Electronic Origin and Kinetic Feasibility of the Lattice Oxygen

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Electronic Origin and Kinetic Feasibility of the Lattice Oxygen Participation During the Oxygen Evolution Reaction on Perovskites Jong Suk Yoo, Yusu Liu, Xi Rong, and Alexie M Kolpak J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00154 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Journal of Physical Chemistry Letters

Electronic Origin and Kinetic Feasibility of the Lattice Oxygen Participation During the Oxygen Evolution Reaction on Perovskites Jong Suk Yoo,‡, a, * Yusu Liu,‡, b Xi Rong,‡, a Alexie M. Kolpak a, * Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA b Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA a

‡ Co-first

authors, contributed equally * Co-correspondence: [email protected], [email protected]

Abstract Density functional theory is employed to investigate the electronic origin and feasibility of surface lattice oxygen (Osurf) participation during the oxygen evolution reaction (OER) on perovskites. Osurf participation occurs via the non-electrochemical pathway in which adsorbed atomic oxygen (O*) diffuses from the transition-metal site to the oxygen site, and then Osurf shifts out of the surface plane to react with O* to form Osurf–O* and a surface oxygen vacancy. The different thermodynamic driving forces of Osurf participation on LaMO3–δ (M = Ni, Co, and Cu) are explained by the changes in the oxidation state of the transition-metal site throughout the reaction. We show that Osurf participation on LaNiO3 cannot be hindered by Osurf protonation in the OER potential range. By including the coverage effect and utilizing the implicit solvent model, we finally show that lattice oxygen mechanism is more feasible than the conventional mechanism for OER on LaNiO3. TOC graphic

The electrochemical splitting of water to produce hydrogen for fuel cells is a potential approach to transitioning from fossil fuels to clean energy sources such as water.1–5 However, significant challenges remain for developing highly active, stable, and inexpensive electro-catalysts to overcome the slow kinetics of the oxygen evolution reaction (OER) at the anode. 6–8 Recently, complex oxides such as ABO3 perovskites (A = alkaline earth metal, B = transition metal), have attracted special interest due to their tunable properties that originate from the large chemical space, and high OER activities compared to precious metal

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based catalysts such as RuO2 and IrO2.9–12 However, a fundamental understanding of the OER mechanisms for different perovskites is limited, hindering the rational design of the optimal catalyst. Recently, experiments employing in situ 18O isotope labelling showed that the O2 gas produced during OER on highly active perovskites such as La1–xSrxCoO3–δ and SrCoO3–δ, contains the lattice oxygen from the perovskites.13 In addition, computational studies employing density functional theory (DFT) showed that the conventional adsorbate evolution mechanism (AEM), in which the adsorbate is sequentially oxidized via OH* O* OOH* O2(g), is favorable for strongly binding perovskites such as LaCoO3, whereas the newly proposed lattice oxygen mechanism (LOM), in which the surface lattice oxygen (Osurf) participates in the reaction, is favorable for weakly binding perovskites such as LaCuO3.14-15 The authors showed that the ideal perovskite with the lowest overpotential requires Osurf participation as the peak of the activity volcano for LOM has been shown to be slightly higher than that for AEM (unpublished results, see Supporting Information (SI) for details). It was also shown that LOM is generally more favorable than another recently suggested lattice-oxygen based mechanism,13 namely Oads–Olatt mechanism or OOM (unpublished results). However, questions remain as to (1) the detailed understanding of the reaction pathway via which Osurf participates in OER, (2) an explanation for different thermodynamic driving forces of Osurf participation on different perovskites, (3) the competition between Osurf participation and Osurf protonation that prevents the desired former reaction, and (4) the feasibility of LOM under OER (oxidizing) condition that accounts for both adsorbatecoverage and solvent effects, especially for moderately binding perovskites where AEM and LOM can compete closely. In this study, we employ DFT to address these issues by taking highly active LaNiO3(001) as an example for moderately binding perovskites (∆GO = 2.46 eV relative to H2O and H2), LaCoO3(001) for strongly binding perovskites (∆GO = 3.21 eV) that prefer AEM to LOM, and LaCuO3(001) for weakly binding perovskites (∆GO = 4.38 eV) that prefer LOM to AEM. The (001) surface plane is chosen for its stability16-17 and experimental availability.18-21 (see Methods) Theoretical insights obtained from this study provide a guiding principle for the development of new perovskites for OER via LOM. Scheme 1a shows the four charge-transfer (electrochemical) steps involved in AEM22,23 vs. LOM11, 13, 15, 24 over NiO2 terminated LaNiO3(001) initially covered with OH*. Scheme 1b shows the detailed reaction pathways possible for Osurf participation, i.e., step (1) in LOM. We observe that Osurf participation may include step 1 in AEM (I I0) depending on the stability of I0 relative to I1, and requires additional non-electrochemical steps such as O* diffusion from Nisurf to Osurf (I0 I1), followed by Nisurf–Osurf bond cleavage to form OO* bound to Nisurf via the original O* atom and a surface oxygen vacancy (VO) (I1 I2) or the cleavages of both the Nisurf–Osurf and Nisurf–O* bonds to form OO* bound to Nisurf via the original Osurf atom and VO (I1 I3-1 I3). Thus, it is essential to first study the kinetic feasibility of all non-electrochemical steps involved in Osurf participation before comparing

