Fuel cell stacks and assemblies consist of individual fuel cells that are combined either in series to provide a higher usable voltage, or in parallel to provide a higher usable current. Each cell in the stack converts the chemical energy from a liquid or gaseous fuel with an oxidant such as air into electricity.
Unlike batteries, fuel cell stacks do not run down and they do not require recharging. Rather, these electromechanical devices generate electricity as long as a fuel and oxidizer are provided; however, corrosion can reduce efficiency over time. Selecting fuel cell stacks involves a consideration of available electrolytes, as well as an analysis of temperature, cost, and application requirements.
Types of Fuel Cell Stacks and Assemblies
Fuel cell stacks are categorized by the fuel cell electrolyte, an ion-conducting material used between the fuel electrode (anode) and the oxidant electrode (cathode). These materials include polymer electrolyte membrane, direct methanol, alkaline, phosphoric acid, molten carbonate, and solid oxide. The type of electrolyte determines the operating temperature, typical stack size, and efficiency for the fuel cells in the stack.
- Polymer electrolyte membrane (PEM) or proton exchange membrane fuel cells use hydrogen as a fuel and a solid polymer as an electrolyte. They require a more expensive catalyst (typically platinum) and are limited to low-temperature applications. Advantages include fast startup times, low sensitivity to orientation, and a favorable power-to-weight ratio, making them favorable in automotive applications. The barrier to these fuel cells is hydrogen storage, as low energy density hydrogen cannot be stored in energy amounts comparative to gasoline or other fossil fuels.
- Direct methanol fuel cells (DMFC) use pure liquid methanol as the electrolyte. Fuel conversion of hydrogen rich fuels takes place within the cell system itself. Because methanol has a higher energy density than hydrogen, fuel storage and transportation problems common to other fuel cells that use hydrogen as a fuel do not exist. DMFC technology is years behind that of other fuel cells however, making the cells less efficient.
- Alkaline fuel cells (AFC) use a solution of potassium hydroxide in water as the electrolyte. AFC stacks can run for as many as 8000 operating hours, but lack the material durability for longer-running applications. Because of this, alkaline cells lack the economic viability for large-scale applications. Some newer products are suitable for both low-temperature and high-temperature use.
- Phosphoric acid fuel cells (PAFC) use phosphoric acid as an electrolyte. Because they handle impurities well, PAFCs are suitable for converted fossil-fuel utilization when other cells might be damaged by carbon monoxide. They can be costly because they require platinum as a catalyst and are less efficient when not used in cogeneration plants (heating and power). They are also larger and heavier than other fuel cells.
- Molten carbonate fuel cells (MCFC) have an electrolyte that consists of a molten carbonate salt mixture suspended in a porous, chemically-inert ceramic lithium-oxide matrix. They operate at high temperatures of roughly 1,200°F and above and do not require an external reformer for fuel conversion, a fact which helps contain costs. They can even utilize carbon oxides as fuel. MCFC fuel stacks are, susceptible to corrosion, however, due to the high operating temperatures and the corrosive electrolyte used. This accelerates component breakdown and significantly decreases cell life.
- Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte. SOFCs can operate at very high temperatures, but require thermal shielding. Because they do not require a precious metal as the catalyst, they are usually lower in cost. This type of fuel cell is also the most sulfur-resistant, a fact that makes SOFC fuel stacks suitable for use with coal-based gases.