Ab initio Simulation Explains the Enhancement. of Catalytic Oxygen Evolution on CaMnO

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1 Article Subscriber access provided by University of Pennsylvania Libraries Ab initio Simulation Explains the Enhancement of Catalytic Oxygen Evolution on CaMnO Tian Qiu, Bingtian Tu, Diomedes Saldana-Greco, and Andrew M. Rappe ACS Catal., Just Accepted Manuscript DOI: 0.0/acscatal.b0 Publication Date (Web): Jan 0 Downloaded from on February, 0 Just Accepted Just Accepted manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides Just Accepted as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. Just Accepted manuscripts appear in full in PDF format accompanied by an HTML abstract. Just Accepted manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI ). Just Accepted is an optional service offered to authors. Therefore, the Just Accepted Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these Just Accepted manuscripts. ACS Catalysis is published by the American Chemical Society. Sixteenth Street N.W., Washington, DC 00 Published by American Chemical Society. Copyright American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

2 Page of ACS Catalysis Ab initio Simulation Explains the Enhancement of Catalytic Oxygen Evolution on CaMnO Tian Qiu, Bingtian Tu, Diomedes Saldana-Greco, and Andrew M. Rappe, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 0-, USA State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China rappe@sas.upenn.edu Abstract Experimental results have shown promising catalytic activity for the oxygen evolution reaction (OER) on the perovskite type material CaMnO. Through density functional theory investigation, we study the OER mechanism on CaMnO, based on a thermodynamic stability approach. Our results reveal that the formation of Mn vacancies caused by the solubility of Mn enhances lattice oxygen activity, which then reduces the energy of the adsorbate *OOH, and therefore loosens the lower overpotential limit predicted from the scaling relationship. This effect suggests that by doping manganite oxides with soluble elements, we could enhance their OER catalytic activities due to the high surface lattice oxygen activity induced by surface metal ion vacancies. Keywords: electrocatalysis, oxygen evolution, calcium manganite, density functional theory, aqueous surface phase diagram

3 ACS Catalysis Page of Introduction The scarcity of non-renewable fossil fuels, along with the increasing demand for energy has attracted significant attention to the development of sources of alternative and renewable energy. One of the most promising alternatives is the field of electrochemical energy conversion, such as fuel cells, metal-air batteries, and electrolysis. It is clear that reducing the energy loss of the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is vital in enhancing overall electrochemical energy conversion efficiency. For the electrolysis of water, IrO and RuO have been reported as promising electrocatalysts due to their high activity for OER. However, since these materials are rare and expensive, current research is focused on searching for inexpensive alternative materials with comparable or even higher catalytic activity. Manganese compounds have shown significant advantages in this competition. Their high abundance and relatively low price ensure their availability for large-scale energy conversion. Studies of the Mn O -cubane show its crucial role in photosynthesis, and recent work shows that nanostructured α-mn O is an excellent bi-functional catalyst for OER and ORR. Perovskites with Mn +/+ as B site cations also exhibit impressive catalysis activity. Du et al. and Kim et al. have found that the required OER potential on CaMnO x decreases from U RHE. V to U RHE. V when x increases from 0 to 0., where x stands for vacancy cocentration, RHE stands for reversible hydrogen electrod, and U RHE stands for electrode potential reference to RHE Noting that the required OER potential on IrO is also. V, it suggests that manganites can provide the highest level of OER catalysis activity. However, to our knowledge, there is no systematic theoretical work investigating the OER process on CaMnO. In order to make further development designing new materials, a thorough understanding of the catalytic mechanism is required. Previous work has shown that there are at least two types of OER mechanisms, 0 namely the adsorbate evolution mechanism (AEM), and the Mars van Krevelen type mechanism (MKM). In a common AEM, oxygen is evolved at one active site on the surface through the intermediates of *OH, *O, and *OOH, and then is released as O. In MKM,

