Preparation And Electrochemical Oxygen Reduction Performance Of Molybdenum-Based Bimetallic Nitrides

Fuel cells are considered to be the most promising new generation energy conversion system due to their high efficiency and environmental protection.However,the oxygen reduction reaction process originating from the fuel cell cathode is very complicated,and the reaction kinetics is very slow,relying heavily on platinum or platinum-based catalysts.However,platinum or platinum-based catalysts are expensive,and their cycle stability and anti-poisoning ability are poor,which severely limits the development of fuel cells.Therefore,it is imperative to find a cheap catalyst with excellent oxygen reduction performance to reduce the cost of fuel cells and achieve large-scale commercial use.This thesis takes transition metal nitride electrocatalysts as the research object,optimizes the electrochemical performance of the material by introducing second-phase metals,carbon materials and doping,and adjusts the operation to affect the conditions of pure-phase synthesis of metal nitrides and improve the nitrides structural characteristics,in order to obtain a catalyst with outstanding performance.The main research contents are as follows:Firstly,metal oxides are prepared by simple hydrothermal synthesis method using metal salts such as cobalt salt,iron salt,and molybdenum salt.On this basis,the metal oxide is nitridated at high temperature to obtain the metal nitride by means of the tube furnace passing ammonia gas.X-ray diffractometer（XRD）characterization results show that the pure phases of Fe₃O₄,Co₃O₄,and Mo O₃ are obtained by hydrothermal,and Fe₂N,Co₄N,and Mo N-Mo₂N are obtained by reaction after high temperature nitridation.Passed the electrochemical workstation to test the oxygen reduction performance of metal nitrides.Metal nitride exhibits oxygen reduction behavior in O₂-saturated electrolyte solution.The initial potential of Co₄N is0.80 V,the limiting current density reaches 4.58 m A/cm²,and the half-wave potential is 0.66 V.Fitting calculations on the number of transferred electrons of metal nitrides found that,compared to the close two-electron reaction process of Mo N-Mo₂N,the numbers of transferred electrons of Co₄N and Fe₂N are 3.6 and 3.5,respectively,which are close to the ideal four-electron reaction of oxygen reduction.process.This indicates that less H₂O₂ intermediate products are produced during the oxygen reduction reaction,which is beneficial to maintain the activity of the catalyst.At the same time,Co₄N showed good stability and methanol resistance,which further proved its potential as an oxygen reduction catalyst.Secondly,based on the introduction of metal nitride into the second phase transition metal element,bimetallic nitride is prepared.Using cobalt chloride and ammonium molybdate as metal salts,explore the conditions for preparing pure-phase bimetallic nitrides.Specifically,the molar concentrations of the metal ions in the cobalt salt and molybdenum salt solutions are equal,the ammonia gas nitriding temperature is 700℃,and the temperature is kept warm.The time is 2 h,and the ammonia flow rate is 50 ml/min.XRD test proved that Co₃Mo₃N with good crystallinity and pure phase was obtained after nitriding.In order to verify the general applicability of this method,other bimetallic nitrides Fe₃Mo₃N and Ni₃Mo₃N were successfully prepared by the same experimental preparation method.In addition,the influence of metal oxide precursors with different morphologies was also explored,and it was found that the oxide precursors with different morphologies can be smoothly converted into Co₃Mo₃N during the high-temperature nitridation reaction.The oxygen reduction test was performed on all pure-phase bimetallic nitrides.Among them,Co₃Mo₃N showed an electron transfer number of 3.75,the initial potential was 0.83 V,and had excellent stability and methanol resistance.Finally,the oxygen reduction performance of bimetallic nitride is further optimized.The specific method is to form a composite material Co₃Mo₃N/NG with graphene oxide nitridation and bimetallic nitride.The effect of different bimetallic loading on performance is explored.The synergistic catalysis between bimetallic nitride and nitrogen-doped graphene oxide has been proved by X-ray photoelectron spectroscopy（XPS）,Raman（Raman）and other tests to prove the successful nitrogen doping of graphene oxide.The electrochemical workstation tested all samples and the results showed that different loadings of Co₃Mo₃N/NG showed excellent oxygen reduction performance.Among them,the initial potential of the 25 wt%Co₃Mo₃N/NG catalyst reached 0.88 V,which was very similar to the 0.87 V of NG.It is close,but the limiting current density reaches 5.0 m A/cm²,which is nearly twice that of NG.In addition,after the optimized 25 wt%Co₃Mo₃N/NG undergoes 15000 s entiostatic polarization,the limiting current density only decays from 5.1 m A/cm² to 4.7 m A/cm²,and the initial potential is almost unchanged,which proves that it is better than commercial Pt/C has better stability.This subject gives full play to the characteristics of metal nitrides,and proposes optimization methods such as the introduction of second-phase transition metal elements and the compounding of nitrogen-doped graphene oxide to realize the oxygen reduction performance,stability and transfer electrons of single-metal nitrides.The increase in numbers has provided an in-depth understanding of the oxygen reduction reaction process of metal nitrides,which will help promote the development of metal nitrides as oxygen reduction catalysts for fuel cells.