Electrochemical Performance Evaluation of Water Electrolysis Enhanced by Magnetic Bubble Control Using a Water-Based Magnetic Nanofluid

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Water (H₂O) is a crucial resource for sustaining human activities in space. In addition to its indispensable role in supporting life, water can be electrolyzed to produce hydrogen (H₂) and oxygen (O₂), where hydrogen serves as a propellant and oxygen supports respiration. If the water resources believed to exist on the Moon can be extracted and electrolyzed in situ (hereafter referred to as water electrolysis), substantial reductions in transportation costs compared to Earth-based supply launches could be realized. Nevertheless, conducting water electrolysis in space poses significant technical challenges. Spacecraft in orbit are in a microgravity environment, and the Moon’s gravity is only about one-sixth that of Earth. As a result, the buoyant force acting on gas bubbles generated at the electrodes during electrolysis is greatly reduced. Consequently, bubbles tend to remain on the electrode surface, hindering the electrochemical reaction and decreasing the efficiency of electrolysis. To address this issue, the present study experimentally investigates the potential for enhancing electrolysis efficiency by facilitating bubble removal through magnetic manipulation. Specifically, a water-based magnetic nanofluid solution was employed. This fluid is an aqueous colloidal suspension in which ferromagnetic nanoparticles, approximately 10 nm in diameter, are stably dispersed. Dispersion stability is achieved through electrostatic repulsion between surfactant molecules adsorbed on the particle surfaces. When subjected to a spatial magnetic field gradient, gas bubbles in the fluid experience magnetic buoyancy, that is, a net force directing them toward regions of lower magnetic field strength. It is hypothesized that this phenomenon can promote effective bubble detachment from the electrode surfaces, thereby enhancing electrolysis performance. The study focuses on evaluating how variations in nanoparticle concentration within the solution affect bubble removal and electrolysis efficiency. A simple planar electrode cell was used. Titanium served as the substrate material, with an iridium catalyst deposited on the anode for the oxygen evolution reaction (OER) and a platinum catalyst on the cathode for the hydrogen evolution reaction (HER). The electrochemically active area was confined to 5 × 10 mm², while the remaining surfaces were coated with epoxy resin to prevent unintended chemical reactions.. The interelectrode spacing was set at 3 mm. The magnetic nanofluid solution used in this study was MSGW10, provided by Ferrotec Material Technologies. To avoid colloidal destabilization due to salting-out induced by the addition of electrolytes, the original stock fluid was diluted to 1/8, 1/16, and 1/32 concentrations. For comparison, pure water (i.e., without magnetic particles) was also used. A buffer solution comprising 0.5 mol/L potassium bicarbonate and potassium carbonate (pH ≈ 10) was employed as the supporting electrolyte. Although magnetic nanofluids are generally prone to particle aggregation under strongly alkaline conditions, the use of buffering ions enabled stable electrolysis. Moreover, the weakly alkaline pH range ensured chemical stability of the dispersed magnetite nanoparticles. Electrolysis performance was characterized via linear sweep voltammetry (LSV), beginning at 1.5 V and scanning at a rate of +5 mV/s. All tests were conducted under both magnetic and non-magnetic field conditions. The LSV results revealed that, under identical operating conditions, the use of magnetic nanofluid solutions led to superior electrolysis performance compared to pure water. Furthermore, when comparing electrolysis under magnetic and non-magnetic conditions, the application of a magnetic field resulted in further enhancement of electrolysis efficiency, indicating the effectiveness of magnetic forces in facilitating bubble detachment via magnetic buoyancy. Higher concentrations of nanoparticles were correlated with increased performance enhancement, likely due to more effective bubble removal under the influence of the magnetic field. However, paradoxically, lower nanoparticle concentrations exhibited even better performance overall. This counterintuitive result may be attributed to the influence of surfactants surrounding the nanoparticles. At higher concentrations, surfactant-induced Marangoni effects may reduce bubble detachment velocity, thereby suppressing electrochemical activity. Additionally, excessive nanoparticle loading may increase the electrical resistance of the solution, further limiting performance. Figure 1

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