| Hydrogen production by water splitting has the advantages of clean,pollution-free,zero carbon emission and sustainable,which is one of the effective methods to solve the shortage of fossil energy.In order to speed up the slow kinetics of water splitting process and improve the efficiency of hydrogen production,the development of efficient and stable electrocatalysts is the key.The traditional method of preparing powder electrode material is complicated,and the addition of conductive agent or binder leads to the agglomeration of active components,which seriously affects the reaction efficiency.Therefore,the development of self-supported electrode materials can increase the specific surface area,expose the active sites fully,improve the utilization rate of active components,and thus improve the overall activity and stability of the catalyst.Wood is an attractive substrate material for the design and synthesis of self-supported electrocatalysts due to its rich hierarchical porous structure.In addition,wood is a kind of natural biomass composite material with various cell shapes,sizes,complex and diverse arrangement,showing significant anisotropy in structure.It is important to clarify the relationship between anisotropy of wood structure and electrocatalytic performance for the design and development of wood-based self-supported electrocatalysts.This paper took masson pine and poplar wood as the research objects,focusing on the anisotropy of wood structure.Firstly,the macroscopic,microscopic and ultrastructural characteristics of wood’s transverse section of plane,radial section of plane and tangential section of plane were studied.On this basis,three kinds of wood electrode of the cross section plane,radial section of plane and tangential section of plane were prepared,and the influence of structural anisotropy on electrocatalytic performance was further analyzed.At last,the cross section plane,radial section of plane and tangential section of plane of masson pine were served as the substrates to synthesize wood-based electrocatalysts through the same preparation process,respectively.The main research contents and results are as follows:(1)It was found that there were obvious differences in,macroscopic structure,microscopic structure and ultrastructural characteristics between masson pine and poplar by using stereomicroscope,light microscope and scanning electron microscope.The anatomical molecule of masson pine is arranged in regular and simple ways,and are composed of tracheids,wood rays,pits,resin channels,etc.The tracheid size is 26.353±4.791 μm.The diameter of pits is 6.396±0.951 μm,and the length of wood ray is 163.359±50.21 μm and the diameter is 6.668±2.046 μm.The anatomical molecular arrangement of poplar is irregular and complex.It is composed of vessels,wood fibers,axial parenchyma cells,pits,etc.The average diameter of vessels is 57.548± 16.572 μm,the average diameter of wood fibers is 11.605±3.168μm,and the diameter of pits is around 2.642±0.441 μm.(2)After carbonization at 900℃,the carbonized wood of masson pine and poplar still perfectly retain the original three-dimensional structure characteristics of wood.The wood are transformed into amorphous carbon and appeared some graphitized structure,which can meet the basic requirements of most electrocatalytic electrode materials.Although the carbonized wood electrode material itself has a certain inertia and low catalytic activity for hydrogen evolution(HER)and oxygen evolution(OER),there are still existing differences in HER and OER performance of the carbonized wood electrodes of cross section plane,radial plane and tangential plane of masson pine and poplar.The overpotential,Tafel slope,electrochemical impedance and electrochemical active area of the cross section plane wood electrodes of both masson and polar were smaller than those of radial plane and tangential plane wood electrodes.These indicate that the structural difference of cell tissue in different directions of wood has a certain influence on the electrochemical performance.(3)The cross section plane of masson pine(Pine-C-W),radial plane of wood(Pine-RW)and tangential plane of wood(Pine-T-W)with structural anisotropy were used as the substrates to synthesize wood-based composite electrocatalysts.After low temperature hydrothermal reaction,Ni(OH)2 nanosheets with abundant pores were uniformly grown on the surface of Pine-C-W,Pine-R-W and Pine-T-W.Then,a large number of nickel particles(Ni NP)and nickel-nitrogen-carbon(Ni-N-C)nanosheets were immobilized on the wood cell wall after carbonization at 900℃.Thus,wood-based electrocatalysts Ni NP@Ni-NC/Pine-C-CW and Ni NP@Ni-N-C/Pine-R-CW and Ni NP@Ni-N-C/Pine-T-CW were synthesized,respectively.The experimental results show that the difference of wood structure has a certain effect on the morphology,structure composition,specific surface area and electrocatalytic performance of wood-based composite electrocatalysts.Ni NP@Ni-NC/Pine-C-CW showed superior electrocatalytic performance,with 81 mV and 262 mV required for 100 mA cm-2 current density for HER and OER,respectively.While the overpotentials required by Ni NP@Ni-N-C/Pine-R-CW are 142 mV and 383 mV,and Ni NP@Ni-N-C/Pine-T-CW are 154 mV and 399 mV,respectively.To sum up,this paper compared and analyzed the original macroscopic,microscopic and ultrastructural characteristics of masson pine and poplar.Then,based on the anisotropy of wood structure,the cross section plane,radial section of plane and tangential section of plane slices were obtained from three special directions.Their corresponding carbonized wood electrodes were obtained through high-temperature carbonization.The influence of woodstructure on its electrocatalytic performance was analyzed by electrochemical test.The wood based electrocatalysts were synthesized by the same preparation process using cross section plane,radial section of plane and tangential section of plane as the structure substrate.It was found that the difference of wood structure had a certain effect on the morphology,structure composition,specific surface area and electrocatalytic performance of wood-based composite electrocatalysts. |
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