Agrafiotis, Christos

Knoblauch, Nicole

Lee, Kangjae

Pein, Mathias

Roeb, Martin

Sattler, Christian

Institute of Transport Research:Publications

Kangjae Lee, Nicole Knoblauch, Christos Agrafiotis, Mathias Pein, Martin Roeb, Christian SattlerLa and Yb Incorporated Zr-Doped Ceria for Solar Thermochemical CO2 Splitting:Thermodynamics and Oxidation Kinetics Study2022 AIChE Annual MeetingPhoenix, AZNovember 15, 2022Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022INTRODUCTION AND RESEARCH OBJECTIVESDLR (CC BY-NC-ND 3.0)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Introduction Step1: Reduction (Qsolar input) Operated at 1300 – 1500 °C, lower pO2 Endothermic MOox becomes MOred (e.g. CeO2 → CeO1.9) O2 released Step2: Oxidation (H2O/CO2 input) Operated at 800 – 1000 °C Exothermic MOred becomes MOox (e.g. CeO1.9 → CeO2) H2/CO produced (H2O/CO2 splitting)Two-step Thermochemical Redox CyclesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022 Agrafiotis, C., Roeb, M., and Sattler, C. "A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles." Renewable and Sustainable Energy Reviews 42 (2015): 254-285.Introduction Current Status CeO2 as a state-of-the-art redox material Scalability: 50 kWth single-reactor system,100 kWth dual-reactor system demonstrated Efficiency: Solar-to-fuel efficiency of 5.25 % at a 4 kWth system, 5.6 % at a 50 kWth system (Figure) Potential outlook Potential redox materials performing better than CeO2 suggested 100 MWth scale-up system concepts available 20 % solar-to-fuel efficiency as a potential objectiveCurrent Status and Potential OutlookKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Zoller, S. A 50 kW Solar Thermochemical Reactor for Syngas Production Utilizing Porous Ceria Structures. Dissertation ETH No. 26451, ETH Zurich (2020) Agrafiotis, C., Roeb, M., and Sattler, C. "A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles." Renewable and Sustainable Energy Reviews 42 (2015): 254-285.Schäppi et al. "Drop-in fuels from sunlight and air." Nature 601.7891 (2022): 63-68.Research ObjectivesBackground and ObjectivesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022 Ceria doped with tetravalent dopants (e.g. Zr4+, Hf4+) Enhanced reducibility due to a lower reduction enthalpy compared to pure CeO2 Poor oxidation kinetics being disadvantageous for the CO2 and H2O splitting reactions The influences of ionic radii or valence on possible structural changes and/orthermodynamic, kinetic properties to establish an associated doping strategy Thus, CeO2-based and Ce(Zr)O2-δ-based materials with additional dopantsare suggested.Research ObjectivesDopant Concentration SelectionKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Selected samples (Abbreviation)CeO2Ce0.9Yb0.1O1.95 (CYO10)Ce0.9La0.1O1.95 (CLO10)Ce0.9Zr0.1O2 (CZO10)Ce0.9Zr0.0547Yb0.0453O1.97735 (CZYO55)Ce0.9Zr0.0547La0.0453O1.97735 (CZLO55)Concentration Concentration Calculated ionic radii [pm]Calculated  ionic radii [pm]Zr M (La/Yb) Zr+La Zr+Yb0.0 1.0 116.0 98.50.1 0.9 112.8 97.10.2 0.8 109.6 95.60.3 0.7 106.4 94.20.4 0.6 103.2 92.70.5 0.5 100.0 91.30.547 0.453 98.5 90.60.6 0.4 96.8 89.80.7 0.3 93.6 88.40.8 0.2 90.4 86.90.9 0.1 87.2 85.51.0 0.0 84.0 84.0La3+: 116.0 pmYb3+: 98.5 pmZr4+: 84.0 pmCe3+: 114.3 pmCe4+: 97.0 pmTHERMODYNAMICSDLR (CC BY-NC-ND 3.0)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA Results: Thermodynamics Reduced at Tred = 1200, 1250, 1300, 1350 and 1400 °C Under pure Ar for the lowest pO2 Under pure Ar + Ar with O2 pump at various flow rate ratio at pO2 = 10-3 – 10-3.75 atm Under Ar with O2 pump at various voltage at pO2 higher than 10-3 atmKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data of a CZLO55 pellet during reduction at various temperatures with O2 pump at maximum voltage (1165 mV)TGA Experimental ConditionsTGA Results: Thermodynamics Oxygen nonstoichiometry (δ) of ceria Close to that from literature Oxygen nonstoichiometry (δ) of all samples Tetravalent dopants: Higher δ than ceria Trivalent dopants: Lower δ than ceria co-doped: δ between ceria and Zr-doped ceria No significant differences between CZYO55 and CZLO55Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Oxygen nonstoichiometry of a) ceria at Tred = 1200 – 1400 °C and b) all materials at 1400 °COxygen NonstoichiometryOXIDATION KINETICSDLR (CC BY-NC-ND 3.0)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA Methods: Oxidation Kinetics Reduced at Tred = 1400 °C under Aratmosphere for 2 hours Oxidized at Tox = 1200 – 500 °C under 0.4 atm CO2 balanced with Ar for an hour (1200 °C) 2 hours (1000 – 500 °C)CO2 Splitting of ceria/doped ceriaKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data of a CeO2 pellet during CO2 splitting at pCO2 = 0.4 atmTGA Results (Δδ): Oxidation Kinetics Reduced at 1400 °C for 2 hours under Ar. Oxidized for 2 hours under pCO2 = 0.4 atm, balanced with Ar.Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Change in nonstoichiometry during CO2 splitting (Left) at 1000 °C, (Mid) at 800 °C, and (Right) at 600 °C.Inset: at early reactions (t = 0 – 480 s)TGA Results: Introducing Reaction Regimes CO2 splitting consists of multiple reaction regimes with different reaction mechanism. First regime (0 s – about 150 s) Second regime (about 150 s until rate becomes almost linear) Third regime (depending on the material, as soon as rate becomes almost linear)CO2 Splitting Reaction RegimesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and reaction regimes of a CZLO55 pellet during CO2 splitting (Tox = 1000 °C)TGA Results: Introducing Reaction Regimes CO2 splitting consists of multiple reaction regimes with different reaction mechanism. First regime (0 s – about 150 s) Second regime (about 150 s until rate becomes almost linear) Third regime (depending on the material, as soon as rate becomes almost linear)CO2 Splitting Reaction RegimesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and reaction regimes of a CeO2 pellet during CO2 splitting (Tox = 1000 °C)TGA Results: Introducing Reaction Regimes CO2 splitting consists of multiple reaction regimes with different reaction mechanism. First regime (0 s – about 150 s) Second regime (about 150 s until rate becomes almost linear) Third regime (depending on the material, as soon as rate becomes almost linear)CO2 Splitting Reaction RegimesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and reaction regimes of a CZO10 pellet during CO2 splitting (Tox = 1000 °C)TGA Results: Introducing Reaction Regimes CO2 splitting consists of multiple reaction regimes with different reaction mechanism. Unrestricted regime Solid surface is not occupied Surface exchange limited regime Solid surface is being occupied by adsorbed CO2 Slow surface exchange is primarily limiting the reaction Surface exchange limited at equilibrium regime Solid surface is fully occupied by adsorbed CO2 Reaction rate is almost linearCO2 Splitting Reaction RegimesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and reaction regimes of a CZLO55 pellet during CO2 splitting (Tox = 1000 °C)TGA Results: Modified Exponential Decay Fit/Slope Factor Conventional exponential decay function cannot be used for the activation energy calculation. Near-linear increase in mass at the surface exchange limited regime requires new fitting function. Exponential decay function was modified to compensate for the linearity.