Influence Of Axial Imidazole Ligands On The Electrocatalytic Performance Of Cobalt Porphyrins For Oxygen Reduction And Water Oxidation

In recent years,due to the excessive use of fossil energy,resulting in energy shortage and environmental pollution,clean energy has attracted much attention.Hydrogen energy has become one of the most promising alternative energy sources due to its high energy density and environmental friendliness.The preparation and utilization of hydrogen energy involves two parts:electrolyzed water splitting and oxyhydrogen fuel cell,and including four important reactions:hydrogen evolution reaction(HER),hydroxide reaction(HOR),oxygen evelution reaction(OER)and oxygen reduction reaction(ORR).Among them,because of the complex reaction and high reaction energy barrier,ORR and OER need to develop catalysts to reduce the reaction energy barrier and improve the reaction rate.Currently,the catalysts that catalyze ORR and OER including noble metal materials such as Pt and Ir,Ru.Therefore we need to develop transition metal catalysts with high catalytic stability and low cost.Transition metal porphyrin and corrole compounds are used for electrocatalytic ORR and OER studies due to their flexible molecular designability and structural stability.In this paper,we simulated cytochrome oxidase(CcO)to synthesize Co porphyrin compounds with an axial imidazole ligand,and studied the effects of axial imidazole ligands on the binding of central metal and O₂,and on electrocatalytic ORR and OER.(1)We synthesized compound 1 with axial imidazole ligand,compound 2 without axial imidazole ligand,and studied the effects of axial imidazole ligands on the binding of pcentral metal and O₂.1 can bind to O₂ at room temperature to form(Li)Co^?-O₂^·-,but 2 cannot.The UV-vis spectrum at various temperaturet proves that the reaction is reversible,which further shows that the product produced by the reaction between 1 and O₂ is(Li)Co^?-O₂^·-.However,2 can not react with O₂ at room temperature and low temperature(such as 213 K).This result indicates that compound 1 with an axial imidazole ligand is more likely to bind to O₂ than compound 2 without an axial imidazole ligand.The axial imidazole ligand is an electron-donating group,and compound 1 with axial imidazole ligand has greater electron cloud density,so it is easy to bind to O₂.This paper proposes for the first time that axial imidazole ligand can promote the banding of central metal and O₂.(2)Study on the electrocatalytic activity of ORR,OER and Zn-air battery of Co porphyrin with axial imidazole ligand.In order to improve the conductivity of the catalyst,we herein report that the noncovalent immobilization Co porphyrin 1 on multiwalled carbon nanotubes(CNT)to form composite catalyst 1/CNT.The electrocatalytic ORR and OER results showed,compared with 2/CNTs,that 1/CNTs has more excellent electrocatalytic ORR and OER performance(larger catalytic current and smaller overpotential).This work applied 1/CNT in Zn-air battery operating in alkaline solution,and achieved smaller charge-discharge voltage gap of 0.88 V,larger energy density of 120 mW/cm²,specific capacity of 785 mAh/g at 20 mA/cm².Moreover,the battery based on 1/CNT displays excellent durability with almost no degradation for>60 h.This paper further confirms that axial imidazole ligands can improve the performance of electrocatalytic ORR and OER;1/CNT with axial imidazole ligands are excellent air electrode catalysts of Zn-air batter,opening up a wider scope for the application of molecular catalysts.