In Situ Preparation Of Electro-catalysts Within Intergranular Defective Pores Of Graphite For Water Electrolysis

Gobal energy field is undergoing tremendous changes,and renewable energy has become an important cornerstone of sustainable development for human society.Hydrogen energy has aroused people's research interest as an ideal energy carrier.Hydrogen production by electrolysis of water plays an important role in the field of renewable energy storage.Developing efficient and low-cost catalysts is the top priority of hydrogen production technology.An industrially attractive electrocatalyst needs to meet a series of criteria such as low price,simple and scalable preparation,robust stability as well as highly efficient catalytic activity.In this thesis,a high pure polycrystalline graphite with intergranular pores was screened to work as a wonderful matrix for in situ synthezing and fixing electrocatalyst nanoparticles.The precursor solutions were directly drip-coated on the graphite surface and infiltrated into the interior of the graphite substrate through the intercrystalline defect pores,where the synthesis and in situ fixation of the catalysts were completed without any binder.Three catalysts prepared as such showed excellent catalytic activity and stability in hydrogen and oxygen evolution reactions.Main research contents and results are as follows:Screening of the matrix of electrocatalysts and the structural characteristics of graphite.The electrode substrate materials were screened among polycrystalline nickel,copper,graphite,glassy carbon,carbon paste,carbon paper and expanded graphite.The nickel-iron precursor solution(Ni(NO₃)₂ and Fe(NO₃)₃)was loaded on the substrate to prepare nickel-iron based catalysts on different substrates.The substrate materials were compared and screened according to the catalytic performance to the oxygen evolution reaction(OER)in alkaline medium.Results show that the polycrystalline graphite with intergranular pores is the optimal matrix for catalysts.The total porosity of the graphite substrate is 15.4%,and the porosity with pore size less than 10 nm is 6.36%.The predominant pore size is 2?3 nm,and the pore volume is 0.030 cm³g^-1 for the pores in a size less than 10 nm.The crystal phase characterization shows that the graphite body is polycrystalline and contains abundant slit type internal channel defects.The abundant pore structure features provide the feasibility of using the lattice gap channels in the conductive matrix as the nanoreactor for loading electrocatalyst.Preparation and characterization of nickel-iron based catalyst for the OER.With high pure polycrystalline graphite as subtrate,the NiFe based catalyst nanoparticles were synthezed within the graphite intergranular pores(NiFe@GIP)for catalyzing the OER in 1.0 M KOH.The linear sweep voltammetry,Tafel method,and electrochemical impedance spectroscopy were used to evaluate the performance of NiFe@GIP for OER in alkaline medium.A series of characterization techniques such as Raman,XRD,XPS,and TEM were used to chacraterize the structure and composition of NiFe@GIP.The NiFe@GIP requires a small overpotential of only 0.210 V to deliver a current density of 10 m A cm^-2 in 1.0 M KOH.The NiFe@GIP maintains the OER activity for 24 h at 10 m A cm^-2 and 100 m A cm^-2 without considerable change in overpotential.Moreover,the precursor ions Ni²⁺and Fe³⁺can infiltrate into the graphite bulk througn intergranular pores up to about 3.6 mm deep.The OER acitivity of NiFe@GIP gradually decreases with inscreasing polished thickness,and tends to the level of blank substrate until a thickness of 3.6 mm is removed.The(Ni,Fe)OOH is proved to be the active species for the OER.Preparation and characterization of trace platinum-based catalyst for hydrogen evolution reaction(HER).With the high pure polycrystalline graphite as subtrate,the Pt@GIP was prepared by drop-casting the precursor(H₂Pt Cl₆ solution)on the graphite surface.The linear sweep voltammetry,Tafel method,cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the performance of Pt@GIP for HER in acidic medium.Raman,XRD,XPS,SEM,and TEM were used for characterizing the morphologies,structures and compsitions of the Pt@GIP.The Pt@GIP needs an overpotential of only 0.025 V at the current density of 10 m A cm^-2 in1.0 M H₂SO₄,which is superior to the benchmark Pt/C.The Pt@GIP catalyst do not show considerable activity decay over 24 h electrolysis at the current density of 10 m A cm^-2,and withstands the large current density of 500 m A cm^-2 for 8 h.What's more,the precursor Pt ions can infiltrate into the graphite bulk with intergranular pores up to about 3.6 mm deep,the HER acitivity of Pt@GIP decrease as the inscreasement of polishing depth,nearly vanish at the polishing depth of 3.6 mm.Data showed that of Pt is the active species for HER,which is electro-reduced from Pt ions in the precursor solution.Preparation and characterization of nickel-iron-sulfur based catalyst for both HER and OER.With the high pure polycrystalline graphite as subtrate,a bifunctional catalyst Ni-Fe-S@GIP was designed and synthesized for highly efficiently catalyzing both HER and OER in the same electrolyte.The Ni-Fe-S@GIP was prepared by drop-casting two precursor solutions,a solution of CH₃CSNH₂ and a mixed solution of Ni(NO₃)₂ and Fe(NO₃)₃,on the graphite surface.The linear sweep voltammetry,Tafel method,cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the performance of Ni-Fe-S@GIP in alkaline surroundings.The XRD,XPS,and TEM techniques were used for characterizing the morphologies,structures and compsitions of Ni-Fe-S@GIP.The Ni-Fe-S@GIP needs an overpotential of only 0.240V for HER and of only 0.197 V for OER to achieve the current density of 10 m A cm^-2in 1.0 M KOH.For overall water splitting,the Ni-Fe-S@GIP requires 1.64 V to reach the current density of 10 m A cm^-2 in 1.0 M KOH,which is compareable to the state of art bifunctional catalysts in basic electrolyte.The Ni-Fe-S@GIP catalyst does not show significant decay in the catalytic activity after subjected to the water splitting for 24 h at10 m A cm^-2 and 100 m A cm^-2.Moreover,the precursor ions S^2-,Ni²⁺and Fe³⁺can infiltrate into the graphite bulk through the intergranular pore channels up to about 3.5mm deep.The HER and OER acitivity of Ni-Fe-S@GIP decreases with increasing polishing thickness,but still higher than the level of the blank substrate as the removed thickness reaches 3.5 mm.The results demonstrate that the active species are different for the HER and OER because the bifunctional catalyst operates in two different potential ranges.The Fe(II)-doped NiS is the HER-active species.,while the the Fe(III)-doped Ni OOH is the active species of the Ni-Fe-S@GIP used in the OER catalysis.In conclusion,three efficient,stable and low-cost electrocatalysts for HER and OER were in situ synthesized in intergranular pores within polycrystalline graphite.The rigid defect structure significantly improves structural integrity and robust stability and minimizes the electron transfer barrier.The strategy proves to be a simple,mild,eco-friendly,and easy to large-scale production.It has great potential,provides a new avenue for catalysts design in industrial applications.