Oriented Construction And Properties Of Wood-based Three-Dimensional Porous Electrocatalytic Materials

Wood is an abundant natural,renewable,and degradable polymer material with an exquisite three-dimensional(3D)hierarchical porous structure and rich surface active functional groups and carbon sources.Hence,it is an ideal material for the preparation of electrolytic water catalysts with large specific surface area,good electrical conductivity,excellent catalytic activity,and stability.Producing hydrogen by water splitting has many advantages such as zero emission,zero pollution,and high purity of the produced hydrogen.Therefore,this technique is beneficial for alleviating energy crisis and solving environmental problems.With a view to improve the efficiency of hydrogen production by water electrolysis,the preparation of efficient catalysts has become the focus of research.At present,the most effective catalysts are precious metals and their compounds;however,their high price and scarce reserves limit their large-scale utilization.In addition,the preparation process of traditional non-noble metal catalyst is complicated.The substrate is derived from petrochemical materials,which has the disadvantages of high cost,high energy consumption,and heavy contamination.Further,the interface coupling of the prepared catalyst is poor.In this study,considering the challenges of complex preparation processes of traditional electrocatalytic materials,expensive substrate materials,and poor surface-interface coupling,the use of wood as the substrate supporting material as was explored.First,the effects of pore structure and carbonization temperature on the electrocatalytic performance of wood were studied;based on the results,non-noble metal electrocatalytic materials with lamellar structures and uniformly dispersed particle structures were synthesized from wood by taking advantage of its natural structure,surface activity,and abundant carbon sources.The main research tasks performed in this work,and the results obtained are as follows:(1)Three-dimensional(3D)porous carbon material was obtained by carbonizing poplar(Populus cathayana Rehd.),balsa(Ochroma lagopus Swartz.),pine(Pinus sylvestris var.mongolica Litv.),and fir(Cunninghamia lanceolata(Lamb.)Hook)at 900℃.The morphology,degree of graphitization,electrical conductivity,and electrocatalytic performance of different carbonized woods were compared.Among the four types of carbonized woods,porous poplar carbon had a rich pore structure,moderate pore size,and good electrical conductivity along with the best catalytic activity.The effect of carbonization temperature(600,700,800,900,and 1000℃)on the catalytic activity of poplar wood was examined.The electrocatalytic activity of the porous poplar wood carbonized at 900℃ was found to be the best.(2)The abundant surface functional groups of poplar wood allow the uniform adsorption of a large amount of metal ions.After carbonization,the 3D porous structure of the wood was retained,and the nickel-iron alloy was wrapped on the wood substrate(NiFe/CW).Furthermore,nickel-iron hydroxides(NiFe-LDHs)were synthesized on NiFe/CW by a hydrothermal reaction,and a self-supporting oxygen-evolution reaction catalyst(NiFe-LDHs@NiFe/CW)with a 3D porous structure and heterostructure was successfully prepared.Because of the abundant heterointerfaces,optimized electronic configuration,and hierarchical pores,the NiFe-LDHs@NiFe/CW has remarkable electrocatalytic activity and stability toward OER with a low overpotential of 212 mV at 50 mA·cm-2 and an insignificant increase in the potential(by 4.0%)at 50 mA·cm-2 after 100 h.Density functional theory calculations further reveal that the heterostructures can optimize the electronic structure and have the lowest adsorption energy,thus accelerating the catalytic kinetics.(3)The abundant pore structure of wood enables the absorption of a large amount of molybdenum ions.During high-temperature carbonization,the 3D hierarchical porous structure of the wood was retained;carbon-containing gas was released and underwent a reduction reaction with molybdenum ions;and β-phase molybdenum carbide with excellent catalytic activity was grown in situ in the pores of the wood substrate(Mo2C/CW).The molybdenum carbide prepared by this simple and environment-friendly method was tightly anchored to the carbon and was extremely small.HER catalyst Mo2C/CW with a 3D porous structure and large specific surface area had the best hydrogen evolution reactivity at a loading concentration of 0.5 mol·L-1.Under alkaline conditions,the current density reached 50 mA·cm-2 at an overpotential of 93 mV,and the corresponding Tafel slope was 62 mV·dec-1.After stability testing after 100 h,the voltage only increased by 2.6%,which indicates excellent stability.In addition,the Mo2C/CW catalyst still has high HER activity in an acidic electrolyte solution.A current density of 50 mA·cm-2 can be achieved at an overpotential of 130 mV,with a corresponding Tafel slope of 65 mV·dec-1.After stability testing for 100 h,the voltage only increased by 4.0%.(4)The prepared NiFe-LDHs@NiFe/CW and Mo2C/CW self-supporting catalysts were assembled as the anode and cathode,respectively,for water splitting.The results show that the NiFe-LDHs@NiFe/CW ‖ Mo2C/CW system needs an external voltage of only 1.59 V to reach a current density of 50 mA·cm-2;this voltage requirement is much less than that of the noble metal system(1.81 V),indicating the excellent electrocatalytic activity of the wood-based self-supporting electrocatalyst.Owing to the natural structure of wood and the abundant surface active functional groups and carbon sources in wood,the prepared self-supporting wood-based electrocatalytic materials have a large specific surface area,abundant active sites,good electrical conductivity,and strong structure.Thus,by using wood-based materials,the problems,such as poor dispersion and poor interface coupling,in the use of traditional catalysts can be overcome.The proposed wood-based material has excellent catalytic activity and stability.This research provides an idea for the comprehensive and effective utilization of sustainable wood resources for preparing high-activity self-supporting electrocatalytic materials,thereby reducing environmental pollution.