Controllable Preparation Of Non-precious Metal Oxygen Reduction Catalysts And Their Application In Energy Devices

The growing conflict between economic development,energy shortage and environmental pollution has prompted a strong interest in the development of green energy systems.These include fuel cells,metal air batteries and water electrolysis.As a new energy system,the Zn-air batteries is considered as the most ideal alternative to fossil fuels because of its high efficiency,renewable and environment-friendly advantages.The oxygen reduction reaction（ORR）in the cathode is the key limiting step for energy conversion efficiency.Currently,Pt-based catalysts exhibit excellent oxygen reduction activity,however,the large-scale production and application of Pt-based catalysts are seriously hindered by high prices and low reserves.In recent decades,efforts have been made to investigate new materials to replace Pt-based catalysts.It has been found that different precursors,heat treatment temperatures,transition metals and preparation processes have a great influence on the oxygen reduction activity and stability of electrocatalysts.Metal-free carbon-based materials,transition metal-doped carbon materials,especially nitrogen-containing materials and specific transition metals as precursors have good performance comparable to Pt-based catalysts.In this paper,we designed experimental schemes based on iron-doped nitrogen,structural modulation,introduction of new transition metals,and heteroatom doping,respectively,and analyzed the effects of the prepared catalysts on the oxygen reduction reaction by corresponding physical and chemical characterization.The prepared catalysts were assembled as air cathode catalysts into zinc vacancy cells and tested for their electrochemical behaviors such as open circuit voltage,power density and specific capacitance.1.Fe,N co-doped non-precious metal oxygen reduction catalysts were prepared using a nitrogen-rich precursor material（g-C₃N₄）as a self-sacrificing template and N source.g-C₃N₄ contains a large amount of unsaturated pyridine nitrogen and generates a large number of defective sites on the carbon substrate after pyrolysis.The Fe source is uniformly anchored on the surface during the pyrolysis process,and the N source is lost with the increase of temperature,resulting in a large number of pore gaps and the formation of dense Fe-N_x active sites.The half-wave potential of the catalyst was optimized to 0.85 V by doping with different levels of Fe sources,which exceeded that of commercial 20%Pt/C catalysts.The prepared catalysts also exhibited excellent electron transfer numbers along with good resistance to methanol neutrophilicity.The prepared catalysts were loaded into Zn-Air battery devices and presented high open-circuit voltage,power density,and specific capacitance performance.2.Oxygen reduction electrocatalysts with multiple heteroatomic doping,abundant defects and porous and layered structures are beneficial for enhancing oxygen reduction reaction（ORR）activity.However,the design and facile preparation of such materials often present great challenges.Herein,Fe,S,N multiple-doped three-dimensional（3D）porous carbon（Fe/S-CN）has been prepared by in-site Na Cl template-assisted ball milling and subsequent high-temperature pyrolysis strategy.Due to the multiple doping,ultra-thin plane and porous carbon framework provide rich active sites and rapid mass/electron transport channels,the Fe/S-CN catalyst with low metal content（0.589wt%）metal species still achieve a high half-wave potential of 0.880 V,exceeding the commercial Pt/C（0.844 V）.The Zn-air battery composed of Fe/S-CN catalysts as air cathodes also exhibited excellent performance.This work exhibition a convenient strategy for the fabrication of low-metal ORR catalysts with reciprocity and layered multi-hole and proves its magni-ficent potential for ORR catalysis and Zn-air batteries.3.Highly active non-precious oxygen reduction（ORR）catalysts are expected to replace precious（Pt）metal oxygen reduction catalysts.An oxygen reduction catalyst（Co@C/Fe-NC）at the bimetallic active site was constructed by doping different types of metals before and after precursor synthesis.This doping method,which can effectively prevent the contact between different metal species.The synergistic effects produced by different metal species are more accurately studied.The prepared Co@C/Fe-NC catalysts have a dodecahedral shape,and the Co source catalyzes the formation of carbon nanotubes on the surface at high temperature.The metal sites are protected by the carbon layer,which can reduce the corrosion of electrolytes and metal agglomeration during circulation,and improve the catalytic activity and stability.The catalyst was tested to have a high onset potential(E_onset=1.02 V),half-wave potential(E_1/2=0.86 V)and stability（negative shift 11 m V after 10,000 cycles）.When assembled into Zn-air batteries,it also exhibits excellent specific capacitance(769.81m Ah g^-1)and power density(100.4 m W cm^-2).The synergistic effect of Co@C and Fe-N_x makes it have significant ORR catalytic activity.