The Structure Regulation And Electrocatalytic Performance Of Wood Cellulose Nanofibrils Based Transition Metal Composites

With the increasing shortage of fossil-resources and the pursuit of green and environmental friendliness,designing and developing biomass-based functional materials based on the abundant agricultural and forestry biomass resource is one of the important research contents of new materials.Cellulose nanofibrils(CNF)can be obtained using nature wood fiber treated by TEMPO-oxidized and high-pressure homogenization.The obtained CNF have the properties of entangled nanostrucutre,high aspect ratio,biocompatibility,degradability and renewability.Meanwhile,CNF has the characteristics of natural fibers and nanomaterials.And the high activity surface of CNF with abundant carboxyl and hydroxyl groups provides the structural and chemical basis for the application of CNF in the fields of structure design,size control and electronic structure regulation of functional materials,which can expand the application of wood in the fields of energy storage,catalytic conversion,bionic response materials and other functional materials.On the other hand,under the background of environment pollution and energy crisis,electrocatalytic water splitting for the hydrogen production has attracted much attention.Although numerous of low-cost transition metal catalysts have been developed and applied in the field of electrocatalysis,the transition metal materials have the problems of easy agglomeration of particles,low conductivity and low catalytic activity.Therefore,how to effectively improve the electrocatalytic performance and efficiency of transition metal catalysts is an important basis for achieving large-scale electrocatalytic hydrogen production.In this paper,the multi-dimensional morphology of transition metal catalysts were regulated by CNF to improve the existing problems of transition metal catalysts.The effects of CNF on the microstructure,physicochemical properties and electrocatalytic performance of transition metal catalysts were studied.The enhancing synergistic mechanisms of CNF on electrocatalytic activity of transition metal catalysts were further discussed,aiming to provide theoretical basis and methods for expanding the application of CNF in the fields of electrocatalysis and electrochemistry,and provide a novel method for the preparation of renewable and"green"electrocatalytic materials based on CNF.The main research contents and conclusions are as follows:(1)CNF suspension was prepared by TEMPO oxidation and high pressure homogenization.Cellulose trend to aggregate and precipitate with TEMPO oxidation pretreatment,and CNF can form stable suspension with further high-pressure homogenization treatment.With the increase of homogenization treatment times,the diameter of CNF decreased,and the CNF suspension became transparent.There are abundant carboxyl and hydroxyl groups on the CNF surface.With the increasing of Na Cl O addition amount during the TEMPO oxidation pretreatment,the carboxyl content on CNF surface increased.The CNF aerogel was prepared with a lamellar porous structure after vacuum freeze-drying,and the CNF carbon aerogel maintained the entangled porous structure.The electrocatalytic oxygen evolution(OER)activity of CNF carbon aerogel was low.(2)Two-dimensional CNF transition metal composites were prepared by hydrothermal and calcination method.Co Fe nanoparticles were anchored and dispersed on the CNF nanocarbon through the chemical adsorption and physical entanglement and the self-assemble of CNF,forming the two-dimensional transition metal composite.With the addition of CNF,the agglomeration of transition metals was avoided effectively,and the size of Co Fe nanoparticles was reduced remarkably.As a result,the specific surface area of resulted Fe-Co S/NC catalyst was increased significantly,which was 7.3 times higher than that of Fe-Co SN catalyst.The addition amount of CNF affected the OER performance of resulted catalysts remarkably.The Fe-Co S/NC catalyst exhibited superior OER performance when the addition amount of CNF was 30%.The overpotential of the as-synthesized Fe-Co S/NC catalyst was 257 m V to deliver a current density of 10 m A cm^-2with a Tafel slope of 46.7 m V dec^-1in 1.0 mol/L KOH electrolyte,outperforming the commercial Ru O₂and other reported Co Fe based catalysts.The Fe-Co S/NC catalyst exhibited excellent stability,which the overpotential to deliver a current density of 10 m A cm^-2was only increased by 2.82%after 50 h chronopotentiometry measurement.