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the thermodynamics of the four charge-transfer steps involved in AEM vs. LOM, to determine which mechanism is more favorable for a particular perovskite.

Scheme 1. (a) The illustration of the four charge transfer steps involved in AEM and LOM over the NiO2 terminated LaNiO3(001) surface covered with OH*. (b) shows the detailed reaction pathways possible for Osurf participation, i.e., step (1) in LOM. Solid arrows indicate electrochemical steps that involve charge transfers from the water solvent, dashed arrows indicate non-electrochemical steps that involve only adsorbates and the surface lattice oxygen (Osurf). Note that we assume that the catalyst surface is initially covered with OH*. This is a reasonable assumption since OER on perovskites operates under oxidizing conditions (e.g. at high potential and high pH).

Fig. 1 shows the calculated reaction energetics of I0 I1 I2/3 over MO2 terminated LaMO3(001) (M = Ni, Co, and Cu). Interestingly, we can see the final state (I2/3) is significantly more stable than the initial state (I0) by ~2.2 eV and ~1.0 eV for LaCuO3 and LaNiO3, respectively, whereas it is conversely less stable by ~0.5 eV for LaCoO3. This can be explained considering the change in the oxidation state of surface M site (Msurf) between the two states. We find that O* in I0 binds to Msurf with a double bond character as indicated by e.g. the Nisurf–O* bond length of 1.68 Å, which is noticeably shorter than the Ni–O bond length of 1.93 Å in bulk LaNiO3. This indicates the oxidation state of Msurf of I0 is slightly more positive than the nominal charge of M3+ in bulk LaMO3, as further supported by our Bader calculations shown in Table S5. On the other hand, OO* in I2/3 binds to Msurf with a single bond character as indicated by e.g. the Nisurf–OO* bond length of 1.93 Å. This indicates the oxidation state of Msurf of I2/3 is fairly close to the nominal charge of M3+ in bulk LaMO3, as further supported by our Bader calculations shown in Table S5. Since late transition metals such as Cu and Ni are electronegative (χ = 1.90 and 1.91 by Pauling scale25), and therefore have large fourth ionization energies26 (IE = 57 eV and 55 eV), compared to early

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transition metals such as Co (χ = 1.88 by, and IE = 51 eV), it is not surprising to find M(3+δ)+ (I0) M3+ (I2/3) more exothermic for LaCuO3 > LaNiO3 > LaCoO3.

