The Design And Synthesis Of Carbon-based Metal-free Electrocatalysts Derived From Biomass Materials For The Application Of Electrocatalytic Reaction

In the hydrogen energy and carbon neutralization scenarios,the transformation of the energy structure puts forward requirements for the innovation of the clean technology chain.Electrochemical energy storage and conversion systems such as fuel cells,water splitting devices and metal-air batteries have attracted much attention because they enable the effective use of renewable energy.The complex and slow oxygen reduction reaction(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)occurring at the internal electrodes often require the use of efficient catalysts to accelerate the kinetic process,so as to improve the energy conversion efficiency.Carbon-based metal-free catalysts are expected to reduce the dependence on expensive precious metals with limited reserves due to their competitive advantages such as high fuel selectivity,good stability and low cost.In the thesis,taking biomass-derived carbon materials as the research object,the preparation and electrocatalytic properties of carbon-based metalfree electrocatalysts were studied.Taking advantage of the structure and composition of biomass precursors and dopants,carbon-based metal-free catalysts with different dimensions and multi-element compositions were prepared from the view of optimizing the pore structure as well as enhancing the intrinsic electrocatalytic activity.The chemical states of active sites were discussed.The influence of the specific surface area and pore structure of the catalyst on the electrocatalytic performance was investigated,providing ideas for the design and synthesis of the carbon-based metal-free catalysts with multifunctional catalytic activity and multi-environmental adaptability.The results achieved are as follows:(1)The in-situ nitrogen-doped hierarchically porous carbon(NHPC)with three-dimensional(3-D)structure was prepared through pre-carbonization,carbonization and activation process using natural and low-cost cattle bone with unique hierarchical structure as the precursor.The specific surface area of NHPC is up to 2182 m2 g-1 and the N content is 4.35 at.%.On this basis,a series of nitrogen and phosphorus co-doped hierarchically porous carbon(N,P-HPC)materials were prepared by co-pyrolyzing NHPC with phytic acid and dicyandiamide as additional P and N sources,respectively,through regulating the pyrolysis temperature and the amount of dopants.The results show that the optimization of N and P co-doping plays an important role in improving the ORR catalytic activity of carbon-based electrocatalysts compared with single doping.When the molar ratio of N to P in the dopant was controlled at 1,the sample prepared at 900? with the optimized content of N(4.46 at.%)and P(3.05 at.%)exhibits excellent ORR catalytic activity.It was found that the halfwave potential of N,P-HPC tested in alkaline electrolyte is 0.853 V,which is 12 mV higher than that of commercial Pt/C catalyst.The kinetic current density of the catalyst is 6.05 mA cm-2 at 0.85 V,1.6 times higher than that of Pt/C.In addition,N,P-HPC electrocatalyst also shows better electrochemical stability and resistance to methanol than Pt/C.Multifunctional electrocatalytic tests reveal the synergistic contribution of P and N atoms in improving the OER and HER electrocatalytic ability for N,P-HPC.(2)In order to further improve the catalytic activity,through adjusting the pyrolysis temperature and the amount of dopant,a series of sulfur,nitrogen and phosphorus ternary-doped hierarchically porous carbon materials(S,N,P-HPC)were prepared by co-pyrolyzing thiourea and N,P-HPC.The oxygen reduction activity of S,N,P-HPC is significantly higher than that of N,P-HPC both under alkaline and acidic conditions,and S,N,P-HPC exhibits excellent electrochemical durability and good anti-poisoning ability.In addition,the OER and HER electrocatalytic activity for S,N,P-HPC is superior to that of N,P-HPC.Results show that the reduced sulfur species doping in the carbon plates which can induce the reconstruction of spin density to create diverse active centers enhance the intrinsic catalytic activity of the carbon materials together with pyridine-N,graphite-N and P-C species.In addition,the 3-D hierarchically porous structure inherited from NHPC and benefitting from the pyrolysis of dopant ensures the high specific surface area(1533 m2 g-1)and developed pores for S,N,P-HPC,which promote the effective mass transfer and the exposure of surface active sites,thus improving the catalytic activity.(3)In order to improve the utilization of effective active sites,twodimensional(2-D)porous carbon materials were designed and prepared by ice/salt double template and red phosphorus assisted methods using watersoluble animal gelatin as carbon/nitrogen source precursor.First of all,the insitu nitrogen-doped ultrathin 2-D porous carbon nanosheets(NCNS)were prepared through controlling the mass ratio of ice to salt template and the carbonization temperature.Relative to the traditional hard templates such as silica and montmorillonite,salt template is eco-friendly and can be removed by washing with water,reducing the damage to the structure of electrocatalyst and the loss of active sites.Results show that the ice template guides the formation of the 2-D structure,while the salt template can not only maintain the integrity of the 2-D structure during the carbonization but confine N atoms and form the mesoscale carbon nanocages rich in defects.The mentioned NCNS was copyrolyzed with red phosphorus,thermogravimetry-mass spectrometry-infrared radiation,pore size and composition analysis show that the sublimation of red phosphorus promotes the activation for carbon nanosheets and the addition of P atoms.The mixture of red phosphorus and NCNS with a certain mass ratio of 3.9 can be heat-treated at 950? to obtain P(1.05 at.%)and N(5 at.%)codoped 2-D ultrathin porous carbon sheets(P-NCNS)with a high specific surface area(500 m2 g-1),exhibiting an optimized electrocatalytic activity.The halfwave potentials of electrocatalytic oxygen reduction for P-NCNS in alkaline and acidic environment are 0.889 V and 0.725 V,respectively.For the OER and HER electrocatalytic activities,the reaction potentials at 10 mA cm-2 are 1.73 V and-0.377 V,respectively.The excellent multifunctional activity and multienvironmental adaptability are profitted from the synergism of active N and P species,the effective utilization of pentagonal carbon defect sites on highly exposed 2-D surfaces,and the promotion of three-phase mass transfer through micro/mesoporous structure.(4)The effects of ammonia etching with simultaneous N-doping on the composition,structure and electrocatalytic properties towards ORR of the biomass-derived carbon materials with 2-D and 3-D structure were researched.The carbon materials treated in ammonia atmosphere at 900? possess increased specific surface area and content of active N species,exhibiting higher electrocatalytic activity for ORR than untreated carbon.DFT results show that pentagon carbon and P-doping defective sites can significantly reduce the overpotential of ORR in alkaline environment,which are considered to be active sites.The performance of the primary zinc-air battery assembled with selfmade carbon-based metal-free electrocatalyst is comparable to that with commercial catalyst.(5)The composition and structure of carbon matrix were controlled by introducing alkali or inorganic salt template,and the synthesis methods of solidphase blending with heat treatment and gas-phase etching with doping were adopted to design and obtain multi-dimensional carbon-based metal-free electrocatalyst through optimizing the mass ratio and the heat treatment temperature.The coupling of multi-doping and the design of both dimension and pore can not only improve the utilization of active sites but promote mass transfer.In addition,the results show that multi-step heat treatment can effectively regulate the active sites and obtain the electrocatalyst with improved apparent activity.