Office Of Basic Energy Sciences Research Articles

Surface Chemistry of Electrocatalysts for CO2 Reduction to Fuels

The surface chemistry of electrocatalysts for the production of fuels by CO2 reduction was explored by surface science model studies using high-resolution spectroscopy and atomic scale imaging, as well as by electrochemical analysis of active electrodes. Using an experimental surface science approach, ultrahigh vacuum conditions were used to facilitate the fabrication of highly characterizable electrode-adsorbate systems. We have utilized a range of techniques for surface analysis to determine the structure and properties of single-crystal surfaces and unique alloys formed by vapor deposition under controlled conditions. Specifically, XPS, LEIS, LEED and STM were used to characterize the composition and structure of III-V semiconductors and bimetallic model catalysts. TPD, UPS, and vibrational spectrocopy using HREELS and IRAS were used to study the binding and electronic structure of adsorbates. Many of our experiments are designed to explain the role of the electrode surface in the high selectivity observed in pyridine and other N-heterocycle-catalyzed CO2 reduction systems. We examined the preferential adsorption sites, bonding interactions, and reactivities of these molecular catalysts and other relevant adsorbates on the surfaces of photoactive electrode materials. Ambient pressure photoemission spectroscopy (APPES), STM, and vibrational spectroscopies were used to study the interaction of water and CO2 reduction catalysts with the surfaces of III-V semiconductors. We have applied these techniques to also study bimetallic alloy electrocatalysts. Bimetallic alloys have been shown to exhibit lower overpotentials for CO2 reduction to fuels when compared to their metal constituents. By tuning the composition of the alloys, one can influence the binding of adsorbates and therefore reaction selectivity. We synthesized bimetallic surface alloys in controlled UHV environments to investigate the relationship between surface composition and electronic structure. We studied the interaction of relevant adsorbed species with the bimetallic alloy surfaces using TPD, IRAS, HREELS, and UPS to begin to understand the surface chemistry of these electrodes. In parallel to these surface science investigations, we studied the electrochemical behavior of single crystals and bimetallic alloys. The purpose of these studies is to correlate CO2 reduction product selectivity to the quantitative measurements of surface chemistry made in UHV discussed above. Specifically we examined the relationship between electrochemical activity, surface composition, and the binding strengths of intermediates along CO2 reduction pathways. This work aids development of a molecular-level understanding of the heterogeneous processes involved the reaction mechanisms of efficient electrocatalytic fuel generation. This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012455.

Nanostructured, Bimetallic, Noble Metal Powders Prepared By Atomic Layer Electroless Deposition for Applications in Sensing and Catalysis

Bimetallic materials exhibit properties that are superior to their constituents in many applications. Examples include alkyne hydrogenation catalysts (selectivity), fuel cell cathodes (higher activity), formic acid oxidation catalysts (resistance to poisoning) and hydrogen storage materials (faster kinetics). A key to improving our understanding of these phenomena is preparation of these materials with nanoscale control of composition. Although recent efforts have focused on controlled synthesis of bimetallic particles, there remains a strong need for a synthetic strategy combining the properties of (i) scalability on high-surface-area substrates, (ii) uniform deposition and (iii) low-temperature processing conditions that preserve nanostructures. Presented herein are details of an Atomic Layer Electroless Deposition (ALED) technique that meets these criteria. By exposing Pd powder to dilute hydrogen in nitrogen gas, sacrificial surface PdH is formed along with dilute interstitial hydride. Once the flow of reagent is terminated, addition of metal salts facilitates galvanic displacement of the hydride, and controlled deposition of an overlayer having sub-nanometer thickness. This process can be cycled to create mesoscale layers. The electroless nature of ALED and its ambient processing conditions, make it an excellent complement to existing strategies of noble metal ALD. We will present high-resolution scanning electron microscopy demonstrating that conformal layers can be formed, and that the degree to which three-dimensional growth occurs is dependent upon deposition conditions. In addition, depth-profiles of bimetallic powders of varying adlayer thickness, generated using energy-resolved X-ray photoelectron spectroscopy, will be presented. We will also show evidence for fractional adlayer coverage based on the infrared spectra of adsorbed organic molecules. These data provide a proof of concept for using materials prepared by ALED as high-selectivity, high-dynamic-range sensors for various target molecules. Finally, we will demonstrate the ability of materials prepared in this manner to act as bi-functional catalysts. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Support for JLS was in part provided by the National Science Foundation, Division of Materials Research, grant #1006747. The authors also thank Mark Homer for STEM sample preparation. Figure 1

