Controllable Synthesis And Electrocatalytic Properties Of Mn-doped Porous Spinel Compounds

Since the industrial revolution,human demand for traditional fossil fuels,such as coal,oil and natural gas,continues to climb.But the excessive use of these traditional fossil fuels could cause serious environmental pollution,destroys the ecological balance of nature and creates an energy crisis.In order to solve the above ecological and energy problems,we urgently need to seek renewable and clean energy.Hydrogen,as a sustainable clean energy carrier,has advantages such as wide sources,zero carbon emissions,and high calorific value.The use of renewable energy as the driving force and the adoption of a zero carbon emission electrolytic water process to produce hydrogen is the only way to achieve a green hydrogen energy source.The electrolytic water process mainly consists of two half-reactions,in which one is the oxygen evolution reaction（OER）occurring at the anode and another one is the hydrogen evolution reaction（HER）occurring at the cathode.However,the multiple electron transfer processes involved in both OER and HER result in slow kinetic rates.Generally,it needs require the use of high performance catalysts to lower the reaction energy barrier.Currently,the noble metal Pt is still regarded as the most efficient catalyst for cathodic hydrogen evolution reaction,and the best catalyst for oxygen evolution reaction is Ru O₂/Ir O₂,but their practical application is somewhat limited by the high price and increasing scarcity of noble metal/noble metal oxide.Therefore,the development of non-precious metal catalysts with low cost,high efficiency and long term stability under harsh conditions for water electrolysis has become a hot research topic worldwide.In recent years,spinel-type compounds have attracted wide attention due to their advantages such as diverse structure,stable performance,abundant resources,low costs and environmental friendliness.The general formula is AB₂X₄,in which A is bivalent cation and B is trivalent cation.Therefore,spinel-type compounds have mixed valence states,which can promote electron jumping between different metal valences.The conductivity of the material has been significantly improved,and thus providing more active sites for electrocatalytic reactions.This thesis mainly discusses how to obtain efficient and stable spinel oxide/sulfide catalysts.The surface structure and electronic distribution state of the catalyst were modulated by means of morphology regulation,element doping,the introduction of vacancy defects,anion tuning,etc.Thus,specific functions are assigned to the catalysts to improve their performance.Meanwhile,the electronic structure of materials and adsorption/desportion ability towards reaction intermediates have been investigated through theoretical calculations.The main research content of this thesis is listed below:（1）Mn-base transition metal alloy ribbons Mn_100-xNi_x（x=30,25,20）were synthesized by vacuum induction melting and single roll quench methods.By chemical dealloying in 1 mol/L（NH₄）₂SO₄ solution,the Mn element could be selectively dissolved from the above initial alloy,and thus spontaneously forming a nanoporous（np）core-shell structure.namely Ni Mn₂O₄/np-Ni（Mn）,can be spontaneously formed;moreover,Enriched oxygen vacancy（O_V）Ni Mn₂O₄/np-Ni（Mn）was obtained by hydrogen annealing at a certain temperature in order to improve the catalytic activity of the material.Analysis of the results of the physical characterization shows that,The Ni Mn₂O₄ shell in O_V-Ni Mn₂O₄/np-Ni（Mn）is enriched with oxygen vacancy,which can optimize the electron conductivity of the material,while the np-Ni（Mn）core has a large specific surface area can provide sufficient mass transfer channels for electrocatalytic reactions.Electrochemical tests show that the O_V-Ni Mn₂O₄/np-Ni（Mn）electrode provides a current density of 10 m A cm^-2 at a low operating potential of only 130m V,as well as an exchange current density as high as 0.5011 m A cm^-2 and long term stability in industrial alkaline hydrogen production environments.Density functional theory（DFT）calculations show that an appropriate amount of O_V in Ni Mn₂O₄could reduces the energy barrier of the rate-determining step（RDS）from 1.59 e V to 1.49 e V,indicating that O_V is the active site for water dissociation in alkaline HER process.（2）The precursor alloys Mn₇₀Ni₁₅Fe₁₅,Mn₇₀Ni₁₀Fe₁₀Co₁₀,Mn₇₀Ni₁₀Fe₁₀Cu₁₀,and Mn₇₀Ni_7.5Fe_7.5Co_7.5Cu_7.5 were designed and prepared.The corresponding alloy ribbons were prepared using rapid solidification melt-spinning technology.Subsequently,a hereditary Mn doped sulfide electrodes,such as S-np-Ni₁₅Fe₁₅（Mn）,S-np-Ni₁₀Fe₁₀Co₁₀（Mn）,S-np-Ni₁₀Fe₁₀Cu₁₀（Mn）,and S-np-Ni_7.5Fe_7.5Co_7.5Cu_7.5（Mn）were prepared by completely chemical dealloying and hydrothermal vulcanization treatment.Through characterization methods such as SEM,XPS and ICP-OES,it can be found that the as-vulcanized electrodes exhibit different morphologies with different constitute of precursor alloy ribbons.The surface of S-np-Ni₁₀Fe₁₀Co₁₀（Mn）exhibited densely grown and uniformly distributed nanorods,which greatly increases the electrochemically active area of the electrode material and exposes more electrochemical active sites.The electrochemical OER performance evaluation also uses 1M KOH as the electrolyte.S-np-Ni₁₀Fe₁₀Co₁₀（Mn）exhibited good oxygen evolution reaction performance,requiring an operating potential of only 138 m V to reach a current density of 10m A cm^-2,acquiring the tafel slope of only 55.1 m V dec^-1,and thus showing good kinetics of the oxygen evolution reaction.After 24 hours of chronopotentiometry testing,S-np-Ni₁₀Fe₁₀Co₁₀（Mn）possessed good stability and durability.