Transition Metal Oxide Surfaces Research Articles

Catalytic Activity of Nitrides and Oxides for Electrochemical Ammonia Synthesis: Computational Guides and Experimental Insights

By mimicking the naturally occurring enzymatic process, ammonia can be produced electrochemically from nitrogen and water at ambient conditions where protons are solvated in the electrolyte by water splitting and electrons are driven to the surface of catalyst via electricity supplied by renewable sources of energy. In this manner, the ammonia synthesis becomes a zero-carbon emitting process and more decentralized compared to the Haber-Bosch. Here we present the result of comprehensive density functional theory (DFT) calculations on the surface of transition metal nitrides and transition metal oxides as two different cathode materials for catalyzing nitrogen conversion to ammonia electrochemically in aqueous solutions and at ambient conditions. The aim was to find candidates that were stable and active toward ammonia formation while simultaneously suppressing the competing reaction of hydrogen evolution. Among the nitrides, the most promising candidates are found to be VN, ZrN, NbN, and CrN1-4, which are predicted to catalyze ammonia formation at overpotentials of 0.50 to 1.40 V versus the standard hydrogen electrode (SHE). However, the most promising oxide candidate is found to be the (110) facets of rutile NbO2, which is anticipated to produce ammonia at relatively low overpotentials of 0.57 V versus SHE5. IrO2 was actually predicted to be the most active oxide catalyst for synthesis of ammonia with only an overpotential of 0.36 V, but this catalyst might contribute a portion of the supplied electrical potential to formation of hydrogen gas too, and thus yields lower rate of ammonia formation. Following these computational guidance, experimentalists investigated the catalytic activity of VN nanoparticles and measured 6% faradaic efficiency (FE)6 towards ammonia synthesis but with significant suppression of hydrogen evolution. Nonetheless, engineering the catalyst surface (as elaborated by the DFT results) is a key factor for enhancement of performance and efficiency on these materials that should be the scope of future works. Also Huang et al. 7got inspiration from our theoretical predictions on oxides and thus tested NbO2 experimentally where they reported 32% FE towards ammonia formation at ambient conditions and in aqueous electrolyte. This is currentlya world record in the field. Both these theoretical and experimental investigations have therefore opened up new avenues for the possibility of improved activity and selectivity of cathode material toward electrochemical ammonia synthesis.

Surface Energy and Reconstruction of Li and Na Based Transition Metal Oxides: A Computational Study

Interfaces between cathode particles and electrolyte have significant influence on performance and longevity of Li-ion batteries. Therefore, for further battery improvement deep understanding of cathode-electrolyte interface structure and its relationship with electrochemical processes inside the battery is strongly required. During the last few years, more and more studies are focused on the interfaces and surfaces of cathode particles, especially for layered transition metal oxides (TMO). For example, it was established that TMO surfaces experience reconstruction from layered to spinel and cubic structures, considerably affecting charge transfer through cathode/electrolyte interface and electrolyte and cathode stability [1]. Though initially reconstruction was considered mostly as a detrimental effect, resulting in increased barriers for Li insertion and cathode degradation, new studies show that reconstructed layer can serve as a natural protection from oxygen loss, preventing decomposition of electrolyte and insertion of water in cathode structure [2]. Though it is evident from experimental studies that the reconstructed layer resembles spinel and rock-salt structures, the exact ordering of atoms is controversial. More importantly, the governing factors for reconstruction are still not fully clear: in some cases, reconstruction occurs during synthesis and in others it proceeds under exposure to electrolyte and/or during cycling. Finally, surface reconstruction remains almost unexplored in promising Na-based TMO cathodes, where its role can be highly important, serving as a protective layer from water insertion [2]. In this work, we develop an algorithm for finding reconstructed structures of surface for transition metal oxides. The algorithm is based on the Monte-Carlo method used for a random swap of alkali atoms, transition atoms, and vacancies, while the density functional theory is used for atomic optimization after each Monte-Carlo step and accurate calculation of energies. The Metropolis method allows to take into account the influence of temperature and predict kinetically controlled surface structures. The algorithm is implemented in our SIMAN package for high-throughput materials modelling and available for free use [3]. Employing the developed algorithm, we consider several oxide compounds, such as LiNiO2, NaNiO2 and NaMnO2 used for pragmatic applications for rechargeable Li-ion and Na-ion batteries. We found that in LiNiO2 and NaNiO2 the formation of surface and subsurface anti-site defects is much easier than that in the bulk region. By making simulation at real synthesis temperatures, we observed kinetically controlled formation of spinel layers for both LiNiO2 and NaNiO2. Under oxidative conditions the transformation of the LiNiO2 surface to a rock-salt structure is thermodynamically favorable. The obtained results explain experimentally observed surface reconstruction to spinel and rock-salt structures in layered oxides. The undergoing study will identify the reasons that control the type of reconstruction depending on the chemical composition and synthesis conditions, providing useful guidelines for improving cathode materials.