Using Smoothed Boundary Method for Simulating Interfacial Transport in Electrochemical Dynamics

The design of electrodes in current Li-ion batteries has long been based on experimental trial and error on electrode characteristics such as thickness, porosity, and composition [1]. Quantitative design criteria have yet to be developed due to the lack of understanding of coupled electronic-ionic transport in complex electrode structures. Physics-based modeling and simulations at electrode microstructural scale will facilitate the design process. Composite electrodes consist of tortuous channels for transport, particle with non-uniform sizes, and irregular particle-electrolyte interfaces. In addition, interfacial transport of Li and electrons along particle-particle contacts can play an important role in the electrochemical processes. Such geometric complexity makes microstructural-level electrochemical simulation a challenge in the research community. Using a diffuse-interface description, we have previously established a framework for electrochemical simulations based on the smoothed boundary method (SBM) [2], which allows efficient simulations of electrochemical dynamics in complex electrodes [3,4]. In this presentation, we describe an extension to the simulation tool that incorporates interfacial transport of Li and electrons on the boundaries between electrode particles or grains, which can enhance or hinder the net transport through the structure. In the SBM, an individual domain is defined by a domain parameter, which equals one inside the particle/grain and equals zero outside. The particle/grain surface is defined by the transition region where the domain parameter is between zero and one. For an agglomerate containing various particles or a polycrystalline electrode, multiple domain parameters can be employed to define individual domains. The surface Laplacian can be calculated, according to the domain parameters, to obtain the fluxes along the boundaries between the particles or grains. Using this method, bulk transport and interfacial transport can be coupled in evaluating the net transport within the electrode. A preliminary simulation of diffusion along grain boundaries of hexagonal grains is shown in Fig 1. We will use this technique to investigate the individual and combined effects of bulk and interfacial transport. Different transport coefficients can be assigned to distinct particles/grains or particle interfaces/grain boundaries to account for the dependence of the coefficients on crystalline orientations of particles/grains. Simulations using this tool will be utilized to examine the effect of tortuosity of interfaces on Li transport in polycrystalline electrode. In the long term, this method will be incorporated into the electrochemical simulation code to examine the role of interfacial Li and electron transport in determining electrode performance, which will provide insights for design and optimization of electrode architecture. Figure 1: SBM simulated concentration profile for grain boundary diffusion in an ideal hexagonal grain structure. Acknowledgement: This material is based upon work supported as part of the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583. [1] Venkatasailanathan Ramadesigan, et al, J. Electrochem. Soc. 159, R31-R45 (2012). [2] Hui-Chia Yu, Hsun-Yi Chen, and Katsuyo Thornton, Modelling Simul. Mater Sci. Eng. 20, 1-41 (2012). [3] B. Orvananos et al., Electrochimca Acta, 137 (2014) 245 [4] B. Orvananos et al.,J Electrochem Soc, 161 (2014) A535 Figure 1

Exploring Li Substituted O3-Structured Layered Oxides As Promising Cathode Materials for Rechargeable Na Batteries

