Figure 4 shows the results of the analysis of the volume composition for conventional CLs with a heterogeneous ionomer distribution. The variation of the local oxygen and ionic transport resistances is examined as a function of porosity for three carbon volume fractions. As shown in
Figure 4(a), there is a strong dependency of
with porosity, which decreases by a factor of two when the porosity is increased from
to
. That is, the ionomer volume fraction is reduced from
to
since
. For a given porosity,
is also reduced with carbon volume fraction due to the decrease of ionomer volume fraction. These results agree with previous works, which showed that the use of moderate ionomer-to-carbon weight ratios (
) is beneficial to enhance oxygen transport at low Pt loading due to a decrease of average ionomer thickness [
12,
15,
34,
43,
46]. Notice also the non-linear dependency of the local oxygen transport resistance with porosity. For high ionomer volume fractions (
),
remains large and rather constant around
. In contrast, at lower ionomer volume fractions (
),
gradually decreases until it settles down around
for CLs with low ionomer volume fractions and high porosities (
,
). Such non-linear behavior highlights the need to design high porosity CLs to mitigate the detrimental effect of local oxygen transport resistance at low Pt loading [
47]. Experimentally, a strong reduction of
has been previously reported for high porosity electrosprayed CLs [
17,
48]. A low ionomer content minimizes the agglomeration of ionomer films, avoiding the formation of locally dense ionomer regions that prevent a proper distribution of oxygen throughout the ionomer surface. CL design with low ionomer fraction and high porosity must be accompanied of a moderate increase of the local ionic transport resistance. As shown in
Figure 4(b),
remains rather constant around
for
(
) but significantly increases beyond
for
(
) when ionomer interconnection approaches the percolation threshold [
49]. Operation close to the ionic percolation threshold of ionomer must be ensured by the incorporation of supporting routes for proton transport, created, for example, by an enhanced water uptake at micro- and meso-scale, as it is the case of optimized electrosprayed CLs [
17,
48]. Alternatively, a delicate arrangement of ionomer might be necessary, as discussed in the next section.