Study On The Performance Improvement Mechanism Of Nickel-based Electrocatalysts For Water Splitting Assisted By Magnetic Field

The integration of magnetic field and electrochemical systems is a new frontier in the field of electrocatalysis.Directly or indirectly,the magnetic field can improve the thermodynamics and kinetics of the hydrogen evolution reaction（HER）or oxygen evolution reaction（OER）by increasing the surface temperature of the catalysts,regulating the spin state of electron and changing the spin state of the intermediates,so as to further improve its activity,selectivity and overall conversion efficiency on the basis of the original catalytic materials.Although some progress has been made in magnetic field-assisted overall water splitting,its development is limited by the lack of a comprehensive understanding of the mechanism on the uniform magnetic field-enhanced OER,the difficulty of catalysts with both magnetism and excellent catalytic activity,and can only work on HER or OER.Therefore,in this paper,an in-situ magnetic field-electrochemical testing device was built to investigate the mechanism of uniform magnetic field-enhanced OER for nickel-based catalysts,and further designs nickel-based catalysts with both magnetic and high catalytic activity to achieve a substantial improvement in the performance of water splitting under the magnetic field.The OER behavior of nickel-based catalysts（Ni（OH）₂,NiO and Ni）derived from Ni（OH）₂ was studied by by designing an electrochemical cell coupled to a vibrating sample magnetometer.Comparing the OER evaluation parameters（overpotential,Tafel slope,charge transfer resistance）of the three nickel-based catalysts under the magnetic field and spin electrons polarization rate,it is found that the increase of OER activity is positively correlated with the polarization rate of spin electrons,revealing that the decisive factor of magnetic field-enhanced OER is to accelerate the polarization transport of spin electrons.Since the Ni catalyst has the largest electron polarizability（8.988%）at a magnetic field of 14000 G,its activity improves most significantly under the magnetic field,and its overpotential decreases by 20 mV when a magnetic field of 14000 G is applied.Moreover,the Tafel slope analysis reveals that the magnetic field-induced spin-polarized kinetics will be more distinct in the case where the first electron transfer step is the rate-determining step of the OER.In addition,the hysteresis effect of ferromagnetic materials can enhance the OER activity of the catalyst for a period of time after removing the magnetic field.Nickel-iron layer double hydroxide/cobalt tetroxide p-n heterojunctions were prepared on the ferromagnetic nickel foam substrate（NiFe-LDH/Co₃O₄/NF）by hydrothermal and subsequent heat treatment process.NiFe-LDH/Co₃O₄/NF displays a promising OER catalytic activity(274 mV at 50 mA cm^-2)and long-term stability（70 h）.Analysis of band structure,surface valence state,and surface potential confirms that the Fermi level difference can lead to the electrons transfer from NiFe-LDH to Co₃O₄.The theoretical calculation proves that interfacial charge polarization promotes the adsorption energy of OER intermediates.Moreover,the OER performance of NiFe-LDH/Co₃O₄/NF can be further enhanced by applying an external magnetic field of 10000 G(247 mV at 50 mA cm^-2).The measurement of magnetic properties,the analysis of the electrochemical data under the magnetic field and the calculation of the spin polarization rate reveal that the existence of the exchange bias effect between the the magnetic nickel substrate and surface-active species（NiFe-LDH）.The interfacial coupling of NF and NiFe-LDH acts as a“spin filter”,screening electrons in a specific direction to generate oxygen in the form of spin triplet states,which thermodynamically reduces the energy barrier of OER.HER and OER bifunctional molybdenum-doped nickel sulfide catalysts supported on a magnetic nickel foam substrate（Mo-NiS_x/NF）were prepared by an in situ spontaneous redox reaction（SRR）and subsequent sulfurization process.NiFe-Mo-NiS_x/NF displays a promising HER(155 mV at 10 mA cm^-2)and OER(332 mV at 50 mA cm^-2)catalytic activity.XPS and in-situ Raman characterization revealed that appropriate Mo doping can optimize the electronic structure of NiS_x,accelerating the dissociation of H₂O and the adsorption of OH^-during HER and OER processes,and promoting the surface reconstruction of the catalyst.Furthermore,the overpotential of HER and OER decreased to 136 mV(10 mA cm^-2)and 307 mV(50mA cm^-2),respectively.The measurements of the magnetic properties of the catalysts reveal that Mo doping can convert NiS_x from paramagnetic to ferromagnetic.The electrochemical data under the magnetic field and the spin polarization rate show that the magnetic activity of Mo-NiS_x/NF can further enhance HER and OER activities under external magnetic field through magnetoresistance（MR）effect and ferromagnetic（FM）spin filter effect.As a result,the electrolytic cell constructed by the bifunctional Mo-NiS_x/NFs as both the cathode and the anode shows a low cell voltage of 1.635V at 10mAcm^-2,the voltage is further reduced to 1.594 V under the action of 10000 G magnetic field,and can continue to electrolyze hydrogen for at least 60 h.