Construction Of 3D Hierarchical Cobalt-based Electrocatalysts And Their Performances For Sensing And Overall Water Splitting

With the rapid development of non-noble metal nanomaterials,cobalt-based catalytic materials have been widely used in the fields of electrochemical sensing and green energy.Due to its unique 3d electronic structure,adjustable chemical valence state,suitable adsorption energy,abundant reserves on the earth,low-cost,thermodynamic stability and envirnment-friendly,cobalt-based electrocatalytic materials have been developed to a kind of potential elecotrode materils and promising application in the fields of electrochemical sensing and electrolysis of water.With the development of new synthesis strategies,characterization techniques and theoretical calculations,the rational design and synthesis of various cobalt-based catalysts with excellent perfermance has become a research focus in the past few years.In this paper,taking the design and synthesis of unique hierarchical-structure cobalt-based electrocatalysts with potential catalytic activity as foothold,working around the optimization of the critical factors restricting catalytic reactions?such as catalytic active site,conductivity and reaction energy barrier?,exploring various strategies to regulate the catalytic activities of cobalt-based materials,and realizing the optimization of the electrochemical behaviors and catalytic activities under the premise in fully exposed active site.Optimizing the surface electronic sturcture of cobalt-based catalysts by means of morphology adjustment,heteroatomic doping and heterogeneous structures construction,designing and synthesizing electrocatalytic materials with high catalytic activity and stability using the relationship among morphology/components-electronic sturcture-catalytic activity,aiming at exploring the catalytic reaction mechanism,and providing a new idea for furhter understanding of the different adjustment strategies to optimize the electrocatalytic performance of cobalt-based catalysts.The specific research contents od this paper include the following points:1.The novel three-dimensional?3D?hierarchical structures of NiCo₂O₄/rGO have been successfully synthesized by hydrothermal process,in which 2D rGO nanosheets were self-assembled and embedded into 3D flower-like architectures NiCo₂O₄.And then mixed with[OMIM]PF₆ ionic liquid,a NiCo₂O₄/rGO/[OMIM]PF₆modified electrode with the highest catalytic oxidation activity for fungicide pyrimethanil?PMT?was constructed,realizing the sensitive detection of PMT.The high catalytic oxidation performance was mainly due to the unique 3D hierarchical structure and synergistic effect among the components,which could increase the contact area of the electrode/electrolyte interface and provide more active sites for electrochemical reactions,promote the electron/mass transfer between the electrode and solution interface,and beneficial for enrichment and selective response for pesticide residue PMT.Based on the experimental results,the catalytic reaction mechanism of PMT on NiCo₂O₄/rGO/[OMIM]PF₆ modified electrode was revealed,and an electrochemical analysis method for the determination of trace PMT was established.Under the optimum conditions,the electrochemical sensor exhibited two linear ranges of 0.1?10.0?mol L^-1 and 20.0?140?mol L^-1 for PMT with a low detection concentration of 11.0 nmol L^-1.Besides,the interference,repeatability and stability measurements were also evaluated.The proposed method was successfully applied to the detection of PMT in water,seawater,fruits and vegetables with good recovery ranging from 93.0%to 105.5%,and possessed potential applications in the analysis of real samples.2.A 3D ultrathin nanosheets network structure bimetallic oxyphosphide self-supporting electrode?CoNi OP/NF?was successfully prepared by low-temperature phosphating ZIF-67/CoNi layered double hydroxides precursor,which was obtained by ion exchange of 2D ZIF-67 nanosheet grafted with 3D ZIF-67polyhedron?2D/3D ZIF-67/NF?integrated architecture,steming from in situ phase transformation of Co?OH?