The Preparation Of One-dimensional Molybdenum Based Nanotubes Arrays And Study In Electrochemical Energy Storage And Conversion Field

Molybdenum based compounds can be used as electrocatalytic hydrogen evolution and supercapacitor electrode materials due to the good conductivity,rich surface valence and high catalytic activity.Furthermore,the capacitance and electrocatalytic activity of the materials molybdenum based compounds will further by construction one-dimensional self-ordered nanotubes arrays because of the high specific surface area,fast ion transport and good stability.In this paper,one-dimensional self-ordered MoO₃ nanotube was synthesized by electrochemical anodization,moreover,MoTaO_x,MoO_x/MoP,MoO_x/MoS₂ and MoO_x/MoON nanotubes were further obtained by a variety of post-treatment.The relationship between the composition,structure,capacitance performance and electrocatalytic hydrogen evolution activity of the electrode materials,as well as the internal mechanism of performance improvement were systematically discussed.One-dimensional self-ordered MoTaO_x nanotubes were formed on Mo-Ta alloy by anodization according to percolation theory,that is,incorporating high corrosion-resistant material Ta oxide into MoO₃ for protecting the entire oxide.Results show the amorphous MoTaO_x nanotubes（A-MoTaO_x）exhibited high thermal stability and cycle stability results from the large number of defects in the amorphous materials for improving the diffusion coefficient of ions.In addition,the loose structure of amorphous state is beneficial to reduce the volume expansion caused by the ion embedding in the charging process.Moreover,the corrosion potential of MoO ₃ is largely increased and corresponding self-corrosion current decreased due to the incorporation of Ta.The electrochemical reaction resistance and electrode ohmic resistance of A-MoTaO_x were greatly reduced results from the transformation of bronze materials H_yMoTaO_x after activation process.The capacitance of A-Mo20TaO_x nanotubes achieves 1484 F/cm³ at a scan rate of 10 m V/s,which is 20time larger than MoO₃and 10⁵ time higher than Ta₂O₅ electrode.The capacitance retention of A-Mo20TaO_x electrode remains 100%of initial capacitance even after10000 cycles.One-dimensional MoO_x/MoP nanotubes arrays were fabricated on Mo foil by anodization and phosphidation treatment for introduction of nonmetal phosphorus.The experimental results show that MoO_x/MoP obtained by phosphating at low temperature has a coaxial heterojunction structure of crystalline MoO₂ and amorphous MoP.The migration of H⁺was accelerated in amorphous MoP resulting the transport process is almost independent with diffusion process.MoO_x/MoP nanotubes achieve 2000 F/cm³（800 F/g）at 1 A/cm³（0.4 A/g）and remain 1400 F/cm³at charging rate of 100 A/cm³.The capacitance is still remained 92.6%after 10000cycles.P-MoO_x800^℃ nanotubes with high crystallinity MoP were fabricated at the phosphidation temperature of 800^℃.One-dimensional structure provides several channels for fast electronic transmission resulting the improvement of electrocatalytic performance.The overpotential of P-MoO_x800^℃ reachs 188 m V and 158 m V at a current density of 10 m A/cm² in 0.5 M H₂SO₄ and 1 M KOH respectively.One-dimensional MoO_x/MoS₂nanotubes arrays with coaxial structure were in situ formatted by sulfidazation in H₂S flow.MoS₂ with high density in MoO_x/MoS₂ out layer provides a large number of active sites for electrochemical hydrogen evolution and protects MoO_x（inner layer）from corrosion by acid electrolyte.The high conductivity of MoO_x in the inner layer promotes the rapid electrons migration.The high-efficiency synergistic effect on one-dimensional ordered heterostructure nanotube array and special interface structure provides conductivity and reactivity.The experimental results show that the hydrogen evolution overpotential of MoO_x/MoS₂ nanotubes in H₂SO₄ solution is 147 m V.After 60 min stability test,the surface morphology and one-dimensional ordered structure was still remained leading to long-term hydrogen evolution current.The MoO_x/MoON coaxial heterjunction nanotubes were fabricated by ammonification treatment on MoO₃ nanotubes,which can be used as supercapacitor negative electrode.The capacitance of MoO_x/MoON nanotubes reached 1987 F/cm³（883 F/g）at a current density of 2.5 A/cm³（1.1 A/g）,which is 0.67 time higher than MoO₃ nanotubes.Moreover,65%of capacitance is remained even the current density increasing to 100 A/cm³（44 A/g）.An outstanding cycling stability with 91.4%retention is retained after 10000 cycles.A variety of characterization results showed the excellent capacitance performance was attributed to the outer amorphous MoON with numbers of active sites and O defects,which promoted the adsorption and embedding of H⁺.And the inner layer crystal MoO_x has a one-dimensional channel crystal structure accommodating H⁺for insertion.The Kelvin probe microscope results exhibited the formation of built-in electric field at the interface of MoO_x/MoON heterojunction due to the different surface work functions of two materials.The electric field direction is from electrode outside to in side that improves the embedding speed of H⁺significantly in charging process.Furthermore,the transfer of electrons and ions were promoted by one-dimensional self-ordered structure.H⁺can be rapidly embedded and removed from the nanotubes by synergisti c effect of electric field built in the interface and high conductivity,which significantly improves the capacitance performance.