Oxygen Reduction Catalytic Activity And Applications Of Zinc-based Metal-organic Framework-derived Cathode Catalysts

The growing problems of energy shortage and environmental pollution have propelled energy storage technology and energy conversion system to the forefront of developing green and renewable energy.Microbial fuel cells(MFCs)have attracted considerable attention as a novel technology for simultaneous purification of wastewater and generation of electricity.Of note,oxygen reduction reaction(ORR)constitutes a key factor in measuring the overall performance and energy conversion efficiency of MFCs.Although precious metal platinum-based catalysts are the most effective ORR electrocatalysts at present,the extensive application of MFCs is limited enormously by their high price,scarce reserves,and poor durability.Therefore,exploring cathode catalysts with low cost,high performance,and good stability plays a crucial role in the widespread use of MFCs.Metal-organic frameworks(MOFs)as functionalized nanomaterials with topological diversity have aroused universal attention at home and abroad.Zeolite imidazoles frameworks(ZIFs),serving as an important subclass of the metal-organic framework,unite the excellent properties of zeolite and traditional MOFs.Notably,ZIF-8 has currently become a research hotspot due to its excellent physicochemical properties,mainly including the controlled structural diversity,hierarchical porous structure,good chemical stability,and thermal stability.The cathode catalyst derived from ZIF-8 can retain the physicochemical properties of the precursors to a great extent and amplify their efficient catalytic activity for ORR,which can significantly improve the electricity production performance and the sewage treatment effect of MFCs.In this paper,a series of cathode catalysts derived from ZIF-8 was prepared by various strategies using ZIF-8 as the precursor template.Besides,this paper further evaluated their catalytic performance for oxygen reduction and the overall performance applied to MFCs.The main results are as follows.1.With ZIF-8 as the precursor self-sacrificing template,the nitrogen-fluorine co-doped carbon-derived cathode catalyst FNC-1:15 was successfully prepared by NH₃ and HF vapor etching treatment.FNC-1:15 catalyst exhibited excellent catalytic activity of oxygen reduction under neutral electrolyte conditions.Its excellent performance was mainly attributed to the high specific surface area,hierarchical porous structure,abundant defect sites,and the synergistic effect of N-F,which contributed to the process of electron transport and proton diffusion,thus significantly accelerating the ORR kinetics rate.The FNC-1:15 catalyst had an excellent onset potential of 0.83 V(vs.RHE),a diffusion limit current of 6.44 m A cm^-2,and an electron transfer number of about 3.97,which was superior to commercial Pt/C catalysts.In addition,the MFC assembled with FNC-1:15 as a cathode catalyst also showed excellent power production performance,with a maximum power density of 460.34 m W m^-2.2.With ZIF-8 as the precursor self-sacrificing template,CoFe_0.2-NC,an efficient and stable cobalt-iron-nitrogen co-doped carbon cage cathode catalyst,was synthesized by impregnation and pyrolysis.The zinc atoms evaporation of ZIF-8 produced a large number of defects,which facilitated the exposure of more active sites.The Co/Fe/N-codoped carbon cage structure avoided the aggregation of metal nanoparticles and provided an ordered hierarchical micropore/mesopore system,which helped reduce the mass transfer resistance.Also,nitrogen doping induced the formation of highly active components of Co-N_x/Fe-N_x as well as nitrogenous functional group,thereby enhancing the ORR catalytic activity and improving the long-term stability of the cathode catalyst.Results of electrochemical performance test showed that CoFe_0.2-NC catalyst exhibited excellent ORR catalytic performance,stability,and tolerance over the full p H range.When it was used in MFCs,it demonstrated a maximum output voltage of 0.48±0.01 V and a maximum power density of 505.53 m W m^-2.The CoFe_0.2-NC catalyst was superior to commercial Pt/C in terms of the aspects above.3.The wastewater treatment effect and cost-effectiveness of ZIF-8-derived cathode catalyst applied to MFCs were analyzed.The results suggested that MFCs have the feasible and stable capacity as well as practical application value with a COD removal rate up to 90%.Moreover,the cost of ZIF-8-derived catalysts was estimated to be only 0.6-1.0%of that of commercial Pt/C catalysts,yet ZIF-8-derived cathode catalysts are superior to the latter in terms of cost value and wastewater treatment efficiency.