19.2.1 Introduction

Today’s growing energy needs in many countries must be satisfied both cost‐effectively and reliably while still conserving our environment and resources for future generations. A technology that supports these requirements is the fuel cell. It will have a major role in the evolution of serving society’s energy needs (Reiser and Schroll 1981).

There are a number of different fuel cell types. They are characterized by the electrolyte used and the operating temperature. Low temperature fuel cells are alkaline fuel cells (AFC), the proton exchange membrane fuel cell (PEMFC), and the phosphoric acid fuel cell (PAFC). These cells operate at 80, 100, and 220 °C, respectively. At this low temperature, the strain on materials is only moderate; however expensive noble metal catalysts are required to facilitate the electrode reactions. High temperature fuel cells such as the MCFC and the solid oxide electrolyte fuel cell (SOFC) operate at 650 °C and 900–1000 °C, respectively. At these temperatures, HTC can become a serious problem, and the choice of the proper materials has great influence on the lifetime and performance of the fuel cells.

In principle, fuel cells function like batteries. The difference is the continuous supply of fuel, in most cases hydrogen or hydrocarbons like methane. The fuel is electrochemically oxidized at the anode, while oxygen is electrochemically reduced at the cathode. When hydrocarbons such as natural gas are used as fuel, a reforming step has to precede the fuel cell reactions. The reforming converts the methane into hydrogen and CO₂ by Reactions 19.2 and 19.3:

19.2

19.3

Reaction 19.2 is strongly endothermic while Reaction 19.3 is slightly exothermic. In total, the reforming requires energy and therefore lowers the net efficiency of the system. To facilitate the reactions, a reforming catalyst is usually employed.

The hydrogen is consumed by the electrochemical fuel cell reaction, which liberates two electrons for each hydrogen molecule. The electrons flow via an electrical load to the cathode where they are involved in the reduction of oxygen. Equations (19.4)–(19.8) describe in a simplified manner the reactions taking place for the different fuel cell types at the anode and the cathode (Table 19.2).

In the fuel cells, the chemical energy of the reactants is directly converted to electrical power and heat.

The electrical efficiency of fuel cell systems ranges from about 40% to 65%. By stacking a number of cells on top of each other, the voltages of the individual cells are added up to technically relevant stack voltages. Nowadays, an individual cell supplies about 800 mV at a load of 160 mA cm⁻². It is an important goal to improve this number in the future by reducing losses in the cells caused by polarization and ohmic resistivities. Fuel cell stacks produce DC power, which is converted to AC power by means of an inverter before it is supplied to the grid.

Reducing our carbon footprint is widely acknowledged as one of modern society’s top priorities, as well as building a sustainable economy based on knowledge and innovation for enduring opportunities of development. MCFCs offer rich potential in these terms as a forward‐looking and highly flexible way to reduce CO₂ emissions, providing more efficient and cleaner, greener energy, making use of both fossil and renewable sources.

MCFCs are a key technology for stationary applications, especially in the size of hundreds to thousands of kilowatts, which is a very interesting power range in view of the increasing decentralization of energy supply and the increased need for high‐quality power independent of the grid. After several years of research programs and extensive demonstration, MCFC‐based systems are now appearing in commercial ventures of multiple megawatts, providing clean energy to commercial and small/midsize industrial customers all over the world. Especially in this phase of early deployment, and with a view to stay at the forefront of smart solutions for the evolving energy paradigm, to improve the technology, increase reliability, and reduce manufacturing costs, a lot of effort is still required from research and development to safeguard the relevancy and make real the enormous potential of MCFC solutions in the near and long‐term future.

The investment cost for MCFC systems is decreasing steadily and is below US$ 4000 kW⁻¹ today, and further cost reduction would be achieved if the production volume increased. MCFC is the most mature high temperature fuel cell technology available nowadays, with several plants already producing energy, as reported above. The installed power is already about 24 MW (around 85 units), with 6 MW being installed in Europe and 18 MW in North America and Asia. Two main companies are responsible for the field trials: Fuel Cell Energy (FCE) in the United States and CFC Solutions in Germany. CFC Solutions recently presented results after 25 000 hours (about three years) of operation for one of their main field trials. The shutdown of the power plant was planned and was not due to any failure. Furthermore, FCE has proved high reliability in their systems and has produced to date more than 20 GWh electrical energy in total. Typical customers are food and beverage industries, hotels, hospitals, prisons, wastewater treatment plants, and manufacturing industries, where key values such as reliability, efficiency, and green technology are important and where the fuel for the power plant is sometimes their own waste.

However, companies such as Ansaldo Fuel Cells in Italy are also not behind, and research centers in South Korea and Japan are developing the technology in close collaboration with industry. Table 19.3 gives a contact list for MCFC deployment.

In the present section the basic principles of MCFCs and the status of the fuel cell development will be considered briefly, followed by a more emphasized analysis on its limitations by corrosion problems and cost issues.

19.2.2 The Operating Principles of an MCFC

The first MCFC was demonstrated by Broers in 1958, and the first MCFC at high pressure was built by Reiser and Schroll in 1980. At present, the MCFC is the most efficient fuel cell, and this will be discussed hereinafter. The MCFC, operating at a temperature between 600 and 650 °C, is generally considered a second‐generation fuel cell. It can be used with coal gas and even more so with natural gas as a fuel (Glasstone and Sesonske 1994).

The electrodes are porous, favorable for gas diffusion in the reaction zone, i.e. at the contact between gaseous reactants, liquid electrolyte, and electrode.

Molten carbonates are an extremely corrosive medium for the majority of metals and alloys at temperatures around 600 °C. Therefore, the choice of materials at a reasonable cost, to be used as stable cathodes under an oxidizing atmosphere such as air or oxygen–CO₂ mixtures, is limited. Only semiconducting oxides are materials with such properties. In the MCFC, the currently used material for the cathode is lithiated nickel oxide. Initially, the cathode is metallic nickel, but during the first period of time of cell operation, a lithiation and oxidation of the nickel occurs spontaneously in the presence of lithium carbonate under an oxidizing atmosphere at high temperature. The original structure of the material is completely changed due to these phenomena. The cathode mass acquires many very small pores, which increase the contact surface area between the electrolyte and the gaseous reactant. The thickness of the NiO cathode ranges from 0.4 to 0.8 mm, and it has an electronic conductivity of approximately 5 Ω⁻¹ cm⁻¹.

The MCFC anode operates under reducing atmosphere, at a potential typically 700–1000 mV more negative than that of the cathode. Many metals are stable in molten carbonates under these conditions, and several transition metals have electrocatalytic activity for hydrogen oxidation. Nickel, cobalt, copper, and alloys in the form of powder or composites with oxides are usually used as anode materials. Ceramic materials are included into the anode composition to stabilize the anode structure (pore growth, shrinkage, loss of surface area) at the time of sintering. An alloy powder of Ni + 2–10 wt% Cr can be used. The initial formation of Cr₂O₃, followed by surface formation of LiCrO₂, can stabilize the anode structure.