4 Page of ACS Catalysis the lattice oxygen can directly participate in the formation of O, and in some cases two cooperative sites could be involved in the formation of a single O molecule. Since surfaces of perovskite-type materials may undergo reconstructions in aqueous environment, multiple local geometries at the active site could be expected on different materials or under different conditions. Therefore, exploring the OER mechanism on reconstructed perovskite-type materials may disclose new features that provide novel opportunities for enhanced catalysis. The required electrode potential for OER is the minimal voltage under which all four steps of intermediate evolution are spontaneous. Knowing that the Gibbs free energy change of the total oxygen evolution reaction H O H + + e + O is independent of the reaction path, the required electrode potential reaches its minimum (U RHE =. V) if four intermediates are evenly spaced on the free energy scale, i.e. all four single-electron transfer reactions have the same reaction free energy. Any deviation from the evenly-spaced case causes at least one single-electron transfer reaction having higher reaction free energy than the average, and therefore requires extra electrode potential referring as overpotential (η) to be spontaneous,,. η is directly related to energy losses during the electrochemical reaction. Recent theoretical studies of AEM have stated that the adsorption energy difference between *OOH and *OH is approximately. ev regardless of metal ion identity at the active site. A direct conclusion from this scaling relationship is that there would be a lower limit for η: η (./.) = 0. (ev). However, since surfaces of perovskite-type materials can undergo reconstructions, it is possible that the local chemical environment at the active site on the surface influences *OH and *OOH in a different way such that this scaling relationship may not apply. Since the thermodynamically stable (oxy)hydroxides are the most relevant structures in OER 0,, we follow the thermodynamic stability based approach to study the OER mechanism on the perovskite-type material CaMnO. The adsorbates could still be identified as *OH, *O, and *OOH on different surface sites to some extent, but the high activity of the lattice oxygens makes them able to host hydrogen atoms as well as interact with protons

5 ACS Catalysis Page of from the adsorbates. We notice that the proton from *OOH has a strong interaction with the lattice oxygen, and even transfers to the lattice oxygen site, while the proton from *OH is less active and could not be further stabilized by transferring to the lattice oxygen site. Since this process lowers the energy of *OOH, and consequently reduces the adsorption en- ergy difference between *OOH and *OH, it may guide the design of catalysts with lower overpotential for OER. Methods and Results Prior to the investigation of the OER mechanism, the bulk stability region of CaMnO in aqueous environment must be evaluated in order to estimate the concentrations of aqueous ions involved in further simulations. The scheme for computing the bulk stability region is well established, based on finding the ph-u region such that the following reaction is spontaneous: A n A H x AO z y CaMnO +n H OH O+n H +H + +n e e () Where the sum accounts for all elements A in the bulk, n A, n H O, n H +, and n e stands for number of A element, H O, H +, and e involved in the reaction, respectively. H x AO z y is the most thermodynamically stable aqueous form of A under given ph and U. Its exact form is determined by selecting the species with the lowest formation energy, according to the following equation: A+n H OH O H x AO z y +n H +H + +n e e () The result is usually plotted as a Pourbaix diagram, (Figure ). Noticing that under experimental condition, U RHE =. ev and ph =, the concentration of aqueous species is approximately 0 M. This value is used in the rest of the calculations.

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7 ACS Catalysis Page of reference surface (R ref ). Under this definition, the surface with the most negative relative formation energy is the most stable one under given ph and electrode potential U. A Hess s law related approach proposed by Rong et al. is selected to calculate G, as shown below. R ref A n A A+n H OH O A A G = G + G n A A+R j G n A H x AO z y +n H +H + +n e e G G can be calculated from density functional theory (DFT), and G can be evaluated from experimental thermodynamic data of free energy per A at standard state relative to standard hydrogen electrode (SHE), G A,SHE G = ( ( G = A A = A H j + A E j + A n A (µ H xao z y kt lna HxAO z y n A H A H ref ) T n A E A E ref ) G H xao z y,she + A ( T A S j + A n A S A µ A)+n H +µ H + +n eµ e n H Oµ H O+.n H +ktph+n e eu SHE kt lna HxAO z y n A S A S ref ).n H +ktph+n e eu SHE where the entropy differences between surfaces are neglected. Next, to study the hydration and (de)hydrogenation processes on surfaces, the relative thermodynamic stability under given ph and U can be established through the reaction free energy G k of the following reaction for each hydration and (de)hydrogenation state k ()

