Résumé
Due to their increased surface area to volume ratio and molecular accessibility, microporous and mesoporous materials are a promising strategy to electrocatalyze the cathodic oxygen reduction reaction (ORR), the key reaction in proton-exchange membrane fuel cells (PEMFC). Here, we synthesized and provided atomically resolved pictures of porous hollow PtNi/C nanocatalysts, investigated the elemental distribution of Ni and Pt atoms, measured the Pt lattice contraction, and correlated these observations to their ORR activity. The best porous hollow PtNi/C nanocatalyst achieved 6 and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement was 4 and 3-fold in mass and specific activity, respectively, over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction, and crystallite size. Furthermore, 100% of the initial mass activity at E = 0.90 V vs RHE (0.56 A mg(-1) Pt) of the best electrocatalyst was retained after an accelerated stress test composed of 30 000 potential cycles between 0.60 and 1.00 V vs RHE (0.1 M HClO4 T = 298 K), therefore meeting the American Department of Energy targets for 2017-2020 both in terms of mass activity and durability (0.44 A mg(-1) Pt, mass activity losses < 40%). The better catalytic activity for the ORR of hollow PtNi/C nanocatalysts is ascribed to (i) their opened porosity, (ii) their preferential crystallographic orientation ("ensemble effect"), and (iii) the weakened oxygen binding energy induced by the contracted Pt lattice parameter ("strain effect").