Carbon corrosion electrochemical oxidation

Manganese and iron oxidation are coupled to cell growth and metabolism of organic carbon. Microbially deposited manganese oxide on stainless and mild steel alters electrochemical properties related to the potential for corrosion. Iron-oxidizing bacteria produce tubercles of iron oxides and hydroxides, creating oxygen-concentration cells that initiate a series of events that individually or collectively are very corrosive. 

One of the common ways in which fuel cell components experience degradation is through corrosion. Carbon particles in the CL are susceptible to electrochemical (voltage) corrosion and contain Pt particles that catalyze oxidation reactions. The carbon fibers in CFPs and CCs and the carbon black in MPLs are not as susceptible to these issues because they are not part of the electrochemical reactions and do not contain Pt particles. However, they can still go through chemical surface (hydrogen peroxide) oxidation by water or even by loss of carbon due to oxidation to carbon monoxide or carbon dioxide [256,257]. 

Corrosion of iron is explained by the position of iron in the electrochemical series of the elements (Fe/Fe2+ —0.44 V). In steel, local anode and cathode areas are found due to the presence of phases containing, for example, carbon, carbides, and oxides. These latent local cells are activated by moisture, oxygen, and current-carrying electrolytes and the following reactions occur between the anode areas consisting of iron, and the cathode areas containing carbides or oxides. 

There are many considerations that must be taken into account when choosing a particular carbon, or carbon structure, as an electrocatalyst support. In hot phosphoric acid at cathodic potentials, the carbon surface is capable of being oxidized to carbon dioxide. The degree of oxidation will depend on the pretreatment of the carbon (for instance, the degree of graphitization), on the carbon precursor, and the provenance. There are two important parameters that will govern the primary oxidation rate for any given carbon material in an electrochemical environment. These are electrode potential (the carbon corrosion is an electrochemical process and therefore will increase rapidly as the electrode potential is raised) and temperature. 

Heckman and Harling57 examined the gas-phase oxidation of carbon black micro-structures and showed that oxidative attack of carbon crystallites was concentrated on the small crystallites, at the edges of layer planes and at lattice defects. Partial graphitization of a carbon black, so that only the outermost surface layers are well-ordered, causes oxidative corrosion within the core of the carbon particle, leaving an outer shell . Consequently, similar behavior can be expected for ungraphitized and partially graphitized carbons in electrochemical environments. 

Also corrosion problems of the carbon support have been considered as a cause of electrocatalyst durabihty loss [32], in particular carbon oxidation can occur through electrochemical oxidation at the cathode, with formation of CO2 (C -I- 2H2O = CO2 -I- 4H -F 4e ), or through water gas shift reaction, with the production of CO (C H2O = CO H2). Both these routes are catalyzed by Pt [56, 57] and subtract caibon useful for platinum loading, with consequent metal sintering and decrease of the electrochemical surface area [58]. 

The important feature of perfluorinated materials for electrochemical application is their excellent or, perhaps, improved, chemical inertness and mechanical integrity in a corrosive and oxidative environment (8, 9). In contrast, the hydrocarbon type material is unstable in this environment, due to the cleavage of the carbon-hydrogen bonds, particularly the a-hydrogen atom where the functional group is attached (9,10) ... 

Furthermore, surprisingly, very few works have been reported on the study of materials degradation phenomena in DAFCs. For instance, a model for carbon and Ru corrosion in a DMFC anode under strong methanol depletion has been very recently proposed by Kulikovsky [195]. The model is based on the mathematical description of the current conservation in the membrane. In the methanol-depleted domain, methanol oxidation reaction is substituted by the carbon oxidation (corrosion). This is supposed to dramatically lowering the membrane potential and to create an environment for electrochemical oxidation of Ru. His calculations show that 50-100 mV loss in the cell potential manifests quite a significant (above 50 %) methanol-depleted fraction of the cell active area (Fig. 8.21). 

Gabriel, A., Laycock, N.J., McMurray, H. N., Williams, G., and Cook, A. (2006) Oxidation states exhibited by in-coating polyaruline during corrosion-driven coating delamination on carbon steel. Electrochem. Solid State Lett., 9, B57—B60. 

Even though the mechanisms of corrosion are not completely understood yet, for the electrochemical oxidation in acids several reaction pathways are proposed either directly to CO2 or via hydroxyl, keto, or carboxylic groups. It is furthermore assumed that the active sites for carbon corrosion are associated with carbon atoms at edges, defects, dislocations, and single-layer planes, that is, amorphous regions within the carbon materials. One strategy to reduce carbon corrosion could therefore focus on removal, reduction, and inhibition of such active sites in order to slow down the carbons oxidation. 

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... 

Both electrochemical oxidation and thermal degradation of carbon in humid air at temperatures <125°C have been reported, and it seems established that these corrosion mechanisms are accelerated by the presence of Pt. Carbon corrosion will first modify the surface of the support, which will become less hydrophobic. It has also been reported that carbon corrosion may enhance the mobility of Pt on the surface, accelerating the Pt sintering discussed above. Further carbon corrosion will degrade the electron-conducting network, rendering ft particles inactive. 

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 ... 

Much of the current research with conductive diamonds in PEMFC research is to develop platinum deposition techniques that can result in more uniform and smaller particle sizes on the conductive surfaces of diamond. However, conductive diamonds for catalyst support applications have often been used to examine the intrinsic properties of catalytic metals because of their inertness to electrochemical processes, lack of surface corrosion, and oxide formation. They remain strong candidates for fuel cell apphcations where catalyst integrity and durability are high priorities, and where typically carbon supports may fail due to the harsh operating conditions and high operating voltages. 

---