Na-ion batteries have recently gained increasing recognition as intriguing candidates for next-generation large scale energy storage systems, owing to significant cost advantages stemming from the high natural abundance and broad distribution of Na resources. Although in terms of absolute energy density, Na-ion battery materials are not comparable with their Li-ion counterparts which are one of the dominating energy technologies in this decade. In particular, Na-ion batteries operating at room temperature could be suitable for applications where specific volumetric and gravimetric energy density requirements are not as stringent as in EVs, namely in electrical grid storage of intermittent energy produced via renewable sources.1 This would also contribute to a significant reduction of the costs connected to the use of renewable sources, which could then penetrate the energy market more easily and make Na-ion technology complementary to Li-ion batteries for stationary storage.2-4 A new O3 - Na0.78Li0.18Ni0.25Mn0.583Ow is obtained by the electrochemical Na-Li ion exchange process of Li1.167Ni0.25Mn0.583O2. The new material shows exceptionally high discharge capacity of 240 mAh g-1 in the voltage range of 1.5-4.5 V, (Fig. 1) thus the total energy density at the materials level reaches 675 Wh kg-1. When cycled between 1.5-4.2 V, the discharge capacity is well maintained around 190 mAh g-1 after 30 cycles. The O3 phase is kept through ion-exchange and cycling process, as confirmed by SXRD (Fig. 2). The stabilized O3 phase could be related to the tetrahedral Li formed upon initial lithiation, and breaks through the critical limitation for most of the O3 compounds. XAS results show that Ni2+/Ni4+ is the main active redox couple during cycling while Mn ions basically stay at tetravalent state. The Na full cell utilizing Na0.78Li0.18Ni0.25Mn0.583Ow as cathode delivers 430 Wh kg-1energy density. Later, the new Li substituted Na layered oxide with pure O3 phase is successfully synthesized by hydroxide co-precipitation method. It demonstrates a considerably high reversible capacity, and excellent rate performance. The phase is stabilized throughout the cycling, which could be the main reason for its good electrochemical properties. Ni2+/Ni4+ is the main active redox couple during cycling, and Co3+/Co4+partially contributes to the charge compensation while Mn ions basically stay at tetravalent state. The findings would contribute to the future improvement and design of practical cathode materials for Na-ion batteries. Fig. 1: (a) Electrochemical profiles of initial delithiation and initial sodiation for Na0.78Li0.18Ni0.25Mn0.583Ow in half cell. (b) Cycling performance for Na0.78Li0.18Ni0.25Mn0.583Ow with 125 mA g-1 current density. Inset is the corresponding electrochemical profile of Na0.78Li0.18Ni0.25Mn0.583Ow during the 1st, 2nd, 10th, 20th, 30th cycles in Na-ion batteries.5 Fig. 2: (a) Ex situ SXRD for Li1.167Ni0.25Mn0.583O2 and Na0.78Li0.18Ni0.25Mn0.583Ow at different states. (b) Schematic of the typical O3 structure. (c) Schematic of the proposed mechanism for sodiation. (In the schematic, transition metal ions are blue, Li+ is red, and Na+ is green.)5 Acknowledgements J. Xu and Y.S.Meng are grateful for the seed funding support from the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, with Award Number DE-SC0012583. The follow up work have been carried out by H. Liu and C. M with other private funding source. H. Liu acknowledges the financial support from China Scholarship Council under Award Number 2011631005. The authors appreciate the kind assistance from Dr Baihua Qu for anode SnS2/rGO synthesis at National University of Singapore (NUS). The XAS and SXRD were collected on 20-BM-B and 11-BM respectively at Advanced Photon Source in Argonne National Laboratory. Reference 1. B. L. Ellis and L. F. Nazar, Current Opinion in Solid State & Materials Science, 2012, 16, 168-177. 2. H. L. Pan, Y. S. Hu and L. Q. Chen, Energy & Environmental Science, 2013, 6, 2338-2360. 3. V. Palomares, M. Casas-Cabanas, E. Castillo-Martinez, M. H. Han and T. Rojo, Energy & Environmental Science, 2013, 6, 2312-2337. 4. M. Valvo, F. Lindgren, U. Lafont, F. Bjorefors and K. Edstrom, Journal of Power Sources, 2014, 245, 967-978. 5. H. D. Liu, J. Xu, C. Z. Ma and Y. S. Meng, Chem Commun, 2015, 51, 4693-4696. Figure 1

An Integrated First Principles and Experimental Approach to Enabling Multi-Electron Lithium-Ion Battery Cathodes

Today’s commercial lithium-ion battery cathodes function on the basis of a single-electron transfer per transition metal. In order to attain significantly higher capacities, particularly in polyanionic compounds with higher voltages, achieving reversible multi-electron transfer per transition metal is necessary. In this talk, we will discuss the efforts of the Northeast Center for Chemical Energy Storage (NECCESS), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294, to identify crystal structures that can reversibly intercalate two lithium ions and understand the implications to the crystal lattice and its stability of inserting those two lithium ions. Specifically, we will present results from our combined first principles and experimental investigations into the thermodynamics, kinetics and electronic structure of ε-VOPO4 (theoretical capacity of 318 mAh/g) over two Li insertion. We find that density functional theory, X-ray photoelectron spectroscopy and cyclic voltammetry all provide evidence of two-phase behavior during the first Li insertion and multiple intermediate phases during the second Li insertion. We will also show that ε-VOPO4 and its lithiated compounds show extremely good thermal stability relative to other well-known cathodes, and discuss our findings on the ionic and electronic limitations in this material.

