Enhancement of electrocatalysis through magnetic field effects on mass transport

The study addresses how external magnetic fields influence electrocatalytic reactions by disentangling kinetic effects from mass transport effects. Renewable energy technologies rely on key electrochemical reactions—HER, OER, and ORR—whose efficiencies determine device performance in electrolyzers, fuel cells, and related systems. While novel catalysts and structures have been developed, the application of magnetic fields remains less explored. Magnetic fields may affect reaction kinetics (via electron transfer or activation energy) and/or mass transport (diffusion, migration, and convection of species). For ORR, polarization curves reveal distinct kinetic, mixed, and diffusion-limited regimes; improvements in mass transport increase the limiting current. Prior observations of enhanced OER on ferromagnetic catalysts raised the possibility of magnetic influences, but conflating kinetic spin-related effects with magnetically induced mass-transport changes has obscured mechanisms. This work isolates mass transport contributions by using nonmagnetic electrodes, positioning only the working electrode within a uniform magnetic field, and designing diagnostics (bubble tracking and pH indicators) to visualize and quantify ion motion. The central hypothesis is that Lorentz forces on ionic species generate magnetohydrodynamic stirring that enhances mass transport, especially under diffusion-limited conditions such as ORR with low dissolved O2 availability.

Previous reports showed direct magnetic enhancement of OER, often attributed to ferromagnetic or antiferromagnetic catalyst properties, but mass transport effects could not be fully excluded (e.g., Garcés-Pineda et al.). Magnetic fields can accelerate or redirect gas bubbles and affect electrodeposition currents. The origin of bubble motion has been debated: some attribute it to Lorentz forces on charges at bubble surfaces and to Kelvin forces in strong field gradients near magnetic catalysts; others propose bubbles drag adjacent ions, generating ion currents subsequently deflected by Lorentz forces. Additional suggestions include interactions of paramagnetic O2 with magnetic fields. However, gas-phase products are not universal and most electrocatalytic mass transport involves ionic diffusion, migration, and convection in electrolyte, warranting further study of magnetic effects on ions themselves. Magnetohydrodynamics (MHD) has long described conducting fluids in magnetic fields and applies to electrolytes, yet some recent studies reported minimal magnetic mass-transport effects, particularly for OER. Clarifying whether and how magnetic fields act on ionic species versus gas bubbles is therefore essential to reconcile the literature and to guide practical utilization.

A magneto-electrochemical setup was built with only the working electrode (WE) placed between uniform electromagnet poles (0–0.55 T; homogeneity better than 1 mT). Reference (HydroFlex) and Pt mesh counter electrodes were outside the magnetic field in a borosilicate H-cell. Nonmagnetic electrodes (Pt and Au) were used to exclude ferromagnetic kinetic effects. Reactions studied: HER (2H2O + 2e− → H2 + 2OH−) and OER (4OH− → O2 + 2H2O + 4e−) in alkaline media to probe bubble dynamics; ORR (O2 + 2H2O + 4e− → 4OH−) to probe ionic mass transport without bubble formation; and chemical O2 generation via spontaneous H2O2 decomposition at an isolated Pt wire as a tracer of electrolyte motion. Working electrodes included: isolated Pt wire tips (bubble stream angle tracking), Pt foil (placed either near the air–liquid interface or deeper in solution), Pt mesh, Pt wire (for ORR limiting current analysis), Pt microelectrode (Ø 10 μm), and Au coil (for ORR with nearby bubble tracer). Magnetic fields of ±0.43 T and 0.215 T were commonly used. Electrolytes: 1 M KOH for OER/HER; ORR in 1 M HClO4 (acidic) and 1 M KOH (alkaline); visualization experiments in 0.1 M Na2SO4 with phenolphthalein (1% in ~85% ethanol; 10 drops) as a local pH (OH−) indicator. Bubble stream angle determination used a J-shaped Pt wire insulated except at the tip; angles between bubble trajectories and vertical were measured from images (ImageJ) while varying current and field direction. For decoupling bubble charging from ionic motion, ORR was conducted on a nearby Au coil (WE), while an isolated Pt wire decomposed H2O2 to produce O2 bubbles; bubble paths were recorded with magnet on/off and varying WE current. pH visualization: ORR at Pt foil (WE) under constant currents (−0.2, −0.4, +0.4 mA) with magnetic field set to 0, +0.43, or −0.43 T; videos were recorded through a hole in a pole piece. Quantitative video analysis converted luma profiles to local pH via a calibrated sigmoidal absorbance–pH relation for phenolphthalein, then to [OH−] concentration profiles versus distance and time. Effective diffusion coefficients Deff were estimated by fitting concentration profiles to a Fickian solution C = C0(1 − x/(2√(Deff t))) and by analyzing diffusion length l = 2√(Deff t). ORR polarization curves (CVs) were recorded with oxygen purging; field dependence and position dependence (near/far from air interface) were quantified. Chronopotentiometry on Pt mesh OER assessed bubble removal effects when switching field on/off.

