Measurement of mass transport losses breakdown: The Pulsed Gas Analysis (PGA) method

Mass transport losses represent a significant limitation for the maximal reachable power density of polymer electrolyte fuel cells (PEFCs). The ability to measure them, in order to minimize them, is of high interest for the design of the structure of fuel cell components. Helox/oxygen voltage gain measurement is a known method to evaluate these losses, though its classical application suffers from measurement artifacts due to the different physical properties of helox (a  mixture of helium and oxygen) and air in terms of water management, and due to the oxidation of the catalyst surface following extended operation with pure oxygen. Our Pulse Gas Analysis (PGA) method [1] solves both issues by operating the cell most of the time with air on the cathode and using short periods of operation (typically 1 second) with helox or oxygen. An optimized in house developed fuel cell test stand ensures a fast, seamless switching between the gases.

 

Measurement with the PGA method can be combined with standard and high resolution neutron imaging, as well as with our multi-cell test rig.
  Appears in  
    GDL MPL CL  
/sites/default/files/2019-04/bulk_diffusion.png Bulk diffusion is the dominating diffusion mode in pores much larger than the mean free path of the gas molecules (~50 nm). The oxygen transport is limited by collisions with nitrogen molecules. x x x Helox gain
/sites/default/files/2019-04/knudsen_diffusion.png Knudsen diffusion is the dominating diffusion mode in pores much smaller than the mean free path of the gas molecules. The oxygen transport is limited by collisions with the material structure.   x x Oxygen gain
/sites/default/files/2019-04/film_diffusion.png Thin film diffusion is represents the transport of oxygen to catalyst particles not having a direct contact to the gas phase, by diffusion through the ionomer material of through water.     x Oxygen gain
Example: Effect of the micro-porous layer (MPL). Performance at high humidity is strongly affected by the presence or absence of a MPL on the cathode. PGA analysis allowed identifying two different modes of mass transport losses. Based on the combination of PGA with neutron imaging, we attributed these to partial flooding of the catalyst layer, and to the formation of a dense water layer in the GDL near the catalyst layer, respectively [2].

[1]

  • Oberholzer P, Boillat P, Siegrist R, Kästner A, Lehmann EH, Scherer GG, et al.
    Simultaneous neutron imaging of six operating PEFCs: Experimental set-up and study of the MPL effect
    Electrochemistry Communications. 2012; 20(1): 67-70. https://doi.org/10.1016/j.elecom.2012.03.038
    DORA PSI

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