Synthesis And Electrochemical Properties Of Iron-Based Phosphides

Transition metal phosphides have been widely investigated as energy conversion and storage devices due to their unique physical and chemical properties.In addition,iron-based phosphides have broad application prospects in high-performance lithium ion batteries and electrocatalysis because of their abundant natural resources,excellent lithium ion storage capacity and catalytic activity.In this paper,we tried to explore,design and synthesize new iron-based phosphides from the aspects of crystal structure,microstructure and electrical conductivity of the materials,and study their electrochemical performances for high performance lithium-ion batteries and catalytic performances for electrochemical water splitting.The main research results of this thesis are summarized as follows:(1)LiFeP,a "111" type iron-based superconductor,was explored as a new high performance lithium ion battery anode material.The lithium ion storage capacity and electrochemical reaction mechanism of LiFeP were studied.LiFeP black powder was successfully obtained by a conventional high-temperature solid-state reaction of metal lithium and FeP precursor powder in a sealed vacuum quartz tube at 850 ℃for 24 h.In this article,LiFeP was investigated as an anode electrode material for lithium ion battery for the first time.LiFeP retains a high reversible capacity of 507 mA h g-1 after 300 cycles at a current density of 0.3 A g-1,and a reversible capacity of 188 mA h g-1 at 1 A g-1 rate.These excellent electrochemical properties are attributed to the initial stable layered crystal structure and good electronic conductivity of LiFeP.Furthermore,LiFeP anode shows a capacity increase during cycling.Mechanism studies reveal that the capacity increase is caused by the occurance of more conversion reactions of LiFeP and the formation of elemental phosphorus during cycling.(2)The three-dimensional graphene framework scaffolded FeP nanoparticles(3D FeP-rGO)composite was designed and synthesized as a new anode material for high performance lithium ion battery.The electrochemical performances at high current density and the feasibility in full cell applications of 3D FeP-rGO composite were investigated.3D FeP-rGO composite was successfully prepared by soluble iron salt and graphene oxide as raw materials through a facial three-step reaction processes of solvothermal,gas phase phosphorization and self-assembly.In half cell,3D FeP-rGO composite displays a stable capacity of 448 mA h g-1 after 500 cycles at a high current density of 3 A g-1.More interestingly,3D FeP-rGO composite can deliver a reversible capacity of 458 mA h g-1 at a high rate of of 10 A g-1.The lithium ion storage performance of 3D FeP-rGO composite is better than those of previously reported FeP-based anode materials.These excellent electrochemical performances of 3D FeP-rGO composite are attributed to the three-dimensional graphene structure with large scale of pores,which can effectively improve the conductivity,promote the rapid transport of lithium ions,and accommodate the volume change of FeP nanoparticles.More impressively,the full cell consisted of 3D FeP-rGO as anode and LiFePO4 as cathode shows a stable lithium ion storage performance at the current density of 0.5 A g-1.After 120 cycles,the revisable capacity of the full cell can maintain at 373 mA h g-1,which is 97%of the first charge capacity.These results reveal that the 3D FeP-rGO composite has excellent lithium-ion storage capacity even at the high current density and can well meet the requirements of anode for high performance lithium ion batteries in terms of energy density and power density.(3)The catalytic performance of porous graphene/FeP nanoparticles composite for hydrogen evolution reaction(HER)in acidic solution,hydrogen evolution reaction(HER)in alkaline solution and oxygen evolution reaction(OER)in alkaline solution are studied.As is known that graphene has a good electronic conductivity,which can effectively increase the electron transfer rate in the catalyst and enhance the kinetics of the catalytic reaction.The results show that the HER catalytic performance of in alkaline solution is the most prominent.The HER overpotential of the composite is only 166 mV in alkaline solution at a current density of 10 mA cm-2.Furthermore,the composite also delivers a low Tafel slope of 141 mV dec-1 and high cycle stability throughout the cycling for HER in alkaline solution.In addition,the composite also displays a good the catalytic performance for HER in acidic solution and OER in alkaline,indicating that the porous grapheme/FeP nanoparticles composite can maintain high catalytic activity for lectrochemical water splitting in a wide range of pH,which is great significant for the practical application of FeP-based materials in electrocatalysis.