Particle-Level Electrochemical Simulation of Phase-Separating Li Intercalation Cathode

Olivine LiFePO4 is believed to form two-phase equilibrium between the FePO4 (FP) and LiFePO4(LFP) phases in large particles. However, in rechargeable batteries, LFP is nanosized to overcome the low electric conductivity. The phase transformation behavior of nanosized LFP remains poorly understood. To address this issue, we have developed models and an electrochemical simulation framework for Li intercalation dynamics at particle/microstructural level to examine the phase transformation dynamics of LFP nanoparticles. The charge/discharge processes of Li-ion batteries involve several coupled physical mechanisms that occur within complex electrode microstructures. To simulate the electrochemical processes at particle/microstructural level, we developed an innovative technique that allows us to efficiently simulate electrochemical (de)intercalation processes at length scales ranging from electrode particle to complex electrode microstructures. In this presentation, we will discuss the smoothed-boundary-method electrochemical simulation framework, which straightforwardly couples the multiple physical mechanisms in the different phases of a composite electrode and incorporates the complex electrode geometries, as well as its applications. For demonstration, we used an experimentally obtained LiCoO2 cathode microstructure and corresponding material properties as simulation input [1]. We applied this simulation framework to investigate the phase-separation dynamics of LFP-nanoparticle electrodes. We first simulated a two-particle system to elucidate the detailed interparticle interactions upon (de)lithiation [2], and identified a unique process of interparticle phase separation. The simulation was extended to a collection of a modest number of LFP nanoparticles, where the particles were not contacting one another and formed a dilute agglomerate [3]. The simulation revealed a group-by-group interparticle phase separation process at low currents. In a further extension, we investigated the dynamics in a densely packed LFP-nanoparticle agglomerate [3], in which direct contact between particles was taken into account. In the dense agglomerate, we found that interparticle phase separation can still prevail at high currents due to the direct Li redistribution across particle interfaces. Finally, we included the consideration of particle-size dependent equilibrium potential into the model [4]. The simulation showed an asymmetry of phase transformation between lithiation and delithiation of an electrode at low currents. Acknowledgement: This material is based upon work supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Numbers DE-SC0001294 and DE-SC0012583. [1] H.-C. Yu et al., in preparation. [2] B. Orvanonos et al., J Electrochem Soc, 162 (2015) A965 [3] B. Orvananos et al., Electrochimca Acta, 137 (2014) 245 [4] B. Orvananos et al., submitted to J Electrochem Soc.

Electrochemical Evolution of the Surface and Subsurface Properties of Layered Nickel-Rich Oxide Cathodes

The limitation and degradation of the ionic and electronic transport across the positive electrode/electrolyte interface can potentially reduce the achievable capacity of Li-ion batteries. This is evident for the layered oxides such as those based on nickel that operate outside the stability of today’s organic electrolytes. The rise in impedance and capacity fade with cycling has been identified as a result of loss of electrical contact originating from the instability of electrode surface and decomposition of the electrolyte. In this talk, we will discuss the surface and subsurface properties of LiNi0.8Co0.15Al0.05O2 (NCA). NCA provides a model system for correlating capacity fade with the formation and evolution of the cathode electrolyte interface with cycling. Our studies directly compare x-ray spectroscopy, electron spectromicroscopy, and high-resolution electrochemical characterization of the same NCA electrodes as a function of both initial charging and extended cycling. This research is supported by the Northeast Center for Chemical Energy Storage (NECCESS), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.

Interfacial Effects of Electrochemical Lithiation of Epsilon-VOPO4 and Evolution of the Electronic Structure

The epsilon polymorph of vanadyl phosphate ε-VOPO4 is a promising cathode material for high-capacity Li ion batteries, owing to its demonstrated ability to reversibly incorporate more than one lithium per redox center, with a total theoretical capacity of 331 mAh/g. As in the case of the olivines, the problem of the inherently low electronic conductivity of ε-VOPO4 is overcome by the use of nano-sized particles within the cathode. At these dimensions the electrochemical reaction can be largely affected by the interfacial chemistry at the nanoparticle surface. An investigation of these surface reactions is critical for obtaining a complete understanding of the mechanism by which lithium intercalates and how it relates to the electrochemical performance. We performed x-ray photoelectron spectroscopy using both soft (XPS) and hard (HAXPES) x-rays to chemically distinguish and depth-resolve the interfacial phase transitions as a function of electrochemical discharge. Core level analysis supports a straightforward two-phase reaction as the first lithium is intercalated. In contrast, the insertion of the second lithium is more complicated with evidence of a significant radial lithium gradient, due to a possible to a disruption of the kinetics. From inspection of the valence band region, we were able to monitor the reversible evolution of ε-VOPO4 to Li2VOPO4 at the surface of our nanoparticles. These assignments were supported by hybrid density functional theory of the three phases. The origin of the radial gradient is likely associated with the presence of stable intermediate phases during the second reaction and warrants further attention. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294. Figure 1

