Preparation And Catalytic Behavior Of Dual-phase High-entropy Alloys

In recent years,the energy crisis and the dye wastewater pollution have created an urgent requirement to find renewable energy sources and ways to treat the water environment.Hydrogen production from water electrolysis can solve the energy problem,and the degradation of dye wastewater can solve the environmental pollution of water resources.Therefore,it is imperative to develop low-cost and efficient catalysts to reduce the reaction barrier.High-entropy alloys（HEA）can optimize surface microstructure and chemical properties through various composition designs due to their unique atomic structure and huge multi-element composition space.In addition,the high mixing entropy of multi-component HEAs contributes to generate a large number of atoms with catalytic activity.It induces a new adsorption mode at the interface between reactant products,opening up a broad prospect for researching and developing new metal catalytic materials.In the present study,Al Fe Cr Mn Ti Zr_0.5HEA powder used for malachite green（MG）solution degradation,Fe Co Ni Mo Cr and FeCoNiMoCu HEA powders used for electrocatalytic oxygen evolution reaction（OER）were prepared by mechanical alloying（MA）technique,all showing the dual-phase structure.Through XRD,laser particle size analyzer,FESEM,TEM,XPS,UV spectrophotometer and electrochemical workstation et al,the phase structure,micro-structure and catalytic behavior of the two HEA systems were systematically investigated,and the relevant catalytic mechanism was discussed.The results are shown below:1.The mechanically alloyed Al Fe Cr Mn Ti Zr_0.5HEA powder with FCC and BCC solid solution structure.It is directly used for decolorizing MG solution without pretreatment,showing excellent degradation performance.Moreover,no new organic matter is produced during degradation.Comparative experiments under light and dark conditions demonstrate that the MG decolorization by HEA powder depends on the effect of light degradation,not just absorption.The rate of reaching degradation saturation and maximum degradation efficiency of the HEA catalyst are much better than those of Fe⁰powder.The powders after catalysis exhibit three characteristic surface morphologies including（P-I）similar to the surface before catalysis;（P-II）surface densely covered by nanoparticles;and（P-III）blooming flower surface superimposed by nanosheets.The results show that the oxides or hydroxides of Al and Mn nucleate and grow up in the powder outside,existing in the form of initial nanoparticles and continuously growing nanosheets during the degradation process.It provides a larger surface and more active sites,thus improving the catalytic efficiency.The exfoliated nano-reactants and exposed fresh powder surface enhance the reusability and sustainability of degradation.The combination of chemical dye decomposition and physical dye adsorption contributes to good catalytic performance.2.The mechanically alloyed Fe Co Ni Mo Cr and FeCoNiMoCu HEA powders have dual-phase structures.Foam nickel（NF）is mainly used as the support material.HEA/NF electrodes were prepared from two unpretreated powders by self-sedimentation.Compared with single NF and noble metal oxide Ru O₂/NF electrodes,it shows excellent electrocatalytic OER performance.Fe Co Ni Mo Cr HEA consists of a Mo-rich BCC phase and a Fe/Cr-rich amorphous phase with a particle size mainly between 2-5μm.FeCoNiMoCu HEA consists of a Mo-rich BCC phase and a Cu-rich FCC phase with a particles size mainly between 6-8μm.The BCC particles in Fe Co Ni Mo Cr HEA powder were distributed in the amorphous matrix,showing a relatively clear BCC-amorphous type stable-metastable interface.The BCC particles in FeCoNiMoCu HEA powder is distributed in FCC matrix,showing a stable crystal interface of BCC-FCC type.Electrocatalytic OER performance test of Fe Co Ni Mo Cr/NF shows that when the current density is 10 m A cm^-2,the overpotential of Fe Co Ni Mo Cr/NF is 298 m V,the Tafel slope is64.7 m V dec^-1,the C_dlis 0.74 m F cm^-2and the R_ctis 1.74Ω,compared with single NF and Ru O₂/NF,the performance improvement is 24%and 14%,respectively.The corresponding series of performance parameters of Fe Co Ni Mo Cu/NF are 345 m V,80.5 m V dec^-1,0.58 m F cm^-2and 8.7Ω,respectively.It indicates that both of them have good electrocatalytic OER performance,and Fe Co Ni Mo Cr/NF has better performance.The results show that compared with the stable crystal interface formed by the addition of Cu elements,the Cr addition in Fe Co Ni Mo is more favorable to the particle size refinement of mechanically alloyed powders.It forms a stable-sub-stable interface with higher interfacial energy and active sites,which is conducive to the adsorption of substances and promotes the electrocatalytic OER reaction in the surface.The metal-oxygen bonding during the reaction of FeCoNiMoCu HEA powder is more difficult than that of Fe Co Ni Mo Cr HEA powder.In addition,the latter reaction surface has a larger variety of valence states of Fe elements,including Fe⁰,Fe³⁺and Fe²⁺,and the binding energy of each valence ion of Co and Mo are smaller than that of the former electrode,which reduces the Rct and promotes the electron transfer during the catalytic reaction.Comprehensive comparative analysis shows that the phase synergy of dual-phase HEA increases the internal structural complexity and surface activity of the material,and the presence of the two-phase interface can provide higher interfacial energy and reduce the energy potential barrier during the reaction.The ionic valence of the principal elements in the dual-phase HEA is abundant,which is easy to form“galvanic cells”.Thus,it promotes the surface electron transport and further optimizes the electrocatalytic performance.