Palladium (Pd) affinity with hydrogen and the subsequent effect on its physical structure is well known in literature. However, the majority of chemical methods for synthesizing Pd-based nanomaterials use sodium borohydride (NaBH₄) as reducing agent, which releases hydrogen in aqueous solution. Pd can readily absorb this hydrogen in its lattice and forms PdH_x hydrides. Alternatively, organic compounds such as ascorbic acid (AA) can be also used for reducing Pd ions in Pd nanoparticles. Pd nanomaterials are used for various reactions as those in fuel cells. In electrocatalysis where the electrode structure plays a key role, the subsequent impact on the catalytic properties should be carefully examined.. In the present work, reducing agent effect on Pd nanomaterials and their long-term chemical stability (during 2 years) were scrutinized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The impact of the hydrogen insertion (into Pd crystalline network) on the electrocatalytic properties of the synthesized Pd nanomaterials is investigated through oxidation of carbon monoxide and oxygen reduction reaction (ORR). It was observed that the surface state of the nanomaterials (Figures 1a-b) prepared by using sodium borohydride as reducing agent (Pd/C-NaBH₄) is radically different from those obtained from L-ascorbic acid (Pd/C-AA) (Figure 1c). ORR starts with enhanced kinetics (E > 1 V vs. RHE) in a 4-electron process, producing p(H₂O₂) < 0.5% associated with excellent durability over 5000 cycles (Figures 1d-e). Both catalysts outperform all reported data for Pd electrocatalysts in alkaline medium. Finally, this work combined ex/in situ XPS and XRD analyses together with ORR as catalytic model towards a clear development in our understanding of Pd affinity with hydrogen and its electrocatalysis.

Figure 1. (a,b) HRTEM micrographs of the as-prepared nanomaterials of Pd/C-NaBH₄ and Pd/C-AA and their corresponding high-resolution Pd 3d XPS spectra. (d) The normalized electrochemical surface area (ECSA) behaviour before and after 5000 cycles of accelerated potential cycling test (APCT: 0.6-1 V vs. RHE at 100 mV s⁻¹ in O₂-saturated 0.1 M KOH). Note that ECSA was evaluated at 50 mV s⁻¹ and 5 mV s⁻¹ scan rates and had the same trend. (e) ORR polarization curves recorded in a O₂-saturated 0.1 M KOH solution at the ring (top) and disc (bottom) for 5 mV s⁻¹ scan rate before (initial) and after APCT (5000 cycles) at RRDE speed of 1600 rpm.

Acknowledgments

The authors gratefully acknowledge financial support from the French National Research Agency ANR-ChemBio-Energy

References

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