The electrode has a particular structure. A mixture of LiAlO₂ and alkali carbonates (typically >50 vol%) is hot‐pressed (about 5000 psi) at temperatures slightly below the melting point of the carbonate salts. In this way, a porous matrix support material of ceramic particles (LiAlO₂) is formed that contains a capillary network filled with molten electrolyte. The ceramic material in the electrolyte structure represents a mechanical resistance, which does not participate in the electrical or electrochemical processes. The prepared electrolyte has a thickness of 1–2 mm, and it is very difficult to produce it in large shapes.

The preferred electrolyte is a binary lithium–potassium carbonate consisting of 62 mol% Li₂CO₃ and 38 mol% K₂CO₃, with a liquidus temperature of 490 °C.

Figure 19.1 shows an outline of an MCFC cell that uses a gaseous mixture of H₂ + CO. The principal electrode reactions are

19.9

19.10

19.11

19.12

19.13

19.14

19.15

A gaseous mixture of H₂ and CO can be obtained by coal gasification or reforming of natural gas. That mixture reacts at the anode with molten ions to produce CO₂ and H₂O vapors, releasing electrons to the external circuit. Also, in the anode compartment, in the presence of metallic cell parts, at 650 °C, CO present in the fuel gas reacts with H₂O vapor, yielding hydrogen, as shown in Eq. 19.3.

At the anode, hydrogen is oxidized more easily than CO, so Reaction 19.10 is more important than Reaction 19.11. For this reason, the main electroactive species is hydrogen.

Carbon dioxide produced at the anode is recycled to the cathode where it reacts with electrons and atmospheric oxygen to regenerate the carbonate ion consumed at the anode. Two mechanisms have been proposed for the anode reaction. The first is

19.16

19.17

19.18

The second is Reaction 19.16 with Reaction 19.17 followed by

19.19

where Reaction 19.17 is the rate‐determining step (r.d.s.).

The cathodic mechanism is complex, and it depends on the melt composition, especially the acidity of the electrolyte, namely, the cation composition. In less acidic electrolytes (see Reaction 19.21), the peroxide mechanism prevails:

19.20

19.21

19.22

where is the peroxide ion and O²⁻ is the oxide ion.

In acidic melts, however, the superoxide mechanism is dominant (Reaction 19.24):

19.23

19.24

19.25

19.26

The relative basicity of the commonly used alkali carbonates is

The cathode reaction mechanism is not so well understood, and more studies are necessary to elucidate it as a function of the electrode material and electrolyte composition.

The reversible cell potential for an MCFC depends on the gas composition at the anode (partial pressures of H₂, H₂O, and CO₂) and at the cathode (partial pressures of O₂ and CO₂):

19.27

where the subscripts a and c refer to the anode and cathode gases, respectively. When the CO₂ partial pressures in the cathode and the anode gases are identical, the cell potential depends only on the partial pressures of H₂, O₂, and H₂O. The values of E⁰ (the standard cell potential) for hydrogen oxidation and for several other reactions at 650 °C are given in Table 19.4.

From Eq. 19.27 it follows that a pressure increase from P₁ to P₂ causes an increase in the reversible cell voltage:

19.28

Therefore, a 10‐fold increase in cell pressure corresponds to an increase of 46 mV in the reversible cell potential at 650 °C. Both the cell voltage and the gas solubility in the electrolyte increase with the gas pressure.

A serious difficulty is the solubility of the nickel oxide cathode in the electrolyte. The solubility of NiO depends on the CO₂ partial pressure, according to the equilibrium

19.29

The equilibrium constant of this reaction is about 5.7 × 10⁻⁶ at 600 °C. The NiO solubility is low in basic electrolytes or at high oxide ion concentrations (O²⁻). Under the conditions of an operating fuel cell, the dissolution reaction that takes place is

19.30

Therefore, in the presence of an excess of O²⁻ ions, the solubility of NiO is suppressed. The solubility of NiO is lowered by a factor of 3 in the Li₂CO₃–Na₂CO₃ mixture compared with the Li₂CO₃–K₂CO₃ mixture. The Ni²⁺ ions formed at the cathode migrate toward the anode under the influence of the electrical field and the concentration gradient. At the anode, the Ni²⁺ ions are reduced and deposited, and for this reason short circuiting of the cell can result after some time. Alternative cathode materials are LiCoO₂, LiFeO₂, and LiMnO₃ (see Section 19.2.4). Recently, many corrosion studies of the materials used in MCFC were made (see Sections 19.2.3–19.2.7). These works led to an increase of cell life up to 50 000 hours.

The internal reforming MCFC has a particular construction. In the anode chamber there is a catalyst for the reforming reaction of natural gas. In this case, the following reactions occur at the anode:

19.31

19.32

19.33

19.34

The main product from the reforming reaction, which is H₂, is consumed in the anode reaction.

Figure 19.2 attempts to show the principle of an MCFC stack. The separator plates (also called bipolar plates) and current collectors function as the single‐cell housing components and provide cell‐to‐cell electronic contact. H₂ + CO enter at the anode side by flowing through the anode corrugated current collector. The oxidizing gas, consisting of O₂, CO₂, and H₂O, flows through a corrugated current collector at the cathode side.

In general, the single operating cell voltage is of the order of 800 mV for current densities of the order of 160 mA cm⁻². To produce larger currents and useful voltages, it is necessary to increase the electrode active area and to stack individual cells in series. It is in this context that the bipolar plates, which are in contact with the anode and cathode current collectors, play a key role, connecting the single or individual cells in series. Therefore, cells with thicknesses of about 5 mm and areas larger than 1 m² can be superposed, composing stacks of many MW. Recent studies of research and development deal with issues of the component materials and their corrosivity in the molten salt environment, as discussed in the following subsections.

19.2.3 MCFC Anode Materials

The MCFC anode is a porous structure like the cathode, which allows diffusion of gases and collects current from triple‐phase boundary (TPB) interfaces at reaction sites. General materials required for MCFC anode have good electrical conductivity and structural stability and suitable catalytic properties for the type of fuel used. Since the reaction kinetics (oxidation of fuel) is faster at the anode side of MCFC at the operating temperature, the base metal catalysis (Ni) is sufficient for hydrogen fuel. Therefore less surface area is acceptable in the case of the anode as compared with the cathode. Since the early stages of development of MCFC, the pure Ni‐based anodes have demonstrated reasonable electrochemical performance with polarization losses of less than 30 mV at 160 mA cm⁻².

