Controllable Preparation Of MOF Derivative M-N/C And Electrocatalytic Performance Of Oxygen Reduction

With the proposal of the"double carbon"strategic goals,the development of new energy hich aims for cleaning and efficient as well as sustainable development will enter the substantive implementation stage.As a new type of energy-converting device,the proton exchange membrane fuel cell（PEMFC）has the characteristics of high power density and low temperature startup as well as environmental friendliness,which was expected to be widely used in electric vehicles and portable electronic equipment.However,the oxygen reduction reaction（ORR）that occurs in PEMFC cathode involves the transfer of multiple electrons.It is a dynamic slow electrochemical response process which is need an urgency to accelerate this reaction with an efficient catalyst.At present,the best precious metal platinum（Pt）base catalyst has problems including expensive price,limited reserves,and poor electrochemical stability,besides,it is difficult to meet the commercial application of high-efficiency PEMFC.Therefore,the development of high performance（activity and stability）and low-cost non-precious metal catalysts is the fundamental solution to reduce battery costs and improve battery performance and life.As a crystalline porous material,the composition and structure of metal organic framework（MOF）have become changing,and it has become an ideal front drive or a self-sacrifice template that builds an ORR catalyst.However,most of the MOF front-drives that have been reported are relatively single in terms of coordination and hole structure.The catalyst activity and selectivity of its derived ORR catalysts need to be further improved.In view of this,we regulate the multistage pore structure of MOF by using the soft template of molecular ordered assembly formed by surfactants,and we use solvent-assisted ligand exchange strategy to regulate the coordination structure of MOF in order to realize the preparation of non-noble metal oxygen reduction catalyst with abundant pore structure and adjustable active site structure and its catalytic performance.The main contents include the following three aspects:（1）Regulating the comparison surface area and multi-level hole structure of the ORR catalyst is an effective way to improve its catalytic activity.In view of this,we use ordered assemblies of molecules formed by surfactants as soft templates,through the guidance of the crystal orientation of the soft template and the effective adjustment of the space distance of the Cu atomic node by the volatile"second metal atom"（Zn）,the high density active site controllable construction of multistage pore Cu-N/C@surfactant and the catalytic oxygen reduction reaction mechanism were studied.Studies have found that the prepared Cu-N/C@Surfactant has a high comparison surface area and a richer hole structure.The ratio of the comparison surface area(1038.4 m²g^-1)of Cu-N/C@TWEEN 20-2 is much higher than that of Cu-N/C’s(768.4 m²g^-1).At the same time,Cu-N/C@TWEEN 20-2 has the ORR catalytic activity(E_1/2=0.86 V)comparable to Pt/C(E_1/2=0.87 V)in alkaline electrolyte,and has higher stability and methanol tolerance than Pt/C.（2）It is an effective way to improve the ORR catalytic activity and selectivity of MOF-74 derived Co-N/C catalyst by controlling the synthesis of MOF-74 structure diversity and electronic regulation of the active site.In view of this,we partially replaced the2,5-dihydroxyterephthalic acid ligand of MOF-74 with dimethylimidazole through a solvent-assisted ligand exchange strategy to study the preparation and ORR catalytic performance of MOF-74@ZIF precursors and Co-O/C and Co-N/C catalysts.Studies have found that the ratio of Co-O/C-1(667.62 m²g^-1)is slightly larger than Co-N/C-1(595.33 m²g^-1).This may be due to the structural recombination of MOF-74 with the introduction of dimethylimidazole,which caused a multistage pore structure MOF-74@ZIF precursor with large surface area.Electrochemical studies showed that when compared the difference between the coordination environment of Co-O/C-1(E_1/2=0.81V)catalyst and the Co-N coordination structure of Co-N/C-1 catalyst,the latter one was more conducive to the adsorption and desorption of oxygen.The results show that Co-N/C-1 catalyst has the same ORR catalytic activity(E_1/2=0.84V)as commercial Pt/C(E_1/2=0.85V)while it has higher electrochemical stability and methanol tolerance,which is expected to be applied to PEMFC and direct methanol fuel cells.（3）It is an effective way to improve the ORR catalytic activity and selectivity of MOF-5derived Co-N/C catalyst by controlling the synthesis of MOF-5 structure diversity and electronic regulation of the active site.In view of this,we partially replaced the terphenyl acid ligand of MOF-5 with dimethylimidazole through a solvo-assisted ligand exchange strategy to prepare MOF-5@ZIF precursors,Co-O/C and Co-N/C catalysts as well as study their ORR catalytic performance.It was found that Co-O/C and Co-N/C catalysts had higher specific surface area and more abundant pore structure.Among them,the specific surface area of Co-O/C-10(715.28 m²g^-1)was larger than that of Co-N/C-10(607.61 m²g^-1),which may be due to the structural reorganization of MOF-5 caused by the introduction of dimethylimidazole to form a multistage pore structure MOF-5@ZIF precursor with a large specific surface area.Electrochemical studies show that the Co-N coordination structure of Co-N/C-10 catalyst is more conducive to oxygen adsorption and desorption,which is different from the Co-O active center coordination environment of Co-O/C-10 catalyst(E_1/2=0.80V).As a result,the Co-N/C-1 catalyst has comparable ORR catalytic activity(E_1/2=0.85V)and higher electrochemical stability and methanol tolerance than commercial Pt/C(E_1/2=0.85 V),which is expected to be applied to PEMFC and direct methanol fuel cells.