Research On Controlable Synthesis And Electrocatalytic Behavior Of Ferrite Series Metallic Nanomaterials

Electrocatalytic water splitting,as the most promising green technology for hydrogen production,shows great application value and prospect in future sustainable and scalable hydrogen manufacturing.The development of efficient hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)electrocatalysts is the key for practical application of water splitting.Due to the high cost of noble-metal catalysts,the urgent requirement is to develop high-performance noble-metal-free catalysts for HER and OER,especially that can operate efficiently in neutral and basic solution.To enhance the efficiency of overall water splitting,it is prior to optimize the catalytic properties of both HER and OER electrodes.The determining factors of the catalytic properties of noble-metal-free catalysts can be concluded as:(1)poor conductivity,(2)insufficient active sites,and(3)poor intrinsic activity.Based on these problems,this doctoral dissertation firstly analyze and summarize the preparation methods and catalytic properties of reported catalysts,and then prepare ferrite series metallic nanomaterials with good electron conductivity on selected two-dimensional or three-dimensional(2D or 3D)conductive substrate as working electrodes for water splitting.Nanomaterials synthesis methods,heteroatom doping and interface engineering are used to tune the catalytic properties of obtained materials.The catalytic mechanisms are experimentally and theoretically investigated.Finally,the overall water splitting performance is evaluated.The details are shown as follows.(1)The preparation of hierarchical nanostructure CoP and its neutral or wide pH HER catalytic properties.3D CoP nanowire network was synthesized on the Hastelloy belt by hydrothermal method and low-temperature phosphorization(CoP NW/Hb).It is demonstrated that the metal belt substrate can reduce the contact resistance between current collector and catalyst.The hierarchical nanostructure boosts the number of active sites.The 3D network and superhydrophilic property facilitate the diffusion and release of hydrogen bubbles,resulting in improved reutilization of active sites.The electrochemical measurements show that the CoP NW/Hb has excellent HER activity with an average onset potential of about 45 mV among wide pH range 0-14.The CoP NW/Hb exhibits an iR-uncorrected ?10 of 121 mV and a Tafel slope of 106 mV dec-1 in neutral media,accompanying with 100 h-long durability.Additionally,the CoP NW/Hb shows excellent flexibility,supported by the large-angle bending measurement and corresponding electrochemical measurements.However,CoP is easily to be oxidized.The surface oxide can influence the stability of the electrode and the investigations of the catalytic mechanism.(2)To investigate the influence of formed oxide,the dissertation prepared a new-type Fe-O-P composite nanorod arrays and studied their neutral and wide pH HER catalytic properties.Firstly,the Fe-O-P nanorod arrays were synthesized on carbon cloth by a partial phosphorization method(Fe-O-P NRs/CC),including amorphous FeOx and nanocrystalline FeP.The influence of the oxide to the HER catalytic property was detailedly studied via constrast experiments.It is demonstrated that the neutral HER activity of FeOx/FeP interface is better than that of FeP.The Fe-O-P NRs/CC exhibit a low ?10 of 96 mV and a small Tafel slope of 47 mV dec-1 in neutral media,accompanying with 60 h robust stability,better than aforesaid CoP,which makes it one of the best neutral HER catalysts.The catalytic behaviors of Fe-O-P NRs in acid and base were further investigated.Finally,the DFT calculations show that the FeOx/FeP interface could enhance the adsorptivity for water molecules,reduce the dissociation energy barrier of water molecules and the AGH*of FeP.(3)After optimizing the catalytic properties of HER,the dissertation started to explore novel OER catalysts and their catalytic properties.The hierarchical nanostructure metaphosphate(Ni2P4O12)was synthesized at the first time on carbon cloth via a low-temperature phosphorylation method(Ni2P4O12/CC).The obtained Ni2P4O12 nacrystal is in the size of less than 10 nm.The electrochemical measurements show that the Ni2P4O12/CC exhibits excellent OER performance in 1 M KOH with a low ?10 of 270 mV and 100 h robust stability.Then experimental and theoretical analyses show that the exposed facets of Ni2P4O12 naocrystals have low energy barriers for oxygenic intermediates adsorption.Further N doping to Ni2P4O12 lattice by a self-doping method was achieved to enhance the OER activity.The studies show that N doping could optimize the electron structure of Ni2P4O12,improve the conductivity and reduce the energy barrier of the formation of O-O bonding,thus increase the OER catalytic activity.However,hydroxide is discovered on the surface of Ni2P4O12 after OER,which may change the active species and catalytic mechanism.(4)To suppress the surface hydroxide,the dissertation designed and prepared a cytomembrane-structure-inspired active Ni-N-O porous interface nanoparticles and studied their OER catalytic properties.The DFT investigations on the catalytic surface show that different with general nitride OER catalysts,the introduction of NiO into Ni3N increases the formation barrier of Ni-OH,thereby suppresses the formation of hydroxide.The in situ oxidation strategy was employed to prepare Ni-N-O porous interface nanoparticles(NiNO INPs),including porous Ni3N matrix and formed NiO nanocrystals on the surface.The characterizations show strong coupling effect on the Ni3N/NiO interface.Compared to single Ni3N NPs,NiNO INPs achieve a reduced Tafel slope and a six-fold enhanced catalytic current at 1.6 V,consistent with the maxmum of 85%reduction of energy barrier in the formation of OOH*on the Ni3N/NiO interface.No amorphous hydroxide layer detected on the surface suggests the active phase is the Ni3N/NiO interface.(5)Although the special structure effect can suppress the surface hydroxide,to general Ni-based OER catalysts,the surface ocyhydrate is a common phenomenon.The investigations on the surface evolution play significant role on the understanding of OER mechanism.In this dissertation,the S doped Ni nitride and phosphide composite(NiNPS)was prepared firstly by multistep chemical reactions.Then combining in situ Raman tracking with ex situ microscopy and spectroscopy methods,the active species generated by surface evolution of NiNPS was identified during OER process.The observations of irreversible phase transformation between NiNPS and a-Ni(OH)2,and reversible phase transition between ?-Ni(OH)2 and ?-NiOOH prior to OER demonstrate that the surface reconstruction starts from the first electrochemical measurement and NiNPS is OER precatalyst despite the displayed excellent OER performance.?-NiOOH is the real active species of NiNPS composite for OER.In addition,there is no direct relation between electrochemical stability and chemical structure stability.The combination of experimental and theoretical investigations demonstrate that Ni-based matrix could optimize the energy barrier for oxygenic intermediates adsorption of surface structure and accelerate the electron transfer via the strong coupling effect.which improves the OER kinetics.Finally,the overall water splitting performance of NiNPS catalyst is evaluated.