The Study On MOF-Derived Oxygen Reduction Catalysts And Their Air Batteries

Zinc-air batteries（ZABs）,which are featured by high theoretical energy density,non-polluting and low cost,constitute a very promising energy conversion system.As the core reaction on the cathode of ZABs,the reaction rate of oxygen reduction reaction（ORR）directly determines the performance of Zn-air batteries.Highly catalytic activity,low-cost non-noble metal oxygen reduction catalysts are required to improve the 4 e^-selectivity of ORR for the commercialization of ZABs.The carbon catalysts containing nitrogen-liganded transition metals（M-N-C）are one of the most important non-noble metal catalysts and remain a hot research topic for ORR catalysts.To address the shortcomings of M-N-C catalysts derived by metal-organic-framework（MOF）material with inhomogeneous decomposition,poor stability and insufficient catalytic activity,efficient and inexpensive carbon-based ORR catalysts were prepared in this paper by using the confined thermal etching strategy and atomic adsorption method,and the relationship between their structure and performance as well as the characteristics of ZABs were systematically investigated.The main contents are as follows:（1）The novel method of preparing single manganese atom catalysts by a confined thermal etching strategy is proposed.Usually,converting Mn-based MOF materials（Mn-ZIF-8）into hierarchical porous graphite-like structure Mn-N-C catalyst（SACs-Mn-1000@g-C₃N₄）with high active site availability,monodispersed Mn-N₃ active centers,and high specific surface area by full gasification of g-C₃N₄ at high temperature.The confined thermal etching effect destroys the metal-ligand bonds of the MOF,causing its metal-organic framework to collapse and transform from a 3D 12-hedral structure to a 2D sheet structure.Unsaturated Mn-N₃ metal active centers can be generated by confinement thermal etching of g-C₃N₄,which provides a simple and effective strategy for the controllable adjustment of the coordination environment of metal centers.The results indicate that:1)The ORR catalyst with monodispersed Mn-N₃ high active sites with ultra-high active site utilization can be generated by the confined thermal etching effect of g-C₃N₄ gas;2)Theoretical calculations show that the Mn-N₃ active center is more favorable for activating the adsorbate,optimizing the electronic structure and reducing the ORR overpotential compared with the planar Mn-N₄ structure.Therefore,the improved ORR activity of the catalyst may be mainly attributed to the defective Mn-N₃ sites with lower free energy barrier and higher intrinsic activity;3)The E_1/2 of the SACs-Mn-1000@g-C₃N₄catalyst in 0.1 mol/L KOH solution is as high as 0.863 V（vs.RHE）,which is comparable to the commercial Pt/C catalyst;4)The maximum power density（226 m W/cm²）and specific energy density(857 Wh/kg _Zn)of the self-assembled primary Zn-air battery are superior to those of commercial Pt/C catalyst.（2）Further,the Mn-N-C catalyst with high stability and high-density Mn-N₅ active centers is designed by an atomic adsorption strategy using the hierarchical porous carbon materials prepared by the confined thermal etching strategy as the carbon matrix.In the first step,a metal-free hierarchical porous graphene-like structure is fabricated by a confinement thermal etching strategy,and the effect of g-C₃N₄ on the pore size distribution of MOF precursors was investigated.The second step greatly increases the doping density of Mn-N₅ active centers through atomic adsorption without introducing redundant metal agglomerates and metal oxides.And the effects of different Mn contents,secondary pyrolysis times and adsorbed metal species on the structure and performance of the catalysts were investigated.The results show that:1)The two-step doping method can increase the degree of graphitization and the doping density of Mn-N₅ centers of the SA-NC/3Mn catalyst to significantly enhance their oxidation corrosion resistance and stability;2)The E_1/2 of the SA-NC/3Mn catalyst in 0.1 mol/L KOH solution is as high as 0.852 V（vs.RHE）,which is comparable to the commercial Pt/C catalyst,but its stability and resistance to methanol poisoning are better;3)The self-assembled primary Zn-air battery exhibits extremely excellent maximum power density（226 m W/cm²）and specific energy density(882 Wh/kg _Zn),which are both superior to commercial Pt/C catalyst.