Design And Preparation Of Noble Metal Nanoalloys And Their Electrocatalytic Properties

The ever-increasing energy crisis and environmental pollution issues have caused severe challenges for sustainable development of human society.It is thus highly urgent to develop renewable and clean energies to substitute traditional fossil fuels,which has become the prominent tasks and research topics in the world.The direct formate fuel cell（DFFC）,a novel energy conversion device,has attracted widespread interest owing to its higher theoretical energy density and discharge voltage compared with other direct liquid fuel cells.In particular,its anodic formate fuel that exists as solid state can be easily transported and stored,which further improves the safety of DFFC devices,making the DFFC be an indispensable energy source in aerospace and other field.Nevertheless,the development of DFFC is currently hampered by the lack of highly active and stable electrocatalysts for formate oxidation reaction（FOR）.Moreover,except for energy conversion devices,exploring the efficient energy storage devices is also strategically significant for the rational deployment of clean energies.In recent years,the newly proposed lithium-carbon dioxide（Li-CO₂）battery has received intensive attention and been regarded as the most promising technique for next-generation energy storage due to its super-high theoretical energy and power densities.However,the research of such Li-CO₂ battery is in its infant stage and there are still many scientific problems to be solved.One of the most important challenge is the sluggish kinetic in decomposing lithium carbonate（Li₂CO₃）discharge products during charge process,which requires high charge voltage,seriously lowering the energy efficiency and cycling life of Li-CO₂ battery.Therefore,based on above-mentioned scientific problems,we well designed and synthesized various advanced nanoalloy electrocatalysts to considerably improve the reaction kinetics of formate oxidation and Li₂CO₃ decomposition,and also systematically elucidate the corresponding mechanism.The specific contributions are listed as following:The ternary PtRhNi nanoalloy aerogels are successfully prepared through the accelerated kinetically controlled strategy in aqueous solution,and used as the highly-efficient electrocatalysts for FOR.The strong reducing capability of NaBH₄ and increased reaction temperature of 60°C together promote the formation of PtRhNi nanoalloy aerogels with unique lamellar porous architecture,providing enormous active sites（metal and defects active sites）and facilitating the mass transportation of reactants during electrocatalysis.Moreover,benefitting from the alloying effect of Rh and Ni atoms,the resulting PtRhNi nanoalloy aerogels show lowered binding energy towards the CO intermediates of FOR and provide abundant oxygen functional groups to further facilitate the desorption of such CO intermediates,which is beneficial to achieve an outstanding FOR activity,stability and anti-CO poisoning capability.In particular,the optimized Pt₆₀Rh₂₈Ni₁₂ nanoalloy aerogels electrocatalyst exhibits a superb FOR mass activity of 1.41 A mg_Pt^-1,which are 9.4 times higher than that of commercial Pt/C electrocatalyst,showing a very broad application prospect.The successful preparation of multimetallic PtRhNi nanoalloy aerogels not only remarkably improve the kinetic of FOR and solve the CO poisoning problem of traditional Pt-based electrocatalysts,but enrich the nanostructural styles of noble metal nanoalloy aerogels,considerably promoting the applications of newly-developed nanoalloy aerogels in the field of energy conversion and storage.The ultrathin PdAgRh alloy nanosheet is for the first time synthesized.The surface reconstruction behavior of such PdAgRh alloy nanosheets and the corresponding electrocatalytic performance towards FOR are then systematically investigated.The ultrathin PdAgRh alloy nanosheets consist of several nanobranches,which provides enormous atomic steps,giving rise to abundant active sites.At conventionally electrochemical potential window of 0.024～1.1 V（vs RHE）,the PdAgRh nanoalloy electrocatalyst shows dramatically enhanced FOR activity and durability compared with commercial Pd/C.More significantly,extending the upper limit potential（ULP）from 1.1 to 1.2,1.3 and 1.4 V（vs RHE）during the electrochemical measurements,the PdAgRh nanoalloy electrocatalyst is revealed to experience the surface reconstruction process,and the reconstructed PdAgRh nanoalloy electrocatalyst exhibits further improved FOR activity and stability.Specifically,at the ULP of 1.3 V（vs RHE）,the FOR mass activity of reconstructed Pd₅₅Ag₃₀Rh₁₅/C electrocatalyst is1.85 times higher than that of pristine Pd₅₅Ag₃₀Rh₁₅/C electrocatalyst.Based on physical characterizations and electrochemical measurements,it can be concluded that higher limit potential triggers the redox reaction of Ag surface atoms in PdAgRh nanlloy,resulting