The deoxidation speed of a solid oxide cathode in molten CaCl2 can be estimated by the PRS steady diffusion model of O2-, which correlates the deoxidation speed with the precursor porosity, P, the metal-to-oxide molar volume ratio, R, and the cathode volume shrinkage S. The PRS model indicates that the porosity of the oxide cathode has important influence on the deoxidation speed, and provides a very simple equation for the prediction of the optimal cathode porosity. For the electrolysis of Ta2O5, the porosity of the cathode is better to be within 50%. The model and its predictions has been well verified by the electrolysis of solid Ta2O5 in molten CaCl2, suggesting the great significance of the PRS model for the high speed and high current efficiency electrolysis of solid compound cathode in molten salts.
Chen G Z, Fray D J, Farthing T W, et al. Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride[J]. Nature, 2000, 407(6802): 361-364.
Norhira T, Yasuda K, Ito Y, et al. Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon[J]. Nature Materials, 2003, 2(6): 397-401.
Yang J Y, Lu S G, Kan S R, et al. Electrochemical preparation of silicon nanowires from nanometre silica[J]. Chemical Communications, 2009, 22: 3273-3275.
Hu X F(胡小锋), Xu Q(许茜). Preparation of tantalum by electro-deoxidation in a CaCl2-NaCl melt[J]. Acta Metallurgica Sinica(金属学报), 2006, 42(3): 285-289.
Zou X L, Lu X G, Li C H, et al. A direct electrochemical route from oxides to Ti-Si intermetallics[J]. Electrochimica Acta, 2010, 55(18): 5173-5179.
Song Q S, Qian X, Kang X, et al. Mechanistic insight of electrochemical reduction of Ta2O5 to tantalum in a eutectic CaCl2-NaCl molten salt[J]. Journal of Alloys and Compounds, 2010, 490(1/2): 241-246.
Jiao S Q, Zhang L L, Zhu H M, et al. Production of NiTi shape memory alloys via electro-deoxidation utilizing an inert anode[J]. Electrochimica Acta, 2010, 55(23): 7016-7020.
Jiang K, Hu X H, Ma M, et al. “Perovskitization”-assisted electrochemical reduction of solid TiO2 in molten CaCl2[J]. Angewandte Chemie International Edition, 2006, 45(3): 428-432.
Schwandt C, Alexander D T L , Fray D J, et al. The electro-deoxidation of porous titanium dioxide precursors in molten calcium chloride under cathodic potential control[J]. Electrochimica Acta, 2009, 54(14): 3819-3829.
Li W, Jin X B, Huang F L, et al. Metal-to-oxide molar molume ratio: The overlooked barrier to solid-state electroreduction and a “green” bypass through recyclable NH4HCO3[J]. Angewandte Chemie International Edition, 2010, 49(18): 3203-3206.
Wu T, Jin X B, Xiao W, et al. Thin pellets: Fast electrochemical preparation of capacitor tantalum powders[J]. Chemistry of Materials, 2007, 19(2): 153-160.
Dring K, Dashwood R, Inman D et al. Predominance diagrams for electrochemical reduction of titanium oxides in molten CaCl2[J]. Journal of the Electrochemistry Society, 2005, 152(10): D184-D190.
Yan X Y, Fray D J, et al. Electrochemical studies on reduction of solid Nb2O5 in molten CaCl2-NaCl eutectic[J]. Journal of the Electrochemistry Society, 2005, 152(1): D12-D21.
Centeno-Sánchez R L, Fray D J, Chen G Z, et al. Study on the reduction of highly porous TiO2 precursors and thin TiO2 layers by the FFC-Cambridge process[J]. Journal of Materials Science, 2007, 42(17): 7494-7501.
Mohanty J, Rath P C, Subbaiah T, et al. Preparation of titanium metal by electrochemical reduction of titanium dioxide[J]. Transactions of the Indian Institute of Metals, 2009, 62(3): 249-253.
Bhagat R, Dye D, Raghunathan S L, et al. In situ synchrotron diffraction of the electrochemical reduction pathway of TiO2[J]. Acta Materialia, 2010, 58(15): 5057-5062.
Alexander D T L, Schwandt C, Fray D J, et al. The electro-deoxidation of dense titanium dioxide precursors in molten calcium chloride giving a new reaction pathway[J]. Electrochimica Acta, 2011, 56(9): 3286-3295.
Yan X Y, Fray D J, et al. Electrochemical studies on reduction of solid Nb2O5 in molten CaCl2-NaCl eutectic[J]. Journal of the Electrochemistry Society, 2005, 152(10): E308-E318.
Kar P, Evans J W, et al. A model for the electrochemical reduction of metal oxides in molten salt electrolytes[J]. Electrochimica Acta, 2008, 54(2): 835-843.
Jin X B, Gao P, Wang D H, et al. Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride[J]. Angewandte Chemie International Edition, 2004, 116(6): 751-754.
Yasuda K, Nohira T, Amezawa K, et al. Mechanism of direct electrolytic reduction of solid SiO2 to Si in molten CaCl2[J]. Journal of the Electrochemistry Society, 2005, 152(4): D69-D74.
Xiao W, Jin X B, Deng Y, et al. Three-phase interlines electrochemically driven into insulator compounds: A penetration model and its verification by electroreduction of solid AgCl[J]. Chemistry-A European Journal, 2007, 13(2): 604-612.
Xiao W, Jin X B, Deng Y, et al. Electrochemically driven three-phase interlines into insulator compounds: Electroreduction of solid SiO2 in molten CaCl2[J]. ChemPhysChem, 2006, 7(8): 1750-1758.
Chen H L, Zeng Y, Li W, et al. A PRS model for accurate prediction of the optimal solid oxide cathode structure for the preparation of metals in molten chlorides[J]. Electrochemistry Communications, 2013, 26: 33-36.
Gao H P, Jin X B, Zou S W, et al. Liquid diffusion of the instantaneously released oxygen ion in the electrolytic porous Fe from solid Fe2O3 in molten CaCl2[J]. Electrochimica Acta, 2013, 107: 261-268.
Ferro P D, Mishra B, Olson D L, et al. Application of ceramic membrane in molten salt electrolysis of CaO-CaCl2[J]. Waste Management, 1998, 17(7): 451-461.