Understanding the Layered Oxides for High-Voltage Intercalation in Alkaline Ion Batteries

Among all candidates of positive electrode materials for lithium ion batteries, the layered oxides have gained growing research interest in recent years, especially the Ni rich layered oxides and the mixed transition metal (Ni,Co,Mn) layered oxides. They can reach more than 220mAh/g capacity when charged above 4.5V, making them one of the highest energy density positive electrode materials among all known intercalation compounds for cathode materials. [1] Being the source of major capacity improvement, the high voltage operation (>4.5V), on the other hand, also cause unexplained phenomena in these layered oxides. In classical layered oxides, the transition metal (TM) oxide host TMO2should retain its host structure during initial discharging–charging, while lithium ions are intercalated–de-intercalated from the host. In contrast, recent studies suggested that after high voltage operation in layered oxides, certain amounts of transition metal ions migrate from the transition metal layers to lithium layers. Our previous work [2] has suggested that the migration may be assisted by oxygen vacancies generated in the late charging states (high voltages), but three critical questions remain unanswered: (1) how can we visualize and quantify the change in structure and chemical content at the atomic level (2) what is the oxygen activity upon high voltage and how does it assist transition metal migration and (3) what are the impact of these high voltage phenomena on rate capability and long-term cycling. LiNi0.8Co0.15Al0.05O2 (NCA), LiNi1/3Co1/3Mn1/3O2 (NCM333) and LiNi0.5Co0.2Mn0.3O2(NCM523) are used as model compounds for the in-depth study. Figure 1 and Table 1 summarize the electrochemical properties of NCA and NCM333 and 532. A suite of powerful experimental tools and computational tools has been deployed. To probe the bulk structural change, morphological and chemical changes, we combined X-ray diffraction (XRD), Neutron diffraction (ND), Pair Distribution Function (PDF) analysis, Electron Energy Loss Spectroscopy (EELS) and Transmission X-ray Microscopy (TXM), analytical Transmission Electron Microscopy (TEM). We will reveal and discuss the factors that determining the high voltage stability of oxides materials in alkaline ion batteries. Acknowledgements This work is supported by the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, with Award Number DE-SC0012583. Reference 1) Z. H. Lu, D. D. MacNeil and J. R. Dahn, Electrochem. Solid- State Lett., 2001, 4, A191–A194 2) K. J. Carroll, D. Qian, C. Fell, S. Calvin, … and Y. S. Meng, Phys. Chem. Chem. Phys., 2013 15, 11128 Figure 1

Thermodynamics and Kinetics of the Multi-Electron ε-VOPO4 cathode Across Two Lithium Insertion

LiVOPO4 is one of the few polyanion cathode materials that can reversibly insert/extract more than one lithium ion per transition metal and has a very high theoretical capacity of 318 mAh/g. Here, we present a detailed first principles and experimental investigation on ε-LixVOPO4 (0 ≤ x ≤ 2) to understand its structural evolution, thermodynamics and kinetics during lithium intercalation. We show that while no intermediate phases are predicted to be stable in the range of 0 ≤ x ≤ 1, first principles calculations predict two intermediate phases in the low voltage regime (1 ≤ x ≤ 2), in agreement with recent experiments.1 These intermediate phases are confirmed by V L-edge soft XAS and XRD data. The calculated voltage profile over 0 ≤ x ≤ 2 is also in excellent agreement with experiments. We predict decreasing phase stability with the incorporation of two Li into ε-VOPO4, suggesting that there are inherent challenges to achieving stable two-electron cycling without structural modification. We will also present a comprehensive analysis of the kinetics (ionic and electronic) of ε-LixVOPO4over the entire lithiation range. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294. 1. Z. Chen, Q. Chen, L. Chen, R. Zhang, H. Zhou, N. A. Chernova and M. S. Whittingham, J. Electrochem. Soc., 2013, 160, A1777

