Particle-size effect of Pt cathode catalysts on durability in fuel cells

Abstract

Monodisperse Pt nanoparticles with three different sizes (d=2, 3, and 4nm, each with a very narrow standard deviation σd of 10%) supported on carbon black (CB) were prepared by the nanocapsule method. Four kinds of 50wt%-Pt loaded catalysts [Pt2nm/CB, Pt3nm/CB, Pt4nm/CB, and a commercial Pt2nm/CB (σd=20%)] were subjected to an accelerated durability test comprising standard potential step cycles between 0.6V and 1.0V vs. RHE, which simulates load cycles for fuel cell vehicles, in N2-purged 0.1M HClO4 solution at 65°C. The oxygen reduction reaction (ORR) activity and H2O2 yield at these catalysts were evaluated from the hydrodynamic voltammograms in O2-saturated 0.1M HClO4 solution at 65°C by the use a multi-channel flow double electrode technique. At the beginning of testing (BOT), all catalysts exhibited the identical value of kinetically-controlled area-specific activity (jk), and the kinetically-controlled mass activity (MAk) was proportional to the electrochemically active area (ECA). With increasing the number of potential step cycles, the MAk values of all catalysts decreased due to the reduction of the ECA, as a result of particle growth, but the jk values were nearly unchanged irrespective of such particle growth. After 30,000 cycles, at which each MAk (or ECA) had reached a stable value (end of testing, EOT), the smallest Pt catalyst (our Pt2nm/CB) still maintained the highest MAk, which was a factor of approximately two higher than those of Pt4nm/CB and the commercial Pt2nm/CB. Contrary to the common view, Pt nanoparticles as small as 2nm were found to be quite durable, maintaining high performance from the BOT to EOT, as long as they were uniform in size (σd≤10%) and highly dispersed over the whole surface of the carbon support.