Hydration dynamics of a Nafion membrane in a running fuel cell

Due to their capability of efficiently converting chemical into electrical energy, proton exchange membrane (PEM) fuel cells could play a major role in future environment-friendly hydrogen-based technology. The amount of water and its spatial distribution in the polymeric membrane of the device under working conditions is a fundamental point to address since the ionic conductivity, and consequently the cell performance, depends on the degree of membrane hydration. The few experimental techniques [1] used so far to measure the water distribution in the membrane suffer from a rather low spatial resolution (if any). Moreover, they have other severe limitations, such as slow response to variations in the degree of hydration, a weak signal resulting in poor accuracy and an indirect dependence only on the quantity of interest.

Researchers from the Istituto di Struttura della Materia of Roma, University of Camerino and the ESRF used very high energy synchrotron radiation diffraction (VHEXD) [2] at beamline ID15B, to perform the first space and time resolved measurements of proton exchange membrane hydration in a running fuel cell. Their approach based on VHEXD allowed in situ measurement of the degree of membrane hydration in an operating cell. A vertical stratigraphy of the membrane was measured from one electrode to the other, corresponding to an imaginary “slicing” of the membrane into a stack of layers (see Figure 1). Both the hydration degree in each layer and the overall amount of water in the membrane could be determined as a function of time, with the highest level of accuracy ever achieved.

Figure 1. Schematic drawing of the fuel cell’s layers.

The observed correlation between the fuel cell voltage and the degree of PEM hydration permits a more accurate description of the how the fuel cell works as well as more realistic models of its performance, to use in further technological improvements.
 

Time-resolved study

To measure the average degree of hydration of the Nafion® 117 membrane (about 140 µm thick), the whole membrane was irradiated by a primary X-ray beam with a vertical cross section equivalent to the PEM thickness. A sequence of diffraction patterns was then collected, keeping the setup geometry unchanged. The patterns were processed taking into account the scattering power of both the water and the polymer and the curve plotting degree of hydration as a function of time was obtained. In Figure 2, the water variations induced by sudden changes of the gas feeding are shown. One can observe the tight correlation between hydration degree and cell voltage (red and green vertical lines) and how even minimal changes of the former influence the latter (red circles).

Figure 2. Time-resolved study of the water content of the whole proton exchange membrane. Upper plot: average water content as a function of time, obtained by processing the diffraction data. Lower plot: Corresponding cell voltage curve.

Space-resolved study

This consisted of a vertical scan of the Nafion® 117 membrane, executed under steady conditions by using a primary X-ray beam with transversal section of about 7 (vertical) x 100 (horizontal) µm² (see Figure 3). The sequence of diffraction patterns produced during the vertical scan of the membrane represents its virtual “slicing” from anode to cathode (repeated in reverse, to test the reliability of the method).

Figure 3. Space-resolved study of the water distribution along the vertical axis in the proton exchange membrane under steady conditions from anode to cathode and back to anode. The diffraction patterns corresponding to the two scanning sequences are reported in the insert.

References
[1] J. St-Pierre, Electrochem. Soc. 157, B724 (2007).
[2] V. Rossi Albertini, B. Paci, A. Generosi, S. Panero, M. A. Navarra, M. Di Michiel Electrochem. Solid-State Lett. 7, A519J (2004).   

Principal publication and authors
V. Rossi Albertini (a), B. Paci (a), F. Nobili (b),   R. Marassi (b), M. Di Michiel (c), Time/space-resolved studies of the Nafion membrane hydration profile in a running fuel cell, Advanced Materials (2008), DOI 10.1002/adma.200801652.
(a) Istituto di Struttura della Materia, C.N.R,. Roma (Italy)
(b) Università di Camerino, Camerino (MC) - Italy
(c) ESRF

Acknowledgement
This research was supported in part by the NUME project (Italian Ministry of University and Research, FISR 2003).