• Energy Research

A series of 16 salt mixtures consisting of potassium carbonate (K2CO3) as the base salt and a secondary salt hydrate in a 20 : 1 mol ratio of anhydrous K2CO3 to anhydrous additive were investigated as potential thermochemical heat storage (TCHS) composites. Those materials were evaluated based on their (de)hydration temperatures and reaction kinetics to find a suitable secondary salt mixture with enhanced phase change behaviour. The improved performance is expected to come from the enhancement of ionic mobility due to the deliquescence of the secondary salt. Based on the screening, we found that deliquescent and highly soluble salts are prone to reacting with K2CO3, forming new compounds that do not impact the behaviour of the base salt. Therefore, the most promising additives were salts that share a common ion with K2CO3 (KF, Cs2CO3) or salts that can react with K2CO3 forming a highly deliquescent salt (CsF, Cs2SO4). Based on the findings, we were able to design a selection procedure that can be applied to other salt hydrates considered for TCHS applications that suffer from poor kinetics and large reaction hysteresis.

This work studied the reversible dehydration of potassium carbonate sesquihydrate (K2CO3·1.5H2O). The study is based on isobaric and isothermal thermogravimetric measurements conducted at a broad range of vapour pressures and temperatures. By controlling both parameters, we examined the influence of both constraints on the reaction kinetics at a wide extent of supersaturations. We have evaluated our experimental findings by employing two thermodynamic theories, classical nucleation theory and transition state theory. By combining both approaches, we were able to establish that: (1) At low supersaturations in a region close to equilibrium, dehydration is limited by nucleation and growth of the anhydrous phase (2) At high supersaturations, dehydration reaches maximum rate and is controlled by the reaction speed. Furthermore, we show that the dehydration of K2CO3·1.5H2O is very sensitive to pressure-temperature conditions and that it does not possess universal activation energy. This publication is part of the project Mat4Heat with project number 739.017.014 of the research programme Mat4Sus which is financed by the Netherlands Research Council (NWO). Peer reviewed

This work investigates the reactions occurring in K2CO3-H2O-CO2 under ambient CO2 pressures in temperature and vapor pressure ranges applicable for domestic thermochemical heat storage. The investigation shows that depending on reaction conditions, the primary product of a reaction is K2CO3·1.5H2O, K2CO3·2KHCO3·1.5H2O, or a mixture of both. The formation of K2CO3·1.5H2O is preferred far above the equilibrium conditions for the hydration reaction. On the other hand, the formation of double salt is preferred at conditions where hydration reaction is inhibited or impossible, as the thermogravimetric measurements identified a new phase transition line below the hydration equilibrium line. The combined X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy study indicates that this transition line corresponds to the formation of K2CO3·2KHCO3, which was not observed in any earlier study. In view of thermochemical heat storage, the formation of K2CO3·2KHCO3·(1.5H2O) increases the minimum charging temperature by approximately 40 °C. Nevertheless, the energy density and cyclability of the storage material can be preserved if the double salt is decomposed after each cycle.

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.

We have experimentally determined the main thermodynamic properties of SrCl2, a potentially promising salt for thermochemical heat storage. We found a high energy density of 2.4 ± 0.1 GJ/m3 and proved full cyclability for at least 10 cycles going from the anhydrate to the hexahydrate without chemical degradation. We have experimentally determined the thermodynamic equilibria for each individual transition and the corresponding metastable zones. We find that the metastable zone is widest for the anhydrate to monohydrate transition and decreases with each subsequent hydration step. We have also established that the observed nucleation kinetics are highly dependent on the preparation of the sample. Depending on the preparation conditions, some seeds of the precursor phase can remain in the sample thereby influencing the induction times for the transition. In heat storage applications we recommend selecting conditions well away from the phase transition lines (at least outside the metastable zone) and to leave some seeds of the phase to be transferred in order to increase the transition speed.

In this work, we evaluate 454 salt hydrates and 1073 unique hydration reactions in search of suitable materials for domestic heat storage. The salts and reactions are evaluated based on their scarcity, toxicity, (chemical) stability and energy density (>1 GJ/m3) and alignment with 3 use case scenarios. These scenarios are based on space heating (T > 30 °C) and hot water (T > 55 °C) to be provided by discharge as well as on heat sources available in the built environment (T < 160 °C) for charging. From all evaluated materials, only 8 salts and 9 reactions (K2CO3 0–1.5, LiCl 0–1, NaI 0–2, NaCH3COO 0–3, (NH4)2Zn(SO4)2 0–6, SrBr2 1–6, CaC2O4 0–1, SrCl2 0–1 and 0–2) fulfil all of the criteria. Provided a suitable stabilisation method is found additional 4 salts and 13 reactions (CaBr2 6-0, CaCl2 6-0, 6-1, 6-2, 4-0, 4-1, 4-2, LiBr 2-0, 2-1, 2-0, LiCl 2-0, 2-1, ZnBr2 2-0) From this selection, only 2 salts/reactions (NaI and (NH4)2Zn(SO4)2) have not been extensively studied in the literature. Moreover, many well-investigated salt hydrates, such as MgSO4 and LiOH, did not pass our screening. This work underlines the scarcity of materials suitable for domestic applications and the need to broaden the scope of future evaluations.

Potassium carbonate (K2CO3) is a promising thermochemical heat storage material (TCM). However, it suffers from hysteresis between (de)hydration temperatures and poor reaction kinetics close to equilibrium conditions. Both aspects are caused by a nucleation barrier and low ionic mobility close to equilibrium. This study investigates the impact of caesium fluoride (CsF) incorporated through recrystallisation on the phase transitions. The composition studies show that K2CO3and CsF react during synthesis, forming KF, which points to the formation of Cs2CO3. The secondary phases are not incorporated into the crystal structure but reside between the main phase's grain cracks due to capillary forces. Because the secondary phases are highly hygroscopic, they promote surface mobility by forming a liquid-like layer even at low water vapour pressures. As the effect of their presence, hydration kinetics are enhanced significantly in all investigated conditions, with the most pronounced impact when hydration of K2CO3is inherently inhibited. The benefits manifest themselves through a faster reaction rate and shorter induction period. The dehydration is enhanced by the presence of the additive mainly far away from equilibrium conditions. Close to the equilibrium, the dehydration of the composite proceeds in an unusual 2-step manner, where the second step is much slower than the dehydration of pure K2CO3. The enhancement of dehydration kinetics is ascribed to the formation of defects during recrystallisation. The lowering of dehydration rates close to equilibrium is attributed to diffusion issues due to excess of a deliquescent phase present in the system.

The addition of bromine complexing agent (BCA) to bromine electrolyte is an accepted method to reduce bromine vapor pressure making bromine-based flow batteries inherently safer. It is well-known that the amine functional group of the BCAs interact with Nafion membranes. The novelty of the current work is that it investigates how this interaction of BCA with the four different membrane chemistries impacts the membrane characteristics and performance of hydrogen bromine flow batteries (HBFBs). The impact of BCA 13 on the system performance is determined by the membrane chemistry. Exposure of Nafion membranes to BCA leads to 60% higher cell resistance, and 55% lower cell power density at 0.5 V at 50% state-of-charge (SOC). This decrease is caused by the strong interaction between the negatively charged sulfonic acid groups in the membrane and the positively charged BCA. Lower SOC, lower bromine concentration and a higher free BCA concentration is detrimental in the cell operation. The use of LC PFSA membranes in the presence of BCA ions should be avoided. while BCA in combination with grafted sulfonated polyvinylidene fluoride (SPVDF) or grafted sulfonated polyethylene (SPE) membranes promising HBFB results are obtained.