Anisotropic magnetoresistance in multiband systems: Two-dimensional electron gases and polar metals at oxide interfaces

Low density two-dimensional electron gases (2DEGs) with spin-orbit coupling are highly sensitive to an in-plane magnetic field, which impacts their Fermi surfaces and transport properties. Such 2DEGs, formed at transition metal oxide surfaces or interfaces, can also undergo surface phase transitions leading to polar metals which exhibit electronic nematicity. Motivated by experiments on such systems, we theoretically study magnetotransport in $t_{2g}$ orbital systems, using Hamiltonians which include atomic spin-orbit coupling (SOC) and broken inversion symmetry, for both square symmetry (001) and hexagonal symmetry (111) 2DEGs. Using a numerical solution to the full multiband matrix-Boltzmann equation, together with insights gleaned from the impurity scattering overlap matrix, we explore the anisotropic magnetoresistance (AMR) in the presence of impurities which favor small momentum scattering. We find that transport in the (001) 2DEG is dominated by a single pair of bands, weakly coupled by impurity scattering, one of which has a larger Fermi velocity while the other provides an efficient current-relaxation mechanism. This leads to strong angle-dependent current damping and a large AMR with many angular harmonics. In contrast, AMR in the (111) 2DEG typically features a single $\cos(2\vartheta)$ harmonic, with the angle-averaged magnetoresistance being highly tunable by a symmetry-allowed trigonal distortion. We also explore how the (111) 2DEG Fermi sufaces are impacted by electronic nematicity via a surface phase transition into a 2D polar metal for which we discuss a Landau theory, and show that this leads to distinct symmetry components and higher angular harmonics in the AMR. Our results are in qualitative agreement with experiments from various groups for 2DEGs at the SrTiO$_3$ surface or the LaAlO$_3$-SrTiO$_3$ interface.

A one-pot surface coating process employing a bifunctional hybrid solution for high-Ni cathode materials for lithium ion batteries

A uniform surface coating and control over residual Li on the surface of high-Ni layered transition metal oxides is crucial for development of highly reliable and high-performance cathode materials for lithium ion batteries (LIBs). To suppress undesirable side reactions at the surface of high-Ni layered transition metal oxides, we propose a one-pot TiO2 surface coating process using a bifunctional hybrid solution. We demonstrate that the uniformity and thickness of a TiO2 coating layer can be controlled, and residual Li can be simultaneously reduced during the surface coating process. Structural and electrochemical characterizations support the utilization of an essential bifunctional hybrid solution composed of titanium (IV) isopropoxide (TTIP) in isopropyl alcohol (IPA) and tetrabutyl-ammonium hydroxide (TBAOH) in water for introducing a uniform TiO2 coating layer and eliminating undesirable Li residues on the surface without yielding significant structural degradation. Our approach provides a practical guide to develop advanced cathode materials and simplify the synthetic process for mass production.

Computational Insights to Charge Transfer Kinetics at the Cathode/Electrolyte Interface

The charge transfer reaction is the fundamental reaction for rechargeable batteries. The energy landscape of this reaction depicts the equilibrium and kinetics of the electrochemical process. Many transition metal oxide surfaces in a high oxidation state have been found to be unstable against common Li-ion electrolytes. It undergoes a spontaneous reduction upon immersion in the electrolyte in open circuit. This process involves the reduction of the cathode surface and degradation of the electrolyte to form a cathode electrolyte interphase (CEI), forming a complex cathode/CEI/electrolyte interface. However, the mechanism of this interaction remains unclear. In this work, a modeling framework was developed to predict the energy landscape of the charge transfer reaction at a LiCoO2/LiF/EC-electrolyte interface, with combined density functional theory (DFT) and molecular dynamics (MD) calculations. The energy profile of this reaction was simulated under different voltage. The mechanism of the interaction between LiCoO2 and EC-electrolyte would be discussed.

