First-principles calculations of bulk, surface and interfacial phases and properties of silicon graphite composites as anode materials for lithium ion batteries.

The high energy density offered by silicon along with its mineralogical abundance in the earth's crust, make silicon a very promising material for lithium-ion-battery anodes. Despite these potential advantages, graphitic carbon is still the state of the art due to its high conductivity and structural stability upon electrochemical cycling. Composite materials combine the advantages of silicon and graphitic carbon, making them promising materials for the next generation of anodes. However, successfully implementing them in electric vehicles and electronic devices depends on an understanding of the phase, surface and interface properties related to their performance and lifetime. To this end we employ electronic structure calculations to investigate crystalline silicon-graphite surfaces and grain boundaries exhibiting various orientations and degrees of lithiation. We observe a linear relationship between the mixing enthalpies and volumes of both Li-Si and Li-C systems, which results in an empirical relationship between the voltage and the volume expansion of both anode materials. Assuming thermodynamic equilibrium, we find that the lithiation of graphite only commences after LixSi has been lithiated to x = 2.5. Furthermore, we find that lithium ions stabilize silicon surfaces, but are unlikely to adsorb on graphite surfaces. Finally, lithium ions stabilize silicon-graphite interfaces, increasing the likelihood of adhesion as core@shell over yolk@shell configurations with increasing degree of lithiation. These observations explain how lithium might accelerate the crystallization of silicon-graphite composites and the formation of smaller nanoparticles with improved performance.

A Porphyrin Complex as a Self-Conditioned Electrode Material for High-Performance Energy Storage.

The novel functionalized porphyrin [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) was used as electrodes for rechargeable energy-storage systems with an extraordinary combination of storage capacity, rate capability, and cycling stability. The ability of CuDEPP to serve as an electron donor or acceptor supports various energy-storage applications. Combined with a lithium negative electrode, the CuDEPP electrode exhibited a long cycle life of several thousand cycles and fast charge-discharge rates up to 53 C and a specific energy density of 345 Wh kg-1 at a specific power density of 29 kW kg-1 . Coupled with a graphite cathode, the CuDEPP anode delivered a specific power density of 14 kW kg-1 . Whereas the capacity is in the range of that of ordinary lithium-ion batteries, the CuDEPP electrode has a power density in the range of that of supercapacitors, thus opening a pathway toward new organic electrodes with excellent rate capability and cyclic stability.

Growth of Stable Surface Oxides on Pt(111) at Near-Ambient Pressures.

Detailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time-dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) we show that a time scale of hours (t≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF-GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430-680 K and an O2 partial pressure of 1 mbar.

Surface Buckling and Subsurface Oxygen: Atomistic Insights into the Surface Oxidation of Pt(111).

Platinum is a catalyst of choice in scientific investigations and technological applications, which are both often carried out in the presence of oxygen. Thus, a fundamental understanding of platinum's (electro)catalytic behavior requires a detailed knowledge of the structure and degree of oxidation of platinum surfaces in operando. ReaxFF reactive force field calculations of the surface energies for structures with up to one monolayer of oxygen on Pt(111) reveal four stable surface phases characterized by pure adsorbate, high- and low-coverage buckled, and subsurface-oxygen structures, respectively. These structures and temperature programmed desorption (TPD) spectra simulated from them compare favorably with and complement published scanning tunneling microscopy (STM) and TPD experiments. The surface buckling and subsurface oxygen observed here influence the surface oxidation process, and are expected to impact the (electro)catalytic properties of partially oxidized Pt(111) surfaces.

Bimetallic alloys in action: dynamic atomistic motifs for electrochemistry and catalysis.

Bimetallic alloys show great promise for applications in a wide range of technologies related to electrochemistry and heterogeneous catalysis. The alloyed nature of these materials supports the existence of surface phenomena and structural motifs not present in single-component materials. These novel features result in electrochemical and catalytic behaviors, requiring entirely new categories of explanations. In this perspective concrete examples are used to illustrate several of these chemical and structural features, which are unique to multi-component metal surfaces. The influence of the surface's structure and surroundings (e.g. adsorbates) on each other provides a common thread, with the emergence of dynamic surfaces as its terminus. In considering three model systems (PtRu, PtNi and AuPd), we discuss not only a selection of surface phenomena relevant to each, but also the implications of these alloy-related behaviors for the electrochemical and catalytic properties of each surface.

Development of a ReaxFF potential for Pt-O systems describing the energetics and dynamics of Pt-oxide formation.

