Construction Of Amorphous Transition Metal Phosphate-Carbon Composite As Oxygen Evolution Electrocatalyst

Oxygen evolution reaction（OER）is an important reaction process in the field of rechargeable metal-air batteries and hydrogen production by electrolysis of water.However,because OER is limited by the slow kinetics of 4 electrons,it will generate a large overpotential on the electrode,which in turn leads to lower energy conversion efficiency and poor cycle stability.For rechargeable metal-air batteries and hydrogen productionby electrolyzing water,the current research hotspot is the development of efficient and stable non-noble metal OER electrocatalysts.Among them,amorphous transition metal compounds are prone to structural reconstruction and able to produce more highly active sites,becoming one of the most promising non-noble metal-based OER catalysts.Nevertheless,the conductivity of the amorphous transition metal compounds is poor,they are generally needed to be loaded or grown on highly conductive materials or frameworks such as graphene,carbon nanotubes,carbon cloth,and nickel foam by ex-situ methods to further improve the electrocatalytic OER performance of the materials.The use of an additional conductive substrate not only complicates the material preparation,but also increases the preparation cost.On the other hand,due to the poor thermal stability of most amorphous transition metal compounds,they can only stably exist at lower temperatures,while carbon-based materials need to be carbonized at higher temperatures to improve conductivity.Therefore,how to prepare an amorphous transition metal compound-carbon-based composite material by an in-situ method is still very challenging.This thesis mainly focuses on the preparation of amorphous transition metal phosphate-carbon composites with different structures by in-situ carbonization,through surface or bulk reconstruction and bimetallic site introduction to further improve the electrocatalyticperformance of the amorphous materials for OER.（1）Nickel and iron-based phytate was prepared by in-situ co-precipitation method,and then transferred to an amorphous nickel and iron-based phosphate-carbon composite material(a-Ni₅Fe₁PO-C₆₀₀)after annealing at 600°C.This experiment focuses on the influence of amorphous and crystalline structures on material structural reconstruction and metal valences.The characterization results show that the amorphous structure is more prone to bulk reconstruction and able to promote the synchronous oxidation of bimetallic sites,thereby forming a nickel and iron（oxy）hydroxide-carbon composite catalyst with high OER activity.Electrochemical tests show that in 1 M KOH solution,the overpotential of 0.1 mg/cm² a-Ni₅Fe₁PO-C₆₀₀ is only 268 m V to reach 10m A/cm² current density,and a high 318 m A/cm² current density could be achieved at1.8 V.The zinc-air battery assembled with a-Ni₅Fe₁PO-C₆₀₀+Pt/C as the air electrode delivers excellent cycling stability,at a current density of 10 m A/cm² with 5 minutes charge-5 minutes discharge,it can cycle more than 100 hours.（2）With the assistance of CTAB,nickel phytate nanospheres with uniform size were synthesized by a reflux method at low-temperature.Those nanosperes could be self-sacrificing template to be iron-doped on the surface area,and then transferred to nickel phosphate-carbon composite nanosphere material（Fe-Ni PO-CNS）when annealing at 600 ^oC.The influences of ferric ion concentration and reaction time on the properties of the material were mainly discussed.Characterizations show that the catalyst synthesized by this method still has an amorphous structure after high temperature annealing and can still maintain a uniform nanosphere structure,and the surface iron-doped nickel phytate nanospheres are prone to surface remodeling during the OER test process,thereby forming a highly active iron and nickel-based oxyhydroxide-carbon electrocatalyst.Electrochemical tests show that the rapid reaction of nickel phytate nanospheres in a high concentration of ferric iron solution is beneficial to further improve the electrocatalytic OER performance of the composite material,in 1M KOH solution with a mas loading of 0.1 mg/cm²,the Fe-Ni PO-CNS can achieve a low overpotential of 270 m V at a current density of 10 m A/cm².The zinc-air battery powered by Fe-Ni PO-CNS+Pt/C exhibited excellent charge-discharge cycling stability,which can be cycled more than 120 hours under the current density of 10 m A/cm².（3）Co_0.8Ni_0.2（OH）₂ nanosheets was used as a template,based on the experimental results that the phytic acid has stronger complexing force with Ni²⁺ions than OH-,and does not complex with Co²⁺in ethanol solution,the surface of Co_0.8Ni_0.2（OH）₂nanosheets is treated with phytic acid to obtain Co_0.8Ni_0.2-x（OH）_y@Ni_xPO two-dimensional nanosheet structure,and then rapid doped with trivalent iron ions,leading to Co_0.8Ni_0.2-x（OH）_y@Ni_xFe_zPO two-dimensional core-shell structured composite material along with high temperature annealing..The effects of phytic acid,iron ion concentration and reaction time on the structure,composition and electrocatalytic OER performance of the final Co_0.8Ni_0.2-x（OH）_y@Ni_xFe_zPO material are mainly discussed to establish the structure-performance relationship.