The Role EBSD Plays in Solid Oxide Fuel Cells

Fuel cells are a main part of the discussion while talking about global energy requirements. Fuel cells, which are highly efficient sources of power, have long-term stability and low emissions.

A fuel cell that runs on hydrogen can be lightweight and compact, generate very low emissions, and have no major moving parts. As a result, they are mainly attractive for applications in remote areas. A fuel cell is an electrochemical conversion apparatus made up of a cathode and an anode separated by an electrolyte. Electricity is generated by introducing the fuel at the anode and when an oxidant is present at the cathode. The oxidant and the fuel react within the electrolyte.

Although there is an extensive range of fuel cell types, this article will concentrate on solid oxide fuel cells, or SOFCs for short, where the electrolyte is a solid oxide. Long-term stability, high efficiency, low emissions, fuel flexibility, and comparatively low cost are the merits of SOFCs. However, the high operating temperature is a major drawback [1]. The fundamental constituents of an SOFC are quite simple; however, the functional needs for the materials used in the different components are very challenging [2].

Electrolyte

The set of properties mentioned below must be taken into account when choosing an electrolyte material:

  • Least electronic conductivity
  • High ionic conductivity
  • Chemical reactivity with the cathode and anode materials
  • Stability in the oxidizing as well as reducing atmospheres
  • Operating temperature
  • Impermeability to prevent mixing of the fuel and oxidant gas feeds

Yttria (Y2O3) stabilized zirconia (ZrO2) or YSZ is the most widely used SOFC electrolyte material. It usually needs an operating temperature of more than 850 °C. This high operating temperature sets strict demands on the materials used in building the fuel cell. The operating temperature is mainly controlled by the nature of the electrolyte, that is, the thickness of the electrolyte layer, and its ionic conductivity. Thus, the operating temperature can be lowered by two possible methods. The first is to decrease the thickness of the electrolyte layer and the second is to look for other electrolyte materials which have higher oxygen ion conductivities [2].

The layer thickness can be decreased using thin-film deposition techniques, which is a domain of ongoing research, specifically for mobile electronics applications. A number of different materials are under research, for example, gadolinia doped ceria, as well as ceria and zirconia doped with other oxides. Lanthanum gallate-based structures [2], strontium titanate [3], and Perovskite-type oxides [4] are also taken into consideration.

Schematic of solid oxide fuel cell

Figure 1. Schematic of solid oxide fuel cell

These materials have different advantages and disadvantages with respect to ion conductivity and durability in the oxidizing and reducing atmospheres present in the system and are thus the center of attention of ongoing investigations. Another area of research is in the optimization of the material microstructure via controlled processing to enhance performance, for example, ion conductivity.

Anode

The anode is a porous layer on the electrolyte where the hydrogen fuel is oxidized by the oxygen ions diffusing through the electrolyte, producing water and electricity. The anode material must fulfill the following set of demands:

  • Porous
  • Electrically conductive
  • Oxidation promotion of hydrogen fuel
  • A similarity with electrolyte thermal expansion
  • Stable in a reducing atmosphere
  • Ion conductive
  • Chemical reactivity with the electrolyte

The anode can be composed of metal since the fuel approaching the anode is a reducing agent. However, the material should not oxidize at the time of operation, thereby restricting the candidate metals to cobalt, nickel, and the noble metals. Nickel is the most frequently used metal due to its comparatively low cost than others. In order to facilitate the flow of the fuel to the electrolyte, the anode must remain porous at the increased operating temperature. Another condition is to adequately decrease the mismatch in thermal expansion between the electrolyte and anode to maintain adhesion at the interface. These two conditions are realized by spreading the nickel within the solid electrolyte material to form a cermet [2].

The preference for anode material may differ mainly because the electrolyte material is varied. Other materials that are taken into consideration include nickel/ceria cermet for ceria-gadolinia based SOFCs, the use of a variety of dopants, and electrically conducting oxides such as LaCrO3 [2] and SrTiO2 [5].