(Invited) Correlation of Degradation Effects and the Thermal Stability of Different Active Materials in Lithium Ion Batteries

Three aspects are considered key for active materials in future lithium ion battery applications: High energy density, long cycle life, and enhanced safety properties. Therein, it is of fundamental importance to investigate and understand specific interactions between electrochemical performance, degradation effects, and their influence on the safety properties. In particular, to guarantee safe operation throughout the whole life cycle, it is crucial to know if degradation effects can cause changes in the safety properties of a lithium ion battery. Even though different active materials have been investigated with regard to their thermal stability in the charged and discharged state, the influence of degradation effects on the thermal stability of active materials is widely unknown. Therefore, this study correlates degradation effects and their influence on the thermal decomposition of different layered, spinel-type, and olivine-type based positive electrodes as well as carbonaceous, silicon composite, and spinel-type based negative electrodes at elevated temperatures. Specific active materials suffer from different degradation effects during charge/discharge cycling that can affect the thermal stability. Thus, in a first step, it is crucial to identify and understand degradation effects for each specific active material in order to deduce the influence on the electrochemical performance and the safety properties of the cell. For this purpose, different positive and negative electrode active materials were investigated in the charged and discharged state after charge/discharge cycling. Therein, thermogravimetric analysis (TGA) and adiabatic reaction calorimetry (ARC) was used to investigate changes in the thermal stability of the electrodes. Negative electrodes based on carbonaceous materials are known to be highly reactive in the charged (lithiated) state. However, the introduction of silicon (Si/C composite) or the use of the spinel-type Li4Ti5O12 (LTO) as active material can largely influence the thermal stability. In general, a thermally stable solid electrolyte interphase (SEI) can impede the thermally induced de-intercalation of lithium in the charged state, leading to an increased thermal stability, hence improved safety properties compared to the pristine state. Considering the positive electrode active materials, post-mortem analysis included, amongst others, structural investigations after heat treatment at different temperatures by X-ray diffraction analysis (XRD) to gain insights into the progression of the thermal decomposition of the active materials. Overall, the high thermal stability of spinel-type LiMn2O4 (LMO) is strongly affected by the presence of other transition metals like e.g. nickel in the high-voltage active material LiNi0.5Mn1.5O4 (LNMO). However, the negligible degradation of the spinel-type and olivine-type (LiFePO4) active materials during charge/discharge cycling results in consistent safety properties. In contrast, the degradation effects on the surface of layered transition metal oxides like LiNi x Co y Mn z O2 (NCM, x+y+z=1) can strongly reduce the thermal stability. In addition, the thermal stability of NCM is strongly affected by the state of charge and by an increasing nickel content (0.33 ≤ x ≤ 0.8). In summary, by combining comprehensive post-mortem analysis and thermal analysis, the degradation effects of different positive and negative active materials could be correlated with the thermal stability, hence the safety properties of different electrode active materials.

Studies on solid state reactions of atomic layer deposited thin films of lithium carbonate with hafnia and zirconia

In this paper, results on the solid state reactions of atomic layer deposited Li2CO3 with HfO2 and ZrO2 are reported. An Li2CO3 film was deposited on top of hafnia and zirconia, and the stacks were annealed at various temperatures in air to remove the carbonate and facilitate lithium diffusion into the oxides. It was found that Li+ ions are mobile in hafnia and zirconia at high temperatures, diffusing to the film–substrate interface and forming silicates with the Si substrate during heating. Based on grazing incidence x-ray diffraction experiments, no changes in the oxide phases take place during this process. Field emission scanning electron microscopy images reveal that some surface defects are formed on the transition metal oxide surfaces during lithium diffusion. The authors also show that lithium can diffuse through hafnia and react with a potential lithium-ion battery electrode material TiO2 residing below the HfO2 layer, forming Li2TiO3.

