Determining Proton Transport in Pseudo Catalyst Layers Using Hydrogen Pump DC and AC Techniques

Abstract

Optimizing electrode morphology with a more uniform ionomer distribution is key to reducing ohmic losses and increasing electrocatalyst utilization in polymer electrolyte fuel cells (PEFCs). Inherent ionomer conductivity, volume fraction and tortuosity determine effective ionic conductivity. We use hydrogen pump (HP) method to measure effective ionic conductivity of a pseudo catalyst layer (PCL) comprised of Vulcan XC-72 carbon black and 3M 825 EW ionomer with ionomer to carbon (I/C) ratios of 0.6, 1 and 1.4 and relative humidity (RH) range of 50 to 120%. These direct current (DC) experiments are then compared with electrochemical impedance spectroscopy (EIS). Both DC and EIS methods show good agreement, indicating that EIS can be used as an alternative to DC method in HP experiment. Ionic conductivity for PCL with I/C of 1 and 1.4 was found to be about one order of magnitude higher than I/C of 0.6 for most of the RH range. At 90% RH tortuosities for I/C = 1 and 1.4 were close to 1, whereas tortuosity for I/C = 0.6 was 3. With decrease in relative humidity tortuosities increased linearly and at 50% relative humidity a PCL with I/C = 0.6 had the highest tortuosity of 6.1.

Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

“…We reported in our earlier study the values of the tortuosities for the AC HP method. 20 Here, these values are reproduced by Figs. 4d-4e along with the calculated values from the EIS method under 50% RH and 100% RH for I/C = 0.6, I/C = 1.0 and I/C = 1.4.…”

“…The cathode layers were single layers of the PCLs made by 3 M company with Vulcan XC72 carbon black and 3 M 800 EW PFSA ionomer with I/C ratios of 0.3, 0.6, 1.0, 1.2 and 1.4. These PCLs I/C ratios were chosen to enable comparison with our previous work 20 but also to represent the range typically used in commercial applications. The measured thicknesses of the cathode PCLs were 8 μm, 11 μm, 12 μm, 12 μm and 14 μm for PCLs with I/C ratio of 0.3, 0.6, 1.0, 1.2 and 1.4, respectively.…”

“…19 Previously, we used a hydrogen pump (HP) technique to measure effective ionic conductivity of the pseudo-catalyst layers (PCLs) without Pt over a relative humidity (RH) range of 50%-120%. 20 For the HP set up, multiple PCLs were placed between two membranes. A typical fuel cell anode and cathode catalyst layers were used on the outer side of each membrane.…”

Electrochemistry under confinement

“…Structures separated by single digit nanometre lengths are hardly realized and often degrade under experimental conditions, although recent developments in nanogap technology and electrochemical sensitivity might change this. 16,[131][132][133] One effect that may arise due to this confinement is the influence of specifically adsorbed molecules on the OHP and ion mobility. Zhao et al used a polyelectrolyte covered nanopore to effectively gate ionic permeability by pH adjustment.…”

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes

“…H 2 Pump EIS Protocol. Electrode ionic conductivity was measured using the H 2 -pump AC technique developed by Sabarirajan et al 44 The pseudo-electrodes, that is, electrodes without any Pt loading, with different ionomer chemistries, and a constant I/C ratio of 0.6, were sandwiched between two membranes. This assembly was placed between the cathode and anode electrodes.…”