With regards to the electrolyte material, the investigation is also being carried out in microstructure optimization. Conductivity is dependent on microstructure, specifically the size and particle size distribution of the solid electrolyte and nickel particles and also the connectivity of the nickel particles in the cermet.

Cathode

The cathode is a porous layer on the electrolyte where reduction of oxygen occurs to produce oxygen ions, which, in turn, are diffused through the electrolyte. Cathodes have a particular set of demands:

  • Electronically conductive
  • Promotes oxygen reduction
  • Porous
  • Stable in an oxidizing atmosphere
  • Similarity with electrolyte thermal expansion
  • Ion conductive
  • Chemically inert with the electrolyte

Maintaining these criteria at high operating temperatures restricts the cathode material option to electronically conductive oxides or noble metals. Owing to the high cost of noble metals, electronically conductive oxides are used exclusively. Strontium-doped lanthanum manganite (LSM) is the most widely used metal and has a coefficient of thermal expansion that is comparable with the YSZ.

Just like the anode, the choice of cathode material may differ chiefly due to the change in the electrolyte material.  Lanthanum strontium cobalt ferrite [1] and LaCoO3 [2] are other materials under consideration. Furthermore, different methods of enhancing performance, for example, electrical conductivity through microstructure optimization are being researched.

Interconnect

The interconnect offers electrical contact between the anode and the cathode. Usually, the interconnect is present between each individual cell to integrate the electrical output of all cells in the stack. It must thus have high electrical conductivity in the oxidizing atmosphere at the cathode as well as in the reducing atmosphere at the anode and be stable in both atmospheres as the operating temperature is increased. Therefore, the interconnect must satisfy the following set of conditions.

  • Highly electrically conductive
  • Chemically inert with the cathode, anode, or electrolyte
  • A similarity with the thermal expansion of the cathode, anode, and electrolyte
  • Impermeable
  • Stable in oxidizing, as well as reducing atmospheres

Such requirements severely limit the choice of materials, particularly at the high operating temperatures of YSZ-based SOFCs. A majority of zirconia-based SOFCs use lanthanum chromite (LaCrO3). Since lower temperature SOFCs are being developed, other materials such as SrTiO2 [5] and Crofer 22 APU [6] are taken into consideration. Crofer 22, a high-temperature stainless steel, has a good thermal conductivity of the oxide layer, a high electrical conductivity, a low coefficient of thermal expansion, and thermal stability at high temperature.

Characterization

EDAX’s Scanning Electron Microscope (SEM)-based instruments are ideal for research and development of SOFCs. Energy Dispersive Spectroscopy (EDS) has the ability to determine chemical composition at submicron length scales. Electron Backscatter Diffraction (EBSD) offers crystallographic information in the range of tens of nanometers.

Instances of EDS research include the chemical composition and spatial distribution of composition within the basic fuel cell materials. The microscopic potential of EDS on the SEM makes it possible to view compositional gradients across the reaction zones at the interfaces between the different fuel cell components. For instance, Figure 2 illustrates a peak in the spatial distribution of chromium at the interface between a Crofer interconnect and its oxide scale.

Schematic of a chromium EDS line scan of a post-tested SOFC showing minimal chromium migration from the stainless steel interconnect into the Mn1.5Co1.5O4 barrier coating.

Figure 2. Schematic of a chromium EDS line scan of a post-tested SOFC showing minimal chromium migration from the stainless steel interconnect into the Mn1.5Co1.5O4 barrier coating. [7]

EDS is also ideal for quantifying the chemical composition of particular characteristics seen in the microstructure. Yet, some phases produce polymorphs, which have different crystallographic structure but the same chemical composition. In such instances, combining EDS with EBSD with the help of EDAX’s unique ChI-Scan™ software enables such phases to be clearly determined. Figure 3 illustrates one such example. In this figure, the EBSD has been used to distinguish the cubic and monoclinic phases of YSZ. Integrating the EBSD results with concurrently collected EDS data illustrates that the cubic zirconia is rich in yttrium in relation to the monoclinic zirconia.

