Synthesis And Energy Storage Characterization Of High Electrical Conductive Metal Phosphide Electrode Material

Electrode materials,as one of a most important factors of supercapacitors,have been attracted extensive attention of the researchers.In this paper,the newest development progress in research of multiple electrode materials of supercapacitors was reviewed,and further explored a new-style metal phosphide electrode materials.Metal phosphides are a sort of submetallic alloy and possess high intrinsic conductivity.For charge-discharge process,electrode materials with high electrical conductivity avail the rapid migration of electrons in large current charge-discharge process,which can improve the rate capability of electrode materials.Accordingly,different kinds of metal phosphide electrode materials were synthesized via different preparation method and studying their energy storage characteristic.The work has been carried out following generally the mainline of monometallic phosphides-bimetallic phosphides-composite materials,studied the influence of electrical conductivity,intrinsic structure of materials,molar ratios of elements for the electrochemical performance.The main contents are presented as follows:（1）By a liquid phase synthesis method,dendritic nickel carbide（Ni₃C）precursor was synthesized via using acetylacetonate hydrate,oleylamine and 1-octadecene as raw material,then high electrical conductive nickel phosphide alloys are controllable synthesized by conversion of Ni₃C into Ni₂P,Ni₇P₃ and Ni₁₂P₅ using a certain amount of trioctylphosphine（TOP）.The Ni₂P inherits particles structure while the Ni₇P₃ and Ni₁₂P₅ appear homogeneous and consistent sphere-cluster-like structure derived from particles accumulation.Each ’sphere’approximate flower-like with a good dispersibility.This structure can lead to large specific surface area in favor of improving the specific capacity of materials.The current-voltage（I-V）curves can analyse the electrical conductivity of materials qualitatively.Results indicate that the conductivity of these nickel phosphide alloys is higher than nickel oxide（NiO）and commercial graphite.Electrochemical testing indicates that Ni₂P possesses the highest specific capacity of 390 C g^-1 while Ni₁₂P₅ held the most excellent rate capability.To further evaluate the electrochemical performance of the nickel phosphide electrodes for practical application,asymmetric supercapacitors were assembled using nickel phosphides as positive electrode and activated carbon as negative electrode.The AC//Ni₂P asymmetric supercapacitor delivers the highest specific capacity of 239.5 C g^-1 at a current density of 0.5A g^-1 and a maximum energy density of 53.2 Wh kg^-1(at a power density of 0.3998 kW kg^-1).（2）Based on one-step hydrothermal synthetic approach,a train of Ni-Co phosphides with different Ni/Co molar ratios(Ni_xCo_2-xP)were synthesized and their microstructure and electrochemical performance was investigated.The research results show that the as-prepared Ni_xCo_2-xP presents particles structure and form abundant tunnels between particles,which provide fast ion/electron transfer.Electrochemical testing indicates that the specificcapacitance of Ni_xCo_2-xP first increases then decreases with the increase of Ni/Co molar ratios,the Ni_xCo_2-xP with a Ni/Co molar ratio of 1:1 shows the most excellent electrochemical performance.The NiCoP as a representative sample is tested and results reveal that NiCoP achieves high electrical conductivity.It achieves a high specific capacity(571 C g^-1 at 1 A g^-1)and out-bound rate characteristic（72.8% capacity retention with a 20-fold increase in current densities）,which possesses better electrochemical performance than monometallic Ni₂P and Co₂P.To inspect the worth of Ni-Co phosphides in practical applications,the asymmetric supercapacitor is fabricated based NiCoP as the positive electrode and AC as the negative electrode.It can achieve a high energy density of 32 Wh kg^-1(at a power density of 0.351 kW kg^-1)and prominent cycling stability with 91.8% initial capacity retention after 3000 cycles.It demonstrates that nickel cobalt phosphides are promising as energy storage materials.（3）The Ni₂P/Co₃V₂O₈ nanocomposite is synthesized by a facile chemical precipitation technique.The effects of Ni₂P:Co₃V₂O₈ molar ratios on the electrochemical properties of the Ni₂P/Co₃V₂O₈ nanocomposite were also investigated.The research results show that the as-prepared composite presents loose granular structure,the larger Ni₂P nanoparticles are serving as the “substrate” and the smaller Co₃V₂O₈ nanoparticles gathered on the surface of Ni₂P nanoparticles.This distinctive structure will not only prevent the largely agglomeration of the Co₃V₂O₈ nanoparticles,but also cause more tunnels to provide fast ion/electron transfer.Electrochemical testing indicates that the specific capacitance of composite first increases then decreases with the increase of Ni₂P,the Ni₂P/Co₃V₂O₈ composite with a Ni₂P:Co₃V₂O₈ molar ratio of 0.8:0.2 shows the most excellent electrochemical performance.The composite improves the electrical conductivity of Co₃V₂O₈ to some extent due to the existence of Ni₂P as conductive additives.The composite combines separately the advantages of the high specific capacitance of Ni₂P and the good rate capability as well as cycle stability of Co₃V₂O₈.To further explore the electrochemical properties of the Ni₂P/Co₃V₂O₈ composite in practical application,an asymmetric supercapacitor（ASC）was fabricated by employing the Ni₂P/Co₃V₂O₈ composite and activated carbon（AC）as the positive and negative electrodes.The maximum specific capacitance of 206.5 C g^-1 and the specific energy of 40.2 Wh kg^-1 are demonstrated at a cell voltage between 0 to 1.7 V,suggesting that the Ni₂P/Co₃V₂O₈ nanocomposite is an ideal supercapacitive electrode material.