Fundamental Aspects of Solid Oxide Fuel Cells

Background and Principles of SOFCs

Fuel cells have emerged as energy conversion devices that produce electrical power directly from electrochemical reactions by combination of gaseous fuel with an oxidant. Fuel cells have first successfully been used for space applications in the 1960s [2]. During the past decades, fuel cells have been developed and offered numerous advantages compared to conventional electrical power generation systems such as high-energy conversion efficiency, high power output, low noise, and zero environmental pollution, which have made them a promising technology for mobile and stationary power generation applications [1,2]. Today, fuel cells are widely utilized in spacecraft, automobiles, home power generation systems, etc.

Nowadays, there are different types of fuel cells classified by chemical characteristics of the electrolyte used, which in turn determines the operating temperature. Table 1.1 illustrates the technical characteristics of the main types of fuel cells that exist today. Fuel cells are categorized as alkaline fuel cell, direct methanol fuel cell, phosphoric acid fuel cell, solid acid fuel cell, proton exchange membrane fuel cell, molten carbonate fuel cell, solid oxide fuel cell (SOFC), and protonic ceramic fuel cell. The first five types have low to medium operating temperatures (50°C-210°C) with relatively lower electrical generation efficiencies (40%-55%). The other three types, however, operate at much higher temperatures (600°C-1000°C) with higher electrical generation efficiencies (45%-60%) [3]. Among the listed types of fuel cells, SOFCs are the most demanding for use as a power generation system from a materials point of view and due to their exceptional features such as:

• • SOFCs offer high energy-conversion and electrical generation efficiencies (fuel input to electricity output).
• • SOFCs have good fuel flexibility (e.g., natural gas and carbon-based fuels) and simplicity of design.
• • Since SOFCs have a solid construction with no moving parts, they operate very quiet with minimal noise, and thereby, they can be installed indoors.
• • The SOFCs’ high operating temperature leads to high-quality byproducts, and exhaust heat is used for co-generation and a variety of processes.
• • Since precious metals are not used in SOFCs, the price is reasonable enough for high-volume manufacturing.
• • The high efficiency and operating temperature of SOFCs result in low CO, emission.
• • SOFCs do not need to work with corrosive liquid electrolyte, making them durable with a life expectancy of 40,000-80,000h [1,3].

TABLE 1.1

Technical Characteristics of Different Fuel Cells

Source: Data with minor modification from Ref. [3].

In 1838, the first fuel cell was invented by Sir William Robert Grove, a Welsh judge, inventor, and physicist, when he found a system that operates in the opposite direction of water electrolysis phenomena. Later in 1839, Grove developed his system to an electrochemical device which combines H, or H₂/CO fuels and an oxidant gas in the presence of an ion-conducting electrolyte and generates electricity and heat directly from the chemical energy [4,5]. Between 1853 and 1932, Friedrich Wilhelm Ostwald provided significant information about the fundamentals and theories of fuel cells, and experimentally determined the roles of fuel cell components [6]. Since then, a worldwide extensive fuel cell research has been carried out on all fuel cell types.

SOFC was first achieved by Emil Baur, a Swiss scientist, and his colleagues in the late 1930s when they conducted many experiments on SOFCs using electrolytes of clay and metal oxides such as zirconium, yttrium, cerium, lanthanum, and tungsten oxide and operating at 1,000°C [3,7,8]. In the 1940s, Davtyan, a Russian researcher, redesigned the SOFC structure to improve its mechanical strength and conductivity by adding monazite sand to a mixture of tungsten trioxide, sodium carbonate, and soda glass; however, this caused unexpected chemical reactions and less cell durability. Later on, by the late 1950s, different companies such as Central Technical Institute in the Netherlands, and Consolidation Coal Company and General Electric in the US were investing on the SOFC research due to its promising high operating temperature which would tolerate carbon monoxide, and its high stability by using a stable solid electrolyte. In 1962, the first federally funded research contract was awarded to Westinghouse to study zirconium oxide- and calcium oxide-based fuel cells. Over the recent decades, by increasing the energy demand and price and developments in materials technology, SOFCs have become more attractive for industry. A recent report indicates that more than 40 companies and many research institutes around the world are working on SOFCs to develop a new cell design and performance for the low-temperature SOFC [3]. Large-scale, utility-based SOFC power generation systems have reached commercial demonstration stages in the US, Europe, and Japan. The most aggressive in commercialization of SOFC is Japan. To establish a hydrogen-based society, Japan’s Ministry of Economy, Trade and Industry (METI) has set a residential-use fuel cell target of 1.4 million units by 2020 and 5.3 million units by 2030 [10]. In 2017. METI has invested on the research and development of industrial use of SOFC systems in order to make them commercially available. Hence, Kyocera Corporation launched the industry’s first 3kW SOFC cogeneration system for institutional applications in this year [3,9,10].

Small-scale SOFCs are being developed for military, residential, industrial, and transportation applications [11,12]. Here are some examples for new relevant projects trending today: (1) establishment of mass hydrogen marine transportation supply chain derived from unused brown coal, (2) development of smart community technology by utilization of hydrogen Co-Generation System, and (3) hydrogen and natural gas co-firing gas turbine power generation facilities R&D for a low-carbon society. SOFC’s history is now about 90 years old, and discoveries are still continuing in electrolyte modification, electrode and interconnecting materials development, ceramic processing to cell thickness with an extended area, operating temperature reduction down to 600°C-700°C, along with developing commercial applications [9].