One of the issues with the pure Ni anode is performance degradation and shrinkage due to creep and sintering. Furthermore, the manufacturing processes and typical operating conditions of MCFC lead to compressive stress at high temperatures on MCFC components including the anode. These conditions are clearly favorable for the creep‐type failures of metallic components in MCFC.

The creep exponents of pure Ni anode are close to the Nabarro–Herring‐type creep (diffusion of vacancies within crystal lattice) or Coble‐type creep (diffusion of vacancies via grain boundaries). Apart from the creep, the decrease in the surface area and shrinkage due to sintering (via neck formation–mass transport) cause the degradation in performance of anodes during the initial few days of MCFC operation. To overcome these problems, alloying of Ni anode with various metals such as Cr, Al, and Cu is a common approach taken by the developers. Arresting the creep by addition of intermetallics is a well‐proven technique used in high temperature metals engineering. Wee et al. (2005) studied the effect of addition of intermetallic powders (Ni₃Al) on the sintering and creep resistance of the NiO anodes. They found that the addition of 5 wt% Ni₃Al phase along with 3 wt% Al decreases the shrinkage and also prevents the porosity collapse.

As in the case of cathodes, an improvement in the electrochemical performance of the anode is desired. As MCFC operation relies on the ionic exchange between the melt and solid electrodes, the wetting of the electrodes (the wetting angle) especially on the anode side is an important parameter determining the MCFC performance. The wetting angle for standard MCFC anodic gas composition is around 50° with (Li/Na)₂CO₃ and 31° (Li/K)₂CO₃. Hence, efforts are being made to improve the wetting of the anode using the coatings or additives. Apart from the material composition and morphology, the electrolyte melt distribution and amount in the anode pores are important characteristics that determine the performance of the anode. For Ni–Cr anode 5–25% electrolyte fill demonstrates the maximum performance with Li/Na carbonate melt as an electrolyte (Yoshikawa et al. 2006). However, some researchers have investigated the use of anodes as an electrolyte reservoir to compensate for the electrolyte loss from the matrix during long‐term operation of MCFC. Youn et al. (2006) reported the use of Ni‐10 wt% Cr anode as an electrolyte reservoir. The anode was coated with boehmite (γ‐AlO(OH)) solution via a dip coating process, which was then converted into Li‐aluminate particles in situ during the cell operation. The surface modifications allowed an increase in the electrolyte filling of anode to 50–60 vol%. The coating resulted in good electrolyte wettability as compared to bare Ni surface, which partially compensates for the decrease in conductivity by providing additional sites for the reaction. The polarization characteristics of a coated cell with additional electrolyte were found to be slightly inferior to a standard cell (25 vol% electrolyte), but the results suggest that the surface modification could be used to modify the anode surface to make it function as an electrolyte reservoir.

As MCFC systems are considered mainly for stationary power generation, significant efforts have been made over the past few decades to develop MCFC systems fueled with “real‐world fuels” such as natural gas, digester gas, and allied hydrocarbon fuels. These fuels can be reformed to H₂ and CO inside the MCFC unit or externally using a separate reformer.

Since heat from the electrochemical oxidation of fuels and steam form can be used for reforming inside the anode chamber, the MCFCs with direct in situ reforming (DIR) are more efficient than those with external reforming. In a high temperature fuel cell such as SOFC operating at temperatures from 800 to 900 °C, the reforming can be achieved using the Ni–YSZ anode itself as a catalyst. In the case of MCFC, DIR is achieved by placing the reforming catalysts into the fuel channels as the catalytic activity of the conventional MCFC anode is not sufficient for reforming because of the lower surface area and lower operating temperature of MCFC.

The placing of the catalyst in fuel channels can be accommodated by modification of cell hardware. Typically, supported metal‐type catalysts are used for internal reforming. Commercial suppliers of MCFC systems (such as FCE and Mitsubishi) and research organizations (CNR‐TAE Institute) developing MCFCs have tested DIR MCFCs with catalysts such as NiO/MgO, Ni/alumina, and Ru/ZrO₂.

Apart from internal reforming catalysts, the anode itself needs to be tolerant of impurities such as sulfur and carbon. To make the anodes more tolerant toward the impurities, the anode materials have been modified by coating with additives or catalysts. Fang et al. (1998) reported the surface alloying of NiO anode with niobium using a molten fluoride process. The surface alloying demonstrated significantly lower corrosion rate (0.02 nm yr⁻¹ compared with 0.17 mm yr⁻¹ for bare NiO) in a carbonate bath and improved electrocatalytic activity toward CO oxidation. The lower solubility is attributed to the formation of a composite phase NiO, Nb₂O₃, and the improvement in the wetting angle and increased surface area after the surface treatment.

Other approaches to mitigate the problem of deactivation of DIR catalysts are indirect internal reforming (IIR) and the use of a separator plate. In IIR, the reforming catalyst unit is kept in thermal contact with the MCFC unit with exit ports of the IIR unit connected to entry port of the anode (Baker 1989). In this arrangement, the catalyst is not exposed to the anodic atmosphere, and thus it increases the life of the catalyst. Furthermore, the IIR arrangement also offers a better thermal gradient in the stack as compared with DIR. However, the efficiency of IIR unit is lower than DIR as hot steam formed at the anode is not used in IIR. To combine the advantages of both IIR and DIR systems, the commercial developers FCE, Inc. and MELCO used a hybrid approach (Vielstich et al. 2007). The partially reformed fuel from IIR catalyst is further reformed at the anode with DIR. The hybrid design improves the thermal distribution inside the cell and thus improves the overall performance and stability.

19.2.4 MCFC Cathode Materials

The cathode is most often made of lithiated NiO, usually oxidized and lithiated in situ. The dissolution of the cathode material has been one of the main issues for MCFC research although posttest characterization of the recently performed long‐term field trials has shown that the problem may be smaller than the models have predicted. The nickel oxide dissolves according to the mechanism described above, in which nickel oxide and carbon dioxide form nickel ions and carbonate ions, as shown in Eq. .

The dissolved nickel ions are then transported from the electrolyte in the pore of the cathode into the matrix. Near the cathode, the nickel ions react with hydrogen, dissolve in the melt, and precipitate as metallic nickel, forming chains that eventually short‐circuit the cell (Yoshikawa et al. 2001). Since the time to short circuit the cell depends on the equilibrium concentration of nickel dissolved in the melt, research has aimed at a reduced solubility of the cathode. This can be done in three ways: changing to another cathode material, stabilizing the present nickel oxide, or changing the melt composition.

