Elena Aleksandrova

Spatial distribution and dynamics of proton conductivity in fuel cell membranes: potential and limitations of electrochemical atomic force microscopy measurements

The proton conductivity of a Nafion 112 membrane is measured with a high spatial resolution using electrochemical atomic force microscopy. Image analysis reveals an inhomogeneous conductivity distribution which is attributed to the limited connectivity of hydrophilic domains. This information relates to the micro-morphology which is due to phase separation of the hydrophobic polymer backbone and the hydrophilic pendant groups. The direct images relate to a different length scale and are complementary to the x-ray diffraction investigations which provide only average information. Furthermore, the measured current values reveal an interesting correlation with the size of the conductive areas. A bimodal conductivity distribution suggests that there are different mechanisms which contribute to the proton current in Nafion. Additionally, time dependence in local conductivity is found and interpreted in terms of redistribution of water in the membrane. A statistical analysis of the current distribution is performed and compared with theoretical simulations. Evidence is found for the existence of a critical current density. On a timescale of seconds the response of the conductive network is probed by applying voltage steps to the atomic force microscope tip.

Observation of Fuel Cell Membrane Degradation by Ex Situ and In Situ Electron Paramagnetic Resonance

Aiming at the development of accelerated stability tests of perfluorinated and polyaromatic proton conducting fuel cell polymer membranes, free-radical degradation was initiated by exposure to .OH radicals generated from hydrogen peroxide. All investigations were performed by electron paramagnetic resonance on various monomer and polymer solutions as well as on bare and coated polymers. No signals indicating an .OH attack on Nafion ionomers were detected, in contrast to all polyaromatic membrane materials. Nevertheless, in situ spin trap measurements with Nafion membranes reveal strong signals which formally belong to trapped H atoms but are not related to degradation. Relative to the Faraday electrons from the electrochemical reaction, they appear on a parts per million level.

Electrochemical AFM Investigations of Proton Conducting Membranes

Perfluorinated, s-peek and s-ppek polymer electrolyte fuel cell membranes are investigated using electrochemical atomic force microscopy. The proton conductivity and the topography are measured simultaneously at a nanoscale resolution. The results are compared with those of Nafion, and significant differences in the proton conductivity are revealed. The hydrocarbon membranes show a small overall conductivity and not the typical conductive and non-conductive regions as observed for Nafion. For the s-peek membrane an anti-correlation between the topography and the conductivity is observed. One type of the perfluorinated membranes in principle behaves similar to Nafion, whereas another type behaves more similar to the hydrocarbon membranes. This suggests that the manufacturing process plays an important role for the morphology of the polymers.

Observation of Membrane Degradation by In-Situ and Ex-Situ Electron Paramagnetic Resonance

The degradation of fuel cell membranes was investigated with perflourinated and with polyaromatic materials. Since radical mechanisms are thought to be responsible for damaging the components in a working fuel cell, •OH radicals were generated by cleavage of hydrogen peroxide by UV light, by heat, and using Fentons reagent. Aiming at the development of accelerated ageing tests for membranes, measurements of radical species were performed by electron paramagnetic resonance ex-situ with monomer and polymer solutions and with membranes, and in-situ on catalyst coated membranes. No signals indicating •OH attack on Nafion were detected, in contrast to all polyaromatic membrane materials. Nevertheless, in-situ spin trap measurements with Nafion reveal strong signals which formally belong to trapped H atoms but are not related to degradation. Relative to the Faraday electrons from the electrochemical reaction they appear on a ppm level.

Proton Conductivity and Micro-morphology of Nafion Fuel Cell Membranes Determined by Electrochemical Atomic Force Microscopy

The proton conductivity of a Nafion 112 membrane is measured with a resolution of ca. 10 nm using Electrochemical Atomic Force Microscopy. The observations are compared with the Gierke model of the micro-morphology due to phase separation of the hydrophobic polymer backbone and the hydrophilic pendant groups. The results demonstrate a highly inhomogeneous conductivity distribution which is attributed to the limited connectivity of hydrophilic domains. A pronounced peak was found in histograms representing the number of conductive areas with a certain current. We suggest that the measured current significantly depends on the formation of a continuous network in the membrane, so that the same conductivity can be measured at all pore exits. These pore exits to some extent seem to be quantized in size, even though they are of irregular shape.