Molecular-Level Investigation of Critical Gap Size between Catalyst Particles and Electrolyte in Hydrogen Proton Exchange Membrane Fuel Cells

Molecular dynamics simulations have been performed to study the structure and transport at the electrode/electrolyte interface in hydrogen-based proton exchange membrane fuel cells. We examine the wetting of catalyst surfaces that are not immediately adjacent to a Nafion membrane, but rather are separated from the membrane by a hydrophobic gap of carbon support surface (graphite). A mixture of Nafion, water and hydronium ions is able to wet small gaps (7.4 Å) of graphite and reach the catalyst surface, providing a path for proton transport from the catalyst to the membrane. However, for gaps of 14.8 Å, we observe no wetting of the graphite or the catalyst surface. Using a coarse-grained model, we found that the presence of a graphite gap of 7.4 Å width slowed down the transport of water by at least an order of magnitude relative to a system with no gap. The implication is that catalyst particles that are not within nominally 1 nm of either the proton exchange membrane or recast ionomer in the electrode leading to the membrane do not possess a path for efficient proton transport to the membrane and consequently do not contribute significantly to power production in the fuel cell.