Evolution of the Surface and Subsurface Chemical Environment of LixNi0.8Co0.15Al0.5O2 electrodes

The nickel-based layered positive electrode, LiNi0.8Co0.15Al0.05O2 (NCA) has a high discharge capacity of ~ 200 mAhg-1 making it promising for potential electric vehicle applications. However, NCA displays drastic capacity fade with cycling. Accelerated cycling tests at elevated temperatures have shown that NCA electrodes undergo significant power fading. The significant power fading has been largely attributed to the formation of insulating and electrochemically inactive phases at the NCA surface. These include Li2CO3 surface species and a NiO-like rocksalt phase at particle surface due to oxygen loss. The literature of NCA surface and subsurface studies have highlighted the complexity of studying electrodes, with many reactions occurring that may not be inherent to NCA. Here we report on our studies of the surface and subsurface of NCA electrodes with and without a binder additive (i.e. PVDF) as a function of initial charging and cycling. The chemical composition and transition metal oxidation states were determined from a combination of x-ray photoelectron and absorption spectroscopy techniques. Our results reveal dramatic differences in the surface environments between our NCA electrodes with and without binder. Our results suggest that the PVDF may play a more significant role in degrading the surface of the NCA i.e. oxygen loss, than previously thought. We also report our surface analysis of our extended cycled NCA binder free electrodes. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.

Ionic Liquid Structure and Dynamics at Charged Graphene Interface

Supercapacitors with room temperature ionic liquid (RTIL) electrolytes and graphene-based electrodes are promising candidates for small electronic devices, especially flexible and wearable ones. The theoretical upper limit for the electric double layer (EDL) capacitance of graphene is the highest in all carbon materials. Also, graphene-based electrodes open up novel designs that are free of binders and inert conductive additives [1]. Furthermore, RTILs stand out as safe, non-volatile electrolytes with a wide potential window. However, our knowledge of these highly dense ionic plasmas at electrified interfaces is still at its infancy due to the lack of in situ experimental data about their potential-dependent EDL structures and dynamics [2]. Here, we use in situ real-time X-ray reflectivity integrated with fully atomistic molecular dynamics (MD) simulations to elucidate the interfacial ionic liquid structure and dynamics at epitaxial graphene electrode during cyclic voltammetry and potential steps [3]. Our results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by MD simulations at various static potentials [4]. The potential-driven transition between the two structures is characterized by an energy barrier (~0.15 eV) that is independent of temperature. The model nicely explains the coexistence of distinct anion and cation adsorbed structures and provides further insights to ultra-slow response of the interfacial structure to potential steps. References 1- Y. Shao et al. Chemical Society Reviews DOI:10.1039/C4CS00316K (2015) 2- M. Fedorov and A. Kornyshev. Chemical Reviews 114, 2978-3036 (2014) 3- A. Uysal et al. Journal of Physical Chemistry C, 118, 569-574 (2014) 4- A. Uysal et al. Journal of Physics: Condensed Matter, 27, 032101 (2015) * This effort was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The research described here was done in collaboration with S. S. Lee, H. Zhou, P. Fenter (Argonne National Laboratory), G. Feng, S. Li, P. Cummings (Vanderbilt University), P. Fulvio, P. Zhang, S. Dai (Oak Ridge National Laboratory), J. McDonough, Y. Gogotsi (Drexel University)