Alternatives to nickel oxide as cathode material should have equal or higher electrocatalytic activity, good conductivity, even lower dissolution, mechanical stability, and low cost combined with an inexpensive manufacturing process. Over the years, several alternative materials have been investigated for use as cathode in MCFC (Bergman et al. 2001; Selman et al. 1990; Uchida et al. 1999; Young 1960). LiCoO₂ and LiFeO₂ have been tested more extensively. LiCoO₂ has a lower solubility than nickel oxide at carbon dioxide pressures below 2 atm and a comparable performance, but the higher cost, brittleness, and the higher contact resistance limit the use of LiCoO₂. On the other hand, LiFeO₂, which has low dissolution and is less expensive, has a too low electrocatalytic activity and conductivity to be used as cathode material. One solution to the problem has been binary or ternary mixtures of the oxides LiCoO₂, LiFeO₂, and NiO (Wijayasinghe et al. 2006). Coatings onto the nickel oxide, most often containing cobalt or iron, have however been the most common approach to stabilizing the nickel oxide with promising results (Escudero et al. 2005), regarding both the solubility of nickel and the performance. To further increase the stability or conductivity, a number of oxides, added to all the three types of cathode materials (pure oxides, mixtures, and coatings), have been investigated (Huang et al. 2004).

The cathode dissolution is also lowered by increasing the basicity of the melt where the basicity is defined as the ability to donate oxide ions. Ota et al. (1992) showed that the degree of basicity for the three carbonates was (in decreasing order) Li₂CO₃ > Na₂CO₃ > K₂CO₃ by varying the melt composition in the region where the acidic dissolution mechanism is valid. Therefore, lithium–sodium carbonate as well as electrolytes with high contents of lithium carbonate is considered to have lower nickel solubility than the traditional lithium–potassium carbonate. However, increased lithium contents result in lower solubility and diffusivity of the gases in the melt, and the gas solubility and reaction rate of sodium containing melts are more temperature dependent. Therefore, a change in the electrolyte composition may cause lower or uneven performance, and the cell design and the operating conditions will determine the melt composition used.

Instead of changing to another alkali carbonate, the basicity of the melt could be increased by adding oxides of alkaline earth metals or lanthanum.

Although lanthanum seems to have the best effect to decrease the solubility of NiO, Matsuzawa et al. (2005) and Mitsushima et al. (2002) showed that a combination of adding MgO to the melt and having a MgO containing cathode results in a synergy effect, leading to even lower nickel dissolution. It is not clear, however, if the effect of the additives will remain during long‐term operation since the segregation of the electrolyte can cause the additives to migrate to the anode side, thus decreasing the additive concentration at the cathode side of the cell (Carlin 1996).

It should be noted though that recent evaluation of long‐term field trials on single cells (25000–40 000 hours) shows a lower effect of the nickel oxide dissolution than the models have predicted. This brings research in the field of nickel dissolution up to date, where the models and predictions have to be reviewed and improved to better predict the results before more experimental work is performed.

19.2.5 MCFC Bipolar Current Collectors

Needless to say, protection of the bipolar plate from corrosion is essential for the entire stability and performance of the cell. From a design point of view, the bipolar plate is frequently composed of three distinct metallic components: the separator plate, the current collector, and the center plate. Schematically, separators are corrugated plates that must fulfill the following main functional requirements: (i) separate fuel and oxidant gas streams; (ii) create flow channels for the gases to pass the electrodes; (iii) provide electrical contact between adjacent cells (in combination with the current collectors and the center plates); and (iv) provide a tight gas flange by extending the electrolyte tile to the plate edges where it is sandwiched between two plates (wet seals) (Figure 19.3). The purpose of the current collectors and center plates is mostly to reduce the contact and corrosion areas of the separator plate with the electrolyte.

Separator plates, current collectors, and center plates must simultaneously satisfy various chemical, electrical, and mechanical requirements, and therefore they are usually made of the same materials. For the sake of simplicity, we will refer to them as a whole with the term bipolar plate.

The most critical requirement is undoubtedly the corrosion resistance as the bipolar plate must tolerate a wide range of aggressive chemical conditions intermediate between the highly oxidizing cathode environment and the highly reducing character of the anode in the presence of a liquid salt. Strictly speaking, the bipolar plate experiences different corrosion conditions along its length. In Figure 19.3, the letters A–H identify such corrosion cell as:

1. Regions in contact with thin layers of molten carbonate in highly oxidizing gas environment.
   
2. Regions in contact with thin layers of molten carbonate in a less oxidizing gas environment.
   
3. Regions in contact with thin layers of molten carbonate in a reducing gas environment.
   
4. Regions in contact with thin layers of molten carbonate in a less reducing gas environment.
   
5. Regions in contact with deep layers of aerated molten carbonate.
   
6. Regions in contact with deep layers of scarcely aerated molten carbonate.
   
7. Regions in contact with deep layers of molten carbonate in a mixed reducing/carburizing gas environment.
   
8. Regions in contact with deep layers of molten carbonate in a poorly reducing gas environment.

In a rather, although widely used, oversimplified approach, these different corrosion areas can be conveniently grouped in (i) cathode region (points A, B, E, F), (ii) anode region (C, D, G, H), and (iii) anode (H) and cathode (F) wet‐seal regions.

Design modifications have been found useful to mitigate the corrosion problems by reducing the wetting areas with electrolyte. For instance, Shimada (1996) describes a “soft” plate that is flexible enough to adsorb the deformation of the active components by means of flat springs contained inside the wet seal that ensure the necessary component pressures. In this way, a pressed current collector with a gas flow channel function could be used instead of a corrugated separator to reduce wetting and, in turn, corrosion areas. A similar pressed plate structure directed to reduce the number of components and the contact areas has been tested with promising results in terms of corrosion and electrolyte loss (Selman et al. 1997).

Another important requirement is that the bipolar plate material should be a metallic conductor. Additionally, the corrosion products must also be sufficiently conductive (σ > 10⁴ S cm⁻¹) and insoluble in the carbonate melt. Finally, several mechanical requirements are associated with fluid flow, high temperature mechanical resistance, proper contact of the components, weldability, and easy formability.

Table 19.5 lists about 60 different high temperature alloys that have been so far evaluated by various developers (Yuh et al. 1995).

The austenitic stainless steels 316L and 310S are the current choices for their appropriate cathode‐side corrosion resistance and a relatively low cost. Regarding the role of alloying elements in the corrosion resistance of commercial steels, chromium is the element that confers the best corrosion resistance under both cathode and anode conditions, whereas nickel is less important or has a slight negative effect in oxidizing environments. Aluminum results in high corrosion resistance but also in corrosion layers with high electrical resistance.

To improve their anode‐side corrosion resistance, Ni‐cladded or Cr‐plated stainless steels can be used there (nickel is thermodynamically stable in the reducing anode gas conditions). Both electroless and electrolytic plating methods have been evaluated, but they are rarely used due to their higher costs and to a lesser corrosion resistance. With the Ni‐clad structure very dense, a 50–100 μm thick layer is adequate to provide the best protection to corrosion, thermal cycles, and interdiffusion. High nickel‐based alloys show appropriate anode‐side corrosion, although they are scarcely used because of their cost and an insufficient resistance in the cathode compartment.

