Nickel-Molybdenum Based Catalyst For Urea Electrooxidation

As a kind of hydrogen-rich chemical fuel,urea can be used in hydrogen production by urea electrolysis or direct urea fuel cell.Therefore,more and more attention has been paid to the electrooxidation of urea(UOR).The development of electrocatalysts with high catalytic activity,low onset potential,high current density and good stability has become the current research hotspot.Among them,nickel-molybdenum based catalysts have attracted much attention because of their rich storage,low cost and good electrocatalytic activity.However,the structure-activity relationship between the design of nickel-molybdenum based catalytic system and catalytic performance still needs to be deeply studied.Through in-depth analysis of the synergistic effect,electronic effect,active site and interface engineering between nickel and molybdenum elements in nickel-molybdenum based catalysts,the understanding of urea electrooxidation can be further realized that will be helpful for the preparation of highperformance electrocatalysts.Therefore,a variety of nickel-molybdenum based hybrid catalysts with high catalytic activity were designed in this thesis.Starting from the Ni-Mo alloy,Ni-Mo oxides,sulfides,selenides and tellurides with heterogeneous structures were constructed,and the effects of different coordination environments on the electrocatalytic performance of Ni-Mo composites were investigated.The main contents are as follows:1.The research of NiMo alloy catalyst for urea oxidation:Different Ni/Mo ratios NiMo alloy catalysts were prepared by hydrothermal and hydrogen reduction method,and the catalyst of NiMo with ratio of 1/1 showed the best catalytic performance.The onset potential for UOR was 1.33 V,and the current density at 1.54 V was 49.6 mA cm-2,that was about 4.5 and 21 times of the Ni nanoparticles and Mo nanosheet catalysts.The electrolyte contructured by NiMoalloy‖Pt/C required a cell potential of 1.41 V to drive 10 mA cm-2,about 200 mV lower than that of water electrolysis.It showed a good stability during the 20 hours operation.The electrocatalyst had large specific surface area and good reaction kinetics and stability due to the synergism of Ni and Mo and faster charge transfer ability.2.NiMo hybrid catalyst for urea oxidation:Three kinds of nickel-based catalysts,namely Ni-NiO/graphene,NiO/MoO2 and NiMoO4,were prepared and their catalytic performance for urea electrooxidation was studied.Through a series of characterization methods and electrochemical kinetic analysis,the results showed that the order of urea electrooxidation activity from high to low followed that microwave hydrothermal NiMoO4m>NiO/MoO2>ordinary hydrothermal NiMoO4-c>Ni-NiO/graphene.The results showed that NiMoO4-m possessed the highest activity in urea electrooxidation,confirming the advantage of microwave-assisted hydrothermal synthesis compared to the traditional hydrothermal synthesis,and the good application prospect for nanostructured electrocatalysts fabrication by microwave-assisted hydrothermal method.At the same time,the effective interfacial hybridization of NiO/MoO2 nanoclusters and the synergistic effect of nickel and molybdenum can effectively improve the electrooxidation performance of urea,and its catalytic ability is much better than that of nickel oxide catalysts.3.Nickel molybdenum heterostructure sulfide catalyst for urea oxidation:Nickel molybdenum heterostructure sulfide was prepared by vulcanization process of NiMoO4 nanorods,and evaluated for urea electrolysis.The material had a rough layered structure,which can realize the full mixing of single metal sulfides in the nano-scale size,thus generating many active sites and nano-interfaces.Spectroscopy and microscopic analysis revealed the formation of high valence states of Mo6+ and Ni3+ in heterogeneous nanorods and the enhancement of the conductive phase of 1 T MoS2.At the same time,due to the formation of nano-interface and the layered rough nanosheets on the surface of nanorods,the electrochemical surface area was increased and more active sites were exposed;Because of the electronic synergy(Ni3+and Mo6+),the catalyst showed much higher catalytic activity,stability,charge transfer ability and catalytic kinetics;the results provided a novel understanding of the synergistic effect for high valence state heterogeneous catalysts in electrocatalysis.4.Nickel molybdenum heterostructure selenide catalyst for urea oxidation:a new type of NiSe2 nanosheet/MoSe2 nanoparticle hybrid microsphere was prepared for urea oxidation.The Nickel molybdenum selenium(NiSe2/MoSe2)microsphere catalyst was prepared by selenization of NiMo MOF,and a hybrid interface and rich defects were found in the catalyst that can significantly improve the activity and stability of urea electrolysis.Specifically,the current density of NiSe2/MoSe2 at 1.54 V was 100.7 mA cm-2,much higher than that of NiSe2 and MoSe.In addition,when the NiSe2/MoSe2‖Pt/C system was used for urea electrolysis,the current density can reach 10 mA cm-2 with the cell voltage of 1.47 V,and the catalyst showed good stability.The efficient catalytic performance might come from many factors such as the synergistic coupling effect of Ni-Se bond and Mo-Se bond,the increased active sites and the number of Ni3+ions.5.Nickel molybdenum heterostructure telluride catalys for urea oxidation:Nickel molybdenum telluride(NiTe-MoTe2)nanospheres were prepared by high temperature annealing and telluridation of NiMo MOF precursor,and studied for urea electrooxidation.The results showed that a strong electronic interaction between NiTe and MoTe2 was found which can enhance the charge transfer and improve the reaction kinetics by reducing the binding strength of the intermediates.NiTe-MoTe2 nanospheres showed excellent UOR activity in alkaline solution.The current density can reach 10 mA cm-2 with the potential of 1.42 V,with a Tafel slope of 56 mV dec-1.In addition,it required a cell voltage of 1.49 V for NiTe-MoTe2‖Pt/C to reach the current density of 10 mA cm-2 in 1 mol L-1 KOH solution containing 0.33 mol L-1 urea,which was much less than that of 1.68 V for water electrolysis.