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Oxygen (O2) reduction reaction on Ba-doped LaMnO3 cathodes in solid oxide fuel cells: a density functional theory study

Oxygen (O2) reduction reaction on Ba-doped LaMnO3 cathodes in solid oxide fuel cells: a density... The oxygen adsorption and subsequent reduction on the {100} and {110} surfaces of 25% Ba-doped LaMnO (LBM25) have been studied at the density functional theory (DFT) with Hubbard correction and the results compared with adsorption on 25% Ca-doped LaMnO (LCM25) and Sr-doped LaMnO (LSM25). The trend in the reduction energies at the Mn cation sites are 3 3 predicted to be in the order LSM25 < LBM25 < LCM25. In addition, the trend in dissociation energies for the most exothermic dissociated precursors follow the order LBM25 < LSM25 < LCM25. The adsorption energies (− 2.14 to − 2.41 eV) calculated for the molecular O precursors at the Mn cation sites of LCM25, LSM25 and LBM25 are thermodynamically stable, when compared directly with the adsorption energies (E = − 0.56 to − 1.67 eV) reported for the stable molecular O precursors ads 2 on the Pt, Ni, Pd, Cu and Ir {111} surfaces. The predicted Gibbs energies as a function of temperature (T = 500–1100 °C) and pressures (p = 0.2 atm) for the adsorption and dissociation on the surfaces were negative, an indication of the feasibil- ity of oxygen reduction reaction on the {100} and {110} surfaces at typical operating temperatures reported in this work. Keywords Gibbs free energies · Superoxide · Peroxide · SOFC · LaMnO Introduction research to more efficient and sustainable conversion and storage technologies [1]. The world’s demand for energy, coupled with the finite sup- Solid Oxide Fuel Cells (SOFCs) are electrochemi- ply of fossil fuels and the environmental and political draw- cal devices for converting chemical energy into electrical backs associated with them, has shifted the focus of energy energy and additional heat. Intermediate conversion of heat to mechanical work required by conventional combustion techniques is largely avoided. SOFCs with electrolyte that * Albert Aniagyei is ionic conducting presents unique opportunities over other aaniagyei@uhas.edu.gh fuel cells, such as cheap constituents, decreased susceptibil- Caroline Kwawu ity to fuel impurities and effectiveness. SOFCs embodies the kwawucaroline@gmail.com cleanest, capable and flexible chemical to electrical energy Ralph Kwakye transfer system [2, 3] providing alternatives to broader con- kwakyer@uhas.edu.gh sumption of hydrogen and carbon-based fuels and regenerat- Boniface Yeboah Antwi ing fuel sources [4, 5]. However, for SOFCs to be explored boni.antwi@yahoo.com on a bigger commercial scale, they must be less expensive Jonathan Osei-Owusu and the electrode fabricated from readily available materi- oseiowusuansahjoe@gmail.com als. One approach to cost reduction is the drastic reduction Department of Basic Sciences, University of Health in the operation temperatures, which suppresses cell deg- and Allied Sciences, Ho, Ghana radation, thermal stress buildup and improvement in cell Department of Chemistry, Kwame Nkrumah University lifetimes [6]. Due to their high catalytic reactivity toward the of Science and Technology, Kumasi, Ghana oxygen reduction reaction, noble metal electrocatalyst [7], Institute of Industrial Research, Council for Scientific such as platinum (Pt), is employed as a cathode. However, and Industrial Research, Accra, Ghana the cost of platinum represents a fundamental problem for University of Environment and Sustainable Development, its application. Somanya, Ghana Vol.:(0123456789) 1 3 15 Page 2 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 Perovskite-type oxides such as LaMnO have attracted Computational details attention for applications built upon their unique electronic and magnetic features. For example, La Sr MnO is the The calculations were carried out within the Kohn–Sham 1-x x 3-δ cathode of choice when the electrolyte material is zirconia DFT formalism [21], using a plane-wave basis set as in SOFCs [7]. This is attributed to the good electrical con- implemented in the Quantum-ESPRESSO [22]. The Per- ductivity, stability, low cost and efficient catalytic activity dew–Burke–Ernzerhof (PBE) generalized gradient approxi- for the oxygen reduction reaction [8]. In SOFCs cathode, mation (GGA) was employed for the exchange and corre- at dopant concentration (x = 0.2 − 0.3), La Sr MnO lation terms [23]. The plane-wave basis set cutoffs for the 1−x x 3−δ is reported to be employed as mixed ionic–electronic con- smooth part of the wave function and the augmented density ducting (MIEC) [9]. were set to 40 and 420 Ry, respectively, which ensured con- For O adsorption and subsequent reduction on pure vergence of the forces to within 0.01 eV/Å. The Brillouin and 25% Ca-doped LaMnO (LCM25) {100} and {110} zone for the bulk LaMnO was sampled using a 4 × 4 × 4 surfaces, Aniagyei et al., [10] reported that adsorption or Monkhorst–Pack k-points mesh [24]. To correct the large reduction processes are more favorable at the Mn sites than self-interaction error inherent in standard DFT-GGA meth- La and Ca sites at the DFT level with Hubbard correction. ods for mid-to-late first-row transition metal oxides [25], The adsorption energies calculated for the {110} surfaces the DFT + U approach [26] with a U value of 4.0 eV for eff 3+ were more favorable and stable than the {100}. Mn ions (mainly Mn ions) provides the best results when Chen et  al., [11] studied the kinetic behavior of the modeling the LaMnO ground-state properties [27]. We have oxygen reduction reaction and diffusion pathways on 25% investigated the symmetric Pm3m cubic structure, because it Sr-doped LaMnO (LSM25) cathode surface. From the is stable under SOFC operating conditions (above 500 °C in spin-polarized DFT and molecular dynamics (MD) cal- ambient air) [28, 29]. All calculations were spin-polarized to culations, O adsorption energies were more stable at the describe accurately the magnetic properties of the LBM25, Mn sites compared to the Sr sites. LSM25 and the triplet ground state of oxygen. 3+ La Ba MnO is a colossal magnetoresistance (CMR) The different possible magnetic orderings at the Mn 1−x x 3 classical compound [12] with a Curie temperature of sites in the LBM25 and LSM25 surface structures were con- 340  K [13]. Dependent on the dopant concentration, sidered and found that the ferromagnetic (FM) ordering is the crystal structure moves from orthorhombic through 0.3 and 0.2 eV, respectively, more stable than the antiferro- rhombohedral (x > 0.13) to cubic (x > 0.35). For the magnetic (AFM) ordering. Hence, all the structures investi- magnetic spin alignment, it shows ferromagnetic behav- gated in this study have FM spin ordering. ior at Ba concentrations of x ˃ 0.15. They are reported The {100} and {110} surface structures were cre- to undergo metal-to-insulator transition at x ≈ 0.20 [13, ated from the fully optimized bulk structure using the 14]. La Ba MnO has received extensive investigation METADISE code [30], which generates different atomic 1-x x 3 of its crystallographic and magnetic properties [14, 15], layer stackings to result in a zero-dipole moment perpen- phase transitions [13, 16, 17], and spin dynamics [18]. dicular to the surface plane, as is required for reliable and For the reduction activity on Ba-doped LaMnO as cath- realistic surface calculations [31]. The