Controlled Synthesis Of Two Dimensional Metal Chalcogenide/phosphide Nano Arrays Toward Enhanced Electrocatalytic Reactions

The development of electrocatalytic conversion technology towards new energy,is a key approach to solve the problems of increasing energy demands and impending climate change.Electrocatalysts play a key role in these energy conversion technologies because they increase the rate,efficiency,and selectivity of the chemical transformations.Transition metal chalcogenides and phosphides(TMCs and TMPs)have been regarded as promising candidates in energy conversion fields due to their plentiful composition,highly conductivity and excellent electrocatalytic activity.However,it is difficult to realize the specific regulation of composition,micro-structure and electron structure in TMCs and TMPs,owing to the scarcity of synthesis methods and strategies.As a result,the control over electrocatalytic reactivity and stability of TMCs and TMPs has been rarely reptorted,which hinders the reasonable design and fabrication of electrode materials.Moreover,how to identify reactive site and further realize its precise regulation and effective dispersion are still remain challenge.Therefore,in this dissertation,controlled synthesis of two dimensional metal chalcogenide/phosphide nanoarrays has been performed,and their electrocatalytic behavior towards hydrazine electrooxidation and water splitting.Firstly,a systematic theoretical study has been carried out to reveal the influence rules toward hydrazine electrooxidation and water splitting.Furthermore,two dimensional metal chalcogenide/phosphide nanoarrays with defined structure and tunable composition have been fabricated through the topo-transformation of layered double hydroxides(LDHs).On one hand,the structured nanosheet arrays can realize the highly dispersion of reactive sites and increase the exposure of effective catalytic surface.On the other hand,by precise tuning the electron structure,the electron transportation and reactive energy barrier can be accelerated and lowered,respectively.As a result,the instinct catalytic performance of the electrode materials was largely enhanced.Therefore,we realized the improved activity of TMCs and TMPs via effective dispersion of active components and precise control of catalytic sites.Follows are the research contents:1.Anionic regulation of TMCs/TMPs and their electrochemical performance toward oxygen evolutionModulating the electronic structure near the active center of electrocatalyst is an effective way to enhance the intrinsic catalytic performance of electrode materials.Specifically,anions serve as electron donors and militate the cations by electron interactions.The same metal ions with various anion ligands exhibit different physical/chemical properties and electrocatalytic activity,due to the different electronegativity between oxygen,sulfur and phosphorus.Sulfur atom has a larger atomic radius than of oxygen atom,which is beneficial to the ionization of electrons.This makes CoNi-sulfide to show a narrower bandgap than CoNi-oxide,which is favorable for electrons transportation in electrode materials.In addition,due to the difference of electronegativity,the coordination environment of metal ion can be tuned by changing the anion composition,which modifies the electronic structure of reactive site and is beneficial to the adsorption of reaction intermediate.The S-doped CoFeP(S-CoFeP)can be obtained through phosphidation followed by sulfidation of CoFe-LDH.The incorporation of S weakens electronic density of the metal sites,which enhances the adsorption of intermediates and reduces the energy barriers during the oxygen evolution reaction.The current density of S-CoFeP displays a 2.5 folds enhancement compared with CoFeP with an overpotential of 400 mV.In addition,Tafel slope decreases from 94 mV dec-1(CoFeP)to 75 mV dec-1(S-CoFeP).2.Design of highly dispersed two dimensional TMCs nanoarray for enhanced electrochemical hydrazine oxidationTMCs have recently received tremendous attention in electrochemical energy storage and conversion systems due to their low cost,high stability and electrical conductivity.However,some problems remain unresolved in material preparation,such as easy agglomeration and low yield,which limits their further application.To overcome these issues,we demonstrate the fabrication of hierarchical CoNi-sulfides nanoarrays via in situ reduction of CoNiLDH nanosheets followed by a subsequent vulcanization process,which serve as a promising catalyst in the electrooxidation of hydrazine.The CoNi-sulfides nanoarray exhibits excellent electrocatalytic performance in hydrazine electrooxidation with a peak oxidation current of 9.64 mA cm-2(0.8 V vs.Reversible Hydrogen Electrode(RHE)),much larger than that of CoNi-oxide(5.06 mA cm-2).A synergistic effect is demonstrated in this unique structure:the conductive CoNi alloy as core provides the highway for electron transfer,and the CoNi-sulfide shell offers highly exposed active sites for electrooxidation of hydrazine.This work provides a new strategy for the preparation of hierarchical TMC materials via structural transformation of LDHs precursor,which can serve as highly efficient noble metal-free electrode toward electrocatalysis.3.Design of highly dispersed two dimensional TMPs nanoarray for enhanced electrochemical oxygen evolutionTMPs have been proven as high performance catalysts with high stability,large conductivity and nearly 100%Faradic efficiency toward hydrogen evoltion in a broad pH range including strong acidic solutions,strong alkaline and neutral media.As a result,I TMPs exhibit enhanced pertormance toward electrocatalytic reactions when compared with corresponding TMCs.By virtue of the so-called topological transformation property of LDHs materials,herein,we have fabricated ultrathin TMPs nanosheets arrays by using LDHs as precursors:well-defined LDHs arrays were firstly prepared via an ultrafast electrosynthesis method,followed by an in situ phosphidation transformation from LDHs to TMPs.The resulting FeCoP exhibits significantly promoted electrocatalytic activity toward both half-reaction in overall water splitting,with an overpotential of 330 mV and 160 mV for oxygen and hydrogen evolution(at a current density of 100 mA cm-2),respectively.Density functional theory(DFT)calculations and experimental results reveal that the electronic structure of Co is modulated via the incorporation of Fe;this benefits the adsorption of water molecule and the dissociation of OH group,accounting for the largely enhanced overall water splitting.We further fabricated hierarchical nanoarrays consisting of TMP(CoNiP)core and LDHs shell by direct growth of NiFe-LDH on CoNiP nanosheet arrays via a facile electrosynthesis method,which shows largely enhanced performance toward overall water splitting.A symmetric two-electrode cell assembled by this material exhibits excellent performance that only requires a cell potential of 1.44 V to drive a current density of 10 mA cm-2,which is the lowest potential among TMPs-based bifunctional catalysts for water splitting to our knowledge.This work provides a promising approach for the design and preparation of high-performance TMPs structured electrode materials,which can be potentially used in energy conversion.