Environmental Engineering Reference
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Pt atoms around a PtCo alloy core. With such a core-and-shell structure, the lowered
drive of the Pt shell atoms to bond to surface oxygen atoms could be now understood
as the result of the involvement of d-electrons of shell Pt atoms in bonding to Co atoms
in the adjacent atomic layer underneath the shell, resulting in lower availability of d-
electrons in the surface Pt atoms for formation of a surface bond to an OH group or an
O atom. This observed correlation between higher ORR activity and lowered surface
affinity to chemisorbed O or OH revealed that the surface affinity to oxygen of un-
alloyed Pt surfaces is apparently somewhat too high to achieve optimized cathode cat-
alytic activity, and, consequently, a slight lowering of the catalyst surface affinity to
oxygen through alloying enhances the rate of the ORR.
Interpretation of this observed correlation between a lowered affinity of the
metal surface to oxygen and a higher rate of ORR measured at a Pt shell over a
Pt-alloy core has also been at the center of recent theoretical work, based primarily
on DFT calculations of electronic properties and surface bond strengths for a
variety of expected ORR intermediates at metal and metal alloy catalysts. The
second part of this chapter contains a discussion of these valuable contributions
and of outstanding issues in tying together this recent theoretical work and ORR
experimental data.
Core-and-shell-type electrocatalyst particles have most recently defined the frontier
in ORR electrocatalysis research. Not only can this structure enable fine tuning of the
electronic properties of surface Pt atoms, it could also allow placing of all the Pt atoms
only on the outer surface of the catalyst particle, i.e., where the catalytic process takes
place, using non-precious metal atoms in the particle core. By having all Pt atoms
located on the outer surface (shell) of a core-and-shell catalyst particle, the mass of
Pt required to generate some given Pt catalyst surface area would drop by a factor
of 5 - 10 compared with catalyst particles built exclusively of Pt atoms. Consequently,
the cost of the catalyst per unit power generated could drop by a similar factor of 5 - 10
if the particle core were made of non-precious metal(s). Several recent demonstrations
of this approach have included a Pt shell over a Ru core as a CO-tolerant anode catalyst
and a Pt shell on a Pd core for the ORR process, both made by Adzic et al. [Zhang
et al., 2004 and references therein]. The most recent work in this area is covered in
Chapters 8 and 9.
A remaining great challenge in the introduction of such atomic-level tailored nano-
particles of electrocatalysts is maintenance of the stability of the preferred surface
atomic structure under fuel cell operation conditions. Encouraging results in this
regard for Pt shell/Ru core anode catalysts tested for 1000 hours were facilitated by
the reducing environment in the anode. The most researched cathode alloy catalyst,
carbon-supported Pt shell/Pt
3
Co core, was first reported to be even more stable
than unalloyed carbon-supported Pt, but more recently a performance decay pattern
reported by Johnson Matthey [Thompset, 2007] showed accelerated decay following
the first 1000 hours, which was explained by loss of near-surface Co atoms, leaving
behind a faulty structure of surface Pt atoms. Basically, under fuel cell cathode oper-
ating conditions, it will be highly nontrivial to achieve the operational stability
required with shell-and-core catalyst particles employing highly electropositive
atoms such as Co, Fe, or Ni, because of the strong tendency of such atoms to leave
the metal alloy crystal and form metal ions.
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