Figure 1. (a) The calculated Gibbs free energies of I0 I1 I2/3 on LaCoO3 (top panel), LaNiO3 (middle panel) and LaCuO3 (bottom panel). All energies are referenced to I1. Note that the transition states for I0 I1 on LaNiO3 and LaCuO3 are not obtained as the reaction is highly exothermic on these surfaces. See Fig. S10 for illustrations of the optimized structures. (b) The changes in the calculated electron densities during I1 I2/3 on LaNiO3. ‘d’ indicates the Msurf–O bond that is being broken.

Fig. 1 indicates O* diffusion from Msurf to Osurf (I0 I1) is highly exothermic by 1.74 eV for LaCuO3, moderately exothermic by 0.72 eV for LaNiO3, whereas it is endothermic by 0.99 eV for LaCoO3. In contrast, the variations in the reactions energetics of the actual bondbreaking and -forming involved in Osurf interacting with O* (I1 I2/3) among the different perovskites are relatively small as indicated by the reaction free energies being generally exothermic (and also the free-energy barriers being generally surmountable at room

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temperature) for all three perovskites. This can be explained considering that there are always some electron densities remaining in the Msurf–O bonds that are being broken (indicated as ‘d’ in Fig. 1b), thus stabilizing the transition states. The significance of these findings is that the extent of O* preferring to diffuse from Msurf to Osurf (e.g. the reaction energy of I0 I1) can be a convenient thermodynamic indicator for the kinetic feasibility of the overall Osurf participation for a given perovskite. Since experimentally synthesized perovskites are often non-stoichiometric,27-30 we investigate the effect of lattice oxygen vacancies on the reaction energetics of Osurf participation, shown in Table S1 in SI. We identify the trend that, when more lattice oxygen vacancies (δ) are introduced to the subsurface layer of LaMO3–δ (M = Ni, Co, and Cu), the reaction energies and activation barriers for I0 I1 I2/3 increase. This can be explained considering that Osurf participation, which involves reversibly subtracting a surface lattice oxygen from the perovskite to form OO* and VO, would be more difficult on more oxygendeficient perovskites. However, the effect of subsurface lattice oxygen vacancies is much smaller for LaCuO3–δ < LaNiO3–δ < LaCoO3–δ (Table S1). Though the effect of the subsurface lattice oxygen vacancies is qualitatively the same for all perovskites, late transition metals with high electronegativities can better tolerate the electron-rich environment created by lattice oxygen vacancies. Fig. 2 shows the changes in the projected density of states (pDOS) for the d states of Msurf of I0 and I1, compared to M3+ in bulk LaMO3 (M = Co, Ni, and Cu). For all three perovskites, electron-withdrawing O* on Msurf of I0 creates more empty states above the fermi level compared to bulk M, i.e., Msurf of I0 is electron-deficient (M(3+δ)+) compared to M3+ in bulk LaMO3. On the other hand, the change in the positions of the d bands after the reaction of I0 I1 is different depending on the identity of M in LaMO3. For LaCoO3, I0 I1 does not effectively eliminate all the unoccupied portions of the d bands, whereas, for LaCuO3 (or LaNiO3), I0 I1 leads to almost all portions of the d bands below the fermi level, electronically recovering from the highly oxidized M(3+δ)+ to more stable M3+. Furthermore, Crystal Orbital Hamilton Populations (COHP) analysis of the Msurf–O* bond (Fig. S7) shows all the unoccupied portions of the d bands in I0 are antibonding states, destabilizing the system for all three perovskites. However, in the case of LaCuO3 (or LaNiO3), I1 has much less occupied antibonding states than I0, substantially stabilizing the system, compared to LaCoO3. This is in agreement with the results shown in Fig. 1. We also note here that similar results are obtained when we extend the same analysis to perovskites containing lattice oxygen vacancies in the outermost subsurface layer (LaMO3–δ where δ = zero, 0.125, and 0.25), as shown in Fig. S1-2, and S8-9. Bader Charge analysis indicates the quantitative differences in thermodynamic driving forces of Osurf participation on different perovskites can be adequately correlated to the variations in the electron density on Msurf throughout the reaction (see Additional Discussion in SI and Fig. S3).

