Fuel cells, basic requirements high-temperature

Phosphoric acid is also used as the electrolyte in so called High Temperature PEFC. In this case, the phosphoric acid is imbibed in a polybenzimidazole polymer matrix. While management of the liquid electrolyte in PAFC requires careful differential pressure control, High Temperature PEFCs are more tolerant Furthermore, the basic nature of the polybenzimidazole matrix prevents electrolyte migration. High Temperature PEFCs therefore are an attractive alternative to PAFC. Nevertheless, phosphoric acid is washed out from PAFC and high temperature PEFC once liquid water can form inside the fuel cell. Therefore, a continuous mode of operation is preferred in both cases. 

The basic principle is the same as for the warm-up humidiflcation method, as a dew point humidifler also depends on water evaporation. However, its structure is much more complex. First, water is evaporated to steam, becoming the humidifier gas then this high-temperature gas is cooled to saturation. Next, the water content of the gas can be accurately calculated according to the saturation vapor pressure at this temperature, after which the gas is heated to the fuel cell temperature and supplied to the fuel cell. The dew point method can accurately control gas humidity, but it consumes a huge amount of energy and requires a complex system. 

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