fully relaxed bulk ode materials toward ORR, to the best of our information, structures were used to create the surfaces to eliminate the no theoretical studies have been shown, which makes this presence of fictitious forces during the surface relaxation. theoretical study timely. The surfaces were modeled using a slab model comprising In this work, an extension to previous studies [10], Hub- of eight atomic layers with a vacuum size of 12 Å introduced bard-corrected DFT approach is applied to study the Gibbs in the z-direction, which is large enough to avoid any spuri- free energies of adsorption of the multiple reaction paths ous interaction between the periodic slab images. associated with the reduction of oxygen at 25% Ba-doped Similar to previous studies [10], all the surface calcula- LaMnO {100} and {110} surfaces (LBM25). The oxy- tions for the interactions between the molecular oxygen spe- gen reduction energetics on LBM25 surfaces are compared cies on the LSM25 and LBM25 surfaces were performed by with LCM25 [10] and LSM25 [see Table S1 and Figure S1 relaxing the top three layers while keeping the bottom five and S2], the main cathode of preference for conventional layers fixed at the bulk parameters. The adsorption energy SOFCs. This is to ascertain whether they could be used as was calculated according to the following relation: an alternative electrocatalyst for oxygen reduction reaction E = E − E + E (1) (ORR) based on their adsorption/reduction energies. The ads surface+O surface O 2 2 adsorption studies were restricted to the {100} and {110} where E is the total energy of the substrate–adsorb- surface+O surfaces of LaMnO because they present the most stable 2 ate system in the equilibrium state, and E and E surface O surfaces [19, 20]. 2 are the total energies of the substrate (clean surface) and 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 3 of 10 15 adsorbate (free O molecule in the spin triplet state), Table 1 Calculated oxygen vacancy formation energies (in eV) of LaMnO- {110} and MnO -terminated {100} LBM25 surfaces with respectively. Hubbard U correction (U = 4 eV) eff Surface models Oxygen-vacancy forma- Oxygen- tion vacancy {110} formation Results and discussion {100} A 4.37 2.63 Description of Ba‑doped LaMnO surfaces B 3.72 2.97 C 4.10 3.35 For reduction reaction on SOFC cathodes, the role played D 4.11 3.48 by oxygen vacancies present on the surface is critical. The LaMnO 4.23 3.60 vacant sites are expected to compete against adsorption and 3 dissociation in the mobility of oxygen. To examine the role of the vacant site on the oxygen reduction reactions at the cathode, LBM25 surfaces with different layer models are La Ba MnO (surface) → La Ba MnO + O (g) 0.75 0.25 3 0.75 0.25 3− 2 shown in Fig. 1. Summarized in Table 1 are the calculated (2) oxygen vacancy formation energies obtained by substitu- 3+ 2+ tion of the host La cation for Ba in the {100} and {110} In previous studies of LaMnO with MnO -terminated 3 2 LaMnO surface models with different layers. The calculated {100} surface at the GGA + U [32] and hybrid B3LYP oxygen vacancy formation energies located on the top layer [33] levels of theory, a calculated oxygen vacancy forma- were based on the reaction of tion energy of 2.2 eV was reported. Surfaces model A of Fig. 1 (Top) A is BaMnO-terminated and B–D are LaMnO-terminated {110} surface models. (Down) MnO -terminated (100) LBM25 models. Color code: green = La, blue = Ba, purple = Mn, and red = O 1 3 15 Page 4 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 the MnO terminated {100} and model B of the LaMnO-For O adsorbed on Mn cation site, the end-on structure 2 2 terminated {110} with the La ion in/near the topmost lay- is changed to a side-on structure after optimization (Fig. 3, ers (Fig. 1) were found to have the lowest oxygen vacancy A1), with an exothermicity of − 2.25 eV. The formed molec- formation energies; thus, we chose these two models for ular precursor A1 is accompanied by significant charge subsequent studies of O –LBM25 interactions. transfer (− 0.34 e from the substrate) to the O orbital, 2 2 In this work, molecular or dissociative chemisorption weakening and elongating the d(O–O) to 1.45 Å compared of O on (1 × 1) and (2 × 1) surface models at various cov- to gas phase and experimental d(O–O) of 1.23 and 1.21 Å erages of Θ = 0.25 ML and 0.50 ML, where a monolayer [34], respectively. For A1, the shortest interatomic distance (ML) refers to an oxygen molecule per active surface cation. d (O–Mn) is 1.838 Å. On the LSM25 surface, O adsorbed Shown in Fig. 2 are the top and side views of the LBM25 end-on and side-on at the Mn sites, and exothermicities of {100} and {110} surface models showing the different 1.59 and 2.41 eV were calculated and the d(O–O) = 1.357 adsorption sites explored for O adsorption. and 1.449 Å, respectively (Table S1 and Figure S1). In both molecular processes, the d(O–O) are elongated by 1.315 and O adsorption on the oxygen‑deficient LBM25 1.480 Å, respectively. The adsorption energies of − 2.14, {110} surface − 2.41 and − 2.25 eV calculated at the Mn cation sites of LCM25, LSM25 and LBM25 are thermodynamically sta- For O adsorption on the {110} surface with coverages of ble, when compared directly with the adsorption energies Θ = 0.25 ML and 0.50 ML, molecular and dissociative sce- (E = − 0.56 to − 1.67 eV) reported for molecular O pre- ads 2 narios for the selected (1 × 1) and (2 × 1) surface models cursors on the Pt, Ni, Pd, Cu and Ir {111} surfaces [35, 3+ 2+ were considered. Substitution of host La with Ba cation 36]. This indicates that LCM25, LSM25 and LBM25 cath- creates a defective surface with a vacant oxygen site. Similar ode materials may be more efficient for O activation than to the undoped surface [10], different adsorption configu- the transition metal surfaces. In addition, since molecular ration and modes were exploited at the {110} and {100} precursors at the Mn cation sites of LCM25, LSM25 and surfaces, that may serve as precursors for O dissociation. LBM25 are associated with stronger bonds compared to Figure 3 shows the optimized adsorption structures, whiles LaMnO [10], this implies Ca, Sr and Ba as dopants in the the adsorption energies, Löwdin atomic charges, interatomic cathode influences the O reduction. The trend in the reduc- bond distances and vibrational stretching frequencies are tion energies on the Mn cation sites are predicted to be in presented in Table 2. the order LSM25 < LBM25 < LCM25. Fig. 2 Schematic representation of the side and top views of the (2 × 1) slab model of (a, b) LBM25 {110} and (c, d) LBM25 {100} supercells showing the corresponding different adsorption sites explored. Color code: green = La, blue = Ba, purple = Mn, and red = O 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 5 of 10 15 Fig. 3 Side view of the optimized geometries of oxygen molecule on (1 × 1) defective LBM25 {110}. Color code: green = La, blue = Ba, pur- ple = Mn, and red = O Table 2 Calculated adsorption − a b Surface Config E (eV)|q| (e ) d(O–Mn) (Å) d(O–La) (Å) d(O − O) (Å) υ(O − O) ads energies (E ), Charge (q), −1 ads (cm ) relevant bond distances (d) of molecular (O ) oxygen on {110} A1 − 2.25 − 0.34 1.838 – 1.446 641 defective {110}-(1 × 1) surface A2 − 1.31 (− 1.50) − 0.29 (2.300) 1.374 (1.407) 878 of LBM25; O−O stretching A3 − 3.78 (− 4.40) − 0.46 1.883 (1.922) 2.377 (2.372) 1.495 (1.496) 514 vibrational frequency (υ) of the adsorbed