𝑦 = 𝑦 + 𝐴 −𝑒𝑥𝑝 𝑥 − 𝑥 /𝑡 + 𝐴 𝑥 − 𝑥 Slope factor A2 represents the slope at the beginning of the second regime.New ExpDecayMod FitKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and fits of CZO10/CeO2 pellets during CO2 splittingTGA Results: Slope Factor Comparison Slope factors vs lattice constants Overall, slope factors are positively proportional to the lattice constant. This trend is possibly due to enhanced surface exchange induced by the stress caused by the larger dopant cations. Exceptions for the CZYO55 at low temperatures (Tox = 600, 700 °C), possibly due to less diffusion limited, or less backward reaction at low temperatures than other materialsOxidation Kinetics and Lattice ConstantsKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Rate of Δδ vs. Lattice constant of doped ceria at various oxidation temperatures5.39 5.40 5.41 5.42 5.43 5.44 5.451E-051E-041E-03Slope factor (log scale) [1/min]Lattice Constant [Å] 600 C 800 C 1000 CZrZr+Yb YbZr+LaLaLarger Dopant Ionic RadiiFaster CO2Splitting KineticsCONCLUSIONS AND OUTLOOKDLR (CC BY-NC-ND 3.0)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Doping Strategy Based on Thermodynamics/Kinetics Some tetravalent dopants such as Zr4+ provide excellent reduction capability, but oxidation kinetics is extremely slow due to limited surface exchange capability induced by smaller effective averaged ionic radius. Doping larger trivalent cations on the Zr-doped ceria lattice results in larger effective averaged ionic radius and significantly enhances surface exchange during CO2 splitting. However, thermodynamic properties become less favorable compared to the case when only tetravalent dopant cations are incorporated. The difference in ionic radius of the trivalent dopant does not influence thermodynamics significantly. Optimization in dopant concentrations, balancing between more tetravalent dopants for higher reducibility and more trivalent dopants with larger ionic radius for faster oxidation kinetics, is crucial to maximize the fuel productivity. Since CO2 splitting kinetics is rather surface exchange limited, techniques to promote the surface exchange (e.g. introducing micro porosity for larger specific surface area, surface coating with catalysts, etc.) will be even more beneficial.Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Acknowledgement DLR Institute of Future Fuels Discussion DLR Institute of Materials Research Material Synthesis/Characterization Funding: DFG Grant No. SCHM 1367/5-1Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022QUESTIONS?DLR (CC BY-NC-ND 3.0)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Introduction Estimation of fuel production costs for solar thermochemical plant Main assumptions Plant capacity: 1000 bpd of jet fuel and 850 bpd of naphtha Solar plant: Concentration 100 €/m², O&M 2 €/m², Tower: 20 €/kWthDNI: 2500 kWh/m² Electricity: 0.04 €/kWhel on site CO2: 108 €/t from air capture (H2O captured concurrently) Thermochemical energy conversion efficiency: 19 % (without vacuum pumping and gas separation) Fischer-Tropsch: 60 % efficiency, 23000 €/bpd (investment), 4 €/bbl (O&M) Interest rates: 5.7 % (debt nominal, 60 %), 11.8% (equity nominal, 40 %) Value of by-product: Price (naphtha) = 80 % × price (jet fuel)Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022 Falter, Christoph, et al. "An integrated techno-economic, environmental and social assessment of the solar thermochemical fuel pathway." Sustainable Energy & Fuels 4.8 (2020): 3992-4002.Economic Analysis and Risk AssessmentIntroductionKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022 Falter, Christoph, et al. "An integrated techno-economic, environmental and social assessment of the solar thermochemical fuel pathway." Sustainable Energy & Fuels 4.8 (2020): 3992-4002.Economic Analysis and Risk Assessment Estimation of fuel production costs for solar thermochemical plant Regional variability of production cost  Levers to achieve low production costsStructural Characterization (XRD) Fig) XRD patterns after heat treatment at 1650 °C Table) Nominal and actual composition via EDSKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Abbreviation Nominal Composition Actual CompositionCZLO55 