It is easily understood that the formation of a corrosion scale with a poor electrical conductivity could result in a voltage loss so that the ohmic drop at the bipolar plate/electrode interface would tend to increase as the corrosion proceeds. The increase of ohmic drop on the cathode side due to scale growth is estimated to contribute to the cell decay rate for less than 0.8 mV/1000 hours (≈1%/1000 hours), if AISI 316L is used (Fujita and Urushibata 1996). However, this number may not be acceptable for a 40 000 hour operation in future MCFC systems, where a cell decay rate of 0.25%/1000 hours has been recently targeted. The following table evidences that the bipolar plate corrosion is one of the most important items to be solved for reducing the cell decay (Tatsumi et al. 1996) (Table 19.6).

Based on posttest analysis of failed MCFCs, the influence of corrosion is not limited to ohmic losses, but many results have indicated that corrosion degrades also the functionality of the cell components (both metallic and active ones). A good presentation of this problem has been discussed by Singh (1983) to which the reader is invited to refer for a more detailed analysis. Here a brief excerpt of his work is presented. The main corrosion factors contributing to the cell performance decay can be grouped in the following categories: (a) dimensional and (b) mechanical change of the plate, (c) loss of electrolyte and gas leakage, and (d) chemical contamination of electrodes and electrolyte.

1. Dimensional changes: A dimensional stability of the bipolar plate may be undetermined if the anodic side of the plate is not being protected by nickel cladding. Uncoated stainless steel in fact experiences excessive scale growth on the anode side, resulting in an intolerable increase of plate thickness. These dimensional changes tend to generate compressive stresses in the cell components causing formation of cracks and breakdown of the most brittle cell components such as tile and electrodes. Excessive scaling also results in the scale crack with anomalies in the gas path geometry. The cell decay rate may be enhanced because of fluctuations in the gas composition and localized use of the fuel.
   
2. Mechanical changes: Mechanical and load‐bearing properties of the bipolar plate components have been sometimes observed to be strongly affected by scaling and carburization processes. Excessive scaling on stainless steels reduces in fact the effective thickness of the metallic component, whereas carburization, i.e. precipitation of chromium‐rich carbides inside the metal matrix and at grain boundaries, may lead to hardening of the component. Both the effects are deleterious because the reduction of mechanical resistance could cause the breaking of the remaining metallic components.
   
3. Electrolyte and gas leakage: The electrolyte loss is caused by reactions between the oxide scale grown on the bipolar plate and the molten carbonate, resulting in a consumption of the alkali metal electrolyte components and formation of reaction products such as LiCrO₂, K₂CrO₄, Li₂CrO₄, LiFeO₂, and others. The increasing and never recovered loss of carbonate melt during cell operation causes an increase of the polarization at electrodes and an increase of the internal resistance at the electrolyte. Likewise, excessive electrochemical corrosion in the wet‐seal area may lead to poor scaling with both gas leakage and electrolyte loss effects.
   
4. Chemical contamination: Formation of both soluble and insoluble oxide layers normally occurs during the corrosion reactions between the metallic components and the carbonate melt. Thus, at the anode compartment, corrosion of stainless steels proceeds with the initial formation of soluble iron oxide, which later precipitates at the melt–gas interface (fluxing dissolution). At the cathode side, under very oxidizing conditions, soluble chromates become the more stable corrosion products. The dissolution of these oxide contaminants into the electrolyte changes the melt chemistry in terms of oxide ion activity (acid/basic properties). Also the NiO cathode electrode can be contaminated by incorporation of aliovalent iron(III) or chromium(III) ions. This changes the defective structure of the cathode, which could result in changes of its electronic conductivity.

19.2.6 MCFC Wet Seal

The cell‐perimeter seal area simultaneously experiences reducing and oxidizing environments (Donado et al. 1984). MCFC seal is provided by a molten electrolyte‐filled matrix (wet seal).

The concept of the wet‐seal flange is nowadays largely applied in the MCFC bipolar plate fabrication as a method of minimizing corrosion and sustaining large differential pressures across the stack. Successive improvements have been performed in the seal design from the pioneeristic work of Davtyan in the 1940s (1946). In the “wet‐seal technique,” the bipolar plate is pressed against the flat surface of the electrolyte tile (i.e. the solid porous support filled with the carbonate mixture). At the MCFC working temperature, the molten electrolyte wets the metallic surface and forms the gas wet seal. Although the width dimension of the wet‐seal area is relatively small (usually 5–10 mm, i.e. about only five times the tile thickness), it has long been realized that corrosion of the wet‐seal area metal is particularly critical and may lead to a poor sealing with consequent gas leakage and rapid decay of the cell performance. An excessive corrosion may also lead to a critical electrolyte loss from the tile causing catastrophic failure (Donado et al. 1984). The use of the wet‐seal technique results in the onset of galvanic couples whose corrosion currents are often limited by the mass transfer rates of O₂ and CO₂. Fe and Ni‐based alloys have been found to offer limited resistance to this kind of attack since their corrosion products are usually too conductive to block the current paths. Thus, for instance, AISI 316L was so severely corroded as anode wet‐seal material during short‐term tests (2000 hours) cannot be absolutely used without protection (Lovering 1982). Corrosion in the cathode wet seal was found to be about 2 orders of magnitude lower than at the anode side (Lovering 1982); therefore AISI 316L could be used without significant problems in the cathode wet seals in short‐term MCFC operation (a few thousand hours).

Methods of minimizing the galvanic corrosion of wet seals are very limited. A review on this subject has been published by Pigeaud et al. (1981). Based on the consideration that an insulating material is desirable to break the corrosion cells, the possibility of using more than one type of material for the bipolar plate was early examined. Aluminum‐containing alloys, such as Kanthal A‐1, reduce the corrosion rate in the anode gas environment by at least 2 orders of magnitude to less than 0.002 cm in 1000 hours with respect to the AISI 316L (Lovering 1982). This is ascribed to the formation of an insulating LiAlO₂ thin surface layer. However, this approach was not pursued for the high costs of fabrication of metallic bipolar plates. Aluminum foil gaskets were also investigated but with unsatisfactory results because the Al melting point (c. 660 °C) is so close to the MCFC operating temperature that even small temperature fluctuations in the cells can melt the gasket (Yuh et al. 1987). Currently, the only followed approach is to protect the stainless steel by deposition of aluminum diffusion coatings in the wet‐seal area. Aluminized stainless steels are in fact known to provide HTC resistance in both oxidizing and reducing environments by forming a dielectric alumina thin film. In the presence of carbonate, alumina converts to LiAlO₂, which is also effective in inhibiting the corrosion cells with a minimal consumption of electrolyte, thus providing the required long‐term stability to the wet seal.

