Magneto-Dendrite Effect: Copper Electrodeposition under High Magnetic Field

Suppressing Competing Solvent Reduction in CO2 Electroreduction with a Magnetic Field

The presence of an external magnetic field is found to affect the competition between the H2O and CO2 reduction reactions by increasing mass transport via the Lorentz force. Increasing the magnetic field strength at the electrode surface from 0 to 325 mT increases the selectivity of CO over H2 by 3×, while an increase in current density from 0.5 to 5 mA/cm2 increases the selectivity of CO production by 5×. Cyclic voltammetry and finite-element simulations reveal that the origin of the enhanced CO selectivity is attributable to a magnetic field lowering the electrode-electrolyte interfacial pH. A drop in interfacial pH enables increased production of CO from CO2 reduction due to a decrease in the activity of H2O reduction and increase in CO2 solubility near the electrode surface. The insight provided in this study offers new opportunities to control reaction selectivity in electrocatalysis with magnetic field vectors.

Excess heat production of the pair annihilation of ionic vacancies in a copper redox reaction using a double bipolar MHD electrode

Through a copper double bipolar magnetohydrodynamic (MHD) electrode (MHDE) producing twice the amounts of ionic vacancies than a conventional single MHDE, the molar excess heat of the pair annihilation of ionic vacancies, 702 kJ mol-1 at 10 T on average was obtained in a copper redox reaction. It was about twice as large as that of a single MHDE, 387 kJ mol-1 at the same magnetic field. This result strongly suggests that a multi-channel bipolar MHDE will produce much greater excess heat. To conserve the linear momentum and electric charge during electron transfer in an electrode reaction, ionic vacancies are created, storing the solvation energy in the polarized core of the order of 0.1 nm, and the pair annihilation of the vacancies with opposite charges liberates the energy as excess heat. The promoted excess heat by the double bipolar MHDE with a diffuser at 10 T was 710 ± 144 kJ mol-1, whereas as mentioned above, 702 ± 426 kJ mol-1 was obtained by the same electrode without such a diffuser. From the theoretical excess heat of 1140 kJ mol-1, the collision efficiencies in pair annihilation were 0.623 ± 0.126 and 0.616 ± 0.374, respectively. From these results, the reproducibility of the thermal measurement was experimentally validated. At the same time, it was concluded that at magnetic fields beyond 10 T, the concentration of ionic vacancy and the collision efficiency take constant uppermost values.

Theory of Chiral Electrodeposition by Chiral Micro-Nano-Vortices under a Vertical Magnetic Field -1: 2D Nucleation by Micro-Vortices

Remarkable chiral activity is donated to a copper deposit surface by magneto-electrodeposition, whose exact mechanism has been clarified by the three-generation model. In copper deposition under a vertical magnetic field, a macroscopic tornado-like rotation called the vertical magnetohydrodynamic (MHD) flow (VMHDF) emerges on a disk electrode, inducing the precessional motions of various chiral microscopic MHD vortices: First, chiral two-dimensional (2D) nuclei develop on an electrode by micro-MHD vortices. Then, chiral three-dimensional (3D) nuclei grow on a chiral 2D nucleus by chiral nano-MHD vortices. Finally, chiral screw dislocations are created on a chiral 3D nucleus by chiral ultra-micro MHD vortices. These three processes constitute nesting boxes, leading to a limiting enantiomeric excess (ee) ratio of 0.125. This means that almost all chiral activity of copper electrodes made by this method cannot exceed 0.125. It also became obvious that chirality inversion by chloride additive arises from the change from unstable to stable nucleation by the specific adsorption of it.

Magnetically active lithium-ion batteries towards battery performance improvement

Lithium-ion batteries (LIBs) are currently the fastest growing segment of the global battery market, and the preferred electrochemical energy storage system for portable applications. Magnetism is one of the forces that can be applied improve performance, since the application of magnetic fields influences electrochemical reactions through variation of electrolyte properties, mass transportation, electrode kinetics, and deposits morphology. This review provides a description of the magnetic forces present in electrochemical reactions and focuses on how those forces may be taken advantage of to influence the LIBs components (electrolyte, electrodes, and active materials), improving battery performance. The different ways that magnetic forces can interact with LIBs components are discussed, as well as their influence on the electrochemical behavior. The suitable control of these forces and interactions can lead to higher performance LIBs structures and to the development of innovative concepts.

