Electrocatalysis of Oxygen Reduction in Polymer Electrolyte Fuel Cells: A Brief History and a Critical Examination of Present Theory and Diagnostics - Fuel Cell Catalysis: A Surface Science Approach - page 2

Specific aspects examined here include insights and conclusions derived from the
most recently performed density functional theory (DFT) calculations, which have
been based on a comprehensive model of the electrochemical interface, and the
strong disagreements (which seem to defy all recent theoretical efforts) that remain
regarding proper interpretation of experimental ORR results and proper identification
of the ORR mechanism in a PEFC cathode employing Pt catalysts.
1.2 KEY MILESTONES ON THE WAY TO THE PRESENT STATE
OF THE ART OF FUEL CELL ELECTROCATALYSIS
When surveying the central milestones in the development of electrocatalysis for
low temperature fuel cells operating in acidic environments, the following, listed in
chronological order, seem to be the most outstanding:
† Establishment of carbon-supported Pt catalysts as a means to achieve higher and
more stable dispersion of the precious metal electrocatalyst on an electronically
conducting support [Petrow and Allen, 1977].
† Implementation of Pt/C catalysts in PEFC technology using recast Nafion w as a
proton conducting and bonding agent [Raistrick, 1986; Wilson and Gottesfeld,
1992].
† Optimization of the catalyst layer composition and thickness in PEFCs for maxi-
mum catalyst utilization in operation on air and on impure hydrogen feed streams
[Wilson, 1993; Springer et al., 1993].
† Advancing from carbon-supported Pt to carbon-supported Pt alloy catalysts, to
enhance the performance per milligram Pt by three- to four-fold [Mukerjee and
Srinivasan, 1993; Mukerjee et al., 1995].
† Moving on from preparation of homogenous Pt alloy particles to tailoring of
core-and-shell alloy particles, targeting (i) further lowering of the mass of pre-
cious metal per unit power output and (ii) further boost of catalytic activity
per square centimeter of catalyst area [Zhang et al., 2004].
Two parallel efforts common to all of the above critical steps and milestones are
(i) maximizing of catalyst dispersion and enhancing electrochemical utilization of
the overall surface area of the catalyst incorporated in the fuel cell electrode and
(ii) further fine tuning of the electronic and, consequently, surface chemistry properties
of Pt catalysts by alloying, typically with electropositive metals such as Co or Ni, to
achieve higher activity per unit surface area of the optimized alloy catalysts. The
former part of the effort led to established methods of preparation of carbon-supported
Pt alloy particles in the diameter range of 2 - 5 nm, i.e., of overall catalyst surface area
between 600 and 1500 cm 2 Pt (or Pt alloy) per square centimeter of electrode cross-
sectional area achieved at a mass loading of only 1 mg Pt (or Pt alloy) per square centi-
meter of cross-sectional area. Such high “electrode surface amplification factors” are
enabled by carefully selected mild reduction processes that generate, typically from