Carbon corrosion reaction

In acid electrolytes, the Tafel slope for the carbon corrosion reaction appears to be indicative of the degree of disorder on the carbon surface. The larger the Tafel slope, the greater the degree of disorder. The influence of heat treatment on the corrosion rate depends on the structure of the parent carbon, particularly on the lattice parameters. Thus, in hot phosphoric acid at cathodic potentials, as... 

Accelerating conditions for carbon corrosion include high potentials as during air/hydrogen fronts and fuel starvation. The role of humidity is still under some dispute. On the one hand, the carbon corrosion reactions require the presence of... 

CB catalyst support is thermodynamically nnstable at typical hydrogen-fed PEMFC and DMFC cathode operating conditions (potential >0.2 V vs NHE at 25 °C)." Despite the fact that carbon corrosion reaction reveals itself to be quite slow at common PEMFC operating temperatures (<120 °C) with a steady nonzero cnrrent demand, severe CB stracture damage has been observed after PEMFC power-cycling and start-up/shut-down operation." " In-line direct gas mass... 

FIGURE 5.33 The three fnel cells in a PEFC with the oxygen in the second half of the anode channel (cf. Figure 5.30). HOR, ORR, and CCR stand for the hydrogen oxidation, oxygen reduction, and carbon corrosion reactions, while the CL abbreviates the catalyst layer. Arrows indicate trajectories of proton (fdled circles) transport. The dashed line is the shape of the membrane phase potential. 

In this section, we show that electrochemical Ru corrosion is induced in domains with strong methanol depletion. Such domains may arise in cell/stack environment due to nonuniformity of methanol supply over the cell surface. In these domains, methanol oxidation is substituted by the carbon corrosion reaction. This strongly lowers the membrane phase potential, thereby activating Ru electrochemical dissolution. 

Physically, the lack of methanol in the second half of the channel forces the anodic reaction to run with carbon as a fuel. Owing to the small exchange current density (Table 5.11), the carbon corrosion reaction requires quite a significant overpotential... 

Corrosion of carbon supports may cause the electrical isolation and aggregation of platinum nanoparticles, causing a decrease in the FCA in the catalyst layers too. The electrochemical behaviors of carbon in different forms have been studied under a variety of conditions there is a comprehensive review of works conducted two decades ago in Kinoshita s classic book (Kinoshita 1988). In aqueous solutions, the general carbon corrosion reaction can be written as (Kinoshita 1988)... 

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form tire metal oxides is accompanied by the formation of carbon solutions or carbides in tire hrst case, and sulphide or sulphates in the second mixture. Since the most importairt aspects of this subject relate to tire performairce of materials in high temperature service, tire reactions are refeiTed to as hot corrosion reactions. These reactions frequendy result in the formation of a liquid as an intermediate phase, but are included here because dre solid products are usually rate-determining in dre coiTosion reactions. 

Of the dissolved gases occurring in water, oxygen occupies a special position as it stimulates the corrosion reaction. Carbon dioxide is scarcely less important this constituent must, however, be considered in relation to other constituents, especially calcium hardness. 

In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at potentials where carbon corrosion occurs. However, in alkaline electrolytes carbon is sufficiently electrocatalytically active for oxygen evolution to occur simultaneously with carbon corrosion at potentials corresponding to charge conditions for a bifunctional air electrode in metal/air batteries. In this situation, oxygen evolution is the dominant anodic reaction, thus complicating the measurement of carbon corrosion. Ross and co-workers [30] developed experimental techniques to overcome this difficulty. Their results with acetylene black in 30 wt% KOH showed that substantial amounts of CO in addition to C02 (carbonate species) and 02, are... 

Depending on the degree of oxygen infiltration, the temperature of the condensate and the presence of other gases such as carbon dioxide, various corrosion reactions may take place in the steam distribution and CR systems. The most basic reaction associated with oxygen infiltration results in oxygen corrosion, which can produce deep pitting in the pipework and is described later in this chapter. 

Electrolyte management, that is, the control over the optimum distribution of molten carbonate electrolyte in the different cell components, is critical for achieving high performance and endurance with MCFCs. Various processes (i.e., consumption by corrosion reactions, potential driven migration, creepage of salt and salt vaporization) occur, all of which contribute to the redistribution of molten carbonate in MCFCs these aspects are discussed by Maru et al. (4) and Kunz (5). 

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... 

Figure 4. Polarization curves of carbon corrosion and oxygen evolution reactions based on measured carbon corrosion kinetics for Pt/Vulcan and Pt/Graphitized-Vulcan and oxygen evolution kinetics for Pt/C catalysts. The upper horizontal dotted line denotes a current density equivalent to oxygen crossover through membrane from cathode to anode.

Equations (18-20) are discretized by the control volume method53 and solved numerically to obtain distributions of species (H2, 02, and N2) concentration, phase potential (solid and electrolyte), and the current resulting from each electrode reaction, in particular, carbon corrosion and oxygen evolution currents at the cathode catalyst layer, with the following initial and boundary conditions ... 

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. 

One of the most controversial issues in AFCs is the formation of carbonates. It is generally accepted that the C02, both originally in the air and formed by corrosion reaction of the carbon catalyst support, interacts with the electrolyte according to the following equation ... 

---