Characterization of CeO_x-decorated Pd/C Catalysts Synthesized By Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

摘要

The high cost and difficult task to eliminate platinum (Pt) catalyst in a Proton Exchange Membrane Fuel Cells (PEM-FCs) are one of the main obstacles preventing wide adoption of fuel cells. The anion exchange membrane fuel cell (AEM-FC) has been proposed as an alternative in which non-Pt metals may be employed. However, obtaining a high-power density Pt-free AEM-FC has been a challenging task because the Hydrogen Oxidation Reaction (HOR) in the electrocatalysis experiences a slow kinetics at the anode. In the present study, with the aim of improving the efficiency of HOR catalysts, various ratios of CeO_x/Pd catalysts were deposited onto a carbon support using Controlled Surface Reactions (CSR) process. A homogenous distribution of CeO_x preferentially attached to Pd nanoparticles (NPs) was expected to achieve highly active CeO_x-Pd/C catalysts for HOR. We present here a comprehensive characterization approach for the synthesized highly active catalyst, and correlate obtained structural/compositional parameters to the performance. The characterization of the catalysts was carried out via Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM) - Energy Dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), and X-ray Photoelectron Spectroscopy (XPS) to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation state and surface chemical states, respectively. The HRTEM images indicated that Pd NPs were uniformly distributed on the carbon support with only some minor agglomeration. Additionally, the achieved high interfacial contact between CeO_x and Pd acquired on single NPs was, for the first time, segmented and calculated using High-resolution STEM-EDS maps and Image J processing program by measuring the overlap intensities between Pd and Ce NPs; the results clearly showed that CeO_x NPs were in intimate contact with Pd and their interfacial contact area increased with the addition of CeO_x, reached a maximum at a ratio 0.38 CeO_x-Pd/C, then decreased due to the formation of large CeO_x islands upon the further addition of CeO_x. The attained interfacial contact area also seems to be much higher than other previously reported Pd-CeO_2 catalysts synthesized by other methods, suggesting that the CSR method resulted in a higher selective deposition of CeO_x on Pd.