Thin Film Model Systems Research Articles

Development of a Thin-Film NMC System for the Investigation and Suppression of Harmful Reactions Occurring on the Surface of Cathode Materials in Li-Ion Batteries

Materials with the stoichiometry Li(NixMnyCoz)O2 (NMC) have emerged as promising candidates for use as cathodes in Li-ion batteries for electric vehicles. This is primarily due to their high capacity, which increases with the nickel content (Ni-rich NMC). However, the increase in nickel also leads to a decrease in the stability of the material, hindering market introduction [1]. Much of these stability issues emanate from the surface of the material and move subsurface towards the bulk during battery operation [2]. The internal electrode architecture of traditional composite battery cathodes, containing active materials, carbon additives, and binders, complicates the study of the surface reactions. For this reason, we developed a thin-film model system of Ni-rich NMC with a high surface area-to-volume ratio to exacerbate the effect of the surface reactions, enabling easier characterization of the surface without influence of the bulk.The interfacial reactions occurring when NMC is exposed to air was investigated using the thin-film system. These experiments were supported by thermodynamic calculations to propose a mechanism of what reactions occur on the surface. Our study showed that the surface of NMC is stable in inert environments, however, when exposed to air (in particular H2O and CO2) the active material at the surface reacts to form Li2CO3, coinciding with loss of Li2O and protonation of the material, forming transition metal oxyhydroxides. We found that this phenomenon can be reversed by performing a heat treatment, moving Li2O back into the film. However, during the annealing process, an additional reaction between the film and substrate takes place. This, once again, leads to the active material being deficient in Li2O. To compensate for this deficiency, Li2CO3 was deposited onto NMC prior to annealing. The thermodynamical properties of each film were approximated from their electrochemical behavior. It was found that the film treated with Li2CO3 before annealing was thermodynamically similar to what is expected for an NMC cathode. These results contribute to the understanding of the interfacial reactions that take place on the surface of NMC cathodes. Additionally, the study proposes strategies to mitigate and compensate for these reactions.

Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction.

Nickel cobalt oxides (NCOs) are promising, non-precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry-dependent electrochemical behavior makes it crucial to understand the structure-OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In-depth film characterization shows the phase transition from Ni-rich rock-salt films to Co-rich spinel films. Electrochemical analysis in 1m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni-rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co-rich spinel films show instantly high activities, while the more sustainable Ni-rich rock-salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure-performance mechanisms.

Revealing the three-dimensional arrangement of polar topology in nanoparticles

In the early 2000s, low dimensional ferroelectric systems were predicted to have topologically nontrivial polar structures, such as vortices or skyrmions, depending on mechanical or electrical boundary conditions. A few variants of these structures have been experimentally observed in thin film model systems, where they are engineered by balancing electrostatic charge and elastic distortion energies. However, the measurement and classification of topological textures for general ferroelectric nanostructures have remained elusive, as it requires mapping the local polarization at the atomic scale in three dimensions. Here we unveil topological polar structures in ferroelectric BaTiO3 nanoparticles via atomic electron tomography, which enables us to reconstruct the full three-dimensional arrangement of cation atoms at an individual atom level. Our three-dimensional polarization maps reveal clear topological orderings, along with evidence of size-dependent topological transitions from a single vortex structure to multiple vortices, consistent with theoretical predictions. The discovery of the predicted topological polar ordering in nanoscale ferroelectrics, independent of epitaxial strain, widens the research perspective and offers potential for practical applications utilizing contact-free switchable toroidal moments.