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Figure 2. Projected density of states for the d states of M3+ in bulk (black), and Msurf of I0 (red) and I1 (grey) for LaCoO3 (top panel), LaCoO3 (middle panel), and LaCuO3 (bottom panel).

To better understand Osurf participation on moderately binding LaNiO3 under oxidizing conditions of OER, we have investigated the effect of OH* and O* coverages on the reaction energetics of I0 I1 I2/3, and found that the presence of these adsorbates on the adjacent Nisurf slightly lowers the reaction energies and activation barriers (Fig. S4). This can be explained considering that the adsorption of the oxygen species on Nisurf destabilizes Osurf neighboring the Nisurf, thus promoting O* diffusion from a nearby Nisurf to the Osurf (I0 I1) and also stabilizing the relatively oxygen-deficient final state (I2/3) that contains VO. Furthermore, we have also investigated Osurf participation on LaNiO3 occurring at higher product (OO*) coverages, and found that I0 I1 I2/3 remains exothermic until the product (OO*) coverage is above 0.50 ML (monolayer), as shown in Table S4 and Fig. S6. In addition, we discuss the possibility of Osurf participation competing with undesirable Osurf protonation on LaNiO3. The left panel of Fig. 3a shows the reaction energy of Osurf protonation obtained in the low coverage regime of 0.25 ML on clean LaNiO3 (no other adsorbates on Nisurf sites). It can be seen that Osurf protonation is highly exothermic (–1.37 eV), and therefore remains as exothermic by –0.14 eV under OER equilibrium potential of URHE = 1.23 V, indicating that Osurf protonation can occur well before Osurf participation on some of the Osurf sites. The electronic origin of Osurf protonation being highly exothermic can be explained by comparing the pDOS for the p states of different oxygen atoms in LaNiO3 surface. For example, the right panel of Fig. 3a shows that the p band of Osurf is not fully filled with electrons, indicating that Osurf is electronically not very stable due to its unsatisfied valence, compared to the protonated surface oxygen (Oprot). With electron donations from the adsorbed proton to Oprot, the p band of Oprot is entirely below the Fermi level, resembling that of the lattice oxygen inside the bulk perovskite (Obulk).

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Figure 3. (a) The calculated reaction free energy of Osurf protonation (left panel) at URHE = 0.00 V (pH = 0) on clean LaNiO3, and the projected density of states for the p states of different oxygen atoms in the LaNiO3 surface (right panel). Obulk indicates the lattice oxygen in bulk LaNiO3, Osurf indicates the surface lattice oxygen, and Oprot indicates the surface lattice oxygen with proton adsorbed on top, and (b) The adsorbed proton coverage dependent reaction energy of Osurf protonation on clean (pink) and 1ML of OH* covered (light blue) LaNiO3 at URHE = 0.00 V (pH = 0). The OH* coverage is defined such that 1 ML of OH* completely saturates the four surface Ni atoms in the supercell, the adsorbed proton coverage is defined such that 2.00 ML of protons completely protonates the eight Osurf atoms in the supercell.

