External Humidification - Principles

Although we have seen that small fuel cells can be operated without additional or external humidification, in larger cells this is rarely done. Operating temperatures of over 60°C are desirable to reduce losses, especially the cathode activation voltage drops described in 

Section 3.4. Also, it makes economic sense to operate the fnel cell at maximum possible power density, even if the extra weight, volume, cost, and complexity of the humidification system are taken into acconnt. With larger cells, all these are proportionally less important. 

The need and the valne of external humidification is shown by revisiting equation 4.6, and taking an example 

Let us take a fuel cell operating at 90°C, moderately pressnrised, with an inlet pressure of 220 kPa and an exit pressure of 200 kPa. We will assume that the air stoichiometry is 2, a very typical value. If we also assume that the inlet is normal air at 20°C and 70% relative humidity, then pntting these numbers into eqnation 4.6, with values from Table 4.1, we find the following  

This is far too low a figure, and these conditions would dry out the PEM very quickly. However, if the inlet air is warm and damp, say 80° C and 90% relative humidity, then the figures become as follows  

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Besides performance, durability is another important concern of fuel cells. To evaluate fuel cell durability, two modes, steady-state (continuous running) and shutdown-restart cycling, are often used. In the steady-state mode, a fuel cell is operated continuously at a fixed current density to measure the voltage decay. Quite different durability data from a few hundred hours to nearly twenty thousand hours have been reported in the literature [2,28,47,55-58]. In principle, the durability is affected by many factors such as the chemical stability and conductivity stability of PEMs and the stability of catalyst layer. Oono and coworkers [56] studied the long-term cell degradation mechanism with a PA-doped PBI membrane (DL is unavailable from the literature). The H2/air fuel cell was continuously tested at 150°C at 0.2 A cm" without external humidification. They found that the fuel cell exhibited extremely... 

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