Accelerating the Development of New Silicon-Based Anode Materials

In this work we have demonstrated the feasibility of using thin film systems as a steppingstone to accelerate the development of commercially relevant silicon-based anode materials for Li-ion batteries. Understanding new materials in conventional particle-based electrode systems is often complex. Lithium and electrolyte interact not only with the active material particles, but also with the different supporting components of the electrode, leaving it difficult to directly assess the electrochemical contribution of the active material. This is further complicated by the electrochemical behavior of such electrodes also being strongly dependent on processing conditions, like casting and drying procedures. Additionally, it is often also a challenge to synthesize such materials in particle form with the required chemical composition and monodispersity in the first place. Therefore, it would be highly beneficial to study and screen new anode materials without having to assume and assess these different factors. That is of particular importance during the optimization stage of the chemical composition for the active material, for which a significant number of samples is typically needed. Thus, evaluating the material in a thin film system is a simple and reproducible method of determining a material’s potential as a future anode material.Such thin films of active materials can reliably be made with a large range of thicknesses, compositions, element combinations and coatings, using multiple flexible and reproducible thin film deposition techniques available. In the present work, we have investigated silicon-based materials made by PECVD and sputtering: pure silicon and sub-stoichiometric versions of silicon nitride, silicon carbide, and silicon phosphide (SiNx, SiCx, SiPx). For evaluation of the applicability of the thin film model system, in several cases we have also compared the results to particles of the same material with similar chemical composition synthesized via a free space CVD process [1].Preliminary evaluation shows good agreement between results for thin film systems compared to what is known for particles of the same material. SiNx particles show good agreement with thin film results where a high nitrogen content decreases the capacity, while increasing the stability due to the formation of a stabilizing matrix around the active silicon [2]. Differences in the differential capacity analysis of similar materials in particle and thin film forms demonstrates the greater complexity in interpreting the data from the more intricate particle-based systems. SiCx is known to react in a similar fashion as SiNx where its coulombic efficiency and capacity decreases with increasing carbon-content, while its stability has been seen to increase due to a similar stabilizing matrix [3]. Reference s : [1] H. F. Andersen et al., ECS Trans., vol. 62, no. 1, pp. 97–105, 2014.[2] A. Ulvestad et al., ACS Nano, vol. 15, no. 10, pp. 16777–16787, 2021.[3] X. D. Huang et al., RSC Adv., vol. 8, no. 10, pp. 5189–5196, 2018.

Phase formation and electrical properties of reactively sputtered Fe1−x O thin films

Wüstite, Fe1−x O, is a crucial phase for the transition to CO2-free steel manufacturing as well as promising for electrochemical applications such as water splitting and ammonia synthesis. To study the effect of interfaces in these applications, thin-film model systems with defined interfaces are ideal. Previous studies lack a description of the growth mechanism to obtain Fe1−x O thin films. Here, we investigate the phase formation of metastable Fe1−x O during reactive magnetron sputtering while systematically varying the O2/Ar flow ratio from 1.8% to 7.2% and the pressure–distance product between 3.5 and 7.2 Pa cm. If bulk diffusion is minimized, thin films containing 96 vol.% wüstite and 4 vol.% Fe as impurity phase were achieved. Therefore, the wüstite phase formation appears to be surface diffusion dominated. To reveal the influence of impurity phases in wüstite on the electrical resistivity, systematic electrical resistivity measurements while cooling in situ were performed for the first time. The electrical resistivity was lower than that of single crystals of the respective iron oxides. This is attributed to the formation of Fe-rich layers at the substrate-film interface, which serve as additional conduction paths.

Element-Specific Study of Magnetic Anisotropy and Hardening in SmCo5-xCux Thin Films.

This work investigates the effect of copper substitution on the magnetic properties of SmCo5 thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimental and theoretical studies show that Cu substitution at the Co3g sites not only stabilizes the formation of the SmCo5 structure but also enhances magnetic anisotropy and coercivity. Density functional theory calculations indicate that Sm(Co4Cu3g)5 possesses a higher single-ion anisotropy as compared to pure SmCo5. In addition, X-ray magnetic circular dichroism reveals that Cu substitution causes an increasing decoupling of the Sm 4f and Co 3d moments. Scanning transmission electron microscopy confirms predominantly SmCo5 phase formation and reveals nanoscale inhomogeneities in the Cu and Co distribution. Our study based on thin film model systems and advanced characterization as well as modeling reveals novel aspects of the complex interplay of intrinsic and extrinsic contributions to magnetic hysteresis in rare-earth-based magnets, i.e., the combination of increased intrinsic anisotropy due to Cu substitution and the extrinsic effect of inhomogeneous elemental distribution of Cu and Co.

