Preparation Of Palladium-based Niobium Oxides And Sulfides With Applications In Electrocatalysis

Renewable and clean hydrogen with the highest gravimetric energy density is regarded as a promising energy carrier.Energy storage and conversion where the hydrogen is used as energy carrier could be realized through the combination of electrolytic cell and hydrogen fuel cell.However,this energy conversion based on electrode reaction is limited due to the overpotential.Therefore,electrocatalysts are necessary for electrode reaction in order to enhance energy conversion efficiency.Pt/C has been considered as a state-of-the-art electrocatalyst.However,Pt is scarce and expensive.In addition,the aggregation of Pt nanoparticles often occurs due to the weak interaction between Pt and carbon,which would lead to the degradation of catalytic performance.It is still a great challenge to develop electrocatalyst with low cost,high catalytic activity and high stability.Because of the similar electronic structures and chemical properties as well as the lower cost compared with Pt,Pd is considered as one of the substitutes for Pt.In order to deal with poor catalytic stability caused by the weak interaction between noble metal and carbon,one method is to stabilize Pd nanoparticles on the support with the help of strong metal-support interaction?SMSI?.On the other hand,Pd could be made to form chemical bonds with the support and accomplish atom-scale dispersion,which would regulate electrical and structural characteristics and enhance both catalytic activity and durability.Niobium oxides and sulfides might have potential applications in catalysis because of their various structures and superior chemical stability.In this study,based upon calculation for chemical thermodynamics and design for crystal structure,Pd-loaded NbO_x?Pd/NbO_x?and Pd-intercalated NbS₂?Pd_xNbS₂?were prepared.Their applications for electrocatalysis were also investigated.The main achievements are listed as follows:?1?We first developed a facile and controllable La-reduced route to synthesize niobium suboxide?NbO_x?nanoparticles,including NbO,NbO₂,Nb₁₂O₂₉ and Nb₂O_5-x.Black Nb₂O_5-x is proved to have more oxygen vacancy compared with white pristine Nb₂O₅ by the characteristics of Raman spectroscopy,EPR and XPS.These resultant NbO_x serve as catalyst support for Pd and demonstrates superior activity toward oxygen reduction reaction?ORR?.Pd/Nb₂O_5-x displays a positive shift in half-wave potential and onset potential,better robustness and higher methanol tolerance in comparison to commercial Pd/C.This great performance of Pd/Nb₂O_5-x for ORR might derive from the SMSI between Pd and Nb₂O_5-x.Charge transfer from Nb₂O_5-x to Pd brought by the SMSI regulates the electronic structure of Pd,which contributes to the stability of Pd on the surface of Nb₂O_5-x and facilitates the absorption of O₂ on Pd and eventually benefits ORR.?2?We incorporated Pd atoms into van der Waals gaps of NbS₂ to form a series of new compounds?Pd_xNbS₂,x=0⁰.23?.The crystal structure of Pd_0.23NbS₂ was identified by single crystal X-ray diffraction.The HER performance is optimized accompanying with the introduction of Pd atoms and the best performance is obtained in Pd_0.23NbS₂.The Pd_0.23NbS₂ catalyst demonstrates a small Tafel Slope(50 mV dec^-1),a low onset overpotential?99 mV?and a low overpotential(157 mV at 10 mA cm^-2)with negligible change in 12 hours.The enhanced HER performance in Pd_xNbS₂ is attribute to the atomic-pillar effect of Pd.The pillared Pd atoms expand the interlayer spacing of NbS₂ and optimize the catalytic activity of neighboring basal-plane S atoms with a lower Gibbs free energy value of 0.06 eV,which facilitates aqueous protons to shuttle into the active interlayer sites and react.Besides,the emerging Pd-S bonding regulates the electronic structure and thus increases the electrical conductivity.In addition,the intercalated Pd atoms stabilize the crystal structure by connecting[Nb-S]interlayers with[PdS₆]octahedra.Thus,the Pd_0.23NbS₂ shows the excellent electrochemical durability.