Carbon corrosion modeling

Gu W, Carter RN, Yu PT, Gasteigta- HA (2007) Start/stop and local H2 starvation mechanisms of carbon corrosion model vs. experiment. ECS Trans ll(l) 963-973... 

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. 

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

Figure 20. The impact of a faster H2/air-front moving through die anode flow-field. 100% relative carbon loss is defined as the localized damage prediction when no pseudo-capacitance is considered in die model. Significantly less carbon corrosion is expected at the anode inlet region as the speed of H2/air-front increases but much less benefit at the anode outlet region.

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... 

Relationships such as that in Eq. (12) offer convenient means of testing the validity of mixed potential models by comparing electrochemically determined parameters (in this case, a reaction order based on measured Tafel slopes) to values measured by other means. One such example would be the corrosion of U02 (nuclear fuel) in aerated neutral solutions containing added carbonate (6). In the presence of carbonate, corrosion product deposits are avoided, since the U02+ corrosion product is solubilized by complexation with the carbonate. Measured Tafel slopes yield a predicted reaction order of n0l = 0.67 with respect to 02 for the overall corrosion reaction ... 

A. A. Franco, M. Gerard. Multiscale Model of Carbon Corrosion in a PEFC Coupling with Electrocatalysis and Impact on Performance Degradation, J. Electrochem. Soc. , 155, B367-B384 (2008). 

In a PEFC, oxygen and hydrogen crossover is important because of the obvious performance loss, the development of a mixed potential, and even durability issues due to hydrogen peroxide generation platinum migration, and possible carbon corrosion [69]. Furthermore, crossover becomes increasingly important as the membranes used become thinner. Presented in this section are the parameters and governing equations to model this phenomenon. 

Fig. 8.25 (a) CGMD model of the CL (b) coarse-grained crmstituents accounted in the simulations (c) example of result from the combination of the CGMD database into a multiscaie simulation package multiscale simulation of cathode CL carbon corrosion. Reprinted from Ref. [209] with permission... 

Franco AA, Gerard M (2008) Multiscale model of carbon corrosion in aPEFC coupling with electrocatalysis and impact on performance degradation. J Electrochem Soc 155 B367-B384... 

Several deterministic corrosion models have been developed, some purely theoretical while others incorporate empirical results. For a review, see for example Nyborg (2002) or Nesic (2007). Many researchers have studied the mechanisms of corrosion on carbon steel in hydrocarbon environments. The purpose of the experiments has mainly been to study the effects of different influencing factors on the corrosion rate. With a theoretical background and mathematics based on physical understanding of the corrosion principles, these models often perform well under controlled experiments. However, their vahdation is limited outside the laboratory. This limited validation for field applications is mainly due to two reasons ... 

Myers JP, Darling RM (2006) Model of carbon corrosion in PEM fuel cells. J Electrochem... 

Takeuchi, N. and Fuller, T.F. (2008) Modeling and investigation of design factors and their impact on carbon corrosion of PEMFC electrodes. J. Electrochem. Soc., 155 (7), B770—B775. 

Mitsushima S, Nishimura Y, Kamiya N, Ota K (2004) Corrosion model for iron in the presence of molten carbonate. J Electrochem Soc 151 A825... 

For the simulation of the degradation, the model accounts for the numerical feedback between the sub-models describing the non-aging mechanisms (e.g., water transport across the porous electrode) and the sub-models describing the aging mechanisms (e.g., carbon corrosion or catalyst dissolution in the case of PEMFCs) (Fig. 14). At each time step of the simulation, the performance part of the model calculates the... 

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