  Achieving high power performance and durability with low Pt loadings are
critical challenges for proton exchange membrane fuel cells. PtCo catalysts
developed on new carbon black supports show promise by simultaneously providing
good oxygen reduction kinetics and local oxygen transport. We investigate the
role of nanoscale morphology in the performance of these catalysts supported on
accessible (HSC-e and HSC-f) and conventional (Ketjen Black) porous carbons
using 3D electron tomography, nitrogen sorption, and electrochemical
performance measurements. We find that the accessible porous carbons have
hollow interiors with mesopores that are larger and more numerous than
conventional porous carbons. However, mesopore-sized openings (>2nm width) are
too rare to account for significant oxygen transport. Instead we propose the
primary oxygen transport pathway into the interior is through 1-2nm microporous
channels permeating the carbon. The increased mesoporosity in the accessible
porous carbons results in a shorter diffusion pathlength through constrictive,
tortuous micropores in the support shell leading to lower local oxygen
transport resistance. In durability testing, the accessible porous carbons show
faster rates of electrochemical surface area loss, likely from fewer
constrictive pores that would mitigate coarsening, but maintain superior high
current density performance at end of life from the improved local oxygen
transport.