Various aluminizing processes have been so far evaluated for their effectiveness, including painting, thermal spraying, vacuum deposition, and pack cementation (Yuh et al. 1987, 1995). At the present time, the ion vapor deposition (IVD) method followed by a diffusion heat treatment is generally considered to offer the most protective and adherent aluminized coating in the MCFC wet‐seal environment. A detailed description of the principles of IVD coating method is given in the Metals Handbook (1982). Diffusion bonding is obtained at 900–1000 °C for one to three hours in a reducing atmosphere. The resultant IVD coating is dense and uniform, mainly consisting of an intermetallic MAl–M₃Al structure (M = iron, nickel plus 5–10 wt% Cr). Concentration of Al in the diffusion layer ranges from the 50 wt% of the outer layer to the 30 wt% of the inner layer, values that are much higher than those obtainable by other methods (Yuh et al. 1987). This confers to the IVD coatings the sufficient long‐term stability and durability required for a 40 000 hour cell operation.

19.2.7 MCFC Electrolyte and Matrix

MCFCs use mixtures of alkali carbonates as electrolyte. Structure and properties of alkali carbonate melts are under study since the beginning of the 1960s (Bloom 1992). During cell operation, the carbonate ions take part in the anode and cathode reactions according to Eq. 19.12. At the cathode, they are created and at the anode they are consumed in equal amounts. Hence, ionic conduction in the MCFC is achieved by transporting carbonate ions from the cathode to the anode. Eutectic or close to eutectic binary (Li/K; Li/Na; Na/K) as well as ternary (Li/Na/K) mixtures of carbonates are suitable for MCFC application. Varying compositions of the ternary system have been investigated at many research institutes. It was found that, regarding cell performance, the blend Li/Na/K (56.8 : 31.2 : 12) gave the best results. Similar results were obtained by the Energy Research Corporation (ERC) (Yuh and Pigeaud 1989).

The performance and lifetime of MCFCs depend to a great extent on the proper choice of the electrolyte. Critical factors for the selection are, for instance, ionic conduction, gas solubility, wetting characteristics in contact with metallic and ceramic surfaces, vapor pressure, viscosity, surface tension, and corrosion stability of materials in contact with the electrolyte. In the past decades, most of the work has been done using mixtures of lithium carbonate and potassium carbonate. The mixture has a eutectic melting point of 761 K (488 °C) at a mole ratio of 62 : 38 (Li/K).

Recently, the Li/Na carbonate electrolyte is getting increasing attention. Earlier reports that this electrolyte leads to increased corrosion have not been confirmed. There are some properties that make the Li/Na carbonate attractive:

• Higher ionic conductivity reduces the cell resistance.
  
• Lower vapor pressure gives longer operation time due to less electrolyte loss.
  
• Lower NiO solubility leads to better cathode stability and prolonged lifetime.
  
• Reduced electrolyte creepage gives less corrosion and improved cell stability.

It can be seen that the Li/Na blend provides many superior characteristics compared with the currently used Li/K blend. However, there are also shortcomings. One is the reduced solubility of gases in the Li/Na melt. This gives rise to higher polarization resistance and lower cell performance.

A direct comparison of the Li/Na and Li/K electrolyte is given by Yuh and Pigeaud (1989). Table 19.7 shows the result. Data for Li/K and gas solubility have been added from other sources.

The table illustrates that five superior (⇑), three approximately equal (⇔), and one inferior (⇓) quality ratings can be assigned to the Li/Na blend as compared with the standard Li/K blend. It is noteworthy that the solubility of the cathode material NiO is 2.5 times smaller in the Li/Na electrolyte.

The high surface tension/large contact angle may lead to problems because it retards the filling of the porous components. In an experimental study, ERC observed this effect with laboratory cells. It could be overcome however by small changes in the assembly and start‐up procedure. Laboratory cells with Li/Na electrolyte were operated up to 5000 hours. After the test, the cell components did not show stronger corrosion as compared with standard electrolyte cells. Little Ni precipitation in the matrix was found, and the particle growth was comparable with Li/K carbonate.

The reduced oxygen solubility of the Li/Na blend gives rise to some concern. Low gas activity in the melt generates high polarization resistance, which lowers the cell performance. The Institute of Gas Technology (IGT) in Des Plaines, IL, seems to have overcome this problem because they reported some time ago that fuel cells using Li/Na carbonate electrolyte routinely exhibit higher performances and lower decay rates than equivalent cells using Li/K carbonate electrolyte when operated at an isothermal temperature of 650 °C. However, Li/Na cells show lower performance than the Li/K cells at temperatures below 600 °C. The loss in performance was determined to be due to increased cathode polarization.

The electrolyte in an MCFC is contained in a ceramic electrolyte matrix structure. Early developers used MgO, which turned out to be not stable enough. Today, lithium aluminate (LiAlO₂) is used, which has a very low solubility in the carbonate melt.

The matrix was formerly manufactured by a sintering process, which resulted in a so‐called electrolyte tile. These structures were very stiff and therefore easily cracked with subsequent gas crossover in the cells. With the advent of the tape casting technology, a more appropriate manufacturing method could be used, which also allowed the manufacturing of larger area components. The matrix obtained by this process is a flat tape with very uniform thickness. The fine LiAlO₂ powder is contained in an organic binder, which gives the tape high flexibility. The matrix is incorporated in the cells in this state. During the start‐up of the cells, the organic binder is burned out, and the remaining fine pores are filled with the electrolyte.

However, there are stability aspects with LiAlO₂ as well, namely, the growth that can take place in number and size of large crystallites at the expense of very fine particles, leading to losses of the surface area. Furthermore, a phase transformation of the LiAlO₂ crystal structure has been observed that leads to increases in bulk volume. Such changes in particle size and shape due to sintering, and the structural expansion of the ceramic matrix as a result of phase transformation, can have an impact on the overall porosity and pore size distribution. This affects both the matrix capillarity and its bulk strength with consequent problems for the distribution of electrolyte between matrix and electrodes.

In the past two decades, there was renewed interest in LiAlO₂ phase stability because long‐term carbonate fuel cell testing (up to 34 000 hours) has indicated particle growth, pore coarsening, and γ‐to‐α phase transformations accompanied by a change in the density during the MCFC operation (Heiming and Krauss 1996; Hyun et al. 2001; Kim et al. 2004; Li et al. 2001; Söllner 1997; Terada et al. 1998; Tomimatsu et al. 1997).

