Materials Science Aspects and Performance of High Power Magnetically Ordered Pseudocapacitors
Authors: Zhitomirsky Igor 
Published: December 2024

This investigation was motivated by increasing interest in magnetically ordered pseudocapacitor (MOPC) materials for applications in high-power pseudocapacitors and water purification devices. MOPCs exhibit capacitance enhancement due to the magnetohydrodynamic effect and interesting magnetocapacitive effects, which result in influence of charge/magnetization on magnetic/pseudocapacitive properties.
Advanced magnetic materials, such as ferrimagnetic spinels, ferrimagnetic hexagonal ferrites and ferromagnetic lanthanum strontium manganates (LSM) were studied. Electrodes with active mass loadings of 40 mg cm-2 were tested in Na2SO4 electrolytes. Chemical precipitation methods, hydrothermal synthesis and high energy ball milling techniques were used for the synthesis of nanoparticles. Capping agents, such as caffeic acid, gallic acid and murexide, were used for synthesis. The chelating groups of the capping agents facilitated their adsorption on particles and the adsorbed capping agents limited particle growth. The use of multifunctional organic capping agents-alkalizers influenced the morphology and phase content of the synthesized materials and facilitated the fabrication of electrodes with enhanced capacitance. In another approach, enhanced capacitance was achieved using redox active dispersants, such as gallocyanine and Celestine Blue, which acted as charge transfer mediators and facilitated charge transfer.
Electrochemical testing was performed using cyclic voltammetry, galvanostatic charge-discharge and impedance spectroscopy. Ferrimagnetic spinels, such as CuFe2O4, Fe3O4, γ-Fe2O3, MnFe2O4 showed capacitances of 2.76, 5.82, 3.78 and 2.67 F cm-2, respectively. The high active mass electrodes showed low resistance, which was below 1 Ohm. The large voltage window, high capacitance and low resistance of the electrodes facilitated the fabrication of devices with enhanced power-energy characteristics. The electrodes were used for the fabrication of asymmetric and symmetric devices. A symmetric device, containing two ferrimagnetic MnFe2O4 electrodes showed a capacitance of 0.92 F cm-2. Asymmetric devices showed capacitances in the range of 2.4-3.2 F cm-2 in a voltage window of 1.6V.
Ferrimagnetic SrFe12O19 and BaFe12O19 showed capacitances of 1.2 and 1.34 F cm-2, respectively. Good capacitive behavior was linked to beneficial effect of high energy ball milling and the use of efficient dispersant, which also acted as a charge transfer mediator. A remarkably high capacitance of 4.59 F cm-2 was achieved at a low resistance for ferromagnetic LSM electrodes. The electrodes showed good capacitance retention at fast charge-discharge rates and excellent cyclic stability.
Magnetic properties of the ferrimagnetic and ferromagnetic materials were analyzed. The charging mechanisms in positive and negative potential ranges were studied. The effect of hard and soft magnetic properties on capacitive behavior was analyzed. The high active mass MOPC electrodes showed good capacitance retention at fast charge-discharge rates and enhanced power-energy characteristics.
The MOFC electrodes tested in this work combine advanced charge storage properties, high spontaneous magnetization, low resistance, giant magnetoresistance and other functional properties. The obtained electrodes are promising for energy storage in supercapacitors with enhanced power-energy characteristics for operation at fast charge-discharge rates, capacitive deionization of water devices and novel applications based on magnetocapacitive effects.