层状Fe-N-C单原子催化剂的制备及其氧还原反应催化性能OA

Development of highly efficient non-precious metal catalysts for the oxygen reduction reaction(ORR)is of paramount importance for advancing fuel cell technologies.Among various candidates,Fe-N-C single-atom catalysts derived from zeolitic imidazolate framework(ZIF)materials have attracted considerable attention owing to their remarkable ORR activity and relatively lower cost compared with platinum-based catalysts.Rational design of precursor structures and precise regulation of active site exposure remain crucial challenges for further improving catalytic performance.Using different zinc sources in an aqueous solvent system was found to yield two distinct layered Zn-MOF structures,demonstrating that the nature of the counter anion played a regulatory role in the growth and assembly of the MOF framework.Specifically,zinc nitrate and zinc acetate cause changes in crystallization behavior and microstructure,indicating that anion coordination and hydrolysis characteristics can influence nucleation and subsequent layer formation.Introduction of ferric nitrate during Zn-MOF synthesis further promoted the formation of layered architectures and reduced the thickness of individual layers,ultimately resulting in the formation of an FeZn-MOF precursor.These observations suggested that Fe species not only served as a source of active metal centers but also participated in structural modulation during framework construction.Systematic adjustment of the molar ratio between total metal ions and 2-methylimidazole revealed a strong dependence of morphology on precursor composition.When the molar ratio of metal ions to 2-methylimidazole was maintained below 1:20,the resulting FeZn-MOF preserved a well-defined layered structure.Increasing the ligand content to a molar ratio of 1:80 induced a structural transformation from a two-dimensional layered morphology to a conventional three-dimensional dodecahedral configuration.Compared with the three-dimensional dodecahedral structure,the two-dimensional layered FeZn-MOF enabled more effective exposure of internal active sites,provided a larger specific surface area,and facilitated mass transport processes.Such structural advantages were expected to enhance accessibility of reactants to catalytic centers and improve overall ORR kinetics. Based on the above research findings,FeZn-MOF(1:12)with an optimized layered structure was selected as the precursor for subsequent pyrolysis to prepare Fe-N-C single-atom catalysts.The calcination temperature was carefully optimized to balance structural integrity,graphitization degree,and active site formation.The material obtained at 975℃,denoted as Fe-N-C-975,exhibited pronounced structural and functional advantages.Scanning electron microscopy(SEM)and high-angle annular dark-field scanning transmission electron microscopy(H AADF-STEM)characterization results indicated the presence of a hierarchically porous architecture,a high density of atomically dispersed Fe-Nx active sites,and enhanced electronic conductivity resulting from an improved graphitic carbon framework.The hierarchical porosity contributed to efficient diffusion pathways,while atomic dispersion of Fe species maximized metal utilization efficiency and prevented aggregation into inactive nanoparticles.Electrochemical measurements were conducted to evaluate ORR performance in an alkaline media.The catalyst prepared using FeZn-MOF synthesized from zinc acetate as a precursor and calcined at 975℃exhibited the highest catalytic activity.Fe-N-C-975 delivered a half-wave potential of 0.891 V and a limiting current density of 4.76 mA/cm2,together with a Tafel slope of 71.8 mV/dec,indicating favorable reaction kinetics.Durability testing further highlighted its robustness,after 5000 potential cycles,the half-wave potential decreased by only 11 mV,a degradation significantly smaller than that observed for commercial Pt/C under the identical conditions.These results highlight the critical influence of anions,metal/ligand ratios,and heat treatment parameters on the structural evolution and catalytic behavior of precursors.Construction of two-dimensional layered FeZn-MOF precursors provides an effective strategy for maximizing active site exposure and optimizing mass transport,thereby enabling the fabrication of high-performance Fe-N-C single-atom catalysts.Such findings offer valuable insights into rational design of cost-effective ORR catalysts and contribute to the development of sustainable fuel cell systems.