Pt Alloy Nanoparticles and Particle Size Effects

Although model catalyst studies show the maximum possible activity obtainable, practical catalyst systems use nanoparticles of Pt or Pt alloys usually 

Trends in calculated oxygen reduction activity plotted as a function of the oxygen binding energy. (Reprinted with permission from ]oumal of Physical Chemistry B, 108, 17886 (2004). Copyright 2004 American Chemical Society.) 

A strong caveat to this prediction is the role of active surface area to the actual activity observed in fuel cells. This is often termed the mass activity— that is, the activity per mass of active metal (usually Pt), which has a direct correlation to cost. Mass activity is a combination of specific activity and surface area  

Mass and specific activities are often quoted at 900 mV cell potential with pure O2 to indicate the kinetic activity in a region as free of mass transport 

Comparison of oxygen reduction specific activity from polycrystaUine Pt, nanostructured thin film Pt, and carbon-supported Pt from RDE measurements in 0.1 M HCIO4. (Reprinted from K. J. J. Mayrhofer et al., Electrochimica Acta, 53(7), 3181. Copyright 2008 with permission from Elsevier.) 

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