Understanding Polymer Structure and Conductivity in Composite Electrolytes

Lithium ion batteries are a crucial technologies for electric vehicles and portable electronics due to their high energy density. However, flammable liquid electrolytes create a safety concern and transitioning to all solid state batteries allows for the potential of higher energy density by utilizing lithium metal anodes. Two classes of solid materials have been explored to replace liquid electrolytes: ceramic and polymer electrolytes. Composite electrolytes, typically in the form of dispersed ceramic particles or nanowires in polymer matrix, combine these two materials to have good ionic conductivity while maintaining the mechanical properties of the polymer. While they have achieved fairly high conductivity (typically 10-4 S/cm), the mechanism for such ion conduction is not well understood.The polymer likely plays a crucial role as the ceramic volume fractions are typically under 20%.1,2 Four possible ways that polymers could increase in conductivity in the composite have been identified: decreased crystallinity, increased free volume, disruption of polymer alignment, and increased lithium ion dissolution due to acidic surface groups on the ceramic.1,3,4 We have developed thin film model systems to probe the interface of the polymer with and ceramic surfaces. We found that when confined to thin films, the conductivity of the polymer electrolyte increased compared to the bulk. We utilized differential scanning calorimetry to identify the crystallinity and found that the small crystallinity variation could not account for the variation in conductivity. Subsequently, we utilized surface conductivity and transference number measurements to understand the effects of acidic surface groups. X-ray Absorption measurements will be similarly utilized to probe the variation in lithium ion environments within these materials to understand any variation in the lithium salt dissolution. It was found that the variation in conductivity based on this effect is small.In order to probe the possible variation in free volume, Small Angle Neutron Scattering will be utilized to quantify the free volume within thin films, bulk polymer, and composite electrolytes. This can quantify the volume associated with each monomer and therefore how much space is between polymer chains for lithium ion movement.5 Lastly, while the polymer chain alignment has not been extensively discussed composite literature, it has been identified that when polymer electrolytes are cast on a surface, polymer electrolytes typically show anisotropy with measurements parallel to the surface having higher conductivity due to the alignment of the chain.6 Therefore, ceramic fillers in the composites may disrupt this packing and allow for higher conductivity perpendicular to the cast surface, the direction in which they are most commonly measured. In order to probe this effect, we measure the anisotropy of both our thin film electrolytes as well as composite electrolytes to compare the significance of the anisotropy and utilize small angle x-ray scattering to measure any anisotropy in the structure.

Effect of Si on the hydrogen-based direct reduction of Fe2O3 studied by XPS of sputter-deposited thin-film model systems

Understanding the effect of gangue elements is of critical importance to optimize the efficiency of hydrogen-based direct reduction (HyDR) of iron ore, as one of the key steps towards climate-neutral steel production. Here, we demonstrate on the example of Si-doped Fe2O3, how thin films can be effectively utilized as a model system to facilitate systematic investigation of the solid-state reduction behavior. In-vacuo X-ray photoelectron spectroscopy (XPS) is used to probe the reduction kinetics by analyzing the chemical state of iron oxide thin films before and after annealing at 700 °C in an Ar+5%H2 atmosphere. It is demonstrated that even low Si concentrations of 3.7 at.% inhibit the HyDR of Fe2O3 by the formation of a SiOx-enriched reduction barrier in the surface-near region.

Enhanced metal exsolution at the non-polar (001) surfaces of multi-faceted epitaxial thin films

Metal exsolution is a dynamic process driven under a reducing atmosphere and at elevated temperatures that results in the self-assembly of nanoparticles at the surface of complex perovskite catalysts. The nanoparticle characteristics of metal exsolution catalysts can be subject to considerable inhomogeneity, and the anisotropic surface properties of ceramic oxides have been identified to have a major influence on the exsolution behavior. We systematically reveal the orientation-dependent anisotropy of the exsolution behavior of Ni in SrTi0.9Nb0.05Ni0.05O3−δ using multi-faceted epitaxial thin films that represent a material system with properties in between functional ceramics and single-crystalline perovskite thin film model systems. Using an approach of combined orientation mapping and surface imaging we study the exsolution behavior with particular focus on the initial exsolution response, i.e. after short annealing times. We find orientation-specific variations in the surface morphology of the thin film facets. In the as-prepared state, surface reconstructions cause the formation of patterned surface structures for all thin film facets apart from (001) surfaces, which exhibit a plain surface morphology as well as an enhanced exsolution response. Surface reconstructions and their inherent energy landscape may hence cause an additional energy barrier for the exsolution reaction that results in orientation-dependent differences in the exsolution kinetics.

Activating Lattice Oxygen in Perovskite Oxide by B-Site Cation Doping for Modulated Stability and Activity at Elevated Temperatures.

