Thermal Decomposition and Reduction of Carbonate Ion in Fluoride Melts

Abstract

The thermal decomposition of dilute solutions of K2CO3 in the ternary fluoride eutectic (Li, Na, K)F (FLINAK) at 750°C was studied both by chemical analysis of quenched melts and in situ by a potentiometric method. The carbonate simultaneously vaporizes as M2CO3 and decomposes: CO32−=CO2+O2− , until a steady state is reached where the major part of the carbonate is retained in the melt. Voltammetric study of the carbonate reduction shows that the reduction is not direct but goes via the prior reduction of alkali metal: (1) M++e=M , (2) 4M+M2CO3=C+3M2O .

Carbon electrodeposition in molten salts: electrode reactions and applications

“…28 Thirdly, CO 3 2À ions can also be reduced indirectly via the prior reduction of alkali metal ions to the metal (reactions (11) and ( 12)). 29 Linear sweep voltammetry performed on a platinum electrode in molten LiF-NaF-KF-K 2 CO 3 (molar ratio: 45.1 : 11.2 : 40.8 : 2.9) at 500 C showed the absence of a CO 3 2À ion reduction peak before the cathodic limit, suggesting CO 3 2À ions were not directly reduced. 29 It was therefore postulated that CO 3 2À ions are not electrochemically reduced but are rather reduced in a chemical reaction following the formation of the alkali metal.…”

“…Carbon deposition was reported on a Ni electrode. 12,17,26,29,34,37,38 Reduction peaks on CVs at À2.0 and À2.4 V vs. Ag in eutectic Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 at 450 C were attributed to the reduction of CO 3 2À ions to carbon and the deposition of alkali metal respectively. The two oxidation peaks observed were assigned to the electrochemical dissolution of the deposited alkali metals and carbon.…”

Capture and electro-splitting of CO2 in molten salts

“…(3) Chemical reduction of CO 3 2 − to carbon via an electrodeposited alkaline metal by reactions (19) and (20) [43] .…”

“…However, no CO was detected in the exhaust gas during electrolysis at low temperatures in Li-Na-K carbonate molten salts, suggesting the second mechanism may not be complete [40] . Speculation for the third mechanism was reported based on the linear sweep voltammetry results, in which a CO 3 2 − reduction peak was not observed before the cathodic limit in the LiF-NaF-KF-K 2 CO 3 molten salt [43] . The observation was consistent with the thermodynamic results that deposition potentials of metals K and Na are close to that of carbon deposition.…”

Studies of Chromium Carbide Electrodeposition in Molten Fluorides

“…The plating of chromium metal from molten salts has been studied by several workers since the original patent by Senderoff and Mellors (4), which mentions only plating from a 8 weight percent (w/o) K3CrF6solution in FLINAK at 750~176 Inman and co-workers (8,9) have reported detailed kinetics to proceed as Cr(III) -~, Cr(II) -* Cr(0), but this work is not directly applicable since it was carried out in LiC1-KC1 at 500~ a temperature too low for CO~ 2-reduction (10). Smith (11) obtained good Cr deposits from a solution of Cr(III) at 800~ produced by anodic dissolution of Cr metal.…”