- Magnetic field deflects and curls bubble streams in HER and OER with an angle roughly proportional to field strength and reaction current; HER and OER bubbles are deflected in opposite directions for the same field, consistent with Lorentz-force-induced flow. The force vanishes when current is off despite field remaining, indicating current-driven ionic motion is essential. - In a configuration where ORR occurs at a nearby Au coil while O2 bubbles form chemically at an isolated Pt wire, bubbles remain vertical unless both ORR is active and a magnetic field is applied; bubble deviation angle increases nearly linearly with ORR current. This demonstrates bubbles are indirectly affected by Lorentz forces acting on ionic species that stir the electrolyte; direct magnetic forces on bubbles are negligible under these conditions. - Phenolphthalein visualization reveals a rotational, asymmetric transport of OH− around the WE under magnetic field; direction reverses with field polarity and with current sign. Ring-shaped high-OH− paths and lateral transport confirm a Lorentz-force-driven transverse component superimposed on diffusion. - Quantification of effective OH− diffusion coefficients (right side of WE, −0.2 mA ORR): Deff,OFF = (6.5 ± 0.1) × 10^−5 cm^2 s^−1; Deff,ON(+) = (9.4 ± 0.1) × 10^−5 cm^2 s^−1 (~+45% vs OFF); Deff,ON(−) = (2.2 ± 0.4) × 10^−5 cm^2 s^−1 (~one-third of OFF). Deff continued to increase over time with field on, while diffusion length plateaued without field. - OER on Pt foil in 1 M KOH shows small current enhancements: ~2.5% at 0.215 T and ~4.0% at 0.43 T across potentials; absolute increases grow with overpotential (e.g., at 2.4 V: ~0.5 mA at 0.215 T, ~0.85 mA at 0.43 T). On Pt mesh, bubble detachment is not improved; overpotential reduction upon field application is marginal (~0 mV at 25–50 mA, ~3.3 mV at 100 mA). - ORR limiting (diffusion-controlled) currents increase substantially under 0.43 T: in both 1 M HClO4 and 1 M KOH at Pt wire, current density rises by ~51% at 0.4 V vs RHE. The enhancement scales approximately with magnetic field (e.g., at 0.4 V vs RHE: +29.8 μA at 0.215 T and +64.7 μA at 0.43 T). - ORR enhancement depends on O2 availability: at a Pt foil near the air–liquid interface (higher O2), enhancement is ~7%; far from the interface (lower O2), enhancement reaches ~30%. At a Pt microelectrode (Ø 10 μm), enhancement is small (~4%), consistent with minimized mass-transport limitations. - Normalized ORR polarization curves (I0.43T/I0T) overlap the 0 T curves across potentials, indicating a primarily mass-transport (not kinetic) effect driven by magnetically induced convection/stirring.

The experiments decouple mass transport from kinetics by employing nonmagnetic electrodes and placing only the WE in a uniform magnetic field. Observations across HER, OER, and ORR consistently indicate that Lorentz forces act on moving ionic species (e.g., OH−), generating transverse flows and rotational motion (magnetohydrodynamic stirring) that modify transport toward/away from the electrode. Bubble deflection is a secondary indicator of this stirred flow rather than a direct magnetic effect on bubbles. The induced convection enhances reactant supply under diffusion-limited conditions (notably ORR with limited O2), increasing limiting currents and effective diffusivity, while offering minimal benefit when reactants are abundant at the interface (e.g., OER/HER solvent availability) or when bubble adhesion governs behavior (Pt mesh). Field polarity and current direction determine the sense of rotation consistent with FL = q v × B. Quantitative pH-imaging analysis supports asymmetric enhancements in OH− transport and an increase in effective diffusion coefficients. Collectively, the results clarify controversies in the literature by showing that magnetic fields can significantly enhance electrochemical mass transport through ionic Lorentz forces, with practical impact dictated by reactant availability, geometry, and mass-transport regime.

The study provides direct visualization and quantitative evidence that magnetic fields enhance electrocatalytic mass transport via Lorentz forces on ionic species, producing rotational flows that can nearly double effective diffusion locally and substantially boost diffusion-limited reaction currents (up to ~51% for ORR at Pt wire; ~30% for Pt foil away from the air–liquid interface). In contrast, enhancements are marginal for reactions with high reactant availability and for bubble removal on porous/mesh electrodes (OER/HER), where mass transport is not the primary limitation. By isolating mass-transport effects from kinetic and magnetic-catalyst contributions using nonmagnetic electrodes and a dedicated magneto-electrochemical setup, the work offers a mechanistic framework and quantitative benchmarks to design magnetic-field-assisted electrocatalysis. Potential avenues include applying controlled fields to diffusion-limited reactions, optimizing electrode placement relative to reactant sources (e.g., air–liquid interface), and tuning field strength/geometry to maximize beneficial MHD while considering device-specific constraints.

- Effects are demonstrated with nonmagnetic Pt and Au electrodes; thus kinetic spin-related enhancements at ferromagnetic catalysts are intentionally excluded and not assessed here. - Magnetic fields up to ~0.55 T were used; bubble removal on Pt mesh remained ineffective, indicating limited capability to dislodge adhered bubbles under these conditions. - Phenolphthalein-based quantification is restricted to its color-change pH window (pH ~8.6–10.6), limiting [OH−] range and causing saturation at high/low concentrations; illumination asymmetry produced shadows that prevented reliable Deff estimation on one side of the electrode. - Deff extraction assumes a Fickian diffusion model without explicitly modeling convection; thus reported Deff values are effective parameters capturing combined magnetic and transport phenomena. - Experiments were conducted at room temperature in specific electrolytes and cell geometries; generalization to other media, temperatures, and reactor designs may require validation.