Study Of Nanostructures Based FeNi-Compounds For Water Splitting

The paper is to be aimed at solving the problems of water spitting based on traditional FeNi-compounds,including poor electron conductivity,unstability,exposing difficulty of catalytic active sites,strict preparation route.In the paper,we introduce the theories of water splitting,summarize the previous works and analyze the present problems.The catalytic performance of water splitting based on FeNi compounds is promoted by carbon coating,doping/loading and porous structure,FeNiOH-double nanotube arrays with high efficient catalytic performance are successfully prepared finally.Alloys,oxides and hydroxides novel nanostructures based FeNi are successfully synthesized through the different routes,which are more convenient and efficient than the previous methods.The detailed information about structures(such as electronic structure in the surface,morphologies etc.)are characterized,the relationship of unique structures and catalytic performance are discussed in detail.Finally,we build the model according the experimental programmer and explain the conclusion by theoretical calculations.The specific work is as follows:(1)To solve the problems about poor electron conductivity and short holes diffusion distance of Fe2O3,which is doped Ni atom,decreased in the particles size and coated by carbon layer to improve the(photo)electrocatalytic performance of water splitting.In the paper,?-CD and Ni atom are introduced in Fe2O3 preparation,which decreases to the particles size of?5 nm.The product is calcined to the final catalyst coated by carbon layer.We investigate and compare the(photo)electrocatalytic performance of water splitting based on them and the catalyst without Ni atom or carbon coating.The results show that the prepared catalyst possesses the ower overpotential(decrease?100 mV)in OER electrocatalysis and obvious photocurrent increasing(0.52 mA/cm2 to 3.3 mA/cm2 at 1.23 V vs RHE)in photocatalysis.The results indicate that thin carbon layer and Ni doping improve the electron conductivity and small particles size promotes the transfer efficiency of holes in the water splitting.(2)To further improve the electron conductivity and active sites of electrocatalysts,FeNi-alloy nanoparticles are embedded in the porous graphic carbon cage,which improves the stability of electrocatalysts.The eletrocatalytic performance for water splitting is investigated based on the alloys with the different Fe/Ni atom ratios.In the synthesis,the filter paper adsorbs FeNi-compounds directly and then they are calcined to the final electrocatalysts for wate spitting in Ar/H2 gas.FeNi ions are reduced to alloys and the fibres of filter paper change into porous carbon cage in high temperature.The porous carbon cage keeps the stability and dispersity of alloys nanoparticles.The electrocatalytic performance of catalysts with different Fe/Ni atom ratios are investigated,the results show the sample with Fe/Ni=1:1 exhibits the best electrocatalytic property,the overpotential are only 319 mVand 127 mV at 20 mA/cm2 for OER and HER respectively.The results indicate that carbon cages hinder the particles reunion,which promote the exposing of active catalytic sites.Moreover,it acts as electron collector to improve the electrons transfer efficiency.(3)To solve the problems about instability of power catalysts and loss of catalytic active sites in the electrode process,the porous nanoarrays are grown on the conductive substrate by in-site route.We design and realize FeNiOH-double nanotubes arrays(FeNi-HDNAs),Fe(OH)3 and Ni(OH)2 nanotubes arrays on Ni foam via an in-situ reaction and Kirkendall effect(ZnO+Mn++H2O-Zn2++ M(OH)n,M=Fe or Ni).The obtained catalysts possess higher specific surface area,more catalytic active sites and better chemical stability for OER.As expected,FeNi-HDNAs exhibit lower overpotential(206 mV at 10 mA/cm2)as well as lower Tafel slope and better durability than the Fe-or Ni-HD peers,they exhibit the superior stability than traditional power catalysts.It is found by XPS and XANES that electron migrations from the Fe 3d orbitals to Ni sites in the FeNi-HDNAs lead to more unoccupied Fe 3d states and a higher oxidation state.DFT calculations elucidate that FeNi hydroxides lower the energy barrier of rate-determining step in OER.Moreover,a high current density of 10 mA cm-2 is obtained at a low potential of 1.49 V using FeNi-HDNAs as the bifunctional electrocatalyst for overall water splitting in basic solution and the overpotential hardly increases in 100 h test.The results indicate that the porous structure promote the exposing of active catalytic sites,in-situ growth on the conductive substrate improve the electrons transfer efficiency and stability of catalysts.(4)To solve the dilemma between short holes diffusion distance and excellent light absorptivity,while to solve the problem of instability of Fe2O3 in the photocatalysiss.Fe2O3/ZnO cable-like array on FTO is successfully prepared by in-situ reaction and Kirkendall effect according the above-mentioned work.The small Fe2O3 nanoparticles in the surface make for holes transfer efficiency,while the arrays promote the light adsorption.The ZnO loading improves the stability of Fe2O3 in the photocatalysis,while Fe2O3 improve the light-absorption efficiency and photocatalytic performance of ZnO.PEC results show that photo current of Fe2O3/ZnO cable-like nanostructure ups to 1.38 mA/cm2 at 0.5 V(vs Ag/AgCl)in neutral solution,which is 1.8 and 4.6 times of ZnO nanorods array and Fe2O3 nanotubes array respectively.It is found clearly that ZnO can improve the stability of Fe2O3 obviously in the comparing results.In conclusion,Ni doping Fe2O3 coated carbon layers,FeNi-alloys limited carbon cages,Fe(OH)3/Ni(OH)2 nanotubes arrays and Fe2O3/ZnO cable-like nanocatalysts are successfully obtained by some facile routes in the paper.The problems about poor electron conductivity,instability and loss of active sites are solved by carbon coating,doping/loading and porous structures,which improve the catalytic performance of traditional FeNi compounds for water splitting.The study is definitely valuable in theory and commercial application.