Impact of polymeric stabilisers on the reaction kinetics of SrBr₂

Thermochemical heat storage (TCHS) in salt hydrates attracts increasing interest due to the high energy density combined with loss-free storage. Strontium bromide hexahydrate (SBH), and composites thereof, are often suggested as suitable materials for this application. Although many aspects of SBH composites have been thoroughly investigated, very little has been done on the fundamental aspects of the hydration reaction and interactions between composite components on a molecular level. In this paper, we examine the interaction between SBH and polymeric additives polydiallyldimethylammonium chloride (PDAC), sodium carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) in previously developed TCHS composites. The primary function of the polymeric additives is enhanced mechanical integrity however this study investigates potential implications on reaction temperature and speed the addition of such components might have. Focus is given to the interaction between SrBr₂ and PDAC since such composites showed (de)hydration behaviour deviating from pure SrBr₂. The reaction kinetics are investigated at several points in the phase diagram through thermogravimetric analysis (TGA), supplemented by powder x-ray diffraction (XRD) studies. Our findings show that there exists an interaction between SrBr₂ and PDAC which manifests itself through shrinkage of crystallite size and increased lattice strain induced by preferential binding of PDAC to SrBr₂. Depending on the PDAC content in the composite we have found out that 1) at excessive amounts PDAC inhibits hydration due to its sequestering properties 2) at low amounts it an enhances reaction kinetics due its hydrophilic nature.