Fabrication And Synthesis Of Functional Carbon Coupled CoFe(Oxy) Hydroxides For Oxygen Evolution

Hydrogen energy,with high energy density and zero pollutant emission,has been considered as a promising clean and renewable energy.Electrochemical water splitting into hydrogen holds a great promise in the future alternative energy fields.Nevertheless,the energy conversion efficiency of this process is often limited by the sluggish kinetics of the oxidative half reaction of oxygen evolution reaction(OER)due to the four proton-coupled electron transfer process.In this case,highly efficient and stable non-noble metal catalysts are required and highly desired to decreasing the overpotential of OER and improving the water splitting efficiency,thus facilitating the industrial application of electrochemical water splitting.In this thesis,we have tried to develop different methodologies and strategies for the design and fabrication of functional carbon coupled CoFe(oxy)hydroxides composites with a focus on the optimization of morphology structure and electronic/coordination structure,which have been demonstrated to improve the exposure and utilization efficiency of active sites as well as the transfer rate of electrolyte ions and electrons,thus realizing the enhancement of the OER activity and reaction kinetics.Two-dimensional(2D)nanosheet-shaped cobalt-iron-layered double hydroxide(CoFe-LDH)are fabricated through the incorporation of Fe3+ into Co(OH)2 matrix.The Fe3+is capable of broadening the interlayer space of the as-made 2D nanosheets from 4.7 A of Co(OH)2 to 7.9 A of CoFe-LDH.It has been found that the broadened interlayer space is helpful for the mass transfer during OER process,thus leading to the enhanced electrochemical activity of CoFe-LDH nanosheets,with an overpotential of 340 mV at 10 mA cm-2.In addition,the 2D sheet-on-sheet binary architectures are configured by assembling CoFe-LDH nanosheets on graphene oxide(GO).The as-made binary architectures,with efficient CoFe-LDH active species and well-interconnected conductive networks,exhibit mass and charge transfer co-enhanced behaviors for OER,in which the overpotential is further reduced to 325 mV to deliver the current density of 10 mA cm-2.GO is partially reduced to fabricate functional graphene through different strategies,with the aim of figuring out the effect of different reduction strategies on the morphology and structure of functional graphene.It has been found that the thermally expanded graphene(TEG)obtained by the thermal expansion reduction strategy is composed of crumpled sheets with numerous corrugations and three-dimensional(3D)open porous structure.In addition,the as-made functionalhod graphene carbon materials are employed as the building blocks to fabricate a series of nanocomposites composed of functional graphene coupled with CoFe-LDH nanosheets by co-precipitation strategy.The effects of different functional graphene on the morphologies and electrochemical properties of the nanocomposites are studied.Of the series of nanocomposites fabricated,the CoFe-LDH/TEG nanohybrid consisting of TEG and CoFe-LDH nanosheets exhibits the best electrochemical activity and kinetics in alkaline medium with a low overpotential of 301 mV at 10 mA cm-2.A novel strategy has been developed for the fabrication of 2D nanocomposites made of nanometer-sized Fe-modulated Co oxyhydroxide(Fe-CoOOH)coupled with graphene via selective etching method from cobalt-iron-aluminum-layered double hydroxide(CoFeAl-LDH)and graphene precursors.The Al species is dissolved in strong alkaline medium based on its amphoteric metal property,thus realizing the transformation in both crystalline structure and microscale from micrometer-sized CoFeAl-LDH sheets to nanometer-sized Fe-CoOOH particles.The effects of Fe species on the morphological and structural transformation are revealed.More importantly,it has been found that the Fe components play a crucial role for the formation of ultrasmall nanometer-sized particles.Meanwhile,the anchoring effect of the graphene substrate also plays a crucial role for improving the dispersion of nanoparticles and the exposure of active sites.Thus,the as-made Fe-CoOOH/G nanohybrids demonstrate a superior electrochemical activity with a low overpotential of 330 mV at 10 mA cm-2.Density functional theory(DFT)calculations further revealed that the Fe dopants in the Fe-CoOOH nanoparticles show an enhanced adsorption capability toward the oxygenated intermediates involved in OER process,thus finally facilitate the catalytic reaction.Co-engineered FeOOH nanosheets have been successfully integrated on carbon fiber paper(Co-FeOOH/CFP)surface with vertical orientation growth via electrochemical deposition method.The surface electronic structure and coordination structure of Co-FeOOH have been analyzed by X-ray atomic absorption spectrometry.In-depth studies reveal that the coordination structure of FeOOH can be modulated by the Co doping,thus leading to the formation of unsaturated FeO6 ligand structures.Further theoretical calculation found that the enriched unsaturated FeO6 ligand structures show an enhanced capability for adsorbing OH-species,which results in the enhanced OER activity.As expected,the Co-FeOOH/CFP hybrids exhibit an extremely low overpotential of 250 mV to reach the current density of 10 mA cm-2.