The Design,Synthesis,and Catalytic Properties Towards Hydrogen Evolution Reaction In Overall Urea-Water Electrolysis Of Multi-Component Composites On 3D Nickel Foam

With the development of society and economy,more requirements are imposed on energy and the environment.Hydrogen（H₂）can be considered as an ideal substitute for fossil fuels because it has many advantages such as rich resources,environmental friendliness,renewable,and high heating value.Water splitting is a kind of environmentally friendly hydrogen production technology that utilizes abundant natural resources to obtain hydrogen energy,including anodic oxygen evolution reaction（OER）and cathodic hydrogen evolution reaction（HER）.At present,the theoretical potential of the OER at anode is too high（1.37 V）,which makes the anode kinetics slow,resulting in low hydrogen production efficiency of the overall water splitting.The theoretical oxidation potential of the urea oxidation reaction（UOR）is low（0.37 V）,so instead of the oxygen precipitation reaction at the anode,a more efficient hydrogen evolution can be achieved.At the same time,urea is the main component of urine in urban wastewater and will be converted into toxic ammonia and nitrogen-containing compounds in natural processes.Through electrolysis,urea can be effectively decomposed,reducing environmental pollution,so fully electrolytic urea can meet the requirements of energy and environment at the same time.The most effective way to reduce energy consumption is to reduce the overpotential of the electrode through a highly efficient catalyst.At present,the ideal electrocatalyst for catalytic hydrogen evolution are the noble metal catalysts,but its natural reserves are scarce and difficult to recycle,which makes its application cost high.Therefore,it is inevitable to develop efficient and low-cost non-precious metal catalysts.3D nickel foam（NF）has the advantages of large specific surface area,good conductivity,and low cost,making it a good substrate for electrocatalysts.Therefore,multi-component composite material is designed on a 3D nickel foam substrate,and the synergy between the materials is used to optimize the performance,which is used in the field of hydrogen evolution of fully electrolytic urea wastewater.The main contents and results are summarized as follows:（1）Preparation of spherical cluster Ni₂P/Ni_0.96S/NF composites on NF and their catalytic performances towards hydrogen evolution reaction in overall urea-water electrolysisUsing nickel foam as the substrate and nickel source,a two-step calcination method is used.The first step is calcining and phosphating to synthesize Ni₂P.And the second step is calcining and vulcanizing to synthesize a composite of nickel phosphide and nickel sulfide(Ni₂P/Ni_0.96S/NF).The composition,valence,and structure of the prepared catalyst were characterized by X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy.At the same time,the electrochemical performance of the catalyst was tested by by linear sweep voltammetry,cyclic voltammetry and chronoamperometry.The experiments show that the content of sulfur source will affect the physical stability of the nickel foam substrate,so the effect of the combination of different sulfur content S/P mass ratios（low sulfur（LS）=1:1,medium sulfur（MS）=1.5:1,high sulfur（HS）=2:1）on the catalytic performance of the catalyst was discussed.The results show that the composite catalyst with medium sulfur content has the smallest potential in urea oxidation reaction at the anode(1.441 V,100 m A·cm^-2),which is much smaller than that of pure electrolytic water(1.661 V,100 m A·cm^-2).At the same time,the hydrogen evolution overpotential at the cathode is also the smallest compared to other catalysts(239 m V,100m A·cm^-2),showing superior electrochemical performance and long-term stability.