Fabrication And Performance Of Ni-and Co-Based Catalysts For Electrochemical Water Oxidation

As a fuel of high gravimetric energy density and carbon neutral,hydrogen is an ideal substitute for traditional fossil fuels.Among many innovative approaches,electrocatalytic water splitting,driven by electric power generated from renewable energy sources such as wind,solar,and tidal energy,etc.,is a promising technology for industrial hydrogen production.Water splitting involves two half reactions,that is,water oxidation to evolve O2 and proton reduction to produce H2.The water oxidation reaction involves the transfer of four protons and four electrons,and it needs to overcome a high thermodynamic energy barrier,which is hence considered to be the bottleneck of the electrochemical overall water splitting reaction.Therefore,the development of cheap,efficient and stable electrochemical catalysts for water oxidation is the key to achieve large-scale and low-cost water splitting.In this thesis,the nonnoble metal Ni-and Co-based materials were employed as the main structures,and the electronic structures and local coordination environments of the central metals were regulated by combining with the semi-metallic B element and doping with Fe,V and Mn elements of different d-band centers,so as to optimize the binding energies of OER intermediates and to realize efficient and stable electrocatalytic water oxidation in alkaline media.The main work of this thesis includes following three parts:(1)The NiBx nanoparticle film was directly grown on the surface of Cu foil by a simple,convenient,and easy-to-scale one-step electroless plating method to fabricate NiBx/Cu electrodes.The effects of the B/Ni atomic ratio and the annealing temperature on the crystal phase composition,electronic conductivity,and electrochemical active area of the NiBx/Cu electrode were investigated.The results showed that when x=0.45 and Tanneal.=250℃,the NiB0.45-250/Cu electrode exhibited the highest electrocatalytic water oxidation activity in 1.0 M KOH solution.The current density of 10 mA cm-2 was achieved with only 296 mV overpotential,and the current density was maintained for more than 60 h.The electrochemical impedance spectra and Tafel curves illustrated that the NiB0.45-250/Cu electrode had low interfacial electron transport resistance and fast water oxidation kinetic process.In addition,energy dispersive X-ray elemental mapping,high-resolution transmission electron microscopy,Raman spectroscopy,and soft X-ray adsorption spectroscopy(XAS)consistently pointed out that a NiB0.45/NiOx core-shell heterostructure was in-situ formed on the surface of NiB0.45/Cu electrode during the alkaline OER process.(2)Multi-metal ultrathin nanosheets(Ni3Fe1-xVx)were arrayed on the surface of hydrophilic carbon fiber paper(CFP)by one-step hydrothermal synthesis.When the atomic ratio was 1:1,the Ni3Fe0.5V0.5/CFP 3D electrode exhibited super-efficient OER activity in 1.0 M KOH solution and only needed 200 and 264 mV overpotentials to achieve current densities of 10 and 100 mA cm-2,respectively,and the stable electrolysis at 100 mA cm-2 was sustained for more than 60 h.The hard X-ray absorption near-edge structure spectra(XANES),Fourier and wavelet transform extended X-ray absorption fine structure(FT/WT-EXAFS)spectra,and density functional theory(DFT)calculation results demonstrated that all doped Fe and V atoms occupied the Ni lattice sites in Ni(OH)2,that is,in a way of substitutive doping.The X-ray photoelectron spectra(XPS)and soft XAS spectra revealed the synergistic electron interactions among Fe,V and Ni ions of different valence electron structures,as well as the role of Fe and V doping in regulating the electronic structures and local coordination environments of metals in the catalysts.In-situ electrochemical Raman and XAS spectra revealed that the host framework in the Ni3Fe0.5V0.5 catalyst was transformed to the active phase of NiOOH in the alkaline OER process,and that the ultra-short V-O bond and the severely distorted[VO6]octahedral structure unit were formed.DFT calculation suggested that the V site located at the aggregation of some Fe and V atoms has an optimized binding energy for the OER oxygen intermediate and the lowest theoretical overpotential.(3)The Mn/V co-doped Co(OH)2 nanowires were in-situ arrayed on the surface of the pretreated hydrophilic CFP by a one-step hydrothermal synthesis method to fabricate CoxV1-yMny/CFP 3D electrodes.Electrocatalytic OER study showed that the optimized C05V0.67Mn0.33/CFP electrode required only an overpotential of 270 mV to achieve a current density of 10 mA cm-2 in 1.0 M KOH solution,and the stable electrolysis of water could be sustained for more than 50 h.Powder X-ray diffraction,XANES and FT-EXAFS spectra demonstrated that the V and Mn atoms occupied the lattice sites of Co in Co(OH)2 in a way of substitutive doping,forming the[MO6]octahedral coordination structural units.XPS spectra indicated that some electrons migrated from Co2+ and Mn3+ to V5+ via the oxo-bridges.Moreover,in-situ Co K-edge XAS and ex-situ Co L2,3-edge XAS spectra showed that the host framework of Mn/V co-doped Co(OH)2 was reconstructed during the alkaline OER process to form CoOOH active species,and that the Co-O bond in the first shell displayed a certain degree of shrinkage.