Synthesis And Electrochemical Performance Of Carbon-Based Functional Materials From Rice Husks

Rice husks is one of the largest biomass resources on earth.The annual output of rice husks in China is over 40 million tons according to statistics,which not only ranks first in the world,but also shows an upward trend year by year.However,in most production areas,rice husks is often discarded or incinerated directly as agricultural waste,which has caused a waste of resources to a certain extent.In addition,some traditional applicatons of rice husk in industry have the problem of low added value and utilization rate.Therefore,it is particularly important to realize the resource utilization,enhance the added value and improve the comprehensive utilization rate of rice husks.According to the structural characteristics of rice husks（it is composed of cellulose,hemicellulose,lignin and nano-silica,containing natural nano-silicon source and carbon source,and the surface is rich in functional groups）,a variety of rice husks-derived carbon-based functional materials were designed and prepared,and their properties as supercapacitor and electrocatalytic electrode materials were investigated.The main research contents and innovations of this paper are as follows:1.This paper presents a method for preparing biomass-based porous carbon with high specific surface area.The dense system composed of cellulose,hemicellulose,and lignin was broken by selectively removing the lignin from biomass,and abundant and uniformly distributed micropores with the diameter of 1～2 nm were formed,which optimized the activation process of KOH.The porous carbon prepared from delignified rice husks shows a hierarchically microporous,mesoporous,and macroporous structure,and has a large specific surface area(2892.5 m² g^-1).The electrode material prepared from the porous carbon shows high mass specific capacitance(310.8 F g^-1,at a current density of 1 A g^-1)and good cycle stability（92%capacitance retention after 10000cycles）in a 6 mol L^-1 KOH.2.A method for preparing metal nano-heterojunction/carbon catalysts was developed by removing lignin from biomass to construct a multi-pore structure to achieve the uniform loading of metal salts and to inhibit the agglomeration of metal particles during pyrolysis.The abundant nano-Mo₂N/Mo₂C heterogeneous interface optimizes the composition and structure of the material,and the rapid transfer of electrons and the rapid desorption/adsorption kinetics of active materials are realized at the interface,and shows a good hydrogen evolution performance in the whole p H range.Especially in alkaline electrolyte,the current density of 10 m A cm^-2 is achieved when the hydrogen overpotential was only 118 m V,and the stability is significantly better than that of commercial Pt/C electrode.DFT calculations showed that the catalytic activity of Mo₂N-Mo₂C hetero-interface was better than that of Mo₂C or Mo₂N alone.This low-cost synthesis method has obvious advantages in large-scale production,resulting in great application potential in the field of hydrogen evolution.3.A universal method for preparing metal single-atom catalysts by anchoring metal ions with oxygen-containing functional groups from biomass has been developed.The mechanism is that the lone pair of electrons on the surface of cellulose were coordinated with the empty orbital of the metal ion to achieve atomic-level"capture"of metals.Carbon-based/Pt and Co single-atom catalysts were prepared from lignin-free rice husks.The Pt single-atom catalysts exhibited high hydrogen evolution activity,the current density reaches 10 m A cm^-2 at an overpotential of 36 m V,the tafel slope is only 29.5 m V dec^-1,and the specific activity of Pt was 11 times higher than that of commercial Pt/C.The Co single-atom catalysts have excellent hydrogen and oxygen evolution performance in alkaline solution,the overall water splitting device delivered10 m A cm^-2 current density at a low voltage of 1.57 V and maintained without decay for 24 hours.Moreover,the preparation method of single-atom catalysts has the characteristics such as universality,low cost,simple process and suitable for large-scale preparation,resulting in great application prospects in the field of biomass carbon-based catalysis.