NDLI: Exploration of molten hydroxide electrochemistry for thermal battery applications

Content Provider	Springer Nature Link
Author	Miles, M.H.
Copyright Year	2003
Abstract	The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag$^{+}$/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH$^{−}$ = H$_{2}$O + O$^{2−}$ where H$_{2}$O is the Lux-Flood acid (oxide ion acceptor) and O$^{2−}$ is the Lux–Flood base. This reaction dictates the minimum H$_{2}$O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li$_{2}$O, Na$_{2}$O or K$_{2}$O. Thermodynamic calculations suggest that Li$_{2}$O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li$_{2}$O$_{(s)}$ + 2 OH$^{−}$ = 2 LiO$^{−}$ + H$_{2}$O whereas Na$_{2}$O acts as a Lux–Flood base, Na$_{2}$O$_{(s)}$ = 2 Na$^{+}$ + O$^{2−}$. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH$^{−}$ to yield oxygen, that is 2 OH$^{−}$ → 1/2 O$_{2}$ + H$_{2}$O + 2 e$^{−}$. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H$_{2}$O] ≫ [O$^{2−}$]) and the reduction of Na$^{+}$ or K$^{+}$ in basic melts. The direct reduction of OH$^{−}$ to hydrogen and O$^{2−}$ is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.
Starting Page	1011
Ending Page	1016
Page Count	6
File Format	PDF
ISSN	0021891X
Journal	Journal of Applied Electrochemistry
Volume Number	33
Issue Number	11
e-ISSN	15728838
Language	English
Publisher	Kluwer Academic Publishers
Publisher Date	2003-01-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Physical Chemistry Industrial Chemistry/Chemical Engineering
Content Type	Text
Resource Type	Article
Subject	Materials Chemistry Chemical Engineering Electrochemistry

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