Jens S. Hummelshøj

Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li–O2 Batteries

We use XPS and isotope labeling coupled with differential electrochemical mass spectrometry (DEMS) to show that small amounts of carbonates formed during discharge and charge of Li-O2 cells in ether electrolytes originate from reaction of Li2O2 (or LiO2) both with the electrolyte and with the C cathode. Reaction with the cathode forms approximately a monolayer of Li2CO3 at the C-Li2O2 interface, while reaction with the electrolyte forms approximately a monolayer of carbonate at the Li2O2-electrolyte interface during charge. A simple electrochemical model suggests that the carbonate at the electrolyte-Li2O2 interface is responsible for the large potential increase during charging (and hence indirectly for the poor rechargeability). A theoretical charge-transport model suggests that the carbonate layer at the C-Li2O2 interface causes a 10-100 fold decrease in the exchange current density. These twin interfacial carbonate problems are likely general and will ultimately have to be overcome to produce a highly rechargeable Li-air battery.

Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries

Non-aqueous Li-air or Li-O(2) cells show considerable promise as a very high energy density battery couple. Such cells, however, show sudden death at capacities far below their theoretical capacity and this, among other problems, limits their practicality. In this paper, we show that this sudden death arises from limited charge transport through the growing Li(2)O(2) film to the Li(2)O(2)-electrolyte interface, and this limitation defines a critical film thickness, above which it is not possible to support electrochemistry at the Li(2)O(2)-electrolyte interface. We report both electrochemical experiments using a reversible internal redox couple and a first principles metal-insulator-metal charge transport model to probe the electrical conductivity through Li(2)O(2) films produced during Li-O(2) discharge. Both experiment and theory show a sudden death in charge transport when film thickness is ~5 to 10 nm. The theoretical model shows that this occurs when the tunneling current through the film can no longer support the electrochemical current. Thus, engineering charge transport through Li(2)O(2) is a serious challenge if Li-O(2) batteries are ever to reach their potential.

Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol

The use of methanol as a fuel and chemical feedstock could become very important in the development of a more sustainable society if methanol could be efficiently obtained from the direct reduction of CO2 using solar-generated hydrogen. If hydrogen production is to be decentralized, small-scale CO2 reduction devices are required that operate at low pressures. Here, we report the discovery of a Ni-Ga catalyst that reduces CO2 to methanol at ambient pressure. The catalyst was identified through a descriptor-based analysis of the process and the use of computational methods to identify Ni-Ga intermetallic compounds as stable candidates with good activity. We synthesized and tested a series of catalysts and found that Ni5Ga3 is particularly active and selective. Comparison with conventional Cu/ZnO/Al2O3 catalysts revealed the same or better methanol synthesis activity, as well as considerably lower production of CO. We suggest that this is a first step towards the development of small-scale low-pressure devices for CO2 reduction to methanol.

Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry

We develop a density functional theory model for the electrochemical growth and dissolution of Li(2)O(2) on various facets, terminations, and sites (terrace, steps, and kinks) of a Li(2)O(2) surface. We argue that this is a reasonable model to describe discharge and charge of Li-O(2) batteries over most of the discharge-charge cycle. Because non-stoichiometric surfaces are potential dependent and since the potential varies during discharge and charge, we study the thermodynamic stability of facets, terminations, and steps as a function of potential. This suggests that different facets, terminations, and sites may dominate in charge relative to those for discharge. We find very low thermodynamic overpotentials (<0.2 V) for both discharge and charge at many sites on the facets studied. These low thermodynamic overpotentials for both discharge and charge are in very good agreement with the low kinetic overpotentials observed in recent experiments. However, there are other predicted paths for discharge/charge that have higher overpotentials, so the phase space available for the electrochemistry opens up with overpotential.

