Corrosion cell components

A prerequisite of long-life sodium/sulfur batteries is that the cells contain suitable corrosion-resistant materials which withstand the aggressively corrosive environment of this high—temperature system. Stackpool and Maclachlan have reported on investigations in this field [17], The components in an Na/S cell are required to be corrosion-resistant towards sodium, sulfur and especially sodium polysulphides. Four cell components suffer particularly in the Na/S environment the glass seal, the anode seal, the cathode seal, and the current collector (in central sodium arrangements, the cell case). 

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). 

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]. 

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. 

The most serious problems with this system are, however, concerned with corrosion of cell components and the development of satisfactory... 

There are corrosion problems of the anode and other cell components in HNO3/HCI. 

Wang, Heli he has worked at the National Renewable Energy Laboratory (NREL) in the United States. He received a Ph.D. in corrosion science and materials chemistry from the Helsinki University of Technology in Finland. From 1998, he had worked with nanostructured semiconductors of metal oxides at the Department of Physical Chemistry, Uppsala University, Sweden. His research work has been in materials, electrochemistry, photoelectrochemistry, as well as fuel cell components. 

Fatigue data of corrosion resistant steels and other relevant high strength materials are easy available for non-corrosive ambiance (air, oil). For the large variety of liquids applied in production processes the data have to be evaluated from special fatigue tests with corrosion cells. This paper answers the question if such uniaxial test results can be applied for the real component design in order to meet three-axial corrosion fatigue reality well. 

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). 

Figure 5.21 shows the expressions for the ac impedance of simple electrical circuits (Fig. 5.21a) applicable to corrosion systems. When the corrosion cell is represented only by resistance, the current is in phase with voltage, 0 = 0 and the impedance has no imaginary component, Z=R. 

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) ... 

Failure to understand the nature of shunt currents in bipolar cell stacks frequently leads to corrosion problems. Corrosion occurs most frequently where shunt currents leave a metal component of the cells or piping system, but corrosion can also occur where the shunt currents re-enter a metal component. Such corrosion can occur at nearly any metal component of an electrolyzer, but the following are the most common cell nozzles, manifold ports, edges of the main cathode in a unit cell adjacent to an inlet or outlet, metal cell components electronically in contact with the cathode and adjacent to an inlet or outlet of the catholyte compartment, and pipe walls adjacent to the flanges of a manifold when these manifold flanges are located near the center of a DC circuit. 

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. 

Atmospheric corrosion takes place by means of an electrochemical process occurring in corrosion cells. A corrosion cell must have the following essential components ... 

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