O ; and calculated A4 − 2.96 (− 3.34) − 0.53 – – 1.507 (1.500) 490 gas-phase d(– O) = 1.229 Å and D1 − 5.99 (− 6.05) − 0.76 1.638 (1.649) – – – −1 the υ(O–O) = 1558 cm D2 − 4.80 (− 5.49) − 0.89 – 2.241 (1.793) – – D3 − 7.83 (− 7.63) − 0.93 1.767 (1.904) 2.410 (2.132) – – {100} A1 − 0.82 − 0.14 1.850 1.380 861 A2 − 1.97 (− 2.88) − 0.36 – 1.491 (1.494) 497 D1 − 3.51 (− 4.12) − 0.44 1.577 (1.851) – – The (2 × 1) values are given in parenthesis a,b The shortest distance between an adsorbed oxygen species and an Mn or La ion (110) surface For O adsorbed on top-La cation, no stable end-on reaction and consistent with experimental observations configuration is obtained because it is changed to a side- [37]. In this work, for the O molecule adsorbed end-on on structure after optimization (A2, Fig. 3) with an exo- and side-on at the top-Mn and Sr cation sites on SrMnO- thermicity of 1.31  eV. The bound O molecule experi- terminated {110} surface, the B (Mn) cation sites were ences a net charge of − 0.29 e and d(O–O) elongation calculated to be more active than A (Sr) cation sites toward of 1.374 Å. The formation of molecular precursor A1 is oxygen reduction reaction (Table S1). This demonstrates energetically more favorable than A2 in terms of adsorp- that Mn cation are the favored on both oxygen-deficient tion energies and stronger bond formation, i.e., shorter d LCM25 [10], LSM25 and LBM25 surfaces for adsorption, (O–Mn) = 1.838 Å than d (O–La) = 2.324 Å showing that in agreement with other reported studies on perovskite Mn are more active than La sites toward oxygen reduction structures [10, 38, 39]. 1 3 15 Page 6 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 Analogous adsorption trends are observed for O were classified based on the d (O–O) and υ(O–O) either as adsorbed side-on at the bridged La–Mn site (A3, Fig. 3). The superoxo and peroxo-like species, which were comparable − 2− adsorption energy of − 3.78 eV is released in that mode, with to O (1.33 Å) and O (1.44 Å) ions [40, 41]. Calculated 2 2 d (O–Mn) and d (O–La) at 1.883 and 2.377 Å, respectively. vibrational frequencies for the superoxide and peroxide are The d(O–O) is significantly elongated (1.50 Å). Compared reliable to O –CeO experimental values [42]. 2 2 2− to the LBM25, O adsorbed side-on at the bridge Sr–Mn For the dissociated oxide ions O on the defective sur- 2− site releases an energy of − 3.31 eV on the LSM25 {110} faces, three possible pathways were exploited with O ions surface, whiles a net charge of − 0.51e is transferred to the adsorbing at the oxygen vacant sites and (i) Top-Mn cation O orbital in the bond formation process. (D1), (ii) Top-La cation (D2), and (iii) Bridged Mn and La When O adsorbate is directly incorporated into the sur- sites (D3). We found these configurations shown as D1, D2 face oxygen vacancy (A4), an energy of 2.96 eV is released. and D3 in Fig. 3 are much more active. The dissociation The d(O–O) in the adsorbate are weakened significantly energies are − 5.99, − 4.80, and − 7.83 eV, respectively, for (1.51 Å). Löwdin population analysis shows a significant D1, D2 and D3 modes. These energies are exothermic than charge transfer of − 0.53 e to the adsorbate upon incor- those calculated for their molecular adsorbed counterparts poration at the surface oxygen vacant site in the LBM25 (A1−A4 in Fig. 3). Dissociated configuration D3 has the substrate. Löwdin population analysis shows a significant highest exothermicity. On the SrMnO-terminated {110} sur- charge transfer of − 0.64 e to the adsorbate upon incorpora- faces, the dissociative configurations (D1, D2 and D3 in Fig- tion at the surface oxygen vacant site in the LSM25 substrate ure S1) have been found to be more stable and have dissocia- (see Table S1) compared with a transfer of − 0.53 e to the tion energies of − 5.98, − 3.18 and − 7.03 eV. The trend in adsorbate on LBM25 (Table 2) and 0.76 e on LCM25 [10] dissociation energies for the most exothermic dissociated surface. The trend in charge transfer from the surfaces to the precursors follow the order LBM25 < LSM25 < LCM25 adsorbate follows in the order LCM25 < LSM25 < LBM25 [10]. Hence, defective surfaces of LCM25, LSM25 and while the reduction energetics at the surface vacant sites LBM25 favors dissociative over associative adsorption. follows the order LBM25 < LCM25 < LSM25. Oxygen dissociation on LBM25 is the most plausible in In all the molecular O precursors (A1–A4), the elon- terms of exothermicity. gated O–O bonds were confirmed to have lower stretch- Similar to adsorption reactions involving the (1 × 1) sur- −1 ing vibrational frequencies: 641, 878, 514, and 490  cm , faces, analogous adsorption trends on the (2 × 1) supercell respectively, compared to the O gas-phase stretching fre- were investigated (Fig. 4). Reported in brackets in Table 2 −1 quencies (1558  cm ). Similar to previous studies 10 the are the calculated adsorption energies and the optimized adsorbed molecular oxygen species (A1–A3 in Table  2) interatomic bond distances. For instance, adsorption Fig. 4 Side views of the optimized geometries of oxygen molecule on defective (2 × 1)-{110} (top) and {100} (down) LBM25 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 7 of 10 15 energies of − 1.50, − 4.40, and − 3.34 eV on the (2 × 1) sur- surface as shown in Fig. 5. From the projected density of face were calculated for O bound side-on at La cation (A2), states (PDOS) for the undoped defective and LCM25 {110} bridged-LaMn (A3) and end-on at the oxygen vacant sites surface [10], La ions contribute negligible states at the (A4), compared to − 1.31, − 3.78 and − 2.96 eV, respectively, Fermi level compared to the Mn ions. Since the density of reported on the (1 × 1) surface. The dissociation energies states around the Fermi energy level roughly determines the (D1 and D2) calculated were highly exothermic compared availability of electrons for a given reaction [43], it can be to their (1 × 1) counterparts. The more exothermic adsorp- inferred that the catalytic activity of the LaMnO {110} sur- tion and dissociation energies calculated on the (2 × 1) com- face should be primarily linked to the surface Mn-d states. pared to the (1 × 1) cells could be due to the lower O cover- This helps to explain why the Mn sites are more active than age found on the (2 × 1) cell that reduces repulsive lateral the La sites for O adsorption. It was also reported that Ca interactions between periodic images. This provides larger doping resulted in a decrease in the Mn-d states around the 2− surface area for the diffusion of the dissociated O ions to Fermi level relative to the undoped surface. As the Mn-d locate more stable sites. states dictates the reactivity of the LaMnO {110} surface, a To provide atomic-level insight into the effect of Ba dop- decrease in their intensity signifies weaker O binding. This ing on the electronic structures of LaMnO surfaces and helps to explain why the Ca-doped surfaces have weaker their implication for catalytic reactivity, we have plotted the O -binding energies than the undoped surfaces. As shown projected density of states (PDOS) for the LBM25 {110} in Fig. 5, Ba doping also causes a decrease in the intensity of the Mn-d states, resulting in weaker O -binding energies (− 2.25 eV) than the undoped defective surface (− 2.32 eV). O adsorption on defective LBM25 {100} surface Similar to adsorption reactions involving the {110} surfaces, analogous adsorption