In-Situ through-Plane Measurements of Ionic Potential Distributions in Non-Precious Metal Catalyst Electrode for PEFC
The present work employs a cyanamide-polyaniline-iron-carbon (CM-PANI-Fe-C) as the ORR catalyst. Nano-scale X-ray computer tomography (nano-XCT) was used to image the electrode in 3D at two resolution levels in phase contrast modes to differentiate the solids (Nafion®, carbon, and Iron) and the pore.2 Staining the electrode in cesium sulfate (Cs2SO4), distinguishes the Nafion® in absorption contrast mode in nano-XCT. The staining ion-exchanges the proton at the Nafion’s sulphonic acid side sites with a higher atomic number Cs+ ion, giving Nafion® a higher absorption contrast. The Nafion® distribution in the cathode morphology is studied for three different Nafion® loadings. Figure 1a shows the 3D reconstruction of Nafion® distribution in CM-PANI-Fe-C electrode with a Nafion® loading of 60 wt%.
The Nafion® loading and spatial distribution can significantly influence the ionic conductivity of electrodes and the catalyst effectiveness. In-situ ionic conductivity and through-plane potential distribution are measured using microelectrode scaffold (MES) diagnostic, as previously developed in our group for Pt electrode.3 A MES consists of a cylindrical working electrode (cathode), a counter electrode (anode) and an electrolyte layer, as shown in Figure 1b. Eight different discrete sensing layers with H2 reference electrodes are placed through the thickness of the cathode at its perimeter, enabling spatio-temporal measurement of the Nafion® potential as a function of the distance through the electrode. Spatially, the slope and the rate of slope change provide us with a measure of the ORR distribution across the electrode. Ionic conductivity is additionally measured using a four-wire resistance measurement technique.4 The potential drop across the sense layer is measured while applying a small perturbation current to the reference electrode for H2 pumping to the anode. The perturbation current is small as to not affect the fuel cell operation. Correlation between the effect of relative humidity and the ionic conductivity with the Nafion® distribution is studied for the three different loading of CM-PANI-Fe-C electrode.
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