Preparation Strategy And Oxygen Evolution Performance Of Silver-functionalized Cobalt-based Nanomaterials

Currently,increasingly severe energy crisis and nonnegligible environmental pollution originated from continuous consumption of fossil fuels have become two main factors that restrict the sustainable development of human society.Worldwide researchers thus invest energy in developing and storing renewable energy.Hydrogen,featured with zero pollution,good environmental compatibility,high energy density and conversion efficiency,has been regarded as an ideal energy carrier and expected to supersede traditional fossil fuels at future energy conversion.Different from hydrogen production by traditional natural gas reformation,electrocatalytic water splitting（EWS）powered by renewable energy is a promising technology for large scale production of high purity hydrogen.Now,EWS is considered as important bond to connect the transition of traditional energy to renewable energy,as well as the key to construct clean and effective continual energy system based on hydrogen,which is vital to achieve the cyclic utilization of hydrogen.The Industrial application of EWS technology relies on the reduction of electricity consumption,and thereby cut down the cost of hydrogen production.However,as semi-reaction of EWS,the oxygen evolution reaction（OER）suffers from slow kinetics because of complex multi-step process with four electrons transfer,and becomes as main challenge restricting EWS overall energy efficiency.Therefore,developing efficient,economical and stable OER electrocatalysts has become a significant direction to promote the development of efficient EWS.A practical OER catalyst requires to feature several necessary characteristics simultaneously,including ample accessible active sites,sufficiently high intrinsic activity,fast electron and mass transfer,robust chemical and structural stability.Many studies have focused on solving one or two aspects,while few efforts have been devoted to the synergistic modulation of all aspects,which is vital for practical applications.This paper focuses on the collaborative application of multiple strategies in the design for the goal that integrate the basic features above into the one system.A series of silver（Ag）functionalized cobalt（Co）based nanomaterials were therewith constructed as efficient OER catalysts.This work will contribute to the rational design of practical OER electrocatalysts,and then facilitates the cost-effective large-scale hydrogen production.The main contents are listed as follows:（1）Ag nanowires（Ag NWs）and Co-based layered double hydroxides（Co-LDH）were dexterously integrated through a simple process of metal coordination and in-situ ion etching to construct a nanocomposite Ag@Co-LDH,which features unique structure and can be used as highly active and stable OER electrocatalyst.Co-LDH with ultrathin sheet-like structure and abundant grain boundary defects effectively provides lots of active sites;High conductivity of Ag NWs and heterointerface between Ag NWs and Co-LDH gravely accelerate electron transfer,abundant cavities meanwhile conduce to mass transfer;The optimized local environment of Co atoms and plentiful amorphous regions leaded by Ag dopants strongly enhance the intrinsic activity of active site.Therefore,the as-prepared Ag@Co-LDH demonstrates distinguished OER activity with a low overpotential of 217 m V at the current density of 10 m A cm^-2.Moreover,benefiting from the unique structure and stable heterointerface,Ag@Co-LDH also exhibits robust cycling stability and long-term durability.This finding provides a practical design direction for high-performance LDH-based OER electrocatalysts.（2）A novel OER electrocatalyst Ag@Co Mo-LDH that in situ integrate Ag nanoparticles（Ag NPs）and Co Mo-based layered double hydroxides nanocages（Co Mo-LDH NCs）was constructed through a sacrificial template method and a subsequent spontaneous strategy.Benefiting from the enlarged layer spacing caused by Mo O₄^2-intercalation,the unique open cage structure and heterogeneous interface generated by in-situ binding of Ag NPs and Co Mo-LDH NCs,Ag@Co Mo-LDH presents accelerated electron and mass transfer,large number of new active sites and optimized activity of original sites during reaction.When used to catalyze OER in alkaline media,the required overpotential of Ag@Co Mo-LDH for affording a geometric current density of 10 m A cm^-2is as low as 205 m V,which is not only significantly lower than that of separate Co Mo-LDH or Ag nanoparticles but also superior to that of most developed OER electrocatalysts reported recently.Impressively,Ag@Co Mo-LDH also exhibited promising practical prospect on account of the remarkable cyclic and long-term stability.This finding demonstrates that pointedly integrating multiple strategies into one system is a promising way to construct new LDH-based OER electrocatalysts with synthetically improved performance,providing a promising model for developing advanced electrocatalysts in energy conversion devices.（3）A promising structure Ag-Co₃O₄/CNT that substituent Ag single atom（Ag NPs）embedded in Co₃O₄nanoparticles（NPs）on the surface of carbon nanotube（CNT）was prepared by co-deposition of metal ions and low temperature pyrolysis,its performance in OER was also probed.The high angle annular dark field-scanning transmission electron microscopy（HAADF-STEM）and X-ray absorption spectroscopy（XAS）demonstrate the successful embeddedness of atomical Ag atom in Co₃O₄lattice,the resultant electronic interaction is conducive to promote charge transfer for OER.Different from hypervalent noble metals like Ru,Ir and Rh,density functional theory（DFT）results clarify that atomical Ag dopant prefers to replace tetrahedral Co²⁺rather than octahedral Co³⁺,which is first reported.In addition,the substitution Ag acts as the active site through Ag-Co bridge for intermediates adsorption and facilitates the desorption process,which improves the turnover frequency（TOF）and boosts the intrinsic activity of Ag-Co₃O₄/CNT.As a result,Ag-Co₃O₄/CNT displays remarkable activity(236 m V@10 m A cm^-2)and robust stability for alkaline OER.This finding offers a potential direction for the design of noble metal SAs involved spinel based OER electrocatalysts.