Preparation Of Carbon-based Electrocatalysts From Microalgae Residue After Lipid Extraction And Nitrogen Transformation And Directional Regulation

As an important renewable energy conversion device to solve the dual needs of environment and energy in the future,one of the important problems limiting its conversion efficiency is the slow dynamics of oxygen reduction reaction（ORR）at the cathode of the cell,especially in the neutral medium of microbial fuel cells（MFCs）,the introduction of electrocatalysts is needed to promote the ORR.Because platinum based electrocatalysts is limited by its high price,it is difficult to develop.Many researchers have developed high ORR activity and low-cost carbon-based electrocatalysts.Nitrogen doped biomass-based carbon materials has been widely used because of its low cost,controllable structure and good electrochemical stability.The carbon electrocatalysts prepared by direct pyrolysis of nitrogen rich biomass can be doped in carbon materials with simple and pollution-free nitrogen atoms by virtue of its own composition and structure.However,the transformation of nitrogen in the process of preparation of N-doped biomass-based carbon materials is unclear,and the mechanism of nitrogen doping on the catalyst of carbon electrode is still controversial.Therefore,it is of great significance to clarify the regulation and transfer of nitrogen in nitrogen doped biomass-based carbon materials,which is of great significance to the preparation of low cost and high-performance carbon electrocatalysts.In this paper,microalgae（MA）and microalgae residue after lipid extraction（MR）were selected as nitrogen rich biomass,and nitrogen self-doped biomass-based carbon materials was prepared by one-step pyrolysis.Through the process parameters such as the pretreatment method of lipid extraction,pyrolysis temperature and heating rate,the different performance of the biomass carbon materials were obtained.The morphology and structure of biomass raw materials and biomass-based carbon materials were characterized,the transformation of nitrogen in the preparation process were explored,and the occurrence of nitrogen was analyzed.The mechanism of nitrogen doping on the ORR performance was analyzed by the electrocatalytic performance test,and the cathode loaded on MFCs was analyzed and tested.Finally,a new type of nitrogen conversion-oriented control method was formed to obtain high performance carbon electrocatalysts.The main conclusions are as follows:（1）The protein content of MA increased after lipid extraction pretreatment.The nitrogen content of residue was 8.31 at%,and the nitrogen rich effect was obvious.The lipid extraction process can make the raw material structure looser,the pore structure of pyrolysis product is more abundant and the specific surface area is larger.In addition,the lipid produced as a high value by-product is an important raw material for biodiesel production.After removing lipids,the content of pyridinic-N and graphitic-N increased,which improved the active site of ORR,and showed excellent ORR performance.In MFCs system,it has better stability and power production performance,and the maximum power density reaches 412.85 m W m^-2.The excellent properties can be attributed to the synergistic effect between its high pyridinic-N（27.8%）,graphitic-N（40.3%）content and abundant multi-stage pore structure.（2）Temperature has an effect on the content and the occurrence of nitrogen.With the increase of pyrolysis temperature from 700℃to 900℃,nitrogen elements gradually separated from carbon skeleton,and nitrogen content decreased.The relative content of pyrrolic-N decreased first and then increased,while the relative content of pyridinic-N increased first and then decreased,while the relative content of graphitic-N continued to increase.This is mainly because in the process of MR pyrolysis,N in protein gradually changes to pyrrolic-N and pyridinic-N with the increase of temperature.With the increase of temperature,pyrrolic-N,pyridinic-N and graphitic-N will change each other.Among them,the thermal stability of graphitic-N is the largest,and the secondary cracking reaction will also cause some n elements to lose gas production and logistics,such as NH₃,HCN,etc.,but these gas states of the products will have a certain self-activation pore making effect on biomass-based carbon materials.The biomass-based carbon materials prepared under the coupling of active sites and pore structure at 800℃has the best ORR performance.Among them,the carbon structure defects caused by nitrogen doping and the active sites provided by pyridinic-N and graphitic-N are beneficial to the ORR activity and catalytic ability.Graphitic-N is beneficial to improve the conductivity of biomass-based carbon materials,and is conducive to the ORR to choose a high-efficient 4e^-reaction pathway;pyrrolic-N makes the ORR pathway tend to 2e^-route,and produces H₂O₂ which is not conducive to the stability of MFCs.（3）The heating mode has an effect on the nitrogen content in biomass-based carbon materials.The increase of heating rate can keep more nitrogen elements in the carbon skeleton,but the relative content of pyridinic-N and graphitic-N will decrease,which will increase the pyrrolic-N content,resulting in the reduction of graphitization degree of the product,and the ORR reaction will be carried out in the direction of 2e^-and the rapid heating rate accelerates the removal of gas products,reduces its self-activation effect,thus making its specific surface area have some degree Based on this,the pyrolysis method of gradient heating is proposed.Through TG-FTIR analysis,the N-gas in the process of heating is mainly lost before about 500℃,and after 500℃,it is mainly nitrogen loss caused by the secondary cracking of biomass-based carbon materials.The higher heating rate before 500℃has obvious effect on nitrogen content,and the higher heating rate promotes the transformation of nitrogen in protein to pyrrolic-N in carbon products,and increases the content of pyrrolic-N.The temperature rising rate decreased after 500℃was beneficial to the transformation of pyrrolic-N to pyridinic-N and graphitic-N.According to the electrochemical test,the initial potential of mrc-5-3 is 0.223 V（vs.SCE）,the maximum output voltage of MFCs reaches 561 m V,and its maximum power density is 843.6 m W m^-2.It is a potential catalyst for the cathode generation of MFCs.