CeO_x-functionalized Pd nanoparticles on single-walled carbon nanotubes for alkaline hydrogen oxidation reaction

The sluggish kinetics of hydrogen oxidation reaction (HOR) in alkaline electrolytes highlight the strong need to develop next-generation catalyst materials for anion-exchange membrane fuel cells (AEMFC) anodes. In this study, CeO_x is sequentially deposited on Pd nanoparticles supported on single-walled carbon nanotubes (SWNT) via atomic layer deposition (ALD). The obtained Pd@CeO_x SWNT_{16 ALD cycles} catalyst shows an excellent alkaline HOR performance with a specific exchange current of 166 mA mg⁻¹_Pd. This is three times higher than the commercially available Pd catalyst and the highest among reported Pd/CeO_x catalyst materials with different CeO_x overlayer coverages. By combining the X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and high-resolution scanning transmission electron microscopy, we confirm that the activity improvement is due to the highly conductive SWNT support enabling the fabrication of high surface area Pd clusters and CeO_x overlayer. These methods reveal that the oxidation state of Ce is varying from Ce³⁺ to Ce⁴⁺ in relation to the CeO_x overlayer thickness and the number of ALD cycles. Density functional theory calculations show that the presence of Ce/CeO_x increases the diversity and population of Pd active sites with improved activity in its vicinity leading to enhanced overall catalytic performance. Moreover, this work provides a new perspective to develop highly active alkaline HOR catalysts for AEMFC.