Effective thermal conductivity of dimagnesium iron hexahydride (Mg2FeH6) for heat storage applications

Abstract The transient plane source method was applied to measure the effective thermal conductivity in dimagnesium iron hexahydride (Mg₂FeH₆) prepared in a high-pressure synthesis of 50 temperature-driven de-/hydrogenation cycles. Temperature- and pressure-dependent measurements of the effective thermal conductivity of the as-synthesized Mg₂FeH₆ powder have been performed. Measurements for as synthesized Mg₂FeH₆ were carried out between 2 and 100 bar in a temperature range from 50 °C to 300 °C and at 70 bar in a temperature range from 480 °C to 520 °C during the cycle test. The effective thermal conductivity of the as-synthesized Mg₂FeH₆ varied between 0.39 W m⁻¹ K⁻¹_, recorded at 50 °C and 2 bar of hydrogen gas pressure, and 0.54 W m⁻¹ K⁻¹, measured at 300 °C and 100 bar hydrogen pressure. The effective thermal conductivity increased with elevated hydrogen gas pressure and temperature. An evidence was found that the presence of iron prevents the sintering of the powder, resulting in a constant effective thermal conductivity during all accomplished cycles. The advantage of a non-sintered material resulting in higher hydrogen diffusion, which leads to a faster reaction time. For 50 measured de-/hydrogenation cycles between 480 °C and 520 °C, the thermal conductivity was found to be constant at around ~ 1.0 W m⁻¹ K⁻¹ in the dehydrogenated state (70 bar/520 °C) and between 0.7 W m⁻¹ K⁻¹ and 0.8 W m⁻¹ K⁻¹ in the hydrogenated state (70 bar/480 °C).