Carbon corrosion hydrophobicity

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. 

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. 

The MPL often suffers more severe carbon corrosion and hydrophobicity loss than the substrate layer, because it is closer to the corrosive substances produced in membrane and CL (e.g., H2O2, HF). Similar to the carbon powders in the catalyst layer, graphitization of carbon powders in MPL also enhances the corrosion resistance. The hydrophobic material (e.g., PTFE) can also protect the carbon from corrosion. Wood and Borup (Wood et al., 2009) found that no significant structure change occurred in MPL after 664 h of durability test at 1 A cm. In order to enhance the durability, Gore CARBEL CL MPL uses PTFE as substrate filled with carbon black (like the concept of reinforced membrane), and it showed good durability in a PEM fuel cell life test (Cleghorn et al., 2006 Wood et al., 2009). 

Electrochemical corrosion of carbon supports was widely studied in the context of phosphoric acid fuel cell development (Antonucci et al. 1988 Kinoshita 1988), but recently also the low-temperature fuel cell community paid more attention to this process (Kangasniemi et al. 2004 Roen et al. 2004). Carbon corrosion in fuel cell cathodes in the form of surface oxidation leads to functionalization of the carbon surface (e.g., quinones, lactones, carboxylic acids, etc.), with a concomitant change in the surface properties, which clearly results in changes of the hydrophobicity of the catalyst layer. Additionally, and even more severe, total oxidation of the carbon with the overall reaction... 

Cathode mass transport Higher mass-transport overpotentials in start/stop-cycled ceU Electrolyte fiooding due to carbon corrosion and resulting changes in hydrophobicity Significant increase of mass-transport overpotentials Severe corrosion of carbon results in loss of structural integrity of cathode catalyst layer and void volume loss... 

The carbon materials attract the increasing interest of membrane scientists because of their high selectivity and permeability, high hydrophobicity and stability in corrosive and high-temperature operations. Recently many papers were published considering last achievements in the field of carbon membranes for gas separation [2-5]. In particular, such membranes can be produced by pyrolyzing a polymeric precursor in a controlled condition. The one of most usable polymer for this goal is polyacrylonitrile (PAN) [6], Some types of carbon membranes were obtained as a thin film on a porous material by the carbonization of polymeric predecessors [7]. Publications about carbon membrane catalysts are not found up to now. 

This indicates that cos GhjO increases with the increase in oxygen content and decreases with the decrease in hydrogen content. This relationship was confirmed by the results of Kinoshita et al. [94] on the electrochemical treatment of carbon blacks. Hydrophilic carbons provide high catalyst utilization factors, whereas hydrophobic carbons (or graphitized carbons) allow easy water removal, avoid flooding of the CLs, and ensure better resistance to corrosion [8,96,97]. 

To control the reaction rate of the acid, retarders such as alkyl sulfonates, alkyl phosphonates and alkyl amines are used to form hydrophobic films on carbonate surfaces. These protective films act as a barrier to slow acid attack. Another method involves the use of foaming agents to stabilize the carbon dioxide foam that is created when CO2 is released as a product of the acidetching reaction. This CO2 foam acts as a barrier to slow acid attack. Yet another method for controlling the acid activity in an oil well is the use of emulsions containing kerosene or diesel as the continuous oil phase and hydrochloric acid as the dispersed aqueous phase. Acid-in-oil emulsions are most commonly used because oil separates the acid from the carbonate surface (and from machine parts, thus reducing the level of corrosion). Moreover, acid reaction rates can be further decreased by surfactant retarders that increase the wettability of the carbonate surface for oil. 

The effective water management at the PTL is achieved by balancing the hydrophilic and hydrophobic properties of the composite structures. The GDL and MPL contain poly-tetrafluoroethylene (PTFE) or other fluorocarbons that bind and partially coat the conductive (carbonaceous) networks. The fluorocarbon films improve mechanical strength and structural integrity, provide corrosion protection, and create hydrophobic regions within the nominally hydrophilic carbon fibers or particles. The overall porosity, pore size distributions, electronic conductivity, and net hydrophobicity are influenced by the amount of fluorocarbon present. 

The GDL sheets are then heated to higher temperatures (2500-3300 K) that are required for graphi-tization (i.e., the phase transition from amorphous carbon to crystalline graphite). This transformation is crucial to enhance the electrical, thermal, and mechanical material properties. It also changes the surface characteristics and chemical response to the fuel cell operating environments (e.g., hydrophobicity and corrosion resistance). Differences in the process conditions (temperature, prevailing atmosphere or vacuum, etc.) can result in large differences in material properties. Hence, care must be... 

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