A planar SOFC consists of three bonded layers: a cathode, an anode, and an electrolyte separating the electrodes. Each electrode is a thin, porous, electronic (e-) conductor. Electrode porosity is required for gaseous diffusion between the electrode's outer surface and the electrode/electrolyte interface. The electrolyte is a thin, fully dense oxygen ion (O=) conductor, but not an electronic conductor. The electrolyte needs to be fully dense to prevent gaseous fuel from contacting air and burning.
In contrast to the low temperature fuel cells for which CO is a poison, SOFC can utilize both H2 and CO in the electrochemical reaction. (H2 and CO are produced by the coal gasification process and by reformation of hydrocarbon fuels, such as CH4, as discussed in more detail later in this section.) Figure 1 illustrates the principles of operation for a SOFC. As shown, gaseous fuel flows across the outer surface of the porous anode, and H2 and CO molecules reach the anode/electrolyte interface by pore diffusion. The fuel-side electrochemical reactions occur at triple points, located at this interface. At a fuel-side triple point the electrolyte (oxygen ion donor), the anode (electronic conductor), and the pore (source of the H2 or CO molecule) meet. Oxygen ions (O=) are extracted from the electrolyte, and the following exothermic reactions occur:
1[H2] + 1[O=] —> 1[H2O] + 2[e-]
1[CO] + 1[O=] —> 1[CO2] + 2[e-]
In the process, two electrons are transferred to the anode (negative electrode) for each H2 or CO molecule reacted, and the reaction products, H2O and CO2, diffuse back toward the outer surface of the porous anode, where they enter the fuel stream.
A competing reaction to the CO electrochemical reaction is the so-called water-gas shift reaction, where CO reacts with H2O to form additional H2 and CO2. Both the shift reaction and the electrochemical reaction do occur but the shift reaction is a strong function of temperature with the tendency to shift towards the reaction products (H2 and CO2) as the temperature is decreased. (Water-gas shift reactors for PEM and PAFC operate in the 200 to 350°C range in order to completely react the CO.)
As also indicated in Figure 1, air flows across the outer surface of the cathode, and O2 reaches the cathode/electrolyte interface by pore diffusion. The air-side electrochemical reactions also occur at triple points, located at the cathode/electrolyte interface. (For the purpose of explanation, the assumption here is that the cathode is a pure electronic conductor. Cathodes with mixed conductivity – both electronic and ionic – are discussed later.) An air-side triple point is where the electrolyte (oxygen ion acceptor), the cathode (electronic conductor), and the pore (source of O2) meet. At these sites, electrons are extracted from the cathode and the following O2 reduction reaction occurs:
1[O2] + 4[e-] —> 2[O=]
These oxygen ions enter and are transported through the electrolyte by solid state diffusion from the high oxygen-pressure side (i.e., the cathode/electrolyte interface) to the low oxygen-pressure side (i.e., the anode/electrolyte interface). For each O2 molecule reacted at the cathode/electrolyte interface, four electrons are extracted from the cathode (positive electrode). At the low end of the SOFC operating temperature range (650 – 700°C), the ability to electrochemically reduce O2 is poor for pure electronic-conducting cathodes, which contributes to poor cell performance. So cathodes having mixed conductivity (oxygen ion and electronic) have been under development for a number of years. Such cathodes improve the electro-catalytic activity of the O2 reduction reaction, particularly at low temperatures, by increasing the number of sites for the reaction to occur
As a result of the reactions at both interfaces and the oxygen ion conductivity of the electrolyte, electrons are transported from the cathode to the anode and a potential difference (voltage) is generated. When the anode is connected to the cathode through an external circuit, as shown in Figure 1, current will continue to flow as long as air flow, fuel flow, and cell temperature are maintained. If the cell temperature drops too low (e.g., below ~650°C), the electrolyte oxygen ion conductivity as well as the air side electro-catalytic activity will become very small, resulting in unacceptably poor cell performance. In addition to being the source of oxygen for the electrochemical reaction, air flow on the cathode side is used to control cell temperatures by removing the thermal energy released during the fuel electrochemical reactions.
As discussed previously, SOFC can utilize both H2 and CO in its electrochemical reaction. However, hydrocarbons (CH4, C2H6, etc.) in fuels such as natural gas and coal-derived fuel gas (referred to as syngas) must first be reformed (i.e., reacted with steam) to produce H2 and CO prior to the fuel side electrochemical reaction. As a result of its high operating temperature, the SOFC is particularly well suited for hydrocarbon reformation. The reformation temperature for methane (CH4) at 1 atm. pressure must be greater than ~700°C for the endothermic (i.e., cooling) reformation reaction to go to completion according to:
1[CH4] + 1[H2O] —> 3[H2] + 1[CO]
Nickel is an excellent catalyst for this reaction. So the cell's anode, which contains nickel, can be used as the catalyst as long as the resultant cell temperature gradients due to this endothermic reaction can be kept in the acceptable range. This is called on-cell reformation or direct internal reformation. Being endothermic, the direct internal reformation process can have the beneficial effect of reducing the air flow requirement for cell cooling, thereby reducing system cost (smaller heat exchanger, main air blower, ducting, etc), and improving system electrical efficiency (reduced air pumping power).