Preparation Of Nickel-iron-based Multistage Composites And Electrocatalyst For Water Splitting Under The Alkaline Conditions

In recent years,energy consumption has been accelerated by economic development.The environmental pollution is seriously exacerbated due to the overloaded use of traditional energy sources,such as oil,coal,or natural gas.It leads to the urgent development of environmentally friendly and renewable energy materials that can replace traditional energy sources.Research progress has found that hydrogen energy?H₂?possesses higher combustion energy density,which is three times that of conventional gasoline.At the same time,the product after combustion is pure water.Therefore,hydrogen energy has aroused widespread attention,focusing on the development of clean energy which subsititute traditional energy.However,the traditional industrial hydrogen production is mainly synthesis through the reforming of methane steam.During the preparation of this method,by-products?CO₂,CO,etc.?are arised to seriously pollute the environment,although hydrogen energy is nonpollution.In view of this problem,the electrochemical water splitting hydrogen production technology?2H₂O=O₂?+2H₂??is considered as one of the most promising energy storage as well as conversion methods.The process of electrochemical water splitting involves two half reactions,one is the oxygen evolution reaction?OER?and the other is the hydrogen evolution reaction?HER?.The OER,as an anode half reaction,has a coupling process of four protons and electron transfer to generate a high overpotential,so the reaction is greatly limited.Meanwhile,the HER belongs to cathode half reaction that of two-electron transfer pathway.In alkaline media,the reaction lacks of hydrogen ions,so it requires water dissociation to provide protons in the early stage,which may introduce an additional energy barrier and cause kinetic retardation.Hence,the development of highly active and stable OER or HER electrocatalyst to achieve efficient and economical water splitting is of great significance for improving the ecological environment and promoting economic development.According to the properties of the materials,water splitting catalysts can be roughly divided into two categories in alkaline condition:one is the noble metal-doped catalysts for example Ru,Ir,or Pt/C;the other is the transition metal catalyst based on Fe,Ni and Co.However,the applications of noble metal materials are limited because of expensive cost and poor durability.On the contrast,transition metal-based catalysts have the advantages of low price and earth-abundant,which becomes a hot research topic.Meanwhile,the Metal-Organic Framework?MOFs?possesses natural porous structure to promote the diffusion of substrate products,greatly accelerate the charge transfer and improve the reaction rate.So it is considered as an excellent electrocatalyst for water splitting.Based on the above analysis,we use simple and feasible experimental methods to design electrocatalysts of the Fe³⁺-regulated MOF-Ni nanoflower composite,the three-dimensional irregular diamond-shaped FeDy@MOF-Ni/CC,and the sea urchin-like nickel-iron dihydroxy oxide bifunctional catalyst.In addition,the catalytic performance,long-term durability as well as practical industrial application prospects are explored in detail combining with a variety of test and characterization measures.And the relevant analysis of the catalytic mechanism is performed.The specific research contents of this article are as follows:1.Fe_xNi_1-x-MOF catalyst:We have developed a controlled hydrolysis strategy to synthetic a series of novel as well as hierarchical porous iron-nickel-based MOFs electrocatalysts on carbon cloth(Fe_x Ni_1-x-MOF).And the OER activities of prepared catalysts are also further studied.The scanning electron microscopy?SEM?and transmission electron microscopy?TEM?analysis showed that the flower structure of MOF-Ni appears morphological collapse before and after hydrolysis.A series of characterizations of the X-ray diffraction?XRD?,Raman spectroscopy,Brunner-Emmet-Teller?BET?,X-ray photoelectron spectroscopy?XPS?reveal that high-priced Fe impurities induce the formation of NiOOH to improve the oxygen evolution active,with the process of Fe³⁺adsorbing on the surface of MOF-Ni.In 1.0 M KOH,the optimal Fe_0.38Ni_0.62-MOF catalyst brings out an ultralow overpotential of 190 mV at a current density of 10 mA cm^-2.Comparing with other catalysts,it also has a smaller Tafel slope(58.3mV dec^-1)and great stability.For MOF-based electrocatalysts,the 3D flower structure of catalyst possesses high surface area to expose more active sites and promote gas release on the material surface.With the formation of NiOOH,the structure of MOF catalyst collapses.When exist a balance between the content of NiOOH and the structural integrity of the MOF framework,the best OER performance is obtained.2.FeDy@MOF-Ni/CC catalyst:We used a two-step hydrothermal method to construct a FeDy-mediated MOF-Ni electrocatalyst with a unique structure for the OER.The catalyst is synthesized by dissolving and regenerating MOF-Ni with co-regulation of Dy₂O₃ and FeSO₄?recorded as FeDy@MOF-Ni/CC?.The chemical composition,morphological characteristics,valence states and other physical and chemical parameters of the materials are determined and characterized by XRD,SEM,TEM,XPS,BET or so on.The FeDy@MOF-Ni/CC exhibits outstanding OER activity,with an overpotential of 251 mV to drive a current density of 10 mA cm^-2,which is comparable to commercial RuO₂.In a two-electrode system,the catalyst also puts up excellent performance of water splitting,requiring only 1.57 V battery voltage to achieve a current density of 10 mA cm^-2.This highly efficient OER performance of catalyst may be due to the strong synergistic effect between Fe and Ni active sites under the action of Dy species.3.FeOOH-NiOOH/NF catalyst:We ingeniously design a simple immersion method to in situ synthesize a serious of highly conductive and active dihydroxy oxide catalysts.The catalysts have bifunctional activity for OER and HER with large current density.Firstly,a Ni-Fe solution is synthesized by hydrothermal reaction.And then the clear nickel foam as a carrier is used to be directly immersed in the above solution.Afterwards the FeOOH-NiOOH/NF composites are directly prepared through ion exchange and adsorption.A series of characterization analysis such as SEM,TEM,XRD,XPS or ICP indicate the elemental composition,apparent morphology and metal valence of the catalysts.Electrocatalytic test results further illustrate the electrochemical performance of FeOOH-NiOOH/NF materials.For the OER,it only offer an extremely low overpotential of 280 mV to reach a high current density of 500 mA cm^-2.For the HER,the preeminent activity is also reflected by an overpotential of 98mV at a current density of 10 mA cm^-2.Furthermore,the FeOOH-NiOOH/NF electrodes are applied to alkaline electrolytic cells in a two-electrode system,attaining current densities of 10mA cm^-2 and 500 mA cm^-2 at low battery voltages of 1.55 and 2.0 V.And it also displays excellent long-term durability with large current density.The electrocatalytic performances are much better than most electrocatalysts that have been reported so far.