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

Abstract

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 anode1–3. In the present study, with the aim of improving the efficiency of HOR catalysts, various ratios of CeOx/Pd catalysts were deposited onto a carbon support using Controlled Surface Reactions (CSR) process. A homogenous distribution of CeOx preferentially attached to Pd nanoparticles (NPs) was expected to achieve highly active CeOx-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 CeOx 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 CeOx NPs were in intimate contact with Pd and their interfacial contact area increased with the addition of CeOx, reached a maximum at a ratio 0.38 CeOx-Pd/C, then decreased due to the formation of large CeOx islands upon the further addition of CeOx. The attained interfacial contact area also seems to be much higher than other previously reported Pd-CeO2 catalysts synthesized by other methods4–6, suggesting that the CSR method resulted in a higher selective deposition of CeOx on Pd.Moreover, the characterized two strong peaks shape resulting from electron transitions12 and the intensity ratio13 of the M4 to M5 edge bands of CeOx obtained from the highest energy resolution (1.1eV) EELS, suggested that most of the CeOx was in the form of CeO2 and Ce4+ was predominantly present in CeOx. However, the presence of Ce3+ cannot be ruled out, specially near the surface of the particles14. The study also found that the already mentioned interfacial contact area was directly correlated to the electrochemical performance reflected on the HOR activity of the CeOx-Pd/C catalysts. From this work, it can be said that when using the CSR method the highest recorded HOR exchange current density, compared to previously reported Pd-CeO2 catalysts, (51.5 mA mg–1 Pd)4–9 can be achieved. AEMFC performance obtained when using CeOx-Pd/C catalyst with Ce/Pd bulk atomic ratio of 0.38 was considerably higher than other cells with analogous HOR catalysts (peak power density of 1169 mW cm–2 mgPd –1 at 333 K)4,5,10,11. This results can be explained by the improved distribution of CeOx onto Pd enhancing OH– spillover from CeOx to Pd, the higher concentration of Pd (IV) sites, and the higher interfacial contact area between CeOx and Pd nanoparticles. REFERENCES: Davydova, E. S., Mukerjee, S., Jaouen, F. & Dekel, D. R. ACS Catal. 8, 6665–6690 (2018).Dekel, D. R. Curr. Opin. Electrochem. 12, 182–188 (2018).Dekel, D. R. J. Power Sources 375, 158–169 (2018).Miller, H. A. et al. Angew. Chemie - Int. Ed. 55, 6004–6007 (2016).Hamish Miller, Francesco Vizza, et al. Nano Energy, Elsevier 293–305 (2017) doi:10.1039/b000000x/1.Yu, H. et al. Nano Energy 57, 820–826 (2019).Yarmiayev, V., Alesker, M., Muzikansky, A., Zysler, M. & Zitoun, D. J. Electrochem. Soc. 166, F3234–F3239 (2019).Bellini, M. et al. ACS Appl. Energy Mater. 2, 4999–5008 (2019).Ralbag, N. et al. Electrochem. Soc. 167, 054514 (2020).Omasta, T. J. et al. J. Electrochem. Soc. 165, J3039–J3044 (2018).Alesker, M. et al. J. Power Sources 304, 332–339 (2016).Goris, B., Turner, S., Bals, S. & Van Tendeloo, G. ACS Nano 8, 10878–10884 (2014).Liu, J. Thesis, Univ. Birmingham (2017).Sims, C. M. et al. Nanotechnology 30, (2019).