Synthesis And Electrocatalytic Water Splitting Performance Of Nickel Iron Phosphate Micro-/nano-materials

Since entering the new century,electrochemical water-splitting technology has attracted widespread interest from researchers because it can produce sustainable and clean hydrogen fuels and oxygen with high specific heat capacity.The electrochemical water decomposition process includes two half reactions:hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Compared with HER,OER is slower due to the occurrence of a four-electron transfer process,and requires a high overpotential to generate a sufficient current density.Therefore,in order to accelerate the oxygen evolution reaction process at a lower overpotential,some precious metal catalysts are usually used,such as RuO2/IrO2.However,due to their high cost and scarcity on earth,their large-scale applications are limited.In order to overcome the above-mentioned problems,substitution with non-precious metal catalyst materials has become an inevitable trend.Among many non-precious metal catalysts,transition metal phosphates are widely used in the field of electrochemical water splitting due to their inherent electrocatalytic activity.Some transition metal phosphate catalysts have been found to exhibit catalytic activity that is close to or better than that of precious metal catalysts.However,little research has been done to further improve its catalytic activity,cycle stability,durability,and mass production.In this paper,a series of nickel-iron phosphate catalysts were synthesized by simple and rapid hydrothermal,ball milling and ultrasonic methods.The method of controlling variables is used to adjust the catalyst's morphology,particle size,and nickel-iron ratio in order to achieve the best electrocatalytic performances.The main content of the paper is shown as follows:1.nickel foam?NF?as matrix,nickel foam?NF?and Fe?NO3?3 as the sources of Ni and Fe,respectively.In the presence of appropriate HCl and urea,the surface porous Ni-Fe Phosphate thin films with nickel foam as substrate made of ultra-thin nanotubes were synthesized by one-step hydrothermal method.The optimal conditions for the synthesis of nickel-iron-phosphate catalysts were investigated by changing the reaction time,temperature,the amount of urea added,and whether or not a phosphate group was introduced.In study,comparison with NiFe-OH/NF catalysts without phosphate groups,the introduction of oxygen can significantly improve the oxygen evolution performance.Meanwhile,Ni_10.73Fe_4.18?PO₄?_11.33/NF,the product was produced at 150?for 90 min in the system with 150 mg of urea,has the best electrocatalytic performance for water decomposition.The overpotentials required to reach the oxygen evolution reaction current density of 10 mA cm^-22 and 50 mA cm^-2are only 206 mV and 246 mV,respectively.In addition,Ni_10.73Fe_4.18?PO₄?_11.33/NF has good catalytic stability.The reaction can continue for 300 hours while maintaining a current density of 20 mA cm^-2,and the overpotential changes only 20 m V.2.NiCl₂ and FeCl₃ were used as iron source and nickel source,respectively,and Na₂HPO₄ was used as source of phosphate group.Nickel-iron phosphate nanoparticles were prepared by ball milling.By controlling the molar ratio of NiCl₂and FeCl₃,the time and speed of ball milling,the morphology of the catalyst and the nickel-iron ratio of the product were adjusted to explore the optimal conditions.When the ratio of NiCl₂ and FeCl₃ was 9:1,the rotation speed was 400 rpm,and the time was 8 h,the product NiFe-Pi had the best oxygen evolution performance in the three-electrode system.X-ray diffraction analysis showed that iron was present in the product as a doped form.The overpotential at a current density of 50 mA cm^-22 was only 232 mV,the overpotential at a current density of 300 mA cm^-2was only 258 m V,and the low overpotential at a large current density was close to the actual production requirements.At the same time,the cycling stability and OER activity of the catalyst hardly changed after 1000 CV cycles and continuously catalyzing 100 h at the current density of 50 mA cm^-2.3.Here,NaH₂PO₄ was used as the source of phosphoric acid,and foamed nickel?NF?and Fe?NO₃?₃ were used as the sources of Ni and Fe.NaH₂PO₄ and Fe?NO3?3 were dissolved in water and foamed nickel were put into solution subsequently.Finally,the nickel iron phosphate catalyst was obtained by ultrasonic method in a certain time.Compared with the catalyst prepared without ultrasound,ultrasound can significantly improve the catalytic OER of the product.By changing the concentration of Fe?NO₃?₃ and the ultrasonic time in the reaction system,the optimal oxygen evolution reaction conditions of the catalyst were investigated.The OER performance of the product catalyst was optimal when the Fe?NO3?3 concentration was 100 mM for 30 minutes.At a current density of 50 mA cm^-2,the overpotential was only 238mV,which is superior to commercial RuO₂ catalysts.The XRD analysis proved that the product was an amorphous phase.According to the literature,the catalytic performance of the amorphous phase catalyst is often better than that of the crystalline phase.The prepared NiFe-Pi had good catalytic stability.After 1000 CV cycling,the LSV curve did not change,and the overpotential of the catalyst had no obvious increase after catalyzing for 90 h.