E. A. Kotomin, R. Merkle, Yu. Mastrikov, J. Maier

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Presentation transcript:

E. A. Kotomin, R. Merkle, Yu. Mastrikov, J. Maier Effects of the surface termination on oxygen reduction rate of cathodes for fuel cells E. A. Kotomin, R. Merkle, Yu. Mastrikov, J. Maier 11/8/2018 Computer modelling school-Moscow-2018

Oxygen/Proton-conducting ceramic fuel cells (PCFC) SOFC PCFC (Zr,Y)O2-x Ba(Zr,Y)O3-x H2O H2 O2 O2- e- H2O H2 O2 H+ e- PCFC: * higher sion at low T * fuel not diluted by H2O  700 °C 300-600 °C Ba(Zr,Y)O3-x 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 (La,Sr)MnO3= LSM one of the first SOFC cathodes well studied (M.Liu 2007-2010; D.Morgan 2009, 2015; E.Wachsman, Watson 2016 Kuklja et al PCCP 15, 5443 (2013) Mastrikov et al, J. Phys. Chem. C 114, 3017 (2010); Wang J.Mat. Res. 27, (2012) main focus was on MnO2 terminated surface as the most stable, a few studies of Sr doping However, it was shown that Sr doping makes (La,Sr)O termination more favourable Piskunov et al PRB 78, 121406 (2008) Orthorhombic phase stable below 750 K (modeling defects in a cubic phase could make problems) 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 Ultimate goals of theory: Atomistic/mechanistic details of oxygen reduction (ORR) at SOFC cathode surface, including O2 adsorption, disociation, O vacancy formation/migration on the surface/bulk; with aim to optimize cathode chemical composition Challenge: what are the rate-determining reaction stages in oxygen reduction reaction Advantage of atomistic modeling: effects on terminating plane; charge redistribution. 11/8/2018 Computer modelling school-Moscow-2018

Methods: 1. Solid state physics Density Functional Theory Plane Wave basis set Generalised Gradient Approximation (Hubbard U) Perdew Wang 91 exchange-correlation functional Projector Augmented Wave method Conjugate Gradient method for structure relaxation Nudged Elastic Bands for energy barriers estimation Bader charge analysis Spin-polarized calculations Slabs 7 – 15 planes with 8x surface unit cell (critically important) 11/8/2018 Computer modelling school-Moscow-2018 5

Computer modelling school-Moscow-2018 2. Quantum chemical approach CRYSTAL 2014 code with LCAO basis set (re-optimised Comp Mat Sci. 29, 165 (2004) For light atoms (O), all-electron basis set (BS) for heavy atoms (Sr, Pb,Ti and Zr), the small-core pseudopotentials Hybrid HF-DFT functionals work very good for band gaps Bond populations and atomic charge analysis Well suited for single slabs without repetition along z axis Ghost basis set on vacant sites 11/8/2018 Computer modelling school-Moscow-2018 6

(La,Sr) (001) terminated surface Problems: it is polar – oppositely charged (+/-1e) LaO/MnO2 planes Stoichiometric slabs reveal macroscopic dipole moment To avoid macroscopic dipole moment, symmetric but nonstoichiometric slabs are used (on both sides-- LaO or MnO2) with even number of planes Dependent on the termination, Mn oxidation state (OX) is varied, which affects Vo formation energy Example: in LSF Vo form.enthalpy changes from 0.7 eV (Fe 4+) to 5 eV (OX < 3). Our goal- to study these effects on ORR on (La,Sr)O termination (Kotomin et al, ECS Trans. 77 (2017), Mastrikov 2018) 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 2 (001) slab terminations LaO (+1) MnO2 (-1) MnO2 (-1) LaO (+1) LaO (+1) MnO2 (-1) LaO (+1) MnO2 (-1) LaO (+1) MnO2 (-1) -- Choice: either non-stoichiometry, or dipole moment 11/8/2018 Computer modelling school-Moscow-2018

Electronic charge distribution orthorhombic (001) LMO, Sr 0%, 25%, Sr 50% 7 planes, 8x surface unit cell bulk LMO: O (-1.27 e), Mn +1.71 e considerable Mn-O bond covalency Variation of formal Mn oxidation state: xSr bulk (La,Sr)O MnO2 0 +3.0 +2.67 +3.25 0.25 +3.25 +3.00 +3.44 0.5 +3.50 +3.33 +3.63 11/8/2018 Computer modelling school-Moscow-2018

