Density functional theory investigation of the electronic structure and defect chemistry of Sr1-xKxFeO3

Abstract

Solid oxide fuel cells (SOFCs) efficiently generate electricity, but high operating temperatures (T op > 800°C) limit their utility. Reducing T op requires mixed ion-electron conducting (MIEC) cathode materials. Density functional theory is used here to investigate the role of potassium substitutions in the MIEC material Sr 1−x K x FeO 3 (SKFO). We predict that such substitutions are endothermic. SrFeO 3 and SKFO have nearly identical metallic electronic structures. Oxygen vacancy formation energies decrease by ∼0.2 eV when x K increases from 0 to 0.0625. SKFO is a promising SOFC MIEC cathode material; however, further experimental investigations must assess its long-term stability at the desired operating temperatures.Solid oxide fuel cells (SOFCs) provide a clean and efficient means of generating electrical power from various fuel sources.[1] However, high operating temperatures (T op > 800°C) reduce the overpotential arising from standard La 1−x Sr x MnO 3 cathodes, but increase material costs and shorten cell lifetimes.[2]Intermediate-temperature (600°C < T op < 800°C) SOFCs have potential to overcome these deficiencies but in turn require alternative, mixed ion-electron conducting (MIEC) cathode materials to increase the active region and reduce the cathode overpotential.[3] Useful MIEC cathode materials must allow for facile electronic and ionic conductivity.La 1−x Sr x Co 1−y Fe y O 3 remains the reference MIEC cathode [4][5][6][7] although many other cathode materials show promising electrochemical behavior. These include Ba 1−x Sr x Co 1−y Fe y O 3 (BSCF), Sr 2 Fe 2−x Mo x O 6 (SFMO), and Sr 1−x K x FeO 3 (SKFO). [8][9][10] Many experimental studies have illuminated the structural properties, oxygen transport kinetics, and electrochemical performance of LSCF, BSCF, and SFMO. [6,[10][11][12][13] Far less is known about SFKO. Hou et al. reported that cathodes of Sr 0.9 K 0.1 FeO 3 (with a La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3−δ , or LSGM, electrolyte and a Sr 2 MgMoO 6 anode) produced a current density competitive with LSCF at 800°C. [9] Repetitive cycling of the cell between open circuit voltage and 0.4 V caused no loss in the observed power density.[9] Furthermore, SFKO cathodes contain no Co, which improves their cost-effectiveness. [14] Additional studies support the use of Sr 0.9 K 0.1 FeO 3 as a SOFC cathode material by offering additional evidence of its stability [15] and demonstrating a viable synthesis method for the material. [16] The small body of experimental evidence supporting SKFO as an MIEC cathode in SOFC applications suggests that further investigation into this material is warranted. [9,15,16] Firstprinciples quantum mechanics methods based on KohnSham density functional theory (KS-DFT) have proven useful in providing rational design principles for related materials. [17][18][19][20][21][22][23][24][25][26] Here we provide a DFT study of SKFO (x K = 0, 0.0625, and 0.125) aimed at answering three questions. First, does potassium substitution (K / Sr in Kröger-Vink notation [27] ) ne...

Modification of LSF-YSZ Composite Cathodes by Atomic Layer Deposition

“…[27][28][29] The high degree of compositional control provided by ALD of Fe 2 O 3 may allow titration of the Sr in the form of SrFeO 3 and re-activation of the electrode. 30,31 Finally, we suggest that ALD has great promise for producing electrodes with complex and potentially promising surface structures. Here, we demonstrated that both pure and mixed-oxide films can be grown, but more complicated structures can be imagined.…”