Self-humidification

Masahiro Watanabe etal., 1998, Polymer Electrolyte Membranes Incorporated with Nano-meter Size Particles of Pt and/or Metal Oxides Experimental Analysis of the Self-Humidification and Suppression of Gas Crossover in Euel Cells. Journal of Physical Chemistry B, 102, 3129-3137. 

Watanabe M, Uchida H, and Emori M (1998), Analyses of self-humidification and suppression of gas crossover in R-dispersed polymer electrolyte membranes for fuel cells ,/. Electrochem. Soc., 145,1137-1141. 

PEMFC humidification technology primarily involves external, internal, and self-humidification. 

FIGURE 10.34 Schematic of self-humidification. (Reprinted from S. Xu et al. 2006. Journal of China Science and Technology. No. 2. With permission.)... 

All seven humidification methods described so far could use the water leaving the cell to do the humidification. One method that does not do this should be mentioned. Watanabe (1996) has described a system called self-humidification, where the electrolyte... 

Watanabe, M., Uchida, H., Seki, Y, and Emori, M., 1998, Polymer electrolyte membranes incorporated with nanometer-size particles of pt and/or metal-oxides experimental analysis of the self-humidification and suppression of gas-crossover in fuel cells, J. Phys. Chem. B 102 3129-3137. 

Air handling unit A self-contained unit for the introduction or removal of air from a space. It consists of one or more of the following plant items heating or cooling coils, filters, fans, damper provision for mixing and recirculation, humidification controls, and acoustic treatment. 

During start up, when the stack temperature and then performance (voltage) are low the passive BOP devices devoted to stream and stack humidification (enthalpy wheels membrane humidifiers) do not work as they are essentially based on the difference of temperature and then are practically excluded by the FCS management on the other hand, at low temperature the requirements for stack humidification are strongly reduced and membrane could self-hydrate if a sufficient water production (proportional to power requirement) occurs. 

It was found that the mass transfer behavior of reactants and products of the stack is more complicated compared with a single cell because of the heat exchange, humidity, and reactant supply effects. Some of the produced water was lost by evaporation, while self-humidifying was found to be more efficient at temperatures above 30 °C. Under laboratory conditions, humidification can be lowered if cooling power is improved to compensate for the heat released by the electrode reactions. In applications, however, cooling power is limited and humidification is a necessity. 

Fig. 16 Various types of humidification (six diagrams showing conventional fuel cell humidification methods used in research (a) the self-humidifying fuel cell with no active external humidification of the reactant streams, (b) the setup featuring liquid water injection into an inactive portion of the fuel cell, (c) a typical dew point humidifier, (d) the evaporation setup, (e) the steam injection system and (f) the flash evaporation method (from Evans, 2003).

The performance of cells using self-humidifying composite membranes has improved considerably compared with that of composite membranes without self-assembled Pt nanoparticles. However, the performance of self-humidifying composite membranes without external humidification is stiU inferior to that of cells with external humidification. At a current density of 900 mA/cm cell potential with external humidification at 80 °C was found... 

An interesting idea is to produce water within a membrane by catalytic recombination of H2 and O2 on a platinum catalyst dispersed in the membrane. One of the difficulties in developing such electrolytes is preventing short circuits, which requires a uniform dispersion of Pt inside the membrane, another is the prohibitive price. Since the water produced within the membrane is insufficient to ensure good performance, external humidification is essential. The advantage of self-humidifying PEMs lies in their ability to reduce the humidity of the reactant gases. 

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