Computational materials science for sustainable energy using nanocatalysts from abundant elements

Resume : La1-xSrxMnO3 (LSM) served as one of the first perovskites employed as SOFC cathode and is still used in composites. We analyzed oxygen adsorption, dissociation and migration on the (001) MnO2-terminated surface [1]. Based on first principles calculations, the oxygen reduction reaction (ORR) rate was estimated for different cases. In pure LaMnO3, the MnO2 (001) termination was found to be the energetically most stable, however with an increase of Sr doping, the (La,Sr)O termination becomes more favorable [2]. In this talk, we compare results of first principles calculations on the elementary steps (oxygen vacancy formation, O2 molecule and O atom adsorption) of the ORR on the two alternative polar (La,Sr)O and MnO2 (001) terminations. Slab calculations were performed using VASP computer code with GGA functionals. The results are compared with hybrid calculations using the CRYSTAL code with LCAO basis set. In contrast to the MnO2 termination, the vacancy formation energy on (La,Sr)O is larger, strongly reducing the surface vacancy concentration. Our approach distinguishes two effects - different surface terminations and slab cation stoichiometry (ratio of (La,Sr) ions to Mn affecting the average Mn oxidation state). The equilibrium oxygen adsorbate concentration was found to be two orders of magnitude larger for (La,Sr)O, but on the other hand, the surface oxygen vacancy concentration is smaller by nearly six orders of magnitude. Therefore, the oxygen reduction rate on (La,Sr)O termination is expected to be significantly lower than on MnO2 [3]. [1] Y.Mastrikov,R.Merkle, E.Heifets, E.A. Kotomin, J. Maier, J.Phys.Chem.C 114, 3017 (2010). [2]. S. Piskunov, E. Heifets, T. Jacob, E.A. Kotomin, E. Spohr, Phys. Rev.B 78, 121406 (2008). [3] Yu. Mastrikov, R.Merkle, E.A.Kotomin, M.M.Kuklja, J.Maier, J. Mater. Chem. A 6, 11929 (2018)

Resume : Photoelectrochemical (PEC) cells can split water directly into hydrogen and oxygen. We study the oxygen evolution reaction (OER) at the Fe2O3-water interface in PEC to identify limiting processes at the interface and to improve performance. Recently, we have developed a general approach to relate electrochemical measurements to the kinetics of multistep reactions at the semiconductor-electrolyte interface.[1] In this approach, the set of microkinetic equations for OER is formulated as a state-space model. By solving the model for steady state, j-V plots and impedance spectra are simulated and are compared to in-house experiments.[1] In this talk, we show how to simulate the time-dependent behavior of the interface using the same framework. Simulated linear sweep voltammetry curves, cyclic voltammetry curves, and chop light measurements will be presented. The sensitivity of these measurements to the reaction rates, voltage-scan-rate, semiconductor parameters, and other interface parameters are investigated. Moreover, the effect of potential drop across the back-contact resistance on these measurements will be discussed. The method is generic and can be easily extended to other materials and interfaces. (1) George, K.; van Berkel, M.; Zhang, X.; Sinha, R.; Bieberle-Hütter, A. J. Phys. Chem. C 2019, 123, 9981–9992.

Resume : Hydrogen constitutes an ideal green fuel but so far it is mainly obtained from fossil resources. Water splitting using TiO2 as a photocatalyst constitutes an alternative although its practical use under visible sunlight is hindered from the too large ?band gap? of the known forms of this oxide. Suitable modifications have been proposed including doping and/or nanostructuring. Nevertheless, finding a practical solution without a deep knowledge of how the properties of TiO2 nanoparticles evolve with size, shape and morphology seems to be an insurmountable task. To make progress in this direction, a systematic study has been carried out regarding the relative stability and properties of differently shaped realistic (TiO2)n nanoparticles containing explicitly up to 1785 atoms or 595 formula units.1 Using all electron, relativistic, density functional theory-based calculations with accurate numerical orbital centered basis sets, we investigated the relative stability of spherical and faceted nanoparticles as a function of size. A crossover between faceted and spherical morphology is found at ~100 TiO2 units where the faceted nanoparticles become more stable than the spherical ones. The present study provides compelling evidence that for stoichiometric nanoparticles of realistic size, a faceted morphology is preferred although spherical shaped ones tend to exhibit smaller optical gap. Therefore, the successful synthesis of spherical nanoparticles, often showing better photocatalytic activity, has to be attributed to the particular route used able to stabilize these energetically metastable structures. The present study suggests that controlling kinetic of particle growth rather than thermodynamic stability is the key towards TiO2 nanoparticles with better photocatalytic activity under sunlight. References: 1. Á. Morales-García, A. Macià, F. Illas, S. T. Bromley, Nanoscale 2019, 11, 9032-9041.