The Role of Ultrathin Coatings on the Interfacial Reactions at LiMn2O4 Surfaces

The spinel lithium manganese oxide (LiMn2O4, or LMO) is an established cathode technology for lithium ion batteries that suffers from a well-known phenomenon whereby Mn dissolves from the surface through the disproportionation reaction of Mn(III), and the subsequent deposition of Mn(II) on the anode. LMO can serve as a model system for exploring various mitigation strategies that will enrich our understanding of electrochemical corrosion at charged interfaces in general and may ultimately lead to better batteries. Consequently, the Center for Electrical Energy Science: Tailored Interfaces (CEES, a DOE-funded Energy Frontier Research Center), is exploring the influence of ultrathin coatings and surface treatments on the interfacial reactivity during lithiation reactions in LMO by combining advanced synthesis, in situ probes, and theory. As part of this effort, we have applied single-layer graphene coatings to LMO films, and discovered that these coatings improve capacity retention following electrochemical cycling. X-ray photoelectron spectroscopy (XPS) depth profiling, reveals that the graphene coating inhibits manganese depletion in the LMO film. Additionally, cross-sectional TEM demonstrates that a stable solid-electrolyte interphase layer is formed on the graphene coating, which screens the LMO from direct contact with the electrolyte, thereby inhibiting dissolution. Density functional theory (DFT) calculations have provided plausible mechanisms for this behavior. In a different effort, we have examined inert metal layers to prevent Mn dissolution. We found that 3-nm-thick Au shells prepared by electroless deposition could reduce Mn dissolution by up to 88%. Similarly, LiAlO2 thin films deposited on LiNi0.5Mn1.5O4 surfaces by atomic layer deposition can improve the electrochemical stability. The LiAlO2 films are 100x more ionically conducting compared to Al2O3, and, consequently, provide a higher discharge capacity compared to Al2O3 and allow for higher rates. Finally, we have explored the role of coherent coatings of a few nm-thick surface LiMn2-xTixO4, deposited on LMO. This approach protects the bulk LMO from acid corrosion but is also electrochemically active and, therefore, maintains the ion and charge transport channels on the surface. The surface-doped LMO has >2-fold improvement in electrochemical performance with respect to uncoated LMO in terms of cyclability and capacity at elevated temperature. In this presentation I will survey these ongoing research efforts into understanding the role of ultrathin coatings on interfacial reactions at LMO surfaces. This work was supported as part of the Center for Electrical Energy Storage: Tailored Interfaces, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

A Single Material Battery

Bulk-type all-solid-state lithium-ion batteries with nonflammable inorganic solid electrolyte are being considered as the ultimate solution for safe lithium-ion batteries. The current all-solid-state lithium-ion batteries suffer from low power density mainly due to strong kinetic limitation at the electrode/electrolyte interface. Here, we report a novel concept of a single-material all-solid-state lithium-ion battery, wherein a single Li10GeP2S12 serves as an electrolyte, an anode, and a cathode, to eliminate the highly resistive interface between the electrodes and electrolyte. The realization of the single-Li10GeP2S12 battery is based on the fact that the Li-S and Ge-S components in Li10GeP2S12 could act as the active centers for lithiation/delithiation as a cathode and an anode, respectively, when electronically-conductive carbon is mixed, while pure Li10GeP2S12 can be used as an electrolyte. This unique concept of a single-material lithium-ion battery can be extended to other solid-state battery systems, providing a new direction for high-power, high-energy, long-cycling solid-state batteries. This work was mainly supported as part of Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160.

Electronic Structure Calculations and Synergistic Experimental Work in the Nanostructures for Electrical Energy Storage (NEES) Energy Frontier Research Center

In this presentation, we will review computational effort and synergistic experimental activities in the Nanostructures for Electrical Energy Storage (NEES) Energy Frontier Research Center, and discuss future research directions. Interfaces and interphases constitute a main theme of our research. Nanoscale structures exhibit enhanced surface areas, and fundamental understanding and smart design of interfaces can help exploit their kinetic advantages while mitigating undesirable parasitic reactions. Thus electrochemical reactions responsible for liquid electrolyte degradation are examined using both large scale ab initio molecular dynamics (AIMD) simulations and complementary ultrahigh vacuum (UHV) measurements. Electrolyte decomposition on lithium peroxide surfaces, a key obstacle in Li-air battery implementation, is examined using analytical/spectroscopic techniques and DFT calculations. Artificial "solid electrolyte interphase" are deposited on amorphous carbon electrodes via atomic layer deposition (ALD) processes, and its electrode-passivating property is characterized by microgravimetric measurements and modelled using DFT calculations coupled with Marcus Theory constructions. This ALD study highlights a second key theme in our research: that necessary fundamental science, computational- and nano-electrochemistry advances emerge naturally in the ostensibly "applied" field of energy storage. We will discuss other examples of method development, such as rigorous voltage calibration in AIMD/DFT calculations, and measurement/control of minute currents associated with single nanowires. Going forward, our new computational approaches will be applied to model solid-solid interfaces pertinent to solid electrolytes, artificially-coated, nanostructured interfaces, and electrode degradation studies.This work was supported by Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DESC0001160. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corpo ration, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The Evolution of a High Capacity Electrode