However, Fig. 3b shows that the reaction free energy of Osurf protonation obtained at URHE = 0.00 V (pH = 0) increases with increasing proton coverage on both clean and OH* covered LaNiO3. As a result, the calculated reaction energy at the adsorbed proton coverage of 0.50 ML is –1.25 eV and –1.65 eV on clean and OH* covered LaNiO3 surfaces, respectively, indicating that many Osurf atoms (precisely, 6/8 Osurf atoms in the supercell) stay unprotonated at URHE = 1.65 V, enabling Osurf participation (see Fig. S11 how the reaction free energy of Osurf protonation becomes endothermic at URHE = 1.65 V). At URHE > 1.65 V or pH > 0, even more unprotonated Osurf atoms would be available for Osurf participation as the reaction condition is highly oxidizing. Thus, we can conclude here that Osurf protonation does not prevent Osurf participation under most OER conditions. We now compare the reaction free energies between AEM and LOM on LaNiO3 surface partially covered with OH* and protons under water solvent implicitly modeled as a homogeneous dielectric medium,31-32 and they are shown in Fig. 4a. First, we can see that

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the inclusion of solvation effect stabilizes relatively polar adsorbates, like H*O-site, OH* and OOH* by 0.1 eV ~ 0.2 eV, whereas it has a negligible effect on relatively unpolar adsorbates, like OO* and O* (Table S2). Consequently, the thermodynamic overpotential for LaNiO3 surface is increased by 0.1 V ~ 0.2 V for both AEM and LOM as the potential limiting step for AEM is OH* O* (step 1 in Scheme 1), and that for LOM is H*O-site * (step (4) in Scheme 1). However, notice LOM is much preferred to AEM as the thermodynamic overpotential for LOM (η = 0.40 V) is found to be lower than that for AEM (η = 0.89 V) with the adsorbatecoverage and solvation effects included.

Figure 4. (a) The free energy diagrams at URHE = 1.23 V for OER via AEM (dark grey) vs. LOM (turquoise) on the LaNiO3 surface covered with 1.00 ML of OH* on the Ni sites and 0.50 ML of adsorbed protons on the O sites (note that this is the surface on which Osurf protonation becomes endothermic under most OER conditions) at the product (OO*) coverage of 0.25 ML (i.e., one of the four adsorbed hydroxyl species being oxidized to OO*) under vacuum (dashed lines) or the water solvent implicitly modeled as a homogenous dielectric medium (solid lines). (b) The comparison of the free energy diagrams for LOM between the two product (OO*) coverages of 0.25 ML and 0.5 ML under the water solvent model.

Furthermore, Fig. 4b shows the dependence of the LOM energetics on the product (OO*) coverage. It can be seen that, as OO* coverage increases from 0.25 ML to 0.50 ML, the overpotential decreases slightly. This indicates LOM remains thermodynamically favorable as long as the amount of VO surrounding a Nisurf does not exceed one (LOM becomes