Resume : Due to the excessive CO2 emission to the atmosphere and inevitable depletion of the fossil fuels, an alternative method for environmentally friendly energy generation is needed. One of the viable methods is the conversion of CO2 to energy carrier molecules making use of the solar energy. This method, however, poses significant challenges to overcome significant thermodynamic stability of the CO2 molecule. This work follows analogous approach to a natural photosynthesis process, where water oxidation and CO2 reduction reactions occur separately. Modified TiO2 (anatase) or Fe2O3 (hematite) are used as the photoanode. These materials are characterized by the optimal photon absorption abilities and low overpotential in water splitting reaction. As a cathode, a graphene supported organic ruthenium complexes are used. Several complexes have been tested with respect to the optimal overpotentnial and product selectivity. In 2-electron reduction process, a Pidko-type Ru-CNC complex displays the best selectivity towards formic acid and appears resistant to deactivation due to interactions with the carbonyl. Ru-porphyrins on the other hand are selective towards methane in relatively low overpotential. Graphene support play here an important role of the charge repolarization, leading to the optimal interaction of intermediates with the Ru site.

Resume : In this talk I will present an overview on the activity of the group devoted to the design of 0D and 2D nanocatalysts for the photo and the electrocatalysis of water in water, through the combination of quantum mechanics with molecular mechanics and with molecular dynamics or by means of continuum models.

Resume : Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. Extended moiré patterns originate from the interface symmetry mismatch. In particular, a stripe moiré pattern is observed when a zigzag direction of graphene is oriented along the [110] direction of the substrate, with parallel physi- and chemisorbed regions of regular width alternating along the graphene armchair direction [1]. Besides characterizing and rationalizing by density functional theory (DFT) the structure and the bonding of this moiré superlattice, we addressed the problem of its stability and evolution. To this aim, a Kinetic Monte Carlo code has been developed on the basis of DFT ground states energies and energy barriers of about fifty single and cooperative elementary processes involving carbon atoms at graphene/substrate interface. Local detachments in the chemisorbed regions experimentally observed upon cooling are explained by peculiar diffusion mechanisms of exceeding carbon segregating from the bulk at the interface region [2]. [1] Z. Zou et al., Carbon 2018, 130, 441-447 [2] V. Carnevali, in preparation

Resume : The scaling relation between *OOH and *OH supposedly slows down the oxygen evolution reaction (OER) [1, 2]. Whereas the ideal separation between the adsorption energies of *OOH and *OH is 2.46 eV, it is around 3.20 eV on most catalysts. Hence, it is currently believed that breaking such scaling relation is crucial to enhance OER electrocatalysts. In this talk, I will test this widespread idea using a large collection of data [3]. The results suggest that breaking the OOH-OH scaling relation is a necessary yet insufficient condition to optimize OER electrocatalysis. As an alternative, I will present the “electrochemical-step symmetry index” (ESSI), the minimization of which is connected to low overpotentials [3, 4]. Finally, I will show how to use ESSI in conjunction with experiments to rationalize activity trends and provide guidelines for the optimization of electrocatalysts [5, 6]. [1] M.T.M. Koper, J. Electroanal. Chem., 660 (2011) 254-260. [2] I.C. Man, H.-Y. Su, F. Calle-Vallejo, H.A. Hansen, J.I. Martinez, N.G. Inoglu, J. Kitchin, T.F. Jaramillo, J.K. Norskov, J. Rossmeisl, ChemCatChem, 3 (2011) 1159-1165. [3] N. Govindarajan, J.M. García-Lastra, E.J. Meijer, F. Calle-Vallejo, Current Opinion in Electrochemistry, 8 (2018) 110-117. [4] N. Govindarajan, M.T.M. Koper, E.J. Meijer, F. Calle-Vallejo, ACS Catal., (2019). [5] M. Retuerto, F. Calle-Vallejo, L. Pascual, P. Ferrer, Á. García, J. Torrero, D. Gianolio, J.L.G. Fierro, M.A. Peña, J.A. Alonso, S. Rojas, Journal of Power Sources, 404 (2018) 56-63. [6] M. Retuerto, L. Pascual, F. Calle-Vallejo, P. Ferrer, D. Gianolio, A.G. Pereira, Á. García, J. Torrero, M.T. Fernández-Díaz, P. Bencok, M.A. Peña, J.L.G. Fierro, S. Rojas, Nat. Commun., 10 (2019) 2041.

Resume : Replacing liquid fossil fuels with renewable fuels produce from water or CO2 is a key challenge for the 21st century. This is linked with both the increase in world energy demand and the climate emergency. Two potential technologies than will alleviate these issues are replacing liquid fuels with hydrogen for transport and capturing & converting CO2 produce in industrial process to useful chemicals. If this is driven by, e.g. solar energy then the processes will be fully renewable. In this presentation we present density functional theory simulations of modifications to TiO2 rutile and anatase by nanoclusters of metal oxides and chalcogenides and explore the possibility for these heterostructures to promote hydrogen evolution (water oxidation) and CO2 activation/reduction. We show that modifying TiO2 with metal chalcogenides (sulfide, selenide) or bimetallics based on Cu can promote hydrogen adsorption to make these heterostructures viable for hydrogen evolution. We also discuss initial work on deposition of these nanoclusters on TiO2 supports. For CO2 conversion we identify a number of modifiers to TiO2 that can activate or dissociate CO2 to CO. This includes bismuth oxide, ceria and very low loadings of copper. Key aspects appear to be the presence of active sites as a result of oxide reduction, the presence of reduced cations and the electronic structure of the modifiers. Comparison with experimental results will also be presented.

Resume : In this work we present first-principles periodic DFT+U calculations on Mn and Fe co-doped BaZrO3 (BaZr0.5Fe0.25Mn0.25O3-δ, BZFM), a promising cathode material for proton-conducting solid oxide fuel cells (PC-SOFCs). Our aims is to individuate perovskite-based material with good mixed proton and electron conductor (MPEC) properties and good electrocatalytic capabilities toward oxygen reduction reaction (ORR) so to be applied as single phase electrode in PC-SOFCs. We have studied BZFM (001) surface for ORR mechanism, identifying the potential-determining steps (PDS) of the ORR using the theoretical standard hydrogen electrode (TSHE) proposed by Nørskov. In particular, we focused on the surface oxygen vacancies as ORR active reaction sites, namely Mn-VO-Fe, Fe-VO-Zr and Mn-VO-Zr. For the low-energy Mn-VO-Fe surface oxygen vacancy, we have studied two possible O2 adsorption pattern: one vertical respect to TMs, and one bridge where the O2 molecule is placed along and above the Mn-VO-Fe line; for Fe-VO-Zr and Mn-VO-Zr, we have evaluated the ORR mechanisms only in the case where O2 adsorption occurs in the vertical configuration. From our calculations on the electrocatalytic activity, for all reaction paths, the PDS is the formation of *OOH intermediate, starting from adsorbed O2. Overpotential for the bridge configuration is on the medium-to-high range ( 0.98 V), while for the vertical configuration is as low as 0.20 V. Still, final overpotentials are on the same range as those calculated for well-known good ORR catalysts such as Pt, for this reason we can assess BZMF as a very promising cathode for PC-SOFCs.

Resume : Photoelectrochemical cells containing iron(III) oxide (Fe2O3) have attracted extensive investigations due to their ability to convert solar energy into chemical energy by water splitting. Recently, fabrication of nanoscaled Fe2O3 has been adopted for photoelectrochemical cells to increase solar energy absorption and reduce slow diffusion length of charge carriers. To understand how nanoscaled confinement influences catalytic efficiency, we performed density functional theory + U calculations of water oxidation on a thin slab of Fe2O3(0001). We considered possible hydrogen vacancies that may appear at high pH and voltage and find that promoting hydrogen vacancy formation improves catalytic efficiency. We conclude that nano-Fe2O3 should be grown on a substrate with a similar lattice constant to reduce strain and improve catalytic efficiency. References: 1. J. Phys. Chem. C, 121(11), 6120 (2017). 2. Advanced Materials, 1706577 (2018). 3. Phys. Chem. Chem. Phys. 19, 17278 (2017).

Resume : Starting from the isolation of Graphene, alternative 2D materials have been manufactured and widely studied due to their promising applications in different technological areas, from nanoelectronics to sensing, from electrochemistry to photovoltaics. Among them, Transition Metal Dichalcogenides present such peculiar optical and catalytic properties that have emerged as electro-catalyst of choice for the hydrogen evolution reaction (HER) in photo-electrochemical device for water splitting. Moreover, recent experiments on vertically stacked van der Waals heterostructures made of MoS2 and WS2 provide new and promising results that combine an efficient inter-layer photo-assisted charge transfer and subsequent catalysis of both HER (on the MoS2 side) and water oxidation to molecular oxygen (on the WS2). To further develop these systems, it is crucial to understand the connection between the chemical structure and electronic properties. To this end, ab initio tools are pivotal in unveiling these structure-property-function relationships and provide new rational design principles. Unfortunately, there is not a unique first-principle method that is accurate enough and computationally feasible in order to address both the optical properties and the surface chemistry of 2D heterojunction. We report a benchmark study on the electronic structure of the MoS2:WS2 heterojunction. We compare semi-local and hybrid DFT results and subsequent GW calculations by varying several numerical parameters, taking in account the spin-orbit coupling term. Our aim is to set a definite scale to assess the efficiency of each level of theory taking as reference the best possible accuracy at the least computational burden.

Resume : The importance of solvation effects on electrochemical reactions is by now a widely accepted fact. On the other hand, sampling all solvent degrees of freedom at potentially expensive levels of electronic structure is often not a feasible approach either. For this reason, implicit solvation models, first pioneered over 80 years ago, are currently undergoing a strong renaissance. Such models generally treat the electrostatic response and possibly dissolved electrolytes on the level of a polarised continuum and the ions' Boltzmann distributions, respectively. The downside to this is that the accuracy of such models generally depend very strongly on a number of effective parameters which, in most cases, cannot be derived from first principles, but rather have to be fitted to a suitable training set. In my talk I present two implicit solvation models, differing in computational cost and complexity, which we implemented in the full-potential numeric atomic orbital code FHI-AIMS. Thereby, I will focus on the parametrisation of the models based on the recently presented Solv@TUM database of experimental solvation free energies. I will highlight some of the intricacies involved the application of popular non-electrostatic correction terms, generally applied to include interactions other than pure electrostatics and to compensate for errors in the solvation model itself. I will also present a new descriptor which to a certain degree can account for structuring of the solvent otherwise absent in pure continuum models. Finally, I will discuss first steps towards new non-electrostatic corrections derived from a compressed sensing machine learning approach used to identify cheaply accessible descriptors for the missing interactions.

Resume : We investigate the solvation effect of water on the overpotentials of the oxygen evolution reaction on rutile TiO2 by applying the thermodynamic integration method on atomistic model interfaces with and without the water molecules. We compare the results at the vacuum interface with the commonly used computational hydrogen electrode method, finding overall good agreement. The effect of the solvent is found to be twofold. First, the explicit treatment of the solvent can lead to equilibrium configurations differing from the relaxed structures without solvent. Second, the overpotentials can be affected by up to 0.5 eV. The energetics are subject to electrostatic effects at the interface rather than to modifications in the hydrogen bond network. These results provide a promising perspective on the use of implicit models for treating the solvent.