19.2.8 MCFC Hardware Materials

The stack module and balance‐of‐plant (BOP) hardware materials undergo temperatures between 200 and 900 °C and can experience various thermal and gas atmosphere transients during operation. The materials are generally less exposed to molten electrolyte and experience less hot corrosion. Nevertheless, excessive oxide spallation may cause undesirable debris formation and fouling. Another important consideration is cost. BOP materials contribute a significant portion of the total power plant material cost. For lower temperature service (<600 °C), lower‐cost standard ferritic stainless steels (FSS) may be acceptable. Standard high Cr austenitic stainless steels may be usable for up to 750 °C. Although alumina‐forming alloys may be usable at temperatures even beyond 800 °C, their high cost may prohibit usage in significant quantities. Therefore, high‐cost materials can only be used sparingly. FCE (Fuel Cell Energy, Inc., Danbury, CT 06813, USA) has accumulated extensive material experience through long‐term, multiyear DFC (FCE’s direct fuel cell power plant) power plant field operations. FCE has also conducted extensive long‐term material tests in simulated environments; oxidation, debris formation, and mechanical properties (yield strength, ductility, and creep) have been measured for numerous heat‐resistant alloys. Higher Cr (>22 wt% Cr) stainless steels demonstrated no corrosion issue, as shown by the gas manifold in field use for more than five years. The medium Cr (∼18 wt% Cr) stainless steels, although having a faster corrosion rate, have shown to be adequate for 20‐year service for thick‐walled piping/equipment application. However, faster corrosion could occur for thin‐walled material such as expandable bellow at certain locations experiencing electrolyte vapor attack.

Al‐coated FSS are potential alloys to reduce cost compared with austenitic alloys. For example, medium Cr austenitic stainless steel used for the module vessel lining has shown significant oxide debris spallation, but at a higher cost. FCE has identified a low‐cost Al‐coated FSS that has demonstrated excellent corrosion and spallation resistance.

Stress corrosion cracking (SCC) that could lead to sudden high gas leakage was also occasionally observed. A sensitized structure is developed during service. The stainless steels inherently become brittle due to high temperature phase transformation as discussed above, and it is well known that temperature, environment, and stress are key factors contributing to SCC (Sedricks 1996). Therefore, thermomechanical stresses could be minimized to avoid such brittle failure. Moisture condensation should also be eliminated to prevent the SCC failure.

19.2.9 MCFC Future Directions

Hot corrosion attack and galvanic corrosion are the major corrosion problems historically afflicting the bipolar plate materials. Although these problems have been solved, at least partially, by appropriate selection of materials and protection techniques, the result is that the capital cost of the current solutions is too high. To reduce material costs it is required to improve cell performance (lower cell decay rates) and possibly operate at higher current densities (∼300 mA cm⁻² against ∼150 mA cm⁻²).

The cost of bipolar materials constitutes a relevant part of the total stack cost so that economical Fe‐based alloys are desirable. However, Fe‐based alloys cheaper than AISI 316L or 310S stainless steels could be used only if sufficiently cost‐effective protection techniques can be individuated. Alternatively, the development of highly corrosion‐resistant alloys specifically designed for MCFC may result in the final application of uncoated but more expensive materials (for instance, Inconel alloys). In these last years, we have noticed a renewed interest in corrosion studies of metals and model alloys to better understand the effects of alloying elements added to the Fe‐based alloys as this appears essential to individuate innovative metallic materials and protective surface treatments (Spiegel et al. 1997).

In this context, it has been carried out a systematic investigation on binary Fe and Ni‐based alloys to evaluate the effect of Al and Ti additions on both electrochemical corrosion behavior and scale conductivity of these alloys. It was found that the addition of 4 wt% Al to a Fe–21Cr alloy decreases drastically the corrosion current, whereas analogous addition of Al or Ti to a Fe–20Ni alloy does not show any effect. The addition of a 4 wt% Ti to the ternary Fe–21Cr–4Al increases the electrical conductivity of the corrosion protective layer without minimizing the corrosion resistance. By a similar approach, a 30Cr–45Ni–1Al–0.03Y–Fe alloy has been developed by Ohe et al. (1996). The alloy shows a much better corrosion resistance than AISI 310S in 300 hour salt coating test under both anode and cathode gas conditions, suggesting that this alloy could be applied without nickel cladding and aluminum diffusion coatings. The alloys proposed by these two works could represent interesting alternative to the use of stainless steels, provided that their cost‐effectiveness would be demonstrated.

A different strategy for material cost savings is to investigate innovative coatings for the wet seals. In particular, aluminization methods, which do not require the expensive post‐deposition diffusion heat treatment, would be highly desirable. Recently, some investigators have used thermal spraying of Al‐containing powders (FeCrAlY, NiAl, Ni₃Al, FeAl) with poor results due to the porous structure of the coatings produced, which are not corrosion protective enough (Yuh et al. 1995). As the corrosion resistance of the Al diffusion coating relies on the in situ formation of an intermetallic iron–aluminum structure, the behavior of a bulk intermetallic alloy FeAl has been extensively studied by Frangini et al. (1996) and Frangini (2000). It has been found that the corrosion resistance of the FeAl aluminide is comparable with that of IVD aluminized 310S steel in both cathode and fuel gas. The use of this alloy for protecting the wet seal deserves further research to individuate a suitable technique to deposit FeAl layers with the desired structure and corrosion properties. Other researchers have focused their attention on suitable ceramic coating materials to protect Fe‐based alloys under anode gas showing that TiN, TiC, and Ce‐based ceramics are promising anode‐side coatings (Keijzer 1997).

It is clearly apparent from this overview that the fundamental mechanisms of hot corrosion and scale fluxing of stainless steels, especially in the anode reducing gas, remain to be better defined. The influence of the different corrosion tests on the final results has been mentioned; much work remains yet to find suitable standardized methods for the purpose of materials screening and long‐term performance predictions. In addition, the corrosion effects on the various cell performance decay modes deserve further attention, especially in the long‐term stack operation (>30 000 hours).

In summary, it is vital to find advanced solutions for cost reduction of metallic materials and coating technologies that could, in turn, further increase the stack performance and extend useful lifetime. Although MCFC is approaching to a mature technology, the search of innovative materials for the new generation of MCFC plants still offers great opportunities for studies to both scientists and developers.

19.3.1 Introduction

The solid oxide fuel cell (SOFC) technology has attracted significant attention due to the fuel flexibility and environmental advantages of this highly efficient electrochemical device. However, typical SOFC operating temperatures near 1000 °C introduce a series of drawbacks related to electrode sintering and chemical reactivity between cell components. Aiming at solving these problems, researchers around the world have attempted to reduce the SOFC operating temperature to 500–750 °C or lower. It would result in the use of inexpensive interconnect materials, minimization of reactions between cell components, and, as a result, longer operational lifetime. Furthermore, decreasing the operation temperature increases the system reliability and the possibility of using SOFCs for a wide variety of applications such as in residential and automotive devices. On the other hand, reduced operating temperatures contribute to increasing ohmic losses and electrode polarization losses, decreasing the overall electrochemical performance of SOFC components. Thus, to attain acceptable performance, reducing the resistance of the electrolyte component and polarization losses of electrodes are two key points. Losses attributed to the electrolyte can be minimized by decreasing its thickness or by using high‐conductivity materials such as doped ceria and apatite‐like ceramics. Regarding electrode losses, the higher activation energy and lower reaction kinetics of the cathode compared with those of the anode limit the overall cell performance. Therefore, the development of new functional SOFC materials with improved electrical/electrochemical properties, combined with controlled microstructures, becomes a critical issue for the development of solid oxide fuel cells. These topics as well as the operating principles of an SOFC, the requirements of interconnect materials, namely, the corrosion resistance, and the oxidation resistance, scale properties, and microstructures of chromium containing alloy/cathode interfaces will be discussed along the following subsections.

