The Effect of Magnetic Field on the Electrochemical Reaction of the Seawater Magnetohydrodynamic Power Generator
Authors: Aoki Makoto , Takeda Minoru 
Published: December 2024

A seawater magnetohydrodynamic (MHD) power generator is a unique system that transforms the kinetic energy of ocean and tidal currents directly into electric energy.1,2 MHD power generator has the advantages of no mechanical parts and being maintenance-free as well as low loss and high efficiency in comparison with a conventional seawater turbine.3 In addition, because seawater electrolysis occurs during seawater MHD power generation, hydrogen gas can be obtained as a by-product. Therefore, seawater MHD generator is expected to be used as an ultimate clean energy machinery.
The chemical reactions, e.g., electrodeposition, electrolysis, electrocatalytic reactions, and control of chiral, under high magnetic field have been studied by many researchers. However, studies focused on the effect of Lorentz force on ions in an electrolyte solution, which are active materials in the electrochemical reaction, have not been reported. In the present study, we developed experimental equipment and an electrochemical channel flow cell which simulated a linear-type seawater MHD power generator, and focused on clarifying in detail how the seawater electrolysis proceeds in high magnetic field.
The electrochemical channel flow cell was connected to the solution tank, liquid sending pump, and flow meter, and set in the room temperature sample bore of the cryostat with a 7 T solenoid-type superconducting magnet. The directions of the solution flow and electric field generated between working electrode (WE) and counter electrode (CE) were perpendicular to the magnetic field direction. To clarify the effect of magnetic field on the anodic current derived from chlorine species and the cathodic current of hydrogen evolution, we measured the linear sweep voltammogram (LSV) at the magnetic field of 0, 1, 3, 5, and 7 T with the NaCl solution flow rate of 0.8 m s–1 and 3.2 m s–1.
Figure 1 shows the magnetic field dependence of the anodic and cathodic current at 1.15 V and –1.15 V, respectively. Anodic current decreased as increasing the strength of applied magnetic field. When the potential was scanned from 0 V to 1.15 V, Cl–, which was a reaction active material, was consumed around WE; then, Cl– was transported from the bulk solution to the vicinity of the WE surface owing to the concentration gradient. However, Cl– was also transported to the vicinity of the CE surface by the Lorentz force. This means that the Lorentz force inhibited the supply of the active material to WE. On the other hand, when the potential was scanned from 0 V to –1.15 V, H+ was transported to the vicinity of the WE surface not only by the concentration gradient but also by the Lorentz force. Thus, the onset potentials of anodic and cathodic reactions positively shifted in accordance with concentration overpotential which is derived by the Nernst equation and Fick's laws of diffusion, resulting in the decreasing and increasing of the anodic and cathodic currents, respectively, as increasing the strength of applied magnetic field. The cathodic current (i.e., hydrogen evolution current) measured at the magnetic field of 7 T with the solution flow rate of 3.2 m s–1 increased by 10.9% compared to the current at the magnetic field of 0 T, indicating that the efficiency of hydrogen evolution reaction was improved under the MHD power generation conditions by H2 bubble disentanglement due to the solution flow and increasing the concentration of H+ around the WE surface owing to the Lorentz force.
This work was supported by the Hyogo Science and Technology Association, the Kansai Research Foundation for technology promotion (KRF), the Sasakawa Scientific Research Grant from The Japan Science Society, and JSPS KAKENHI Grant Number JP22K14764. Financial support was also obtained from Mr. Takahiro Itakura, via ‘academist’, crowd funding site for scientific research.
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Figure 1