（2）Preparation of folded tremella-like MoP@NiCo-LDH/NF and their catalytic performances towards hydrogen evolution reaction in overall urea-water electrolysisBased on foamed nickel,MoP was formed on the foamed nickel through a hydrothermal process followed by phosphating,and then a nickel-cobalt double-layer hydroxide（NiCo-LDH）was deposited on MoP/NF by electrodeposition,and finally formed MoP@NiCo-LDH/NF multiple complex.In this process,the effects of different electrodeposition times（10 min,20 min,30 min）on the formation of NiCo-LDH were discussed.At different electrodeposition times,NiCo-LDH’s load growth density on MoP/NF is also different,and their electrocatalytic performance is also different.The composition,structure,and corresponding electrochemical properties of the materials were analyzed by similar physical characterization methods and electrochemical tests.The results show that when the electrodeposition time is 20 minutes,the formed wrinkled tremella-like MoP@NiCo-LDH/NF has a more suitable specific surface area,and thus has better electrocatalytic performance.It has lower potential(1.392 V,100 m A·cm^-2)and overpotential(255 m V,100 m A·cm^-2)than the other materials in the anode urea oxidation reaction and the cathode hydrogen evolution reaction.When MoP@NiCo-LDH/NF was used the anode and cathode in a catalytic two-electrode system（MoP@NiCo-LDH/NF（+,-））,it has a lower driving voltage than other catalysts(1.405 V,100 m A·cm^-2).And compared with MoP/NF without the introduction of NiCo-LDH,and NiCo-LDH/NF alone,the electrocatalytic performance of MoP@NiCo-LDH/NF is significantly improved,showing a good bifunctional catalytic activity and stability.（3）Preparation of interlaced rosette-like MoS₂/Ni₃S₂/NiFe-LDH/NF composites and their catalytic performances towards hydrogen evolution reaction in overall urea-water electrolysisNickel foam used as the substrate and nickel source,a two-step solvothermal method was used.The first step is hydrothermal synthesis of MoS₂/Ni₃S₂on foamed nickel,and then hydrothermally compound nickel-iron double-layer hydroxide（NiFe-LDH）to form MoS₂/Ni₃S₂/NiFe-LDH/NF.In order to explore the influence of the ratio of Ni/Fe on the morphology of NiFe-LDH,the composite materials with different molar ratios of Ni and Fe were synthesized within a controlled range.After a series of physical characterization and electrochemical tests,the best catalytic performance was screened for the ratio of Ni/Fe（molar ratio=4:1）.At this ratio,MoS₂/Ni₃S₂/NiFe-LDH/NF has lower anode potential(1.396V,100 m A·cm^-2)and as both cathode overpotential(261 m V,100 m A·cm^-2)than other materials.And it was used simultaneouslythe anode and cathode in a catalytic two-electrode system（MoS₂/Ni₃S₂/NiFe-LDH/NF（+,-））it has a lower driving voltage(1.343 V,100m A·cm^-2).And by comparison,when the Ni₃S₂,MoS₂and NiFe-LDH components are present at the same time,the electrocatalytic performance of MoS₂/Ni₃S₂/NiFe-LDH/NF is better than other individual components due to the synergy between multiple materials.And it exhibits excellent bifunctional catalytic activity and stability.Adopt a two-step solvothermal method with foamed nickel as the substrate and nickel source.The first step is to synthesize MoS₂/Ni₃S₂on the nickel foam,and the second step is to hydrothermally compound NiFe-LDH to form MoS₂/Ni₃S₂/NiFe-LDH/NF multiple composites.The composition,structure and corresponding electrochemical properties of the prepared materials were analyzed through a series of physical characterization and electrochemical tests.It is found that the electrocatalytic performance of the multi-component composites（MoS₂/Ni₃S₂/NiFe-LDH/NF）was significantly improved compared to the single-component materials（Ni₃S₂/NF,MoS₂/Ni₃S₂/NF and NiFe-LDH/NF）.In order to explore the influence of Ni and Fe input on the entire composite material during the formation of NiFe-LDH,the input of different molar ratios of Ni and Fe within the control range（molar ratios:Ni/Fe=3:1,4:1,5:1）was discussed.The results show that when the molar ratio of Ni/Fe is 4:1,MoS₂/Ni₃S₂/NiFe-LDH/NF has a lower anode potential(1.396 V,100 m A·cm^-2)and a lower cathode overpotential than other materials(261m V,100 m A·cm^-2).And it was used simultaneouslythe anode and cathode in a catalytic two-electrode system（MoS₂/Ni₃S₂/NiFe-LDH/NF（+,-））it has a lower driving voltage(1.343 V,100 m A·cm^-2),showing better bifunctional catalytic activity and stability compared to other catalysts.