Co9S8 and Mn3O4 Supported on Carbon Material Catalyze Oxygen Reaction and Their Electrocatalytic Mechanism

Author:Li Wan Qing

Supervisor:peng feng


Degree Year:2019





As one of the new energy technologies,zinc-air battery has the characteristics of high energy density,light weight,strong safety and high recycling efficiency.The by-products produced during the charging and discharging processes are H2O and O2,which are environmentally friendly.Therefore,zinc-air battery has attracted extensive attention.However,the current commercial product of such a technology is only applied as a disposable button battery for hearing-aid.As a secondary charge and discharge battery,it is only used in the zinc-air battery car in a presentation level.The large-scale application still has a long way to go.One of the main reasons is that air electrodes involves oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)during charging and discharging of batteries,where large amounts of Pt,Ir or Ru are introduced as catalysts.These precious metals are expensive and extremely unstable under the battery operation condition.Transition metal oxides,sulfides and phosphatides,combined with carbon materials,can effectively catalyze OER and ORR in alkaline electrolyte,with good toxicity resistance and long-term stability.However,due to the complex configuration of this type of materials,the active center and the reaction mechanism are still ambiguous.In this dissertation,the Co9S8 and Mn3O4 carbon composites have been investigated through experimental designs,Density Functional Theory(DFT)calculations and in-situ characterizations.The structures and properties of transition metal-carbon-based composite catalysts and the mechanism of electrocatalytic ORR/OER reactions were systematically studied,aiming to guiding the optimization design of this type of catalysts for the development of Zn-air battery.The dissertation mainly includes the following contents:(1)A simple and effective method is applied for synthesizing Co9S8-porous carbon spheres composite catalyst(Co9S8/CS)with cheap raw materials of thiourea(nitrogen and sulfur sources),glucose(carbon source)and cobalt nitrate.The material structures of N,S co-doping carbon combined with Co9S8 have been evidenced by a variety of characterization methods.In electrocatalytic ORR,Co9S8/CS has an electron transfer number near 4 and a low hydrogen peroxide yield,which is similar to Pt/C.Co9S8/CS can effectively catalyze ORR via a direct four-electron pathway,and the anti-toxicity and the long-term stability are better than Pt/C.Furthermore,Co9S8/CS-800 shows a favorable reversible OER/ORR potential gap of 0.78 V(ΔE=Ej=10–E1/2),which is much lower than Pt/C(0.93 V)and superior to most of the bifunctional electrocatalysts reported to date including metal-free carbon hybrids and transition metal doped carbon materials.In OER,the Co9S8/CS-800 catalyst shows a lower overpotential and Tafel slope than Pt/C,showing a good OER performance.Because of the interaction between Co9S8 and N,S co-doped carbon material,Co9S8/CS-800 exhibits a good stability and excellent activity than commercial Pt/C for ORR and OER.This work provides ideas for the further development of the transition metal sulfides.(2)In the second heat treatment process of preparing Co9S8/CS,a certain amount of sodium hypophosphite was added for high-temperature phosphating,and the phosphorus-doped Co9S8/P@CS composite was successfully prepared.The Co9S8/P@CS-1:2 catalyst performance toward the oxygen evolution in alkaline media exhibits an ultralow overpotential of 233 mV at 10 mA/cm2 and a small Tafel slope of 48 mV dec-1,which is better than the precious metal catalyst IrO2/C(414 mV and 96.58 mV dec-1,respectively).The optimized Co9S8/P@CS-1:2 catalyst was assembled as the air electrode catalyst into a rechargeable zinc-air battery.The battery shows the maximum power density of 142.50 mW/cm2 at 0.56 V with the current density of 255.47 mA/cm2,which is significantly higher than the Pt/C-IrO2(105.70mW/cm2,0.73 V and 145.01 mA/cm2,respectively).After 350 hours(1050 cycles)of charge and discharge cycles,there is no obvious attenuation of the performance.Its cycle stability is better than that of the precious metal catalyst(Pt/C+IrO2),with a good prospect of application and development.The mechanism of electrocatalytic OER was analyzed by in situ FTIR.The active center of the catalyst was revealed by the change of the characteristic peak intensity of O2-at different potentials.(3)Mn3O4@CS/CP was prepared by hydrothermal reaction.The Mn3O4 nanoparticles coated by carbon was supported on porous carbon sheets.The Mn3O4@CS/CP has a high specific area and porosity.The electrocatalytic performance of ORR and OER was studied in alkaline electrolyte.The results show that Mn3O4@CS/CP exhibits not only similar catalytic ORR activity as commercial Pt/C catalyst,but also a better reaction kinetics,methanol resistance and stability.The overpotential(η10,10 mA/cm2)of OER catalyzed by this catalyst is only 290 mV,which is lower than the representative RuO2 catalyst(380 mV).In particular,the Mn3O4@CS/CP catalyst has a lower reversible potential difference(ΔE=0.72 V)for electrocatalytic ORR/OER reactions,which is superior to precious metal catalysts and most reported materials.In situ FTIR and DFT calculations were used to analyze the active sites of hybrid materials and the interaction between Mn3O4 and carbon substrate,revealing the relationship between structure,performance and the catalytic mechanism of ORR/OER.The theoretical calculation results are consistent with the electrochemical tests.