EBSD phase map and an EDS elemental map for yttrium obtained simultaneously, showing the correlation between phase and yttrium content.

Figure 3. EBSD phase map and an EDS elemental map for yttrium obtained simultaneously, showing the correlation between phase and yttrium content. [8]

As EBSD data can be gathered quickly and automatically with the help of the most recent Orientation Imaging Microscopy (OIM™) systems, the spatial distribution of crystallographic orientation can be mapped within the microstructure. An example of YSZ is demonstrated in Figure 4. The map is colored in accordance with the accompanying scale. The grains colored in red have [001] axes perpendicular to the section plane, the green ones are [101] type, and the blue ones are [111] type. Here, there is no desired orientation. With such information, scientists can associate properties with the orientation aspects of the microstructure.

EBSD orientation map of YSZ

Figure 4. EBSD orientation map of YSZ

The relation between oxidation rate and orientation is one example. By integrating microelectrochemical experiments with EBSD, Davenpor and König [9] have demonstrated that oxidation is a function of orientation. Subsequent to scanning the measurement area using EBSD, photoresist microelectrodes were applied to particular grains in a polycrystalline titanium sample. The electrochemical and EBSD results are outlined in Figure 5 for a potential of 10 V.

(a) Orientation map with specific grains labeled. (b) Chart showing the variation in current density as a function of orientation from the microelectrochemical measurements on the labeled grains in (a).

Figure 5. (a) Orientation map with specific grains labeled. (b) Chart showing the variation in current density as a function of orientation from the microelectrochemical measurements on the labeled grains in (a). [9]

There is obviously a huge difference between the results with basal planes that are parallel to the surface and those that are normal to the surface. The authors could further associate the quality of the EBSD patterns with the thickness of the oxidation layer. As the oxidation rate is anisotropic, the properties of the fuel cell materials can be adjusted by enhancing the microstructure through process control. The objective would be to form a desired orientation in order to increase the fraction of grains with oxidation-resistant planes parallel to the surface or perpendicular based on the particular application.

Ions try to pursue the lowest energy path through the crystal structure. Therefore, ion conductivity is normally an anisotropic material property. Moreover, since the bonds are different in ceramics, the grain boundaries are not as dense as seen in metals. Therefore, the diffusion of ions is usually more rapid along and across grain boundaries than within the grains [10].

Some authors have examined the effect of grain boundaries on ion conductivity [3,5]. However, this is a field that requires further research. For instance, Shih et al. [3] have examined the role of the population of Σ3 type boundaries in ion conductivity of SrTiO3 using EBSD. Σ3 boundaries are unique in the sense that they have a high fraction of coincidence at the boundary between two crystal lattices. Figure 6 demonstrates the distribution of special boundaries in the YSZ sample illustrated in Figure 4(a) as measured by OIM™. Here, the fraction of special boundaries is not large; most of the grain boundaries are random high angle boundaries. It must be noted that OIM™ can also quantify the preferred orientation or crystallographic texture. In this case, the crystallographic texture is basically random as illustrated in Figure 6(b).

(a) Distribution of special grain boundaries and (b) an inverse pole figure texture plot for the YSZ EBSD data shown in Figure 4.

Figure 6. (a) Distribution of special grain boundaries and (b) an inverse pole figure texture plot for the YSZ EBSD data shown in Figure 4.

By gaining insights into how electrical current or charged ions are conducted through the microstructure, the materials engineer can customize the microstructure to realize the optimal performance for a specific application. For instance, one can consider the three conditions mentioned below.