8 Page of ACS Catalysis R ref +n H OH O R k +n H +(H + +e ) G k () where the most stable surface predicted from the metal-ion exchange step can be chosen as R ref. The number of electrons (or protons) that are released in this reaction can be treated as a label n k of the state R k on an oxidization ladder. Knowing that it is always possible to write OER as a sequence of four single-electron transfer reactions: R0 R e (+H O) H + (+H O) H + R e e (+H O) H + (+H O) H + R e R0 +O, in which two H O molecules (or OH ) are included at certain steps, the label n k of each state R k is increased by one successively from R 0 to R, due to the removal of an electron at each step. With this relationship, the OER path can be naturally determined instead of being arbitrarily selected. To determine the OER path, notice that all four reactions must be made spontaneous by applying the required electrode potential, i.e. G 0 G G G G 0 e(u φ O /H O ) under ph and U, it can be immediately concluded that the state Rk with the lowest Gk under ph and U 0 takes place of R in the OER cycle. Once R is determined, the labels n k of the other three intermediates are automatically determined, and therefore the identity of each of them can be determined by selecting the most thermodynamically stable surface with the correct label n k for this OER step. The structure and energetics of CaMnO with different surface reconstructions are com- puted with first-principles DFT using the Perdew-Burke-Ernzerhof functional revised for solids (PBEsol) as implemented in the QUANTUM ESPRESSO package. The calcula- tions account for spin-polarized electronic densities. All atoms are represented by norm- conserving, optimized, designed nonlocal 0 pseudopotentials generated with the OPIUM package, treating the s and p of O, s, p, d, and s of Ca, and s, p, d, s, and p of Mn as semi-core and valence states. In addition to the inclusion of semi-core states, the Mn pseudopotential also includes nonlinear core-valence interaction via the partial core correction scheme. A rotationally invariant DFT+U+J scheme is applied for Mn. U eff

9 ACS Catalysis Page of =. V is chosen from Ref [ ] and J 0 =.0 V is calculated from the linear response method., All calculations are performed with a 0 Ry plane-wave energy cutoff. The Brillouin zone is sampled using Monkhorst-Pack k-point meshes for the slab calculations. The R nine-layer supercell is chosen for all surface configurations. Surface-adsorbates and bare surface structures are relaxed with Å vacuum and a dipole correction to cancel the artificial interaction between the system and its periodic images. MnO -terminated (00) surfaces and CaO-terminated (00) surfaces with various O adatoms, O vacancies, Mn vacancies and Ca vacancies are considered. A subset of the involved surfaces is shown in Figure (). The most stable surface reconstructions are reported as a phase diagram in Figure ( a). Surfaces are named with the format termination ± #atom, where +, - represent adatoms and vacancies, respectively, and # represents number of atoms introduced / removed per supercell relative to a reference surface with fixed termination, which can be either Mn O or Ca O. Figure ( a) shows that there are two surface reconstructions whose stability regions extend above the O /H O equilibrium line, namely Mn O +.0O and Mn O -.0Mn -.0O. Hence the hydration and (de)hydrogenation process are evaluated on both surfaces. Applying the scheme mentioned in equation (), results of the most stable hydration and oxidation states are shown in Figure ( b) and Figure ( c). Conditions are named with the format #H O ± #H, where # is the number of H O and H introduced to or removed from the surface per supercell. For the case of Mn O -.0Mn -.0O, since H O - H is in the stability region at or above the O /H O equilibrium line, we first assume this state to be the intermediate R in the OER cycle on surface Mn O -.0Mn -.0O. Since the position on the oxidization ladder is only determined by ± #H, R0, R, and R should take the form of #H O + H, #H O + H, and #H O + 0H, respectively, and their exact identities are listed in Table (). Intermediates R k and R k are built up from different surface reconstructions listed in the first row. Once the OER cycle is determined, the required electrode potential could be calculated accordingly. For this reaction cycle on

