(156e) Solar Thermochemical CO2-Splitting Using Redox Cycles of Cr-Doped Mn-Based Perovskites

Finding novel strategies to enable the transition away from a fossil fuel-based energy economy to the efficient use of renewable energy resources is a global challenge of modern society.The solar-to-fuel technology is a promising approach to transform one of the most abundant renewable sources into fuels, while mitigating CO₂ emissions.¹ Solar-driven thermochemical cycles operate on redox processes of oxides in which H₂O and CO₂ are converted to syngas (H₂ and CO). In this technology, solar energy is collected in concentrated solar power plants and stored chemically in synthetic fuels, through a 2-step thermochemical cycle^1,2. Currently, the benchmark material used for the solar-to-fuel process is CeO₂. Despite the promise of CeO₂, the high reduction temperature of 1500 °C to reach usable fuel production yields opposes challenges for reactor design, long-term microstructural degradation and cost. In addition, traditionally redox cycling is operated in temperature-swing regimes. Namely, oxidation is performed at 500 ºC lower than reduction, which poses high energy penalties for the process. Alternatively, thermochemical redox cycles could be operated under isothermal conditions benefiting from faster kinetics and long-term materials stability.¹ Recent advances proved that perovskite-type oxides could operate at significantly decreased temperature.^2,3,4 Namely, by utilizing solid solutions of La_0.6Sr_0.4Cr_0.8Co_0.2O₃ thermal reduction could be carried out at a 300 °C lowered temperature, while producing the same level of CO when compared to state-of-the-art CeO₂.⁵ However, such materials exhibited slower kinetics than CeO₂ and further research is needed in order to find more efficient perovskite materials based on abundant and inexpensive elements that could work in more kinetically favorable conditions.

Consequently, in this work we explore La_0.6Sr_0.4Cr_xMn_1-xO₃ perovskite compositions due to favorable thermodynamics of Cr- and Mn-based perovskites for CO₂ splitting in accordance to recent thermodynamic models.^4,5,6,7 La_0.6Sr_0.4Cr_xMn_1-xO₃ perovskite powders were synthesized through a modified Pechini method covering a compositional range of chromium from x=0 to 1. Although O₂ release decreased with increasing addition of Cr, results showed that La_0.6Sr_0.4Cr_0.85Mn_0.15O₃ outperformed CeO₂ and La_0.6Sr_0.4MnO₃ under isothermal redox cycling operation. Namely, La_0.6Sr_0.4Cr_0.85Mn_0.15O₃ yields 350 µmol/g, whereas La_0.6Sr_0.4MnO₃ 250 µmol/g for 1400 ºC and pCO₂=0.5 atm. Conversely, it was observed that Cr-rich perovskites performed poorer than La_0.6Sr_0.4MnO₃ under temperature-swing operation.

These results illustrate the beneficial influence of Cr-doping on the fuel production for isothermal operation, confirming trends obtained from thermodynamic modeling by Bork et al..⁷ Furthermore, results corroborated the high operational flexibility that easy-tunable perovskite materials offer. Accordingly, proper materials design based on thermodynamic models and further experimental confirmation are crucial for determining the most efficient compositions at given working conditions.

1. Muhich, C. L., Ehrhart, B. D., Al-Shankiti, I., Ward, B. J., Musgrave, C. B., & Weimer, A. W. Wiley Interdisciplinary Reviews: Energy and Environment, 5(3), 261–287. (2016).

2. Kubicek, M., Bork, A. H.& Rupp, J. L. M. J. Mater. Chem. A, in review (2017).

3. McDaniel, A. H., Miller, E. C., Arifin, D., Ambrosini, A., Coker, E. N., O’Hayre, R.,et al. Energy & Environmental Science, 6(8), 2424. (2013).

4. Yang, C.-K., Yamazaki, Y., Aydin, A., & Haile, S. M. Journal of Materials Chemistry A., 2, 13612–13623. (2014)

5. Bork, A. H., Kubicek, M., Struzik, M. & Rupp, J. L. M. J. Mater. Chem. A3, 15546–15557 (2015).

6. Bork, A. H., Povoden-Karadeniz, E. & Rupp, J. L. M. Adv. Energy Mater.7, 1601086 (2017).

7. Bork, A. H., Povoden-Karadeniz, E. & Rupp, J. L. M. to be submitted.