Design and Operation of SOFCs

SOFCs are different from other fuel cells in many aspects such as using a solid- state electrolyte and materials, which results in no restriction on cell configuration, and higher operation temperature where certain oxide electrolytes become highly oxygen-ion conducting [3]. SOFCs are mainly being configured in two different ways: tubular/rolled tube and flat-plate cells. The latter has recently been adopted by electronics companies [9].

The SOFC structure generally consists of a dense solid-state oxygen-ion-conducting electrolyte sandwiched between two porous electrodes (i.e., anodes and cathodes). Figure 1.1 shows a schematic diagram of SOFC. Hydrogen fuel is fed to the anode side in which hydrogen is combined with the oxygen, from the air, entering the cell through the cathode side. On the anode side, the hydrogen-containing fuel burns which results in a drastic reduction of the oxygen concentration on the cathode side. The oxygen ions, passing through the crystal lattice of the ceramic electrolyte [e.g., yttria-stabilized zirconia (YSZ)], react with the oxidized fuel, thereby producing electrons. The generated electrons then pass through the external circuit (from the anode to the cathode). Pure water and heat are the only byproducts of this process. The SOFC reactions are as follows:

At the cathode side:

At the anode side:

The required hydrogen can be extracted from natural gas through either external or internal reforming. The internal reforming of the fuel within the fuel cell

eliminates the necessity of external fuel reforming as it is needed for other types of fuel cells [3,5,13-15]. The fuel reforming reaction is an endothermic reaction. The required heat for the fuel reforming reaction can be provided by the overpotential loss and entropy change heat with the high-temperature fuel cells such as SOFCs. For instance, the internal methane steam reforming reaction and the water gas-shifting reaction are as follows:

Methane reforming:

Gas-shifting:

The CO is oxidized by oxygen ions at the anode side which produces CO, and electrons [15]:

Figure 1.2 shows the internal reforming of the hydrocarbons in the SOFC system. As can be seen, some parts of the hydrocarbon fuel are internally reformed in an indirect reforming unit; however, the other parts are directly reformed within the fuel cell. The heat generated due to electrochemical reactions is utilized for the internal reforming process. The depleted combustible fuel containing hydrogen and CO is sent to a combustor for oxidation.

SOFC requires to operate at high temperatures (600°C-1000°C) in order to facilitate oxygen ions migration through the electrolyte, thus achieving high ionic conductivity (~0.1 S cm^-1 at operating temperature) [5]. Lowering operating temperature of SOFCs not only causes reduction in ionic conductivity (i.e., increase in ohmic loss) but also decreases catalytic activity of the electrodes which has a negative impact on the cell performance. However, there are main challenges with operation of SOFCs at very high temperatures (800°C-1000°C) which are:

• • Reaching the operating temperature increases fuel burning time period, thus a long start-up time.
• • Exerting sealing problems and a need for expensive materials to be used as interconnects for the SOFC stack.
• • Inducing thermal stresses on the SOFC materials at the electrolyte- electrode interfaces [5].

Lowering the SOFC operating temperature down to 600°C and even below can properly address the abovementioned problems. Hence, research has recently focused on lowering the SOFC operating temperature with regards to its potential barriers. In the past decade, there has been a growing interest toward proton-conducting ceramics due to their high proton conductivity with low activation energies at temperatures below 600°C [16,17]. Figure 1.3 shows the schematic of an SOFC operating with the proton-conducting electrolyte. The proton-conducting ceramic materials, termed as high-temperature proton conductors (HTPCs), exhibit proton conductivity under hydrogen and/or steam atmospheres, and the material that is commonly used as the

electrolyte is based on doped BaCeO, and BaZrO, materials [18-22]. HTPC electrolytes must meet a wide range of requirements, including high ionic (protonic) conductivity, excellent thermodynamic stability, high ceramic and mechanical qualities, and acceptable thermal and chemical compatibility with other functional materials [23,24]. However, the state-of-the-art HTPC representatives still possess such disadvantages as the highly conductive cerates, (BaCeO,), and low chemical stability against C- and S-containing gas components [24]. The reaction with C-gas components causes severe degradation of the electrolyte and precludes applications in fuel cells based on hydrocarbon fuels [25-27].

The optimization of proton conductivity significantly depends on the type of dopants that are used to enhance proton transport and chemical stability of the materials. The determination of proton conductivity relies on the introduction of defects into the perovskite structure and their distribution in the crystalline lattice. To improve the key features of BaCeO,, persistent efforts have shown that dopants with a low ionic charge (higher ionic size) can increase its proton conductivity because doping with an acceptor admixture (In-^,+ and Y³⁺) compared to the lower ionic size Ce⁴⁺ or Zr*⁺ leads to the formation of oxygen vacancies [28]. Low-temperature solid oxide fuel cells (LT-SOFCs), from 400°C to 650°C, have seen considerable research and development and are widely viewed as the “next-generation” technology [29]. Low operating temperature is also potentially useful for reversible SOFCs because reducing the temperature can shift the H₂0-C0₂ co-electrolysis product composition to one with substantial CH₄.

In order to achieve a better insight into decreasing the SOFC operating temperature, one must explore and assess kinetics of reactions and losses occurring in the SOFC, as well as thermodynamics and electrochemistry of the reactions. The following section will discuss these aspects of SOFCs.