Design,Controllable Synthesis,and Electrocatalytic Performance Of Single-atom Catalytic Materials

Single-atom catalysts（SACs）are a new type of heterogeneous catalysts in which metal atoms are isolated on a solid support.Compared with traditional nanoparticle catalysts,SACs exhibit strong metal-support interaction and unsaturated coordination environment due to their unique structure.They have excellent catalytic activity,selectivity,and stability in multiple reaction systems.In addition,SACs with maximizing atom utilization have become an important research field for future green chemistry and sustainable development.SACs enable new opportunities for the rational design of catalysts and the precise control of active sites,but also faces many challenges:（ii）large-scale preparation of SACs with high active site density;（ii）precise control of the metal atom coordination fine structure in SACs;（iii）analysis of the active site structure in SACs;（iv）Catalytic mechanism of SACs.The uncontrollability of the preparation methods of precise atomic structure analysis make the study of the catalytic mechanism of SACs greatly challenging.This paper aims to address the above scientific issues of SACs,develop a series of SACs with high active site density and controlable the active site structure,and reveal their relationship between structure and electrocatalytic perfromance.The potential applications in energy conversion and storage,such as zinc-air（Zn-air）batteries and proton exchange membrane fuel cells（PEMFCs）have been demonstrated.The specific research content is as follows:（1）A multilayer stabilization strategy is demonstrated to achieve controllable preparation of dense SACs.Firstly,a double layer of perfluorooctanoic acid（PFTA）is constructed in a mixed solution of water and ethanol due to the both hydrophilic and hydrophobic groups in PFTA.The organometalic precursor is then confined in the PFTA double layers.To further stabilize the metal precursor,the PFTA double layers are coated with polypyrrole layers.Thus,the migration of metal during thermal treatment is effectively restricted,achieving atomical dispersion of metals.A series of nitrogen,sulfur,and fluorine co-doped graphene-like carbon-supported single-atom catalysts（M-SA-NSFC）have been successfully prepared,with both non-precious and precious metals（Fe,Co,Ru,Ir,and Pt）.The metal loading can reach～16 wt%.Fe-SA-NSFC,as an efficient oxygen reduction catalyst,has a half-wave potential(E_1/2)of 0.91V and 0.82 V in alkaline and acidic solutions,respectively.Moreover,as an air electrode for a Zn-air battery,Fe-SA-NSFC exhibits high peak power density(247.7 m W cm^-2)and excellent long-term stability.Density functional theory（DFT）analysis shows that S and F co-doping can improve the intrinsic activity of Fe active site towards ORR.This work provides a controllable synthesis method for developing high-loading SACs for various applications.（2）An in-situ trapping strategy using nitrogen-rich molecules is demonstrated to improve the ORR activity of Fe-N-C by simultaneously improving the site density and accessibility of Fe-N₄ moieties in hierarchically porous carbons.By adding nitrogen-rich molecules such as melamine（MA）,dicyandiamide（DCD）,and phenanthroline（Phen）in zeolitic imidazolate frameworks（ZIF-8）,Fe ions are coordinated with dimethylimidazole to form Fe-N₆ structures in ZIF-8.These Fe-N₆ sites are converted into atomically dispersed Fe-N₄ sites in the subsequent pyrolysis process.The resulting Fe-N-C/MA material possesess a high Fe content of 3.5 wt%and a large surface area of 1160 m² g^-1.The material shows higher ORR activity in 0.5 M H₂SO₄ electrolyte,with a E_1/2of 0.83 V,and demonstrated good performance in a H₂-air PEMFC system,with a current density of 80 m A cm^-2 at 0.8 V.This work provides a promising method for high-performance Fe-N-C ORR electrocatalysts and also offers a new idea for in-situ capture of nitrogen-rich molecules.（3）Heteroatoms such as N,O,S,and P doping in Fe-N-C catalysts or direct coordinating with Fe atoms can enhance their ORR performance by balancing the adsorption and desorption of intermediates on Fe N_x sites.Fluorine（F）,as the highest electronegative element,should have a more significant impact on Fe atoms.However,in literature reports,F often reduces the performance of Fe-N-C.Here,a synthesis method using high-temperature pyrolysis of PFTA-modified Fe-doped ZIFs is demonstrated for the peparation of F-doped Fe-N-C（F-Fe NC）,with improved ORR performance.The modification by PFTA causes changes in the catalyst structure,including F doping,increased microporous and specific surface areas(up to 1085 m² g^-1),and a high density of Fe N_x sites.Compared to the original Fe NC material(E_1/2=0.80 V),F-Fe NC has a higher E_1/2of 0.83 V.It subsequently shows high durability.DFT calculations reveal that the strongly electronegative F atoms doped oncarbon can optimize the electronic structure of Fe active center and reduce the adsorption energy of ORR intermediates,thus enhancing their ORR activity.（4）Increasing the site density of Fe N₄ in Fe-N-C is a key for improving its ORR performance.On the one hand,this increases the current density,and on the other hand,shortening the distance between active sites(d _Fe-Fe)enhances spin-coupling of single Fe atoms,which leads to lower adsorption capability of Fe-N₄ for oxygen intermediates,and thus improves the intrinsic activity of ORR.However,whether continuously increasing the site density to d _Fe-Fe<5?can further improve the activity?Evidence provided in this article tends to challenge this assumption.DFT calculations show that reducing d _Fe-Fe to less than 5?increases the adsorption of ORR intermediates,leading to a decrease in ORR activity.To address this dilemma,a new ultra-high-density catalyst,u Fe-N-C/hemin,was synthesized using two Fe precursors,Fe³⁺and hemin,with d _Fe-Fe of 3?and a six-coordination number of Fe site.The maximum peak power density of this catalyst in a 2.0 bar H₂-O₂ PEMFC is 920 m W cm^-2.DFT calculations suggest that introducing axial ligands（such as side-on adsorbed O₂ molecules）to form a six-coordinated Fe structure can offset the strong adsorption of intermediates,further improving catalytic activity.This study explores the effect of small d _Fe-Feon ORR in Fe-N-C,providing a deeper understanding of the catalytic behavior of high-density SACs.