Cathode catalyst layers with ionomer to carbon mass ratios in the range 0–2 studied by electrochemical impedance spectroscopy, cyclic voltammetry, and performance measurements

• Cathode catalyst layers containing Pt/C and ionomer were studied in I/C mass ratio range 0–2.
• Pt ESCA and electrode low frequency capacitance were found to be independent of I/C.
• Ionic resistance of CCL increases with the increase of I/C up to I/C ∼ 0.3.
• Mass specific activity of platinum reaches maximum at I/C ∼ 1.
• Ionic resistance of the ionomer-free CCLs strongly depends on the electrode potential.

Characteristics of the cathode catalyst layers (CCL) containing HiSPEC 9100 Pt/C catalyst and ionomer (I) in ionomer to carbon mass ratio (I/C) range 0–2 were studied. Pt electrochemically active surface area (ECSA) and electrode low frequency capacitance were found to be independent of I/C value. Ionic resistance of CCL was found strongly dependent of I/C value. It reaches maximum value at I/C∼0.3. Ionic resistance of CCL with I/C = 0.05 increased 7 fold with the shift of electrode potential (E) from 0.4 V to 1.05 vs. RHE. Ionic resistance of an imitating layer, which contained ionomer-free Ketjenblack EC-300J carbon, increased by a factor of 20 with the potential shift from 0.1 V to 1.05 V vs. RHE. Ionic conductivity in ionomer-free CCLs is ascribed to the presence of protons which originate from ionization of oxygen containing acidic surface groups of carbon support. Application to the CCL of potential positive relative to potential of zero charge of carbon support (pzc) draws protons to the counter electrode, decreasing the ionic conductivity of the CCL. Pt mass specific activity (Im) dependence on I/C mass ratio reaches maximum at I/C ∼ 1.