “Mass transport in gas-diffusion electrodes: A diagnostic tool for fuel-cell cathodes,”Journal of the Electrochemical Society, 145 (1998) 1, 5.

Figure Caption:

Figure 1.  Four limiting cases of Polarization-Change Curves (PCC), which are constructed from experimental cell data by taking the difference between two different polarization curves (e.g., from two different cells or the same cell under different operating conditions).  Four theoretically-predicted limiting cases are shown here, which were derived using a simple model: 1) Kinetic (solid line), 2) Ohmic (dashed line), 3) Transport (dotted curved line), and 4) Leak (dash and dot curve).

Figure 1