(486i) Creating a Redox Materials Database for Solar-Thermochemical Air Separation and Fuels Production

Creating
a Redox Materials Database for Solar-Thermochemical Air Separation and Fuels
Production

J. Vieten^a, P. Huck^b,
D. Guban,^a M. Horton^b, B. Bulfin^c,
M. Roeb^a, K.A. Persson^b,
C. Sattler^a

^aInstitute of Solar Research, German Aerospace Centre
(DLR), Cologne, Germany

E-mail address: Josua.Vieten@dlr.de

^b Environmental Energy Technologies Division,
Lawrence Berkeley National Laboratory, Berkeley, USA

E-mail address: kapersson@lbl.gov

^c Professorship for Renewable Energy Carriers, ETH
Zurich, Zurich, Switzerland

E-mail address: bulfinb@ethz.ch

Fig.
1:
Thermochemical cycles for the production of solar fuels or for air separation
and ammonia production. Adapted from [1].

Converting
heat from renewable sources into other forms of energy is considered an
essential factor in the reduction of greenhouse gas emissions. For instance,
high temperatures can be reached using concentrated solar power (CSP), and the
thus-captured energy can be converted into so-called solar fuels via
thermochemical processes. These consist of the partial reduction of a redox
material, usually a metal oxide, at high temperatures following the exothermic
re-oxidation of this material at a lower temperature level using steam or CO₂,
which are thus converted into hydrogen or carbon monoxide, respectively. These
two gases can be combined to generate syngas for the production of hydrocarbons
(see Fig. 1). Through the same process, a stream of mostly inert gas can be
produced by re-oxidation with air, allowing air separation using renewable
energy sources. Hydrogen production and air separation can also provide the
feedstock for ammonia production through the Haber-Bosch process, as the
achieved oxygen partial pressures can be kept low enough to avoid catalyst
poisoning. [2] Ammonia produced through this method can be used for fertilizer
production, or as a fuel for energy storage.

Achieving
efficient air separation and fuels production through solar-thermochemical
processes is challenging but possible. Finding suitable redox materials
depending on the respective process conditions through evaluation of the
materials thermodynamics is a key point in reaching high process efficiencies.
[1, 3-5] Within a materials screening for these
applications, we prepared perovskite solid solutions with the general
composition A_xB_1-xM_yN_1-yO_3-¦Ä
with A, B = Ca, Sr and M, N = Ti,
Mn, Fe, Co, Cu using a modified Pechini
method. [5] Their redox enthalpies and entropies as a function of the
non-stoichiometry ¦Ä can be tuned by adjusting
their composition. We obtained experimental data gathered
via equilibrium oxygen uptake and release measurements using thermogravimetric
analysis, and theoretical data gathered via density functional theory (DFT)
calculations. The experimental data, i.e., redox enthalpies and entropies, are
fit using a novel empirical model, in order to generate interactive isotherms,
isobars, as well as graphs at constant non-stoichiometry which are referred to
as isoredox plots (see Fig. 2). In a joint effort between
the German Aerospace Center and the
Lawrence Berkeley National Laboratory,
the data is used to create a search engine for redox materials data based upon
the infrastructure of The Materials
Project. [6] The data is included into MPContribs [7], which is the
framework for external contributors to publicly share their data on the
Materials Project website. Many of the functions included in this contribution
are based on the databases included in The
Materials Project.

Fig. 2: Interactive isotherms, isobars, and ¡°isoredox¡±
plots as they are integrated into MPContribs (here for SrFeO_3-¦Ä).

Fig. 3: Combination of experimental and theoretical data within The Materials
Project.

Moreover,
theoretical data is collected for a large set of perovskite redox materials,
including solid solutions in a large compositional range. This data is
generated by pre-evaluating possible stable candidate materials using the
Goldschmidt tolerance factor and subjecting this data to Density Functional
Theory (DFT) based calculations. The resulting structural data contains the
energies of all atoms in the structure and allows to calculate redox
enthalpies, and, via the elastic tensors of the materials and their composition,
redox entropies. Thus, a full model of thermodynamic properties is generated.
Both experimental and theoretical datasets are used to create a redox materials
database. By defining specific target process conditions, such as the reduction
and oxidation temperatures and oxygen partial pressures, it is possible to find
the most efficient redox material for each specific application using the novel
perovskite search engine.

These
factors, in summary, accelerate the finding of new materials by replacing large
sets of experiments by a computer-based pre-selection step. By finding many new
redox materials and selecting the best candidates for further studies, we allow
a major leap towards more efficient renewable energy conversion and storage,
including ammonia production and solar fuels generation.

References

[1]  J. Vieten, B. Bulfin, F. Call, M. Lange,
M. Schmuecker, A. Francke, M. Roeb, C. Sattler,
Journal of Materials Chemistry A, 4, 13652-13659 (2016)

[2]  B.
Bulfin, J. Lapp, D. Guban, J. Vieten, S. Richter, S. Brendelberger, M. Roeb, C.
Sattler, submitted, (2018)

[3]  J.
Vieten, B. Bulfin, M. Senholdt, M. Roeb, C. Sattler, M. Schmuecker,
Solid State Ionics 308, 149-155 (2017)

[4]  B.
Bulfin, J. Vieten, C. Agrafiotis, M. Roeb, C. Sattler, Journal of Materials Chemistry A, 5, 18951-18966 (2017)

[5] 
J. Vieten, B. Bulfin, M. Roeb, C. Sattler, Solid State Ionics 315, 92-97
(2018)

[6] 
A. Jain, S.P. Ong, G. Hautier, W. Chen, W.D.
Richards, S. Dacek, S. Cholia,
D. Gunter, D. Skinner, G. Ceder, K.A. Persson, APL Materials, 011002 (2013)

[7] 
https://materialsproject.org/mpcontribs,
RedoxThermoCSP
contribution under construction.