trends on the {100} surfaces were investigated. For O adsorbed on top-Mn cation, no stable end-on configuration is observed because it is changed to a side-on structure after optimization. In that configuration, an adsorption energy of 0.82 eV is released. The formed molecular precursor A1 is accompanied by a slight length- ening of the d (O–O) to 1.28 Å. The d (O–Mn) is shorter at 1.850 Å for A1. For O directly incorporated into the surface oxygen vacancy (A2, Fig.  6), the adsorption energy of 1.97  eV is released. The incorporation process is accompanied by Fig. 5 Projected density of states of (PDOS) of 25% Ba-doped LaMnO (110) a net charge gain of − 0.36e by the adsorbate from the Fig. 6 Side view of the optimized geometries of oxygen molecule on (1 × 1) defective LBM25 {100}. Color code: green = La, blue = Ba, pur- ple = Mn, and red = O 1 3 15 Page 8 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 oxygen-deficient substrate, which weakens the calculated where  is the surface coverage. The expression for the d(O–O) to 1.49 Å in relation to the gas phase. Gibbs energy of adsorption then becomes On the {100} surface, configuration D1 has the high- gas ads ads ΔG(T,p) =ΔE +ΔE − dH ZPE est adsorption energy, shortest interatomic distance d T,0 (6) (Mn–O) = 1.577 Å and the d (O–O) = 2.835 Å, making this  ads gas − T ΔS + S − S − Nk T ln conf vib configuration the most stable. On the (2 × 1) supercell, the direct incorporation into with the adsorption energy defined as the surface oxygen vacancy releases an energy of 2.88 eV ads (surf +O2) surf O2 ΔE = E − E − NE , p = 0.2 and p = 1 atm. (Fig. 4, A1). However, when adsorbed at the top-Mn site, To account for errors in the binding energies of O , 1.36 eV/ either in the end-on or side-on configuration, it is dissoci- O obtained from the fitting of experimental formation ated into oxide ions after geometry optimization. One atomic enthalpy and calculated oxide formation energies [44] is oxygen is adsorbed on top-Mn cation whiles the other is added to the calculated Gibbs free energies for the most incorporated into surface vacant site to give D1. The process stable molecular and dissociated configurations. The zero- releases the energy of 4.12 eV. The calculations indicates point vibrational energy (ZPE) is calculated as the difference that adsorption and dissociation processes of O on both between the ZPE correction of the adsorbate on the surface pure and oxygen-deficient LCM25, LSM25 and LBM25 and in the gas phase according to the following equation: at the {100} surface are less competitive than that at the {110} surface because of a weaker adsorption. This sug- 3n 3n hv hv i i ads gests that the {110} surface is catalytically more active for ΔE = surf − gas (7) ZPE 2 2 i=1 i=1 O reduction. where h is the Planck constant and vi are the vibrational ads Gibbs free energies 1G T, p of the O frequencies. adsorption/dissociation Figure 7 shows the predicted Gibbs free energies against temperature plotted for the most stable molecular and dis- To assess the relevance of the calculated adsorption and dis- sociative structures of O on the {110} and {100} surfaces sociation energies of the most stable molecular and dissoci- ads of LBM25. It is evident from the plot that ΔG (T, p) is ated configurations at the typical operating temperatures of always negative, an indication that the oxygen reduction SOFCs (T = 500–1100 °C) in SOFCs, we have calculated reaction is feasible at the typical operating temperatures. In ads the Gibbs free energies, ΔG (T , p) using the Gibbs free addition, it is worth stating that ΔGads (T, p) values become energy relation more negative with increasing temperature, signifying that ads surf+adsorbate surf oxygen reduction reactions are more possible at higher tem- ΔG = G −G −N (3) gas peratures, partly explaining why higher temperatures are ads where N represents the number of molecules adsorbed in the involved with SOFCs operations. In addition, the ΔG (T, reaction. If the enthalpy or the entropy of the solid is not p) are more negative on the {110} compared to {100} sur- changed considerably by the presence of the adsorbates, faces; hence, oxygen reduction reactions are more favored these terms cancel out. The vibrational entropy of the on the {110} surfaces at higher temperatures. ads adsorbates, S , and the coverage-dependent configurational vib entropy, Δs , only contribute to entropy of the surface/ conf adsorbate. The vibrational entropy of the adsorbates is given Conclusion by We have studied the adsorption and reduction of O on ads − S = Nk ln 1 − e (4) the {110} and {100} surfaces of LSM25 and LBM25 as vib −1 a SOFC cathode material using the Hubbard-corrected DFT approach. The molecular precursors at the Mn cat- where  = and  = h is the total vibrational energy k T ion sites of LCM25 [10], LSM25 and LBM25 are associ- of the adsorbent obtained from normal-mode analysis DFT ated with stronger bonds compared to LaMnO [10], this calculations [45], and the coverage-dependent configura- implies Ca, Sr and Ba as dopants in the cathode influ- tional entropy may be included as ences the O reduction. The trend in the reduction ener- 1 − gies at the Mn cation sites are predicted to be in the order Δs = k ln (5) conf LSM25 < LBM25 < LCM25. In addition, the trend in dis- sociation energies for the most exothermic dissociated pre- cursors follow the order LBM25 < LSM25 < LCM25 [10]. 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 9 of 10 15 Fig. 7 Gibbs free energies ΔG (T, p) of the most stable molecu- m d lar ( O ) and dissociative ( O ) 2 2 structures at {110} and {100} surfaces of defective 25% Ba-doped LaMnO (LBM25). Color code: A = ΔG (eV) O at LBM25 {110}; B = O at LBM25 {110}; C = O at LBM25 {100}; D = O at LBM25 {100} included in the article's Creative Commons licence, unless indicated Thus, defective surfaces of LCM25, LSM25 and LBM25 otherwise in a credit line to the material. If material is not included in favor dissociative over associative adsorption. The disso- the article's Creative Commons licence and your intended use is not ciated configurations on the {110} and {100} surfaces of permitted by statutory regulation or exceeds the permitted use, you will LCM25, LSM25 and LBM25 have higher energies, showing need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . that these adsorbed configurations are thermodynamically ads the most stable. The predicted ΔG (T, p) is negative, sug- gesting that the oxygen reduction reactions on LCM25 [10] and LBM25 are feasible at any of the typical operating tem- References peratures of SOFCs and consistent with the high tempera- ads tures employed in operating conditions SOFCs. The ΔG 1. Minh, N.Q., Takahashi, T.: Science and technology of ceramic (T, p) are more negative on the {110} compared to the {100} fuel cells. Elsevier, Amsterdam (1995) surfaces; hence, oxygen reduction reactions are more favored 2. Singhal, S.C.: Solid oxide fuel cells for stationary, mobile, and on the {110} surfaces at higher temperatures. military applications. Solid State Ionics 152, 405–410 (2002) 3. Singhal, S.C.: Advances in solid oxide fuel cell technology. Solid State Ionics 135, 305 (2000) 4. Yang, L., Wang, S., Blinn, K., Liu, M., Liu, Z., Cheng, Z.: Author contributions All the authors contributed equally to the prepa- Enhanced sulfur and coking tolerance of a mixed ion conduc- ration of the manuscript. AA performed the theoretical calculations. tor for SOFCs: BaZr Ce Y Yb O . Science 326, 126–129 0.1 0.7 0.2-x x 3-δ The initial draft of the manuscript was written by AA with input and (2009) suggestions from all the co-authors. All the authors commented on 5. Yang, L., Choi, Y., Qin, W., Chen, H., Blinn, K., Liu, M., Liu, P., previous versions and approved the final manuscript. 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Oxygen (O2) reduction reaction on Ba-doped LaMnO3 cathodes in solid oxide fuel cells: a density functional theory study

