Screening Ruddlesden-Popper (n=1) Oxide Materials for Thermochemical Water Splitting By Density Functional Theory

Thermochemical redox reactors store concentrated solar power by thermally inducing an oxygen deficiency within a metal oxide structure. The metal oxide’s affinity for reoxidation allows it to facilitate the splitting of H2O or CO2 to produce H2 or CO for syngas formation.[1] Typically, high temperatures (>1400 °C) are used to drive the reduction of the benchmark material, CeO2, [2] however there is motivation to lower this temperature and investigate new metal oxides capable of larger fuel productions. Perovskite materials have been thoroughly investigated due to their crystallographic stability and ability to accommodate relatively large oxygen deficiencies. Emery et al. [3] conducted a wide computational screening of this family of materials based on a few simple thermodynamic parameters originally proposed by Meredig and Wolverton. [4] Herein, we extend these previous studies and investigate the A2BO4 Ruddlesden-Popper (RP) oxides family [5], layered perovskites, that have previously demonstrated fast redox kinetics and large oxygen storage as solid oxide fuel cell cathodes.[6] A combination of screening parameters based on charge neutrality, Goldschmidt tolerance and computed defect formation energy, identified 38 possible RP candidate materials. One of which - Ca2MnO4-δ – was taken forward to experimental testing due to its Earth abundant elements. The powder was synthesized via a modified Pechini method and thermal analysis experiments demonstrated thermally driven oxygen evolution from 800 to 1200 °C equating to a non-stoichiometry of δ=0.18. High-temperature X-ray diffraction alluded to the formation of a CaMn2O4 secondary phase above 1075 °C, therefore consequent thermochemical water splitting cycles were carried out at a maximum of 1000 °C. Oxidation under steam at 800 °C demonstrated good hydrogen production volumes, although gas production on further cycles was limited by particle sintering observed by scanning electron microscopy. Further investigations aim to understand and improve the cyclability and kinetics under different reaction temperatures and humidity. This study outlines, with experimental validation, how computational screening can be used to find future suitable RP candidates for thermochemical water splitting whose performance can be further improved with doping strategies and morphological adaptation. References [1] Scheffe J. R. and Steinfeld A., Materials Today, (2014), 17, 341-348 [2] Chueh W. C., et al., Science, (2010), 330, 1797–1801 [3] Emery A., et al., Chem. Mater. (2016), 28, 5621−5634 [4] Meredig B. and Wolverton C., Phys. Rev. B: Condens. Mater. Phys. (2009), 80, 245119. [5] Ruddlesden S. N. and Popper P., Acta Crystallogr., (1957), 10, 538-539 [6] Ghorbani-Moghadam T., et al, Ceram. Int., (2018), 44, 21238–21248