Environmental Engineering Reference
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operation at such level of power density and at a conversion efficiency higher than
60%, i.e., 1 mg Pt/W, or 1 g Pt/kW, translates at the present market price of Pt
metal to $50/kW. Such a cost of the catalyst component of a power source, may
possibly enable technology implementation for some stationary or portable power appli-
cations, where the cost of the relevant incumbent power technology is in the hundreds and
thousands of dollars per kilowatt, respectively. The same catalyst cost is prohibitive, how-
ever, in transportation applications, where the overall cost per kilowatt of the incumbent
technology, i.e., the internal combustion engine, is similar to, or lower than the cost of just
the Pt catalyst required per kilowatt of fuel cell stack at the present state of PEFC electrode
technology and the present market price of Pt. Consequently, the greatest drive during the
last few years to lower the catalyst loading in a PEFC stack to 0.1 - 0.2 g/kW, i.e., 5-10
times lower than is possible with Pt/C catalysts, has come from programs and teams
pursuing transportation applications of PEFCs.
Pt alloy catalysts: While driven technologically by the demanding cap on Pt usage
per unit power generated, the development of Pt alloy catalysts of higher ORR activity
has been driven scientifically by the pursuit of the fundamental physical parameter(s)
that determine the electrocatalytic activity of metals and metal alloys in the ORR. As
this aspect of electrocatalysis research is covered extensively in this topic, mention will
be made here only of recent central achievements in (i) the actual introduction of Pt
alloy/C catalysts into PEFC cathodes, demonstrating for such PEFCs an advantage
in cell power per mg Pt of about fourfold compared with the non-alloyed Pt/Ccatalyst,
and (ii) the coupled theoretical and experimental research efforts since the year 2000,
leading to an understanding of the origin of the higher ORR activity observed follow-
ing proper alloying of Pt and, indeed, to forecasts on possible further improvements in
ORR catalysis.
The specific Pt alloy that has been introduced more than any other as a carbon-
supported cathode catalyst in PEFCs has been PtCo. It belongs to the first group of
catalysts considered as possible ORR activity enhancers in PEFCs, a group that also
included Pt
3
Cr and Pt
3
Ni [Mukerjee and Srinivasan, 1993; Mukerjee et al., 1995].
In fact, this choice was inspired by the similar ORR activity enhancement observed
for this group of alloys in earlier development of cathode catalysts for the phosphoric
acid fuel cell (PAFC). Incorporation of these alloys, starting more than 20 years ago, in
cathodes of commercial PAFC power units fabricated by UTC (Connecticut, US), was
based on the superior activity of the alloy per unit mass of Pt compared with the
unalloyed Pt/C catalyst. When the first measurements of the changes in electronic
properties of Pt on alloying with Co, Ni, or Cr were reported [Mukerjee and
Srinivasan, 1993; Mukerjee et al., 1995], it was still not clear why an increase
should be observed in Pt d-band vacancy following alloying and why that would
necessarily lead to higher ORR activity compared with unalloyed Pt. What helped
here was electrochemical surface characterization by cyclic voltammetry, which
revealed a shift of the onset of OH or O electrosorption to higher anodic potential
as a result of alloying, suggesting a lowered tendency of the metal alloy surface to che-
misorb O or OH by water discharge, as compared with unalloyed Pt. Such lowered
affinity for oxygen would be difficult to rationalize for a surface containing, e.g.,
Co atoms in addition to Pt atoms; however, experimental evidence then appeared of
spontaneous formation on such PtCo alloy particles of a single-atom-thick shell of
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