Thesis presented April 29, 2021

Abstract:
The PEM Fuel cell technology has attracted great attention for future energy conversion and storage applications. Its commercialization is still a matter of challenge due to the durability and cost limitations of the catalyst. In order to enhance their achievement on the market, it is mandatory to optimize the high costly catalyst and improve its durability and stability or to innovate a new structure of electrodes. Commercial Pt/C catalyst (nanoparticles 3–5 nm) supported carbon black is considered as the current standard catalyst offering high surface areas and high specific activity. Unfortunately, they are limited by various degradation barriers: the cost; the corrosion of the carbon support, and the Pt dissolution/agglomeration through electrochemical Ostwald ripening mechanism, which is reflected by a fast and significant loss of electrochemical surface area overtime during fuel cell operation.
This thesis focuses on reducing the amount of platinum and improving the performance and durability of membrane electrode assemblies MEAs, aiming to reduce their cost and encourage the development of PEMFCs. Previous studies have demonstrated that Pt-based catalysts such as PtCo, PtCu, and PtNi, etc., have shown to exhibit higher oxygen reduction reaction ORR activity than platinum for PEMFC and very promising enhancement in performance and stability for low Pt loading.
We aspired in this work to develop and control a new carbon-free architecture of the cathode, made only with vertically aligned PtNi nanowires and nanotubes supported onto a Nafion^® membrane to improve performance and durability of the PEMFC at low Pt loading.
The fabrication process is based on a multistep process of fabrication: Elaboration and geometry tuning of the nanoporous anodic aluminum oxide used as a sacrificial template by the double- anodization technique; The pulsed electrodeposition process as an electrochemical route of Ni nanowires growth inside the aluminum oxide template; Controlled galvanic displacement process, which is an electrochemical process involving the oxidation of the transition metal by the ions of a platinum salt at different concentrations of hydrochloric acid at different concentrations and temperatures: the process also involves a thermal treatment and acid leaching leading to the formation of nanowires and nanotubes (~50 nm in diameter and ~400 nm in length); The latter are transferred onto a Nafion^® membrane using an optimized hot-pressing process. Electrodes were integrated into a complete Membrane Electrode Assembly (MEA) to characterize their electrochemical and transport limitations in real operating conditions. Ultraviolet spectrophotometric determination of platinum content showed a loading of ~0.1 mgPt/cm² for the nanowires and ~15 µgPt/cm² for the nanotubes. For comparison, we have elaborated a homemade Pt/C conventional with low Pt loading (~35 µgPt/cm²), which exhibits a close catalyst surface area to the PtNi nanostructures. The performance of the MEA was quantified by recording the polarization curves and electrochemical impedance spectroscopy measurements. Accelerated durability tests gave an insight into the interest, stability, and limitations of these nanostructures compared to conventional electrodes.

Keywords:
Catalyst, Electrode, Nanostructures, Nanoporous Alumina, ORR Transport, PtNi

On-line thesis. ​