The Atomic Dispersion Of Low-Pt/Pt-free Catalysts:Controllable Preparation And Electrocatalytic Performance

Proton exchange membrane fuel cell(PEMFC)is a promising technology which can be used in transportation and other fields,due to its high zero emission,high power density and high efficiency.Oxygen reduction reaction(ORR),the kinetically sluggish cathode reaction,is a more important research topic in PEMFC that requires more attention and effort.So far,Pt and Pt-based alloys are the most active catalysts for ORR.However,the high cost and scarcity of Pt limit the application of PEMFC.To overcome this,it is of great significance to develop highly active Platinum group metal-free(Pt-free)or low-Pt catalysts for ORR.Based on these,the preparation of Pt-free catalysts with higher catalytic activity and development of low-Pt catalysts with high utilization of active sites are the focus and main direction of current research on PEMFC electrocatalysts.Firstly,we demonstrate a“eutectic salt-assisted semi-closed carbonization”technique for fabricating high-density active-sites hierarchically porous Fe/N/C catalyst by using ZnCl₂/KCl eutectic salt as template.Our technique allows for the pyrolysis of Fe/N/C precursor under the protection of the molten ZnCl₂/KCl eutectic salt,which can not only provide an ion liquid-confined space to suppress the large weight loss and N evaporation of precursor in a very wide temperature range from 390°C to 923°C,but also play a key role in modulating the porous structure,specific area and graphitization degree of Fe/N/C catalyst.Accordingly,the as-prepared Fe/N/C catalyst exhibits excellent ORR activity and stability in both acidic(half-wave potential of 0.803V versus reversible hydrogen electrode)and alkaline(half-wave potential of 0.918V versus reversible hydrogen electrode)media.More importantly,real cathodes made from the Fe/N/C catalysts further demonstrated superior performance in H₂-O₂ fuel cells and Zinc–O₂batteries,respectively.This strategy provides a new avenue for the design and development of advanced porous carbon materials for different applications.Secondly,atomically dispersed Zn-N-C nanomaterials are promising Pt-free catalysts for oxygen reduction reaction(ORR).However,the fabrication of high Zn loading Zn-N-C catalyst remains a formidable challenge due to the high volatility of Zn precursor during high-temperature annealing.Here,we report that the atomically dispersed Zn-N-C catalyst with an ultrahigh Zn loading of 9.33 wt%can be successfully prepared by simply adopting a very low annealing rate of 1°/min.The Zn-N-C catalyst exhibits comparable ORR activity with Fe-N-C catalyst,and significantly better ORR stability than Fe-N-C catalyst in both acidic and alkaline media.Further experiments and DFT calculations demonstrate that Zn-N-C catalyst is less susceptible to protonate than Fe-N-C catalyst in acidic medium.DFT calculations reveal that the Zn-N₄ structure is more electrochemically stable than the Fe-N₄ during ORR process.Finally,while single-atom catalysts(SACs)are drawing wide attention because they offer properties that differ from those of conventional nanoparticle(NP)-based catalysts,the lack of neighboring metal centers to cooperate in catalysis limits their real application in many important chemical processes.Here,we report the synthesis of a multi-atom platinum(Pt)catalyst that consists of cross-linked Pt-Pt metal centers stabilized by atomically dispersed Zn Fe-N-C support through Pt-N bonds.X-ray absorption fine structure analysis reveals that each Pt atom in the multi-atom Pt catalyst coordinates with?2.6 N atoms and?4.3 Pt atoms.This novel multi-atom Pt catalyst combines the merits of SACs and NP,resulting in extremely high Pt efficiency,excellent stability and high activity for the oxygen reduction reaction(ORR)in both acidic and alkaline media.With an ultralow Pt loading of 0.035 mg cm^-2 at the cathode,the fuel cell assembled by this catalyst delivers a 1.02 W cm^-2maximal power output.Density functional theory(DFT)calculations revealed that the strongly coupled Pt-N bond is critical for stabilizing the cross-linked Pt.This cross-linked multi-atom catalyst provides a new direction to reduce metal usage and enhance the activity and stability of supported catalysts.