Preparation Of Ni-Co/C-N Composites As Electrocatalysts For Alcohol Oxidation And Oxygen Reduction Reaction

The oxidation of alcohol?AOR?and oxygen reduction reaction?ORR?are two important reactions of fuel cells.Development of novel AOR and ORR electrocatalysts is one of the main work in the field of fuel cells.Although the best catalysts for AOR and ORR are still Pt or Pt based composite materials,their practical applications are seriously limited by their high cost and the easy poisoning caused by CO.Therefore,considerable attention has been paid to the study on non-noble metal catalysts,which is found to present excellent electrocatalytic activity for AOR and ORR.Ni/Co metals show a good catalytic effect on both AOR and ORR in acid,alkaline and neutral solutions.And the performance of the catalysts can be further improved after doping C,N,S and other hetero atoms.At present,research on zinc-air battery focuses on alkaline solution as electrolyte,but alkaline electrolyte will lead to the corrosion of the anode zinc,carbonization of electrolyte and formation of zinc dendrites,which will reduce the performance of battery.Neutral solution can be used as a substitute for alkaline solution in zinc-air batteries due to the weak corrosion.In this work,novel Ni/Co based electrocatalysts have been prepared by using Ni/Co as metal source and different carbon-nitrogen materials as C-N sources.Their electrocatalytic activity for oxygen reduction reaction and alcohol oxidation reaction has been studied.In addition,a zinc-air battery with KNO₃ as the electrolyte and the NiCo/C-N catalysts as the cathode have been studied.?1?The C-N composites containing Ni and Co?Ni₁Co₁/C-N,Ni₃Co₁/C-N,Ni₆Co₁/C-N,Ni₉Co₁/C-N,Ni/C-N and Co/C-N?can be produced by direct pyrolysis of the NiCo-doped polyaniline?PANI?precursors in N₂ atmosphere at 800?,and show efficient electroactivity for oxygen reduction reaction?ORR?in alkaline media.Distribution and compositions of the catalysts were characterized by SEM,TEM,EDS and XRD techniques.The catalysts were loaded on carbon paper to prepare gas diffusion electrodes,which electrocatalytic activity for ORR in alkaline media was investigated by voltammetric techniques.The ORR current density on these carbon paper-supported NiCo/C-N catalysts exhibits a linear increase with the negative shift of ORR potential.The ORR onset potential is around-0.2 V?vs Ag/AgCl?in alkaline media.Among the prepared catalysts,the catalyst Ni₆Co₁/C-N presents the largest ORR current density,which is 68.5 mA cm^-2?-0.8 V,vs Ag/AgCl?in alkaline media.Moreover,Ni₆Co₁/C-N catalyst also presents good electrocatalytic activity stability for ORR.?2?The catalyst precursor Ni₃Co₁/C-N was firstly fabricated by direct pyrolysis of Ni-Co salts/polyaniline?PANI?composite at 800?under N₂ atmosphere.Then the obtained Ni₃Co₁/C-N and multi walled carbon nanotubes?CNT?particles were ground together to synthesize the Ni₃Co₁/C-N/CNT catalysts with different CNT mass percentages:Ni₃Co₁/C-N/CNT?15%?,Ni₃Co₁/C-N/CNT?25%?,Ni₃Co₁/C-N/CNT?35%?,Ni₃Co₁/C-N/CNT?45%?.The catalysts were characterized by SEM,TEM,XRD and XPS techniques.Cyclic voltammetry?CV?,chronoamperometry?CA?and electrochemical impedance spectroscopy?EIS?were used to study the electrocatalytic activity of the prepared catalysts for oxidation of methanol and ethanol in alkaline media.Results show that among the prepared catalysts,the Ni₃Co₁/C-N/CNT?35%?presents the best catalytic activity for oxidation of both methanol and ethanol.The current density at 0.85 V?vs Ag/AgCl?for oxidation of methanol and ethanol on the Ni₃Co₁/C-N/CNT?35%?reaches as high as 213 mA cm^-2 and 181 mA cm^-2,respectively,which has exceeded the results on the state of the art catalysts.For oxidation of both methanol and ethanol,the Ni₃Co₁/C-N/CNT?35%?catalyst also exhibits smaller electron transfer resistances,larger reaction rates and more stable electrocatalytic activity.?3?Preparation of Ni/Co-doped C-N nano-composite catalysts?NiCo/C-N?from direct pyrolysis of Ni/Co salt,polyaniline?PANI?,carbon nanotube?CNT?and dicyandiamide was carried out.Distribution and compositions of the catalysts were characterized by SEM,TEM,XRD,EDS and XPS techniques,and the electrocatalytic activity of the catalysts for ORR in0.5 mol L^-1 KNO₃ media was investigated by voltammetric techniques.Among the as-synthesized catalysts,Ni₁Co₃/CN-3 presents high and stable ORR current density of5.27 mA cm^-2 in 0.5 mol L^-1 KNO₃ solution at 2000 rpm.Electron transfer number of ORR on Ni₁Co₃/CN-3 catalyst is 3.78,showing a nearly complete reduction of O₂ to H₂O.?4?A neutral Zn-air battery with the prepared Co/CN,Ni₁Co₃/CN-1,Ni₁Co₃/CN-2 and Ni₁Co₃/CN-3 composite coated on carbon paper as the air electrode catalyst and metal Zn as the anode was constructed in 0.5 mol L^-1 KNO₃ solution.For the catalyst Ni₁Co₃/CN-3,the open circuit voltage of the neutral Zn-air battery is 1.16 V,and the maximum power density is38.5 mW cm^-2.The galvanostatic discharge time is 345.7,160.6,18.1 and 1.3 h as the discharge current density keeps 10,50,100 and 150 mA cm^-2 respectively.A stable voltage platform arises at various discharge current densities.The neutral Zn-air battery can repeatedly discharge after the zinc anode has been replaced,indicating the synthesized ORR catalyst is an excellent cathode material for the neutral Zn-air battery.