Trends in High-Temperature Polymer Electrolyte Fuel Cells
The HT-PEFC, based on polybenzimidazole type membranes doped with phosphoric acid, has a typical operating temperature of 160 °C. As a result of the high temperature level, it has a high CO tolerance, which means that it is particularly suitable for operation in combination with reformers. Compared to Nafion-based polymer electrolyte membrane fuel cells, water is not required for the ionic conductivity of the membrane; therefore the gases do not need to be humidified. Another advantage of HT-PEFC technology is that due to the high temperature difference between the stack and the ambient temperatures, cooling can be designed in a much more compact manner than is necessary in classical PEMFC systems.
However, for all these advantages, prevention measures must be in place to ensure that the acid is not discharged from the membrane. Operating conditions that could create liquid water in the cells should be avoided. Save operation below 120°C is possible only with pure hydrogen. Due to the decreasing CO tolerance with decreasing temperature operation above 120°C is necessary if the fuel contains typical CO concentrations of 1%. Operation in low temperature ranges should therefore be avoided. This has been taken into account by operating strategies and the specific areas of application. In this context the oxygen reduction reaction and operation behaviour of cells and stacks will be discussed.