SOFC electrodes

FIGURE 6.1 Triple-phase boundaries (TPBs) in SOFC electrodes at which electrochemical reactions take place. Cathode mixed conductor materials have larger potentially electrochem-ically reactive surface areas (entire particle surfaces rather than only the TPBs). 

In addition to the use of composite anodes and cathodes, another commonly used approach to increase the total reaction surface area in SOFC electrodes is to manipulate the particle size distribution of the feedstock materials used to produce the electrodes to create a finer structure in the resulting electrode after consolidation. Various powder production and processing methods have been examined to manipulate the feedstock particle size distribution for the fabrication of SOFCs and their effects on fuel cell performance have also been studied. The effects of other process parameters, such as sintering temperature, on the final microstructural size features in the electrodes have also been examined extensively. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

The sputtering technique has been used in the preparation of SOFC electrodes, and is generally combined with photolithography in the production of thin-lilm patterned electrodes that are mainly used in fundamental reactivity and mechanistic studies [120], Although it is a versatile technique that allows for excellent control of composition and morphology, and relatively low temperatures that help to prevent unwanted reactivity observed at higher temperatures, its major limitations lie in the equipment costs and in the slow deposition rates ( 5 pm/h) [120],... 

The main difference in SOFC stack cost structure as compared to PEFC cost relates to the simpler system configuration of the SOFC-based system. This is mainly due to the fact that SOFC stacks do not contain the type of high-cost precious metals that PEFCs contain. This is off-set in part by the relatively complex manufacturing process required for the manufacture of the SOFC electrode electrolyte plates and by the somewhat lower power density in SOFC systems. Low temperature operation (enabled with electrode supported planar configuration) enables the use of low cost metallic interconnects which can be manufactured with conventional metal forming operations. 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

While the various strategies described above have proven promising, SOFC electrodes remain largely empirically understood and far from optimized and suffer from numerous short- and long-term degradation problems. Reported performances vary tremendously with many unknown variables at work and limited understanding as to how materials properties and microstructure relate to performance and long-term stability. ... 

In the case of SOFCs, a large volume of work shows that for many SOFC electrodes, overall performance scales with the ID geometric length of this three-phase boundary. As such, the TBP concept and electrode performance models based on it have proven to be some of the most useful phenomenological concepts for guiding design and fabrication of SOFC cathodes, particularly the microstructure. 

The University of Patras works on design and synthesis of materials, characterization of materials, catalyst development and evaluation, advanced electrochemical reactors, SOFCs, electrodes, and the reforming of fuels. 

It must be remembered that in aqueous systems the redox process occurs over the entire electrode area, whereas in solid electrolyte systems the redox process occurs only in the three-phase or charge-transfer region. The technique has been used with solid electrolyte systems for sometime to study the oxidation and reduction of metals and metal oxides in inert atmospheres,94,95 the behaviour of solid oxide fuel cell (SOFC) electrodes and has also been applied to the in-situ study of catalysts.31,32,95... 

Usually, SOFC electrodes are composed of two (or sometimes more) layers, where the first (the porous anode in Figure 3.3) has mainly a structural function, and the second is a functional layer (called the reaction zone in Figure 3.3), with the main aim of promoting the electrochemical reaction. 

The mass diffusive flux m, of Equation (3.2) generally depends on the operating conditions, such as reactant concentration, temperature and pressure and on the microstructure of material (porosity, tortuosity and pore size). Well established ways of describing the diffusion phenomenon in the SOFC electrodes are through either Fick s first law [21, 34. 48, 50, 51], or the Maxwell-Stefan equation [52-55], Some authors use more complex models, like for example the dusty-gas model [56] or other models derived from this [57, 58], A comparison between the three approaches is reported by Suwanwarangkul et al. [59], who concluded that the choice of the most appropriate model is very case-sensitive, and should be selected, according to the specific case under study. 

