**3. Solid oxide fuel cell (SOFC)**

The solid oxide fuel cell (SOFC) offers a highly efficient and fuel-flexible technology for distributed power generation and combined heat and power (CHP) systems, and it is obviously promising technology for utilizing biofuels. In this section, SOFC technologies are briefly reviewed from fundamentals to current status of development.

(Kishimoto et al., 2007), synthetic diesel (Kim et al., 2001), crude oil and jet fuel (Zhou et al., 2004). Highly efficient fuel cells operated by fossil fuels can certainly contribute to the suppression of environmentally harmful emissions, but in view of exhaustion of fossil resources, the utilization of renewable bio-energies should be more promoted. Direct feeding of biofuels to SOFC gives an environmental-friendly, compact and cost-effective energy conversion system. Biogas derived primarily from garbage is one of the most attractive bio-energies for SOFC (Van herle et al., 2004a; Shiratori et al., 2008, 2010a, 2010b). Recently, Shiratori et al. (2010) has demonstrated the stable operation of an IRSOFC operating on non-synthetic biogas over one month using an anode-supported button cell. On the other hand, the use of liquid biofuels is also attractive due to their easy storage and transportation with high energy density. Tran et al. (2011) has demonstrated the stable operation of an IRSOFC operating on practical palm-biodiesel over 800 h, also using an

In this chapter, performances of IRSOFCs operating on biofuels are summarized and roadblocks to overcome for the realization of this type of highly-efficient carbon-neutral fuel

**2. Sustainable society using internal reforming SOFC (IRSOFC) running on** 

Although the conventional large scale power system provides us with a stable electric power supply, the associated large consumption of fossil fuels and release of large amount of waste heat are unfit for social and environmental needs in recent years. Now, the role of biomass, having the largest exploitation potential among renewable energy resources, becomes very important. Of course, the use of edible plants is highly restricted, but the use of organic wastes is highly desirable. Biogas and biodiesel fuels (BDFs) are attractive alternative fuels which can be produced from bio-wastes, and their spread will generate synergistic effects to create new industries and employment in their production and

Among fuel cells, the SOFC is the only technology capable of converting bio-energies directly to electricity without an external fuel reformer (Staniforth et al. 1998). As for the low temperature fuel cells like polymer electrolyte fuel cell (PEFC), the external reforming process is essential prior to electrochemical conversion of biofuels to electricity. Superiority of an IRSOFC running on biofuels (hereafter called *Bio*-SOFC) is described in Fig. 1. By selecting red arrows in this figure social needs are satisfied. Our final goal is to establish a microgrid system as shown in Fig 2 using *Bio*-SOFC as major distributed generators providing heat and power on site. These distributed *Bio*-SOFCs can contribute to leveling of the unstable power supply from solar and wind energies. However, breakthroughs are

The solid oxide fuel cell (SOFC) offers a highly efficient and fuel-flexible technology for distributed power generation and combined heat and power (CHP) systems, and it is obviously promising technology for utilizing biofuels. In this section, SOFC technologies are

briefly reviewed from fundamentals to current status of development.

anode-supported button cell.

cell are mentioned.

**biofuels (***Bio***-SOFC)** 

refinement processes.

necessary to realize the *Bio*-SOFC system.

**3. Solid oxide fuel cell (SOFC)** 

Fig. 1. Superiority of IRSOFC running on biofuels (*Bio*-SOFC), a promising candidate as a distributed generator in the next generation.

Fig. 2. Microgrid system using *Bio*-SOFC as a major distributed power source.

#### **3.1 Operation mechanism**

A fuel cell in general converts chemical energy of the fuel directly into electrical energy without converting it to mechanical energy. Therefore, the fuel cell has potential of attaining higher electrical conversion efficiency than those of conventional technologies such as heat engines limited by Carnot efficiency. Fig. 3 shows the principle of SOFC operation. The basic unit of an SOFC, i.e. cell, consists of an electrolyte sandwiched with two electrodes, anode and cathode. In the electricity generation process, an oxide ion O2- is generated from oxygen in air via the cathodic reaction (1).

$$\text{V}\_2\text{O}\_2\text{ (g)} + 2\text{e}^- \rightarrow \text{O}^{2-} \tag{1}$$