Doping perovskite oxide with different cations is used to improve its electro‐catalytic performance for various energy and environment devices. In this work, an activated lattice oxygen activity in Pr0.4Sr0.6Co x Fe0.9− x Nb0.1O3− δ (PSCxFN, x = 0, 0.2, 0.7) thin film model system by B‐site cation doping is reported. As Co doping level increases, PSCxFN thin films exhibit higher concentration of oxygen vacancies (Vo••) as revealed by X‐ray diffraction and synchrotron‐based X‐ray photoelectron spectroscopy. Density functional theory calculation results suggest that Co doping leads to more distortion in Fe—O octahedra and weaker metal—oxygen bonds caused by the increase of antibonding state, thereby lowering Vo•• formation energy. As a consequence, PSCxFN thin film with higher Co‐doping level presents larger amount of exsolved particles on the surface. Both the facilitated Vo•• formation and B‐site cation exsolution lead to the enhanced hydrogen oxidation reaction (HOR) activity. Excessive Co doping until 70%, nevertheless, results in partial decomposition of thin film and degrades the stability. Pr0.4Sr0.6(Co0.2Fe0.7Nb0.1)O3 with moderate Co doping level displays both good HOR activity and stability. This work clarifies the critical role of B‐site cation doping in determining the Vo•• formation process, the surface activity, and structure stability of perovskite oxides.

Sr Surface Segregation; Implications for Oxygen Electrode Behavior and Routes to Mitigate Impact

The activation energy of the oxygen evolution/oxygen reduction process at perovskite type oxygen electrode materials is slightly higher than that of oxygen transport in the classical electrolyte materials, and also of that of the steam-splitting/hydrogen oxidation process at the fuel electrode. Today at 750 oC – 800 oC state of the art oxygen electrodes do not limit overall SOEC/SOFC performance, but when pushing the operation temperature lower, towards 600 oC the oxygen electrode can become limiting. Hence, there is still a need to develop high performing and importantly, durable, oxygen electrodes.One mechanism that has been reported to reduce oxygen exchange rates at Sr-containing perovskite type electrode materials is Sr segregation to the surface. This has especially been studied and observed on model systems like thin films [1, 2] and bulk materials [3, 4]. A surface enrichment in Sr will potentially make an electrode more vulnerable to degradation by reaction with gas-phase impurities like CO2 and Sulphur-species, but also to Cr-poisoning. Hence, it is important to study what drives such surface enrichment and how to mitigate its consequences.We have studied this in some detail on La0.6Sr0.4FeO3 both on thin films [2] and on bulk samples [4]. Interestingly, on the thin film model system, one can to some extent alleviate the consequences of the enrichment by leaching away excess Sr [1,2]. On the bulk samples, we have found that the oxygen exchange kinetics decrease with aging at low temperature (650 oC), but that the samples can be rejuvenated by a short high temperature treatment [4].In this contribution, we shall present the relation between the oxygen exchange rates measured on La0.6Sr0.4FeO3 surfaces and the chemistry of the surface. It turns out, that Sr enrichment to the surface occurs very fast and the “rejuvenation” treatment does not affect the Sr content at the surface, but rather in which phase/environment the Sr is found.We shall further discuss the implications of the learnings on above presented model-type-systems to the technical level, where often the materials are applied in form of composites.[1] Z. Cai, M. Kubicek, J. Fleig and B. Yildiz, Chem. Mater., 2012, 24, 1116–1127[2] Tripković, D., Küngas, R., Mogensen, M. B. & Hendriksen, P. V., 2020, Physical Chemistry Chemical Physics. 22, 27, p. 15418-15426[3] Edith Bucher, Werner Sitte, Solid State Ionics 192 (2011) 480–482.[4] Đ. Tripković, R. Küngas, M. B. Mogensen and P. V. Hendriksen, J. Mater. Chem. A, 2019, 7, 11782–11791. Figure 1

Thin Film Model Systems: Thin Film NCM Cathodes as Model Systems to Assess the Influence of Coating Layers on the Electrochemical Performance of Lithium Ion Batteries (Adv. Mater. Interfaces 9/2021)

Thin film model systems allow to analyse the properties of cathode materials without the influence of additives. In article number 2002074, Matthias T. Elm and co-workers demonstrate that probing the cathode-electrolyte-interface of pristine and coated NCM thin films with surface sensitive techniques enables to elucidate the beneficial effect of protective layers.

