Study On Oxygen Reduction Electrode Of Zinc/Air Batteries

Zinc/air batteries become promising candidates for portable mobile power,energy storage system,electric vehicle?EV?and wearable electronic devices due to their good safety,high energy density and low cost.The oxygen reduction electrode is the core component of the zinc/air battery.Furthermore,a high performance and long lifetime oxygen reduction electrode is the key to the industrial application of zinc/air battery.At present,a lot of research work has been focusing on improving the activity of oxygen reduction catalyst,few studies on the relationship between electrode structure and performance.In this paper,focusing on the problems of large polarization loss and low stability of oxygen reduction electrode,a new oxygen reduction electrode was designed and prepared.The relationship between structure and performance of the electrode were systematically investigated via the microscopy,spectroscopy and electrochemical techniques.The corrosion behavior of MnO₂/C catalyst in without switching three electrode systems was also investigated.The oxygen reduction electrodes are usually consisted of three parts,which are gas diffusion layer?GDL?,current collecting layer?CCL?,and catalytic layer?CL?.In this study,the gas diffusion layer was carbon fiber felt?CFF?treated by polytetrafluoroethylene?PTFE?emulsion.The effects of PTFE content on the physical properties of GDL were investigated by measurement of contact angle,gas permeability and water breakout pressure.Electrochemical properties of electrodes and zinc/air batteries with above GDLs were also studied by electrochemical analysis.The results showed that the contact angle of CFFs were further than 90°after hydrophobic treatment with good hydrophobicity.With the increase of PTFE content,the gas permeability of GDL decreased and the water breakout pressure rised.Then the electrochemical performance of the corresponding electrode decreased with the increase of PTFE content.Porous nickel foam was often usually used as CCL of oxygen reduction electrode.In this study,nickel foam was hydrophobic treated by filling conductive carbon powder and PTFE in three dimensional pores.The physical properties of the hydrophobic nickel foam were tested by scanning electron microscopy?SEM?,contact angle,resistivity,gas permeability,water breakout pressure and hydrophobic porosity,besides which also assembled into electrodes for electrochemical test.The results showed that the hydrophobic nickel foam has good hydrophobicity,high permeability and good gas permeability.After optimization,it was found that when the total loading of carbon powder and PTFE in nickel foam was 0.027 g·cm^-2,and m?C?:m?PTFE?is 2:3,the CCL had the lowest resistivity,proper water breakout pressure,and showed the best electrochemical performance.The pressure in the process of electrode preparation also has an important effect on the performance of the electrode.It was found that the oxygen reduction electrode had the best performance with the pressure of 10 MPa.A new oxygen reduction electrode with the hydrophobic pores distributed continuously and gradient was designed and prepared and characterized by the SEM combined with energy dispersive X-ray spectroscopy?EDS?.The electrochemical performance of electrode was tested by electrochemical workstation and battery testing system.The results showed that the maximum power density of the new electrode was 252 mW·cm^-2,which was higher than the traditional one 17 mW·cm^-2;while the lifetime was 3600 h,which was 3¹0 times of the traditional electrode;the maximum power density of the new electrode was about 1.6 times that of the commercial electrode,while the lifetime was about 6 times of that.The reasons for improving the performance and lifetime of the new electrode were further explored by SEM,EDS,Thermogravimetric Analysis?TG?,electrochemical impedance spectroscopy?EIS?and battery testing respectively.It was found that the new CCL was helpful to prevent the water accumulation,and inhibiting the formation of carbonate.Focusing on oxygen reduction catalyst was easily corroded in the without switching three electrode rechargeable zinc/air batterries.The stability of MnO₂ and carbon supports in different potential ranges was tested by separating manganese based catalysts.The relationship between the electrode potential and the catalyst were also explored.The results showed that the activity of MnO₂ catalyst in the high potential range increased first and then decreased,in the medium potential range increased continuously,and decreased continuously in the low potential range,while the carbon carrier in the high,middle,low and three potential ranges showed the trend of the performance decreased gradually.The above results showed that the performance of the Mn based oxygen reduction catalyst in the without switching three electrode rechargeable zinc/air battery was related to MnO₂ and carbon support.Moreover,the structure-activity relationship needed further investigation.