Normally, SOFCs are operated in the temperature range between 600 and 900 oC in which electrolytes composed of doped metal oxides can exhibit rather high oxygen ion conductivity. Oxygen ions generated in the cathode are transported to the anode side through the dense electrolyte and are used to electrochemically oxidize a fuel, here hydrogen, in the anodic reaction (2).

$$\rm H\_{2}\rm (g) + O^{2-} \rightarrow H\_{2}O \text{ (g)} + 2e^{\cdot} \tag{2}$$

$$\rm{CO} \text{ (g)} + \rm{O^{2-}} \rightarrow \rm{CO\_2} \text{ (g)} + 2\rm{e} \tag{3}$$

In the high temperature SOFC, not only hydrogen but also carbon monoxide can contribute to the generation of electricity (3). Hydrogen and carbon monoxide can be produced by steam reforming or partial oxidation of hydrocarbon fuels on the Ni-based anode material, therefore in principle hydrocarbon fuels can be directly supplied to SOFC without using a pre-reformer.

Fig. 3. Principle of SOFC operation.

The electromotive force of fuel cell, *E*, derived from the difference in the partial pressure of oxygen, *p*(O2), between cathode and anode sides can be expressed by the Nernst equation (4).

$$E = \left< \mathbf{RT}/4F \right> \ln \left< p(\mathbf{O}\_2, \mathbf{c}) / p(\mathbf{O}\_2, \mathbf{a}) \right> \tag{4}$$

A fuel cell in general converts chemical energy of the fuel directly into electrical energy without converting it to mechanical energy. Therefore, the fuel cell has potential of attaining higher electrical conversion efficiency than those of conventional technologies such as heat engines limited by Carnot efficiency. Fig. 3 shows the principle of SOFC operation. The basic unit of an SOFC, i.e. cell, consists of an electrolyte sandwiched with two electrodes, anode and cathode. In the electricity generation process, an oxide ion O2- is generated from oxygen

Normally, SOFCs are operated in the temperature range between 600 and 900 oC in which electrolytes composed of doped metal oxides can exhibit rather high oxygen ion conductivity. Oxygen ions generated in the cathode are transported to the anode side through the dense electrolyte and are used to electrochemically oxidize a fuel, here

In the high temperature SOFC, not only hydrogen but also carbon monoxide can contribute to the generation of electricity (3). Hydrogen and carbon monoxide can be produced by steam reforming or partial oxidation of hydrocarbon fuels on the Ni-based anode material, therefore in principle hydrocarbon fuels can be directly supplied to SOFC without using a

O2

H2 CO

The electromotive force of fuel cell, *E*, derived from the difference in the partial pressure of oxygen, *p*(O2), between cathode and anode sides can be expressed by the Nernst equation

 *E* = (*RT*/4*F*) ln { *p*(O2, c)/*p*(O2, a)}, (4)

½ O2 (g) + 2e- → O2- (1)

CO (g) + O2- → CO2 (g) + 2e- (3)

(2)

O2- *External*

*e-*

H2O CO2 *e-*

*electric load*

**3.1 Operation mechanism** 

in air via the cathodic reaction (1).

hydrogen, in the anodic reaction (2).

pre-reformer.

*Fuel*

(4).

H2 (g) + O2- → H2O (g) + 2e-

*Air*

*Cathode*

*Electrolyte*

*Anode*

and/or

CH4 H2O

(e.g. CH4)

Pre-reformed gas (H2, CO, CO2, H2O)

unreformed hydrocarbon

Fig. 3. Principle of SOFC operation.

*Cell*

where *R* is the gas constant and *T* is absolute temperature. a and c denote anode and cathode sides, respectively. Theoretically, *E* is approximately 1 volt, and the ideal electrical efficiency can be calculated by *ΔG*/*ΔH*, where *ΔG* and *ΔH* are Gibbs free energy change and enthalpy change of the hydrogen combustion reaction (5), respectively. At the operating temperature of 800 oC ideal efficiency becomes 70%LHV.

$$\text{H}\_2\text{ (g)} + \text{}^\downarrow \text{O}\_2\text{ (g)} = \text{H}\_2\text{O (g)}\tag{5}$$

The actual electrical efficiency of an SOFC (40-50 %LHV) is always lower than the ideal value because fuel utilization (*U*f) can not be increased up to 100 % in the practical SOFC system and the contribution of the internal resistances such as resistances of the materials themselves, contact resistances and electrode reaction resistances is not negligible. However, in the small size fuel cell systems, the heat generation, including the intrinsic heat release, *ΔH -ΔG*, can be utilized effectively on site leading to overall efficiency above 80 %LHV.

