Enhancement of electrocatalysis through magnetic field effects on mass transport

The urgent need for sustainable energy solutions and carbon emission reduction necessitates advancements in renewable energy technologies. Electrochemical energy sources, which utilize electrons to drive chemical reactions and store energy in chemical bonds, play a central role. The hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are crucial electrochemical reactions for sustainable energy production, underpinning fuel cells, batteries, and electrolyzers. Optimizing the efficiency of these reactions is paramount for developing sustainable energy sources. While significant progress has been made with new catalyst materials and structures, unconventional methods, such as applying magnetic fields, remain relatively unexplored. The application of magnetic fields in electrocatalysis offers a unique approach to potentially enhance reaction rates and efficiency. However, it is crucial to differentiate between the effects of magnetic fields on reaction kinetics and mass transport. This study specifically addresses this challenge by employing a meticulously designed experimental setup that allows for the separate quantification of these effects.

Previous research has shown a direct enhancement of the OER under a magnetic field, attributed to changes in the catalyst's magnetic properties. However, the influence of magnetic fields on mass transport couldn't be entirely excluded. It's known that magnetic fields can affect the movement of gas-phase reaction products (bubbles) and electrodeposition currents. The exact mechanism behind the effect on bubbles has been debated, with some studies suggesting the involvement of Lorentz forces on charged bubble surfaces, Kelvin forces with strong magnetic field gradients, or ion currents near bubbles influenced by Lorentz forces. However, the impact of magnetic fields on the movement of ionic species in the electrolyte—a major component of mass transport in electrocatalysis—requires further investigation. Existing studies have presented conflicting results regarding the significance of magnetic field effects on mass transport, highlighting the need for a comprehensive and controlled experimental approach.

This work utilized a custom-built magneto-electrochemical system designed to thoroughly investigate the influence of magnetic fields on electrocatalytic reactions. Non-magnetic electrodes (Pt and Au) were used to eliminate the confounding effects of the electrode material's magnetic properties. The working electrode (WE) was strategically positioned within the magnetic field while the reference and counter electrodes remained outside to avoid interference. Several electrocatalytic reactions were studied: the OER and HER, which involve gas-phase products; and the ORR, which provides insights into the magnetic field's impact on ionic species' mass transport without bubble influence. To visualize mass transport, indicators such as O2 bubbles from H2O2 decomposition and pH-sensitive phenolphthalein dye were employed. The experimental protocols were designed to quantitatively and qualitatively assess the effects of Lorentz forces on mass transport. For instance, the HER (2H₂O + 2e⁻ → H₂(g) + 2OH⁻) and OER (4OH⁻ → O₂(g) + 2H₂O + 4e⁻) were used to investigate the effect of magnetic fields on gas bubble movement. The ORR (O₂(aq) + 2H₂O + 4e⁻ → 4OH⁻) was studied in both acidic (0.1 M HClO₄ with 0.49 M H₂O₂) and alkaline (1 M KOH) media to understand its influence on ionic species. The experiments involved varying the magnetic field strength and direction, reaction current, and electrode type (Pt wire, Pt mesh, Pt foil, Pt microelectrode) to analyze the mass transport behavior. A quantitative analysis of mass transport was conducted using video analysis of phenolphthalein dye distribution under different magnetic field conditions. The resulting luma (brightness) values were correlated with OH⁻ concentration, allowing for the determination of the effective diffusion coefficient under different magnetic field strengths and directions.

The experiments revealed that the Lorentz force acting on moving ionic species is the primary cause of the magnetic field-induced mass transport enhancement. The magnetic field creates a whirling motion in the electrolyte around the electrode, improving diffusion and convection. The movement of gas bubbles is a secondary effect, influenced by the surrounding electrolyte's movement. The magnetic field significantly enhanced the ORR current (up to 51% at 0.4 V vs. RHE in both acidic and alkaline conditions), especially when oxygen availability was low. In contrast, the enhancement in the OER current was far less significant (around 2.5% under 0.215 T and 4% under 0.43 T), primarily because the reactant (water or hydroxide ions) is abundant near the electrode surface. The magnetic field's influence on bubble removal, while noticeable, was not substantial enough to significantly increase the overall reaction rate for the OER, especially with electrodes like Pt mesh where bubbles tended to be trapped. Quantitative analysis with phenolphthalein dye showed that the magnetic field could increase the effective diffusion coefficient of OH⁻ by approximately 45% in one direction, while decreasing it to one-third in the opposite direction, leading to a clear asymmetry in ion distribution. The enhancement rate in the ORR varied depending on the electrode type and position: around 4% for Pt microelectrodes, 7% for a Pt foil at the liquid-air interface, and nearly 30% for a Pt foil far from the interface. This variation highlights the impact of reactant concentration on the magnetic field's effectiveness in boosting reaction rates.

These findings clarify the role of mass transport in magnetic field-enhanced electrocatalysis. The observed differences in the extent of magnetic field enhancement between the OER and ORR underscore the importance of considering reactant availability. The significant enhancement in the diffusion-limited ORR, compared to the minor improvement in the OER, emphasizes the impact of the Lorentz force on ion movement. The results challenge the notion that magnetic field effects on mass transport are solely attributed to gas bubble removal. The study's detailed quantification of the effects expands our understanding beyond previous observations of gas bubble movement, showcasing the importance of the Lorentz force's influence on ionic species in determining overall reaction rate. The methodology, employing non-magnetic electrodes and carefully controlled conditions, enhances the reliability and interpretation of the results, addressing the limitations of previous studies that lacked precise separation of kinetic and mass transport contributions.

This research demonstrates that magnetic fields can enhance electrocatalysis primarily through their influence on mass transport via the Lorentz force, creating a stirring effect on electrolyte ions. The impact is particularly pronounced in diffusion-limited reactions with low reactant availability, such as the ORR. The enhancement in gas-phase product reactions like the OER is significantly less pronounced, suggesting that bubble removal alone is not a sufficient mechanism for substantial rate improvement. This work provides a deeper understanding of the underlying mechanisms of magnetic field enhancement in electrocatalysis and highlights the importance of considering both kinetic and mass transport effects when designing and optimizing electrochemical systems for sustainable energy applications. Future research could focus on exploring the potential of magnetic fields in combination with other strategies for further enhancing the performance of electrocatalytic processes and expand these findings to different catalyst materials and reaction systems.

While this study offers comprehensive insights, some limitations exist. The experiments were conducted at room temperature, and the results may not directly translate to higher temperature conditions prevalent in industrial applications. The model used to fit the diffusion profiles, assumes a constant concentration at the electrode surface and does not account for the effect of magnetic fields on convection which could potentially lead to underestimation or overestimation of the effective diffusion coefficient values. Furthermore, the use of the phenolphthalein dye concentration as a proxy for OH- concentration is an approximation, which may introduce some level of uncertainty in the quantitative analysis of mass transport. Further investigation is needed to explore the generalizability of these findings to a wider range of electrochemical reactions and catalyst materials.