Decomposition pathway of KAlH4 altered by the addition of Al2S3

Abstract

Altering the decomposition pathway of potassium alanate, KAlH₄, with aluminium sulfide, Al₂S₃, presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH₄–Al₂S₃ during heating under dynamic vacuum began at 185 °C, 65 °C lower than for pure KAlH₄, and released 71% of the theoretical hydrogen content below 300 °C via several unknown compounds. The major hydrogen release event, centred at 276 °C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 Å, β = 122.88°) and hexagonal (a = 10.079, c = 7.429 Å) unit cells, respectively. Unlike the 6NaAlH₄–Al₂S₃ system, the 6KAlH₄–Al₂S₃ system did not have M₃AlH₆ (M = alkali metal) as one of the intermediate decomposition products nor were the final products M₂S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 °C. The measured enthalpy of hydrogen absorption (ΔH_abs) was in the range −44.5 to −51.1 kJ mol⁻¹ H₂, which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H₂ release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties.