Construction And Electrocatalytic Performances Research Of Single Atom Electrocatalysts Based On Metal Phthalocyanine Precursors

The development of clean and efficient electrochemical conversion technology can provide an effective solution to energy and environmental problems in sustainable social development.Electrocatalyst s are important for energy conversion efficiency and atomic efficiency of electrochemical conversion.Non-noble metal single-atom electrocatalysts(SACs)with metal-nitrogen-carbon(M-N-C)structure have attracted significant attention in the field of electrocatalysis due to their excellent catalytic performance,high atom utilization rate,and low cost.However,it remains challenging in efficiently constructing single-atom catalysts and establishing the relationship between catalyst structure and performance.In this dissertation,metal phthalocyanines(Me Pcs)were used to construct SAC by two methods.High-temperature pyrolysis through Me Pcs encapsulated in zeolite imidazole frameworks(ZIFs).Carbon nanotube(CNT)hybridization for molecularly dispersed electrocatalysts(MDEs)as model SACs.These catalysts were applied for the oxygen reduction reaction(ORR)and carbon dioxide reduction reaction(CO₂RR),and molecular engineering was utilized to optimize the catalytic performance.Combining microstructure characterization,electrochemical measurements and theoretical calculations,the structure-performance relationship for these catalysts was systematically studied.A facile method for preparing Fe-N/C SACs with high metal content was developed by using iron phthalocyanine(Fe Pc)derivatives as metal precursors.Fe Pc molecules were encapsulated into the ZIF structure,and subsequently converted into SACs by pyrolysis.It was found that the introduction of cyano groups on Fe Pc molecules(Fe Pc-CN)could promote the introduction of phthalocyanine molecules and the formation of corresponding active sites,thus increasing the activity for ORR.Adding Fe³⁺ions to Fe Pc-CN/ZIF could further enhance the ORR activity by increasing the content of Fe-Nx sites on the catalyst surface.The optimized Fe-N/C catalyst showed half-wave potentials of 0.81 and0.91 V versus reversible hydrogen electrode(RHE)under acidic and alkaline conditions,respectively.A series of single-atom electrocatalysts were constructed by pyrolyzing the cyano-substituted metal phthalocyanines(Me Pc-CN,Me=Fe,Co and Ni)in ZIFs.Compared with their counterparts synthesized with metal nitrate precursors,Me Pc-CN molecules could effectively construct single-atom electrocatalysts with higher metal loading and little variation when changing the metal center.It was found that the activities and selectivities of the SACs constructed by the Me Pc precursors were higher than those of the metal nitrate counterparts.In a gas-diffusion electrode(GDE),the SACs derived from Ni Pc-CN exhibited CO Faraday efficiencies(FEco?96%)at current densities from-10 to-200 m A cm^-2,and could be operated stably under-200 m A cm^-2 for 16 hours.In order to overcome the problem of structural heterogeneity and u ncertainty of active sites in Me-N-C electrocatalysts prepared by pyrolysis,a series of nickel phthalocyanine molecules are further designed and dispersed on the sidewalls of CNTs to construct model SACs(Ni Pc-MDE)for CO₂RR.In terms of stability,activity and selectivity,Ni Pc-MDE is superior to Ni-N/C prepared by pyrolysis and the aggregated Ni Pc molecular catalyst.By molecular engineering,Ni Pc-MDE with methoxy functional groups(Ni Pc-OMe-MDE)solves the problem of poor stability of the original nickel phthalocyanine.Ni Pc-OMe-MDE exhibited high selectivities of>99.5%for catalyzing CO₂ to CO at reduction current densities up to 300 m A cm^-2 and it could be stably operated for 40 h at the current density of-150 m A cm^-2.Combined with theoretical calculations,the relationship between catalyst structure and performance was revealed.Ni Pc-MDE and Fe Pc-MDE model SACs were applied for the study of ORR with two-electron and four-electron transfer pathways,respectively.Among them,Ni Pc-MDE exhibited peroxide selectivities of>83%under alkaline conditions,which is better than Ni Pc molecules and Ni-N/C prepared by pyrolysis.By molecular engineering,it was found that cyano functionalization could enhance selectivity and activity for the production of hydrogen peroxide,and the peroxide selectivities in the potential range of 0.70?0.20 V were above 92%.Fe Pc-MDEs exhibited preferred four-electron transferred selectivity.A series of substituted Fe Pc-MDEs were studied and Fe Pc-CN-MDE exhibited a half-wave potential of0.92 V under alkaline conditions.In Zn-O₂ batteries,Fe Pc-CN-MDE exhibited the maximum power density of 124.7 m W cm^-2 at the current density of 206 m A cm^-2.Me Pc precursors can not only construct high-performance SACs,but also benefit the optimization of catalytic performance by molecular engineering of Me Pcs.The constructed Me Pc-MDE model SACs are unique with indentified and easily tunable active sties,which is beneficial to reveal the relationship between catalyst structure and performance and promote the rational design of SACs.