19.3.2 The Operating Principles of an SOFC

The heart of the high temperature SOFC is an yttria‐stabilized zirconia (YSZ) Zr(Y)O₂ film, which acts as a solid electrolyte, allowing high conductivity for O²⁻ ions at about 1000 °C. The addition of yttrium stabilizes the cubic fluorite structure of ZrO₂, which otherwise has a monoclinic → tetrahedral → cubic phase transition with increasing temperature. Yttrium doping up to 98 mol% also increases the ionic conductivity by introducing oxygen vacancies.

The perovskite (La,Sr)(Mg,Ga)O₃ (LSMG) and related compositions have higher ionic conductivities than YSZ and are potentially more compatible with a wider range of cathode materials (Ishihara et al. 1994). The main drawbacks of LSMG are the high reactivity with the commonly used Ni–YSZ anode and the uncertain cost of Ga sources (Feng et al. 1996; Singhal 2013). Doped ceria‐based oxides such as Ce_1 − xGd_xO₂ are considered the most promising electrolyte materials for intermediate temperature (<600 °C) SOFC due to their ionic conductivity at low temperatures (Steele 2000). At higher operating temperatures, these materials suffer from high electrical conductivities under reducing conditions (Mogensen et al. 2000). These electrolyte materials, whose main purpose is to transport oxide ions from the cathode to the anode, should possess high ionic conductivity, low electronic conductivity, good chemical stability in the oxygen partial pressure gradient between the anode and the cathode, and good sinterability to enable a fully dense structure to be fabricated. The electrolytes are dry solids. This property eliminates many engineering problems of water management, which tend to complicate the design and operation of other types of fuel cells, considering that the critical temperature of water is 374.15 °C, so it cannot exist in the liquid phase above this temperature. Waste heat is produced at high temperature and can be used for different purposes, making the overall efficiency of electricity and heat production very high. In fact, the waste heat produced in this type of fuel cell can even be used to produce electricity in a conventional heat engine. In this mode of cogeneration, fuel can be converted to electricity at an overall efficiency exceeding 60%.

The SOFC anode material should have catalytic activity toward electrochemical oxidation of the fuel, a high electrical conductivity, and high stability in the reducing environment. In addition, if the electrode is to be used in a fuel cell running of hydrocarbon fuels, the anode should display catalytic activity toward the water shift and reforming reactions and a tolerance to sulfur as well. Nickel is a relatively cheap metal that has a very high catalytic activity toward the oxidation of H₂ (Setoguchi et al. 1992). However, nickel cannot be used on its own due to a high coefficient of thermal expansion (CTE) and problems with Ni grain growth, leading to microstructural coarsening (Singhal 2013). The solution has been to make a porous ceramic‐metal composite (cermet) of Ni and YSZ. The role of the YSZ, in addition to lowering the CTE and hindering Ni coarsening, is to extend the TPB area at which the anode reaction can take place, as illustrated in Figure 19.4, for the boundary where the fuel gas, Ni, and YSZ phases meet (Mogensen and Skaarup 2000).

Carbon deposition covers the active sites of the anodes, resulting in the loss of cell performance. In high carbon activity environments, iron, nickel, cobalt, and alloys based on these metals could corrode by a process known as metal dusting (see Section 9.4). Metal dusting involves the disintegration of bulk metals and alloys into metal particles at high temperatures (300–850 °C) in environments that are supersaturated with carbon. The Ni corrosion process strongly depends on the temperature and the gas composition, and, in general, the Ni corrosion rate increases with temperature.

The presence of sulfur (primarily in the form H₂S) in the fuel gas can also affect the performance of Ni–YSZ cermet anodes. The electrode polarization resistance (Ω cm²) increases by a factor of 2 by adding only 5 ppm of H₂S at 950 °C in 97% H₂/3% H₂O system. The poisoning effect of sulfur‐containing fuel gas on electrode performance depends on the total sulfur content and the temperature (Singh and Minh 2004). Several alternative anode materials have been suggested, for example, Cu–CeO₂ composites (Brett et al. 2005) and La_xSr_1−xTiO₃ (Marina et al. 2002). Nevertheless, Ni–YSZ remains the most commonly used anode material today.

The cathode material in an SOFC should have high catalytic activity toward oxygen dissociation and reduction and high electrical conductivity and be chemically stable in the oxidizing environment. Most economically viable candidate materials belong to the perovskite family, with lanthanum strontium manganite (LSM) being the most popular choice to date. By doping LaMnO₃ with Sr, a CTE closely matching YSZ and an appreciable electrical conductivity can be achieved (Yang 2008). The electrocatalytic activity of LSM is modest but may be significantly improved by infiltration or impregnation with catalytic nanoparticles (Jiang 2006). Because LSM is nearly a pure electronic conductor, it is often mixed with YSZ in order to increase the TPB area at which the oxygen reduction reaction can take place (Ji et al. 2005). Cathode materials with mixed ionic and electronic conductivity would be preferable since the electrochemically active region would then be extended from the cathode–electrolyte interface to the whole cathode surface. A promising material in this respect is (La,Sr)(Co, Fe)O₃ (LSCF). A challenge with LSCF is that it reacts with YSZ to form insulating SrZrO₃ (Simmer et al. 2006); however, this can be prevented by using doped ceria as the electrolyte material or as a buffer layer between the cathode and YSZ electrolyte (Choi et al. 2012).

A major challenge for all the conventional cathode materials is sensitivity toward chromium poisoning (Park et al. 2014). Volatile Cr(VI) species released from the metallic interconnect have been found to deposit both specifically on the electrochemically active TPB sites (Konysheva et al. 2006) and randomly along the cathode–electrolyte interface (Jiang et al. 2005). Although the exact deposition and poisoning mechanisms are under debate, it is clear that the cell voltage degrades faster than usual in the presence of volatile Cr species (Jiang and Chen 2014). There is however a large difference in how sensitive the different cathode materials are toward Cr poisoning, and new cathode materials with reportedly higher Cr tolerance are under development (Park et al. 2014).

The cathode process (the air electrode) in an SOFC normally involves the reduction of molecular oxygen to oxygen anions (2O²⁻) using electrons external to the cell:

19.35