Vacancies in different planes No Sr doping 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 Two terminations show opposite trends: ΔE = 1.2 eV transforms into difference in surface [VO..] of 5 orders of magnitude at 1000K in agreement with D.Morgan (PRB 2015) for formation energy estimate, use as the reference the bulk with the same OX! 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 25% Sr doping Smaller DEVo.. Different reference energies in the bulk! Similar results for 50% Sr 11/8/2018 Computer modelling school-Moscow-2018

oxygen adsorption sites on AO atomic: molecular: La-O 2.2 Å (bulk 2.7 Å) O-O 1.46 Å = peroxide O(2-) adsorption in "hollow" site between 2 La, not ontop of La Charge -1.2 e is close to bulk, unlike -0.5 e on MnO2 for O(1-) Ads. energy 4.3 eV is much larger than 1.1 eV on MnO2 11/8/2018 Computer modelling school-Moscow-2018

O2 dissociation, VO.. migration estimated VO.. migration barriers (via subsurface jumps): 1.4-1.3-1.2 eV (Sr 0-25-50%) Compare with 0.9 eV in bulk and 0.6 eV on MnO2 Dissociation barriers: 0.2-0.6-0.8 eV (Sr 0-25-50%) compare ~ 1 eV on LaO La2NiO4 (Kilner 2016) Large enough supercell is critically important Our adsorbate coverage 12% 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 O2 dissociation on AO initial state = peroxide transition state +0.60eV next frame -0.14eV 1.90 Å 2.44 Å 1.49 Å 2.30 2.35 2.47 2.64 2.22 2.32 2.31 2.50 2.14 2.29 2.37 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 LMO- LaO termination, slab with initial Mn oxidation state 2.67+ DE O2 La La high O- coverage surface almost poisoned by O- very low surface [VO..] 10-7 relative to MnO2 termination with same ox. state at 1000 K  O incorporation is rate determining step O-O 1.46 Å peroxide -5.0eV La La O- O La La O O -4.3eV 1.4eV La La La -O O- O- La La 2(-0.75eV) molecular adsorption dissociation low barrier encounter of adsorbed O and VO.. via subsurface La O2- La incorporation 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 La0.75Sr0.25MnO3 (La,Sr)O termination, slab with initial Mn oxidation state 3.0+ DE O2 La La O-O 1.46 Å peroxide high O- coverage surface almost poisoned by O- very low surface [VO..] 10-5 relative to MnO2 termination with same ox. state at 1000 K  O incorporation is rate determining step -4.0eV La La O O La La O- O 0.6eV -2.9eV 1.3eV La La La -O O- O- La La 2(-1.4eV) La O2- La molecular adsorption dissociation encounter of adsorbed O and VO.. via subsurface incorporation 11/8/2018 Computer modelling school-Moscow-2018

still higher than on MnO2 still low surface [VO..] La0.5Sr0.5MnO3 (La,Sr)O termination, slab with initial Mn oxidation state 3.3+ moderate O- coverage about 10-30% still higher than on MnO2 still low surface [VO..] 10-4 relative to MnO2 termination with same ox. state at 1000 K  O incorporation is rate determining step DE O2 La La O-O 1.38 Å superoxide La La O O La La O- O -2.5eV 1.2eV 0.8eV -1.3eV La La La -O O- O- La La 2(-2.4eV) molecular adsorption dissociation encounter of adsorbed O and VO.. via subsurface La O2- La incorporation dissociation barrier increases with increasing Sr content,  less negative overall reaction enthalpy, less electron transfer to ads. O2 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 Our goal is modelling of all steps of the ORR in order to find limiting one. Entropy effects important under realistic operation conditions (1000 K). MnO2 (001) termination: Mastrikov et al, J. Phys. Chem. C 114, 3017 (2010); Wang J.Mat. Res. 27, 2012 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 CONCLUSIONS Surface termination, surface dipoles and Mn oxidation states (nonstoichiometry) play very important role in slab calculations of polar surfaces O (O2) adsorption energy on (La,Sr)O surfaces much larger than on MnO2 (1-2 order larger coverg.) BUT concentrations of VO.. is 4-7 orders smaller Thus, ORR rate at (La,Sr)O surface is much slower than on MnO2 termination could be relevant for many similar polar surfaces 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 Acknowledgments Many thanks to: E. Heifets R. Evarestov D.Gryaznov R.Dovesi 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 Joint experimental- theoretical study on BSCF-LSCF solid solution solutions Recent review articles related to SOFC: L. Wang et al, J.Mater. Res. 27, 2000 (2012) M. Kuklja et al, PCCP (review) 15, 5443 (2013) R.Catlow (ed.) Computational Approaches to Energy Materials, Wiley, 2013, Chapter 6. Yu. Mastrikov et al, PCCP 15, 911 (2013) D. Fuks et al, J. Mater. Chem. A1, 14320 (2013) 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 For comparison: LM (Mastrikov, 2010) MnO2 termination DG O O DE Mn O Mn O- +0.7eV O O2 +0.6eV Mn O Mn -O O- O- Mn O Mn O Mn Mn Mn Mn O Mn O O -1.1eV Mn O Mn 2(-1.5eV) O- -1.6eV O +0.6eV Mn O Mn Mn O2- Mn +0.7eV -O O- O- Mn O Mn O Mn Mn Mn 2(-1.5eV) rate determining: dissociation or encounter of adsorbed O and VO.. Mn O2- Mn 11/8/2018 Computer modelling school-Moscow-2018