In Situ Lithiated ALD Niobium Oxide for Improved Long Term Cycling of Layered Oxide Cathodes: A Thin-Film Model Study

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, within 3D cathode composites it is difficult to isolate the effect of the coating because of additives and non-ideal interfaces. In this study we investigate niobium oxide (NbOx) as cathode coating in a thin-film model system, which provides simple access to the cathode-coating-electrolyte interface. The conformal NbOx coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO2 cathodes. The cathode/coating stacks were annealed to lithiate the NbOx and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling with respect to the uncoated cathode. At a NbOx thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode film. Elemental analysis using TOF-SIMS and XPS revealed a bulk and surface contribution of the NbOx coating. These results show that in situ lithiated ALD NbOx can significantly improve the performance of layered oxide cathodes by enhancing interfacial charge transfer and inhibiting surface degradation of the cathode, resulting in better rate performance and cycle life.

Thermal stability of thin Au films deposited on salt whiskers

Thin metal films deposited on patterned or rough substrates play an increasing role in microelectronics, sensing, catalysis, and other areas of nanotechnology. However, the thermal stability and solid state dewetting of thin metal films with complex three-dimensional architecture is still poorly understood. In this work we employed a model system of nanocrystalline Au thin films deposited on prismatic single crystalline KCl whiskers to study the solid state dewetting of thin films in a three-dimensional setting. The arrays of KCl whiskers were grown on porous substrates under well-defined humidity and temperature conditions. Single crystalline prismatic KCl whiskers with a very high aspect ratio, [001] axis and {100} side facets were obtained. The whiskers were coated with thin conformal Au films of 20-30 nm in thickness. The annealing of these core-shell whiskers at the temperature of 350oC resulted in solid state dewetting of the Au film, with the dewetting processes occurring much faster along the whisker edges than on the side facets. The orientation relationships between Au and KCl were determined by employing similarly prepared thin Au films deposited on the flat KCl (100) substrates. Inspired by our experimental results, we developed a numerical model describing the curvature-gradient driven and surface diffusion-controlled growth of a hole in the thin film deposited on a curved substrate. The model predicted the growth of anisotropic elliptical holes elongated along the whisker axis. We discuss the experimental results in terms of the proposed model, indicating the importance of the change in orientation relationship between the Au grains and KCl whisker along the whisker edges.

Fast Charge Transfer between LLZO Solid Electrolyte and LCO Cathode Thin Films with a Modified Interface

Thin film deposition of solid electrolytes can be useful for the fabrication of better solid-state batteries by reducing the thickness of the electrolyte material to a few hundred nanometers. This facilitates ionic conductance for faster charge-discharge and reduces the total volume of inactive material, therefore increasing energy and power density. Thin film batteries can also be valuable model systems to investigate interface charge dynamics and to serve as playgrounds for interface engineering, without the complexity of bulk systems. Here we investigate the charge transfer properties between the lithium garnet Li7La3Zr2O12 (LLZO) solid electrolyte and the LiCoO2 cathode using a thin-film model system, with the aim of reducing the interface resistance and allowing high charge-discharge rates.Bulk LLZO electrolyte has come under the spotlight owing to its high ionic conductivities (10-4 - 10-3 S/cm) as well as a wide electrochemical stability window (against metallic lithium anode and high potential cathode materials). We developed a method for fabricating crystalline LLZO thin films using magnetron co-sputtering followed by an annealing step at 700°C (significantly below the standard processing temperatures). The resulting 500 nm-thick Ga-doped LLZO thin films show densities and ionic conductivities (2×10-4 S/cm) comparable to the values observed in bulk ceramic pellets.[1]Based on this thin film fabrication process, we fabricated an all-thin-film model system to investigate the LLZO / LCO cathode interface. This architecture provides an easy access to the interface for characterization, allowing one to identify the degradation processes taking place at the interface under high-temperature co-sintering. Introducing an in situ-lithiated Nb2O5 diffusion barrier at the interface, it was possible to lower the LLZO / LCO charge transfer resistance to about 50 Ω cm2, a 3-fold reduction with respect to previously reported values. The low interfacial resistance combined with the high conductance through the LLZO thin-film electrolyte allows charge transfer at high charge–discharge rates up to 40 C (0.9 mA/cm2).[2] References [1] Sastre, Jordi, et al. "Lithium Garnet Li7La3Zr2O12 Electrolyte for All‐Solid‐State Batteries: Closing the Gap between Bulk and Thin Film Li‐Ion Conductivities." Advanced Materials Interfaces (2020): 2000425.[2] Sastre, Jordi, et al. "Fast Charge Transfer across the Li7La3Zr2O12 Solid Electrolyte/LiCoO2 Cathode Interface Enabled by an Interphase-Engineered All-Thin-Film Architecture." ACS Applied Materials & Interfaces 12.32 (2020): 36196-36207.

