Enhancing reactivity of Na2Zn(SO4)2 hydrates by doping for thermochemical energy storage

Abstract

The thermochemical properties of Na2Zn(SO4)2 are systematically analyzed under dynamic conditions at various partial vapor pressures ranging from 50 mbar to 700 mbar, aiming to evaluate its potential as a material for Thermochemical Energy Storage (TCES). At 50 mbar, dehydration to Na2Zn(SO4)2 starts at ~94.5 ◦C and rehydration begins at 60.6 ◦C. At 700 mbar, onset temperatures shift to 105 ◦C for dehydration and 117.9 ◦C for hydration. Na2Zn(SO4)2 exhibits a favorable charging temperature interval and high cycle stability during the transition from dihydrate to anhydrate, indicating its promise for TCES applications. The introduction of dopants significantly influences system performance, resulting in: (a) a reduction in dehydration onset temperatures by 7–20 ◦C; (b) an increase in hydration onset temperatures by 3–7 ◦C; (c) complete vapor uptake reaching the stoichiometric 2 mol H2O/mol Na2Zn(SO4)2; and (d) shorter induction times with accelerated reaction kinetics. LiCl-doped samples also show improved stability and reduced mass loss over multiple hydration cycles. However, the complete hydration of tetrahydrate is not fully achieved, suggesting a need for further investigation into optimal dopant compositions and their effects on hydration kinetics. Overall, addressing thermodynamic challenges and optimizing hydration processes position Na2Zn(SO4)2⋅2H2O as a potential candidate for efficient and sustainable thermal energy storage, contributing to advancements in the field of TCES.