CatApp: A Web Application for Surface Chemistry and Heterogeneous Catalysis

Solid catalysts form the backbone of the chemical industry and the hydrocarbon-based energy sector. Most catalysts and processes today are highly optimized, but there is still considerable room for improvements in reactivity and selectivity in order to lower energy consumption and waste production. In addition, the development of sustainable energy solutions is a tremendous challenge to catalysis science and engineering. The ability to store solar energy as a fuel calls for new catalysts, as does the development of a sustainable chemical industry that is based on biomass and other non-fossil building blocks. The development of new catalysts could be accelerated significantly if we had access to systematic data for the activation energies of elementary surface reactions. Once the key parameters that determine the activity or selectivity of a certain process have been established through experiments or calculations, such a database would enable searches for new catalyst leads. Ideally, data would come from detailed, systematic experiments, but it is generally not possible to find such data. Electronic structure calculations provide a powerful alternative. The accuracy is not such that detailed predictions of absolute rates of elementary reaction steps can be made, but for classes of interesting catalysts (such as transition metals) it is possible to create systematic data with sufficient accuracy to predict trends in reactivity. Herein, we introduce such a set of calculated reaction energies and activation energies for a large number of elementary surface reactions on a series of metal single-crystal surfaces, including surfaces with defects such as steps. We also introduce a simple web application (CatApp) for accessing these data. The data will be part of a larger database of surface reaction data that are being developed under the Quantum Materials Informatics Project. The database includes reaction energies for all surface reactions that involve C C, C H, C O, O O, O H, N N, C N, O N, N H splitting for molecules with up to three C, N, or O atoms on close-packed face-centered cubic fcc(111), hexagonal close-packed hcp(0001), and body-centered cubic bcc(110) surfaces, as well as stepped fcc and hcp surfaces. The metals included in the database are Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Re, Rh, Ru, Sc, V. The data have been compiled from previous reports, where details of the calculations can be found. The key point is that the values have all been calculated with the same code (DACAPO), the same exchange-correlation energy functional (GGARPBE), and similar calculational parameters. Therefore one adsorption energy or reaction barrier can be compared to another with some confidence. Gas-phase CO2 and O2 , for which the RPBE functional performs poorly, were corrected as described in Refs. [25] and [26], respectively In cases where there are no calculated data for a given reaction, we use the recently developed scaling relations to provide an estimate. The scaling relations link the adsorption energies of different molecules that contain varying amounts of hydrogen. In a similar fashion, we exploit the fact that transition-state energies are quite generally found to scale with reaction energies. We have developed a simple visual query tool for accessing the data presented above. On our homepage, we maintain a list of hyperlinks to the available versions of the tool, together with a list of references to the scientific data it employs. The tool is a web application implemented in JavaScript, SVG, and HTML, and runs in modern web browsers without any plug-ins. The application can be easily used on computers and portable devices with a touch interface. By running the application, one can choose a surface and an elementary reaction and be presented with a reaction path that reports the reaction and activation energy. If a DFT value is not found for a given reaction and a scaling estimate is used, the value is shown in italic type. In Figure 1 we have shown an example of the use of the application. The energy barrier needed to break the N2 bond on two different surface orientations of ruthenium, Ru(0001) and stepped Ru(0001), are extracted. The plots immediately show the structure dependence of this important step in the Haber–Bosch process (N2+ 3H2!2NH3). Our web application will allow anyone to download data such as that shown in Figure 1, and to quickly explore whether there may be other metals or structures where the N2 bond is broken more readily. All code and data are downloaded when the application is accessed for the first time and is kept in the local storage of the browser. This feature allows the application to be used even when the user has no internet connection. More importantly, it guarantees the user complete privacy, since all queries are performed locally in the browser and not by connecting to our server. The only information that is delivered from the user to our server is an anonymous [*] Dr. J. S. Hummelshoj, Dr. F. Abild-Pedersen, Dr. F. Studt, Dr. T. Bligaard, Prof. J. K. Norskov SUNCAT Center for Interface Science and Catalysis SLAC National Accelerator Laboratory 2575 Sand Hill Road, Menlo Park, CA 94025 (USA)

The Computational Materials Repository

The possibilities for designing new materials based on quantum physics calculations are rapidly growing, but these design efforts lead to a significant increase in the amount of computational data created. The Computational Materials Repository (CMR) addresses this data challenge and provides a software infrastructure that supports the collection, storage, retrieval, analysis, and sharing of data produced by many electronic-structure simulators.