ReaxFF force field parameters describing Pt-Pt and Pt-O interactions have been developed and tested. The Pt-Pt parameters are shown to accurately account for the chemical nature, atomic structures and other materials properties of bulk platinum phases, low and high-index platinum surfaces and nanoclusters. The Pt-O parameters reliably describe bulk platinum oxides, as well as oxygen adsorption and oxide formation on Pt(111) terraces and the {111} and {100} steps connecting them. Good agreement between the force field and both density functional theory (DFT) calculations and experimental observations is demonstrated in the relative surface free energies of high symmetry Pt-O surface phases as a function of the oxygen chemical potential, making ReaxFF an ideal tool for more detailed investigations of more complex Pt-O surface structures. Validation for its application to studies of the kinetics and dynamics of surface oxide formation in the context of either molecular dynamics (MD) or Monte Carlo simulations are provided in part by a two-part investigation of oxygen diffusion on Pt(111), in which nudged elastic band (NEB) calculations and MD simulations are used to characterize diffusion processes and to determine the relevant diffusion coefficients and barriers. Finally, the power of the ReaxFF reactive force field approach in addressing surface structures well beyond the reach of routine DFT calculations is exhibited in a brief proof-of-concept study of oxygen adsorbate displacement within ordered overlayers.

Surface stability of Pt3Ni nanoparticulate alloy electrocatalysts in hydrogen adsorption.

Nanoparticles of Pt/Ni alloys represent state of the art electrocatalysts for fuel cell reactions. Density functional theory (DFT) based calculations along with in situ X-ray absorption spectroscopy (XAS) data show that the surface structure of Pt3Ni nanoparticulate alloys is potential-dependent during electrocatalytic reactions. Pt3Ni based electrocatalysts demonstrate preferential confinement of Ni to the subsurface when the electrode is polarized in the double layer region where the surface is free of specifically adsorbed species. Hydrogen adsorption triggers nickel segregation to the surface. This process is facilitated by a high local surface coverage of adsorbed hydrogen in the vicinity of the surface confined Ni due to an uneven distribution of the adsorbate(s) on the catalyst's surface. The adsorption triggered surface segregation shows a non-monotonous dependence on the electrode potential and can be identified as a breathing of the catalyst as was proposed previously. The observed breathing behavior is relatively fast and proceeds on a time scale of 100-1000 s.

Direct Integration of Strained-Pt Catalysts into Proton-Exchange-Membrane Fuel Cells with Atomic Layer Deposition.

The design and fabrication of lattice-strained platinum catalysts achieved by removing a soluble core from a platinum shell synthesized via atomic layer deposition, is reported. The remarkable catalytic performance for the oxygen reduction reaction (ORR), measured in both half-cell and full-cell configurations, is attributed to the observed lattice strain. By further optimizing the nanoparticle geometry and ionomer/carbon interactions, mass activity close to 0.8 A mgPt -1 @0.9 V iR-free is achievable in the membrane electrode assembly. Nevertheless, active catalysts with high ORR activity do not necessarily lead to high performance in the high-current-density (HCD) region. More attention shall be directed toward HCD performance for enabling high-power-density hydrogen fuel cells.

Development of a ReaxFF reactive force field for aqueous chloride and copper chloride.

Copper ions play crucial roles in many enzymatic and aqueous processes. A critical analysis of the fundamental properties of copper complexes is essential to understand their impact on a wide range of chemical interactions. However the study of copper complexes is complicated by the presence of strong polarization and charge transfer effects, multiple oxidation states, and quantum effects like Jahn-Teller distortions. These complications make the experimental observations difficult to interpret. In order to provide a computationally inexpensive yet reliable method for simulation of aqueous-phase copper chemistry, ReaxFF reactive force field parameters have been developed. The force field parameters have been trained against a large set of DFT-derived energies for condensed-phase copper-chloride clusters as well as chloride/water and copper-chloride/water clusters sampled from molecular dynamics (MD) simulations. The parameters were optimized by iteratively training them against configurations generated from ReaxFF MD simulations that are performed multiple times with improved sets of parameters. This cycle was repeated until the ReaxFF results were in accordance with the DFT-derived values. We have performed MD simulations on chloride/water and copper-chloride/water systems to validate the optimized force field. The structural properties of the chloride/water system are in accord with previous experimental and computational studies. The properties of copper-chloride/water agreed with the experimental observations including evidence of the Jahn-Teller distortion. The results of this study demonstrate the applicability of ReaxFF for the precise characterization of aqueous copper chloride. This force field provides a base for the design of a computationally inexpensive tool for the investigation of various properties and functions of metal ions in industrial, environmental, and biological environments.

ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell.

In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H2/Ni/YSZ and C4H10/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC.