Preparation Of Nickel-molybdenum Alloy Catalyst And Its Performance In Water Splitting

Hydrogen as a good medium of energy conversion,have many advantages such as high energy density,environmental protection,which is an effective energy to replace fossil fuels.Electrocatalytic water splitting hydrogen evolution reaction(HER)is the most effective method of preparation of hydrogen.However,because of the overpotential of real reaction,its consumption of electric energy could greatly increased.In order to enhance the energy conversion efficiency,it is necessary to develop a cheap and efficient catalyst for HER.Due to the excellent performance of hydrogen evolution reaction in alkaline solution and its lower price than noble metal,nickel molybdenum alloy has received the widespread attention.In this paper,we build NiMoO4 nanorods arrays through hydrothermal reaction,after decomposition in hydrogen,we get NiMo alloy.The HER performance and decomposition process of catalysts were studied by introducing different content of oxygen vacancies.In addition,the influence of morphology on the performance of the system also discussed.The specific research contents are as follows:1.Using nickel foam as the support,in the hydrothermal synthesis process of NiMoO4 precursor,introducing 0.5 MPa different atmospheres:hydrogen,oxygen,air.As a result,we introduce different contents of oxygen vacancies in precursores.Using Raman,EPR and XPS to characterize content of oxygen vacancies and the impact on the valence state.Then,we calcine different NiMoO4 in 600℃ under the hydrogen/nitrogen atmosphere to facilitate NiMoO4 broken down to produce interior MoO2 and the spherical NiMo alloy on the surface.XRD characterizated Ni3MO in the hydrogen samples after calcination.As the contrast,most alloy particles are Ni4MO in the rest of the samples,which proves that the reducing ability of atmosphere could influence the crystal phase.Using electrochemical double layer capacitance to characterize the active surface area,we found active surface of the oxygen sample is small,which proves that the oxidizing atmosphere effects on the surface area.In 1 M KOH solution,the HER performance of air samples(72.5 mV)is the best and that of oxygen sample(84.3 mV)is better than that of hydrogen sample(125.5 mV).The significant difference in performance proves the effectiveness of the atmosphere regulation and the effect of crystal phase is stronger than that of active sites.The free energy of H*adsorption and water adsorption on Ni4Mo(200)and Ni3Mo(101)are studied by DFT calculation.It is thermodynamically proved that the HER performance on Ni4Mo(200)is better than the Ni3Mo(101).Using in-situ XRD to characterize structural changes in the calcinated process of hydrogen and oxygen samples.the peak position of hydrogen sample overall shift to the left,which proves Mo atoms are more likely to be reduced and facilitate Mo atoms diffusion to the outside surface,that lead to other crystal lattice fringe be widen.Another obvious difference can be embodied in contour diagram,hydrogen sample has MoO2(002)surface all the time.To the contrary,the surface of oxygen sample the crystal disappeared in high temperature.We think this is caused by the internal growth resistance of different catalysts,it further illustrates the atmosphere regulation can effectively change the stability of the crystals,which further affects the decomposition process.2.Nickel foam was used as the substrate,ammonium fluoride was added to corrode Nickel foam as the Ni source in the hydrothermal process,ammonium molybdate was added as the Mo source.NiMoO4 precursor was obtained by hydrothermal process in 150℃ for 6 h.By adjusting the amount of ammonium fluoride and ammonium molybdate,the morphology of NiMoO4 can be adjusted.There are two main morphologies observed by SEM:tripyramids and thin flakes.The catalytic performance can be improved by adjusting the loading amount of ammonium molybdate.Ammonium fluoride can affect the catalytic performance by adjusting the morphology.When more ammonium fluoride was added,nucleation bursted out at the early stage of synthesis reaction,as a result,many small tripyramids were formed.On the contrary,larger flakes are more likely to form when less ammonium fluoride added.XRD was used to detect the crystal phase composition of samples,which after calcinating in hydrogen.It can be seen that the sample calcined at 400℃ basically maintained the NiMoO4 structure.However,when the calcined temperature reached over 500℃,the characteristic peaks of the precursor disappear completely,the characteristic peaks of MoO2 and active phase of HER-Ni4Mo appeared.In addition,the performance of the catalyst was also tested.It was found that with increasing the content of ammonium fluoride and ammonium molybdate,the OER performance of the catalyst was significantly improved.As for the performance of HER,the catalyst with too high surface area will degrade due to the high temperature calcination of the precursor.After screening,the overall water splitting catalyst was obtained and the performance of the catalysts were tested.The stability test was carried out at the current density of 10 mA cm-2 and the electrolytic voltage was 1.7 V,It was found that the catalysts have good stability.