Feb 03 2011
The polymer electrolyte membrane fuel cell (PEMFC) is widely regarded as a potential power source for portable and mobile applications, due to its noteworthy features of high efficiency and zero emission.
Successful start-up from subzero temperatures is of paramount importance for the commercialization of PEMFCs for practical applications, such as backup power and automotive applications.
Under freezing environmental conditions, water produced at the cathode has a tendency to freeze in open pores in the catalyst layer and GDL, rather than be removed from the fuel cell, thus creating mass transport limitations which, eventually, end the ability for operation.
Even though various external heating methods can be used to ensure the cold start capability, the volume and weight of the system, as well as the operation complexity and installation costs, all increase with the increment of the external heating power requirement.
Analysis of the various cold start processes to achieving optimal design and operating strategy is therefore critical to simplify or cast off the external heating system.
While the characteristics of PEMFC dynamics have been studied by several groups, research on PEMFC cold start-up is relatively new and some aspects of degradation caused by freeze/thaw cycling are discussed controversially in literature.
Water is produced during the operation of a fuel cell; in PEMFCs, water within the cell is necessary to ensure high protonic conductivity of the polymer electrolyte membrane. During normal operation, the generated waste heat is sufficient to keep the water within the cell above the freezing point, even at ambient temperatures significantly belov 0°C. Howevwr, when fuel cells are switched off under sub-zero conditions, the volume expansion by ice formation within the cell can lead to structural damage.
In order to ensure good gas diffusivity and to extend the electrochemically active surface area, the materials of the gas diffusion layers, micro-porous layer and electrodes are highly porous. If water freezes within these media, volume expansion can lead to cracks in their structure and a change in the pore size distribution of the electrodes.
The physical state of water within the membrane seems to be a key issue in membrane degradation under freezing conditions. Due to the high capillary pressure in small pores, the freezing point of water within the membrane can fall below 0°C. In a Nafion membrane, besides free and loosely bound freezable water, non-freezing water can be present, which is still moveable even at –20°C.
For the membrane/electrode interface, some authors reported delamination of the electrodes from the membrane due to freeze/thaw cycling, while some others didn’t find any indication for delamination of catalyst layers. Performance degradation in PEMFCs is highly dependent on the cell components.
Purging with reactant gases (dried or humidified) seems to be a promising approach to prevent degradation of PEMFCs caused by freeze/thaw cycling. Such purging procedures are applied before switching off the cell, in order to remove residual water from the porous media.
In summary, to avoid degradation at low temperatures, water has to be removed before freezing from the cell or the PEMFC components have to be redesigned with greater material flexibility to allow volume expansion at the phase transition of liquid water to ice.
What do you think is the best mitigation strategy in order to allow a cold start-up, preventing a cell degradation at sub-zero temperatures?
No responses yet