Shaping of vertically aligned carbon nanotubes as a novel microporous layer structure for gas  diffusion layer in PEMFCs

Proton exchange membrane fuel cells (PEMFCs) are promising for transportation due to their high power density and low carbon emissions. They convert the chemical energy of hydrogen and oxygen into electricity, producing water and heat. Within the cell, the gas diffusion layer (GDL), located between the catalyst layer and the bipolar plate, simultaneously ensures reactant distribution, liquid water removal, electronic and thermal conduction, and mechanical support of the membrane-electrode assembly (MEA).
Conventional GDLs generally consist of a macroporous carbon-fiber substrate associated with a microporous layer. This architecture must simultaneously meet sometimes antagonistic requirements: ensure gas transport, evacuate liquid water while maintaining MEA hydration, provide good electrical and thermal conductivity, and maintain its properties under operating conditions. Liquid water removal relies in particular on hydrophobic agents such as PTFE, whose durability remains an issue. The development of new materials and architectures to control these transport phenomena is therefore important for improving PEMFC performance and durability.
Carbon nanotubes (CNTs), owing to their hydrophobic, chemical and electrical properties, offer the possibility of producing self-supported GDLs without fluorinated treatment. Vertically aligned carbon nanotubes (VACNTs) were synthesized by aerosol-assisted catalytic chemical vapor deposition (AACCVD), then dispersed and filtered to form a thin, porous paper made of interlaced CNTs.
In this context, this thesis is part of the PEPR H2, specifically the PEMFC95 project, which aims to increase the nominal operating temperature of PEMFCs from 80°C to 95°C for heavy-mobility applications.
Conducted between CEA Saclay and CEA Grenoble, this thesis focuses on the development of an innovative CNT-based GDL in the form of a porous, electrically and thermally conductive, hydrophobic paper known as a buckypaper.
The objective of this study is both technological and scientific: to develop CNT-based GDL architectures adapted to the targeted operating conditions and to establish relationships between their structure, transport properties and electrochemical performance, particularly under the project conditions. Multiscale characterization combined with differential-cell testing was used to relate material properties to transport mechanisms and cell performance.
Optimization of the cathodic GDL led to a bilayer architecture combining a densified buckypaper with a macroporous fibrous layer. This configuration achieved, and under certain conditions exceeded, the performance of the commercial reference GDL without PTFE-based hydrophobic treatment. Optimization of the anodic GDL also demonstrated that its architecture can significantly influence cell performance, highlighting the importance of this electrode in maintaining adequate membrane hydration while preserving efficient reactant transport.
This thesis highlights the potential of CNT-based architectures as an alternative to conventional GDLs and provides insights into the relationships between porous structure, liquid and vapor water transport, reactant transport and PEMFC performance. These results could also provide useful input data for validating and refining transport models of PEMFCs.