Ultrahigh purification in concentrated NaOH by electrowinning for solar cell application

•Ultra-purification of highly concentrated sodium hydroxide via simple electrowinning.•The large-scale manufacturing of impurity-free silicon wafers for solar cells.•Selective and complete removal of iron and nickel ions by cathodic deposition.•Optimal current density, appropriate electrodes and reaction temperature obtained.

High purity sodium hydroxide (NaOH) solution is extremely important in the large-scale manufacturing of impurity-free silicon (Si) wafers for solar cells. In this paper, we demonstrate the purification of highly concentrated NaOH via electrowinning. By optimizing temperature, current density, and the type of electrode for both anodes and cathodes, we maximized the selectivity toward cathodic deposition of Fe and Ni. Our results suggest that removal of metal impurities in the concentrated 50 wt.% NaOH is highly dependent on the reactor temperature (>90 °C) due to enhanced reaction kinetics and decreased solution viscosity. Meanwhile, current density has limited effect on the metal removal efficiency. We further demonstrate that the cathodic deposition of Fe and Ni strongly relies on the type of electrode pair used, with platinum (Pt) and nickel (Ni) as the anode and cathode, respectively, exhibiting the best removal performance. The good electrochemical performance arises from the high catalytic activity of Pt anode and good stability of Ni cathode from the highly corrosive concentrated alkaline conditions. Following these results, we recommend future scientific and technical studies on the use of electrowinning as a possible alternative to the costly membrane-based purification techniques.

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