In and ex situ link : water transport and distribution through the working ion exchange membrane – INEXTREMIS

INEXTREMIS is a fundamental research project, exploratory type, which aims to contribute to the understanding of coupled charge/mass (proton/water) transport phenomena involved in the operation of polymer electrolyte fuel cells. That in order to establish relevant links between the evolution of the electrolyte water content and the electrochemical performances of the fuel cell.

Three complementary experimental methods, all based on radiation scattering, will be used for the determination of water concentration profiles through the thickness of the working electrolyte. Raman scattering is an up-and-coming technique for studying such problems, characterised by an excellent spatial resolution (few µm3). Small angle radiation scattering (neutrons and x-ray) have been successfully used to obtain water profiles through ionic polymers. Neutron scattering measurements will be used as reference, to corroborate results obtained from x-ray and Raman scattering. It should be noted that, because of the limited accessibility to radiation sources, neutrons scattering can be employed usually only for specific, targeted, studies. This justifies the need to develop new in situ characterisation methods, less onerous from the implementation point of view.

The hydration state of the electrolyte during operation is highly heterogeneous. In this project, the distribution of water through the membrane thickness will be investigated at the same time as a function of the cell working conditions and at different locations in the heart of the electrochemical system. Namely, at the membrane placed in the gas distribution channels as well as between the current collection ribs.

Among the expected outcomes of the project, we would like to emphasize the determination of local water diffusion coefficients (as a function of the local water content through the membrane thickness) and the electro-osmotic drag coefficients, for Nafion and Aquivion membranes. Furthermore, new membranes with hierarchical structure will be obtained from commercial ones. Their electrochemical properties will be investigated and in situ Raman analysis will allow evaluating the impact of the multilayer structure on the water repartition through the thickness.

Modelling and simulation of water transport in polymer membranes will be improved by using the experimentally determined water concentration profiles and diffusion coefficients (water diffusion, electro-osmotic drag). As a function of simulation results, the design of new membranes with hierarchical structure will be proposed. The efficacy of this strategy to improve water management in fuel cells will be verified by studying the behaviour of the first samples of multilayer materials obtained from Nafion and Aquivion.

INEXTREMIS is an exploratory project, including different research approaches: from the carrying out of new experimental set up until the modelling of transport phenomena and the shaping of new membranes. The detailed analysis of the highly coupled transport phenomena occurring in the heart of PEMFCs, strongly affecting the electrochemical performances of the system, needs the joining of several ex and in situ measurements, realized on or of fuel cell operation. The approach proposed in this project is, therefore, not only to use emerging measurement techniques, which can give improved knowledge, but also to separate the whole physical phenomena involved through targeted, complementary experiments and methods. The comparison of a wide amount of in and ex situ data will allow verifying whether or to what extent results obtained from measurements on membranes under equilibrium and homogeneous conditions can be useful to model the fuel cell electrolyte behaviour under working conditions. This is, in our opinion, the only realistic approach to establish relationships between the structural features of the electrolyte, its physical properties (especially hydration) and the experimental conditions.