The SOFC has the following advantages because of its high operating temperature. Various kinds of fuel, such as natural gas, liquefied petroleum gas, kerosene and biofuels, etc. can be utilized with a simple fuel processing system. Even direct feeding of such practical fuels is theoretically possible. Higher electrical efficiency above 50 %LHV can be obtained by setting higher fuel utilization. Overall efficiency can be enhanced by using the heat released from the cell for the fuel reforming process, in which endothermic steam reforming proceeds as a main reaction. This kind of energy recycle is possible because the operational temperature of SOFC is nearly the same as that of the reformer. In addition, further enhancement of electrical efficiency is expected by using residual fuel and water vapor in a downstream gas turbine and steam turbine. High quality heat from the high temperature SOFC system can also be utilized effectively for hot water supply as well as reformer.

#### **3.2 Component materials of SOFC and stack configurations**

In Table 1, requirements for component materials of a cell and interconnector are summarized. Typical materials are also listed in this table. The electrolyte has to be gas-tight to prevent leakage of fuel and oxidant gases. Both electrodes have to be porous to provide electrochemical reaction sites. The interconnector plays a role of electrically connecting the anode of one cell and the cathode of the adjoining cell, and also separating the fuel in the anode side from the oxidant in the cathode side. Component materials must be heat resistant and durable in the highly oxidative and reductive atmospheres for cathode and anode sides, respectively.

To fabricate a cell, powders of these component materials are formed into desired shapes by general ceramic processing such as extrusion, slip casting, pressing, tape casting, printing and dip coating (Stöver et al., 2003). Subsequently, the resulting "green" ceramics undergo heat-treatments. High temperature sintering processing above 1300 oC is normally required to obtain a dense and gas tight electrolyte layer.

In an SOFC, all solid state fuel cell, various types of cell configuration have been designed and classified by support materials and shapes as summarized in Table 2. The SOFC has a laminate structure of thin ceramic layers, therefore a support material is necessary to ensure the mechanical stability. Anode-supported, electrolyte-supported, cathode-supported, metal-supported and nonconductive ceramic-supported (segmented-in-series) types have been developed. From the viewpoint of cell shape, there are roughly three types, i.e., planar, flat tubular and tubular types.


Table 1. Requirements for component materials and typical materials used to fabricate SOFC.


\* Cells are formed in series on a nonconductive porous ceramics.

Table 2. Various SOFC configurations.

Y2O3-stabilized ZrO2 (YSZ), Sc2O3-stabilized ZrO2 (ScSZ), Gd2O3 doped CeO2 (GDC), (La,Sr)(Ga,Mg)O3 (LSGM))

> Ni-YSZ, Ni-ScSZ, Ni-GDC

(La,Sr)MnO3, (La,Sr)(Fe,Co)O3

(La,Sr)CrO3, (La,Ca)CrO3 (Sr,La)TiO3, Stainless steel

Kyocera Acumentrics,

TOTO

Siemens Power Generation, TOTO

Mitsubishi Heavy Industries

**Component materials Requirements Typical materials** 

Ionically conductive

Porous, Electrochemically active, Electronically conductive

Porous, Electrochemically active, Electronically conductive

Electronically conductive

Table 1. Requirements for component materials and typical materials used to fabricate

**Planar Flat tubular Tubular** 

Siemens Power Generation

Rolls-Royce Fuel Cell Systems, Tokyo gas

Electrolyte Dense (Gas tight),

Interconnector Dense (Gas tight),

**Support materials Cell shape** 

Versa Power Systems, Ceramic Fuel Cells, Topsoe Fuel Cell, Nippon Telegraph and Telephone, NGK Spark Plug

Hexis, Mitsubishi Materials

Cell

SOFC.

Anode-supported

Electrolytesupported

Cathodesupported

Metal-

Nonconductive ceramicsupported (Segmented-in series \*)

supported Ceres Power

Table 2. Various SOFC configurations.

\* Cells are formed in series on a nonconductive porous ceramics.

