Corrosion of carbon materials

Corrosion of carbon materials in PEMFCs is believed to proceed via two different routes electrochemical (as described above) and chemical, due to the... 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Simonov, P.A., Zaikovskii, V.I., and Savinova, E.R. (2010) Microstmcture effects on the electrochemical corrosion of carbon materials and carbon-supported Pt catalysts. Electrochim. Acta, 55,... 

Electrochemical corrosion of carbon material in aqueous acid electrolytes could follow the reaction ... 

Especially for HT-PEM MEAs, the higher operation temperature of 160 °C and the harsh oxidizing H3PO4 electrolyte inducts a very pronounced corrosion of carbon materials. This carbon corrosion phenomenon leads to the formation of surface oxides. The surface oxides are causing a decrease in hydrophobicity of the electrode material. In the case of PAFC or HT-PEM MEAs, which are based on liquid electrolytes, this decrease of electrode hydrophobicity causes an increase in electrolyte loss and in mass transport limitation due to flooding of the electrodes. 

Although it can be concluded from the above evidence that greater edge plane exposure and number of defected sites usually have a positive impact on catalytic activity of the carbon nanostructured supports, it is also known that the corrosion of carbon materials is initiated at these edge planes. This relationship between catalyst support enhancement and carbon corrosion needs to be kept in mind when developing novel nanostructured carbon catalysts. Unfortunately, the influence of defects in CNTs and CNFs on the durability of these structures is stiU not quite known and little research is published in this area. 

Uniform and pitting-type corrosion of various materials (carbon steels, stainless steels, aluminum, etc.) could be characterized in terms of noise properties of the systems fluctuation amplitudes in the time domain and spectral power (frequency dependence of power) of the fluctuations. Under-film corrosion of metals having protective nonmetallic coatings could also be characterized. Thus, corrosion research was enriched by a new and sufficiently correct method of looking at various aspects of the action of corrosive media on metals. 

MSO is unsuited for treating materials with high inert content, such as asbestos, concrete, soils, and rubble. There is concern over emissions from MSO relating to particulate mercury content and radioactivity. MSO is inappropriate for wastes with high tritium levels. MSO pilot programs have encountered problems with carbon monoxide (CO) emissions. The corrosion of reactor materials by molten salt has remained a concern for the long-term operability of the system. The viscosity and volatility of the melt have to be controlled. There have been problems with material from the melt plugging air exhaust and feeder systems. 

Other examples of such mixed potential models include that developed by Macdonald and Urquidi-Macdonald to predict water radiolysis effects in thin condensed water layers on metal surfaces (24), and the models of Marsh and Taylor (25), and Kolar and King (22) to predict the corrosion of carbon steel and copper waste containers surrounded by a low permeability material such as clay. 

Dean, S.W., G.D. Grab. Corrosion of Carbon Steel Tanks in Concentrated Sulfuric Acid Service. Materials Performance 25, 7(1986) p. 48. 

From the practical and economic point of view atmospheric corrosion is closely associated with centers of population. Three factors here coincide high pollution level, high density of population, which in turn means great use of materials. The rate of atmospheric corroion decreases sharply with increasing distance from the emission source. This may be illustrated by the corrosion of carbon steel as function of the distance from the stack of a polluting industry in Kvarntorp, see FIG.8 (26). 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

Simonov et al. [260] attempted to find common ground in the literature data and suggested a correlation dependence of the corrosion rate on substructural characteristics of carbon materials determined from x-ray analysis. This is represented in Figure 12.8, which shows that specific corrosion rate increases with the substructural parameter, defined as (/ooz/ to) x dooi/Lc (these are defined in Section 12.2). This empirical parameter approaches zero for highly ordered carbon materials, since 1002, ho, and doo2 are constant and h is large but increases for amorphous carbons. 

Figure 12.8 Influence of substructural parameters of carbon materials on specific corrosion currents measured after 100 minutes at 1.0 V at 443 K in 85% H3PO4 [260]. (Experimental data are replotted from refs. 8 and 249, with permission from John Wiley Sons and from Elsevier, respectively.)...

In PEMFCs, carbon materials are exposed to conditions that favor their corrosion. These are the positive values of the electrode potential, the acidic environment (pHelevated temperature (333 to 363 K), but also the presence of electrocatalysts such as metal nanoparticles. Since the electrode potential, water content, and Pt mass fraction are higher at the cathode of a PEMFC, this may explain why stronger degradation of the carbon support is usually reported at this electrode [266]. The rate of corrosion of carbon in PEMFCs has been reported to increase with an increase in the relative humidity [97,255,256], but Borup et al. [273] arrived at the controversial conclusion that the rate of carbon corrosion increases with decreasing relative humidity. 

The use of carbon materials in electrochemical systems started in 19 century, when carbon electrodes replaced copper ones in Volta batteries and Pt electrodes in Grove Cells. Nowadays, carbon materials are used in many electrochemical applications because of their high electrical and thermal conductivity, low density, high corrosion resistance, low elasticity adequate strength and high purity. In addition, carbon materials are available in a variety of physical structures (powders, fibres, cloths), have a low cost and can be fabricated into composite structures. 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

Other environmental related tests include ASTM D-904, Standard Practice for Exposure of Adhesive Specimens to Artificial (Carbon-Arc Type) and Natural Light ASTM D-1828, Standard Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures ASTM D-1879, Standard Practice for Exposure of Adhesive Specimens to High Energy Radiation and ASTM D-3310, Standard Test Methods for Determining Corrosivity of Adhesive Materials. 

On most of the electrocatalysts, oxygen reduction takes place by the formation of high-energy intermediate, peroxide, which is then further reduced to H2O. This is probably a consequence of the high stability of the 0—0 bond, which has a dissociation energy of 494 kj mol. In contrast, the dissociation energy of the 0—0 bond in H2O2 is only 146 kJ mor In order to obtain maximum efficiency and to avoid corrosion of carbon supports and other materials by peroxide, it is desired to achieve a... 

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