Carbon corrosion mechanism

Similar ECA loss phenomena have been observed in PEMECs. Understanding ECA loss and carbon corrosion mechanisms may help with designing more durable... 

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]. 

Web of Knowledge (accessed 21 May 2015) with search string carbon corrosion, mechanism, fuel cell ... 

Electrochemical ex situ studies [131, 144, 145] in the temperature range 25-80 C have shown that at a potential higher than 0.3 V versus RHE, COsmf starts to form irreversibly oti the carbon particle surface. One specific species is the quinone group that is electrochemically active with a redox peak at 0.55 V versus RHE that can be identified in cyclic voltammetry. The presence of Pt catalyzes the subsequent oxidation to CO2. The carbon corrosion mechanism consists of the following steps ... 

The issue of carbon corrosion has received considerable attention in recent years. There are several drivers for this (1) the cost drivers for commercialization require the use of high performance catalysts with less durable carbon catalyst supports, (2) the need for system simplification and low cost prevents additional control systems to be implemented to avoid the carbon corrosion conditions, and (3) the use of the fuel cells subjected to "real world" conditions as opposed to carefully controlled demonstration projects, with very dynamic duty cycles and many start-up/shutdown cycles. This increased attention has resulted in new or improved measurement techniques and several studies and reviews on the high cathode potential and associated carbon corrosion mechanism [39,40,48-51]. 

Another catalyst degradation mode is catalyst carbon support corrosion or oxidation. Figure 10.14 shows a schematic of the carbon corrosion mechanism [56]. 

Carbon Corrosion Mechanism, Kinetics, and its Correlation to Cell Voltage Loss... 

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

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 ... 

The major issue found in testing is the corrosion of the foam material and resultant contamination of the membrane. The high manufacturing cost of the metal or carbon foam with the required pore shape, size, and distribution also is a challenge. Further study and testing of the corrosion mechanism, selection of appropriate coating, a capillary process involved in the tiny pores, and related water retention are necessary to identify whether the new material and concept can be finally applied in the plate. 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

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... 

The mechanism of carbon corrosion is still not fully established. It has been suggested to proceed via the sequence of an electrochemical step followed by a chemical step namely, an electron transfer step followed by hydrolysis of the resulting C(s)+ surface sites, finally yielding CO2. The latter either escapes from... 

The corrosion mechanisms of metals and alloys in carbonate melts are quite different... 

Sweet Corrosion. It is caused by the presence of dissolved COj in the prodnced llnids. CO2 reacts with water to form carbonic acid (H2CO3), which dissociates to form hydrogen ions and the carbonate ion. At the anodic sites, the metal atoms give np electrons and dissolve to form metal ions. These electrons are taken up by hydrogen ions at cathodic sites to form atomic hydrogen. Bicarbonate ions, however, react to form a protective iron carbonate film, and the rate of corrosion depends on the stability of this film [2]. The corrosion mechanism can be represented by the sketch shown in Figure 11.7. 

The diffusion constant D is determined by the concrete quality. At the carbonation front there is a sharp drop in alkalinity from pH 11-13 down to less than pH 8. At that level the passive layer, which we saw in Chapter 2 was created by the alkalinity, is no longer sustained so corrosion proceeds by the general corrosion mechanism as described in the Chapter 2. 

Another critical mechanism of electrochemical activity loss is due to carbon corrosion. Carbon corrosion has been cited as a major concern in higher temperature environments, such as phosphoric acid fuel cells [40] but is relatively benign at potentials less than IV RHE at the temperatures at which PEM fuel cells normally operate. Several papers have noted that in the case of gross fuel starvation, cell voltages can become negative, as the anode is elevated to very positive potentials, and the carbon is consumed instead of the absent fuel [41]. 

Three sintering mechanisms have been proposed to explain ECSA loss of fuel cell catalysts catalyst dissolution/reprecipitation, migration of Pt particles, and carbon corrosion [85]. 

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