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Abstract

The oxygen adsorption and subsequent reduction on the {100} and {110} surfaces of 25% Ba-doped LaMnO (LBM25) have been studied at the density functional theory (DFT) with Hubbard correction and the results compared with adsorption on 25% Ca-doped LaMnO (LCM25) and Sr-doped LaMnO (LSM25). The trend in the reduction energies at the Mn cation sites are 3 3 predicted to be in the order LSM25 < LBM25 < LCM25. In addition, the trend in dissociation energies for the most exothermic dissociated precursors follow the order LBM25 < LSM25 < LCM25. The adsorption energies (− 2.14 to − 2.41 eV) calculated for the molecular O precursors at the Mn cation sites of LCM25, LSM25 and LBM25 are thermodynamically stable, when compared directly with the adsorption energies (E = − 0.56 to − 1.67 eV) reported for the stable molecular O precursors ads 2 on the Pt, Ni, Pd, Cu and Ir {111} surfaces. The predicted Gibbs energies as a function of temperature (T = 500–1100 °C) and pressures (p = 0.2 atm) for the adsorption and dissociation on the surfaces were negative, an indication of the feasibil- ity of oxygen reduction reaction on the {100} and {110} surfaces at typical operating temperatures reported in this work. Keywords Gibbs free energies · Superoxide · Peroxide · SOFC · LaMnO Introduction research to more efficient and sustainable conversion and storage technologies [1]. The world’s demand for energy, coupled with the finite sup- Solid Oxide Fuel Cells (SOFCs) are electrochemi- ply of fossil fuels and the environmental and political draw- cal devices for converting chemical energy into electrical backs associated with them, has shifted the focus of energy energy and additional heat. Intermediate conversion of heat to mechanical work required by conventional combustion techniques is largely avoided. SOFCs with electrolyte that * Albert Aniagyei is ionic conducting presents unique opportunities over other aaniagyei@uhas.edu.gh fuel cells, such as cheap constituents, decreased susceptibil- Caroline Kwawu ity to fuel impurities and effectiveness. SOFCs embodies the kwawucaroline@gmail.com cleanest, capable and flexible chemical to electrical energy Ralph Kwakye transfer system [2, 3] providing alternatives to broader con- kwakyer@uhas.edu.gh sumption of hydrogen and carbon-based fuels and regenerat- Boniface Yeboah Antwi ing fuel sources [4, 5]. However, for SOFCs to be explored boni.antwi@yahoo.com on a bigger commercial scale, they must be less expensive Jonathan Osei-Owusu and the electrode fabricated from readily available materi- oseiowusuansahjoe@gmail.com als. One approach to cost reduction is the drastic reduction Department of Basic Sciences, University of Health in the operation temperatures, which suppresses cell deg- and Allied Sciences, Ho, Ghana radation, thermal stress buildup and improvement in cell Department of Chemistry, Kwame Nkrumah University lifetimes [6]. Due to their high catalytic reactivity toward the of Science and Technology, Kumasi, Ghana oxygen reduction reaction, noble metal electrocatalyst [7], Institute of Industrial Research, Council for Scientific such as platinum (Pt), is employed as a cathode. However, and Industrial Research, Accra, Ghana the cost of platinum represents a fundamental problem for University of Environment and Sustainable Development, its application. Somanya, Ghana Vol.:(0123456789) 1 3 15 Page 2 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 Perovskite-type oxides such as LaMnO have attracted Computational details attention for applications built upon their unique electronic and magnetic features. For example, La Sr MnO is the The calculations were carried out within the Kohn–Sham 1-x x 3-δ cathode of choice when the electrolyte material is zirconia DFT formalism [21], using a plane-wave basis set as in SOFCs [7]. This is attributed to the good electrical con- implemented in the Quantum-ESPRESSO [22]. The Per- ductivity, stability, low cost and efficient catalytic activity dew–Burke–Ernzerhof (PBE) generalized gradient approxi- for the oxygen reduction reaction [8]. In SOFCs cathode, mation (GGA) was employed for the exchange and corre- at dopant concentration (x = 0.2 − 0.3), La Sr MnO lation terms [23]. The plane-wave basis set cutoffs for the 1−x x 3−δ is reported to be employed as mixed ionic–electronic con- smooth part of the wave function and the augmented density ducting (MIEC) [9]. were set to 40 and 420 Ry, respectively, which ensured con- For O adsorption and subsequent reduction on pure vergence of the forces to within 0.01 eV/Å. The Brillouin and 25% Ca-doped LaMnO (LCM25) {100} and {110} zone for the bulk LaMnO was sampled using a 4 × 4 × 4 surfaces, Aniagyei et al., [10] reported that adsorption or Monkhorst–Pack k-points mesh [24]. To correct the large reduction processes are more favorable at the Mn sites than self-interaction error inherent in standard DFT-GGA meth- La and Ca sites at the DFT level with Hubbard correction. ods for mid-to-late first-row transition metal oxides [25], The adsorption energies calculated for the {110} surfaces the DFT + U approach [26] with a U value of 4.0 eV for eff 3+ were more favorable and stable than the {100}. Mn ions (mainly Mn ions) provides the best results when Chen et  al., [11] studied the kinetic behavior of the modeling the LaMnO ground-state properties [27]. We have oxygen reduction reaction and diffusion pathways on 25% investigated the symmetric Pm3m cubic structure, because it Sr-doped LaMnO (LSM25) cathode surface. From the is stable under SOFC operating conditions (above 500 °C in spin-polarized DFT and molecular dynamics (MD) cal- ambient air) [28, 29]. All calculations were spin-polarized to culations, O adsorption energies were more stable at the describe accurately the magnetic properties of the LBM25, Mn sites compared to the Sr sites. LSM25 and the triplet ground state of oxygen. 