Corrosion cell components electrolyte

Electrolyte loss occurring in long-term operation of MCFC is another problem to be solved for practical application of MCFC. For commercialization, the MCFC should show stable performance over 40,000 hours. Electrolyte loss in MCFC is caused by various factors, e.g., corrosion of components, creepage, reaction with cell components and direct evaporation. These... 

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). 

Mechanical and Chemical Stability. The materials must maintain their mechanical properties and their chemical structure, composition, and surface over the course of time and temperature as much as possible. This characteristic relates to the essential reliability characteristic of energy on demand. Initially, commercial systems were derived from materials as they are found in nature. Today, synthetic materials can be produced with long life and excellent stability. When placed in a battery, the reactants or active masses and cell components must be stable over time in the operating environment. In this respect it should be noted that, typically, batteries reach the consumer 9 months after their original assembly. Mechanical and chemical stability limitations arise from reaction with the electrolyte, irreversible phase changes and corrosion, isolation of active materials, and local, poor conductivity of materials in the discharged state, etc. 

Opportunities for application of new materials as components in electrochemical cells (electrodes, electrolytes, membranes, and separators) are discussed in this section. In addition, electrochemical processing is considered in the sense that it presents opportunities for the synthesis of new materials such as electroepitaxial GaAs, graded alloys, and superlattices. Finally, attention is focused on the evolution of new engineering materials that were developed for reasons other than their electrochemical properties but that in some cases are remarkably inert (glassy alloys). Others that are susceptible to corrosion (some metal-matrix composites) and more traditional materials that are finding service in new applications (structural ceramics in aqueous media, for example) are also considered briefly. 

The technical challenges posed by these systems are different from those facing low- to medium-temperature cells. For instance, there are no severe kinetic limitations at the electrodes or poisoning of electrocatalysts by impurities (other than sulfur) in the fuel gas. Instead, material science issues arise with (i) sintering of the electrodes and the electrolyte matrix, (ii) corrosion of cell components in molten salt electrolytes (MCFC), (iii) electrolyte migration in the external manifolds of MCFCs and (iv) differential expansion coefficients of the materials of construction in all-solid-state systems (SOFCs). 

For taking place corrosion, the formation of a corrosion cell is essential. A corrosion cell is basically composed as an electrolytic cell. It includes the following four components (Figure 10.3) ... 

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

A fuel cell decays with time, and the rate of decay determines its durability. The decay is related to the aging of the fuel cell components, especially the membrane electrolyte, the catalysts, and the catalyst support. The decay of the membrane will cause its thinning and mechanical property deterioration. The loss of its mechanical properties often causes a fuel cell to fail prematurely and catastrophically. The decay of the catalyst is normally due to the particle size increase and the particle dissolution and redistribution. Catalyst decay rarely causes a sudden failure of a cell. The decay of the catalyst-support is often related to its corrosion. Corrosion makes the electrode more prone to flooding and accelerates the growth and redistribution of the catalyst particles. 

The high operating temperature of MCFCs provides the opportunity for achieving higher overall system efficiencies and greater flexibility in the use of available fuels compared with the low temperature types. Unfortunately, the higher temperatures also place severe demands on the corrosion stability and life of cell components, particularly in the aggressive environment of the molten carbonate electrolyte. 

In general, these points serve to illustrate the heterogeneous nature of corrosion reactions and indicate the need, whenever possible, to identify the component electrodes and electrode reactions of the corrosion cell . Furthermore, it can be seen that compositional changes in the electrolyte are always involved both in facilitating corrosion and as a result of the process. 

In this section, the relationship of corrosion inhibitors to anodic and cathodic polarization wiU be explained. Of the four components of a corrosion cell (anode, cathode, electrolyte, and electronic conductor), three may be affected by a corrosion inhibitor to retard corrosion. The inhibitor may cause ... 

At the beginning of PAFC development, diluted phosphoric acid was used in PAFCs to avoid corrosion of some of the cell components. Nowadays with improved materials available for cell constmction, the concentration of the acid is nearly 100%. The acid is usually stabihzed in a matrix based on SiC. The higher concentration of the acid increases the conductivity of the electrolyte and reduces the corrosion of the carbon-supported electrodes. 

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