Design,Optimization And Performance Study Of Fe-N-C Catalysts For Oxygen Reduction

Oxygen reduction reaction(ORR)is a key reaction of energy conversion devices such as fuel cells and zinc-air battery.However,the kinetics of ORR is slow,which significantly restricts the energy conversion efficiency and performance of this devices.Thus,designing and fabricating electrocatalysts for efficient ORR is highly desired.At present,Pt-based nanomaterials are widely considered to be the state-of-the-art electrocatalyst for the ORR,and Pt/C is a commercial catalyst.However,the high cost and scarce of Pt,as well as the low stability,restrict their large-scale commercialization,which necessitate the development of low-cost but high-performance catalysts to supersede commercial Pt/C catalysts.To achieve this goal,great efforts have been devoted to developing novel cost-effective catalysts.Generally,there are two main research directions in this field,including developing platinum group metal-free(PGM-free)catalysts and reducing the Pt usage of Pt-based catalysts.Several advantages of carbon-based materials make them popular as electrode materials in electrochemistry field,such as low-cost,controllable morphology,high conductivity,and various doping types.Therefore,we designed and synthesized a series of carbon-based ORR catalysts from metal-free carbon-based catalysts to Fe-N-C catalysts and Pt-decorated Fe-N-C catalysts,and studied their catalytic performance and mechanism,to provide theoretical basis and technical support for the development of high-performance and low-cost catalysts for ORR.The main research contents and results are listed as follows:(1)Considering that the porous structure of conventional carbon-based materials is predominantly composed of micropores,a novel carbonaceous material with helical mesoporous nanotube structure was achieved by the rational design and facile synthesis,which results in the significantly enhanced mass transport kinetics.A bola-type cationic gelator derived from amino acids can be self-assembled into left-hand twisted nanoribbons in anhydrous ethanol.Then it was used as a template to prepare helical resin-silica nanotubes,where silica serves as pore-maker and resin is used as carbon source.The as-design novel structure endows the material with large specific surface area and enlarged exposure to electrolyte,which efficiently improve the mass transport.Additionally,the N-doping allows the formation of ORR active sites.Benefiting from these properties,the as-prepared catalyst demonstrates excellent catalytic performance with a half-wave potential of 0.77 V(vs.RHE)in alkaline media,and attractive stability as well as good tolerance to methanol.By controlling the morphology and composite,we studied the effect of mesoporous structure and N-doping on the ORR performance.(2)Since the metal-free carbon-based catalyst is still not strong enough for commercialization,transition metal such as Fe was introduced into it to create high-active sites for ORR.In this work,sequential cubic Na Cl template coated with a uniform complex film containing organic and ferric salts was fabricated through a freeze-drying method.After a pyrolysis process under Ar atmosphere,the precursor was transformed to Fe-N-C with a3D interconnected network structure.The as-prepared material demonstrated excellent performance with a half-wave potential of 0.89 V(vs.RHE).Additionally,with advanced and comprehensive characterization techniques as well as the DFT computation,the significant role of Fe₄N on boosting the catalytic activity of Fe-N₄ moieties was confirmed.(3)Based on the strategy that the activity of Fe-N₄ moieties can be effectively adjusted by interaction.In this work,covalent organic framework(COF)was employed as the template and precursor to prepare the Fe-N-C catalyst.Since the N sites on COF can serve as ligands for anchoring Fe cations and the periodic porous structure with separated units can inhibit agglomeration of Fe atoms,the material prepared here was composed of atomically dispersed Fe-N₄ sites and Fe atomic clusters consisting of a few atoms.The as-prepared catalyst exhibits a high half-wave potential of 0.912 V in alkaline environment,and it performed better than commercial Pt/C when they were used in zinc-air batteries.Experimental measurements and DFT calculations show that the Fe-N₄ site is the main active site but the Fe nanocluster can further boost its activity.Our fifindings illustrate a way of incorporating nanoclusters onto single-atom catalysts toward enhancing the oxygen reduction activity.(4)The thermodynamic instability of metal atoms makes high density of active sites in the single-atom catalysts difficult to achieve.Additionally,it is important to clarify that not all single-atom active sites contribute to ORR activity because a lot of active sites are usually located within the carbon matrix.Thus,increasing the utilization of the active sites by structure engineering is highly desired.In this work,we designed a facile synthetic route that involves a controllable etching process of zeolitic-imidazolate-framework-8(ZIF-8),allowing the fabrication of Fe-N-C single-atom catalysts with hierarchically porous structure for mass transfer and concave shell for increasing external surface area,which enable the dramatically enhanced utilization of previously blocked internal active sites.The as-prepared catalyst shows high catalytic activity with a half-wave potential of 0.926 V(vs.RHE)in alkaline media and 0.8 V(vs.RHE)in acidic media.(5)Despite the great progress in their performance in alkaline solution,it is still a huge challenge to achieve high performance for Fe-N-C catalysts in acidic media.In light of this fact,Pt-based ORR catalysts with improved utilization efficiency and reduced cost may offer more immediate opportunities to commercialize proton exchange membrane fuel cells(PEMFCs).In this work,we designed and successfully synthesized a highly active ORR electrocatalyst composed of core-shell Pt alloy nanoparticles on an Fe-N-C substrate with atomically dispersed single Fe atoms.This catalyst has multiple active sites for enhancing the catalytic activity,and therefore reducing the Pt usage and cost.It demonstrates significantly enhanced activity and durability in both acidic RDE measurements and fuel cell tests.The as-developed ORR catalyst achieves a half-wave potential of 0.923 V(vs.RHE)in acidic media and a PEMFC constructed using it demonstrates a peak power density of 1.31W cm^-2.In summary,we designed and prepared various carbon-based ORR catalysts and studied their ORR catalytic performance and mechanism.We investigated the influential factors that affect the performance of metal-free carbon-based catalyst,and the strategy to adjust the intrinsic activity of single-atom active sites in Fe-N-C catalysts.Additionally,we developed an effective strategy for reducing the Pt usage of Pt-based catalysts.Our works will promote the development of cost-effective and high-efficiency ORR catalysts,which is important for the commercialization of various promising energy devices.