Magnetic Field-enhanced Electrocatalytic Water Splitting Performance Of Ferromagnetic Alloy Catalysts

Green energy technologies such as water splitting,fuel cells,and metal-air batteries are mainly driven by redox reactions between water,oxygen,and hydrogen,namely,oxygen reduction reaction（ORR）,hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）.At this stage,noble metal Pt and noble metal oxides such as Ru O₂ and Ir O₂ are state-of-the-art catalysts for ORR,HER,and OER,which exhibit excellent catalytic activity and long-term stability under extreme working conditions.However,the high cost and scarcity of precious metals are serious barriers to widespread applications of these technologies.Therefore,researchers have been working on the design and modification of non-noble metal-based catalysts to achieve efficient catalytic activity and excellent stability.Despite considerable progress in transition metal-based catalysts,most non-noble metal catalysts are still inferior to noble metal catalysts in terms of catalytic performance.To solve this problem,external fields such as electric field,magnetic field,optical field,and strain,etc.are introduced into the field of electrochemistry,which provides more degrees of freedom for the improvement of electrocatalyst performance.Therefore,we can design catalysts using defect engineering,doping,exposure of active sites,etc.,while introducing an external field to further tune the performance of catalysts.In this paper,two novel ferromagnetic alloy catalysts were first deposited on porous substrates by pulsed laser deposition（PLD）,which increased the active area of the catalysts.Then,on the basis of studying the electrochemical performance of the catalyst,the magnetic field was used to further improve the performance of the catalyst.Finally,a mechanistic explanation was given for the regulation results.The main research content and conclusions are as follows:1.NiMnIn ferromagnetic alloy films（NiMnIn/NF）were deposited on Ni foam（NF）by PLD and used for HER in alkaline medium.While exploring the HER performance of NiMnIn/NF,magnetic field was used to further enhance its HER activity.The self-activation of NiMnIn/NF occurred during the HER process,and its overpotential at a current density of-10 m A cm^-2 decreased from 189.4 m V to 133.4m V with the chronopotentiometry test.This was because during the reaction process,a large number of nanosheets were formed in situ on the surface of the NiMnIn film,which enlarged its active area.At the same time,the oxide layer on the surface of the film was leached,which may also be the reason for the increase in performance.We further used the magnetic field to tune the properties of NiMnIn/NF.At an overpotential of 275 m V,the current density of NiMnIn/NF was increased by 10%and 7%by applying perpendicular and parallel magnetic fields,respectively.In order to explore the reason why the magnetic field enhances the electrocatalytic HER,NiMnIn/Si（NiMnIn deposited on Si）and Cu/NF（Cu deposited on NF）were prepared by PLD.By comparing the effect of magnetic field on the HER performance of NiMnIn/Si,NF,and Cu/NF,the spin-selection effect was considered to be the main reason for the magnetic field to promote the HER rate of NiMnIn/NF.2.CrMnFeCoNi high-entropy alloy（HEA）thin films（As-deposited HEA/CC）were deposited on carbon cloth（CC）by PLD and used for OER in alkaline medium.While exploring the oxygen evolution activity of HEA,it was further regulated by magnetic field.When exploring the performance of HEA,it was found that the OER activity of HEA/CC electrode was significantly improved by using CV activation technology（HEA/CC-a1）.Further using electrochemical measurement technology,XPS,and SEM,it was found that the performance improvement may originate from the active hydroxide layer formed on the catalyst surface and the increase of Ni²⁺/Ni^3+/4+transformation.At the same time,we found that the aging of the catalyst was greatly delayed by periodic rejuvenation steps in chronoamperometry.Through XPS and SEM,it was found that the active hydroxide on the surface of the catalyst was rejuvenation during the rejuvenation step,that is,the surface was restructured and the catalyst showed the characteristics of rejuvenation.Next,the performance of as-deposited HEA/CC and HEA/CC-a1 was tested under the magnetic field,and it was found that at a current density of 35 m A·cm^-2,the overpotentials of as-deposited HEA/CC were reduced by 15 m V and 16 m V under the vertical and parallel magnetic field,respectively.While the magnetic enhancement effect of HEA/CC-a1 was almost negligible.We explained this phenomenon by exploiting the behavior of the outer electrons and the spin selection effect during the OER process of the catalyst.