| Oxygen reduction reaction?ORR?is an important reaction process of fuel cells.Study on ORR catalysts with excellent catalytic performance is one of the main contents of fuel cells.Although lots of efforts were carried out for the ORR catalysts,the current best ORR catalysts are still a precious metal catalyst?Pt/C,etc.?.However,precious metal catalysts belong to scarce resources,and they have high cost and are easy to be poisoned by alcohol,which seriously hinders their commercial promotion.Therefore,it is particularly important to find a catalyst that is rich in source,low in price,and has good alcohol resistance to replace precious metal catalysts.Studies have shown that certain transition metals?such as Fe,Co,Ni and Mn,etc.?have excellent catalytic activity.Among them,Co/Ni metal has a good catalytic effect on ORR in both alkaline and neutral environment.The catalytic activity of the catalyst can be further improved by doping nitrogen with carbon material as carrier.Alcohol fuel cells and zinc-air batteries are important members in fuel cells.The application of catalysts with good catalytic performance is of great significance for promoting the development of fuel cells.In this paper,carbon-doped Ni-Co-N catalysts with excellent properties were prepared by using honeycomb carbon and carbon spheres as catalyst carriers,nickel-cobalt salts as metal sources and dicyandiamide as nitrogen sources.The ORR catalytic activity and alcohol resistance of the catalyst were investigated.In addition,the catalyst was applied to an alkaline alcohol fuel cell and a zinc-air battery in various environments,and a series of battery discharge tests were performed.?1?First,a honeycomb carbon having a regular hexagonal structure was prepared by a PMMA template method.Then,NiCo-doped HC and C-N nano hollow tubes were prepared by thermal decomposition using honeycomb carbon?HC?,dicyandiamide and cobalt acetate/nickel acetate as raw materials.In a three-electrode system,GC and carbon paper coated with a Ni3Co1-N/HC catalyst were used as working electrodes to measure its ORR performance.The structure and composition of the prepared catalyst were analyzed by SEM,TEM,XRD and other characterization techniques.Finally,Linear scanning voltammetry?LSV?was used to test the polarization curves of Ni3Co1-N/HC-40 catalyst in the presence of ethanol,n-propanol and isopropanol,respectively,and that of oxygen reduction?ORR?in neutral and alkaline environments.In an alkaline medium,the Ni3Co1-N/HC-40 catalyst has an optimum ORR electroactivity with a maximum current density of 4.04 mA·cm-2.Similarly,in neutral medium,Ni3Co1-N/HC-40 catalyst also has the best ORR electroactivity,and its maximum current density is 5.7 mA·cm-2.In addition,the prepared catalysts,especially Ni3CO1-N/HC-40 catalyst,are more resistant to alcohols?ethanol,n-propanol and isopropanol?than the Benchmark Pt/C catalysts.The effect of the addition of alcohol on the catalytic activity of the prepared catalyst was minimal in the range of potentials examined.?2?A neutral Zn-air battery in 0.5 mol·L-11 KNO3 solution was assembled with Ni3Co1-N/HC-20,Ni3Co1-N/HC-30 and Ni3Co1-N/HC-40 as cathode materials and Zn sheets as anode materials.A alkaline Zn-air battery was also assembled in 6 mol·L-11 KOH solution.The power polarization curves and discharge performance of the batteries were tested.Among them,the Zn-air battery with the Ni3Co1-N/HC-40 cathodic electrode presents excellent ferformance.Under alkaline conditions,the open circuit voltage is 1.42 V,and the maximum discharge current density and maximum power density are 545 mA·cm-22 and 276 mW·cm-2,respectively.Under neutral conditions,the open circuit voltage is 0.68 V,and the maximum discharge current density and maximum power density are 344 mA·cm-22 and 49 mW·cm-2,respectively.The step discharge of the battery at different current densities shows that the zinc-air battery has good discharge stability.In addition,compared with the Benchmark Pt/C catalyst,the performance of the Ni3Co1-N/HC-40 catalyst exceeds the Pt/C catalyst