Designed Fabrication Of Ferrite Series Metallic Catalysts For Oxygen Evolution Reaction

With the depletion of fossil fules and severe environmental issues,developing clean,efficient and sustainable new energy to meet the ever increasing energy demand has become one of the research hot spots.Hydrogen gas?H₂?,as one of the most potential alternative to fossil fuels,is an energy carrier with considerably higher energy density compared with most hydrocarbons and zero-carbon emission during combustion.Compared with other techniques,hydrogen production by electrolysis of water can directly convert electrical energy into chemical energy and achieve zero carbon emission,which is considered to be one of the effective ways to obtain hydrogen.The electrochemical water-splitting reaction can be divided into two half reactions:hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?,occurring simultaneously at the two electrodes of an electrolytic cell.Nevertheless,compared to the two-electron-involved process of HER,OER,a four-electron process,is an inherently kinetic hysteresis step,which can dramatically diminish the overall energy conversion efficiency.Thus it is particularly urgent to develop highly-active,clean but low-cost catalysts to reduce the energy barrier of the oxygen evolution reaction and improve the conversion efficiency.Noble-metal oxides,such as RuO₂ and IrO₂,are the state-of-art OER electrocatalysts,whereas their high cost and element scarcity significantly hinder their practical applications.Transition metal-based catalysts,especially those Fe-,Co-,Ni-based catalysts,are considered as potential alternatives to precious metals due to their various electronic states,abundant species,tunable structure,good catalytic activity and stability.In this work,the cobalt?nickel,iron?-based transition metal catalysts are synthesized and the effects of morphology and structure on the catalytic performance are systematically investigated.In order to solve the problems of low specific surface area,insufficient active sites,agglomeration and dissolution at highly potential during the process of electrocatalysis,we improve the electron transport efficiency and increase the number of active sites of metal nanoparticles by modifying the morphology and composition.Firstly,we prepared iron/nickel alloy nanoparticles embedded in N-doped hierarchically porous carbon(Fe_0.64Ni_0.36@NC),by simultaneously adsorbing metal precursors and dopamine on the surface of SiO₂ macroporous hard templates,followed by annealing the system and etching off the templates.The SEM images showed that the silica spheres were evenly distributed with a diameter of about 300 nm and a narrow size distribution.Thus,NC material prepared by using this pellet as a template has uniformly dispersed macropores,which is advantageous to rapid mass transfer.Meanwhile,iron/nickel alloy nanoparticles were uniformly dispersed in the carbonitride layer,which not only could protected the metal species from dissolution and agglomeration but also is beneficial to rapid electron transfer during the reaction.In electrochemical measurements,Fe_0.64Ni_0.36@NC shows a superior OER activity in alkaline solution,with an overpotential as low as 286 mV to deliver a current density of 10 mA·cm^-2,significantly lower than the value of 380 mV for RuO₂.Besides,the catalyst displays negligible activity decrease after 2000 cycles of continuous CV scanning,confirming its excellent durability.The observed nice performances of the alloy catalyst in alkaline solution can be ascribed to four critical structural features:?1?the macroporous structures made by stacking of SiO₂ microspheres endow the carbon framework with relatively thin layer,thus the embedded iron/nickel alloy particles can well activate the surrounding carbon layer to expose copious active sites;?2?the hollow spherical structure can provide channels for the release of O₂,and further increased the specific surface area;?3?the synergistic effect between the Fe_0.64Ni_0.36 alloy particles and the Fe atoms could enhance the electrochemical activity for OER;?4?the graphitized N-doped carbon layers are able to protect the alloy nanoparticles from corrosion,thus improving the durability of the catalysts.This work may give some insight into the design and synthesis of highly efficient non-noble-metal OER catalysts.Due to the effect of interlayer van der Waals force,transition metal sulfides are prone to stacking,agglomeration,etc.,which reduces the number of active sites and affects the exposure of effective active sites.In addition,the transition metal sulfide has poor conductivity,which seriously affects the charge transfer process and restricts its oxygen evolution activity.In this work,the exposure of abundant active sites is achieved via constructing a single layer interface sheet structure.Meanwhile,FeCo₂O₄-FeCo₂S₄/NF heteronanosheets directly grown on conductive nickel foam substrateis beneficial to the rapid electron transport and thus enhanced oxygen evolution activity.Firstly,Fe-Co double metal oxide nanosheets precursor was synthesized by a facile hydrothermal method,followed by a sulfidation process to obtain FeCo₂O₄-FeCo₂S₄/NF heteronanosheets and FeCo₂S₄ nanosheets dependent on the sulfidation extent.Comparative studies demonstrated that sulfidation degrees can alter the materialcomposition and thus dominate the catalytic activity of the products.Morphology characterization of FeCo₂O₄-FeCo₂S₄/NF heterononosheets revealed that the entire electrode surface was very rough,and then numerous nanosheets were coated with nanowire array stretching out,which can significantly increase the specific surface area,favor faster charge transfer,benefits the catalytic reaction.Electrochemical tests showed that the FeCo₂O₄-FeCo₂S₄/NF heteronanosheets require an overpotential of 232mV to achieve a current density of 50 mA·cm^-2.To afford a higher current density of100 mA cm^-2,only an overpotential of 247 mV is required.After 40 hours of continuous electrolysis,the electrode output remained stable and the shape of the electrode material remained substantially unchanged,exhibiting excellent oxygen evolution activity and stability.The excellent oxygen evolution activity and stability of FeCo₂O₄-FeCo₂S₄/NF heteronanosheets can be attributed to the following advantages:?1?the interaction between the tightly combined FeCo₂O₄ and FeCo₂S₄ create an unique active interface region,which enables sufficient contact with the electrolyte and provides abundant accessible active sites;?2?A integrated electrode structure is constructed by directly growing FeCo₂O₄-FeCo₂S₄/NF heteronanosheets on the surface of 3D interconnected Ni foam,and this structure is able to ensure uniform growth of the nanostructure and is robust enough to fall apart,which can take full advantage of the structural superiority and stability of the catalysts.