Preparation And Electrochemical Performance Of N-Rich Carbon-Confined Catalysts

Under the background of "30 carbon peak·60 carbon neutral",the upgrading of energy conversion technology is imperative.Electrocatalytic conversion is expected to achieve de-fossilization,low-carbon green and distributed development of energy resources.If electrocatalysts with the required activity can be developed,a low-carbon energy conversion system can be constructed by combining with renewable energy through electrochemical processes.Liquid methanol has the advantages of high volumetric energy density,easy storage and transportation,and the ability to use existing infrastructure.Therefore,methanol fuel cells are a good choice for a smooth transition from fossil fuels to renewable energy.This project has developed the surface and interface controllable methods such as atomic(nano)-level interface engineering,heteroatom doping,steric hindrance effect,confinement synthesis,etc.and.Some novel nitrogen-rich carbon-confined(N-C)catalysts with high catalytic activity,high selectivity and high stability were successfully developed through simple methodology,and the electrocatalytic activity for oxygen reduction(ORR)and methanol oxidation(MOR)was investigated.The research content includes three parts,as follows:1.TiN nanoparticles with uniform size and distribution were synthesized in the nano-confined environment of nitrogen-rich mesoporous graphitic carbon nitride(g-C3N4,C/N=0.75).The composite catalyst TiN?g-C3N4 with high specific surface area was prepared with 2D graphitic g-C3N4 served as template and reactant to generate TiN("reaction template").TiN nanoparticles(～5 nm)were embedded in the amorphous g-C3N4 structure,which significantly improved the electrical conductivity of TiN?g-C3N4 and well preserved the 2D lamellar structure of g-C3N4.Abundant nanoscale crystalline/amorphous heterointerfaces were generated during the preparation process.The catalytic activity of ORR has been greatly improved by coupling g-C3N4 with TiN in a simple experimental method.The half-wave potential E1/2 of TiN?g-C3N4 is 0.789 V,which is better than other reported similar materials.The excellent electrocatalytic activity is attributed to the abundant nanoscale heterointerfaces and the charge redistribution caused by wrinkles at the interfaces.TiN?g-C3N4 with high methanol resistance has stable structure and high corrosion resistance,and has the potential as a support.In view of this,for the secondary development of TiN?g-C3N4 catalyst,after loading a small amount of Pt NPs on its surface,the obtained Pt/TiN?g-C3N4 also showed good MOR activity.It means that the bifunctional(Pt)TiN?g-C3N4 catalyst system was successfully developed,and the high selectivity of the cathode/anode catalyst was realized.It can effectively suppress the problem of battery activity decline caused by methanol permeation and reduce the cost of catalyst development and use.It is worth mentioning that the entire material development process is only achieved through simple parameter control(precursor atomic ratio and heating rate).2.To induce the formation of microstructures to facilitate mass transfer and tune the microelectronic structure,the mesoporous amorphous CoSnO3 nanocubes with high curvature were employed as templates,precursors and Co atoms source with ordered distribution.Uniform and dense ZIF-67 thin layer was formed on the inner and outer surfaces of the mesoporous CoSnO3 nanocubes through a simple and green solvent-free sublimation crystallization process.The pre-anchored Co atoms in the mesoporous CoSnO3 framework can facilitate the interaction between 2-methylimidazole vapor and uniformly dispersed Co atoms to generate uniformly distributed ZIF-67.Then a new Co/CoO/SnO@N-C nanocube catalyst with high ORR electrocatalytic activity was prepared after in-situ high temperature pyrolysis.The design-synthesis strategy of the catalyst has the following merits:(1)The existence of bimetallic(Co&Sn)with a certain stoichiometric ratio in the ternary transition metal oxide CoSnO3 plays a positive role in the construction of atomic-level interface engineering in the product;(2)The coupling of mesoporous CoSnO3 and porous ZIF-67 can not only effectively increase the specific surface area of the target sample and expose more active sites.In addition,the chemical bond-based mutual interaction between ZIF-67 and CoSnO3 ensures that the structure of Co/CoO/SnO@N-C does not collapse during in-situ high temperature carbonization;(3)CoSnO3 nanocube acts as a template,and the ZIF-67 attached to the inside and outside serves as a carbon source.After high-temperature pyrolysis carbonization,the N-C confined Co/CoO/SnO@NC nanocubes were prepared.The active materials were encapsulated inside by the exposed carbon material derived from ZIF-67,which can ensure the stability of the catalyst.The Co/CoO/SnO@NC catalyst with rich atomic-level interface exhibits high ORR electrocatalytic activity with an initial potential of 1.01 V and a half-wave potential E1/2 of 0.832 V,which is only 24 mV different from the commercial Pt/C catalyst(TKK,0.856 V).It is worth mentioning that the catalyst is still not deactivated at high methanol concentration(8M).3.In view of the aforementioned research,this project aims to simply couple a bimetallic organic framework(ZIF-8/67)with MWCNTs and prepare the N-C confined catalyst Co/Co-N-C/MWCNTs with high ORR electrocatalytic activity after in-situ high temperature pyrolysis.The half-wave potential(E1/2)of Co/Co-N-C/MWCNTs is 0.854 V,which is comparable to that of Pt/C.The effects of composition,carbonization temperature and amount of MWCNTs on the properties of the catalyst were investigated.In this study,it was found that the amount of MWCNTs had an obvious impact on the size of the precursor ZIF-8/67.The size ZIF-8/67 is affected by the steric hindrance of self-growth,and decreases obviously as the amount of MWCNTs increased.When there are enough MWCNTs,the size of ZIF-8/67 is no longer changed.It was found that the presence of MWCNTs had a significant effect on the microstructure of Co-N-C polyhedra.The MWCNTs with high toughness alleviated the shrinkage and collapse of the surface structure of the ZIF-8/67 polyhedron during the high-temperature carbonization process,and played the role of structural support.In addition,the Zn precursor has a spatial isolation effect on the Co precursor,which can effectively inhibit the sintering of the Co-based active material during the carbonization process.Meanwhile,the existence of MWCNTs facilitates the diffusion of the reactant O2 into the active sites in the catalyst layer.And form a global conductive pathway for electron transfer,making the C-Nx and Co-Nx active sites an interconnected conductive network.The 1D MWCNTs/3D Co-N-C coupled structure construction brings about an all-domain conductive network and an open microstructure,which is beneficial to charge and mass transfer and is an important guarantee for the high electrocatalytic activity.Co/Co-NC/MWCNTs also exhibit very good durability and methanol resistance.