Controllable Preparation Of Biomass-Based Porous Carbon Materials For Electrocatalytic Oxygen Reduction And Hydrogen Evolution Reactions

In order to promote the construction of modern energy system and alleviate the contradiction between rapid economic development,energy crisis and environmental protection,energy cleanliness has become a research hotspot.Based on this,energy storage and conversion devices have emerged.Fuel cells as the core device can lead the development of hydrogen energy and efficiently convert hydrogen energy into electricity.At the same time,how to produce hydrogen in a cost-efficient and green way is the key to securing the development of new energy sources.Electrolytic water process is an effective way to achieve hydrogen production.In order to achieve a high level of energy recycling,it is necessary to reduce the overpotential of the electrode reactions associated with fuel cells and water electrolysis device.Therefore,the development of highly efficient electrocatalysts to reduce the reaction energy barriers of the oxygen reduction reaction（ORR）at the cathode of fuel cells and the hydrogen evolution reaction（HER）at the cathode of water electrolysis devices has become a major research objective.Biomass-based carbon materials from a wide range of sources have unique physicochemical properties,such as controllable structure,good electrical conductivity and corrosion resistance.After modification,they can not only be used as catalytic active phases,but also be used as composite phases to synergistically improve catalytic performance,thus attracting much attention in the field of electrocatalysis in recent years.The natural structural characteristics of biomass make it an ideal carbon precursor and the development of biomass modification tools is necessary to achieve a high value conversion of biomass.In this thesis,chemical,biological and physical methods were used to modify the surface,interior and whole of the biomass-based materials,and the modification principle is studied in depth,respectively.A series of carbon-based electrocatalysts with excellent performance were constructed by finely regulating the pore structure and active sites of porous carbon derived from biomass.Combined with physicochemical characterization,the relationship between the microstructure and electrocatalytic performance of carbon materials was investigated from a structure-property perspective.The specific research contents of this thesis are as follows:（1）The surface of biomass was modified by chemical degreasing method to synthesize high efficiency ORR electrocatalyst.The"lotus effect"on the surface of biomass prevents hydrophilic chemicals from binding to biomass.The saponification reaction can change the micromorphology of biomass,affect the surface functional groups of materials,and then promote the accessibility of biomass with activators and nitrogen dopants.Loquat leaves were used as carbon source and degreased with 1%Na OH.The contact angle test showed that the biomass surface changed from hydrophobic to hydrophilic structure（contact angle changed from 103.9°to 70.28°after degreasing）.Then the degreased loquat leaves were mixed with activator and nitrogen dopant,and the nitrogen-doped micro/mesoporous carbon-based catalyst with large specific surface area(1654.5 m² g^–1)and rich nitrogen atom content（4.51 at%）was successfully prepared.The catalyst exhibited excellent oxygen reduction activity with onset potentials of 1.01 and 0.85 V vs.RHE,half-wave potentials of 0.87 and 0.71 V vs.RHE and limiting current densities of 5.48 and 6.14 m A cm^–2 in alkaline and acidic electrolytes,respectively.In addition,by selecting different biomass（persimmon leaves,papaya peel and kapok fiber）as carbon sources,it was further verified that the chemical degreasing method has biological universality for enhancing the oxygen electrocatalytic activity of carbon-based nanomaterials.（2）The catalyst with excellent ORR activity over a wide p H range was prepared by using the dissolution effect of enzymatic hydrolysis to modify the internal structure of biomass.In view of the dense lignocellulosic structure inside the biomass,hemicellulase or cellulase was used to react with cypress branches and leaves.The methylene blue adsorption experiment was used to quantize the adsorption level of the modified biomass,which effectively proved that the enzyme hydrolysis method has a loose effect on the lignocellulosic skeleton:the methylene blue adsorption capacity of the enzymatically treated biomass was 16.87 mg g^-1（hemicellulase）and 19.48 mg g^-1（cellulase）,respectively,which was significantly higher than that of the non-enzymatically untreated biomass(9.82 mg g^-1).By comparing biochar and nitrogen-doped porous carbon nanomaterials,it was clearly found that biological enzymes expanded the pore size within the biomass,and the abundant pore space allowed the potassium salts and nitrogen dopants to be more efficiently embedded in the carbon skeleton,improving the efficiency of nanopore formation and nitrogen doping,and thus improving the electrocatalytic performance of oxygen reduction.The electrocatalytic ORR performance of the enzyme-modified biomass-based nanoporous carbon materials were superior to that of the non-enzyme-treated materials in alkaline,neutral and acidic electrolytes,and even comparable to that of commercial Pt/C.Thus,this work opens up new avenues for loosening the internal structure of lignocellulosic biomass and improving the electrochemical performance of biomass-based materials from a“green”perspective.（3）The effect of ultrasonic cavitation on the whole biochar was investigated and prepared p H-universal ORR electrocatalysts.Using coconut leaves as the carbon source,melamine as the nitrogen source and KOH/KHCO3 as the activators,the carbon-based materials with hierarchically porous structure and high-level nitrogen doping were synthesized by ultrasonic-assisted method.The pulsed waves,microjets and free radicals formed by ultrasonic cavitation are beneficial to carbon layer exfoliation,promoting interfacial mass transfer and accelerating the mixing of microscopic molecules,which have a positive impact on activation pore formation and nitrogen doping.In addition,the effects of activation temperature,nitrogen dopant and activator on the electrocatalytic activity of the carbonaceous materials were investigated in depth in this work.The optimal carbon-based catalysts successfully synthesised were rich in micro/mesoporous structure with the highest density of pyridinic-N and graphitic-N per unit surface area(0.015 at‰g m^–2).The onset potentials of this material in alkaline,neutral and acidic media were 1.01,0.91 and 0.84 V vs.RHE,and the half-wave potentials were 0.87,0.74 and 0.66 V vs.RHE,respectively.The ORR activity of this material was superior to that of 20%commercial Pt/C.（4）Biomass-based nitrogen-doped porous carbon was used as carbon carrier to support metal active sites,realizing the multifunctional application of porous carbon and successfully synthesizing efficient HER electrocatalyst.Firstly,different transition metal-based(Co²⁺,Mn²⁺,Ni²⁺,Cu²⁺)HER materials were prepared to investigate the differences in HER properties among different transition metals,and the electrocatalyst with the best HER properties was screened out（C-N-Co@NPC）.In addition,C-N-Co@NPC was compared with pure phase cobalt-based nitrides（C-N-Co）to explore the specific efficacy of porous carbon substrates in the preparation of highly active HER materials.Electrochemical impedance tests revealed that C-N-Co@NPC has a more rapid electrocatalytic kinetics than C-N-Co.The large specific surface area of nitrogen-doped porous carbon and the abundance of nitrogen atoms inside contribute to anchoring and dispersing Co metal atoms on the carbon framework,increasing the effective active sites for hydrogen evolution.Its inherent high electrical conductivity accelerates mass and charge transfer during hydrogen production catalysis.In 0.5 M H2SO4 and 1 M KOH,C-N-Co@NPC had overpotentials of 186 m V and 160 m V,respectively.Furthermore,the catalytic performance of the catalyst did not decrease significantly after continuous operation for 15 h,and the current remained stable,demonstrating the potential to replace commercial Pt/C.