Anode

Cathode

Fig. 4. FESEM images of (a) anode-supported cell and (b) electrolyte-supported cell.

Resistance of the electrolyte dominates the internal voltage loss in many cases, thus making thinner electrolytes is a key technology to achieve better performance especially at lower operating temperatures. For example, an anode-supported cell (Fig. 4a), in which a thin electrolyte layer with a thickness of around 15 μm is formed on the anode substrate with a thickness of 1 mm, enables operation of an SOFC around 200 K lower in temperature as compared to an electrolyte-supported cell (Fig. 4b) (Steele & Heinzel, 2001).

There are also various types of stack structures with different flow directions of electric current, fuel and air, as shown in Fig. 5. The planar type (Fig. 5a) can achieve higher current density and lower manufacturing cost, but it has lower tolerance to thermal stress. Flat tubular type (Fig. 5b) exhibits higher durability than planar type because the area for gas seal is smaller compared to planar type without changing the direction of electric current. This type enables downsizing of a SOFC stack but is not suitable for hundred kW class large systems due to the mechanical properties of the interconnector material. The segmented-in-series type (Fig. 5c) has the advantage of scalability because gas seals and electrical connections between the adjacent cells are already completed. However, this structure is complicated and a lot of optimization is required for precise fabrication.

Fig. 5. Structures of SOFC stack; (a) anode-supported planar type (b) anode-supported flat tubular type and (c) segmented-in-series tubular type.

Fig. 6. Flow diagram of 200 kW-class SOFC-micro gas turbine combined system developed by Mitsubishi Heavy Industries (Yoshida et al., 2011).

Large scale SOFC systems are being developed aiming for a distributed electrical power plant with high energy conversion efficiency. Mitsubishi Heavy Industries (JP) has developed a 200 kW class micro gas turbine hybrid system as shown in Fig. 6 with a maximum efficiency of 52.1% LHV, and a maximum gross power of 229 kW-AC was achieved with natural gas as a fuel. Their final goal is to achieve electrical efficiency of 70% by developing large-scale power generation system in which the SOFC integrates with gas turbines and steam turbines (Yoshida et al., 2011). Rolls-Royce Fuel Cell Systems (GB) is designing a stationary 1 MW SOFC power generation system based on their segmented-inseries cell stack named Integrated-Planar SOFC technology (Haberman et al., 2011; Gardner et al., 2000). FuelCell Energy (US) is developing SOFC power plants, currently utilizing SOFC stacks developed by Versa Power Systems (CA). Their ultimate goal is to develop Multi-MW SOFC power plants suitable for integration with coal gasifiers and capable of capturing > 90% of carbon in coal syngas (Huang et al., 2011).

Fig. 7. Appearances of installation sites of residential SOFC CHP systems under demonstrative research project in Japan (Hosoi et al., 2011).

Small scale SOFCs of 1 -2 kW class are now being developed all over the world, aiming at early commercialization of residential CHP systems (Fig. 7). City gas is generally used as a fuel with high overall efficiency more than 80 %LHV obtained by utilizing both electricity and heat on site. In Japan, a demonstrative research project is now being carried out in which Kyocera (JP), Tokyo gas (JP) and TOTO (JP) participate as manufacturers of SOFC stacks. More than 200 systems have been installed on actual residential sites as of FY2010. In this project, reductions of primary energy consumption and CO2 emission by 16 and 34 %, respectively, were demonstrated (Hosoi et al., 2011). Until now, long-term operation over 25,000 h has been demonstrated, and potential of 40,000 h durability has been confirmed. Ceramic Fuel Cells (AU) is manufacturing the residential system called BlueGen which can deliver initial electrical efficiency of 60 %LHV at 1.5 kW-AC, and exhibited 55 % efficiency after 1 year operation (Föger et al., 2010; Payne et al., 2011). Hexis Ltd. (CH) had operated 1 kW class system for 28,000 h (Mai et al., 2011). Acumentrics (US) (Byham et al., 2010) and Ceres Power (GB) are also developing residential SOFC systems (Leah et al., 2011).

For the spread of these SOFC systems, further enhancements of electrical efficiency, fuel flexibility and thermomechanical reliability are essential. In this chapter, these challenges are summarized taking our effort, application of biofuels to SOFC, as a good example of advancements.