₂ nanosheets on Ni foam?NF?.This sturcture had an extremly rich margin active site,and the anisotropic structure with high surface volume ratio of 2D nanosheets could effectively improve the chemical reaction interface and shorten the electron/ion transport distance,thus achieving the optimization of catalytic reaction kinetics.In the process of hydrogen evolution reaction?HER?reaction,the highly electronegative P atom and the bimetal active sites in CoNiOP were the proton and hydride acceptors,respectively,which could make the catalyst have high HER activity;in oxygen evolution reaction?OER?reaction process,CoNiOP as a real catalytic active site could promote the charge transfer.The simultaneously formation of metal-P and metal-O bonds in CoNiOP can optimize the electronic structure and balance surface free energy for reactant adsorption/desorption,and thus enhance the catalytic activity of splitting water.Benefiting from the ingenious structure design and the synergistic effect of bimetalic phosphide,the intergrated electrode of CoNi OP/NF showed excellent electrocatalytic activities of HER and OER,needing 82 mV and 294 mV overpotential to drive 10 mA cm^-2 and 30 mA cm^-2 in alkaline solution,respectively.As an assembled electrolyser it required a cell voltage of 1.60 V to achieve a splitting current density of 30 mA cm^-2 in alkaline solution with robust durability.3.A new 3D porous N-doped carbon layers encapsulated bimetallic phosphides integrated electrode?CoNiP@CN/NF?was designed and successfully synthesized by in situ pyrolysis and phosphorization of multi-layer metal organic framework precursor?CoNi ZIFs@ZIF-8?on highly porous and conductive NF,which was obtained by two-step electrodeposition.During the pyrolysis,CoNi ZIFs turned into uniformly dispersed Co NiP nanoparticles and ZIF-8 became a porous carbon layer due to the evaporation of metallic Zn under high temperature.The results showed that the introduction of Ni atom and electronegative P atom could increase the metallic properties of catalyst and thus improve the conductivity,while optimate the electronic structure and thus reduce the energy barrier of the catalytic reaction.N doping could activate adjacent C atoms,facilitate the adsorption of protons or oxygen-containing intermediates,and improve the catalytic performance of HER/OER.Meanwhile,the unique 3D hierachical structure combining with no-binder conductive substrate could facilitat charge/mass transfer,expose more active sites,and thus accelerate the catalytica reaction.Due to the synergistic effect of bimetallic phosphides,N-doped porous carbon layer and conductive substrate,CoNiP@CN/NF showed outstanding electrocatalytic performance during HER and OER in 1 mol L^-1KOH,needing 87mV and 258 mV to drive 10 mA cm^-2 and 30 mA cm^-2,respectively,which were superior to the control samples of single metal phosphide.As an electrolyser for overall water splitting,it required a cell voltage as low as 1.59 V to achieve a water splitting current density of 30 mA cm^-2 in alkaline solution with robust durability.4.A binder-free intgrated electrode with a free-standing 3D Co embedded within nitrogen-doped carbon nanotube arrays?Co@N-CNT/NF?was successfully synthesized via self-boosting catalytic pyrolysis of 3D ZIF-67/2D Co?OH?₂/NF precursor in the presence of melamine.The constructed Co@N-CNT electrocatalyst with metallic Co encalsulated in the nanotube end could expose more catalytic active site;and the unique bonding structure of pyridine N-Co could generate synergistic catalysis at the contact interface,while the strong coupling among Co-N-C could optimize the adsorption free energy between the active site and the intermediates of HER and OER,thus improve the electrocatalytic activity.Additionally,the unique 1D nanotube/2D nanosheet heterogeneous structure could accelerate charge transfer and bubble release,while in-situ growth on the conductive NF substrate could ensure the electrode conductivity and stability.The Co@N-CNT/NF intergrated electrode showed excellent HER and OER electrocatalytic activities,needing 64 mV and 248mV overpotential to drive 10 mA cm^-2and 30 mA cm^-2in alkaline solution,respectively.As an electrolyser for overall water splitting,it required a cell voltage as low as 1.58 V to achieve a current density of 30 mA cm^-2 in alkaline solution and still maintained robust stability after 24 h electrolysis.According to a series of DFT calculations,it indicates that the cooperation of appropriately numbered pyridinic N atoms and Co atom play very important role in HER and OER activities,which further reveals the electro-catalytic splitting water mechanism of as-prepared Co@N-CNT.Meanwhile,this universal method can be extensively applied to fabricate well-defined monometal or bimetal encapsulated into N-CNTs for various electrochemical energy applications.