Understanding the Origin of Magnetic Field Dependent Specific Capacitance in Mn₃O₄Nanoparticle Based Supercapacitors

Abstract

Contrary to what has been recently reported for electrode material MnO 2 , Mn 3 O 4 actually shows reduction in the specific capacitance values under magnetic field. This observation cannot be explained by the earlier suggested reasons such as varying magnitude of Lorentz force, Nernst layer, ion-concentration at solid electrolyte interphase and the probability of intercalation/de-intercalation probability. An additional factor viz., magneto-dielectric constant, has to be invoked to understand the suppression of specific capacitance. The paper also presents a new template-free room temperature synthesis protocol for obtaining Mn 3 O 4 nanoparticles. These particles, when used in three electrode configuration, return a specific capacitance of ∼290 F g −1 at a scan rate of 10 mV s −1 and ∼221 F g −1 at current density of 0.5 A g −1 in combination with 1 M Na 2 SO 4 as electrolyte. Nearly 50% change in specific capacitance is observed as a function of magnetic field strength. The underlying reasons are presented. The paper shows that the use of magnetic transition metal oxides based electrochemical capacitors near magnetic field will have to be revisited.

Toward the Origin of Magnetic Field-Dependent Storage Properties: A Case Study on the Supercapacitive Performance of FeCo2O4 Nanofibers

“…The GCD and CV results complement each other and clearly indicate an improvement in the capacitance values when an external magnetic field is applied. Similarly, magnetic field-dependent supercapacitive properties of metal oxides/composites have been reported in the past. − …”

“…Wang et al and Pal et al have also seen similar results; however, they stressed upon the importance of surface area of the electrode material, in particular, of the magnetic nature and its compositing with carbon. , Furthermore, the filling of a magnetic material into the space between two layers of carbon was proposed to increase the interfacial area which results in an improved storage performance. ,, The change in the supercapacitive properties of the material under the magnetic field was ascribed to the variation in the dimensions of the Nernst layer formed at the interface of the electrode/electrolyte . Later, the same group observed a decrease in the dielectric constant as well as in the capacitance of a material under the magnetic field . However, in contrast, surprisingly, Shi et al showed enhancement in the dielectric constant of a material at the electrode/electrolyte interface, followed by a change in the supercapacitive properties of the material …”

“…Furthermore, gravimetric and volumetric capacitances of SCs can be tuned/improved by modifying/controlling the pore size and its distribution, structural integrity, effective active surface area, and surface chemistry of electrode materials. − Keeping the above facts in focus, in the last few years, researchers have been interested in developing an improved supercapacitive performance using transition-metal oxides and chalcogenides. − The storage performance of these oxides is found to be moderate only because of their low conductivity and large volume variation in the structural unit cell during charging and discharging processes. , However, the storage capability of these materials is improved by reducing the interfacial resistance at various interfaces in SC devices. Compositing with conductive materials and designing different nanostructured materials of high surface area and optimum pore sizes are some of the used strategies. − Recently, there has been increasing interest in the studies on the coupling of the electrochemical performance of SCs with external physical fields, − which implies that the associated interfacial resistance and the redox mechanism are being investigated under an external magnetic field. It is worth mentioning that the effect of an external magnetic field on biomaterials and aqueous solutions has been studied in the past because the applied magnetic field changes the Lorentz force acting on moving charges/ions under the electric field, and therefore, the concentration of charge varies at the electrode/electrolyte interface as compared to the bulk of electrode, thus a different storage mechanism may be expected. − Motivated by the above facts, Zhu et al showed enhancement in the storage capability of the used material under a magnetic field which was attributed to the reduction in the interfacial resistance and thereby improvement in the electron/ion transportation .…”

Electrode Materials with Highest Surface Area and Specific Capacitance Cannot Be the Only Deciding Factor for Applicability in Energy Storage Devices: Inference of Combined Life Cycle Assessment and Electrochemical Studies

“…With the increase in the scan rate, specific capacitance decreased, which is a normal trend in metal oxide based supercapacitors. This happens because the faradaic behavior decreases and EDLC contribution starts dominating the overall specific capacitance values rate. , To have an idea of the faradaic and nonfaradaic contribution in the specific capacitance of the electrode materials, quantification of pseudocapacitive and double layer portions was performed. When the scan rate was increased by 20-times, that is, 200 mV s –1 , specific capacitance retentions were 77%, 63%, and 84% for CO-NS, CO-ND, and CO-SNS, respectively.…”

Theoretical Model for Magnetic Supercapacitors—From the Electrode Material to Electrolyte Ion Dependence

“…Very recently, there have been few studies on energy storage technologies such as supercapacitors and Li-batteries, where appreciable modifications in the electrochemical performance have been reported. − These effects are quite prominent in supercapacitors fabricated using metal oxides that have at least one magnetic ion. Most of the authors, including us, have attributed the change to the changing Lorentz force and/or magneto-hydrodynamic effects. − However, the literature lacks a suitable theoretical model that explains the experimental data and incorporates the impact of the radius of electrolyte’s ion, which is critical in the case of magnetic supercapacitors. The size of the electrolyte ion should be considered because the kinetic energy of the ions will be directly modulated by the Lorentz force experienced by them.…”