Resume : PEEK and PEKK are important polymers with varied applications including those in aerospace. With regard to the various applications of these polymers, it would be beneficial to understand their welding behaviour, especially the molecular aspects. Molecular dynamics (MD) simulations of these polymers provide useful insight into their various properties. The welding behaviour of PEEK and PEKK can be investigated through molecular dynamics simulations. These welding simulations would involve large systems that would require significant computational time and resource if all-atom samples are considered. Therefore, in order to simulate these large systems, it is necessary to coarse-grain the polymers. This work describes our coarse-graining scheme for PEEK and PEKK. The coarse-graining scheme involves 3 beads per monomer of PEEK or PEKK. Various properties of the detailed sample and coarse-grained sample have been examined. Static properties such as end-to-end length, radial distribution function, glass transition temperature and compressibility have been compared between the all-atom sample and the coarse-grained sample. Dynamic properties such as mean square displacement (msd) of the chain centres have also been investigated. As expected, the match between dynamic properties of the all-atom and coarse-grained samples is poor and scaling has been performed between them.

Resume : Major sources for human-made CO2 emission comprise the energy and the industrial sector including cement production. One of the most appropriate concepts to capture CO2 from fossil plant exhausts is the oxyfuel combustion. However, most of known highly permeable ceramic membrane materials show unwanted chemical instability against CO2 and other flue gas components. Here the first principles calculations could be helpful in order to understand adsorption of CO2 on atomistic level. We performed detailed theoretical study of the effect of CO2 gas on surface properties of the (La,Sr)FeO3 perovskite membranes based on the hybrid density functional (PBE0) calculations as implemented in CRYSTAL computer code [1]. The role of dispersion interactions (PBE0-D3 [2]) is also addressed, and, in particular, for the surfaces containing oxygen vacancies. We could analyze carefully the correlation between the adsorption enthalpy and stability of different atomic configurations for the CO2 molecule adsorption and membrane basic properties. Thus, the Sr-content influences the Fe average oxidation state which is reflected in the adsorption properties. The behavior of surface vacancies as well as formation of carbonate phases are analyzed through charge redistribution near surface, chemical bond lengths and bond populations. [1] ] R. Dovesi, A. Erba, R. Orlando, et al., WIREs Comp. Mol. Sci. 8 (4), (2018). doi: 10.1002/wcms.1360. [2] S. Grimme, J. Anthony, S. Ehrlich, H. Krieg, J. Chem. Phys. 132, 154104 (2010).

Resume : Modern progress in material engineering allows producing new complex materials with improved ferroelectric and piezoelectric properties. The ability of the ABO3 perovskite structures (e.g., BaTiO3 — BTO) to accept isovalent dopants on both A- and B-sites is the way of enhancement of the electromechanical properties of lead-free materials. For example, our recent calculations show that Ba(1-x)SrxTiO3 (BSTO) and Ba(1-x)CaxTiO3 (BCTO) solid solutions exhibit significantly enhanced piezoelectric properties, in a comparison with pure BTO [1–3]. In this work, we presented the results of ab initio (first-principles) computations and local structural analysis for BTO, SrTiO3 (STO), CaTiO3 (CTO) perovskites and (Ba,Sr/Ca)TiO3 perovskite solid solutions. Calculations were performed with the CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using PBE0, B1WC and B3LYP advanced hybrid functionals of the density-functional-theory (DFT). Different chemical compositions are considered for the ferroelectric (tetragonal) phase of solid solutions by means of supercell calculations. Both Sr and Ca off-centering within the dodecahedra and the Ti atom displacement within the TiO6 octahedra in solid solutions are studied. The relation of these structural distortions with piezoelectric properties of solid solutions is discussed. [1] L.L. Rusevich, G. Zvejnieks, A. Erba, R. Dovesi, E.A. Kotomin, J. Phys. Chem. A 2017, 121, 9409. [2] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, M. Maček Kržmanc, A. Meden, Š. Kunej, I.D. Vlaicu, J. Phys. Chem. C 2019, 123, 2031. [3] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, Solid State Ionics 2019, 337, 76.

