Correlation between cathode geometry and aluminium deposit retention in molten chloride electrolysis

This is the authors' abstract. We don't add key points for this paper.

Non-uniform aluminium deposition and deposit detachment reduce apparent current efficiency during molten chloride electrolysis for aluminium scrap upcycling. Here, the influence of cathode geometry on aluminium deposit morphology, retention behaviour and current efficiency was examined at 500 °C in an MgCl 2 -NaCl-KCl-AlF 3 molten electrolyte using an aluminium-alloy anode. Planar cathodes showed high total current efficiency when detached deposits were included, but the apparent current efficiency decreased during prolonged electrolysis because aluminium-rich deposits detached from edge regions. The estimated Al balance, calculated from the quantified metallic impurities, exceeded 99.6 mass% for both retained and detached deposits. Together with the low measured Mg, Na and K contents, this result indicates that the apparent efficiency loss mainly reflected deposit retention, although residual chloride- or fluoride-containing species were not quantified. The cylindrical-cathode/annular-anode configuration improved deposit retention and maintained a higher apparent current efficiency while suppressing extended edge-localized growth. Comparisons of curvature radius and immersion depth further showed that cathode geometry controls the spatial difference in deposition behaviour. Finite-element simulations were used only to analyse the primary ohmic field and current-line distribution in a homogeneous electrolyte. Because the primary current density is proportional to the electric-field strength under ohmic conditions, the simulations identify where geometry causes field-line and current-line convergence, greater local charge-transfer demand and stronger Al-species consumption. They do not solve concentration gradients, species flux, electrode kinetics, convection, diffusion-layer evolution or deposit growth. The final morphology should therefore be interpreted as the coupled outcome of geometry-induced field-line and current-line convergence, Al-species availability and flux distribution, mass transport, concentration polarization and morphology evolution. These results identify electrode geometry and cathode–anode arrangement as practical design parameters for improving aluminium deposit retention in molten chloride electrolysis.