TOPICAL DAY - Nanoscale water dynamics in operando fuel cells

Orateur : Nicolas Martinez (CEA Grenoble)

Proton exchange fuel cells (PEMFC) represent one of the most promising technologies for powering small portable devices and automobiles in an environmental friendly way. The energy resulting from the reaction between H2 and O2 is transformed in electrical current that can be used to power small devices or automobiles in an environmental friendly way. The key element of the PEMFC is the Membrane Electrode Assembly (MEA), which consists of a proton exchanging membrane inserted between two electrodes. The membrane’s role is to act as an electrical insulator while conducting protons produced at the cathode to the anode. This membrane is composed of a polymer hydrophobic matrix containing hydrophilic SO3 groups. As a consequence, the membrane swells when put in contact with water, forming a nano-separated phase of polymer matrix and water. This water nanophase is responsible for the transport of hydrogen produced at the cathode towards the anode. The low cross sections of the metallic gaskets holding the MEA and the huge incoherent cross section of hydrogen make neutron scattering and imaging techniques an ideal tool to perform operando measurements of water distribution and dynamics. QENS in particular can probe water transport properties in the ps-Å window. Previous studies have shown that in ex-situ conditions, protons form two dynamically distinct populations [1] [2]. While a majority of these protons undergo fast bulk-like dynamics, the other population exhibits a slow dynamical behavior. Both these populations exhibit a confinement due to the interaction with the SO3 groups and the nanophase segregation. This study showed that slow/fast protons ratios, diffusion coefficients and confinement radii greatly depend on the hydration state of the membrane. While these results help us greatly understand the transport properties of water in these membranes, there is no evidence that this behavior stays unchanged in a functioning fuel cell, or if alters these properties. The talk will focus on how in-situ measurements compare to ex-situ ones, in order to conclude on whether ex-situ measurements can be used to draw conclusions on operando systems.

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