Resume : Atomic and electronic structures of undoped and Tb-doped CeO2-δ were calculated from first principles with inclusion of strong correlation effects based on the Hubbard model (PBE U) and hybrid PBE0 exchange-correlation functional. To correctly account for possible oxidation states of Tb and Ce (3 and 4 ), distinct values of Hubbard U-parameter were simultaneously applied to Ce and Tb ions. Multiple configurations were obtained, with electrons localizing on different number of cations in response to a presence of an oxygen vacancy (Vo) near a Tb or Ce ion. Site symmetry approach was combined with the group theory analysis to successfully identify the ground state configuration for localization of excess electrons on different number of cations. Our results indicate that in Tb-doped CeO2 the difference in total energy between the systems in which Tb ion has either 3 or 4 oxidation state is very small and thus we predict that, unlike Gd-doped ceria, these states can co-exist without Vo formation. In Tb-doped CeO2-δ, Gibbs formation energy of Vo is diminished by a factor close to four, compared to undoped CeO2-δ. The lowest formation energy for the small polaron corresponds to a configuration in which two Ce3 (undoped CeO2-δ) are the nearest neighbors to VO, or one Tb3 and one Ce3 ions (Tb-doped CeO2-δ) are, correspondingly, the nearest and the third nearest neighbor to Vo [1, 2]. Such configuration is consistent with optical measurements from literature. [1] R.A. Evarestov, D. Gryaznov, M. Arrigoni, E.A. Kotomin, A. Chesnokov, J. Maier, Phys. Chem. Chem. Phys. 19 (2017) 8340–8348. [2] A. Chesnokov, D. Gryaznov, E. Kotomin, Opt. Mater. 90 (2019) 76–83.

Resume : Engineering the electronic energy band structure of hybrid nanostructured semiconductor materials through judicious control of their atomic composition is a promising route to increase visible light photoresponse, which is necessary prerequisite for efficient photocatalytic hydrogen production from water. In our study we have carried out the first principle calculations to simulate the electronic structure of hybrid GaN/WS2 nanotubes. Ab initio modeling reported here have been performed within the formalism of hybrid Density Functional Theory and Hartree-Fock method when using HSE06 exchange-correlation functional properly adapted and verified relatively to properties of GaN and WS2 bulk and nanosheets. The calculation results show that the studied hybrid GaN/WS2 nanotube with the diameter of about 2 nm is a semiconductor with a band gap of about 2.2 eV. GaN/WS2 nanotubes found to be suitable for photocatalytic water splitting under influence of solar light. The edges of their band gaps correspond to the range of visible spectrum. The top of the valence band and the bottom of the conduction band of considered GaN/WS2 nanotubes have been properly aligned relative to the oxidation and reduction potentials necessary for water splitting under visible light irradiation. Funding from Latvian Council of Science fundamental and applied research project Nr. LZP-2018/2-0083 is greatly acknowledged.

Resume : Nowadays, scientific advancement has a growing impact on the natural environment. It is most significant in the fields of industry, transportation, and the energy production, where air pollution is most noticeable. Hence, more and environment-friendly and alternative energy sources are pursued. One of the most interesting and promising solutions are fuel cells, which directly convert the fuel (hydrogen or hydrocarbons) to electrical energy by the electrochemical reaction without a combustion process. The high-temperature fuel cells, such as Molten Carbonate (MCFC) and Solid Oxide Fuel Cell, are most prospective. Typically in MCFC the electrodes are made of Ni/NiO. Numerous of addition and changes in chemical composition of electrodes were studied and it is found that, the promising candidate is Bi2O3 addition. Due to that, in current studies, we used density functional theory (DFT) calculations to investigate the chemisorption and stability of O2 and CO2 at Bi2O3 surface. Moreover, the partial density of states combined with the effective bond order were used to analyse the bonding nature of adsorbed molecules. We applied the climbing image nudged elastic band method (CI-NEB) to estimate energy barriers in reactions taking place on Bi2O3 surface. The obtained results were compare with analogical reactions occurring on the Ni/NiO surfaces.

Resume : The search for semiconductor co-catalysts requires the calculation of the alignment of valence and conduction bands of the co-catalyst with respect to the redox levels of liquid water. In this context, we analyze two transition metal-based co-catalyst, namely NiO and Ni2P, which have been proven to exhibit high hydrogen evolution rates when used in conjunction with metal-organic frameworks, acting as photo-catalysts. We perform hybrid-functional calculations to compute the band structure of the two semiconductors. Both a molecular and a dissociative model of the water adsorbed at the semiconductor surfaces are considered. Finally, molecular dynamics simulations of the NiO/H2O and Ni2P/H2O interfaces are used to determine the band edge alignment. Our results aim at giving an explanation for the role of NiO and Ni2P as co-catalysts for the photo-catalyst MIL-125-NH2.