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 4

Figure 1.1 The nature of a composite, Pt/C/recast ionomer layer with a structure that enables
high electronic and gas mobilities as well as sufficient proton mobility [Gasteiger, 2005].

layer by the three participants in the electrochemical process—gaseous reactant, pro-
tons, and electrons—all at the rates called for by the demand current density. Fulfilling
the latter condition requires a composite catalyst layer structure in which the electron
mobility, the proton mobility, and the effective gas diffusivity across the thickness
dimension of the catalyst layer are all sufficient, at the demand current, to access the
maximum fraction of the catalyst particles dispersed uniformly in the (5 - 20 mm
thick) catalyst layer. The type of structure satisfying high catalyst utilization in catalyst
layers of PEFCs is shown in Fig. 1.1.
The figure shows catalyst layer porosity at both micrometer and nanometer scales.
The larger pores form as a result of the highly structured nature of the carbon support,
that prevents closer packing of the submicrometer carbon particles, facilitating
gas transport through the catalyst layer. It is easy to see that the carbon structure
obtained will support electron percolation with the roughly 30% volume fraction occu-
pied by the carbon particles. However, achieving higher proton mobility is usually a
bigger challenge. As can be seen from Fig. 1.1, the proton-conducting component of
the catalyst layer, typically applied as a solution of the ionomer which recasts around
carbon particles [Gasteiger, 2005], needs to reach a volume fraction that would satisfy
the ionic conductivity demand, but, at the same time, would leave the network of
micropores sufficiently open to gas transport. The specific conductivity of the ionomer
is several orders of magnitude smaller than that of the carbon and, as a result, the
performance of the membrane/electrode assembly will typically be limited by the
effective protonic conductivity within the catalyst layer, thereby diminishing the effec-
tive overpotential at catalyst particles located away from the surface of the ionomeric
membrane [Springer et al., 1993]. A reverse situation, in which catalyst particles away
from the gas diffusion layer are the least well utilized, will apply when the gas
permeability through the catalyst layer becomes the main transport limiting factor.
The most recent improvements in Pt catalyst utilization U by optimization of cata-
lyst layer composition and structure have led to catalyst utilizations as high as 80%, or
more, determined as the ratio between measured ORR current per geometric square
centimeter of electrode area and the current expected from the total measured Pt
surface area per geometric square centimeter of the electrode, i.e.,

J ORR (E, T)
A Pt J ORR (E, T)

U ¼ (1 : 1)