Exploration of molten hydroxide electrochemistry for thermal battery applications

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⁻ = H₂O + O²⁻ where H₂O is the Lux-Flood acid (oxide ion acceptor) and O²⁻ is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH⁻ = 2 LiO⁻ + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O²⁻. 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 O₂ + H₂O + 2 e⁻. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] ≫ [O²⁻]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH⁻ to hydrogen and O²⁻ 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.