Magnetohydrodynamic Interface-Rearranged Lithium Ions Distribution for Uniform Lithium Deposition and Stable Lithium Metal Anode

Uneven lithium (Li) electrodeposition hinders the wide application of high-energy-density Li metal batteries (LMBs). Current efforts mainly focus on the side-reaction suppression between Li and electrolyte, neglecting the determinant factor of mass transport in affecting Li deposition. Herein, guided Li⁺ mass transport under the action of a local electric field near magnetic nanoparticles or structures at the Li metal interface, known as the magnetohydrodynamic (MHD) effect, are proposed to promote uniform Li deposition. The modified Li⁺ trajectories are revealed by COMSOL Multiphysics simulations, and verified by the compact and disc-like Li depositions on a model Fe₃ O₄ substrate. Furthermore, a patterned mesh with the magnetic Fe-Cr₂ O₃ core-shell skeleton is used as a facile and efficient protective structure for Li metal anodes, enabling Li metal batteries to achieve a Coulombic efficiency of 99.5 % over 300 cycles at a high cathode loading of 5.0 mAh cm^-2 . The Li protection strategy based on the MHD interface design might open a new opportunity to develop high-energy-density LMBs.

Long-Term Electrodeposition under a Uniform Parallel Magnetic Field. 1. Instability of Two-Dimensional Nucleation in an Electric Double Layer

Under a parallel magnetic field, after long-term copper deposition from an acidic copper sulfate solution, numerous spherical secondary nodules of 10 to 100 μm diameters were formed one upon another in dendritic mode. This is a new type of micro-magnetohydrodynamic (MHD) effect arising from the unstable growth of three-dimensional (3D) and two-dimensional (2D) nuclei by specific adsorption of hydrogen ions (second micro-MHD effect). From the viewpoint of instability in electrodeposition, though 3D nucleation in the diffusion layer is always unstable, with ionic specific adsorption such as hydrogen ions, stable 2D nucleation turns unstable after long-term deposition. The resultant competitive growth of 3D and 2D nuclei produces spherical nodules as their composite, leading to their dendritic growth. Furthermore, though negligibly small, nonequilibrium fluctuations occurring in 2D nucleation migrate with the laminar solution flow caused by Lorentz force (MHD flow). Depending on whether the ionic adsorption is specific or nonspecific, the traveling asymmetrical fluctuation changes the direction to the upstream or downstream side, respectively.

Excess heat production in the redox couple reaction of ferricyanide and ferrocyanide

In order to establish the universality of the excess heat production in electrochemical reaction, under a high magnetic field, as one of the most fundamental electrochemical reactions, the case of ferricyanide-ferrocyanide redox reaction was examined, where ionic vacancies with ± 1 unit charge were collided by means of magnetohydrodynamic (MHD) flow. As a result, from the pair annihilation of the vacancies with opposite signs, beyond 7 T, excess heat production up to 25 kJ·mol⁻¹ in average at 15 T was observed, which was attributed to the liberation of the solvation energy stored in a pair of the vacancy cores with a 0.32 nm radius, i.e., 112 kJ·mol⁻¹. Difference between the observed and expected energies comes from the small collision efficiency of 0.22 due to small radius of the vacancy core. Ionic vacancy initially created as a by-product of electrode reaction is unstable in solution phase, stabilized by releasing solvation energy. Ionic vacancy utilizes the energy to enlarge the core and stores the energy in it. As a result, solvated ionic vacancy consists of a polarized free space of the enlarged core surrounded by oppositely charged ionic cloud. The accuracy and precision of the measured values were ascertained by in situ standard additive method.

Excess Heat Production by the Pair Annihilation of Ionic Vacancies in Copper Redox Reactions

In the pair annihilation of ionic vacancies with opposite charges, a drastic excess heat production up to 410 kJ mol⁻¹ in average at 10 T (i. e., 1.5 times larger than the heat production by the combustion of H₂,  285.8 kJ mol⁻¹) was observed, which was then attributed to the emission of the solvation energy stored in 0.61 nm radius vacancies with two unit charges. Under a high magnetic field, using Lorentz force, we made ionic vacancies created in copper cathodic and anodic reactions collide with each other, and measured the reaction heat by their annihilation. Ionic vacancy is initially created as a byproduct in electrode reaction in keeping the conservation of linear momentum and electric charge during electron transfer. The unstable polarized particle is stabilized by solvation, and the solvation energy is stored in the free space of the order of 0.1 nm surrounded by oppositely charged ionic cloud. The collision of the ionic vacancies was carried out by circulation-type magnetohydrodynamic electrode (c-type MHDE) composed of a rectangular channel with a pair of copper electrodes and a narrow electrolysis cell.