Preparation Of Nickel-iron Bimetallic Compounds And Their Electrolytic Water Properties

As a renewable and clean energy source,hydrogen has attracted wide attention.Its high calorific value and zero-carbon emissions are considered to be the best carriers to replace traditional fossil fuels.Currently,hydrogen generally acquired from water-gas shift reaction or natural gas in industrial production.However,this method needs a large amount of fossil energy,which will eventually produce CO2 and increase the emissions of greenhouse gas.The earth is rich in water resources,and the preparation of hydrogen from electrolyzed water is an ideal choice to alleviate the energy crisis and global warming.The overall water splitting consists of two half reactions of oxygen evolution reaction(OER)occurring at the anode and hydrogen evolution reaction(HER)occurring at the cathode.Among them,the slow kinetic process,the four proton/electron transport processes and the high activation energy to form oxygen-oxygen double bonds resulted in a large overpotential in the OER process compared to the HER process.It is well known that noble metal-based catalysts(such as Pt,Ru,Ir and corresponding oxides)have excellent electrocatalytic performance,but their high cost and rarity have severely restricted their widespread industrial application.Therefore,designing a bifunctional electrocatalyst with low cost,high efficiency and high stability is the key to accelerating the development of electrolyzed water technology.Transition metal compounds have a wide range of applications in electrocatalysis,photocatalysis,supercapacitors,fuel cell and lithium ion batteries because of their excellent catalytic performance,earth-abundant,and low cost.Numerous studies have shown that bis/polymetallic compounds tend to exhibit superior electrochemical performance than single metals,and their combination with conductive substrates can significantly increase the electron transport rate,thereby further increasing the catalytic activity.In this paper,the OER,HER and overall water splitting performance was investigated by preparing different Ni-Fe bimetallic compounds on nickel foam substrate.The specific research contents are as follows:(1)A heterostructured NiFe-LDH@Ni3S2 composite array on nickel foam substrate was prepared by using a two-step hydrothermal method.Different from the traditional structure of nanosheets on the surface,the as-synthesised electrocatalyst has a special structure of an on-chip epitaxial growth sheet which not only provides enhanced interfacial bonding,but also a large number of exposed active sites.Meanwhile,the catalyst directly contact with nickel foam also increases the electron transport rate.Electrochemical test results show that the electrocatalytic activity of heterostructured Ni1Fe10-LDH@Ni3S2/NF for oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)is better than that of single component Ni1Fe10-LDH and Ni3S2.For OER,it only needs an overpotential of 223 mV to drive a current density of 20 mA cm-2,while exhibiting a Tafel slope of only 48 mV/dec.For HER,it only requires 172 mV to obtain a catalytic current density of 10 mA cm-2.In addition,in the two-electrode system,NilFe10-LDH@Ni3S2/NF as both an anode and a cathode,which requires only a voltage of 1.65 V to achieve a current density of 10 mA cm-2 and still keep stable after a long time stability test.(2)The iron-nickel precursor on the nickel foam substrate was prepared through a hydrothermal method,then prepared the NiFe-P/NF nanosheets array with different iron-nickel ratios by low-temperature phosphating method.The morphology and electrochemical performance of NiFe-P/NF arrays were characterized.The test results show that Ni6Fe4-P/NF exhibits the highest electrochemical activity and the lowest Tafel slope.For the oxygen evolution reaction,Ni6Fe4-P/NF requires an overpotential of only 321 mV and 415 mV to achieve current densities of 50 mA cm’2 and 100 mA cm-2.For hydrogen evolution reactions,Ni6Fe4-P/NF only requires 186 mV and 278 mV overpotential to drive a current densities of 20 mA cm-2 and 50 mA cm-2.In the standard two-electrode system test,Ni6Fe4-P/NF was used as both anode and cathode for overall water splitting,it only required a voltage of 1.57 V and 1.65 V to reach a current density of 10 mA cm-2 and 20 mA cm-2.The doped of P and the synergistic effect of bimetallic cations is vital for the improved catalytic performance.