Oxygen Defect Formation Thermodynamics of CaMnO₃: A Closer Look

Abstract

Hydrogen is one of the most promising future energy sources. [1] In contrast to conventional energy sources, only water is produced during its combustion. At present, a large amount of hydrogen is still obtained from hydrocarbons by steam reforming. [2] However, for every two molecules of hydrogen, one molecule of carbon dioxide is also produced, therefore this process is not climate neutral. Therefore, the sustainable production of hydrogen is of primary interest for the future of the energy industry. [3] One possibility is electrolytic water splitting. [4] The problem, however, is the high demand for environmentally friendly-generated electricity, which is not yet sufficiently available. A promising alternative is solar or solar-thermal water splitting by electrolysis, thermochemistry or photocatalysis. This is not yet feasible on a large scale due to technical and economic challenges. Photocatalysis, on the other hand, does not depend on an external supply of electricity or heat, but requires special catalysts that are often too expensive or resource critical.In this respect, the combination of chemical and electrical potentials is advantageous. If an anode material is used which is itself redox active, the potential required for electrolysis can be reduced. Perovskites ABO 3 are suitable materials due to their stability and temperature-dependent reversible loss of oxygen and high oxygen mobility in the lattice. [5,6] One of the perovskites which have recently attracted interest is CaMnO 3 . Poeppelmeier et al. discuss the low-temperature synthesis of oxygen-deficient CaMnO 2.5 starting from polycrystalline CaMnO 3 . [7] X-ray and neutron diffraction patterns were used to determine the crystal structure and distribution of oxygen vacancies in CaMnO 2.5 . Jung et al. determined the electronic band structures of CaMnO 3 and LaMnO 3 measuring reflectivity spectra at room temperature. [8] Wollan and Koehler studied the magnetic properties of perovskite-type compounds ½La ð1ÀxÞ , Ca x MnO 3 uses neutron diffraction. [9] Several antiferromagnetic (AFM) spin states were found for x ¼ 0 and x ≥ 0.5. Between x ¼ 0 and x ¼ 0.5, they observed simultaneous occurrence of ferromagnetic and AFM structures. In a narrow range around x ¼ 0.35 the structure was found to be ferromagnetic. The measured magnetic moments decrease with higher x values. Bakken et al. measured the variation of oxygen nonstoichiometry of CaMnO 3-δ with the oxygen partial pressure at 1223, 1273, and 1323 K. [10] The dependence of partial molar enthalpy and entropy from δ was derived. Bulfin et al. measured the equilibrium nonstoichiometry of CaMnO 3 and Ca 0.8 Sr 0.2 MnO 3 for temperatures in the range 400-1200 °C and different oxygen partial pressures. [11] Ca 0.8 Sr 0.2 MnO 3 showed a lower enthalpy of reduction and was also robust against phase transition, different from CaMnO 3 at high temperatures. Also a number of computational studies on CaMnO 3 using density functional theory (DFT) have been published. Molinari et al. calculated the structural, e...

Defect Formation Thermodynamics of (A,A′)(B,B′)O3 (A = Mg,Ca,Sr and B = Ti,Mn,Cr,Fe,Mo) Perovskites

“…The average OVF energy of the first two defects () for (here SG 20) is 180.3 kJ mol −1 , which is 27 kJ mol −1 larger than that was determined previously for in SG 62. [ 12 ] The difference is related to the different structures of the two space groups. It can also be due to the less intensive search for the most stable OV configuration for .…”

Effect of Surface Energetics on Phase Stability of CaMnO3

“…[ 24 ] The global hybrid functional PW1PW together with triple‐zeta basis sets optimized by our group [ 25 ] was used because this combination proved to give the most accurate results for solid‐state calculations of CaMnO 3 . [ 9 ] The integral truncations (TOLINTEG) were set to 10 −7 and 10 −14 atomic units. Furthermore, a 4 × 4 × 4 Monkhorst–Pack grid was employed for bulk calculations, while a 4 × 4 × 1 grid was used for surface calculations.…”

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Perovskites are frequently discussed as anode material for electrolytic water splitting [1][2][3][4][5] due to their structural stability, which allows reversible oxygen exchange with the environment. [6,7] Hydrogen evolution reactions (HERs) with perovskites have also recently studied by Sokolov et al [8] In previous work, [9] we theoretically studied bulk properties of CaMnO 3 , which is considered as a promising catalyst. CaMnO 3 allows oxygen defect formation in a typical range for high-temperature electrolysis but has the drawback of decomposing into CaMn 2 O 4 and Ca 2 MnO 4 .

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“…In previous work, [ 9 ] we theoretically studied bulk properties of CaMnO 3 , which is considered as a promising catalyst. CaMnO 3 allows oxygen defect formation in a typical range for high‐temperature electrolysis but has the drawback of decomposing into CaMn 2 O 4 and Ca 2 MnO 4 .…”

“…In our previous study, [ 9 ] we identified two orthorhombic polymorphs with space groups (SG) no. 20 and 62 as most stable.…”