Scandia-stabilized zirconia electrolyte with standard SOFC electrodes is used to construct the cells. Stacks are constructed using stainless steel interconnects. The surfaces of the stainless steel are treated to provide an electrically conductive scale with low-scale growth rate. The performance characteristics of a single cell (2.5 cm active area) and a 25-cell stack (active area of 64 cm per cell) are shown in hgure 3.5 and hgure 3.6. The performance stability of the stack is shown in hgure 3.7. 

Figure 15.1. Illusuation of the difference in location of the electrode reaction on two different SOFC electrode types. Upper In an electrode where the electrode material is exclusively an electronic conductor, the reaction zone is restrained to the vicinity of the triple phase boundary (TPB). Lower In a mixed ionic-electronic conductor (MIEC) the electrode reaction can take place on the entire electrode surface...

Another Ni-based solid oxide fuel cell (SOFC) electrode was developed on which a YSZ (yttria-stabilized zirconia) cermet and Lanthanum chromite were deposited by a slurry coating method. It was also suggested that a plasma spraying process can be used for the cermet deposition on the electrodes. The following reactions are expected to take place in a fuel cell employing a natural gas source, where internal reforming takes place on the Ni-YSZ electrode ... 

Therefore, the macrohomogeneous concept can also be adequately extended to the whole cell. For instance, a framework for macrohomogeneous modeling of porous SOFC electrodes is possible by taking into account multicomponent diffusion, multiple electrochemical and chemical reactions, and electronic and ionic conduction. The concept applies to both porous anodes and cathodes. The derivation of the model is illustrated by considering different chemical and electrochemical reaction schemes. The framework is general enough so that additional chemical and electrochemical reactions can be accounted for. 

Key words Nanocrystalline Powder/SOFC Electrode/Catalyst/Cermet Anode... 

Fourier transform infrared spectroscopy (FTIR) has been widely used for in situ analysis of adsorbed species and surface reactions. Infrared spectroscopy techniques have being used for the characterization of solid oxide fuel cells (SOFCs). FTIR is utilized to identify the structure of the SOFC electrode and electrolyte surface (Resini et al., 2009 Guo et al., 2010). Liu and co-workers (Liu et al., 2002) were pioneers in the in situ surface characterization by FTIR under SOFC operating conditions. 

Shearing, P. R., Brett, D. J. L. Brandon, N. P. Towards intelligent engineering of SOFC electrodes a review of advanced microstructural characterisation techniques. International Materials Reviews 55, 347-363, doi 10.1179/095066010xl2777205875679 (2010). 

The wider range of fuel choice for SOFC also calls in for advanced electrode materials for achieving the maximum performance. There is a considerable effort going on aimed at improving the thermo-catalytic, structural, and electrical properties of SOFC electrodes [54,17]. In a noted review, it is said that, Although cost is clearly the most important barrier to the widespread SOFC implementation, perhaps the most important technical barriers currently being addressed relate to the electrodes, particularly the fuel electrode or anode [37]. 

Yamahara K, ShoUdapper T Z, Jacobson C P, Visco S J, de Jonghe L C (2005) Ionic conductivity of stabilized zirconia networks in composite SOFC electrodes. Sol Stat Ionic 176 1359-1364... 

Infiltration is a weU-known method in heterogeneous catalyst applications. Infiltration, also known as impregnation, is commonly used for the preparation of supported catalyst systems. It involves the dispersion of a metal salt component into a support. It has recently been appHed to produce enhanced SOFC electrodes in order to solve the problems mentioned above. This chapter is intended to provide an overview of the application of infiltration in manufacturing SOFC electrodes, the procedures that are implemented to impregnate the electrode materials and the improvements achieved with infiltrated electrodes. 

This is followed in Section 26.4 by a discussion of mesoscale modehng of the SOFC electrodes in which the SOFC electrodes are explicitly resolved and the detailed reactive transport and electrochemistry is modeled. Section 26.5 briefly describes nanoscale approaches for modeling the transport and reactions of species in the SOFCs, which are suitable for elucidating kinetic and mechanistic issues relevant to SOFC performance. 

