Mechanism Study Of Effect Of Magnetic Field/Strain On Electrochemical Performance

The traditional strategies to improve the electrochemical performance of energy materials mainly focus on regulating the morphology,structure and composition of materials.These strategies develop with the material themselves.With the development of scientific research,introduction of external physical field to enhance the reactivity can add an extra degree of freedom to the design of electrochemical processes.The external physical method can not only tailor the intrinsic activity of materials,but also modulate the local environment of the reactant-electrolyte interface.Therefore,the coupling of external physical field with electrochemical is very promising in both the catalytic field and the energy storage field.Among them,magnetic field and strain field have a good effect on the modulation of preparation of materials and the electrochemical process.The research on the coupling of magnetic field and strain field with electrochemical is still in the preliminary stage.The research on the mechanism of action is not in-depth enough.Therefore,this paper takes the influence of magnetic field and strain field on the electrochemical performance of materials as the subject.Simple systems were used and the mechanism of magnetic field and strain field with electrochemical was discussed in study.The specific research content is as follows:（1）First,an external magnetic field was used to modulate the dealloying process of the Ni-Al alloy in alkaline solution.The magnetic field first prolongs the Ni rearrangement process when the magnetic flux density increases from 0 T to 0.2 T.When the magnetic flux density becomes greater than 0.3 T,the magnetic field has the opposite effect-it shortens the Ni rearrangement process.The much finer morphology of nanoporous Ni can be prepared by adjusting the external magnetic flux density.This behavior could be attributed to the mass transfer induced by the magnetic field.The nanoporous Ni prepared with/without magnetic field was used as a supercapacitor electrode(np-Ni₀and np-Ni₅₀₀).The electrochemical performance of np-Ni₀and np-Ni₅₀₀is different.The np-Ni₅₀₀prepared under an external magnetic field of 500 m T exhibits a much better electrochemical performance,in comparison with that（np-Ni₀）prepared without applying a magnetic field.Furthermore,the specific capacitance of the np-Ni₀electrode could be further enhanced when we increase the magnetic flux densities from 0 T to 500 m T,whereas the np-Ni₅₀₀electrode exhibits a stable electrochemical performance under different magnetic flux densities.（2）Second,an external magnetic field was used to modulate the hydrogen evolution reaction（HER）at nickel surface in alkaline solution.The variation of reaction current was tuned when adjusting the external magnetic field.The magnetic-induced change in reaction current is strongly dependent on the overpotential.A theoretical model was built up by introducing the term of magnetic field into the traditional kinetic rate equation of HER.The model was based on the Volmer-Heyrovsky mechanism for HER and the Frumkin adsorption isotherm equation at nickel surface.The agreement between theoretical analysis and experimental result supports that the activation enthalpy as well as the adsorption enthalpy are magnetic-dependent.At equilibrium potential the reaction is controlled by the magnetic dependence of activation enthalpy.At large overpotential it is a weighted sum of magnetic-dependent activation enthalpy as well as adsorption enthalpy.The weighting factor emerges as strongly dependent on the hydrogen coverage.（3）Last,mechanical stretching as a non-conventional method modulated the electrochemical double layer capacitive behaviour of an amorphous carbon cloth surface.The capacitive potential is shifted positively while the corresponding current is shifted negatively when tensile strain is applied.More importantly,the discharging time is shortened when mechanically stretched.This phenomenon suggests that the specific capacitance is strain-dependent,and tensile strain decreases the specific capacitance of carbon cloth.This dependence could be attributed to a change in the surface free energy of carbon cloth when it is mechanically stretched.External mechanical work can modulate the typical capacitive process on not only metal or metal-oxide surfaces with specific lattice structures but also the surfaces of amorphous materials.Mn O₂was then deposited on the stretched carbon cloth.The mechanical stretching can improve the deposition of Mn O₂compared without stretching for the same time.