LDHs-based Nanostructural Electrodes For Electrocatalytic Hydrogen Evolution Coupled With Organic Oxidation

Hydrogen,which has the characteristics of high energy density,clean combustion products,and abundant resources,is known as the "ultimate energy"to deal with energy and environmental crises.Our country has also clearly identified hydrogen energy as a new type of energy and incorporated it into energy strategic development plan.Compared with the hydrogen producted by fossil resources,the electrocatalytic water splitting to produce hydrogen can convert intermittent renewable energy into hydrogen energy,becoming one of the most potential sources of green hydrogen.However,the efficiency of industrial electrocatalytic water splitting still is low,resulting in high energy consumption and cost.From the nature of the water splitting reaction,the main factor affecting the efficiency of water electrolysis is the slow thermodynamics of the anodic oxygen evolution reaction(OER)involving four-electron transfer,which directly determines the high overpotential of the reaction system.Therefore,in order to improve the reaction efficiency of the electrocatalytic process,the design and preparation of an efficient and stable oxygen evolution electrocatalyst has become the key to the development of electrocatalytic hydrogen production technology.Moreover,the industrial value of O2 is limited,and the mixing of H2 and O2 in the electrochemical cell gives rise to potential safety issues.Controlling the reaction and synthesizing high value-added chemicals are also of great significance for further reducing the cost and promoting the industrial application of hydrogen production by electrocatalytic water splitting.In response to the above two key scientific issues,this thesis is based on typical electrocatalytic water splitting materials such as layered double hydroxides(LDHs),and proposes the design of LDHs structured electrodes to promote the reaction dynamics.It also involves doping,surface and interface construction,defect control and other methods to achieve intrinsic activity control,thereby significantly improving the electrocatalytic water splitting performance.Moreover,organics are introduced into the reaction system for further reducing the anode overpotential by coupling hydrogen production with organic oxidation,while achieving the synthesis of high-value chemicals.Construction of a new system of electrocatalytic oxidation of small organic molecules coupled with hydrogen production can be achieved by revealing the changes of reaction intermediates and the mechanism of the catalyst during the electrocatalytic reaction.The specific research content and results of this paper are as follows:1.Ultra-thin NiLa-LDH array for efficient oxygen evolution reactionAlthough NiFe-LDH electrocatalysts have received widespread attention due to their excellent performance,how to develop new LDHs and further improve their catalytic performance still faces huge challenge.In this work,rare earth element is introduced into the LDHs laminate,which effectively regulates the electronic structure of LDHs and promotes its electrocatalytic water splitting performance.At the same time,in view of the poor conductivity and easy embedding of active sites of the LDHs powder catalyst prepared by the traditional method,the NiLa-LDH nanosheet array material was grown in situ on the conductive substrate by a simple electrochemical synthesis method.The study found that the introduction of La reduced the binding energy of Ni 3p and O Is.The electrocatalytic performance of the OER of the NiLa-LDH nanosheet array shows a volcanic function relationship with the amount of La.The NiLa-LDH(Ni:La=5:1)nanosheet array exhibits the best performance,which only requires an overpotential of 209 mV to reach 10 mA cm-2,and can remain stable for more than 30 hours.Furthermore,effective regulation of the loading of NiLa-LDHs nanosheets can be achieved by changing the time of electrosynthesis.Finally,the controllable synthesis of the new LDHs-based electrocatalyst and the assembly of structured electrode were realized,and its hydrogen production performance was significantly improved.2.LDHs-derived electrode for electrocatalytic overall water splittingAlthough LDHs show relatively excellent electrocatalytic water splitting activity,their efficiency in electrocatalytic hydrogen evolution is still low,which restricts the overall water splitting performance.In order to further improve the electrocatalytic overall water splitting efficiency,we design and synthesize the heterostructures consisting of(Ni,Fe)S2 nanoboxes and MoS2 nanoarrays via a hydrothermal synthesis based on NiFe-LDH precursor,which display largely improved activity and durability towards water-splitting process.Typically,the(Ni,Fe)S2@MoS2 heterostructures exhibit low overpotentials of 270 mV and 130 mV to reach the current density of 10 mA cm-2 for OER and HER respectively,which is better than commercial Pt/C and IrO2/C catalysts.In combination with in situ Raman spectra,we demonstrate that the constructed interfacial active sites are favorable to the formation of S-Hads,which synergistically lower the chemisorption energy of the intermediates of HER and OER,thereby facilitating the electrocatalytic overall water splitting.This work provides new ideas for the design of electrocatalytic water splitting electrode materials.3.Surface defects regulation of LDHs nanostructural electrode for electrocatalytic organic oxidation coupled with hydrogen productionAccording to high anode overpotential and low added value of oxygen products,we aim to produce hydrogen by coupling with electrocatalytic synthesis of high value-added organic chemicals.5-hydroxymethylfurfural(HMF)oxidation to 2,5-furandicarboxylic acid(FDCA)was introduced into the anode reaction system,and by regulating the surface defects of the LDHs structured electrode,the electrocatalytic organic oxidation coupled with hydrogen production performance was significantly enhanced.First,based on the "memory effect" of LDHs,through the topological transformation of baking restoration,a multi-level structure of LDHs nanosheet arrays rich in metal defects are prepared.X-ray photoelectron spectroscopy(XPS),X-ray absorption spectroscopy(XAS)and other techniques show that Ni vacancies(VNi-CoNi-LDH)are formed during the calcination restoration process,resulting in changes in the metal coordination structure of the LDHs laminate.Electrochemical tests indicate that after the introduction of metal vacancies,VNi-CoNi-LDH exhibits excellent FDCA selectivity(100%)and Faraday efficiency(97%).Density functional theory(DFT)calculation studies show that the existence of Ni vacancies reduces the energy barrier of the rate determining step,thereby improving the conversion selectivity and Faraday efficiency.The electrocatalytic HMF oxidation pathway was determined by chromatographic analysis and isotope labeling.In situ Raman proved that the oxidation of HMF was achieved through the valence of metal of VNi-CoNi-LDH.This work reveals the catalytic process of the catalyst and reactant in the electrocatalytic HMF oxidation,and provides a certain theory for the design and mechanism research of catalysts for electrocatalytic small organic oxidation.4.LDHs-derived nanostructural electrode for electrocatalytic organic oxidation coupling with hydrogen productionIn order to further suppress the occurrence of the OER,we used CoNi-LDH as a precursor,and Se was introduced on the surface through selenization treatment to obtain structured ordered(CoNi)0.85Se multi-level nanosheet array.High-resolution transmission electron microscopy(HRTEM),XPS,XAS and other characterization techniques confirmed that the maintenance of the(CoNi)0.85 Se hierarchical structure array morphology and the uniform dispersion of active components.The prepared(CoNi)0.85 Se shows excellent electrocatalytic HMF oxidation activity(FDCA selectivity:100%,Faraday efficiency:99%).The potential window(1.3?1.6 V vs.RHE)that maintains more than 90%of FDCA selectivity is wider than CoNi-LDH(1.55?1.6 V vs.RHE),due to the suppression of OER side reaction.At-0.3 V vs.RHE,the HER current density of the(CoNi)0.85Se structured electrode is two times higher than that of CoNi-LDH.(CoNi)0.85Se can maintain the stability of 10 cycles,with practical industrial application prospects.This work further provides new ideas for the design of catalysts for electrocatalytic organic oxidation coupling with hydrogen production.