Fuel cell batteries are devices that directly convert the chemical energy of fuel into electrical energy. They are a promising, clean, and efficient method of power generation, often referred to as the distributed power source of the 21st century. The working principle of a fuel cell is similar to the reverse process of water electrolysis. Hydrogen-based fuel is fed to the anode (negative electrode) of the fuel cell and converted into hydrogen ions. Oxygen from the air is fed to the cathode (positive electrode). Negative oxygen ions travel through the electrolyte between the electrodes to the anode and combine with hydrogen ions to form water, while an electric current is generated in the external circuit.
Typically, a complete fuel cell power generation system consists of a battery stack, a fuel supply system, an air supply system, a cooling system, a power electronic converter, and protection, control, and instrumentation systems. The battery stack is the core component. Low-temperature fuel cells should also be equipped with a fuel reformer. High-temperature fuel cells have internal reforming capabilities and do not require a reformer.
Phosphoric acid fuel cell (PAFC) is currently the most mature and commercially available fuel cell technology. Large-capacity pressurized 11MW units and portable 250kW units are now being produced. The second-generation fuel cell, molten carbonate fuel cell (MCFC), operates at high temperatures (600–700℃), allowing the reforming reaction to occur internally. It can be used for large-scale power generation and is currently undergoing megawatt-scale verification testing. Solid electrolyte fuel cell (SOFC) is considered the third-generation fuel cell. Because the electrolyte is a solid electrolyte such as zirconium oxide, it can be used for coal-based fuel power generation in the future. Proton exchange membrane fuel cells are the most promising power source for electric vehicles. Fuel cells have the following advantages:
1) High efficiency: Hydrogen-based fuel cells theoretically achieve 100% power generation efficiency. Molten carbonate fuel cells achieve an actual efficiency of 58.4%. Through combined heat and power (CHP) or combined cycle systems, the overall thermal efficiency of fuel cells can be expected to reach over 80%. The power generation efficiency of fuel cells is largely independent of scale; even small-scale equipment can achieve high efficiency.
2) When in hot standby mode, the fuel cell has a very strong ability to follow load changes, and can follow 50% load changes within 1 second.
3) Low noise; can achieve practically zero emissions; saves water.
4) Short installation period, flexible installation location, and can save the need to build a new power transmission and distribution system.
The high cost of fuel cell batteries currently hinders their large-scale application, and they will take time to become economically competitive with conventional power generation methods. Key technologies for fuel cell batteries involve aspects related to battery performance, lifespan, scaling, price, and commercialization, with a significant focus on new electrolyte materials and catalysts. Molten carbonate fuel cells (MCFCs) suffer from reduced lifespan due to liquid electrolyte loss and corrosion leakage at high temperatures, limiting their large-scale and practical application. Addressing issues such as corrosion of battery components and performance degradation caused by changes in electrode pore structure is crucial.
Solid oxide fuel cells (SOFCs) use solid electrolytes and operate at very high temperatures, placing special requirements on their constituent materials and processing. To obtain a chemically stable and dense (gas-free) electrolyte at high temperatures, Y₂O₃ is added to zirconium oxide to generate yttrium-stabilized zirconium oxide. To lower the operating temperature, the electrolyte film thickness should be minimized. Electrolyte films are typically prepared using methods such as melt spraying, sintering, and electrochemical evaporation coating.
The thickness of practical electrolyte membranes is 0.03–0.05 mm. More advanced membranes have reached 0.01 mm. Such thin electrolyte ceramic materials, besides requiring sufficient mechanical strength, must possess high gas tightness; otherwise, the performance of the fuel cell will be compromised. The fuel electrode uses heat-resistant metal ceramics such as nickel-zirconium, and nickel is also used as a catalyst in fuel reforming. The air electrode is subjected to high-temperature oxidation during operation, making it difficult to use common metals. Platinum has good stability but is expensive; alternative materials are needed, such as electronically conductive ceramics. Another important research direction to reduce operating temperature is to find low-temperature proton-conducting electrolytes. If the operating temperature can be reduced to below 700°C, the cost of SOFCs can be significantly reduced.
