Magnetohydrodynamic effect during electrodeposition at conically structured electrodes

Micro- and nano-sized conical structures hold great potential in tailoring the surface wettability and in enhancing electrocatalytic applications. However, their manufacturing process remains a formidable challenge. This work investigates whether and how magnetic fields could support the manufacture of such surfaces by electrodeposition. Through a combination of experiments and numerical simulations, we systematically investigate the magnetic field structuring effect at single dia- and ferromagnetic cones of mm size and at neighboring cones at varying distances. The results provide a first experimental proof of the magnetic-field-driven flow caused by the magnetization of the cones themselves and on the impact of neighboring cones, which helps to understand the resulting mass transfer at different scales. When a uniform external magnetic field is oriented perpendicular to the substrate, the magnetic gradient force enabled by the magnetization of ferromagnetic cones can generate a beneficial electrolyte flow and support the conical growth, thereby often dominating over the Lorentz and buoyancy forces arising from the electrode reactions. With a magnetic field of 200 mT and a current density of 16 mA/cm2, the deposit thickness at the cone tip can be increased by 73% for ferromagnetic and 9% for diamagnetic cones after 10 s of deposition. At smaller cone distances, the magnetic-field-driven flow becomes weaker. However, the magnetic gradient force could sustain a supportive flow to promote conical growth. Our study confirms that the magnetic gradient force is a promising means during electrodeposition for generating flow-enhanced mass transfer to support conical growth at arrays of small surface elevations.