Electrochemical Behavior of Graphite in KF–AlF[sub 3]-Based Melt with Low Cryolite Ratio

Abstract

The electrochemical behavior of graphite in KF-AlF 3 -based melt with a cryolite ratio ͑molar ratio of KF to AlF 3 ͒ of 1.3 as well as a low oxygen content at 750°C was investigated by cyclic voltammetry, potentiostatic electrolysis, sampled-current voltammetry, and chronopotentiometry. Aluminum deposited on graphite at a potential of ca. 0.25 V ͑vs Al͒ and the electrochemical intercalation of potassium into graphite occurred at ca. Ϫ0.1 V ͑vs Al͒. The intercalation of potassium into graphite enhanced the formation of Al 4 C 3 . The deposition of aluminum on graphite was a linear diffusion-controlled process, while its reverse reaction involved kinetic limitations. Sampled-current voltammetry results indicated that the aluminum deposition on graphite was depressed in the potential region extending from about 0.1 to 0.03 V ͑vs Al͒. Chronopotentiometry results revealed that the critical cathodic current density for potassium intercalation was about 400 mA cm −2 .

Towards Understanding the Cathode Process Mechanism and Kinetics in Molten LiF–AlF3 during the Treatment of Spent Pt/Al2O3 Catalysts

“…Tao et al [29] came to the conclusion that the formation of Al 4 W in the NaF-KF-AlF 3 -Al 2 O 3 melts is predominantly controlled by mass transport while the deposition of aluminum is primarily controlled by charge transfer. The diffusion coefficient of Al(III) ions found in [26] was 1.77 × 10 −8 cm 2 •s −1 , which is rather low compared to the value 3.34 × 10 −6 •cm 2 •s −1 (even though the temperature was 110 • lower) obtained by Liu et al [30] who also stated that the aluminum reduction in the KF-AlF 3 melt is diffusion-controlled. Al 4 W alloy existed on the electrode surface and was tightly attached to the tungsten electrode surface after potentiostatic electrolysis.…”

Cathode Process at the Electrolysis of KF-AlF3-Al2O3Melts and Suspensions

“…These are close to the D values obtained for graphite and platinum electrodes in previous works. 21,22 The addition of the solid Al 2 O 3 and increases in its content in the KF-AlF 3 -Al 2 O 3(sat) suspensions lead to decreasing D values at the same temperature. sen due to the lower liquidus temperature 19,29 and larger limit current densities at the same other conditions.…”

“…[18][19][20] The results of the study supplement scarce data on the kinetics of aluminum electrowinning from KF-AlF 3 -Al 2 O 3 melts and suspensions. [21][22][23]…”

Carbon Electrodes for K-Ion Batteries

“…Pure graphite’s failure in NIBs may have discouraged progress in using graphite as an electrode in K-ion batteries (KIBs). Electrochemical potassium insertion into graphite was conducted in molten KF or KF/AlF 3 at above 700 °C. , Wang et al reported reversible electrochemical insertion of potassium into nongraphitic carbon nanofibers at room temperature . In fact, KC 8 , the stage-one K-GIC, as the first discovered alkali metal GIC, has been readily synthesized by potassium vapor transport or by soaking graphite in a nonaqueous solution with potassium metal solvated. − Surprisingly, it remains unknown whether K-GICs can be formed in an electrochemical K/graphite cell at room temperature.…”