(Invited) Tracing Reactivity through Outgassing in Ni-Rich and Li-Rich Li-Ion Cathode Materials

Outgassing of active materials in Li-ion batteries provides a route to quantitatively study degradation processes that occur during cycling. In particular, we are primarily interested in quantifying the individual and coupled decomposition/transformations of the cathode – a lithiated transition metal oxide (TMO) – and the electrolyte – most commonly carbonate blends (ethylene carbonate, diethyl carbonate, etc.) with lithium hexafluorophosphate (LiPF6) as the salt. Previous observations of high-voltage instabilities include TMO surface reconstruction, transition metal dissolution, electrolyte decomposition, and formation of surface species. However, this picture is still incomplete, with the dependence on electrolyte and TMO composition not yet fully understood. We will present results in which isotopic labeling of 18O in Ni-rich and Li/Mn-rich NMCs is combined with quantitative gas evolution analysis to show that residual solid lithium carbonate (Li2CO3) on the surface of TMOs has a direct impact on electrolyte and electrode degradation. In particular, oxygen release from the TMO lattice is related to the amount of Li2CO3 present in the cathodes. Our results suggest that the role of impurities on interfacial reactivity in batteries is critically important and should be a key parameter considered in similar future studies.

Interaction of a bioactive molecule with surfaces of nanoscale transition metal oxides: experimental and theoretical studies

Schematic diagram of metal oxide–BTT interaction and the associated changes in experimental UV-Vis spectra. BTT adsorbed α-Fe2O3 is represented by red spectra, while green spectra represent BTT adsorbed NiO. Black spectra represent pure BTT spectra.

Enhanced photoelectrochemical performance and stability of Si nanowire photocathode with deposition of hematite and carbon

Tremendous efforts have been dedicated to the development of transition metal and/or metal oxide surface coating materials to enhance the activity and stability of photoelectrodes. Nanostructured Si photoelectrodes have shown outstanding photoelectrochemical (PEC) performance due to their effective photon absorption and charge generation, separation, and mobility. While the chemical stability and surface reaction efficiency of Si photoelectrodes still need improvement before commercial application. Herein, we report the design and synthesis of a composite Si photoelectrode with a configuration of C/α-Fe2O3/Si nanowires, which presented a stable photoelectrochemical hydrogen production in neutral electrolyte. The p-Si nanowires were prepared by metal-assisted chemical etching for enhanced optical absorption and decorated with a mesoporous α-Fe2O3 thin film (∼80 nm) through pyrolysis of ferrocene. A thin carbon passivation layer (∼20 nm) was further deposited through ion sputtering further increasing the stability of the composite structure and low bias photocurrent. The role of α-Fe2O3 and carbon layer have been discussed. The composite photoelectrode shows a stable photocurrent of ∼−27 mA/cm2 in 2 h and an anodic onset potential shift of ∼0.33 V relative to the bare Si in the neutral solution.

Kinetics-Based Computational Catalyst Design Strategy for the Oxygen Evolution Reaction on Transition-Metal Oxide Surfaces

Density functional theory was used to examine the oxygen evolution reaction on a large number of active sites formed by doping three different surfaces of Co3O4 with various 3d transition-metal atoms. By combining the scaling and Bronsted–Evans–Polanyi (BEP) relationships that are determined for these sites, it is shown that the activity of a site is controlled by the redox potential for oxidation of the site. On the basis of this, a kinetics-based design strategy is presented for identifying the optimal active site at a given electrode potential. This design strategy is shown to be valid regardless of whether the rate-limiting water addition step occurs electrochemically or nonelectrochemically, as long as certain conditions are met. Another finding is that the BEP relations are sensitive to the structure of the active site, with sites reacting through μ3-oxo species having the most favorable relations. Finally, the kinetics-based design strategy is compared with the commonly used thermodynamics-based de...

Adsorption of a superoxo O[formula omitted] species on the pure and Ca-doped Sr3Ru2O7(001) surface

Only recently, the activation of oxygen molecules on clean defect-free transition metal oxide surfaces has been reported, for example on the CaO-terminated surface of the Ruddelsden-Popper perovskite Ca3Ru2O7(001). In this work we show that oxygen molecules adsorb as an activated superoxo species on a clean SrO-terminated surface of Sr3Ru2O7(001). At all coverages, the electrons activating the molecule originate from the subsurface RuO2 layer. At low coverages, the presence of a Ca dopant in the terminating SrO layer slightly increases the adsorption energy. At high coverage, DFT predicts a flat potential energy surface and a preferred adsorption of the O2- near surface cations. Advanced many-electron calculations (RPA) predict adsorption energies of −0.99 eV and −0.49 eV per O2- molecule for low and high coverages, respectively, and a preference for forming line-like structures in the latter case.