(3)Based on the two-dimensional structure,the three-dimensional porous CNF based transition metal composite(Ni Fe Cr P/NC)was further designed by hydrothermal method and calcination method using CNF as the reaction platform.The agglomeration of transition metal nanoparticles was avoided effectively with the addition of CNF.The resulted Ni Fe Cr P/NC catalyst exhibited porous structure,and the size of transition metal nanoparticles was reduced when the CNF concentration was 0.3%.Moreover,the specific surface area of prepared Ni Fe Cr P/NC catalyst was increased significantly,which was 9.7 times of that of Ni Fe Cr P catalyst.When the CNF concentration was 0.3%and the addition amount of CNF was 30%,the as-synthesized Ni Fe Cr P/NC catalyst exhibited superior OER performance.The overpotential of Ni Fe Cr P/NC catalyst was 249m V to deliver a current density of 10 m A cm^-2with a Tafel slope of 51.2 m V dec^-1in 1.0 mol/L KOH electrolyte,outperforming the commercial Ru O₂.The conductivity of Ni Fe Cr P/NC catalyst increased and the electrochemical impedance(EIS)decreased significantly.The electrochemical active surface area of Ni Fe Cr P/NC catalyst increased significantly,which was 4.7 times of that of Ni Fe Cr P catalyst.The Ni Fe Cr P/NC catalyst exhibited excellent performance and structure stability,which the overpotential to deliver a current density of 10 m A cm^-2was only increased by 2.5%after 40 h chronopotentiometry measurement.(4)The three-dimensional porous M3PCA transition metal catalyst was synthesized by hydrothermal and phosphating method using CNF as the sacrificial template and dispersant.The resulted M3PCA catalyst exhibited three-dimensional porous structure with abundant pores and cracks on the surface.With the addition of CNF,the agglomeration of transition metal nanoparticles was avoided,and the size of transition metal nanoparticles were reduced.Moreover,the specific surface area of the M3PCA catalyst was increased,which was 2.7 times higher than that of M3P catalyst.the M3PCA catalyst exhibited superior OER and hydrogen evolution reaction(HER)performance in the 1.0 mol/L KOH electrolyte.The overpotentials of M3PCA catalyst were 265 m V and 186 m V for OER and HER with the Tafel slopes of68.0 m V dec^-1and 73.8 m V dec^-1.When the M3PCA catalyst was used in overall water splitting as a bifunctional catalyst,it needs only 1.58 V to deliver a current density of 10 m A cm^-2,which is 110 m V lower than that of M3P catalyst,outperforming other reported bifunctional catalysts.The M3PCA catalyst exhibited excellent stability,which the overpotential to deliver a current density of 10 m A cm^-2was only increased by 1.8%after 40 h electrocatalytic water splitting reaction.(5)The synergistic mechanism of CNF as a reaction platform and template on transition metal catalyst was discussed from physical-chemical dispersion mechanism,high activity surface synergistic mechanism and electron efficient transfer mechanism.The transition metal nanoparticles were dispersed and anchored by the physical-chemical anchoring and dispering of CNF,providing a foundation for the synthesis of two-dimensional and three-dimensional porous CNF-based transition metal composites.The transition metal catalysts prepared using CNF as the reaction platform or using CNF as the template and dispersant exhibited smaller nanoparticle size and increased specific surface area,which accelerate the disperse of transition metal nanoparticles,avoided agglomeration and exposed more catalytic active sites.All these endow the synthesized catalysts high active surface,promoting the catalytic reaction efficiency.At the same time,the electrochemical impedance of the catalyst was significantly reduced,which promoted the efficient transfer of electron.The research results provide ideas for the dispersion of transition metal nanoparticles,which promotes the development of green catalysts.Moreover,the results also provide ideas and methods for the dispersion of metal nanoparticles,inorganic nanoparticles and organic polymers,as well as the synthesis of specific morphologies,expanding the application fields of CNF and wood resource,which meets the needs of green development.