We modified Nafion  mass loading, which results in a change in the ratio of Cu to Nafion  loading (referred to as mratio) within the agglomerate catalyst structure. Fig shows how the CL's porosity evolves as we increase Nafion  loading. Generally, we observe a decrease in porosity as mratio decreases. The increase in Nafion  ionomer amount from 0.5 to 0.7 mg⋅cm -2 reduces the catalyst layer porosity by 50%.