N-Doped Carbon:Tuning Of Porous Structures For Enhanced Electrochemical Processes

Fuel cells have gained wide attention for their high energy conversion efficiency and zero emissions as one of the most promising clean energy batteries.The Platinum-based catalysts used in fuel cells,however,are scarce and expensive,which greatly hampered the commercialization of PEMFC.Developing low-cost and highly efficient alternatives for platinum-based catalysts has become the key issue to promote the widespread application of fuel cells.Nitrogen-doped carbon was considered to be one of the alternative materials.The nitrogen atom has the similar atomic radius with carbon atom,as well as the conjugate effect of the lone pair electrons in the N atoms and extended pi bond in the carbon atomic lattice,making nitrogen-doped carbon material good electrochemical catalysts.Normally the carbon catalysts are constructed into porous structures,with one purpose to increase the specific surface areas and another to provide mass transfer channels.While,although the extensive reports on the porous structures,there is still deficient of a systematic investigation on the influence of detailed porous structures on the ORR performances.The catalytic performance of N-dope Carbon is still less comparable to that of Pt-based catalysts.Thus how to improve its intrinsic activity and the utilization of active sites by tuning the porous structure is a significant step for the carbon ORR catalytic investigation at present.Based on this,we have carried out the following two studies here:Firstly,to satisfy the mass transfer and increase the active sites of ORR catalysts,we designed a set of ordered porous network structure combined with macropores and mesopores,by using SiO₂ microspheres as macroporous template,12 nm of colloidal silica as mesoporous template,dopamine as carbon and nitrogen precursors,and FeCl₃as iron source.Nitrogen-doped Macro-Mesoporous carbon FeNC-X-12（X represents the macropore size）catalysts were obtained via pyrolyzing polydopamine followed by removing the Fe element and SiO₂.In order to systematically study the influence of macropore and mesopores on the ORR performance,we synthesized SiO₂ microspheres with different diameter sizes（100,300,500 nm）as the macropore templates and synthesized the FeNC-X and NC-X-12 type catalysts.Electrocatalytic testing results indicating that the existence of mesopores leads to better performances,and Fe doping results in additional Fe contained active sites and micropores,in comparison to the metal-free N-doped materials.Besides,when increasing the macropore sizes from 100to 500 nm,there has a little enhancement in ORR performance,but the two materials with macropore sizes of 300 nm and 500 nm show no significant difference.The obtained FeNC-500-12 material displays the nice ORR catalytic performance,by showing a half-wave potential of 0.82V in 0.1 M KOH.Secondly,porous microstructure could significantly change the catalytic performance.Theoretically,designing a hierarchically-porous structure which possesses macropores,mesopores and micropores will greatly enlarge the specific surface area and improve the utilization rate of active sites,enhancing the oxygen reduction activity of catalysts.When using ZnCl₂ as microporous template,it will form a large number of micropores in the catalyst matrix after gasification in high temperature,providing plentiful micropores and exposed catalytic active centers.In addition,introducing ZnCl₂ can also effectively avoid pyrolysis loss,surface sintering and collapse of porous structure during carbonation process,which greatly improves the yield of catalyst and efficiency of nitrogen doping.Moreover,the residual ZnCl₂ can be completely removed by a facile washing step.Therefore non-metal,hierarchically macro-meso-microporous NC-X-12-ZnCl₂ carbon materials were fabricated by using SiO₂ microspheres as macroporous template,12 nm of colloidal silica as mesoporous template and ZnCl₂ as microporous template.Particularly,the specific surface area of catalysts increase about2³ times after introducing the micropores.NC-300-12-ZnCl₂ catalyst shows the best electrochemical performance for which the half-wave potential reaches 0.86 V in alkaline medium,being about 20 mV higher than that for commercial Pt/C.For comparison purpose,we also synthesized NC-300-ZnCl₂,NC-12-ZnCl₂,NC-300 and NC-300-12 catalysts.The materials characterization and electrochemical test results tell that:1)The macropores are indispensable for a high catalytic performance;2)The copious micropores make large specific surface areas,contributing to enhancd catalytic performance;3)The achieved hierarchically porous structures exhibit superb catalytic property and excellent stability.As a bifunctional catalyst,NC-300-12-ZnCl₂ can also be used in supercapacitor as the electrode material.When the current density is 0.5 A/g,the specific capacitance of NC-300-12-ZnCl₂ reaches 182.5 F/g,showing good electrochemical properties.