Acidity Constants of the Hematite-Liquid Water Interface from Ab Initio Molecular Dynamics.

The interface between transition metal oxides (TMO) and liquid water plays a crucial role in environmental chemistry, catalysis, and energy science. Yet, the mechanism and energetics of chemical transformations at solvated TMO surfaces is often unclear, largely because of the difficulty to characterize the active surface species experimentally. The hematite (α-Fe2O3)-liquid water interface is a case in point. Here we demonstrate that ab initio molecular dynamics is a viable tool for determining the protonation states of complex interfaces. The p Ka values of the oxygen-terminated (001) surface group of hematite, ≡OH, and half-layer terminated (012) surface groups, ≡2OH and ≡1OH2, are predicted to be (18.5 ± 0.3), (18.9 ± 0.6), and (10.3 ± 0.5) p Ka units, respectively. These are in good agreement with recent bond-valence theory based estimates, and suggest that the deprotonation of these surfaces require significantly more free energy input than previously thought.

DFT+U Study on Catalysis by Co3O4: Influence of U Value and a Surface–Bulk Bi-U Strategy

The DFT+U method provides an effective approach to correctly reproduce the material bulk properties of strongly correlated transition metal oxides (TMO) such as magnetic ground states, electronic structure, and redox reaction energies. Therefore, the catalysis processes over these strongly correlated TMO surfaces are frequently simulated utilizing the DFT+U method with the U value fit by these reported bulk properties. However, it is generally acknowledged that the calculated results utilizing the DFT+U method depend on the applied effective U value. Due to the significantly different coordination environments, the fit U value based on the bulk properties might not be proper for the simulation of the catalysis process. Moreover, the possibly caused calculation errors still remain elusive. To clarify it, we systematically investigate the influence of the Hubbard U value on the molecule adsorption at different sites, CO oxidation, and first C–H activation of methane over Co3O4(110). It is found that with th...

Outgassing of Ni- and Li-Rich Li Ion Battery Cathode Materials: The Importance of Impurities

Outgassing of active materials in Li-ion batteries provides a route to quantitatively study degradation processes that occur during cycling. In particular, we are primarily interested in quantifying the individual and coupled decomposition/transformations of the cathode – a lithiated transition metal oxide (TMO) – and the electrolyte – most commonly carbonate blends (ethylene carbonate, diethyl carbonate, etc.) with lithium hexafluorophosphate (LiPF6) as the salt. Previous observations of high-voltage instabilities include TMO surface reconstruction, transition metal dissolution, electrolyte decomposition, and formation of surface species. However, this picture is still incomplete, with the dependence on electrolyte and TMO composition not yet fully understood. We will present results in which isotopic labeling of 18O in Ni-rich and Li/Mn-rich NMCs is combined with quantitative gas evolution analysis to show that residual solid lithium carbonate (Li2CO3) on the surface of TMOs predominantly accounts for CO2 and CO evolved. Furthermore, O2 evolution from the TMO lattice is related to Li2CO3 present in the cathodes. These results indicate that electrolyte degradation negligibly contributes to gas evolution up to 4.8 V vs. Li/Li+ on the first charge.

Bias-Dependent Scanning Tunneling Microscopy Signature of Bridging-Oxygen Vacancies on Rutile TiO2(110).

