Abstract
This paper describes the synthesis, crystal structure, and NH3 sorption properties of Mg1-xMnx(NH3)6Cl2 (x = 0–1) mixed metal halide ammines, with reversible NH3 storage capacity in the temperature range 20–350 °C. The stoichiometry (x) dependent NH3 desorption temperatures were monitored using in situ synchrotron radiation powder X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The thermal analyses reveal that the NH3 release temperatures decrease in the mixed metal halide ammines in comparison to pure Mg(NH3)6Cl2, approaching the values of Mn(NH3)6Cl2. Desorption occurs in three steps of four, one and one NH3 moles, with the corresponding activation energies of 54.8 kJ⋅mol-1, 73.2 kJ⋅mol-1 and 91.0 kJ⋅mol-1 in Mg0.5Mn0.5(NH3)6Cl2, which is significantly lower than the NH3 release activation energies of Mg(NH3)6Cl2 (Ea = 60.8 kJ⋅mol-1, 74.8 kJ⋅mol-1 and 91.8 kJ⋅mol-1). This work shows that Mg1-xMnx(NH3)yCl2 (x = 0 to 1, y = 0 to 6) is stable within the investigated temperature range (20–350 °C) and also upon NH3 cycling.
Highlights
Energy storage materials and methods have gained high interest to ensure the transition to carbon-free future
The mass loss ratio 4:1:1 of the Kissinger analysis was performed on the differential scanning calorimetry (DSC) heat flow signals for the three desorption events monometallic and mixed hexammines (Supporting Information, Table S2) corresponds to the moles measured for Mg(NH3)6Cl2, Mn(NH3)6Cl2 and Mg0.5Mn0.5(NH3)6Cl2 to determine the activation energy of NH3 desorbed in each desorption event, i.e., four moles of NH3 released in the first desorption and investigate their NH3 desorption kinetics
All Mg1−x Mnx (NH3 )6 Cl2 solid solutions crystallize in a cubic unit cell with space group symmetry Fm-3m and unit cell parameters intermediate that of the two monometallic materials, Mg(NH3 )6 Cl2 and Mn(NH3 )6 Cl2
Summary
Energy storage materials and methods have gained high interest to ensure the transition to carbon-free future. The ammonia storage properties and crystal structures of the CaCl2 -CaBr2 , SrCl2 -SrBr2 and SrCl2 -SrI2 solid solutions have been investigated, and intermediate ammonia storage properties of the mixed anion metal halides were observed [28,29,30]. These studies show the possibility of forming mixed metal halides with tunable ammonia sorption properties. The thermally induced ammonia release for the mixed metal halide ammines is discussed: three NH3 desorption events are observed and the crystal structures of the intermediate ammine phases are identified and structurally characterized. The results presented in this work show that by changing the relative Mg/Mn ratio the NH3 sorption properties can be tuned and optimized depending on the application
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