in the alkaline zinc-air battery,even in the neutral zinc-air battery,it is also close to the Benchmark Pt/C catalyst.?3?A membrane-free direct alcohol fuel cell was constructed with ternary Pd7Ag2Sn2 as the anode catalyst,Ni3Co1-N/HC as the air electrode catalyst,and ethanol,n-propanol and isopropanol as fuels.The prepared catalyst particles were coated on carbon paper to prepare an air electrode.The polarization curves of the battery and the discharge current density were recorded in an alkaline medium.At the same time,the variation of battery voltage with time under constant discharge current density was obtained to study the stability of the battery.From the discharge performance test,the cell voltage and power density of the cell with the cathode catalyst Ni3Co1-N/HC-40 are superior to those of the other three catalysts.The maximum cell current density of the cathode catalyst Ni3Co1-N/HC-40 is 42.5 mA·cm-2,while the maximum cell current densities of Ni3Co1-N/HC-20,Ni3Co1-N/HC-30 and Pt/C are 32.1,37.5 and 26.7 mA·cm-22 respectively.In addition,the maximum power density of Ni3Co1-N/HC-40 is 4.9 mW·cm-2.The membrane-free fuel cell with the cathode catalyst Ni3Co1-N/HC-40 has a good discharge stability.For alcohol fuel cells,when the battery current density is 10 and 20 mA·cm-2,the battery exhibits a relatively stable voltage plateau corresponding to the respective discharge time of 27 h and 5 h.For n-propanol or iso-propanol fuel cells,a relatively stable voltage plateau can also occur at different constant current densities.?4?First,carbon spheres with a smooth surface and regular shape were prepared by the St?ber method.Then,NiCo-doped CS and C-N nano hollow tubes were prepared by thermal decomposition using carbon spheres?CS?,dicyandiamide and cobalt acetate/nickel acetate as raw materials.Three catalysts of Ni1Co1-N/CS,Ni2Co1-N/CS and Ni3Co1-N/CS were synthesized.In the three-electrode system,the ORR electroactivity of the prepared catalyst coated on the GC and carbon paper was investigated,and the linear sweep voltammetry?LSV?of the catalyst in the presence of ethanol,n-propanol and iso-propanol was also tested to study their resistance to alcohol.ORR polarization curves in a neutral environment and an alkaline environment were obtained.The structure and composition of the prepared catalysts were analyzed by SEM,TEM,XRD and other characterization techniques.In an alkaline medium,the Ni3Co1-N/CS catalyst has an optimum ORR electroactivity with a maximum current density of 4.33 mA·cm-2.Similarly,in neutral medium,Ni3Co1-N/CS catalyst also has the best ORR electroactivity,and its maximum current density is 6.37 mA·cm-2.In addition,the prepared catalysts,especially the Ni3Co1-N/CS catalyst,are better resistant to alcohols?ethanol,n-propanol and iso-propanol?than the Benchmark Pt/C catalyst,The effect of the addition of alcohol on the catalytic activity of the prepared catalyst is minimal in the potential range tested.?5?Zinc-air batteries in both 0.5 mol·L-11 KNO3 solution and 6 mol·L-11 KOH solution were assembled by using Ni1Co1-N/CS,Ni2Co1-N/CS and Ni3Co1-N/CS as cathode materials,Zn sheets as anode material,and their power polarization curves and discharge performance were tested.This zinc-air battery has a high open circuit voltage and discharge current density either in neutral or alkaline medium.Under alkaline condition,the open circuit voltage is1.49V,and the maximum discharge current density and maximum power density are 560mA·cm-22 and 283 mW·cm-2,respectively.Under neutral conditions,the open circuit voltage is1.0 V,and the maximum discharge current density and maximum power density are 382mA·cm-22 and 50.2 mW·cm-2,respectively.The step discharge of the battery at different current densities shows that the zinc-air battery has good discharge stability.It can be concluded that the Ni3Co1-N/CS catalyst exceeds the reference Pt/C catalyst in alkaline zinc-air battery,and it also exceeds the reference Pt/C catalyst in neutral zinc-air battery. |
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