In addition to molecular difiusion, Knudsen diffusion can also be a significant transport mechanism in the p)ore space of SOFC electrodes. In Knudsen diffusion, the interactions of gas molecules with the pore walls are of the same frequency as the interactions between gas molecules. Knudsen diffusion is typically formulated as Fickian diffusion [Eq. (26.2)], with the Knudsen diffusion coefficient being used in place of the binary diffusion coefiicient. The Knudsen diffusion coefficient of a species is independent of the other species in the system and is derived from the molecular motion of the gas molecules and the geometry of the pores [8, 11, 12). Owing to the small average pore radii of SOFCs ( 10 m [13-15]), diffusion in the pore space of the electrodes usually falls within a transition region where both molecular and Knudsen diffusion are important [16]. To model the transition region. Pick s law can be used with an effective diffusion coefficient to account for... 

Depending on the resolution of the mathematical model, different forms of the species conservation equations may be considered in the porous electrodes. For instance, in the multi-scale modehng of Khaleel et al. [18], a mesoscale lattice Boltzmarm model of the electrodes resolves the species transport in the gas, on the surface of the electrode, and through the bulk solid of the electrode. In this model, Eq. (26.1) is solved in three separate domains with corresponding transport properties and source terms. In contrast, in the macroscale distributed electrochemistry model of Ryan et al. [19], the porous medium of the SOFC electrodes is not explicitly resolved but is included in the model via effective properties. In the effective properties model, the diffusion coefficient of Eq. (26.1) is replaced with an effective diffusion coefficient, which is discussed in Section 26.3.3. 

Sufficient fuel and oxidant flows throughout the SOFC stacks. Depletion of fuel or oxidant can cause oxidation and reduction of the SOFC electrodes. 

In an effective properties model, the porous microstructures of the SOFC electrodes are treated as continua and microstructural properties such as porosity, tortuosity, grain size, and composition are used to calculate the effective transport and reaction parameters for the model. The microstmctural properties are determined by a number of methods, including fabrication data such as composition and mass fractions of the solid species, characteristic features extracted from micrographs such as particle sizes, pore size, and porosity, experimental measurements, and smaller meso- and nanoscale modeling. Effective transport and reaction parameters are calculated from the measured properties of the porous electrodes and used in the governing equations of the ceU-level model. For example, the effective diffusion coefficients of the porous electrodes are typically calculated from the diffusion coefficient of Eq. (26.4), and the porosity ( gas) and tortuosity I of the electrode ... 

Percolation theory [53] is also used to calculate the effective properties such as the ionic conductivity in the SOFC electrodes. The effective conductivity of a composite electrode is less than that of the pure material due to the composite structure and porosity of the electrode. Percolation theory calculates an effective ionic conductivity that accounts for the tortuous path of the electrolyte phase in the electrodes and is based on the probability of finding a percolated chain of the electrolyte phase through the electrode [53]. 

Macroscale cell-level models are able to provide a great amount of insight into the operation and performance of SOFCs. With the newer mesoscale electrochemistry models, information about the conditions within the SOFC electrodes and electrolytes can even be resolved. However, due to the continuum-scale treatment of the SOFC, these models stiU rely on effective parameters, which need to be determined through smaller scale modehng or by fitting the models to experimental data. 

Mesoscale models provide valuable insight into the operation of SOFCs and how the micrometer-scale phenomena translate into the macroscale behavior of the SOFC. By discretely modeling the gas phase and solid phase of the SOFC electrodes, they can investigate the surface reactions and transport in SOFCs, which could lead to advances in the design of the electrodes to improve the electrochemical performance of the SOFC. They are also able to provide macroscale models with effective properties for the transport and reaction parameters based on the local microstracture and physics of the SOFC. 

---