HRTEM Characterization of Non-Precious Metal-Based Catalyst Cathode Electrodes in PEM Fuel Cells

In Proton Electrolyte Membrane Fuel Cells (PEMFCs) the structure and morphology of the electrode layer play an important role in the reduction of the electrochemical resistance, which impacts the single cell performance.  In the cathode layer, specifically, the oxygen reduction reaction takes place, involving complex mass and charge transport processes (diffusion of oxygen, migration and diffusion of protons, migration of electrons, water transport by diffusion, permeation, electroosmotic drag, as well as vaporization/condensation of water) ¹.

An understanding of the structure of the electrode layer at the various hierarchical levels, and furthermore, of its influence on the cell performance is far from complete ². Moreover, factors like the type of the supported  catalyst, the type and amount of ionomer added, the type of dispersion medium used during ink preparation, and the fabrication conditions (temperature, pressure) have proved to influence the properties of this microstructure¹.

Over the last four decades, intensive research has been devoted to the replacement of the platinum catalyst with non-precious metals, posing a new challenge in terms of electrode layer structure and properties. Given the lower activity of these materials compared to Pt and Pt-based materials, achievement of comparable results  requires an increase in the ~10µm current state-of-the-art electrode thickness, leading to mass-transport limitations ³.

In previous studies performed in our lab, catalysts based on Cu, Co, Ni, Mn and Fe complexes of substituted triazoles supported on BP2000 carbon black have been synthetized. The higher activity towards the oxygen reduction were obtained with the Co and Cu based catalysts ⁴.

The main objective of this study is to evaluate the microstructure characteristics of Co and Cu-based cathodes for its application in PEMFCs operation. To achieve this goal, membrane electrodes assemblies (MEA) were prepared using cathodes with different catalyst loading and ionomer-catalyst ratio. Platinum electrodes with a 0.2 mg/cm² loading were used as anodes.  The performance of the MEAs was evaluated in single cell, and the structure and morphology were analyzed before and after testing through Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM).

References

1.             Zhang, J., PEM Fuel Cell Electrocatalysts and Catalyst Layers. Fundamentals and Applications. Springer London: 2008.

2.             Holdcroft, S., Fuel Cell Catalyst Layers: A Polymer Science Perspective. Chemistry of Materials 2013.

3.             Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T., Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B: Environmental 2005, 56 (1–2), 9-35.

4.             Goenaga, G.; Brooksbank, J.; Dabke, C.; Belapure, A.; Papandrew, A.; Foister, S.; Zawodzinski, T. In Synthesis and electrochemical characterization of Co, Cu, Fe, Ni and Mn-based catalysts for ORR in PEM fuel cells, Honolulu, HI, Honolulu, HI, 2012; pp 1749-1757.