In-Situ through-Plane Measurements of Ionic Potential Distributions in Non-Precious Metal Catalyst Electrode for PEFC

Expensive platinum (Pt) or Pt alloys are typically used as oxygen reduction reaction (ORR) catalyst in the cathode of acidic polymer electrolyte fuel cells (PEFC).  The high Pt loading in the cathode required for satisfactory performance is an obstacle for commercialization due to the expensive raw material. Hence, there is significant interest in non-precious metal catalyst (NPMC) for cathodes. The activity and durability of ORR NPMC have significantly improved in the past decade.¹ However, in order to generate current densities comparable to those obtained with conventional Pt cathodes, the lower volumetric activity requires the NPMC cathodes to be an order of magnitude thicker. Increasing the electrode thickness increases the proton and oxygen transport resistance, as well as makes the electrode more susceptible to liquid-water flooding. Therefore, understanding the relationships between the cathode architecture and performance is crucial for optimizing the performance of thick-format NPMC cathodes. The present work investigates the proton conduction through these electrodes by resolving the 3D ionomer distribution in the electrode and by directly measuring the corresponding conductivity and electrolyte potential through the cathode. 

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. 

References 

1. G. Wu, K. L. More, C. M. Johnston, and P. Zelenay, Science, 332, 443–447 (2011). 

2. S. Komini Babu, H. T. Chung, G. Wu, P. Zelenay, and S. Litster, ECS Trans., 64, 281–292 (2014). 

3. K. C. Hess, W. K. Epting, and S. Litster, Anal. Chem., 83, 9492–8 (2011). 

4. S. J. An and S. Litster, ECS Trans., 58, 831–839 (2013). 

See more of: D0-1 Non-PGM Cathode Catalysts 1
See more of: I05: Polymer Electrolyte Fuel Cells 15 (PEFC 15)
See more of: Fuel Cells, Electrolyzers, and Energy Conversion

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