Preparation And Catalytic Performance For Oxygen Reduction Reaction Of Boron-doped Carbon Materials

The increasingly severe energy crisis and the environmental pollution make the development of fuel cells,a new green energy technology,inevitable.Fuel cells have many advantages,such as high energy conversion efficiency,low environmental pollution,low noise,light weight and so on.It has great application prospects in the field of fuel cell vehicles.Cathode catalysts are the key materials of fuel cells.However,the traditional commercial Pt/C catalyst is not only expensive and low in reserves,but also sluggish in oxygen reduction reaction,poor stability and poor methanol tolerance,which greatly restrict the large-scale commercial application of fuel cells.In this dissertation,the research aim is to develop high performance Pt-free catalysts.The emphasis is to further improve the catalytic performance towards oxygen reduction reaction of boron doped carbon materials.We have done this in two ways by improving the intrinsic activity and increasing the specific surface area of the catalysts,and by developing a suitable preparation method and optimizing the doping process.The main research results are as follows:(1)A series of boron-free pure carbon materials with different content ratios of sp2-C to sp3-C(sp2/sp3 value)were prepared utilizing hot filament chemical vapor deposition technique by changing the volume ratio of methane to hydrogen.When the sp2/sp3 value increases to 3.21,the oxygen reduction peak potential of boron-free pure carbon materials gradually shifts to-0.273 V,and then remains unchanged;The peak current density and the transferred electron number increase first and then decrease,respectively reaching the maximum values of 1.825 mA/cm2 and 2.76(-0.6 V)at the sp2/sp3 value of 3.21.Thus,it is of great significance to optimize the content ratio of sp2-C to sp3-C in boron-free carbon-based materials,i.e.,to regulate the microstructure of carbon materials,in order to enhance the catalytic activity for oxygen reduction reaction.(2)Boron-doped diamond materials with different doping concentrations were prepared by changing the addition of boron source using hot filament chemical vapor deposition technique.The electrochemical performance tests show that the oxygen reduction peak potential of all boron doped diamond materials remains at-0.790 V,while the peak current density increases gradually with the increase of boron-doped concentration,and reaches the maximum value of 0.54 mA/cm2 at the boron-doped concentration of 3.037×1020 cm-3,and then begins to decrease.Raman analyses show that the diamond is not graphitized,excluding the effect of sp2 hybrid carbon on the electrocatalytic performance of boron doped diamond.The variation of peak current density with boron-doped concentration is attributed to a combined effect of the increase of defects and the decrease of(220)crystal plane.Compared with the electrocatalytic performance of boron-free pure carbon materials,it is found that the intrinsic activity of boron-free carbon-based materials has a more significant effect on catalytic performance than the doping.(3)Fixed the volume ratio of methane to hydrogen corresponding to sp2/sp3 value of 3.21,the catalysts with controllable specific surface area were successfully synthesized by the method combining hot filament chemical vapor deposition technique with air oxidation etching.The effect of oxidation parameters on the catalytic performance of boron doped carbon materials for oxygen reduction was systematically studied.It is found that with the increase of oxidation temperature,the specific surface area of boron-doped amorphous carbon catalytic materials oxidized for 60 min at 380?470 ? increases first and then decreases,and the maximum value is obtained at 450?;The oxygen reduction peak potential increases gradually;The peak current density and transferred electron number increase first and then decrease,and reach the peak value at 450?,corresponding to the largest specific surface area of the catalytic material.The peak potential,peak current density and transferred electron number of boron doped amorphous carbon catalysts oxidized at 450? for 15?75 min all increase first and then decrease with the prolongation of oxidation time,and respectively reach the maximal values of 0.285 V,4.129 mA/cm2 and 3.24(-0.6 V)at 45 min.The catalytic performance is better than that of boron-free pure carbon material with sp2/sp3 value of 3.21.(4)The boron doped carbon films were deposited directly on the foam nickel substrates with different porosities using hot filament chemical vapor deposition technique.It is found that that the porosities(52.2%?57.5%)of prepared porous boron doped carbon catalytic materials are proportional to that(80%?95%)of substrates.With the increase of the porosity of catalytic materials,all oxygen reduction peak potentials are at-0.32 V,while the peak current density increases gradually,which has a positive correlation with porosity.The peak potential is mainly determined by the nature of the material,while the peak current density is mainly determined by the surface state(surface active sites,porosity,etc.)of the catalyst.The limiting current density and the onset potential of the catalytic material with the biggest porosity are higher than those of other samples,showing the highest catalytic activity.The relative current of the porous boron doped carbon catalysts with the porosities of 52.2%?57.5%is maintained in the range of 94.1%?86.9%after 36000 s.According to the influence of porosity on the catalytic activity,it can be seen that the stability and the catalytic activity for oxygen reduction changed in the opposite direction with the increase of porosity.(5)Boron doped carbon materials with controllable boron doped content were synthesized on the nucleating agents using liquid phase method by adjusting the B2O3 concentration in the methanol reaction solution containing nucleating agent and B2O3 dopant.There are two boron-containing compounds of BC3(B-C)and BC2O(O-B-C)in the synthesized materials,proving that boron atoms have been successfully doped into the structure of the deposited carbon.Oxygen atoms can easily lead to the inactivation of B-C catalytic sites,so that B-C bond is the effective bonding form instead of O-B-C bond.With the increase of the B2O3 concentration,the boron doped content increases gradually at first and then decreases,reaching the maximal value of 0.53 at.%when the concentration is 23.94 mol/mL.The variation of specific surface area is consistent with the boron doped content,and the peak value is 18.476 m2/g when the concentration is 14.36 mol/mL.Among the prepared boron doped carbon materials,the catalyst prepared at the B2O3 concentration of 14.36 mol/mL has boron doped content of 0.47 at.%and specific surface area of 18.476 mn/g.Its oxygen reduction peak potential,peak current density,onset potential and current density at-0.6 V are-0.296 V,2.76 mA/cm2,-0.016 V and 3.98 mA/cm2,respectively,showing the best oxygen reduction catalytic activity.This result is attributed to the synergistic effect of boron doped content and specific surface area.(6)To solve the deactivation of B-C catalytic sites caused by oxygen atoms bonding with boron atoms,a two-step anaerobic solution combining hot filament chemical vapor deposition technique and inert gas protected heat treatment is proposed.Substance BC2O(O-B-C)is not detected in the prepared boron doped carbon materials,indicating that the method effectively solves the problem of oxygen atoms occupying B-C active sites.Within two hours of annealing,with the prolongation of annealing time,the boron doped content in boron-doped carbon materials increases from 0.35 at.%to 1.11 at.%.If the time is too long,the B-C bond in the catalysts is converted into the B=C bond,causing the catalytic activity for oxygen reduction of the catalysts to begin to decay.The catalyst synthesized by annealing for 2 h exhibits the best catalytic performance for oxygen reduction.The oxygen reduction peak potential,the peak current density,the onset potential and the current density at-0.6 V is-0.368 V,1.05 mA/cm2,-0.063 V and 2.03 mA/cm 2,respectively.The boron doped content of the catalyst is 1.11 at.%,which is higher than 0.47 at.%of the sample prepared by liquid phase method at the 14.36 mol/mL of B2O3,but its catalytic activity is slightly poor.This shows that boron doped content is not the only factor affecting the catalytic performance,and the specific surface area also plays a non-negligible role.