  1. For a shape anisotropic property — meaning the property is better in the grain elongation direction than in the transverse direction — the optical microstructure would be that illustrated in Figure 7(a). This microstructure is also optimal for a material property which is stronger along the grain boundaries than through the grains. For instance, if diffusion of ionic species is more rapid along grain boundaries than through the grains, then this microstructure would enhance diffusion in the vertical direction. Actually, a more optimized microstructure, in this case, would be made up of very thin rods stretching through the whole thickness of the layer.
  2. For a material property where the strongest anisotropy is crystallographic — meaning it is stronger in one crystallographic direction when compared to the other — aligning this crystallographic direction with the direction in the bulk would be better, as demonstrated in Figure 7(b).
  3. If specific types of boundaries are more appropriate to the material property than others, aligning the good boundary types in the direction of preferred performance improvement would be favorable as demonstrated in Figure 7(c).

Idealized microstructures for a desired material property where (a) the property is stronger in the grain elongation direction or along grain boundaries as opposed to through grain interiors, (b) the property is stronger in a specific crystallographic direction or (c) the property is stronger for certain grain boundary types (white) and weaker for others (black).

Figure 7. Idealized microstructures for a desired material property where (a) the property is stronger in the grain elongation direction or along grain boundaries as opposed to through grain interiors, (b) the property is stronger in a specific crystallographic direction or (c) the property is stronger for certain grain boundary types (white) and weaker for others (black).

Practically, it is not possible to realize all microstructures, and properties may also differ crystallographically both within grains and with grain boundary type so that some hybrid structure would offer the best overall performance. Furthermore, in nearly all cases, the goal is to improve one property without causing adverse effects on other properties. Therefore, a balanced approach must be considered. The potential of EBSD to characterize grain orientation, as well as grain boundary misorientation makes it a perfect tool for characterizing the microstructures to make sure that the goals of the microstructure engineering are being realized. The automated EBSD systems allow characterization of these aspects of microstructure with statistical reliability.

Conclusions

EBSD is a tool appropriate for the characterization requirements of solid oxide fuel cell research and development, particularly in the characterization of orientation and also grain boundaries in polycrystalline microstructures. Moreover, the combination of EBSD with EDS increases the potential of both approaches to fulfill a broad range of materials characterization requirements at the microscopic scale.

Reference

[1] Wikipedia – Solid Oxide Fuel Cells

[2] R. M. Ormerod (2003) “Solid oxide fuel cells.” Chemical Society Reviews 32, 17-28.

[3] S.-J. Shih, C. Bishop & D. J. H. Cockayne (2009). “Distribution of 3 misorientations in polycrystalline strontium titanate.” Journal of the European Ceramic Society 29: 3023-3029.

[4] N. Sammes & Y. Du () Intermediate-Temperature SOFC Electrolytes in Fuel Cell Technologies: State and Perspectives Proceedings of the NATO Advanced Research Workshop on Fuel Cell Technologies: State and Perspectives Kyiv, Ukraine 6–10 June 2004, eds. N. Sammes, A. Smirnova & O. Vasylyev, Springer: Netherlands, 19-34.

[5] F. Horikiri, L. Q. Han, A. Kaimai, T. Otake, K. Yashiro, T. Kawada & J. Mizusaki (2006). “The influence of grain boundary on the conductivity of donor doped SrTiO3Solid State Ionics 177: 2555-2559.

[6] thyssenkruppvdm.de/_pdf/Crofer22APU_e.pdf

[7] S.P. Simner, M.D. Anderson, G-G Xia, Z. Yang & J.W. Stevenson (2005). “Long-Term SOFC Stability With Coated Ferritic Stainless Steel Interconnect” Ceramic Engineering and Science Proceedings 26: 83-90

[8] P. M. Delaforce, J. A. Yeomans, N. C. Filkin, G. J. Wright, & R. C. Thomson (2007). “Effect of NiO on the Phase Stability and Microstructure of Yttria-Stabilized Zirconia” Journal of the American Ceramic Society 90: 918–924.

[9] B. Davepon, J. W. Schultze, U. König & C. Rosenkranz (2003). “Crystallographic orientation of single grains of polycrystalline titanium and their influence on electrochemical processes.” Surface and Coatings Technology 169-170: 85-90

[10] C. B. Carter, M. G. Norton (2007) Ceramic Materials, Springer: New York, pp. 197-198

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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