Thesis presented March 17, 2023

Abstract:
Proton-Exchange Membrane Fuel Cells (PEMFC) have emerged as promising emission free energy conversion devices. However, the ionomer membrane at the heart of the device fails to continually perform over the desired time (8000 h for transportation and 50000 h for stationary) with a high temperature (100–150 °C vs. 80 °C for std Nafion) and at low humidity (30 %RH) ranges. The aim of this work is to improve existing membranes by introducing stabilizing additives by Sol-Gel (SG) chemistry. SG precursors are selected to diffuse and selfcondense inside commercial membranes allowing for the introduction and immobilization of the stabilizing groups without influencing the proton conductivity, mechanical properties and permeability of the membranes. The SG phase is designed to protect against oxidation and limits dimensional changes that lead to ageing of the membrane during in-cell operation. An extensive list of antioxidant functions exists, and it was decided to use the functions thiourea, thiol, phenol hindered as sacrificial stabilizing functions for their high reactivity in the presence of hydrogen peroxide already presented in the literature. Stabilization by inserting a regenerable radical inhibitor, such as cerium ions, is also envisaged. The morphology (size, interaction/dispersion, connectivity) and localization (polar/apolar regions) of the SG phase will depend on the chemical affinity and are parameters expected to be crucial for properties (H⁺ conductivity, water uptake), durability (H₂O₂-accelerated aging tests to assess the effectiveness of the reactive SG phase) and performances (FC operation) of the hybrid membranes. Consequently, this study explores the morphology obtained (mass fractal structure vs. dispersed spherical aggregates vs. interconnected ones) at all relevant length scales using a combination of direct space (AFM/SEM/TEM) and reciprocal space (contrast variation SANS/SAXS) techniques (dimensional scale covered: from a hundred to a few nanometers). H₂O₂-accelerated ex situ aging tests and fuel cell tests show promising operability of the hybrid membranes and the potential of the SG phase to inhibit the chemical ageing of sPEEK.

Keywords:
Polymers, PEMFC, Characterisation, Energy, Materials

On-line thesis. ​