(10f) Creating a Redox Materials Database for Solar-Thermochemical Processes

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

^a Institute
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.