Correlating the Improved H₂/Air Performance of Mesoporous Graphitic Spheres with their Tailored Pore Structure

The carbon support structure strongly contributes to the performance of a proton exchange membrane fuel cell (PEMFC). It is well known that the porous structure of high surface area carbons can mitigate the interaction of ionomer and internally deposited Pt particles, but concurrently significantly inhibits oxygen transport to the active surface. This study investigates ∼350-400 nm diameter mesoporous graphitic spheres (MGS) as carbon support for Pt-based cathode catalysts, whereby the Pt particles are dispersed homogeneously throughout the MGS. Compared to Vulcan or Ketjenblack carbon supports, MGS-based cathodes demonstrate significantly improved H₂/air performance at all current densities and over a broad range of operating conditions. This is attributed to the well-defined MGS pore network with a high amount of large mesopores, which apparently enables an efficient transport of protons and oxygen to the internal Pt particles that show a high oxygen reduction activity. We demonstrate that the enhanced high-current density performance is primarily attributed to the substantial decrease in local oxygen transport resistance, arising from optimal catalyst accessibility. Finally, to establish a link between its exceptional H₂/air performance and the distinct properties of a Pt/MGS-based cathode catalyst layer, we utilized scanning transmission electron microscopy in combination with gas physisorption measurements.