Solid Oxide Fuel Cells (SOFCs) operate at temperatures higher than molten carbonate fuel cells, with a working temperature ranging from 800 to 1000°C. In this type of fuel cell, the electromotive force originates from different oxygen partial pressures on both sides of the cell. The individual cell consists of two electrodes (the fuel electrode as the negative electrode and the oxidant electrode as the positive electrode) and an electrolyte. The main functions of the anode and cathode are to conduct electrons and provide diffusion pathways for reaction gases and product gases.
The solid electrolyte separates the gases on both sides. Due to the different oxygen partial pressures on both sides, a chemical potential gradient of oxygen is generated. Under the influence of this chemical potential gradient, oxygen ions that have gained electrons at the cathode move toward the anode through the solid electrolyte. At the anode, electrons are released, creating a voltage potential between the two poles.
Solid Oxide Fuel Cells (SOFCs) are touted as the third generation of fuel cells, with a solid, non-porous metal oxide as the electrolyte, through which oxygen ions shuttle within the crystal to transport ions. The technology has now reached a mature stage. However, due to the limited number of materials capable of operating at high temperatures and their high cost, there is a shift towards the development of intermediate-temperature fuel cells.
Principle
When Solid Oxide Fuel Cells (SOFCs) operate with reformate gas (a mixture of hydrogen and CO) as the fuel, the following reaction occurs within the fuel cell:
At the cathode, oxygen molecules gain electrons and are reduced to oxygen ions, i.e.,
O2+4e−→2O2−
Under the influence of the potential difference and concentration driving force on both sides of the electrolyte membrane, oxygen ions, through the oxygen vacancies in the electrolyte membrane, undergo directed transition to the anode side and engage in oxidation reactions with the fuel, i.e.,
H2+O2-→H2O+2e-
CO+O2-→CO2+2e-
Overall Reaction:
H2+CO+O2→CO2+H2O
compositon
To smoothly fulfill the function of converting electrical energy, an SOFC (Solid Oxide Fuel Cell) stack should include the following components:
(1) An electrochemical conversion device consisting of a solid electrolyte and both cathode and anode. Among electrolyte materials, yttria-stabilized zirconia is the most maturely developed.
(2) A fuel reformer. This device includes a catalyst, a carrier, and a container. It converts the fuel into small gaseous molecules, such as methane, and is positioned at the front end of the cell stack to exchange heat generated during fuel cell operation.
(3) Gas and fuel transport channels (or gas distributors). Metals are commonly used as conduit materials to ensure optimal diffusion and transport of reactants.
(4) Current collectors, also known as electrical brushes, typically made of metals or materials with good electronic conductivity, are essential for efficient conduction.
(5) Sensors. Various commercially available sensors can be used to monitor the temperature, current, compound types, and output voltage of the cell.
(6) Thermal control devices, such as insulating layers, coolers, heat exchangers, and ventilation systems.
(7) Metal or glass-ceramic housing. Materials usable at room temperature, such as stainless steel 304, are employed. High-temperature-resistant materials are required for internal contact with the SOFC, making commercial metal alloys favorable for reducing manufacturing costs.
Characteristics
Solid Oxide Fuel Cells (SOFCs) are an ideal type of fuel cell, not only possessing the high efficiency and environmentally friendly advantages of other fuel cells but also featuring the following prominent characteristics:
(1) SOFCs have a fully solid structure, eliminating the corrosion issues and electrolyte loss problems associated with the use of liquid electrolytes, offering the potential for long-term operation.
(2) Operating at temperatures between 800 and 1000°C, SOFCs not only eliminate the need for precious metal catalysts but also can directly use natural gas, syngas, and hydrocarbons as fuel, simplifying the fuel cell system.
(3) SOFCs release high-temperature waste heat, which can be utilized in combined cycles with gas turbines or steam turbines, significantly improving overall power generation efficiency.
