Iron-group Metal Compounds Micro/nano-structures: Controlled Synthesis And Electrocatalytic Performances For Water Splitting

It is well known that,with the rapid development of economy,the demand for energy is increasing.Hydrogen has been recognized as the most potential clean energy in the future owing to its high energy density and zero pollution.Also,it conforms to the concept of green new energy for sustainable development.Among many hydrogen production technologies,the electrolysis of water,which is consisted of the cathodic hydrogen evolution reaction(HER)and the anodic oxygen evolution reaction(OER),can produce the high purity hydrogen with the advantages of simple preparation and green process.In the actual hydrogen production process,however,the working voltage between the two electrodes is often much higher than the theoretical value of 1.23 V due to the existence of overpotential.Thus,a huge amount of electric energy will be consumed during the large-scale electrolytic reaction for a long time.When a highly efficient catalyst is used,nevertheless,the electrolysis efficiency of water can be improved owing to the reduction of overpotential.Thus,the cost of electricity consumption can be reduced.Therefore,it is necessary to develop the efficient,stable and low-cost electrocatalysts for hydrogen evolution and oxygen evolution.At present,iron-group metal(Fe,Co,Ni)compounds have focused much research interest in the field of electrocatalysis because of their unique electronic structure,outstanding electrical conductivity and excellent catalytic performance.Therefore,iron-group metal compounds micro/nano-structures were selected as the research objects in the dissertation.Through the strategies of morphology regulation,doping of heterogeneous elements,multi-metal synergistic effect,constructing heterogeneous interface,and coupling conductive substrates,we anticipated to construct electrocatalysts with high activity and high stability for hydrogen and oxygen evolution reaction.The main contents are shown as follows:Ferrite MFe₂O₄(M=Fe,Co,Ni)tubular microstructures were successfully synthesized through a simple two-step strategy.The tubular Fe OOH microstructures were firstly obtained by a heating reflux route at 120°C for 4 h,employing Mn O₂ nanowires and Fe²⁺as the sacrificial templates and Fe metal sources,ethylene glycol and deionized water as the mixed solvents.Then,tubular ferrite MFe₂O₄(M=Fe,Co,Ni)microstructures were produced by a solvothermal method with different metal salts M²⁺at 200°C for 12 h.Experiments showed that as-obtained MFe₂O₄ catalysts demanded the overpotentials of-201?-359 m V or?340-432 m V to afford the current density of+/-10 m A cm²for HER or OER in 1.0 M KOH solution.Among them,Ni Fe₂O₄ tubular microstructures presented the best electrocatalytic activities for HER and OER with a long-term durability of 50 h due to the largest BET specific surface area,the highest electrochemical active surface area and the smallest charge-transfer resistance.Furthermore,when simultaneously employing the Ni Fe₂O₄ as anode and cathode electrocatalyst in a two-electrode setup in 1.0 M KOH,a cell voltage of 1.78 V was needed to reach a current density of 10 m A cm^-2.Fe-doped Co-Mo-S microtubes were successfully synthesized through a multistep synthetic route,employing MoO₃ microrods as the self-sacrificing template,Co(NO₃)₂and Fe SO₄ as the metal sources,2-Methylimidazole as the ligand and thioacetamide as the S^2-ion source.Experiments showed that as-obtained Fe-doped Co-Mo-S microtubes catalyst demanded overpotentials of?105 and 268 m V to afford the current density of-10 m A cm^-2for HER and 10 m A cm^-2 for OER with a durability of 60 h in 1.0 M KOH solution,respectively.The excellent catalytic performance was attributed to the interface engineering formed between Co-S and Mo-S,which produced many lattice dislocation and defects,and could provide additional active sites.Also,the addition of trace Fe increased the conductivity and catalytic activity of the catalyst.In a two-electrode water-splitting device,the as-prepared Fe-doped Co-Mo-S microtubes acted as both anode and cathode electrocatalyst simultaneously.To deliver the current density of 10 m A cm^-2,a cell voltage of 1.605 V was required in 1.0 M KOH solution.After continuously catalyzing overall water splitting for 60 h,the overpotential hardly changed.ZIF-derived hollow CoS_x nanoparticles grown on CoFe-LDH microtubes(CoFe-LDH@CoS_x)were successfully prepared by a mild synthesis route,employing MoO₃microrods as the sacrificial template,Co(NO₃)₂ and Fe(NO₃)₃ as the metal sources,2-methylimidazole as the ligand.Experiments showed that as-obtained CoFe-LDH@CoS_xelectrocatalyst exhibited excellent OER performance and good durability.To deliver current densities of 10,100 and 400 m A cm^-2 in the 1.0 M KOH distilled water solution,the electrocatalyst required overpotentials of 229,270,and 308 m V,respectively.In a two-electrode water-splitting device with the CoFe-LDH@CoS_x as the OER electrocatalyst and the 20%Pt/C as the HER electrocatalyst,the low voltages of 1.495,1.600 and 1.770 V were separately required to achieve the current densities of 10,100 and 400 m A cm^-2 for overall water splitting with remarkable stability for 50 h at the current density of 50 m A cm^-2.The superior electrocatalytic activity should be attributed to its special tubular structure and the synergistic effect between CoS_x and CoFe-LDH.Furthermore,the as-constructed composite electrode could display good OER electrocatalytic performances in alkaline tap water and natural river water,respectively,implying its outstanding potential in the practical application.Hierarchical core-shell NiCo₂S₄@CoNi-LDH nanoarrays grown on carbon cloth(CC) with outstanding electrocatalytic activity for OER were successfully constructed by a three-step hydrothermal strategy.Owing to the self-supporting electrode without adding binder and the special hierarchical core-shell array structure,more active sites could be exposed,and produced gases easily escaped,too,which effectively promoted the OER electrocatalytic activity of the catalyst.Electrochemical measurements showed that the OER catalytic activity of as-obtained NiCo₂S₄@CoNi-LDH catalyst could be tuned by the molar ratio of Co/Ni in CoNi-LDH.Experiments uncovered that NiCo₂S₄@Co₁Ni₄-LDH catalyst prepared at the Co/Ni molar ratio of 1:4 exhibited the strongest OER electrocatalytic activity.To deliver the current densities of 100 m A cm^-2,the as-obtained NiCo₂S₄@Co₁Ni₄-LDH catalyst required the overpotential of 337 m V in 1.0 M KOH solution.After continuously catalyzing for 40 h,the voltage curve hardly waved,presenting excellent long-term stability.Furthermore,the NiCo₂S₄@Co₁Ni₄-LDH/CC electrode possessed the stronger OER catalytic activity against its single component,which should be attributed to the synergistic effect between NiCo₂S₄ and Co₁Ni₄-LDH in the NiCo₂S₄@Co₁Ni₄-LDH/CC electrode.