performance
The chemical reactions occurring inside lithium-ion batteries and fuel cell batteries determine the battery's reversible electromotive force (EMF). For an electrochemical reaction, its reversible EMF can be calculated using the formula:
This refers to the change in Gibbs free energy of an electrochemical reaction under standard conditions. The change in Gibbs free energy reflects the thermodynamic probability of the electrochemical reaction, and its magnitude is determined by the nature of the reaction itself, the concentrations of reactants and products, and the reaction temperature. n is the number of moles of electrons transferred per mole of reactant, and F is the Faraday constant. Under standard conditions, the reversible electromotive force (EMF) of a fuel cell is approximately 1.25V, and it decreases with increasing temperature. For lithium-ion batteries, however, the reversible EMF changes continuously due to the ongoing structural changes in the positive and negative electrode materials during the reaction. The reversible EMF of the battery is related to the extent of the reaction; therefore, the state of charge (SOC) of the battery can be determined by measuring the OCV based on the OCV-SOC curve. The performance curves of fuel cells and lithium-ion batteries during actual use are shown in Table 2.
Energy density
Electric vehicles rely entirely on batteries for power, emphasizing driving range after charging and thus focusing on battery energy density. The improvement of lithium-ion battery energy density is limited by theoretical bottlenecks in battery materials. Currently, the positive electrode material for domestically produced electric vehicle lithium-ion batteries is mainly lithium iron phosphate (LFP), while the negative electrode material still primarily uses graphite, with a specific energy of approximately 90-140 Wh/kg. Fuel cell batteries, on the other hand, are power generation devices with a much higher energy density than lithium-ion batteries. Regarding the driving range, which is directly related to energy density, the luxury electric vehicle Tesla has just reached 500 km; while fuel cell vehicles, such as the Toyota Mirai and Hyundai ix35, have driving ranges exceeding 500 km. Therefore, in terms of energy density, fuel cell batteries are superior to lithium-ion batteries.
life
The performance of both fuel cell batteries and lithium-ion batteries deteriorates with increased usage. Furthermore, start-stop and acceleration/deceleration cycles constitute a significant portion of a vehicle's overall operating conditions, resulting in a wide range of battery operating currents and a very large rate of current change, which undoubtedly shortens battery life. Therefore, researching the lifespan of fuel cell batteries and lithium-ion batteries has become one of the key issues for their application.
cost
Currently, the cost of domestic lithium-ion battery systems is around 1800 yuan/kWh, while the cost of fuel cell stacks (excluding fuel systems and other accessories) is around 5000 yuan/kW. For a typical sedan, assuming it's an electric vehicle with a 60kWh battery (the BYD DE6 has a 60kWh battery), the cost is around 96,000 yuan. If it's a fuel cell vehicle with a 100kW power output (the Toyota Mirai has a 114kWh battery), the stack cost is around 500,000 yuan.
The cost of fuel cell batteries is currently significantly higher than that of lithium-ion batteries, which is a bottleneck restricting their development. It is generally believed that the higher cost of fuel cell batteries is mainly due to the use of the precious metal phosphorus (Pt). The actual cost of Pt is calculated as follows: the current high Pt loading level is 0.4 mg/cm², with an electrical performance level of [email protected]/cm², or 0.96 W/cm². A 100kW fuel cell system uses 41.67g of Pt. Assuming a Pt price of 500 yuan/g, the cost of the Pt used is 41.67 * 500 = 20,833 yuan. The total cost of a 100kW fuel cell stack is over 500,000 yuan, with Pt accounting for only about 4% of the total cost. The high cost of fuel cell batteries is due to the current immaturity of materials and system technology; however, with commercialization, the cost will inevitably decrease significantly.
Safety and related regulations
The safety of power lithium batteries is a primary concern and issue to be addressed in the development of electric vehicles. Improving the safety of power lithium-ion batteries requires establishing a series of technical measures, encompassing materials, batteries and key components, and system safety assurance. With the increasing size and modular use of individual battery cells, the safety of power lithium-ion battery systems faces new challenges. Fuel cells use hydrogen, a flammable and explosive gas, leading to widespread market concerns about its safety. However, in reality, hydrogen's safety is not inferior to that of gasoline and natural gas.
The safety design of single-cell fuel cell systems is less complex than that of lithium-ion batteries. System-integrated fuel cell systems are more complex than lithium-ion battery systems. Due to the use of combustible hydrogen gas, additional hydrogen leakage protection designs are required. To prevent the effects of insufficient proton exchange membrane wetting, changes in internal humidity must be monitored by monitoring internal resistance. The relevant safety designs for fuel cell systems and lithium-ion batteries are shown in Table 4.
In power lithium-ion batteries, the reducing agent and oxidizing agent are stored in the same device, separated only by a membrane with a thickness of only a few micrometers. In contrast, in fuel cell batteries, the reducing agent and oxidizing agent are placed separately on the outside of the battery. In principle, fuel cell batteries are safer than lithium-ion batteries. However, through a series of safety protections, the safety of both types of batteries is within an acceptable range.
To ensure the safety of power lithium batteries, the state has formulated a series of standards for power lithium-ion batteries and fuel cell batteries, thereby ensuring the safety and reliability of power lithium batteries. Fuel cell batteries have fewer standards than lithium-ion batteries, were issued earlier, and their compliance with current conditions is not as good as that of lithium-ion batteries. Electric vehicles have corresponding type approval test procedures, such as "GB/T18388-2005 Electric Vehicle Type Approval Test Procedure," while type approval procedures for fuel cell vehicles, as an essential standard for the automotive industry regarding the type approval of new energy vehicle products, are urgently needed.