Effects of LSO surface charge AO surface charge smaller than for MnO2 part of MnO2 surface dipole compensated by different surface rumpling  11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 DG LaMnO3 LaO termination, slab with initial Mn oxidation state 2.67+ DE O2 La La high O- coverage (but plateau 50%?) surface almost poisoned by O- very low surface [VO..] 10-7 relative to BO2 term. with same ox state at 1000 K  O incorporation is rds -5.0eV La La O- O La La O O La La O- O La La O O La La La -O O- O- La La -4.3eV La O2- La 1.4eV La La La -O O- O- La La 2(-0.75eV) molecular adsorption dissociation low barrier encounter of adsorbed O and VO.. via subsurface La O2- La incorporation 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 DG La0.75Sr0.25MnO3 (La,Sr)O termination, slab with initial Mn oxidation state 3.0+ DE high O- coverage (but plateau 50%?) surface almost poisoned by O- very low surface [VO..] 10-5 relative to BO2 term. with same ox state at 1000 K  O incorporation is rds O2 La La La La O O La La O- O -4.0eV La La O O La La O- O La La La -O O- O- La La 0.6eV -2.9eV La O2- La 1.3eV La La La -O O- O- La La 2(-1.4eV) La O2- La molecular adsorption dissociation encounter of adsorbed O and VO.. via subsurface incorporation 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 La0.5Sr0.5MnO3 (La,Sr)O termination, slab with initial Mn oxidation state 3.3+ moderate O- coverage (still higher than on BO2) coverage 10-30% still low surface [VO..] 10-4 relative to BO2 term. with same ox state at 1000 K  O incorporation is rds DG DE O2 La La La La O O La La O- O La La La -O O- O- La La La La O O La La O- O -2.5eV 1.2eV 0.8eV -1.3eV La La La -O O- O- La La La O2- La 2(-2.4eV) molecular adsorption dissociation encounter of adsorbed O and VO.. via subsurface La O2- La incorporation dissociation barrier increases in parallel to less negative overall reaction enthalpy from LM to L5S5M 11/8/2018 Computer modelling school-Moscow-2018

Charge redistribution near surfaces Effective atomic and plane charges, MnO2 termination (a) xSr=0 (b) xSr=0.50 La Mn O plane Sr T 1.69 -1.20 -0.71 1.77 -1.14 -0.51 2.09 0.89 1.59 -1.18 0.66 1.83 -1.21 -0.59 1.88 -1.16 -0.44 Central 2.08 -1.25 0.83 1.57 0.58 Bulk 1.78 2.10 1.58 1.85 (La,Sr)O termination (a) xSr=0 (b) xSr=0.50 La Mn O plane Sr T 1.99 -1.34 0.64 2.00 1.57 -1.33 0.45 1.59 -1.29 -0.99 1.78 -1.24 -0.70 2.07 -1.30 0.77 2.09 1.58 0.60 Central 1.69 -1.27 -0.85 bulk -1.6 -1.3 1.79 11/8/2018 Computer modelling school-Moscow-2018

Charge redistribution Comparison for the same Mn oxidation state: surface O ions more negative for AO than MnO2 (-1.34 vs -1.20 e), La charge less positive (+1.99 vs +2.09 e) surface MnO2 layers more covalent than bulk (decreased Mn and O charges) 11/8/2018 Computer modelling school-Moscow-2018

Density of states for different Sr doping (La,Sr)O (La,Sr)O termination: Reference: deep O 2s level E Fermi decreases with OX (Sr content) VB top consists of Mn t2g, eg states overlap with O 2p Mn ions in 2-, 4-planes close to bulk 11/8/2018 Computer modelling school-Moscow-2018

Computer modelling school-Moscow-2018 DOS vs Sr content MnO2 termination: Surface Mn t2g is wider and split off Mn ions in 3 plane close to bulk Sr doping does not affect splitting MnO2 11/8/2018 Computer modelling school-Moscow-2018

Variation of oxygen exchange rates for MnO2 termination . or 11/8/2018 Computer modelling school-Moscow-2018 32

Computer modelling school-Moscow-2018 11/8/2018 Computer modelling school-Moscow-2018