Study On Preparation Of Doped Nanocarbon Catalysts From Lignin And Their Performances For Oxygen Reduction Reaction

Lignin is the second largest organic substance,its content in terrestrial plants is second only to cellulose.Due to the high carbon content of lignin,mesoporous carbon materials prepared from lignin are widely used in supercapacitors and electrocatalysts.In this paper,non-metallic carbon doped nano-carbon materials were prepared from lignin with wide source and low price.The influence of preparation conditions on the carbon doped materials was explored.The electrocatalytic oxygen reduction performance of the materials was evaluated.The main research contents and conclusions are as follows:?1?Using bagasse lignin as carbon precursor,graphene-like carbon nanosheets were successfully prepared by one-step annealing by adjusting the ratio of melamine to urea.The specific surface area,pore size distribution,nitrogen-sulfur doped amount and doped form of carbon nanosheets were controlled by the preparation conditions.Under the optimum ratio of melamine to urea of 1:4:4 and pyrolysis temperature of 1000 ^o C,the fully wrinkled carbon nanosheets?LC-4-1000?with high graphitization degree were obtained.LC-4-1000has a high specific surface area of 1208 m² g-¹ and pore volume of 1.40 cm³ g-¹.In alkaline medium,the half-wave potential of LC-4-1000(E_1/2=0.860 V)is 10 mV higher than that of commercial Pt/C(E_1/2=0.850 V),and its limited current density?4.67 mA cm-²,0.1V?is very close to that of Pt/C?5.00 mA cm-²,0.1V?.In acidic medium,the half-wave potential of LC-4-1000 is only 60 mV negative to that of Pt/C,and its limited current density?6.18mA cm-²,0.1V?obviously superior to that of Pt/C?5.00 mA cm-²,0.1V?.In addition,LC-4-1000 has better durability and methanol resistance than Pt/C.The experiment confirmed that LC-4-1000 catalyzing ORR follows a direct four-electron oxygen reduction pathway.Studies on the structure and properties of carbon nanosheets show that graphitic N ratio plays a key role in catalyzing ORR in acidic conditions.As for thiophene sulfur,it has catalytic activity for ORR in alkaline conditions,but has low activity in acidic condition.?2?N,S,Cl co-doped three-dimensional porous carbon catalysts were prepared by using commercial low sulfur lignin as carbon precursor,melamine and urea as nitrogen sources and ZnCl₂/NaCl eutectic salt as double template.The effects of NaCl and ZnCl₂ on material structure,nitrogen-sulfur content,specific surface area and element composition were investigated.Carbon layer spacing,thiophene sulfur and graphitic nitrogen were controlled by adjusting the ratio of ZnCl₂/NaCl.A three-dimensional porous carbon catalyst?LN-3-1?with N,S and Cl co-doping for electrocatalytic ORR was prepared at the optimal ratio of 3:1?ZnCl₂:NaCl?.The specific surface area and pore volume of LN-3-1 reached1289 m² g-¹ and 2.81 cm³ g-¹,respectively.LN-3-1 showed better catalytic activity and kinetic properties than Pt/C both in alkaline and acidic medium.Especially in acidic medium,the half-wave potential of LN-3-1 is 49 mV higher than that of commercial Pt/C,and its Tafel slope is only 43 mV dec-¹.LN-3-1 follows the direct four-electron reduction pathway for ORR and has better cycle stability than Pt/C.It is a non-metallic oxygen reduction catalyst with excellent performance in acidic medium at present.?3?For the first time,H₂-O₂ proton exchange membrane fuel single cell was assembled using lignin-derived N,S,Cl co-doped three-dimensional porous carbon as ORR catalyst.The maximum power density of 779 mW·cm^-2 was obtained.It is one of the highest power densities among proton exchange membrane fuel single cell assembled with non-metallic carbon-based materials as ORR catalyst.It has a great application prospect.