Controlled Synthesis Of Two-Dimensional Sandwich-like Mesoporous Materials By Block Copolymer Self-assembly In Solution For Energy-related Applications

Mesoporous materials have shown great potential in various applications,including energy storage and conversion,catalysis,drug delivery,adsorption,separation,owing to their high specific area,tunable pore structure and pore size,stable architecture and rich framework compositions.With the rapid development of self-assembly technology,block copolymer self-assembly in solution provides a versatile strategy for efficient preparation of mesoporous materials.Especially,employing block copolymer self-assembly on two-dimensional（2D）interfaces,such as graphene,enables the construction of 2D sandwich-like mesoporous materials which may afford abundant active sites,enhanced electrical conductivity as well as short mass transfer distance,therefore showing great advantages in energy storage and conversion.Nevertheless,controlled synthesis of 2D sandwich-like mesoporous materials with well-defined pore structures has still remained a great challenge.In this work,we designed and fabricated multiple types of 2D sandwich-like mesoporous materials with controllable morphologies,through the interface self-assembly of block copolymers on graphene substrates,as organic cathodes for lithium-ion batteries（LIBs）or catalysts for oxygen reduction reaction（ORR）.Furthermore,we studied the effect of pore structure on electrochemical performance.The contents and results are summarized as follows:（1）Organic cathode materials have attracted great attention for LIBs,benefiting from their high theoretical specific capacity and flexible design of molecular structures.In this work,we fabricated 2D sandwich-like mesoporous materials as organic cathodes for enhanced electrochemical performance.By employing spherical and cylindrical micelles formed by the self-assembly of polystyrene-block-poly（ethylene oxide）（PS-b-PEO）or poly（ethylene oxide）-block-poly（propylene oxide）-block-poly（ethylene oxide）（PEO-b-PPO-b-PEO,including F127 and P123）in aqueous solutions as the soft templates,we patterned the surfaces of rGO with mesoporous PDA（mPDA）of spherical pores（denoted as mPDA/rGO-1&2 for different pore sizes）or cylindrical pores（mPDA/rGO-3）.The resulting mPDA/rGO nanocomposites possess average thicknesses of 14-26 nm,mean pore sizes of 8 to 14 nm,and large surface areas up to 356 m²g^-1.Serving as cathode materials of LIBs,mPDA/rGO showed improved specific capacity and better rate performance compared with nonporous PDA/rGO,verifying the remarkable advantage of mesoporous architecture.Furthermore,the different mPDA/rGO nanocomposites afford a chance for the study on the effect of pore structure on their electrochemical performance.Among the three samples,PDA/rGO-2 with spherical pores of the smaller mean diameter（8 nm）and the largest SSA(356 m² g^-1)exhibits the highest specific capacity of 151 mA h g^-1 at 50 mA g^-1,the best rate performance and satisfied stability with 88.3%capacity retention after1000 charge-discharge cycles at 2000 mA g^-1.（2）2D mesoporous nitrogen-doped carbon materials have been promising ORR catalysts due to their high specific areas and the consequent abundant catalytic sites.In this work,we fabricated 2D mesoporous N-doped carbon/reduced graphene oxide nanosheets（denoted as mNC/rGO）via the self-assembly of PS-b-PEO block copolymer in solution with m-phenylenediamine（mPD）as the nitrogen-containing carbon source.the resultant mNC/rGO nanosheets possess a mean pore size of 19 nm,an average thickness of 35 nm,a high specific surface area of 812 m² g^-1,and a nitrogen doping content of 2.64 wt%.Benefiting from the unique structure,the mNC/rGO nanocomposites show excellent catalytic performance as a metal-free catalyst for ORR in an alkaline medium,with a half-wave-potential(E_1/2)of 0.77 V versus reversible hydrogen electrode（RHE）and a limiting current density（J_L）of 5.2 mA cm^-2,which is well comparable with that of the commercial Pt/C catalyst and is also better than those of many reported N-doped carbon materials.