Preparation And Water Splitting Properties Of Ternary Hydrotalcite Ultrathin Nanosheets

Serious environmental problems and depletion of fossil fuels have prompted us to explore sustainable clean energy,and electrochemical water splitting to produce clean fuel hydrogen is one of the most promising methods because water is an abundant renewable resource.The electrolytic reaction of water consists of two half reactions:oxygen evolution reaction?OER?and hydrogen evolution reaction?HER?.But the slow kinetic process and the high reaction energy barrier restrict the wide application of water sources.Currently platinum-based materials are the best catalysts for HER,iridium-based and ruthenium-based materials are the best OER catalysts.However,high cost and scarcity limit their large-scale application.Therefore,it is urgent to develop efficient,abundant and cheap electrocatalysts for renewable energy production.Layered double hydroxide have attracted much attention due to their unique structure and physicochemical property.In this work,several kinds of Ni-Fe layered double hydroxide nanocomposites with different sizes and morphologies were prepared from nickel and iron resource,and their electrochemical properties were studied.The main works are as follows:?1?By using nickel foam?NF?as the supporting carrier,the ternary NiFeMo-LDH ultrathin nanosheet supported on Ni foam?NiFeMo-LDH@NF?was synthesized by hydrothermal method.NiFeMo-LDH nanosheet grows vertically on the surface of Ni foam.The ordered nanosheet structure has a large specific surface area,which can increase the number of active centers and promote the rapid release of gas products.The doping of high valence molybdenum significantly improves the OER and HER performance of layered double hydroxide.As OER electrocatalysts,NiFeMo-LDH@NF only needs 206 mV overpotential when the current density reached 10 mA cm^-2 in 1 M KOH solution,and the tafel slope is 40.5 m V dec^-1.And NiFeMo-LDH@NF has good stability.The catalytic activity of NiFeMo-LDH@NF is not significantly attenuated at a constant voltage of 1.50 V 60 h test.As HER electrocatalyst,it needs 122 mV overpotential when the current density reached 10 mA cm^-2.Using Ni FeMo-LDH@NF as bifunctional catalysts for overall water splitting,a current density of 10 mA cm^-2 was achieved at a voltage of 1.424 V,and 100 mA cm^-2at 1.45 V with excellent durability,which is much better than that of NiFe-LDH?+,-?electrodes.As an overall water splitting catalyst,NiFeMo-LDH@NF also shows good durability.?2?The ultrathin LDH nanosheets and their nanocomposites with controllable size and morphology were prepared by permeation-diffusion method in gel matrix.By controlling the concentration of metal ions and gel strength,the size and morphology of NiFe-LDH nanosheets can be controlled,and the catalytic performance of the NiFe-LDH can be changed.The size of the nanosheets prepared by this method ranges from 500 nm to 700 nm.The ultrathin structure makes the LDH nanosheets have more active sites exposed.NiFe-LDH-NS exhibited good OER activity,with an overpotential of 295 mV at 10 mA cm^-2 in 1 M KOH solution.But the pure NiFe-LDH has poor conductivity and stability.To make up for its deficiency,we chose to compound it with GO and reduced GO?rGO?,prepared NiFe-GO and NiFe-rGO by diffusion method.The electron transport efficiency was greatly improved by the uniform load on the graphene surface.The overpotential of Ni_2/3Fe_1/3-rGO was only247 mV at the current density of 10 mA cm^-2,and the tafel slope was 37.7 mV dec^-1.The catalyst also has good stability.?3?A core-shell NiFe-LDH catalyst?CuSe@NiFe-LDH?was prepared by electrodepositing NiFe LDH on the CuSe surface with copper selenide as support.CuSe support improved the conductivity of the catalyst,and the uniform array of NiFe-LDH ultrathin nanosheets was beneficial to the exposure of active sites,the proximity of OH ions and the overflow of oxygen.CuSe@NiFe-LDH has good electrocatalytic performance for OER,and the overpotential is 188 mV at the current density of 50 mA cm^-2.The tafel slope is 42.1 mV dec^-1.The catalyst also has good electron transport capacity and stability,and has no obvious attenuation in the 80-hour chronopotential and 22-hour multi-step chronopotential curve tests.This study shows that CuSe-@NiFe-LDH has a broad application prospect as a low-cost and efficient OER catalyst.