Preparation Of Tungsten-Based Transition-Metal Nitride/Carbon Composites And Their Oxygen Reduction Electrocatalytic Activity

The large-scale commercial application of proton exchange membrane fuel cells(PEMFCs)is greatly limited by their cathodic oxygen reduction catalysts,which is expensive and easy to be poisoned.Therefore,it is necessary to search a new type of low-cost cathodic catalyst with high catalytic activity and long-term stability.Tungsten nitride is known as“platinum-like”catalyst because the electron arrangement on the surface is similar to that of Pt,so that they have similar catalytic properties.Transition metal W has strong electronegativity and high unsaturated 5d orbit.During the ORR,tungsten components are easy to coordinate with various oxygen-contain groups,and release the active sites occupied,then the reaction is facilitated.However,its catalytic activity can't meet the demand,so a lot of work is needed in order to improve the activity.In this paper,a series of experiments was conducted to focus on the modification of tungsten nitride,and it is expected to provide a novel scheme to solve the problem of excessive dependence on the noble metal.The main contents of this article are as follows:1.Using sodium sulfate as the morphology control agent and sodium tungstate as the metal source,the granular,rod-like,needle-like and urchin-like tungsten oxides were prepared by hydrothermal method combined with controlling the time of the hydrothermal treatment.After the thermal treatment under NH₃,high purity tungsten nitride was obtained.The formation mechanism of tungsten oxide with different morphology was studied.We analyzed the condition of heat treatment under NH₃ and the relationship between the morphology of WO₃ and the ORR activity of WN.WN converted from the needle-like WO₃ shows the best ORR activity,and its limiting current density is 3.7 m A cm^-2.2.In order to improve the catalytic activity of WN,we loaded the WN on carbon materials,prepared WN/C composites.The WO_x/GO composites were prepared by the hydrothermal method using sodium tungstate as the tungsten source and the graphite oxide prepared by the Hummers'method as the carbon support.After thermal treatment under NH₃,the WN/GO composite was obtained.WN particles are distributed on the graphene uniformly,and its onset potential of ORR is0.82 V,the half-wave potential was 0.75 V,the limiting current density is 3.3 m A cm^-2.In addition,WO_x/OMC composite was prepared by solvent induced evaporation with F127 as the soft template,sodium tungstate as the tungsten source and resin as the carbon source.After heat treatment under NH₃,WN/OMC composite with high specific surface area(549.3 m2 g^-1)was synthesized.The electrochemical test shows that its onset potential of ORR was 0.87 V,the half-wave potential was0.80 V,and the limiting current density reached 4.1 m A cm^-2.3.The ORR activity of WN is further improved by introducing a second-phase metal.Carbon-coated tungsten nitride-metal/carbon nanotubes composite was prepared through a high-temperature solid phase reaction under the N₂ atmosphere,using ammonium tungstate as the tungsten source,metal acetylacetonate as the metal source and dicyandiamide as the carbon and the nitrogen source at the same time.The effect of different metal on WN catalytic activity was studied,and the result shows that metal Co is the most suitable doping metal.The effect of thermal treatment temperature on the electrocatalytic performance was investigated,and 800?C was found to be the most optimum temperature.The ORR onset potential of WN-Co@C/CNTs is 0.89V,the half-wave potential is 0.85V,the limiting current density reaches 4.5 m A cm^-2 and the tafel slope is 80 m V Dec^-1.In order to improve the dispersion of WN-Co particles,DF-WN-Co was prepared by electrospinning.TEM shows that the dispersion of WN-Co particles is improved significantly and the particle size was reduced.The electrochemical test shows that its onset potential of ORR was 0.89 V,the half-wave potential was 0.84 V,and the limiting current density reached to 5.0 m A cm^-2.4.Structural control of tungsten nitride at the atomic scale was carried out,and the process of thermal treatment from binary transition metal oxide to binary transition metal nitride discussed in detail.A novel high-purity binary transition metal nitride Co₃W₃N was successfully prepared for the first time.First,CoWO₄ was prepared by the hydrothermal method with sodium tungstate as the tungsten source and cobalt chloride as the cobalt source.Then,the binary transition metal nitride Co₃W₃N was prepared by temperature programmed thermal treatment under the NH₃ atmosphere.We investigated the effects of preheating temperature and atmosphere,heat treatment rate,and the speed of NH₃ flow on the synthesis of Co₃W₃N.The electrochemical test shows that the ORR onset potential of Co₃W₃N is 0.81 V,the half-wave potential is 0.74 V,and the limiting current density is 4.3m A cm^-2.5.The electrocatalytic activity of Co₃W₃N was further improved by constructing a synergistic catalytic system and increasing the amount of active sites.CoWO₄/CNTs was synthesized by hydrothermal method using a mixture of sodium tungstate,cobalt chloride and acid oxidized CNTs.After heat treatment under NH₃,Co₃W₃N/CNTs composite was obtained.The XPS test shows that the nitrogen content in CNTs is 3.7%.The electrochemical test shows that the onset potential is 0.86 V,the half-wave potential is 0.81 V,the tafel slope is 84 m V Dec^-1,and the limiting current density reached 4.9 m A cm^-2.Replacing CNTs with GO,we synthesized Co₃W₃N/GO composite through the same method.The nitrogen content of GO was increased to 6.4%,and the electrochemical test shows that the ORR onset potential is 0.91V,the half-wave potential is 0.86 V,the tafel slope was 76 m V Dec^-1,and the limiting current density reached to 5.5 m A cm^-2.A series of modifications about WN is carried out to improve its ORR catalytic activity in this article.The methods may have some practical and guiding significance for improving the catalytic activity of WN.In addition,the proposed in-situ conversion method of binary transition metal nitride could offer some reference for the preparation of similar catalysts system.