Carbon corrosion kinetics

Figure 4. Polarization curves of carbon corrosion and oxygen evolution reactions based on measured carbon corrosion kinetics for Pt/Vulcan and Pt/Graphitized-Vulcan and oxygen evolution kinetics for Pt/C catalysts. The upper horizontal dotted line denotes a current density equivalent to oxygen crossover through membrane from cathode to anode.

Recent kinetic studies indicate that carbon corrosion can be significant under normal transient operation.56,57,60-62 The rate of voltage change, common in the automotive application, enhances cathode carbon-support corrosion.16 Hence, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling. Currently, the relationship between fuel cell performance decay and accumulated carbon-support loss is only empirical.22 More effort has to be made to incorporate mechanisms that can accurately quantify voltage decay with carbon-support loss.31,32... 

To account for the cathode CL (PEM) transport properties changes induced by carbon corrosion (ionomer degradation) we can use, for example, spatially-averaged fractal representations of the CLs to describe the impact of carbon (ionomer) mass loss on the microstructural properties changes, such as the evolution of the carbon instantaneous surface area or effective diffusion coefficients in the CL. We have used this approach for example ini d68,i79 relate the temporal evolution of the cathode thickness and carbon surface area with the carbon corrosion kinetics, by representing the carbon phase as a two-dimensional Sierpinski carpet projected in the cathode thickness direction (Fig. 11.11). 

Example of CGMD calculation of the cathode CL re-organization induced by carbon corrosion kinetics. (Source Ref. 269.)... 

In this chapter, we attempt to evaluate state-of-the-art commercial conventional-carbon-support MEAs for their carbon corrosion kinetics, the relationship between cell voltage loss and carbon-support weight loss, and the fife projection of the catalyst support under automotive operating conditions. These operational conditions include steady-state operation, transient, start/stop, and unintended deviations from nominal run parameters. On the basis of these analyses, we elucidate (1) which operational conditions result in severe carbon corrosion, (2) whether current conventional-carbon-support MEAs are robust enough to meet automotive durability targets, and (3) if a state-of-the-art corrosion-resistant carbon-support MEA is absolutely required for improving automotive fuel cell durability. 

Fig. 3 The log-log plot of the fitted carbon corrosion kinetics (2) versus experimental carbon corrosion rates determined by CO evolution from a N -purged working electrode at various temperatures, corrosion times, and potentials (vs. RHE). (Reproduced by permission of Makharia et al. (2006), The Electrochemical Society)...

Carbon corrosion kinetics of commercial conventional-carbon-supported MEAs were studied at various potmtials and tempoalures. The lifetime projectiMi of OMventional-carbon-supported MEAs in the automotive fuel cell system was then analyzed using the kinetics shown in this chapto. It is found that these conventional-carbon-supported MEAs are not likely to meet automotive fuel cell durability targets under the severely dynamic automotive operational conditions. Automotive fuel cell systan start/stop and local anode starvation are beheved to be two of the rntgor contributors... 

Fundamental model analyses incorporating the measured carbon corrosion kinetics were developed for conditions of start/stop or local starvation. The combination... 

The mechanism of carbon corrosion has been investigated in MEAs and in liquid electrolytes. Carbon itself is thermodynamically unstable toward oxidation at higher potentials, but this oxidation is kinetically limited ... 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

The kinetic model described by Eq. (8) can be used to evaluate the material impact on the carbon corrosion rate. As shown for 50% Pt/Vulcan in Fig. 4 and Table 3 (first row), its carbon corrosion rate is essentially equal to the 02 crossover current. Since the... 

We use the coupled kinetic and transport model to predict when local H2 starvation occurs and how it affects carbon corrosion rate. 

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

It has been known for a long time that iron corroding in the presence of hydrogen sulfide will be covered by an iron sulfide film. One would therefore have expected that this film somehow affects the kinetics of the corrosion process. Sardisco and coworkers (35) carried out classical investigations of the corrosion kinetics of iron as a function of the partial pressure of hydrogen sulfide and carbon dioxide. The authors measured the overall kinetics of the corrosion as well as the rate of iron sulfide film formation and came to some conclusions with respect to the protectiveness of the iron sulfide film. The results, however, were rather confusing and could not be fitted into conventional rate theory. 

Goidanich, S., Lazzari, L., Ormellese, M., Pedeferri, M. Influence of AC on corrosion kinetics for carbon steel, zinc and copper. Corrosion 5 and 2 (2005)... 

As mentioned earlier, CB is prone to oxidation, the so-called carbon corrosion, which results in the loss of surface area, changes in the pore structure and finally also leads to sintering of the supported nanoparticles and eventually their loss from the support surface. This affects both the kinetics of the reaction and the electrode s mass transport behavior resulting in a significant loss of performance with operation time. Consequently, carbon support durability is considered to be a major barrier for the successful commercialization of fuel cell technology in the automotive sector. So much so, during the last decade, more than 60 publications dealt with carbon corrosion mechanisms in fuel cell apphcation [82]. 

Weight loss corrosion rates, which represent an average of corrosion over the test period, are useless from a predictive point of view, but are often used in conjunction with other measurements for quality assessments. Corrosion kinetics can be measured in different ways. Most favored are electrochemical techniques. They are, however, contrary to common belief, indirect techniques and must be properly calibrated and interpreted to be useful. If corrosion products are soluble in solution (as, for instance, iron carbonate), the buildup of such in solution can be used to monitor how corrosion progresses. Hydrogen, a byproduct of anaerobic corrosion, can also be used to monitor kinetics. Less common, but equally direct, are methods that use the removal of radioactivity from irradiated surfaces. Kinetic measurements have also been carried out with electrical resistance probes. As a general principle, no one method is in itself without some problems and should, therefore, always... 

Generally, carbon has been used as a support in fuel cell systems since it allows one to decrease Pt loading from ca. 4 to 0.2 mg cm [31, 32]. However, thermodynamically favorable and kinetically slow electrochemical oxidation of carbon support during start/stop of a simple fuel cell (Eq. 10) or carbon corrosion that could proceed during recharge of the... 

The kinetics of this reaction are in the focus of research groups worldwide. The studies of Kinoshita in PAFC-based Systems showed that the carbon corrosion of the most commonly used Vulcan XC-72 catalyst support material starts at potentials higher than 0.8 V and temperatures higher than 65° C. When the carbon material is at room temperature, the oxidation starts at potentials higher than 1.0 V. So the corrosion of... 

For example, within this context, CGMD simnlations have been used to build up a stmctural database for CLs with different carbon contents in terms of interpolated mathematical functions describing the impact of the carbon mass loss (induced by corrosion) on the evolution of the ionomer coverage on Pt and carbon, the electronic conductivity of the CB, the effective O2 diffusion properties, the carbon surface area and the Pt surface area (which re-organizes during the carbon corrosion process) (see Fig. 11.9, 11.12). These functions are then integrated into the MEMEPhys kinetic model to simulate the impact of carbon corrosion on the MEA performance decay. ... 

---