In-situ observation of operating polymer electrolyte fuel cell (PEFC) by grazing incident small-angle neutron scattering; Contrast variation by using deuterium gas

Koizumi, Satoshi; Putra, A.; Yamaguchi, Daisuke; Zhao, Y.

no journal, , 

In order to visualize water distribution in an operating fuel cell, we combined two different methods using neutron as a probe, i.e., a combined method of small-angle & ultra-small-angle scattering (SANS) and radiography imaging. SANS observes water distribution in a membrane electrolyte assembly (MEA), whereas radiography observes bulk water appeared in a gas flow channel (so called "flooding"). The polymer electrolyte fuel cell (PEFC) was specially designed suitable for small-angle neutron scattering by replacing materials with aluminum in order to decrease background scattering. We employed hydrogen gas (H) and deuterated gas (D) as a fuel for operation. With exchange of H and D, we aim to perform a contrast variation as for polyelectrolyte film (Nafion). When D gas is used as a fuel, DO is produced at the cathode and diffuses back to the film. Then the film, originally swollen by HO, exhibits change of coherent scattering contrast. By changing a fuel gas from H to D, SANS quantitatively detected decrease of scattering intensity at scattering maximum originating from the ion-channel in the electrolyte. After quantitative analyses on scattering intensity, which is related to water ratio (HO/DO) in the ion channel, we found that 30% of the total water is replaced by DO by changing the gas from H to D. In a stationary state of fuel cell operation using D, the scattering intensity rhythmically oscillates (respiration of fuel cell). The rhythmic oscillation found for the peak intensity is a non-equilibrium and non-linear phenomenon, in which "flooding" in a flow field is a feedback mechanism to slow down chemical reaction or water production by affecting mass transportation of air at the cathode. A valance between two diffusions, (1) back diffusion of DO from the cathode to the electrolyte and (2) diffusion of HO supplied as humidity, determines a time interval of