Salt hydrates are thermochemical materials capable of storing and releasing heat through reversible binding with water, [1]. In a heat battery, millimeter-sized salt hydrate particles ensure a relative stable permeability of the whole system. We need to understand how these salt particles hydrate to make better thermochemical storage materials.

In this study, we show that the hydration timescale of salt particles is inversely proportional to the effective diffusion coefficient of water vapor through the porous particle (Deff). From gravimetric measurements done on SrBr2·6H2O and CaC2O4, we calculated the reaction coefficients k and Deff, and found that they validated a front-diffusion limited hydration hypothesis; indeed, the found Deff values are compatible with literature references [2] and depend only on the particles’ porosities. We modeled the numerical factor that represents the spread in power output that can be expected between salts and found that the spread is related to the equilibrium water vapor pressure.

This result highlights that the power output of particles can only be increased by accelerating the water transport into the particle and not by increasing the primary particle reaction rate.

In order to increase the water transport, we are presently investigating the effect of adding highly hygroscopic salts (CsF, KF, CH3OOH, HCOOH, Cs2CO3, Rb2CO3) [3,4] into the pores of the particles. Their efficacy is evaluated in terms of increment of the power output, based on which we made hypothesis on the mechanism giving this contribution, with the help of gravimetric analysis of the water uptake, electron microscopy and helium pycnometry.

Strategies to circumvent the diffusion limitation and research on the mechanical stability of mm-sized particles might greatly benefit the development of next generation heat batteries.