Structure Modulation Of V-based Oxide Electrocatalysts For Enhanced Water Splitting Performance

Hydrogen is becoming a new type of energy,which can replace traditional fossil energy due to its high efficiency,cleaning and sustainability.Electrocatalytic water splitting technology is a promising sustainable hydrogen production method because the advantages of low energy consumption and high stability.At present,the best electrocatalysts for water splitting are noble-metal such as Ir,Ru,Pt and Pt-based materials,but the development and application are limited due to their high price and scarce reserves.In contrast,transition metal oxides(TMOs)have the advantages of excellent conductivity,low density,cost-effectiveness and more readily available.However,the overall efficiency of TMOs is very low because of the high voltage barrier required during the water splitting process.Therefore,the development of high efficiency,clean and non-precious electrocatalytic materials is the key issue to improve the technology of hydrogen production by water splitting.Vanadium oxide is one of the most representative non-precious metal oxide materials in TMOs,which owns the advantages of low price,sufficient resources and good conductivity.Therefore vanadium oxide is gradually becoming an ideal alternative to replace of noble metal catalysts.However,the activity and stability of most vanadium oxide electrocatalysts are difficult to satisfy the requirements of industrial applications.The main problems limiting the electrocatalytic property are as follows:the limited electrochemical active area,poor catalytic performance at large current density and the unoptimized electronic structure.In view of these problems,in this paper,a series of vanadium oxide electrocatalyst materials with high active sites and excellent electronic structure were constructed by designing nano-scale arrays with different morphologies,establishing heterojunction,and developing self-supported electrodes in-situ grown on nickel or cobalt foam substrate.Moreover,vanadium-based oxide catalysts showed excellent bifunctional electrocatalyst performances and long-term stability at the industrial-grade current density.Through the regulation of the phase,morphology,interface structure and electronic structure,the mechanism of improving the performance of electrocatalytic reaction is established,which lays an important foundation for large-scale commercial application.The specific research conclusions of this paper are listing as follows:(1)A novel "all in one" V2O5·H2O/NF with unique nanoribbon self-assembled micro-flower array structure was synthesized in-situ on nickel foam(NF)via a mild one-step hydrothermal method.This "one-pot" 3D nanostructure exposed more active sites on the catalyst surface and improved the mechanical stability in the catalytic reaction.Compared with V2O5/NF catalyst without polyaniline,the electronic environment between V and Ni was changed,the valence state was adjusted(from V4+to low-valent ions V3+),and the electronic structure of the vanadium site was optimized.The current densities of 100 mA cm-2 and 1000 mAcm-2 obtained in 1.0 mol/L KOH alkaline solution only required extro-low overpotentials of 113 mV and 330 mV,as well as maintaining the HER catalytic stability for at least 70 h at a large current density of 600 mA cm-2·V2O5·H2O/NF has the potential for large-scale development.(2)The V4O9/NF nanobeam array materials were successfully synthesised by a facile one-step hydrothermal method in alkaline solution at a large current density.The addition of oxalic acid changed the charge of V ions in V4O9/NF,effectively regulated the electronic structure,improved the intrinsic activity and enhanced the adsorption and desorption ability of the intermediates.By constructing the microstructure of 3D self-assembled nanobeams,the catalyst exposed more active sites,increased the electrochemical active area,and accelerated the electrocatalytic reaction kinetics.Therefore,the V4O9/NF catalyst exhibited a high efficiency of industrial-grade hydrogen evolution,and delivered the HER current densities of 100 mA cm-2,500 mA cm-2 and 1000 mA cm-2 only need 222 mV,385 mV and 453 mV in an alkaline medium,respectively,which were better than the current commercial Pt/C/NF electrodes.(3)A new vanadium oxide electrocatalyst material,V6O13/NF nanosheet self-assembled micro-flower array was prepared on NF substrate by a simple one-step hydrothermal method.Results indicated that through controlling the content of vanadium source and the precise introduction of acidic oxidant,the morphology and electronic structure of the catalyst were regulated,and the electrocatalytic active sites were effectively increased and the reaction kinetics was improved.The nano-hierarchical structure of micro-lower assembles by multi-layer nanosheets exposed more numbers of active sites and increases the area contacting with electrolyte,which was more conducive to the rapid escape of H2 bubbles.Therefore,V6013/NF maintained great stability and high hydrogen evolution performance under the large current density.At the current densities of 100 mA cm-2 and 1000 mA cm-2,HER overpotential is very low(125 mV and 298 mV,respectively),together with an extraordinary long-term durability for 90 h at the current densities of 50 mA cm-2,which is better than commercial Pt/C/NF counterpart and most reported vanadium oxide materials recently.(4)CoVOx nanosheet morphology was grown in-situ on the cobalt foam(CF)substrate through a facile one step hydrothermal method.The OER/HER properties of CoVOx/CF nanosheet arrays in alkaline environment were investigated.At the same time,by comparing with CoVOx,C-CoVOx/CF material was synthesized through adding CoCl2·6H2O.Results showed that the extra Co regulates electronic structure and active site,and enhanced the electron transfer between Co and V.Compared with the in-situ CoVOx/CF,the electronic structure of C-CoVOx/CF was more optimized,and the Co-V bimetal synergically enhanced the catalytic performance and improved the intrinsic activity of the material.Therefore,CoVOx/CF exhibited excellent bifunctional catalytic performance in alkaline environments,requiring very low overvoltages of 373 mV to drive the OER current densities of 100 mA cm-2,and maintained the OER stability at least 60 h at the current densities of 150 mA cm-2.At the same time,HER performance was also excellent,at the current density of 100 mA cm-2,the overpotential was only 202 mV,along with the HER extraordinary durability at the current densities of 50 mA cm-2 for at least 50 h.(5)A novel nanoparticle anchored micro-spheres composite electrocatalyst material(Co3V2O8/Co3V/CF)with the unique morphology was prepared by one-step solvothermal method on the cobalt foam as conductive substrate and the methanol as solvent.The electronic structure,morphology and OER/HER properties were regulated by changing or removing the V source.The electronic structure of the material was optimized due to the rich heterojunction structure and the synergistic effect of Co3V2O8 and Co3V,thus improving the intrinsic catalytic activity.Co3V2O8/Co3V/CF drive the high OER current densities of 100 mA cm-2 and 1000 mA cm-2,only requiring the overpotential of 228 mV and 465 mV,respectively and can maintain at least 15 h at the high current density of 200 mA cm-2.Co3V/CF catalyst with better HER performance was prepared by replacing VCl3 to NaVO3.Under the large current density of 100 mA cm-2 and 1000 mA cm-2,the overpotential only are 299 mV and 454 mV,respectively,and the stable operation was at least 50 h at the current density of 100 mA cm-2.