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unfavorable at OO* coverage higher than 0.50 ML (Table. S4 and Fig. S6) because it requires more than one VO forming around a Nisurf. To conclude, we have shown, based on DFT calculations, that Osurf of a perovskite can participate in OER via the non-electrochemical route in which adsorbed O* diffuses from Msurf to Osurf and then Osurf shifts out of the surface plane to form OO* and VO . This reaction pathway is more favorable for more weakly binding perovskites like LaCuO3. We have also found that the reaction energy of O* diffusion from Msurf to Osurf can be a convenient thermodynamic indicator for the kinetic feasibility of overall Osurf participation. Therefore, a promising route to developing highly active perovskites that prefer LOM to AEM is to promote O* diffusion from Msurf to Osurf. Additionally, we have found that the origin of Osurf participation can be understood as Msurf recovering from the highly oxidized state to the electronically more stable, lower oxidation state throughout the reaction. The different thermodynamic driving forces of Osurf participation on different perovskites can be explained by the differences the transition metals’ tolerance towards high oxidation states. To better understand Osurf participation on moderately binding LaNiO3 under highly oxidizing conditions of OER, we have investigated the effect of OH* and O* coverages on the reaction energetics of Osurf participation, and found that the presence of these adsorbates on the adjacent Nisurf sites makes Osurf participation more favorable. Furthermore, we have also investigated the possibility of Osurf protonation hindering Osurf participation, and found that many of the Osurf atoms stay unprotonated under most OER conditions, thus enabling Osurf participation. Finally, we have compared the reaction free energies between AEM and LOM on LaNiO3 surface partially covered with OH* and protons in implicit water solvent. Therefore, we have shown that LOM is much preferred to AEM as the overpotential for LOM is found to be much lower than that for AEM, thus contributing to a more complete understanding of catalytic mechanisms on different perovskites for water-splitting. Methods Spin polarized calculations were performed using VASP33 with PAW34 pseudopotentials and the RPBE35 functional. The fast algorithm, accurate precision, and 4×4×1 Monkhorst-Pack36 k-point mesh were used for all calculations with the energy cutoff of 500 eV and Gaussian smearing of 0.1 eV. The periodic slab model for the MO2 terminated LaMO3 (M = Ni, Co, and Cu) (001) surface was built from the bulk cubic phase using a (2×2×4) supercell separated by more than 17 AT of vacuum perpendicular to the surface plane. Note that this slab size has been employed in many previous OER studies on perovskites23, 37-39. The (001) surface plane is particularly chosen as it is generally known to be the most stable for simple ABO3 perovskites in cubic structures.16-17 In addition, (001) surfaces are widely used in experiments. One possible reason is that (001)-oriented epitaxies (thin films) are easy to grow on substrates compared to other orientations.18-21 The top two oxide layers of the slab models as well as the adsorbates were allowed relax until the forces on the individual relaxed atoms were less than 10-3 eV/AT . The adsorption energies of all adsorbates were calculated in the low coverage limit of 0.25 ML (i.e. one of the four surface metal sites

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occupied), unless noted otherwise, and they were all referenced to gas-phase H2O and H2. The projected density of states were calculated using a denser 12×12×1 Monkhorst-Pack kpoint mesh for higher accuracy. The Bader-charge analysis was conducted using the gridbased algorithm40-43. Crystal Orbital Hamilton Populations (COHP) analysis was conducted using the LOBSTER suite44-47. The activation barriers were determined using the climbing image nudged elastic band (CI-NEB) method48-49. We also crosschecked our results by using RPBE+U (Ueff = 6.4 eV for nickel50), and found that applying the U parameter significantly weakens O* adsorption compared to OH* or OO* adsorption, and therefore induces a higher oxidation state for Msurf of I0 compared to e.g., that of I2/3, further strengthening all the conclusions made in this study (Fig. S5). The solvation effects were investigated using the VASPsol31-32 patch that implements the continuum solvation model with the dielectric constant of water. The reaction free energies were determined using the following equation: ௥ ∆‫ ܧ∆( = ܩ‬+ ∆ܼܲ‫ ܧ‬− ܶ∆ܵ)୴ୟୡ − ܷ݁ୖୌ୉ + ∆‫ܪ‬ୱ୭୪ (ܷ݁ୖୌ୉ term was not considered for nonelectrochemical reactions). The zero-point-energy (∆ZPE) and entropic (T∆S) corrections were determined for LaNiO3 (Table S3), and were used for all perovskites. The calculated ௥ ∆‫ܪ‬ୱ୭୪ values for different reaction intermediates on LaNiO3 are shown in Table S2. Finally, we also considered (2×2×7) supercells with identical surface terminations on both sides of the slab models (thus, non-polar in z direction) in calculating adsorption energies, and found that the results are similar to those obtained with (2×2×4). Supporting Information Available: structure details, additional tables, figures and discussions. Acknowledgement J.S.Y and X.R. acknowledge support from the Skoltech-MIT Center for Electrochemical Energy. Computations were performed using computational resources from XSEDE and NERSC.

References (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (2) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science 2002, 297, 2243–2245. (3) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253–278. (4) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141–145. (5) Meyer, T. J. Catalysis: The Art of Splitting Water. Nature 2008, 451, 778–779.

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Electronic Origin and Kinetic Feasibility of the Lattice Oxygen

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