in the formation of Ag surface oxide species,and then constructing the highly efficient metal/metal oxide interfaces as active sites,which would further promote the oxidation kinetic of FOR.The in-depth study of surface reconstruction of nanoalloy electrocatalyst clearly traces and uncovers the realistic states of nanoalloy surface atoms during the electrocatalytic process,giving more insights into the understanding of catalytic mechanism of nanoalloys,which provides comprehensive and powerful experimental evidence for the better design the high-performance FOR electrocatalysts in the future.To further enhance the FOR performance,the ternary porous Pd Ag Ir nanoflowers（PdAgIrNFs）is successfully designed and prepared,and the unique functionality of Ir element in improving FOR performance is systematically investigated.The representative TEM results indicate that PdAgIrNFs electrocatalyst shows the typical nanoflower architecture with considerably high electrochemical active areas and abundant pore channels,which could give rise to massive active sites and remarkably facilitate the mass transportation of reactants during catalytic process.According to electrochemical results,it can be seen that the Ir element in PdAgIrNFs electrocatalyst plays pivotal role in promoting the reaction kinetic of FOR.The PdAgIrNFs electrocatalyst exhibits the most negative onset and peak potentials,as well as the broad platform of peak current density,which considerably reduces the overpotential of FOR,then significantly enhancing the practical cell voltage and energy density of DFFC devices.XPS experiments demonstrate that the d-band center of PdAgIrNFs nanoally is shifted far away from the Fermi level owing to its advantageous electronic structure,which weakens the binding energy of H_ad intermediate on Pd active sites,dramatically improving the FOR performance and displaying a novel catalytic mechanism towards FOR.The present study represents a breakthrough in the research of FOR,making considerable contribution for developing high-performance DFFC devices.Bimetallic ruthenium-cobalt nanoalloy nanoparticles dispersed on carbon nanofibers（i-RuCo/CNFs）are prepared through the solvothermal combined with heat-treated methods,which are then for the first time used as the cathodic electrocatalysts in Li-CO₂ battery.The i-RuCo nanoparticles with solid solution phase are uniformly anchored on the surface of CNFs,forming highly efficient and stable nanostructure.Moreover,the electronic structure of i-RuCo nanoalloy can be precisely modified because of the robust interactions between Ru and Co atoms in i-RuCo solid solution nanoalloy,facilitating the reaction kinetic of discharge and charge processes in Li-CO₂ battery.Systematical battery testing results illustrate that the Li-CO₂ battery based on bimetallic i-RuCo/CNFs electrocatalyst shows enhanced performance in comparison to that of Li-CO₂ battery based on monometallic Ru/CNFs electrocatalyst.Furthermore,the Raman and gas chromatograph measurments show that the Li-CO₂ battery based on the i-RuCo/CNFs electrocatalyst completely complies the reversibly electrochemical reaction of 4Li⁺+3CO₂?2Li₂CO₃+C,which dramatically decreases the voltage in decomposing Li₂CO₃products during charge process,improving the discharge-charge performance of Li-CO₂ battery.This work elucidates the discharge-charge mechanism of Li-CO₂ battery with Ru-based nanoalloy electrocatalysts and provides sufficient experimental basis for developing more advanced and efficient cathodic electrocatalysts in Li-CO₂ battery.Based on aforementioned alloying strategy,the i-RuCu nanoalloys can be further precisely and controllably synthesized by incorporing Cu as more efficient alloying element,and then the resulting i-RuCu nanoalloys are systematically investigated as cathodic electrocatalyst of Li-CO₂ battery.The electrons effectively transfer from Cu atoms with lower work function to Ru atoms with higher work function,leading to the formation of electron-rich Ru in i-RuCu nanoalloys,which facilitates to achieve the optimized electronic structure.Impressively,the Li-CO₂ battery based on i-RuCu/CNFs bimetallic cathodic electrocatalyst shows the remarkably lowered charge voltage and considerably enhanced cycling performance.For example,the Li-CO₂ battery based on i-Ru₄Cu₁/CNFs electrocatalyst shows a much low charge voltage of only 3.7 V（Li/Li⁺）at a constant current density of 100 mA g^-1,which is much superior to that of the monometallic Ru/CNF and bimetallic i-RuCo nanoalloys based Li-CO₂ batteries.In addition,quantitatively regulating the number of edge defects in the cathodic electrocatalysts could remarkably improve the discharge-charge performance of Li-CO₂ battery,proving that the edge defect is a kind of effective active sites in Li-CO₂ battery,which shed new insights into developing high-performance cathodic electrocatalysts for Li-CO₂ battery and greatly accelerate their commercialization.