Electrodeposition of Silicon with a Liquid Gallium Cathode in Molten Salts

Abstract

Further development of solar power depends largely on the availability of inexpensive solar grade silicon. Electrodeposition is a candidate method to produce high purity silicon at a reasonable cost. We have previously studied electrodeposition of silicon from molten calcium chloride based electrolytes and molten fluoride electrolytes. Recent experiments were carried out by using a liquid gallium cathode. The approach is based on studies by Stephen Maldonado and coworkers [1] of electrodeposition of silicon by using a liquid gallium cathode in an organic based electrolyte. Silicon has a low tendency to alloy with gallium. Electrochemical studies and electrolysis to deposit silicon from molten eutectic KCl – KF with 2 mol% K2SiF6 were carried out at 650 oC. Silver wire and glassy carbon rod were used as working electrodes, while glassy carbon rod or silver wire were counter electrodes. A silicon flag reference electrode was used. Liquid gallium in a small alumina tube with tungsten lead was the cathode during electrolysis while glassy carbon or silicon was the anode. Silicon deposition is diffusion controlled on silver cathode, while silicon deposition is charge transfer controlled on liquid gallium cathode. Silicon was found to deposit using a liquid gallium cathode, both during galvanostatic and potentiostatic electrolysis. The current efficiency was found to be quite high, and consistently above 80 %. Some silicon particles were found on top of gallium after cooling down. A much larger amount of gallium was used in recent experiments. This helped to form silicon particles inside the liquid gallium cathode. Silicon was recovered after experiments by physical separation from liquid gallium followed by dissolution of gallium in concentrated HNO3. High purity silicon was obtained. Gallium can form alloys with many impurity elements which may cause a refining effect of silicon. The use of a liquid gallium cathode is promising for the prospect of developing a new process for producing high purity silicon by electrolysis in molten salts.