EAGER: Feasibility of the Solid Oxide Membrane-Based Electrolysis Process for Solar Grade Silicon Production

Trustees Of Boston University, Boston

Investigators

Abstract

PI: Pal, Uday Institution: Trustees of Boston University Proposal Number: 1210442 Title: EAGER: Feasibility of the Solid Oxide Membrane-Based Electrolysis Process for Solar Grade Silicon Production This is a one-year early-stage exploratory research program aimed at producing silicon from silica. If successful, it will lead to a larger research program producing solar grade silicon and achieving the industrial goal for solar power of $1/Wp (watt at peak exposure) which could enable widespread grid parity. Intellectual Merit: This membrane-based silica electrolysis process will employ a solid oxygen-ion conducting membrane (SOM), such as stabilized zirconia, to electrolyze silica and other impurity oxides. The feasibility studies will involve dissolving silicon dioxide with controlled amounts of impurity oxides (such as those of P, Fe and Cr) in a molten fluoride flux and increasing the applied electrical potential between the electrodes to first deposit more electronegative impurities, followed by the deposition of pure Si at the cathode. The anodic reaction during the electrolysis process will produce pure oxygen gas at an inert anode, protected from the molten flux by the SOM. In order to demonstrate feasibility, the following research activities will be undertaken: - Employ appropriate molten fluoride flux system for performing SOM electrolysis of silica with controlled amounts of impurity oxides. Research will include survey and selection of the fluoride flux systems using differential thermal analysis, impedance and thermo-gravimetric measurements.�� - Use a simple electrolysis cell design with appropriate reference electrodes to measure and analyze current response as a function of the applied electrical potential. This information will be used to model mass-transfer resistance in the system due to chemical diffusion, migration and convection and understand the charge-transfer mechanism during the deposition process. - Characterize the microstructure and chemical composition of the metal deposit (silicon) and relate it to the process model. In the next phase of the program, more detailed process characterization employing multiple gas bubbling tubes, thermocouples and cathodes will be performed for impurity removal and solar grade silicon deposition. The chemical and physical properties of the deposit will be characterized to assess its suitability for solar grade application. A finite element SOM electrolysis process model will be developed. The process model will combine electric current density with heat transfer, fluid flow, and diffusion for use as a tool for process design. It will run in three dimensions, and calculate boundary layer structures in order to estimate changes in concentration polarizations and mass transfer resistances. Other model features will include determining: multiple species deposition using boundary layer profiles from mass transfer calculations to estimate deposition rates for more- and less-electronegative species as well as for silicon; shape evolution of the growing silicon cathode in order to optimize electrode placement for favorable product form; new stirring methods in addition to argon bubbling or rotating electrodes such as employing DC magnetic field. By using this model ?MOxST? will explore electrode geometry and placements, and stirring conditions for process scaleup. Broader Impact: Solar-grade silicon (SoG-Si) represents a large fraction of the cost of solar cells. This process is expected to significantly lower the cost of SoG-Si, reduce Green House Gases (GHG), and increase energy efficiency compared to the currently used methods of silicon production. While this study is focused on SoG-Si, the process can be used for production of other energy-intensive metals (Li, Ti, Al, etc.), leading to increased overall energy efficiency and reductions in GHG emissions. The project will provide research opportunities for two graduate students, increasing expertise in the areas of electrochemical processing and energy/environmental systems. Students will be explicitly recruited by working in conjunction with under-represented student organizations, and through our contacts with Howard University. Undergraduates will be supported through a supplementary REU proposal and through BU?s matching grant programs.