Ce0.9Zr0.0547La0.0453O1.97735 Ce0.9Zr0.06La0.04O1.98CZYO55 Ce0.9Zr0.0547Yb0.0453O1.97735 Ce0.9Zr0.05Yb0.05O1.975CZO10 Ce0.9Zr0.1O2 Ce0.9Zr0.1O2CLO10 Ce0.9La0.1O1.95 Ce0.91La0.09O1.96CYO10 Ce0.9Yb0.1O1.95 Ce0.9Yb0.1O1.95Structural Characterization (Raman Spectroscopy) Fig. Left) Normalized F2g Raman peak of all materials Inset) Disorder bands (D1 and D2 band) Fig. Right) F2g Raman peak position vs. lattice constant of all materialsKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022D1 band~550 cm-1Related toO2 vacancyD2 band~600 cm-1Not related toO2 vacancyTGA Results: Thermodynamics Reduced at Tred = 1200, 1250, 1300, 1350 and 1400 °C For 2 h at 1200 – 1300 °C and for 4 h at 1350 –1400 °C Under pure Ar for the lowest pO2 Under pure Ar + Ar with O2 pump at various flow rate ratio at pO2 = 10-3 – 10-3.75 atm Under Ar with O2 pump at various voltage at pO2 higher than 10-3 atmKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022a) Partial molar enthalpy and b) partial molar entropy vs. δPartial Molar Enthalpy/EntropyTGA Results: Thermodynamics Reduced at Tred = 1200, 1250, 1300, 1350 and 1400 °C For 2 h at 1200 – 1300 °C and for 4 h at 1350 –1400 °C Under pure Ar for the lowest pO2 Under pure Ar + Ar with O2 pump at various flow rate ratio at pO2 = 10-3 – 10-3.75 atm Under Ar with O2 pump at various voltage at pO2 higher than 10-3 atmKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Gibbs free energy for a) reduction and b) oxidation calculated at δ = 0.007.Gibbs Free EnergyTGA Results (Δδ): Oxidation Kinetics Reduced at 1400 °C for 2 hours under Ar. Oxidized for 2 hours under pCO2 = 0.8 atm, balanced with Ar.Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Change in nonstoichiometry during CO2 splitting (Left) at 1000 °C, (Mid) at 800 °C, and (Right) at 600 °C.Inset: at early reactions (t = 0 – 480 s)TGA Results: Introducing Reaction Regimes CO2 splitting consists of multiple reaction regimes with different reaction mechanism. Unrestricted regime (0 s – about 150 s) Surface exchange limited regime (about 150 s until equilibrated) Surface exchange limited at equilibrium regime (depending on the material, as soon as it is equilibrated)CO2 Splitting Reaction RegimesKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022TGA raw data and reaction regimes of a CZO10 pellet during CO2 splitting (Tox = 1000 °C)TGA Results: Slope Factor Comparison Slope factor prediction Slope factor of CeO2 and CLO10 were calculated only at low temperatures (below 600 °C). Predicted data for CLO10 at high temperatures are available based on the calculated rate constant. Slope factor trend Slope factors of La-doped samples were higher than those of Yb-doped samples. CZO10 was the lowest in the slope factor. Dopant ionic radius likely determines CO2 splitting kinetics.Slope Factor ComparisonKangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022Slope factor (Rate of Δδ) vs. CO2 splitting temperaturesMeasured dataPredicted dataBased on Rate Const.Plans For Future Works Surface exchange and diffusion coefficients will be determined via Secondary-Ion Mass Spectrometry (SIMS) oxygen isotope experiments, and compared to the TGA results. Surface exchange promoting techniques will be tested (developing composites, making foams or other porous samples, etc.). Optimized materials with respect to thermodynamics and oxidation kinetics will be proposed and syngas productivity using them will be experimentally demonstrated.Kangjae Lee, 2022 AIChE Annual Meeting, November 15, 2022

La and Yb Incorporated Zr-doped Ceria for Solar Thermochemical CO2 Splitting: Thermo-dynamics and Oxidation Kinetics Study

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La and Yb Incorporated Zr-doped Ceria for Solar Thermochemical CO2 Splitting: Thermo-dynamics and Oxidation Kinetics Study

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