Basic composition of solid oxide fuel cell (SOFC)

The main components of a single fuel cell are electrolyte, anode, cathode and interconnect or bipolar separator, as shown in Figure 1.5.

The various component materials that make up the fuel cell should have good stability in oxidizing and (or) reducing atmospheres, including chemical stability, crystal stability and stability of external dimensions; chemical compatibility with each other; appropriate electrical conductivity and similar thermal expansion coefficients. At the same time, the electrolyte and the interconnect are required to be completely dense to prevent the penetration and mixing of fuel gas and oxygen; the anode and cathode should be porous to facilitate gas penetration to the reaction site. The specific requirements of the various components of the fuel cell are shown in Table 1.6.

I. Electrolyte materials

The electrolyte in SOFC is the core of the battery, and is generally made of oxide ceramics, that is, sintered solid solution electrolyte-fully stabilized ZrO2. The performance of the electrolyte directly determines the operating temperature and performance of the battery.

The resistivity of pure ZrO2 at 1000 ℃ is 107 Ω/cm, which is close to that of insulating materials. At present, solid electrolytes based on ZrO2, which are widely used in SOFC, use certain divalent or trivalent oxides to dope ZrO2 to replace the position of Zr^(4+) with low-valent metal ions. As a result, not only does ZrO2 (fluorite structure) have a stable phase structure from room temperature to high temperature (1000℃), but also more O^(2-) vacancies are generated in it due to charge compensation, thereby increasing the ionic conductivity of ZrO2 to 10^(-2)S/cm, and at the same time expanding the oxygen partial pressure range of ionic conduction. In this stabilized ZrO2, O^(2-) vacancies are used as a medium, that is, the vacancy mechanism is used to show O^(2-) conductivity.

At present, the most common material used as electrolyte is Y2O3-stabilized ZrO2 (abbreviated as YSZ), whose ion conductivity does not change significantly when the oxygen partial pressure changes by more than ten orders of magnitude. At present, how to prepare YSZ film with suitable performance is a hot spot and difficulty in people's research.

II. Anode material

The electrode material of SOFC is first of all a catalyst. The anode material requires high electronic conductivity, stability in a reducing atmosphere and good air permeability. Therefore, platinum is usually used, but platinum is expensive. The use of metal materials such as nickel and cobalt will cause thermal expansion mismatch and adhesion problems, and long-term high-temperature operation will also reduce its porosity. The current research direction is to use metal ceramics as anode materials, and the more ideal one is Ni-composite YSZ. The main task is to study reasonable processes and prepare Ni-YSZ composite materials with suitable performance.

III. Cathode material

The cathode of SOFC is similar to the anode and should also be a porous electronic conductive film. Since the cathode of the battery works in a high-temperature oxidizing atmosphere and plays the role of transferring electrons and transmitting oxygen, the requirements for the cathode material are relatively stringent. The cathode material should have high electrical conductivity, high-temperature oxidation resistance and high-temperature thermal stability, and should not react chemically with the electrolyte. The traditional material is metal platinum, and the recent development is doped oxide ceramics - LaMnO3. As the cathode material of SOFC, a large number of experiments have proved that La1-xSr xMnO3 is the preferred cathode material.

IV. Connector Material

The electrolyte and electrode materials together form a three-in-one single battery unit. The power of a single battery is limited and can only generate a voltage of about 1V. In order to obtain a high-power battery pack, several single batteries must be connected together in various ways (series, parallel, and mixed), which requires connector materials and sealing materials. In SOFC, the connector component is required to have good electronic conductivity and stability at high temperatures. At present, only a few oxides can be used as SOFC connector materials, such as lanthanum chromate (LaCrO3) with a perovskite structure. High-temperature alloy materials used as SOFC connector materials are also a research hotspot.

V. Sealing materials and others

Sealing materials are used to connect electrolyte materials and connector materials together. They are required to be resistant to high temperatures. Under the battery reaction temperature (700~1000℃), they are generally prepared by glass-ceramic mixed melting. In addition, other auxiliary materials are required, such as corundum tubes as oxygen gas chambers and quartz tubes as fuel gas chambers. They all have air inlets and outlets and need to be sealed and connected.

VI. Single cell assembly

Large-scale SOFC is a battery pack composed of single cells stacked in various structures. Currently, four types of battery packs have been developed, including tubular, serial, block, and flat. In practical applications, individual fuel cells are connected in series and/or in parallel to form a battery pack to meet specific applications.

In the single fuel cell shown in Figure 1.5, the anode, electrolyte, and cathode form a three-in-one composite structure. In actual research and processing, four different structural types have been formed: seal less tubular design, segmented-cell-in-series design, monolithic design, and flat-plate design. The structural schematic diagram is shown in Figure 1.6.