Preparation Of Mushroom-derived Carbon Support And Design Of Catalytic Active Center

Due to world energy shortages and environmental pollution,we urgently need to find cleaner renewable energy fuels to replace fossil fuels,and more efficient energy conversion devices to improve energy efficiency,and to address low utilization and pollution problems from fuel sources and using process.In order to form a complete energy storage and conversion system,hydrogen and oxygen can be first produced by electrolyzing water,and then used as a reaction raw material of a hydrogen-oxygen proton exchange membrane fuel cell to supply clean and efficient power generation equipment.However,energy conversion devices such as fuel cells and electrolyzed water to produce hydrogen require efficient and inexpensive electrocatalytic materials to reduce the reaction barrier,activation energy and overpotential of the electrochemical process,thereby reducing energy loss and improving utilization.Therefore,the development of excellent and stable electrocatalytic materials is of great significance for solving the two major problems of energy utilization and environmental protection.In this paper,biomass-based porous carbon was used as the support,to load the electrocatalytic materials of three different scale active units of oxide nanoparticles,metal-nitrogen-carbon clusters and platinum single atoms,and the regulation of their composition and structure on properties was studied.In the whole research process,the interface structure of carbon-loaded oxide composites was taken as the breakthrough point.The strong interaction and electron transfer between the supported material and the support were analyzed in detail,and the close relationship between newly formed covalent bond at the interface and electrocatalytic properties was obtained.Based on this,we designed high-activity metal-nitrogen-carbon groups according to the host-guest strategy,and through a large number of experimental characterization and density functional theory calculations,we obtained the activity origin of 3d transition metal nitrogen-carbon materials and the design principles of this type of materials.Then,we have prepared a single-atom platinum-nitrogen-carbon multifunctional material by photochemical solid phase reduction method,which can be applied to electrolysis of water to produce hydrogen and fuel cells.This material has high activity and low cost,laying a good foundation for big-scale commercialization.The main results of this study are as follows:(1)After pre-carbonization,carbonization and activation,we have prepared a mushroom-based hierarchical porous carbon with a specific surface area of 3332 m2 g-1.When the mushroom carbon is activated by ammonia gas,the contents of pyridinic-N and graphitic-N are significantly increased,resulting in more active sites.The test results of oxygen reduction performance show that the mushroom carbon has an E1/2 of 0.88 V(vs.RHE),a kinetic current density of 8.51 mA cm-2,a very low H2O2 yield and an oxygen reduction path close to four electrons.After post-doping with protoporphyrin,the E1/2 of mushroom carbon is increased to 0.91 V(vs.RHE).Mushroom carbon has a high specific surface area,abundant heteroatoms and a suitable pore structure,which is beneficial to the deposition of different active units.(2)In situ nucleation and growth under the ethanol system,we prepared a pure phase of lanthanum manganate.Based on the suitable space confinement effect of mushroom carbon,anchoring effect of heteroatoms and large specific surface area,the lanthanum manganate particles with a size of about 4 nm were further uniformly loaded on the carbon,and its ORR activity exceeded Pt/C.The activity enhancement mechanism of oxygen reduction of the carbon-supported lanthanum manganate composite can be described as a strong chemical interaction between the oxide and carbon support interface,forming a C-O-Mn chemical bond and generating electron migration from the substrate to the supported particles.(3)Based on the host-guest strategy,direct pyrolysis of the mushroom-based porous carbon as host and Hemin as guest can construct a three-dimensional iron-nitrogen-carbon(Fe-N/MC)catalyst with rich active sites.STEM,XPS and XANES confirmed Fe atom coordinates with atbfsdjacent four N and eight C atoms to form a Fe-N4-C8 plane structure.The ORR test showed that its E1/2 reaches 0.93 V(vs.RHE),and the E1/2 only negatively shifts by 2 mV after 5000 cycles of cyclic voltammetry.The zinc-air battery based on the Fe-N/MC has an open circuit voltage of 1.5 V,a maximum power density of 99 mW cm-2 and excellent rate performance,and the voltage retention rate is over 90%after continuous discharge for 80 hours at a current density of 25 mA cm-2.(4)Density functional theory(DFT)calculation shows that*OH to form OH-is the theoretical rate-determining step of the ORR process catalyzed by the Fe-N-C,and U=0.87 V is its onset potential.By comparing the ORR free energy level diagrams of some 3d transition metals,the onset potentials of the Fe,Co,and Cu-based samples are consistent with the order of oxygen reduction activities.By calculating the Mulliken charge of the d-band center of the metal and its adjacent C atoms,the positive charge on the surface of the active C atom increases with the increase of the d-band center of the doped Cu,Co and Fe,and the catalytic activity of the sample is more high.(5)Highly loaded and well-dispersed Pt single atoms on NPC(Pt1/NPC)was prepared by photochemical solid phase reduction.The XPS,XAFS and DFT results give clear evidence of the formation of a pyridine-type Pt-N4 coordination sites in the catalyst.In the electrocatalytic activity test,the sample exhibited excellent electrochemical performance for HER.At 10 mA cm-2,its overpotential was 25 mV(vs.RHE).Under this overpotential,the specific mass activity of Pt1/NPC is 2.86 A mg-1pt,which is 24 times that of Pt/C(0.12 A mg-1Pt).At the same time,the Pt1/NPC sample showed good ORR activity.