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 5

where J ORR (E, T) is the ORR current per cm 2 Pt at cathode potential E, determined by
an independent measurement at a Pt electrode of known surface area (and well-defined
mass transport characteristics) and A Pt is the number of cm 2 Pt per geometric cm 2
of the fuel cell cathode, determined from the hydrogen adsorption charge or the
CO desorption charge as measured by voltammetry. The ability to achieve catalyst
utilizations as high as 80% in PEFC cathodes (even more easily achieved on
the anode side) by optimizing the composition and structure of the catalyst layer,
means that this degree of freedom in enhancing further the net electrocatalytic activity
has been exhausted to a significant degree. This said, some remaining opportunity
for further enhancing the rate of the cathode process through better catalyst utilization
has been argued very recently, based on (i) higher utilizations determined
from Reaction (1.1) demonstrated for types of carbon support material of higher
surface area [Toyota R&D Center, 2007] and (ii) the argument that A Pt , the “electro-
chemically active” surface area determined by voltammetry, may not include the area
of Pt catalyst particles dislodged from the carbon support during catalyst layer
preparation. This argument implies that the performance of the air cathode in the
fuel cell can likely be further improved, at some given temperature, by a factor of
about 2 with further perfection of the carbon/PFSA ionomer composite catalyst
layer composition and mode of fabrication. Such a performance gain is not insignif-
icant, but, at the same time, leaves ample room for further, larger reductions of air
electrode losses.
Pt demand per kW versus PEFC performance and cost targets: The presently
achieved PEFC air cathode (initial) performance with Pt/C catalysts is basically
defined, as explained above, by a Pt cathode catalyst surface area of 600 - 1500
cm 2 /mg Pt and by the intrinsic ORR catalytic activity at the (bulk) Pt/ionomer inter-
face at some given temperature and ionomer hydration level, J ORR (V cath , T cell , RH)/
cm 2 Pt. At the lower end of catalyst dispersion specified above, the intrinsic ORR
activity per cm 2 Pt is maintained similar to that of the bulk metal. Reading the quality
of catalytic activity obtained state-of-the-art dispersed Pt/C catalyst is done in some-
what different ways from a PEFC technology perspective and from an ORR electro-
catalysis science perspective. From a technology implementation perspective, the
question is: “How close is this PEFC technology based on carbon-supported Pt cata-
lyst to market-entry targets of PEFC stack performance and cost?” From the perspec-
tive of electrocatalysis fundamentals, the question is: “With the cathode remaining the
largest source of PEFC voltage loss, by far, what can be further done to improve per-
formance by moving away from the Pt/C ORR catalyst to another catalyst of higher
intrinsic electrocatalytic activity?” Starting from the technology perspective, in a
PEFC operating at 80 8C, the electrocatalytic activity derived from state-of-the-art cat-
alyst dispersion and catalyst utilization described above, the PEFC cathode activity
measured at V cath ¼ 0.90 V is 0.11 A per mg Pt and increases 10-fold at V cath ¼
0.84 V [Gasteiger, 2005]. This would translate to nearly 1 W per mg Pt at V cath ¼
0.84 V, enabling one to achieve with 1 mg Pt per square centimeter of electrode geo-
metric area an areal power density of 1 W/cm 2 , translating to a bulk power density of
the cell of around 1 W/cm 3 , i.e., a respectable power density of 1 kW per liter of the
stack at a cell voltage as high as 0.8 V. However, the Pt catalyst mass required for