Carbon Corrosion in PEM Fuel Cells during Drive Cycle Operation

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation.  For transportation applications, the fuel cell will be subjected to frequent power cycling.  For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown  as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport.  Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance.

To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO₂ in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell.  CO₂ present in air was removed by a lime bed prior to introduction to the fuel cell.  We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential.  The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min.  The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). 

The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential.  A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V.  Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides.  Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. 

The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion.  The magnitude of the spikes decrease as the lower cell potential is raised.  These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials.

The peak in CO₂ evolution occurs when the cell potential increases from high power operation to low power near open circuit.  This correlates with when CO₂ evolution is observed during cyclic voltammograms, which occurs during the positive sweep at  ~ 0.55 to 0.60 V.  The evolution of this CO₂ peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface.

During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume.  This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours.  Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion.

These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation.  To minimize the carbon corrosion, the size of the steps in potential should be minimized.  For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports.

Acknowledgments

Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland