MOF-Derived Nickel/Cobalt-Based Bimetallic Compounds For Electrocatalysis

Environmental pollution and resource shortages ecourage people to develop new energy conversion and energy storage technologies.Among them,water splitting and zinc-air batteries have attracted much attention.On the one hand,hydrogen,with high calorific value and non-polluting properties,is considered as an ideal substitute to the traditional fossil fuels.However,the slower reaction kinetics and excessively high potential of the hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）in water splitting hinder its development.On the other hand,zinc-air battery,as an energy storage technology,plays an important role in ensuring energy supplement and sustainability owing to the high theoretical energy density,low cost and environmental friendliness.However,the oxygen reduction reaction（ORR）,which determines the reaction rate of the cathode,involves the transfer of four electrons,leading to a sluggish kinetics,which severely hinders the commercialization process of zinc-air batteries.Although Pt-based and Ir-based materials are the best electrocatalysts for ORR/HER and OER,respectively,their high cost,scarcity,and single functional property limit their large-scale application.Therefore,designing and developing the electrocatalysts with earth-abundant property and high electrocatalytic performance is of great significance to the application of electrolyzed water and rechargeable zinc-air batteries.Metal organic framework（MOF）-derived transition metal-based catalysts are widely used in the field of electrocatalysis due to their simple preparation,controllable structure,high conductivity and excellent theoretical performance.In this paper,nickel and cobalt-based materials derived from metal organic framework are selected as the research objects,and the physicochemical properties of the materials are regulated through different synthesis strategies.Simultaneously,the application of water splitting and zinc-air batteries for catalysts is studied:（1）NiRu alloy nanoparticles encapsulated into nitrogen-doped carbon matrix were obtained by the controlled pyrolysis of nickel ruthenium-hexamethylenetetramine（HMT）composite（NiRu@NC）.The obtained catalyst shows extremely high activity and durability in hydrogen evolution reaction in all pH media,even exceeding the commercial 20%Pt/C.The excellent catalytic performance can be attributed to the high conductivity of nitrogen-doped carbon and the electronic synergy between NiRu alloys.In addition,after a simple pyrolysis of NiRu@NC in air,the obtained Ni-RuO₂@NC owns excellent electrocatalytic performance for OER under various pH conditions than commercial RuO₂due to the coexistence of multiple active components.The two-electrode water-splitting device constructed by NiRu@NC（for HER）and Ni-RuO₂@NC（for OER）also exhibited the possibility of large-scale applications in solar-hydrogen and wind-hydrogen systems.（2）Using CoFe-HMT metal organic framework as precursor,Co-Fe-P-Se nanoparticles supported on two-dimensional nitrogen-doped carbon nanosheets were prepared by selenization and phosphating in pyrolysis.The obtained material possesses excellent catalytic activity for OER and ORR,also performs well in rechargeable zinc-air batteries.A series of experiments and characterization results comfirm that the two-dimensional nitrogen-doped carbon nanosheets are beneficial to electron transfer and avoid the aggregation of metal particles during the low-temperature pyrolysis process;Meanwhile,the three-dimensional morphology with abundant channels is in favor of electrolyte transport and diffusion of gas;Furthermore,the phosphide and selenide which were formed during the pyrolysis process can serve as active sites to improve the catalytic performance.（3）Based on the intrinsic characteristics of the high N content of HMT and the controllable metal sites,the rapidly synthesized Ni-Co-based bimetallic organic framework is pyrolyzed and vulcanized at a low temperature to obtain the three-dimensional nitrogen-sulfur co-doped Ni-Co-S/NSC.After a series of characterization analysis,it is found that Ni-Co-S/NSC exhibits excellent catalytic performance for ORR and OER in alkaline electrolyte due to the synergy between nickel and cobalt,the porous structure and the introduction of sulfur.In addition,it also has excellent performance in rechargeable zinc-air batteries,which shows higher energy density,power density and long-term stability compared with commercial Pt/C+RuO₂.