There is an acute need for the development of long-lasting, efficient and portable power sources for further technology improvement in automobiles, commercial electronics devices, military and stationary applications. These systems all require the power source to be energy-efficient, and able to operate for long periods of time without refueling. Fuel cells are known for high energy density, a variety of fuel sources, and ease of scaling for application-specific requirements.
Temperature homogeneity in a fuel cell stack is critical for optimal performance. The temperature in a fuel cell stack may not be uniform due to gas flow, water phase change, coolant temperature, the trapping of water and catalyst layer heating. The membrane must be hydrated for proper ionic conduction through the fuel cell. If the fuel cell is heated too much, water will evaporate and the membrane will dry out -- causing the performance of the fuel cell to suffer. If too much water is produced on the cathode side, water removal can affect the overall cell heat distribution, which ultimately leads to fuel cell performance losses. Fuel cell transport phenomena and phase changes complicate the temperature distribution in traditional PEM fuel cell stacks.
Most fuel cell stacks require a cooling system to maintain temperature homogeneity throughout the fuel cell stack. Any deviation from the designed temperature range can result in lowered fuel cell efficiency. Higher temperatures mean faster kinetics and an increase in voltage. Operating at lower temperatures means slower kinetics and greater voltage losses; however, there are less water management issues.
Today’s fuel cell stack heat management systems are still created using water or coolant systems. These systems are messy, inefficient and costly because they require a recirculator, an external cooling system, and cooling plates. The traditional balance-of-plant system adds bulk and complexity to the system, and adds substantial cost to the system. Water cooling designs are complex because they require an oil-free water pump, and must be maintained within a narrow temperature range. If the fuel cell stack is used in freezing conditions, it must be warm enough to allow the water to remain in the liquid state, which means it is critical that the proper temperature is maintained in the fuel cell stack. A comparison of commonly used cooling methods is shown in Table 1.
Table 1. Comparison of Cooling Systems for PEM Fuel Cell Stacks
• Known systems for coolant use
• Wide variation of flow designs
• Hard to control
• Costly subsystems required for use
• Often uses simpler system than coolant
• Unique cooling systems can be created using the water generated by the cathode catalyst layers
• Cannot be used effectively in HT PEM systems due to the required low operating temperature
• Two-phase issues
• Can be used in HT PEMFCs at high stoichiometric flow rates
• Uses lower-cost systems to circulate air
• Known systems for air recirculation
• Wide variety of flow field plate designs can be used
|• Cannot be used in low-temperature PEM fuel cells over 10 kW due to membrane dehydration|
Another commonly used cooling method is air cooling. For small fuel cells, natural convection cooling is extremely simple -- with an open channel structure on the cathode side. Forced convection air flow is also used to remove waste heat from the stack. Often high air flow capacity or wide gas channels are necessary to remove waste heat. In PEM fuel cells, the membrane needs to be humidified, which limits the air flow through the stack because the air will dry it out. This complicates the fuel cell stack because it will require a separate reactant air supply and cooling system. Air cooling is limited in PEM fuel cell stacks to under 10 kW. Over 10 kW, either water or coolant must be used.
There are advantages and disadvantages of every fuel cell type – depending upon the stack design, surface area and number of cells. With the right design, high-temperature PEM fuel cells can alleviate many of the issues that have plagued conventional low-temperature PEM fuel cells for decades. These novel fuel cells offer the following technical advantages over conventional low-temperature fuel cells:
• Reduced membrane cost: Commercially-available high-temperature membranes use alternative polymers that are widely-available and low-cost, which enables a significant drop in membrane cost. The high costs of traditional fluoropolymer-based membranes limit cost reduction opportunities for low-temperature PEMFCs.
• Elimination of liquid cooling systems within the stack: The higher temperature difference between the fuel cell and the ambient conditions enable more efficient fuel cell stack heat rejection to the atmosphere, which reduces the heat transfer area and enables the use of air cooling through the cathode channels at high power levels. This reduces the complexity of the fuel cell stack design because only hydrogen sealing is required (low-temperature stacks need to seal hydrogen, air and the coolant loop). This simplification can lead to a significant reduction in fuel cell stack cost and reliability.
• Elimination of liquid water generation in the cathode channels: Since the HT PEMFC operates at a temperature higher than the boiling temperature of water, the water generated by the fuel cell does not accumulate in the cathode. Since all fluids are gaseous, this simplifies the stack design and results in increased reliability of the fuel cell stack. Unlike low-temperature PEM fuel cell systems, the high-temperature system does not require an air compressor, humidifiers, or heat exchangers.
• Wide temperature range of operability: High-temperature PEM fuel cells have good current and power density over a wide range of operability, which can be beneficial for a wide range of low-cost system designs. The wide temperature range does not limit the material and component selection to only specific high-temperature materials, which allows the selection of lower-cost, readily available components.
Higher operating temperatures also introduce the necessity of proper heating and cooling of the stack to maintain stack temperature uniformity. If the stack temperature is too high, the durability of the fuel cell stack components is compromised, and the agglomeration of the platinum particles in the electrodes increases. Material degradation of engineering materials, such as the flow field plates, seals and gaskets become greater, and the rate of corrosion could increase. Therefore, minimizing temperature fluctuations in the HT PEM fuel cell stack is necessary to meet the requirements for successful commercialization and integration into several fuel cell applications.
Thermal uniformity, water management, and humidity control are significant contributors to the long life, system complexity, and high cost of traditional PEMFC systems. By increasing the operating temperature of PEMFCs -- these issues can potentially be resolved. High-temperature fuel cell operation enables a reduction in the required heat transfer area for stack heat rejection and provides the opportunity to simplify the fuel cell system and the stack by using the cathode air channels for heat rejection.