nanomaterials Article Cycle Stability and Hydration Behavior of Magnesium Oxide and Its Dependence on the Precursor-Related Particle Morphology Georg Gravogl 1,2, Christian Knoll 2,3 , Jan M. Welch 4, Werner Artner 5, Norbert Freiberger 6, Roland Nilica 6, Elisabeth Eitenberger 7, Gernot Friedbacher 7, Michael Harasek 3 , Andreas Werner 8, Klaudia Hradil 5, Herwig Peterlik 9, Peter Weinberger 2 , Danny Müller 2,* and Ronald Miletich 1 1 Department of Mineralogy and Crystallography, University of Vienna, Althanstraße 14, 1090 Vienna, Austria; [email protected] (G.G.); [email protected] (R.M.) 2 Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (C.K.); [email protected] (P.W.) 3 Institute of Chemical, Environmental & Biological Engineering, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] 4 Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria; [email protected] 5 X-ray Center, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (W.A.); [email protected] (K.H.) 6 RHI-AG, Magnesitstraße 2, 8700 Leoben, Austria; [email protected] (N.F.); [email protected] (R.N.) 7 Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (E.E); [email protected] (G.F.) 8 Institute for Energy Systems and Thermodynamics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] 9 Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria; [email protected] * Correspondence: [email protected]; Tel.: +43-1-5880-1163-740 Received: 31 August 2018; Accepted: 2 October 2018; Published: 7 October 2018 Abstract: Thermochemical energy storage is considered as an auspicious method for the recycling of medium-temperature waste heat. The reaction couple Mg(OH)2–MgO is intensely investigated for this purpose, suffering so far from limited cycle stability. To overcome this issue, Mg(OH)2, MgCO3, and MgC2O4·2H2O were compared as precursor materials for MgO production. Depending on the precursor, the particle morphology of the resulting MgO changes, resulting in different hydration behavior and cycle stability. Agglomeration of the material during cyclization was identified as main reason for the decreased reactivity. Immersion of the spent material in liquid H2O decomposes the agglomerates restoring the initial reactivity of the material, thus serving as a regeneration step. Keywords: particle morphology; magnesium hydroxide; magnesium carbonate; magnesium oxalate; magnesium oxide; cycle stability; in-situ powder X-ray diffraction (PXRD); hydration reactivity; thermochemical energy storage; thermochemistry 1. Introduction Energy management is a major challenge for our society, requiring equal measures of political and scientific involvement [1]. Energy supply, sustainable, environmentally benign energy production, and efficient utilization are key issues in managing global energy use [2]. Energy management may, in many cases, be better expressed as ‘heat management’, as heat is the most ubiquitous form of energy. Nanomaterials 2018, 8, 795; doi:10.3390/nano8100795 www.mdpi.com/journal/nanomaterials Nanomaterials 2018, 8, x FOR PEER REVIEW 2 of 14 Nanomaterials 2018, 8, 795 2 of 14 management may, in many cases, be better expressed as ‘heat management’, as heat is the most ubiquitous form of energy. In nearly all types of electrical power plants, as well as in most industrial Inprocesses, nearly all heat types is used of electrical as the driving power plants,force and as welloperating as in mostmedium. industrial Within processes, this context, heat the is usedutilization as the drivingof waste force heat, and accounting operating for medium. two-thirds Within of thisoverall context, global the energy utilization production, of waste is heat, an extensively accounting forinvestigated two-thirds field of overall [3]. The global use energyof waste production, heat flows is includes an extensively several investigated aspects, one field of [them3]. The being use oftemporal waste heatdecoupling flows includes of waste several heat availability aspects, one and of demand, them being as the temporal two are not decoupling necessarily of wastecorrelated. heat availabilityThe necessary and storage demand, may as be the realized two are using not materials necessarily for correlated.sensible, latent The, or necessary thermoch storageemical maystorage be realizedof energy using (heat) materials [4–9]. forAll sensible, three energy latent, orstorage thermochemical concepts offer storage advantages of energy (heat)in specific [4–9]. Allareas three of energyapplication storage [6,9,10] concepts. offer advantages in specific areas of application [6,9,10]. Thermochemical energy storage (TCES) features long long-term-term storage, a wide range of compatible temperatures, applicability as a heat heat pump pump system system,, and and finally, finally, high energy energy storage storage dens densitiesities [10 [10––13]13].. Based on on these these aspects, aspects, medium medium-temperature-temperature waste waste heat heat (up (up to 450 to 450°C and◦C extensively and extensively available available from fromindustrial industrial processes) processes) is perfectly is perfectly suitable suitable for TCES for systems. TCES systems. An attractive An attractive TCES material TCES for material medium for- medium-temperaturetemperature applications applications is the system is the Mg(OH) system Mg(OH)2–MgO with2–MgO a storage with a storagetemperature temperature around around350 °C ◦ 350[14]. C[Both14 ].Mg(OH) Both Mg(OH)2 and 2MgOand MgOare industrial are industrial base base materials materials and and are are,, therefore therefore,, available available in in large quantities at low prices. Mg(OH)22––MgOMgO as aa TCESTCES materialmaterial is is well well known known for for this this purpose, purpose, with with many many aspects aspect relateds related to itsto applicationits application in energy in energy storage storage already already investigated investigated in literature. in literature. Kinetic investigations Kinetic investigations of dehydration of anddehydration rehydration and [ 15rehydration,16], mechanistic [15,16] aspects, mechanistic of the conversionaspects of the [17 ,conversion18], modification [17,18] of, modification the material byof additionsthe material of lithiumby additions salts [of16 lithium,19,20], bysalts coating [16,19,20] or use, by of coating composite or use material of composite [21,22], bymaterial dotation [21,22] [23],, andby dotation finally also[23], applicabilityand finally also in formapplicability of a chemical in form heat of a pumpchemical [24 ]heat were pump reported. [24] were Nonetheless, reported. twoNonetheless, key issues two preventing key issues industrial preventing application industrial remain unaddressed:application remain First, rehydrationunaddressed: reactivity First, (completeness),rehydration reactivity and second, (completeness) the cycle stability., and second, Whereas the for thecycle limited stability. cyclability Whereas observed for the thus limited far, no satisfyingcyclability solutionobserved has thus been far found,, no satisfying the rehydration solution reactivity has been is addressedfound, the byrehydration the addition reactivity of lithium is saltsaddressed [16,19 b,20y the], which addition are of quite lithium expensive. salts [16,19,20] On a molecular, which are level, quite reactivity expensive. could On a also molecular be tuned level, by 2+ 2+ dotationreactivity of could Mg(OH) also2 bewith tuned Ca by-ions dotation [23]. of Mg(OH)2 with Ca -ions [23]. OnOn an an industrial industrial sca scalele MgO MgO is isproduced produced via via calcination calcination of Mg(OH) of Mg(OH)2 or 2MgCOor MgCO3 [25]3. Both[25]. Bothprecursors precursors are found are foundin natural in natural deposits, deposits, but whereas but whereas MgCO3 MgCOis an industrially3 is an industrially mined raw mined material, raw material,Mg(OH)2 is Mg(OH) produced2 is from produced serpentinite from serpentinite or processing or seawater processing [26] seawater. However, [26 aerobic]. However, calcination aerobic of calcinationany other Mg of anycompound other Mg may compound result in formation may result of in MgO formation by stepwise of MgO decomposition. by stepwise decomposition. In Scheme 1, Inthis Scheme is shown1, thisat the is example shown atof thethe mentioned example of industrial the mentioned precursor, industrial as well precursor,as for magnesium as well oxalate as for magnesiumdihydrate. oxalate dihydrate. Scheme 1. ThermalThermal decomposition decomposition of ofvarious various MgO MgO precursors: precursors: (1) (1)Mg(OH) Mg(OH)2, (22), (2)MgCO MgCO3, (3)3, (3)MgC MgC2O42∙2HO42·O2H. 2O. All soso farfar performed performed investigations investigations on theon rehydrationthe rehydration of MgO of MgO for thermochemical for thermochemical energy storageenergy · purposesstorage purposes have largely have largely neglected neglected the origin the oforigin theMgO. of the AsMgO Mg(OH). As Mg(OH)2, MgCO2, MgCO3, MgC3,2 MgCO4 2H2O24O,∙2H and2O, MgOand MgO crystallize crystallize in crystallographically in crystallographically and stereochemically and stereochemically different different systems systems (Table 1(),Table and feature1), and notablyfeature notably