Study On The Electronic Structure Regulation Of Metal-Organic Frameworks For Water Splitting

With the rapid development of society,the energy consumption and environmental pollution issues have become more and more serious.To meet the sustainability of human society,development of electrcatalytic materials that facilitate electrochemical energy conversion and storage technology has become a major research focus.Electrocatalytic water splitting（including the oxygen evolution reaction（OER）and hydrogen evolution reaction（HER））is considered to be one of the effective strategies to replace traditional fossil fuels.State of the art materials for water splitting are mainly based on Pt-based（or Ru）-based noble metals,but thei rarity and high cost are big hurdles for their widely practical application.Therefore,the pursuit of electrocatalysts with high activity and low cost for water splitting has received increasing attention.Metal-organic frameworks（MOF）recently emerge as one of promising electrocatalysts for energy storage and conversion,because of their permanent porosity,high surface area,tunable functionality and versatile framework topologie.However,the poor conductivity and low stability seriously hinder their practical application in electrocatalysis fields.It is of significant importance to develope effctive strategies to regulate the electronic structure of MOFs to improve performance of MOF for electrocatalysis.In this thesis,we have developed several strategies to modulate the electronic structure of MOF for futher improving their electrocatalytic water splitting（OER and HER）performance,including cation exchange,constructing conductive interlayers,building heterostructure and heteroatom doping.The details are described as follows:（1）Regulating electronic structure of ZIF-67 via caion exchange for boosting electrocatalytic water oxidation.ZIF-67 was selected as a model for regulation of the electronic structure.Various transitional metal ions,including Mn²⁺,Fe³⁺,Ni²⁺and Cu²⁺have been successfully incorporated into ZIF-67 by ion exchange reaction.It not only retains the crystal structure and morphology of the pristine Co-ZIF-67,but also introduces hetero metal ions to modulate the electronic structure of Co（Ⅱ）to promote OER activity.Specifically,the ZIF-67/Fe-1 delivers a minimum overpotential of 287m V to reach the current density of 10 m A cm^-2 with a small Tafel slope of 40.7 m V dec^-1 and exhibits high durability after 2000 CVs testing.More importantly,XRD,XPS,TGA and Raman measurments were performed to characterize the structural and chemical changes,deciphering that the possible origination of the high activity was electrochemically transformed metal hydroxides and oxyhydroxides.（2）Constructing conductive interfaces between carbon paper and MOF for efficient electrocatalytic oxygen evolution reaction.The electrode is built by cathodic electrodeposition of a Ni interlayer on carbon paper（CP）followed by a solvothermal these of MOFs on the Ni interlayer,where single,bimetallic or trimetallic MOF can be well-controlled.The conductive interlayer is an effective intermediate layer to enhance the electron transport between the electrocatalyst and the electrode,and MOF with porous structure can reduce mass transfer resistance,both of which contribute excellent electrocatalytic performance.The optimal Ni Fe-MOF/Ni//CP nanosheets exhibits remarkable catalytic activity toward the oxygen evolution reaction（OER）,affording high current densities of 50 m A cm^-2 at low overpotentials of 230 m V.More importantly,a low voltage of 1.716 V is achieved for the Ni Fe-MOF/Ni//CP-based water splitting cell to reach 50 m A cm^-2 with high stablity.More importantly,a portable overall water splitting device is demonstrated through the integration of a water-splitting cell which is fabricated based on Ni Fe-MOF/Ni//CP.（3）Building heterostructure in 2D MOF nanosheets to promote performance in electrocatalytic hydrogen evolution.2D porous CoP/Co-MOF heterojunction nanosheets is prepared by partially phosphorization of Co-MOF.As a highly efficient integrated electrode,the optimal 2D CoP/Co-MOF exhibits excellent HER activities over a wide p H range,which only requires 52,106 and 26 m V overpotentials at-10m A cm^-2and small Tafel slope of 49,89 and 53 m V dec^-1 in acidic（0.5 M H₂SO₄）,neutral（1.0 M PBS）and alkaline（1.0 M KOH）solution,respectively.In addition,negligible overpotential decay is achieved after 20 h.The superior p H universal HER property of 2D Co-MOF/CoP is attributed to the unique two dimensional porous structure,nanometer array and heterostructure,collaborative ly optimizing the electronic structure of composite catalyst and strengthening the electronic interaction interface.（4）Constructing 2D porous NiCoP/CoP heterojunction nanosheets by a controlled phosphorization of bimetallic organic framework.CoP/NiCoP heterostructure results in the promoted HER activity in a wide p H range,which can be applied to build a tandem hydrogenation system for hydrogenation of organics.Layer Co（OH）₂ as both the template and precursor was used to fabricate vertically-oriented NiCo-MOF materials.Furthermore,bimetallic NiCoP porous materials with ultrathin nanosheet arrays were synthesized by low temperature phosphatization methods.The as-prepared NiCoP/CoP catalyst exhibits low potentials of 129,177 and 120 m V to derive the current density of 100 m A cm^-2 in 0.5 M H₂SO₄,1.0 M PBS and 1.0 M KOH solutions,respectively.Moreover,superior operation stability is also obtained for the NiCoP catalyst.Moreover,the NiCoP nanosheet arrays self-supported electrode directly was used as working electrode for the hydrogenation of styrene and nitrobenzene under hydrogen atmosphere at room temperature,an efficient water splitting device with hydrogenation for organic reduced.The combination of electrocatalytic hydrogen evolution reaction and organic hydrogenation system can achieve a separated cascade reaction of electrocatalytic organic hydrogenation.