The electrochemical reduction processes of solid compounds in high temperature molten salts

Abstract

Solid electrode processes fall in the central focus of electrochemistry due to their broad-based applications in electrochemical energy storage/conversion devices, sensors and electrochemical preparation. The electrolytic production of metals, alloys, semiconductors and oxides via the electrochemical reduction of solid compounds (especially solid oxides) in high temperature molten salts has been well demonstrated to be an effective and environmentally friendly process for refractory metal extraction, functional materials preparation as well as spent fuel reprocessing. The (electro)chemical reduction of solid compounds under cathodic polarizations generally accompanies a variety of changes at the cathode/melt electrochemical interface which result in diverse electrolytic products with different compositions, morphologies and microstructures. This report summarizes various (electro)chemical reactions taking place at the compound cathode/melt interface during the electrochemical reduction of solid compounds in molten salts, which mainly include: (1) the direct electro-deoxidation of solid oxides; (2) the deposition of the active metal together with the electrochemical reduction of solid oxides; (3) the electro-inclusion of cations from molten salts; (4) the dissolution-electrodeposition process, and (5) the electron hopping process and carbon deposition with the utilization of carbon-based anodes. The implications of the forenamed cathodic reactions on the energy efficiency, chemical compositions and microstructures of the electrolytic products are also discussed. We hope that a comprehensive understanding of the cathodic processes during the electrochemical reduction of solid compounds in molten salts could form a basis for developing a clean, energy efficient and affordable production process for advanced/engineering materials.

Electroreduction of Iron(III) Oxide Pellets to Iron in Alkaline Media: A Typical Shrinking-Core Reaction Process

“…[20][21][22][23][24][25][26][27][28][29][30][31][32] Moreover, the gaseous reduction of iron oxides to metal iron by hydrogen gas has also been recognized as an environmental friendly way. [33][34][35][36] However, high cost of hydrogen production is still a challenge for iron and steel industry.…”

Influences of graphite anode area on electrolysis of solid metal oxides in molten salts

“…The cathode potential change from reaction (1) to (3) is 0.335 V, and it should be greater for the reduction of the pseudo-metal oxides. However, even with this consideration, cathode polarisations alone still cannot account for the significant difference between the theoretical and practically applied cell voltages.…”

“…Electrochemical reduction of various solid metal oxides to the respective metals and alloys has been successfully carried out in CaCl 2 -based molten salts at mild temperatures (500~1000°C) by many researchers since 2000 [1][2][3][4][5]. This process, known as the Fray-Farthing-Chen (FFC) Cambridge Process, is usually brought about by applying a constant cell voltage between typically a cathode fabricated from the metal oxide or a mixture of two or more metal oxides and a carbon or graphite anode.…”

Quantitative Analysis of Oxygen Gas Exhausted from Anode through In Situ Measurement during Electrolytic Reduction

“…The cathode (F in Figure 2) was used by assembling the Ni conductor container and the rectangular basket (75 mm × 25 mm × 204 mm) made of three-ply layered (20-325-100, Nichidai)-STS wire meshes. Porous simfuel pellets with a tap density of 3.59 g/cm 3 (density of single pellet of 6.89 g/cm 3 , bulk density 62.86%) and a cylindrical shape ( 6.4 × 6.2 mm) were used by loading the STS wire mesh cathode basket. Its components and composition are listed in Table 1; it was composed of UO 2 , rare earth oxides, noble Science and Technology of Nuclear Installations metal oxides, and salt-soluble fission products.…”

“…Electrolytic reduction (also called oxide reduction or OR) of solid compounds to solid products in molten salt electrolytes has attracted widespread interest in metallurgy because it is simple, cost-effective, and environmentally friendly [1][2][3]. This technique has been applied to the reduction of various metal oxides such as TiO 2 [4] [10][11][12], Cr 2 O 3 [13,14], and CeO 2 [15].…”