Structural Regulation Of Fe-N/C Catalysts And Its Performance For Oxygen Reduction Reaction

At present,with the increasing demand for portable mobile devices such as notebook computers and mobile phones,higher requirements have been put forward for clean and efficient power supply equipment.Direct formate fuel cell（DFFC）has attracted extensive attention from scholars at home and abroad because of its unique advantages,such as room temperature start-up,stability,high efficiency,non-toxic,and ease of storage and transportation.However,the expensive ion-exchange membrane and high-load noble metal cathode catalyst in the DFFC structure make the high cost of DFFC.In recent years,a large number of studies have shown that cheap carbon-based catalysts are highly selective for oxygen reduction（ORR）,which can reduce the cost of the cathode while achieving membrane-less DFFC,which provides the possibility for its economical and commercialization.Nevertheless,carbon-based ORR catalysts still have challenges such as limited exposure to the active site,easy aggregation,poor performance,and difficult pore size control.To solve these problems,based on multiple interdisciplines such as electrochemistry and materials science,this article aims to improve the performance of carbon-based ORR catalysts,then analyze the synthesis,pore size control,and regulation mechanism of nanocarbon,and the transport characteristics of air cathode in membrane-less DFFC.The main contents include:（1）Porous nano carbon shells were obtained by using sodium 3-hydroxybutyrate as the carbon source in one step,and the pores were controlled by thermal process control method to prepare thin-walled nano carbon shells with hierarchical porosity.The effects of different pore distribution on active site loading and ORR activity of carbon nanoshells were investigated.（2）The formation mechanism of the morphology and pores of the nano carbon shell support and the principle of controlling the pores by the thermal process control method were clarified.By comparing the microscopic morphology and pore distribution of the carbon support obtained by varying heating rates at different temperature stages,the effective temperature range of low heating rate was clarified,which improved the preparation efficiency of the carbon support.（3）The nano carbon shell ORR catalysts with different pore distributions were prepared as electrodes by the spraying method.The performance and mass transfer characteristics of the series of electrodes in DFFC were studied,and the optimal electrode structure and operating conditions were obtained.On this basis,and the optimal electrode structure and operation condition were obtained.On this basis,the influence of catalysts with different pore distribution on the performance of DFFC was evaluated.The main conclusions are as follows:1)The thermal process control method proposed in this paper is an important means to control the surface morphology and pore distribution of carbon materials.Reducing the heating rate can greatly increase the number of micropores and macropores in nano carbon shell,thus increasing the specific surface area and pore volume of carbon materials.The abundant pore structure provides a large number of interfaces and channels for the loading of active material Fe Pc,and is conducive to oxygen and ion transport in ORR process.The specific surface area and pore volume determine the doping effect of Fe Pc particles.The rich micropores of the carbon support provide sufficient specific surface area,and the large pore volume ensures the smooth entry of Fe Pc into the micropores,so that Fe Pc can be uniformly loaded on the microporous area inside the carbon nanoshell to form effective active sites.The low specific surface area and pore volume of the carbon support lead to the Fe Pc particles are mainly loaded on the surface of the nano carbon shell in the form of agglomeration.Fe/N-C-1(E_onset=29 m V、E_1/2=-29 m V、I_limited=5.67 m A cm^-2)showed the best ORR catalytic activity compared to other samples and commercial Pt/C catalysts(E_onset=-5m V,E_1/2=-78 m V,I_limited=4.72 m A cm^-2).Meanwhile,the Fe/N-C-1 sample has the highest kinetic current density,the lowest Tafel slope and the highest electrochemically active surface area.In addition,Fe/N-C catalysts exhibited excellent selective catalytic properties in formate resistance tests.2)During the carbonization process,sodium 3-hydroxybutyrate spontaneously generated a large number of sodium carbonate crystals encapsulated in the carbon structure.After the carbonization,it was removed by pickling,forming a shell-like morphology and pores on the surface of the carbon material.The carbonization process affects the growth of sodium carbonate crystals,and the heating rate of carbonization determines the size of the sodium carbonate crystals,thus affecting the surface morphology and pore distribution of the nano carbon shells.At a lower carbonization heating rate,the sodium carbonate crystal grain size formed by the precursor during the carbonization process is smaller（<100 nm）,so the nano carbon shell prepared at a low heating rate has a more uniform shell-like structure and a more abundant pore distribution.Sodium 3-hydroxybutyrate has two stages of weight loss in the carbonization process.Among them,at 400-500℃,sodium 3-hydroxybutyrate has the largest weight loss in the carbonization process,and it is also the stage of the formation and growth of sodium carbonate crystal.Therefore,adjusting the carbonization conditions in this temperature range is an important means to change the microscopic morphology and pores of the carbon support.During the carbonization process,the carbonization temperature range of 400-500℃is the effective temperature range under the effect of low heating rate.When the heating rate is adjusted to 1℃in the range of400-500℃,the pore distribution of this sample can be similar to that of C-1,and the catalyst with similar performance to Fe/N-C-1 can be obtained.Compared with constant heating rate,the preparation time of carbon carrier with variable heating rate can be greatly shortened（from 13.2 h to 4.6 h）while its specific surface area and pore volume are ensured.At the same time,it saves energy consumption,which is beneficial to economy and Practicality.3)Based on the high-performance ORR catalyst,two different structures of sheet cathodes,CMD and CDM,were prepared.The CMD electrode has better performance in DFFC,and its maximum power density reaches 16.9 m W cm^-2,which is 28%higher than that of the CDM electrode.This is mainly due to the shorter distance of oxygen transmission through the electrolyte on the CMD electrode.When the concentration of supporting electrolyte KOH is 4 M and the concentration of sodium formate fuel is 2 M,the battery performance is the best.This is the result of the effect of the concentration of potassium hydroxide and sodium formate on the conductivity,viscosity,cathode ion adsorption and anode activity of the solution.The Fe/N-C-1 catalyst with more developed pores has better battery performance than other Fe/N-C catalysts.This is because more reaction area provides a large number of highly efficient catalytic sites,and the developed pore volume forms a large number of material transport channels,which ensures the rapid supply of electrolyte and oxygen,and reduces the polarization loss of Fe/N-C-1 electrode.The maximum power density is 14%higher than that of commercial Pt/C.The current density of Fe/N-C-1 cathode can reach 102.4 m A cm^-2,which is 20%higher than that of Pt/C(84.5 m A cm^-2)at 0.4 V（vs.RHE）.