3+ La Ba MnO is a colossal magnetoresistance (CMR) The different possible magnetic orderings at the Mn 1−x x 3 classical compound [12] with a Curie temperature of sites in the LBM25 and LSM25 surface structures were con- 340  K [13]. Dependent on the dopant concentration, sidered and found that the ferromagnetic (FM) ordering is the crystal structure moves from orthorhombic through 0.3 and 0.2 eV, respectively, more stable than the antiferro- rhombohedral (x > 0.13) to cubic (x > 0.35). For the magnetic (AFM) ordering. Hence, all the structures investi- magnetic spin alignment, it shows ferromagnetic behav- gated in this study have FM spin ordering. ior at Ba concentrations of x ˃ 0.15. They are reported The {100} and {110} surface structures were cre- to undergo metal-to-insulator transition at x ≈ 0.20 [13, ated from the fully optimized bulk structure using the 14]. La Ba MnO has received extensive investigation METADISE code [30], which generates different atomic 1-x x 3 of its crystallographic and magnetic properties [14, 15], layer stackings to result in a zero-dipole moment perpen- phase transitions [13, 16, 17], and spin dynamics [18]. dicular to the surface plane, as is required for reliable and For the reduction activity on Ba-doped LaMnO as cath- realistic surface calculations [31]. The fully relaxed bulk ode materials toward ORR, to the best of our information, structures were used to create the surfaces to eliminate the no theoretical studies have been shown, which makes this presence of fictitious forces during the surface relaxation. theoretical study timely. The surfaces were modeled using a slab model comprising In this work, an extension to previous studies [10], Hub- of eight atomic layers with a vacuum size of 12 Å introduced bard-corrected DFT approach is applied to study the Gibbs in the z-direction, which is large enough to avoid any spuri- free energies of adsorption of the multiple reaction paths ous interaction between the periodic slab images. associated with the reduction of oxygen at 25% Ba-doped Similar to previous studies [10], all the surface calcula- LaMnO {100} and {110} surfaces (LBM25). The oxy- tions for the interactions between the molecular oxygen spe- gen reduction energetics on LBM25 surfaces are compared cies on the LSM25 and LBM25 surfaces were performed by with LCM25 [10] and LSM25 [see Table S1 and Figure S1 relaxing the top three layers while keeping the bottom five and S2], the main cathode of preference for conventional layers fixed at the bulk parameters. The adsorption energy SOFCs. This is to ascertain whether they could be used as was calculated according to the following relation: an alternative electrocatalyst for oxygen reduction reaction E = E − E + E (1) (ORR) based on their adsorption/reduction energies. The ads surface+O surface O 2 2 adsorption studies were restricted to the {100} and {110} where E is the total energy of the substrate–adsorb- surface+O surfaces of LaMnO because they present the most stable 2 ate system in the equilibrium state, and E and E surface O surfaces [19, 20]. 2 are the total energies of the substrate (clean surface) and 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 3 of 10 15 adsorbate (free O molecule in the spin triplet state), Table 1 Calculated oxygen vacancy formation energies (in eV) of LaMnO- {110} and MnO -terminated {100} LBM25 surfaces with respectively. Hubbard U correction (U = 4 eV) eff Surface models Oxygen-vacancy forma- Oxygen- tion vacancy {110} formation Results and discussion {100} A 4.37 2.63 Description of Ba‑doped LaMnO surfaces B 3.72 2.97 C 4.10 3.35 For reduction reaction on SOFC cathodes, the role played D 4.11 3.48 by oxygen vacancies present on the surface is critical. The LaMnO 4.23 3.60 vacant sites are expected to compete against adsorption and 3 dissociation in the mobility of oxygen. To examine the role of the vacant site on the oxygen reduction reactions at the cathode, LBM25 surfaces with different layer models are La Ba MnO (surface) → La Ba MnO + O (g) 0.75 0.25 3 0.75 0.25 3− 2 shown in Fig. 1. Summarized in Table 1 are the calculated (2) oxygen vacancy formation energies obtained by substitu- 3+ 2+ tion of the host La cation for Ba in the {100} and {110} In previous studies of LaMnO with MnO -terminated 3 2 LaMnO surface models with different layers. The calculated {100} surface at the GGA + U [32] and hybrid B3LYP oxygen vacancy formation energies located on the top layer [33] levels of theory, a calculated oxygen vacancy forma- were based on the reaction of tion energy of 2.2 eV was reported. Surfaces model A of Fig. 1 (Top) A is BaMnO-terminated and B–D are LaMnO-terminated {110} surface models. (Down) MnO -terminated (100) LBM25 models. Color code: green = La, blue = Ba, purple = Mn, and red = O 1 3 15 Page 4 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 the MnO terminated {100} and model B of the LaMnO-For O adsorbed on Mn cation site, the end-on structure 2 2 terminated {110} with the La ion in/near the topmost lay- is changed to a side-on structure after optimization (Fig. 3, ers (Fig. 1) were found to have the lowest oxygen vacancy A1), with an exothermicity of − 2.25 eV. The formed molec- formation energies; thus, we chose these two models for ular precursor A1 is accompanied by significant charge subsequent studies of O –LBM25 interactions. transfer (− 0.34 e from the substrate) to the O orbital, 2 2 In this work, molecular or dissociative chemisorption weakening and elongating the d(O–O) to 1.45 Å compared of O on (1 × 1) and (2 × 1) surface models at various cov- to gas phase and experimental d(O–O) of 1.23 and 1.21 Å erages of Θ = 0.25 ML and 0.50 ML, where a monolayer [34], respectively. For A1, the shortest interatomic distance (ML) refers to an oxygen molecule per active surface cation. d (O–Mn) is 1.838 Å. On the LSM25 surface, O adsorbed Shown in Fig. 2 are the top and side views of the LBM25 end-on and side-on at the Mn sites, and exothermicities of {100} and {110} surface models showing the different 1.59 and 2.41 eV were calculated and the d(O–O) = 1.357 adsorption sites explored for O adsorption. and 1.449 Å, respectively (Table S1 and Figure S1). In both molecular processes, the d(O–O) are elongated by 1.315 and O adsorption on the oxygen‑deficient LBM25 1.480 Å, respectively. The adsorption energies of − 2.14, {110} surface − 2.41 and − 2.25 eV calculated at the Mn cation sites of LCM25, LSM25 and LBM25 are thermodynamically sta- For O adsorption on the {110} surface with coverages of ble, when compared directly with the adsorption energies Θ = 0.25 ML and 0.50 ML, molecular and dissociative sce- (E = − 0.56 to − 1.67 eV) reported for molecular O pre- ads 2 narios for the selected (1 × 1) and (2 × 1) surface models cursors on the Pt, Ni, Pd, Cu and Ir {111} surfaces [35, 3+ 2+ were considered. Substitution of host La with Ba cation 36]. This indicates that LCM25, LSM25 and LBM25 cath- creates a defective surface with a vacant oxygen site. Similar ode materials may be more efficient for O activation than to the undoped surface [10], different adsorption configu- the transition metal surfaces. In addition, since molecular ration and modes were exploited at the {110} and {100} precursors at the Mn cation sites of LCM25, LSM25 and surfaces, that may serve as precursors for O dissociation. LBM25 are associated with stronger bonds compared to Figure 3 shows the optimized adsorption structures, whiles LaMnO [10], this implies Ca, Sr and Ba as dopants in the the adsorption energies, Löwdin atomic charges, interatomic cathode influences the O reduction. The trend in the reduc- bond distances and vibrational stretching frequencies are tion energies on the Mn cation sites are predicted to be in presented in Table 2. the order LSM25 < LBM25 < LCM25. Fig. 2 Schematic representation of the side and top views of the (2 × 1) slab model of (a, b) LBM25 {110} and (c, d) LBM25 {100} supercells showing the corresponding different adsorption sites explored. Color code: green = La, blue = Ba, purple = Mn, and red = O 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 5 of 10 15 Fig. 3 Side view of the optimized geometries of oxygen molecule on (1 × 1) defective LBM25 {110}. Color code: green = La, blue = Ba, pur- ple = Mn, and red = O Table 2 Calculated adsorption − a b Surface Config E (eV)|q| (e ) d(O–Mn) (Å) d(O–La) (Å) d(O − O) (Å) υ(O − O) ads energies (E ), Charge (q), −1 ads (cm ) relevant bond distances (d) of molecular (O ) oxygen on {110} A1 − 2.25 − 0.34 1.838 – 1.446 641 defective {110}-(1 × 1) surface A2 − 1.31 (− 1.50) − 0.29 (2.300) 1.374 (1.407) 878 of LBM25; O−O stretching A3 − 3.78 (− 4.40) − 0.46 1.883 (1.922) 2.377 (2.372) 1.495 (1.496) 514 vibrational frequency (υ) of the adsorbed O ; and calculated A4 − 2.96 (− 3.34) − 0.53 – – 1.507 (1.500) 490 gas-phase d(– O) = 1.229 Å and D1 − 5.99 (− 6.05) − 0.76 1.638 (1.649) – – – −1 the υ(O–O) = 1558 cm D2 − 4.80 (− 5.49) − 0.89 – 2.241 (1.793) – – D3 − 7.83 (− 7.63) − 0.93 1.767 (1.904) 2.410 (2.132) – – {100} A1 − 0.82 − 0.14 1.850 1.380 861 A2 − 1.97 (− 2.88) − 0.36 – 1.491 (1.494) 497 D1 − 3.51 (− 4.12) − 0.44 1.577 (1.851) – – The (2 × 1) values are given in parenthesis a,b The shortest distance between an adsorbed oxygen species and an Mn or La ion (110) surface For O adsorbed on top-La cation, no stable end-on reaction and consistent with experimental observations configuration is obtained because it is changed to a side- [37]. In this work, for the O molecule adsorbed end-on on structure after optimization (A2, Fig. 3) with an exo- and side-on at the top-Mn and Sr cation sites on SrMnO- thermicity of 1.31  eV. The bound O molecule experi- terminated {110} surface, the B (Mn) cation sites were ences a net charge of − 0.29 e and d(O–O) elongation calculated to be more active than A (Sr) cation sites toward of 1.374 Å. The formation of molecular precursor A1 is oxygen reduction reaction (Table S1). This demonstrates energetically more favorable than A2 in terms of adsorp- that Mn cation are the favored on both oxygen-deficient tion energies and stronger bond formation, i.e., shorter d LCM25 [10], LSM25 and LBM25 surfaces for adsorption, (O–Mn) = 1.838 Å than d (O–La) = 2.324 Å showing that in agreement with other reported studies on perovskite Mn are more active than La sites toward oxygen reduction structures [10, 38, 39]. 1 3 15 Page 6 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 Analogous adsorption trends are observed for O were classified based on the d (O–O) and υ(O–O) either as adsorbed side-on at the bridged La–Mn site (A3, Fig. 3). The superoxo and peroxo-like species, which were comparable − 2− adsorption energy of − 3.78 eV is released in that mode, with to O (1.33 Å) and O (1.44 Å) ions [40, 41]. Calculated 2 2 d (O–Mn) and d (O–La) at 1.883 and 2.377 Å, respectively. vibrational frequencies for the superoxide and peroxide are The d(O–O) is significantly elongated (1.50 Å). Compared reliable to O –CeO experimental values [42]. 2 2 2− to the LBM25, O adsorbed side-on at the bridge Sr–Mn For the dissociated oxide ions O on the defective sur- 2− site releases an energy of − 3.31 eV on the LSM25 {110} faces, three possible pathways were exploited with O ions surface, whiles a net charge of − 0.51e is transferred to the adsorbing at the oxygen vacant sites and (i) Top-Mn cation O orbital in the bond formation process. (D1), (ii) Top-La cation (D2), and (iii) Bridged Mn and La When O adsorbate is directly incorporated into the sur- sites (D3). We found these configurations shown as D1, D2 face oxygen vacancy (A4), an energy of 2.96 eV is released. and D3 in Fig. 3 are much more active. The dissociation The d(O–O) in the adsorbate are weakened significantly energies are − 5.99, − 4.80, and − 7.83 eV, respectively, for (1.51 Å). Löwdin population analysis shows a significant D1, D2 and D3 modes. These energies are exothermic than charge transfer of − 0.53 e to the adsorbate upon incor- those calculated for their molecular adsorbed counterparts poration at the surface oxygen vacant site in the LBM25 (A1−A4 in Fig. 3). Dissociated configuration D3 has the substrate. Löwdin population analysis shows a significant highest exothermicity. On the SrMnO-terminated {110} sur- charge transfer of − 0.64 e to the adsorbate upon incorpora- faces, the dissociative configurations (D1, D2 and D3 in Fig- tion at the surface oxygen vacant site in the LSM25 substrate ure S1) have been found to be more stable and have dissocia- (see Table S1) compared with a transfer of − 0.53 e to the tion energies of − 5.98, − 3.18 and − 7.03 eV. The trend in adsorbate on LBM25 (Table 2) and 0.76 e on LCM25 [10] dissociation energies for the most exothermic dissociated surface. The trend in charge transfer from the surfaces to the precursors follow the order LBM25 < LSM25 < LCM25 adsorbate follows in the order LCM25 < LSM25 < LBM25 [10]. Hence, defective surfaces of LCM25, LSM25 and while the reduction energetics at the surface vacant sites LBM25 favors dissociative over associative adsorption. follows the order LBM25 < LCM25 < LSM25. Oxygen dissociation on LBM25 is the most plausible in In all the molecular O precursors (A1–A4), the elon- terms of exothermicity. gated O–O bonds were confirmed to have lower stretch- Similar to adsorption reactions involving the (1 × 1) sur- −1 ing vibrational frequencies: 641, 878, 514, and 490  cm , faces, analogous adsorption trends on the (2 × 1) supercell respectively, compared to the O gas-phase stretching fre- were investigated (Fig. 4). Reported in brackets in Table 2 −1 quencies (1558  cm ). Similar to previous studies 10 the are the calculated adsorption energies and the optimized adsorbed molecular oxygen species (A1–A3 in Table  2) interatomic bond distances. For instance, adsorption Fig. 4 Side views of the optimized geometries of oxygen molecule on defective (2 × 1)-{110} (top) and {100} (down) LBM25 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 7 of 10 15 energies of − 1.50, − 4.40, and − 3.34 eV on the (2 × 1) sur- surface as shown in Fig. 5. From the projected density of face were calculated for O bound side-on at La cation (A2), states (PDOS) for the undoped defective and LCM25 {110} bridged-LaMn (A3) and end-on at the oxygen vacant sites surface [10], La ions contribute negligible states at the (A4), compared to − 1.31, − 3.78 and − 2.96 eV, respectively, Fermi level compared to the Mn ions. Since the density of reported on the (1 × 1) surface. The dissociation energies states around the Fermi energy level roughly determines the (D1 and D2) calculated were highly exothermic compared availability of electrons for a given reaction [43], it can be to their (1 × 1) counterparts. The more exothermic adsorp- inferred that the catalytic activity of the LaMnO {110} sur- tion and dissociation energies calculated on the (2 × 1) com- face should be primarily linked to the surface Mn-d states. pared to the (1 × 1) cells could be due to the lower O cover- This helps to explain why the Mn sites are more active than age found on the (2 × 1) cell that reduces repulsive lateral the La sites for O adsorption. It was also reported that Ca interactions between periodic images. This provides larger doping resulted in a decrease in the Mn-d states around the 2− surface area for the diffusion of the dissociated O ions to Fermi level relative to the undoped surface. As the Mn-d locate more stable sites. states dictates the reactivity of the LaMnO {110} surface, a To provide atomic-level insight into the effect of Ba dop- decrease in their intensity signifies weaker O binding. This ing on the electronic structures of LaMnO surfaces and helps to explain why the Ca-doped surfaces have weaker their implication for catalytic reactivity, we have plotted the O -binding energies than the undoped surfaces. As shown projected density of states (PDOS) for the LBM25 {110} in Fig. 5, Ba doping also causes a decrease in the intensity of the Mn-d states, resulting in weaker O -binding energies (− 2.25 eV) than the undoped defective surface (− 2.32 eV). O adsorption on defective LBM25 {100} surface Similar to adsorption reactions involving the {110} surfaces, analogous adsorption trends on the {100} surfaces were investigated. For O adsorbed on top-Mn cation, no stable end-on configuration is observed because it is changed to a side-on structure after optimization. In that configuration, an adsorption energy of 0.82 eV is released. The formed molecular precursor A1 is accompanied by a slight length- ening of the d (O–O) to 1.28 Å. The d (O–Mn) is shorter at 1.850 Å for A1. For O directly incorporated into the surface oxygen vacancy (A2, Fig.  6), the adsorption energy of 1.97  eV is released. The incorporation process is accompanied by Fig. 5 Projected density of states of (PDOS) of 25% Ba-doped LaMnO (110) a net charge gain of − 0.36e by the adsorbate from the Fig. 6 Side view of the optimized geometries of oxygen molecule on (1 × 1) defective LBM25 {100}. Color code: green = La, blue = Ba, pur- ple = Mn, and red = O 1 3 15 Page 8 of 10 Materials for Renewable and Sustainable Energy (2021) 10:15 oxygen-deficient substrate, which weakens the calculated where  is the surface coverage. The expression for the d(O–O) to 1.49 Å in relation to the gas phase. Gibbs energy of adsorption then becomes On the {100} surface, configuration D1 has the high- gas ads ads ΔG(T,p) =ΔE +ΔE − dH ZPE est adsorption energy, shortest interatomic distance d T,0 (6) (Mn–O) = 1.577 Å and the d (O–O) = 2.835 Å, making this  ads gas − T ΔS + S − S − Nk T ln conf vib configuration the most stable. On the (2 × 1) supercell, the direct incorporation into with the adsorption energy defined as the surface oxygen vacancy releases an energy of 2.88 eV ads (surf +O2) surf O2 ΔE = E − E − NE , p = 0.2 and p = 1 atm. (Fig. 4, A1). However, when adsorbed at the top-Mn site, To account for errors in the binding energies of O , 1.36 eV/ either in the end-on or side-on configuration, it is dissoci- O obtained from the fitting of experimental formation ated into oxide ions after geometry optimization. One atomic enthalpy and calculated oxide formation energies [44] is oxygen is adsorbed on top-Mn cation whiles the other is added to the calculated Gibbs free energies for the most incorporated into surface vacant site to give D1. The process stable molecular and dissociated configurations. The zero- releases the energy of 4.12 eV. The calculations indicates point vibrational energy (ZPE) is calculated as the difference that adsorption and dissociation processes of O on both between the ZPE correction of the adsorbate on the surface pure and oxygen-deficient LCM25, LSM25 and LBM25 and in the gas phase according to the following equation: at the {100} surface are less competitive than that at the {110} surface because of a weaker adsorption. This sug- 3n 3n hv hv i i ads gests that the {110} surface is catalytically more active for ΔE = surf − gas (7) ZPE 2 2 i=1 i=1 O reduction. where h is the Planck constant and vi are the vibrational ads Gibbs free energies 1G T, p of the O frequencies. adsorption/dissociation Figure 7 shows the predicted Gibbs free energies against temperature plotted for the most stable molecular and dis- To assess the relevance of the calculated adsorption and dis- sociative structures of O on the {110} and {100} surfaces sociation energies of the most stable molecular and dissoci- ads of LBM25. It is evident from the plot that ΔG (T, p) is ated configurations at the typical operating temperatures of always negative, an indication that the oxygen reduction SOFCs (T = 500–1100 °C) in SOFCs, we have calculated reaction is feasible at the typical operating temperatures. In ads the Gibbs free energies, ΔG (T , p) using the Gibbs free addition, it is worth stating that ΔGads (T, p) values become energy relation more negative with increasing temperature, signifying that ads surf+adsorbate surf oxygen reduction reactions are more possible at higher tem- ΔG = G −G −N (3) gas peratures, partly explaining why higher temperatures are ads where N represents the number of molecules adsorbed in the involved with SOFCs operations. In addition, the ΔG (T, reaction. If the enthalpy or the entropy of the solid is not p) are more negative on the {110} compared to {100} sur- changed considerably by the presence of the adsorbates, faces; hence, oxygen reduction reactions are more favored these terms cancel out. The vibrational entropy of the on the {110} surfaces at higher temperatures. ads adsorbates, S , and the coverage-dependent configurational vib entropy, Δs , only contribute to entropy of the surface/ conf adsorbate. The vibrational entropy of the adsorbates is given Conclusion by We have studied the adsorption and reduction of O on ads − S = Nk ln 1 − e (4) the {110} and {100} surfaces of LSM25 and LBM25 as vib −1 a SOFC cathode material using the Hubbard-corrected DFT approach. The molecular precursors at the Mn cat- where  = and  = h is the total vibrational energy k T ion sites of LCM25 [10], LSM25 and LBM25 are associ- of the adsorbent obtained from normal-mode analysis DFT ated with stronger bonds compared to LaMnO [10], this calculations [45], and the coverage-dependent configura- implies Ca, Sr and Ba as dopants in the cathode influ- tional entropy may be included as ences the O reduction. The trend in the reduction ener- 1 − gies at the Mn cation sites are predicted to be in the order Δs = k ln (5) conf LSM25 < LBM25 < LCM25. In addition, the trend in dis- sociation energies for the most exothermic dissociated pre- cursors follow the order LBM25 < LSM25 < LCM25 [10]. 1 3 Materials for Renewable and Sustainable Energy (2021) 10:15 Page 9 of 10 15 Fig. 7 Gibbs free energies ΔG (T, p) of the most stable molecu- m d lar ( O ) and dissociative ( O ) 2 2 structures at {110} and {100} surfaces of defective 25% Ba-doped LaMnO (LBM25). Color code: A = ΔG (eV) O at LBM25 {110}; B = O at LBM25 {110}; C = O at LBM25 {100}; D = O at LBM25 {100} included in the article's Creative Commons licence, unless indicated Thus, defective surfaces of LCM25, LSM25 and LBM25 otherwise in a credit line to the material. If material is not included in favor dissociative over associative adsorption. The disso- the article's Creative Commons licence and your intended use is not ciated configurations on the {110} and {100} surfaces of permitted by statutory regulation or exceeds the permitted use, you will LCM25, LSM25 and LBM25 have higher energies, showing need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . that these adsorbed configurations are thermodynamically ads the most stable. The predicted ΔG (T, p) is negative, sug- gesting that the oxygen reduction reactions on LCM25 [10] and LBM25 are feasible at any of the typical operating tem- References peratures of SOFCs and consistent with the high tempera- ads tures employed in operating conditions SOFCs. The ΔG 1. Minh, N.Q., Takahashi, T.: Science and technology of ceramic (T, p) are more negative on the {110} compared to the {100} fuel cells. 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Journal

Materials for Renewable and Sustainable Energy – Springer Journals

Published: Dec 1, 2021

Keywords: Gibbs free energies; Superoxide; Peroxide; SOFC; LaMnO3

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