Enhanced Oxygen Reduction Activity on Silver-Palladium Alloyed Thin Film Electrocatalysts in Alkaline Media

The oxygen reduction reaction (ORR) requires a large overpotential, which is a factor limiting the large-scale deployment of clean energy technologies such as hydrogen fuel cells (H2 FCs), direct methanol FCs, and metal-air batteries. Designing strategies to enhance catalyst activity, stability, and selectivity for the ORR is one method towards enhancing H2 FCs technology. Many predicted active catalysts are not stable in acidic media, resulting in limited catalyst options for standard proton exchange membrane H2 FCs. Some theoretically active, yet acid-unstable, catalysts, such as Ag, can be stabilized in alkaline media making anion exchange membrane (AEM) fuel cells an attractive alternative. Ag-rich catalysts are specifically ideal because of their low cost and Ag’s high abundance relative to other catalysts materials such as Pt. We have previously shown that Cu atoms in Ag-Cu alloys can be modified by Ag to produce an enhanced activity for the ORR at high Cu atomic loadings. Better understanding the nature of intrinsic enhancements for the ORR on alloys will help in the development of new ORR alloy electrocatalysts.Besides, Ag-Cu alloys, several other Ag-bimetallics (including Ag-Pd, Ag-Mn, Ag-Fe, and Ag-Co) have been predicted to be good catalysts for the ORR. Alloying could lead to activity enhancements via changes in electronic structure, strain, and/or localized geometry. The activity of Ag-based catalysts for the ORR needs to be enhanced. One strategy to do this is to alloy Ag, which binds oxygen adsorbates too weakly, to a metal such as Pd, which binds oxygen adsorbates too strongly and is also very active. In this work, we use a thin film model system to show intrinsic activity enhancements for the ORR on Ag-Pd catalysts, resulting in activities higher than those of both parent materials even after reducing the Pd content by half.Ag-Pd thin films of different compositions are prepared using electron beam physical vapor co-deposition. X-ray diffraction indicate alloying across the full compositional range tested and atomic force microscopy shows that the films are highly flat (roughness factor < 1.02). Electrochemical testing in 0.1 M KOH shows significant activity improvements for the Ag-Pd films. The films with +40 at% Pd have activities, at 0.9 V vs RHE, greater than the linear combination of pure Ag and Pd activities. On a Pd content basis, the thin films with +50 at% Pd exhibit higher kinetic current densities, at 0.9 V vs RHE, than both pure Ag and Pd. Ag10Pd90 is the most enhanced thin film, both on a surface area and Pd loading bases, demonstrating a 5-fold activity enhancement relative pure Pd. By correlating density functional theory with experimental activity and physical characterization measurements, we hypothesize that Ag is modifying Pd’s surface electronic structure to adsorb OH more weakly via ligand and strain effects. These enhanced Ag-Pd ORR catalysts could facilitate the large-scale implementation of efficient clean energy electrochemical systems such as fuel cells and metal-air batteries.

X-ray photoelectron spectroscopy and rotating disk electrode measurements of smooth sputtered Fe-N-C films

Electrocatalysts for the oxygen reduction reaction (ORR) based on complexes of iron and nitrogen in a carbon matrix (Fe-N-C) are a promising alternative to platinum group metal (PGM) based catalysts in polymer electrolyte membrane (PEM) fuel cells. Further improvements of Fe-N-C catalysts would benefit from model thin film studies of activity and stability of catalytic sites, but synthesis of Fe-N-C model thin films is challenging. Here we report on synthesis and characterization of Fe-N-C thin films produced by co-sputtering iron and carbon in a reactive nitrogen atmosphere onto removable glassy carbon rotating disk electrode (RDE) tips. Scanning electron microscopy (SEM) measurements indicate that the Fe-N-C films deposited at high temperature are smoother than the films annealed at high temperature. Electrocatalytic activity measured on the thin Fe-N-C films is greater for both high-temperature samples than for the room-temperature sample. From the analysis of X-ray photoelectron spectroscopy (XPS) data, exposure of the films to high temperatures results in increased graphitization of the carbon within the Fe-N-C films, and increased relative amount of graphitic and hydrogenated nitrogen species. Overall, the results of this study demonstrate the feasibility of a thin film model system approach for studying active sites in PGM-free catalysts.

Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide

Low-cost transition metal oxides (TMO) has received extensive attention as new-generation oxygen evolution reaction (OER) catalysts. In recent years, introducing oxygen defects via various strategies emerged as an efficient way to increase electro-catalytic activity of TMO. Despite many successful examples, the physical and chemical origin of such enhanced OER activity induced by oxygen defects is still not well understood. In this work, we systematically investigate the correlation between the concentration of oxygen vacancies and OER activity by using a NiO thin film model system grown by pulsed laser deposition. Oxygen vacancies are introduced into NiO thin films by Ar plasma treatments with different power (50 W, 100 W, and 150 W). The OER activities of NiO thin films are found to be significantly enhanced after plasma treatment. The improvement in electrochemical OER performance is attributed to the creation of oxygen defects, which lead to the strongly modified surface adsorption properties, as confirmed by X-ray photoelectron spectroscopy and contact angle measurements. Our results demonstrate that plasma treatment as a promising method for precisely tuning physical and chemical properties of TMO through oxygen defects engineering to obtain high performance electro-catalysts.

Direct Measurement of Crystal Growth Velocity in Epitaxial Phase-Change Material Thin Films.

Central to the use of Ge-Sb-Te based phase-change materials for data storage applications is their crystallization capability since it determines memory writing time. Although being intensively studied to identify intrinsic limits and develop strategies to enhance memory performance, the crystallization process in these materials is still not fully explored. Therefore, this study focuses on the determination of crystal growth dynamics in an epitaxial phase-change material thin film model system offering the advantage of high crystalline quality and application-relevant sizing. By introducing a method that combines time-resolved reflectivity measurements with high-resolution scanning transmission electron microscopy, crystal growth velocities upon fast cooling after single ns-laser pulse irradiation of the prototypical phase-change material Ge2Sb2Te5 are determined. As a result, an increase in crystal growth velocity from 0.4 to 1.7 m/s with increasing laser fluence is observed with a maximum rate of 1.7 m/s as the upper detectable limit of the studied material.

(Invited) A Tutorial on Atomic Layer Deposition and Its Application in Lithium-Ion Batteries

Atomic layer deposition (ALD) is a deposition technique where thin films are deposited from sequential gas-surface reactions. Thanks to the self-limiting nature of these reactions, ALD provides interesting deposition characteristics such as atomic scale thickness control and deposition onto complex shaped substrates (including powders) with excellent uniformity and conformality. These features have enabled ALD to become a well-established tool in semiconductor manufacturing, and there is a growing consensus that other domains such as catalysis and energy storage can also strongly benefit from these unique coating characteristics. In this tutorial, two different types of potential applications of ALD in the field of lithium-ion batteries will be discussed: - ALD of the active components of a lithium-ion battery (positive and negative electrodes, solid electrolytes), either as functional layers in thin film batteries, or as thin film model systems. Novel battery concepts such as the 3D thin-film lithium-ion microbattery are promising for applications such as on-chip energy storage, and ALD could play an important role in the deposition of critical components of these battery stacks onto 3D nanostructured substrates, in particular for the interfaces and solid-electrolyte layers. Moreover, the excellent film quality, control over composition and crystallinity provided by ALD enable the synthesis of thin, planar model systems. Such thin film model systems with well-controlled properties provide simplified one-dimensional currents and are unbothered by the complex interplay of various components of the classical battery, such as binder, conductive additive, particle size, solvent composition etc, and can thus reveal potentially beneficial battery chemistries which would otherwise be obscured, or provide insights into otherwise very complex systems. - ALD modification of surfaces and interfaces. In particular, ALD of protective coatings onto electrode materials has been explored extensively to stabilise interfaces and thus enhance cell cycle life. As battery ageing for traditional lithium-ion batteries can already be problematic, novel architectures, battery concepts and material chemistries pose increasingly greater challenges in this direction, and surface/interface engineering have become important strategies to overcome ageing and improve battery performance. To obtain a high quality coating on the complex surface of battery electrodes, the low processing temperature in combination with the thickness control, uniformity and conformality provided by ALD can become key advantages.