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13 ACS Catalysis Page of Table : Reaction free energy for each step in OER Mn O -.0Mn -.0O Mn O +.0O Reaction Step G RHE /ev Reaction Step G RHE /ev () R 0 R. () R 0 R.0 () R R.0 () R R. () R R 0. () R R 0. () R R 0 +O. () R R 0 +O..0 ev +. ev =. ev (referenced to RHE) is close to the proposed value (. ev) in the scaling relationship. The OER process on Mn O -.0Mn -.0O surface is different from what is described in the AEM as mentioned above, but it is still possible to identify the *OH, *O, and *O in R 0, R, and R, respectively, as they all occupy the same site. The adsorbate at the active site in R is different from *OOH, but we can treat it as *OOH with the proton transferred to the lattice oxygen site, and investigate the influence of this proton transfer phenomenon. Adding up the reaction free energy of step () and step () shows that the free energy difference between R 0 and R is. ev +.0 ev =. ev. This value is notably smaller than the expected free energy difference between *OOH and *OH (. ev), suggesting the violation of the scaling relationship. In order to confirm whether the violation is caused by the proton transfer, we move the hydrogen back to the adsorbed oxygen site (Figure a) and let the structure relax (Figure b). The energy is increased by 0. ev, but there is still strong interaction between the lattice oxygen and the proton. In order to eliminate this interaction, we first let the lattice oxygen bond with another hydrogen atom pointing away from *OOH, remove one hydrogen atom from one lattice oxygen, and only allow the three atoms in *OOH to relax, shown in Figure ( c) and Figure ( d). Then, we replace the *OOH in Figure ( b) with the relaxed one in Figure ( d) to achieve the final geometry, see in Figure ( e), and perform the scf calculation for this geometry. From Figure ( b) to Figure ( e), the energy is further increased by 0.0 ev, meaning that the lattice oxygen hydrogen interaction reduces the energy by approximately 0. ev in total. This suggests that the proton transfer process indeed makes a significant contribution to the

14 Page of ACS Catalysis weakening of the scaling relationship. The lattice oxygen hydrogen transfer process is driven by the strong activity of the lattice oxygen, which itself could be attributed to the formation of Mn vacancies. Surface atoms are less coordinated, compared with atoms in the bulk, and the dissolution of Mn ions further reduces the coordination number of the neighboring lattice oxygen atoms. In order to compensate for the missing lattice Mn, lattice oxygen interacts with the hydrogen atoms, as can be seen by noting the contrast (Figure ) that half of the surface lattice oxygen are hydrogenated in R 0, while all surface lattice oxygen atoms are hydrogenated in R 0, the surface with Mn vacancies. It is also worthwhile to consider whether hydrogen transfer occurs between *OH and lattice oxygen or not. An examination of Figure ( a) and Table shows that reaction () has 0. ev higher reaction free energy than reaction (). This difference would be even larger if the hydrogen were bonded to *OO to make it *OOH in R since it would make R less stable. Although the over 0. ev reaction free energy difference could not be fully attributed to differences in O H bond strength between *OH and *OOH, it still exhibits that the hydrogen from *OH is much less likely to transfer to the lattice oxygen, compared with the hydrogen in *OOH. The enhanced activity of lattice oxygen also induces the formation of the MnO triatomic ring as the intermediate of O formation, rather than *OOH standing on the surface, which could be another factor leading to reduced energy difference between *OH and *OOH. If we trace back further for the reason of Mn vacancy formation, the high solubility of MnO ion plays the role. Following this analysis, we suggest that doping the B site element with a more soluble element could be a possible way of reducing the overpotential for the OER process on perovskite type materials. It is constructive to compare our conclusion with the work in Ref [ ], where A site cation deficiency in perovskite type materal LaFeO enhances the OER and ORR performance. They attribute the improvement to the formation of lattice oxygen vacancies on the surface induced by A site vacancies. Our claim, however, is based on the enhanced activity of lattice oxygen atoms surrounding the metal ion vacancies that

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16 Page of ACS Catalysis the OER overpotential and increase the OER catalytic activity. Therefore, we suggest that doping perovskite type materials with soluble metal elements to induce metal ion vacancies on the surface in aqueous environment could enhance their catalytic activity for the OER process. Acknowledgement T.Q. was supported by the Department of Energy Office of Basic Energy Sciences, under grant number DE-FG0-0ER0. B.T. was supported by National Natural Science Foundation of China, under grant number. D.S.G. was supported by the National Science Foundation, under grant number DMR-. A.M.R. was supported by the Office of Naval Research, under grant number N The authors acknowledge computational support from the High-Performance Computing Modernization Office of the U.S. Department of Defense, as well as the National Energy Research Scientific Computing center. References () Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. J. Electroanal. Chem. 00, 0,. () Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 00,,, PMID:. () Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. J. Phys. Chem. Lett. 0,, 0, PMID:. () Dismukes, G. C.; Brimblecombe, R.; Felton, G. A. N.; Pryadun, R. S.; Sheats, J. E.;

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