The rutile TiO2(110) surface has long-served as a well-characterized, prototypical transition-metal oxide surface used in heterogeneous catalysis and photocatalytic water splitting. Naturally occurring defects on this surface, called bridging-oxygen (BO) vacancies, are important as they determine the overall reactivity of the surface. Herein, we report a bias-dependent, scanning tunneling microscopy (STM) signature of the BO vacancies on TiO2(110): for sample bias voltages past a threshold of +3 V, the bright vacancies are flanked on either side (along the oxygen row) by two dark spots approximately shaped like half-moons. The BO vacancies have a bright aspect below the threshold bias also but are not surrounded by half-moon dark depressions. Using generalized gradient approximation calculations with Hubbard correction (GGA + U) for projected density of states (DOS) and simulated STM images, we find that the bias-dependent STM signature originates from (i) local DOS maxima of all BOs (lighter background that occurs above the threshold bias) and (ii) the increased separation between the first and second BO atoms neighboring the vacancy which leads to an apparent dip between these neighboring oxygens. These results offer a new striking example of the STM signature that appears without switching the polarity of the bias. Similar approaches can be employed for seeking distinguishing features on the surfaces of other large band gap semiconductors and insulators.

In situ ambient pressure XPS observation of surface chemistry and electronic structure of α-Fe2O3 and γ-Fe2O3 nanoparticles

Fundamental understanding of charge transfer reaction is essential for the surface and interface engineering of transition metal oxides. In this study the chemical reactivity towards oxygen and hydrogen (13 Pa) under applied thermal conditions (423–673 K), of two polymorphic forms of Fe2O3 nanoparticles (γ-Fe2O3 and α-Fe2O3) are investigated with the combination of in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and near edge X-ray absorption fine structure spectroscopy (AP-NEXAFS). Our data show that the reactivity of these two polymorphs has a similar character based on the contribution of oxygen vacancy defect states and related material non-stoichiometry. Their exposure to hydrogen at increased temperature results in both cases in the surface reduction. However, γ-Fe2O3 exhibits more covalent character and undergoes the reduction preferentially with a contribution of metallic Fe0 than Fe2+, in contrast to α-Fe2O3. Further, upon introduction of oxygen at low temperature of 423 K, rapid re-oxidation process takes place at the Fe2O3 nanoparticles surface. Prepared γ-Fe2O3 and α-Fe2O3 nanostructures exhibit in general high n-type and p-type sensor response towards hydrogen, respectively, in a wide concentrations range.

Electron Transfer at the Metal Oxide/Electrolyte Interface: A Simple Methodology for Quantitative Kinetics Evaluation

While quantitative models exist for measuring rate constants for electron exchange at the surface of semiconductors to or from redox couples in solution, their use is very limited for studying transition metal oxide electrodes. Taking advantage of the possibility of tuning the doping level of WO3 by thermal treatment, we measured the rate constants for electron injection from soluble probes in the so-called reverse mode. Hence, we could demonstrate that intermediate states located below the conduction band take part to the oxidation process and that the rate constant is bimolecular with a first order dependence on the concentration of redox probes and the concentration of intermediate states. A kinetics model was thereby developed to describe this reverse mode and a zone diagram established, which can then be used to quickly assess rate constants for interfacial electron transfer on the surface of transition metal oxides semiconductors that are critical for applications such as solar energy conversion.

Image potential states at transition metal oxide surfaces: A time-resolved two-photon photoemission study on ultrathin NiO films

For well-ordered ultrathin films of NiO(001) on Ag(001), a series of unoccupied states below the vacuum level has been found. The states show a nearly free electron dispersion and binding energies which are typical for image potential states. By time-resolved two-photon photoemission (2PPE), the lifetimes of the first three states and their dependence on oxide film thickness are determined. For NiO film thicknesses between 2 and 4 monolayers (ML), the lifetime of the first state is in the range of 28--42 fs and shows an oscillatory behavior with increasing thickness. The values for the second state decrease monotonically from 88 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms of coupling of the unoccupied states to the layer-dependent electronic structure of the growing NiO film.

Understanding How Nitriles Stabilize Electrolyte/Electrode Interface at High Voltage.

Nitriles have received extensive attention for their unique ability in stabilizing electrolytes against oxidation at high voltages. It was generally believed that their anodic stability originates from a monolayer of chemisorbed nitrile molecules on transition-metal oxide surface, which physically expels carbonate molecules and prevents their oxidative decomposition. We overturn this belief based on calculation and experimental results and demonstrate that, like many high voltage film-forming electrolyte additives, nitriles also experience an oxidative decomposition at high voltages, and the high oxidation stability of nitrile-containing electrolytes is merely the consequence of a new interphasial chemistry. This important mechanistic correction would be of high significance in guiding the design of new electrolytes and interphases for the future battery chemistries.