Silicon can store Li+ at a capacity 10 times as high as that of graphite anodes. However, to harness this remarkable potential for electrical energy storage, one must address the multifaceted challenge of volume change inherent to high capacity electrode materials. In this talk, we present novel chemical strategies to address this challenge. Based on fundamental understanding of nanostructure interface, we have synthesized non-cracking silicon-carbon nanotube composite electrodes that electrically connect amorphous silicon beads. This synthesis is made possible by a propagative chemistry which creates regular bands of alkylcarboxyl groups along the nanotubes. The functional bands anchor the beads such that they exhibit a symmetric “radial breathing” around the CNT string, remaining crack-free and electrically connected throughout lithiation-delithiation cycling. We further show that, solely by chemical tailoring of the Si-C interface with atomic oxygen, the cycle life of the electrodes can be substantially improved, by 300%, even at high mass loadings. The evolution of this work culminates in the design and fabrication of a high capacity composite electrode that is weavable. The engineered andoe simultaneously attains high areal capacity (3.86 mAh/cm2), high specific capacity (922 mAh/g based on the mass of the entire electrode), and excellent cyclability (80% retention of capacity after 160 cycles), which are among the highest reported. This work was mainly supported as part of Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. We acknowledge cumulative contributions from many collaborators particularly K. Karki, M. Okada, H. Liao, H. Zhu, Z. Jia, E. Baker, Y. Zhang, K. Gaskell, A. Ghemes, P. Wang, J. Cumings, L. Hu, Y. Inoue, T. Li, Y. Qi, and G. W. Rubloff.

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

Thin Film Photovoltaics with Organic Metal Halide Peroviskites

Organic metal halide perovskites have now emerged as new and promising candidates for developing low cost thin film solar cells with efficiencies in the range of 20.1% certified efficiency. The recent advances of CH3NH3PbI3 based thin film solar cells have manly focused on the design architecture and improving the stability. Both TiO2 and Al2O3 mesoscopic architectures provide robust support to design high efficiency thin film solar cells. The excited state dynamics leading to the charge separation remains to be fully understood. Through comparative spectral analysis between the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH3NH3PbI3 across the transient absorption spectrum recorded following laser pulse excitation. The excited state of CH3NH3PbI3 characterized through transient absorption spectroscopy show dual behavior, viz., charge transfer complex state and charge separated state.1,2 The interplay between these two different excited states leads to efficient charge separation. Insights into the charge recombination kinetics and band filling aspects of CH3NH3PbI3 provide new ways to tune the efficiency of perovskite solar cells. These spectroscopic studies provide further insight into the optoelectronic behavior of methyl ammonium lead iodide films. Excited state properties that elucidate the exceptional photovoltaic properties will be discussed. Acknowledgments The research described herein is supported by the Office of Basic Energy Sciences, U.S. Department of Energy. Selected Publications Christians, J. A.; Fung, R.; Kamat, P. V., An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. J. Am. Chem. Soc. 2014, 136, 758–764.Manser, J. S.; Kamat, P. V., Band Filling with Charge Carriers in Organometal Halide Perovskites. Nature Photonics 2014, DOI: 10.1038/NPHOTON.2014.1171.

Surfactant-Templated Nanoporous Metal Films and Powders

At sufficiently high concentrations, many surfactants can direct electrodeposited or chemically reduced metals to form nanoporous structures. The surfactants can be small molecules, block copolymers, or polymers with functional endgroups. Under some conditions, the pores exhibit uniform, tunable sizes and long-range ordering. We will present a brief overview of demonstrations of these phenomena from the literature, and then focus on our own efforts to produce and characterize various forms of nanoporous palladium, a material of interest for energy storage, hydrogen storage, gas separation and sensing, and catalysis applications. We have used liquid-cell transmission electron microscopy and small-angle neutron scattering to study the mechanisms of particle growth in the presence of surfactants. Our results suggest that Pd nanoparticles form and sinter around micelles present in the aqueous phase. Changing the size of the surfactant molecules affects the size of these micelles, and ultimately determines the pore dimensions. Careful control of surfactant self-assembly and particle growth conditions is required to obtain long-range order. However, porosity that is more disordered, but still of quite uniform size distribution, can be obtained under much more relaxed conditions, and lower surfactant concentrations. We believe that interactions between the surfactants and the metal surfaces are more influential in these cases. The flexible range of conditions under which surfactants are able to induce nanoporosity suggests that this technology could prove to be a valuable synthetic method for energy materials with high power densities, as well as chemical engineering materials with high surface reaction rates. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. The Lujan Neutron Scattering Center at LANSCE is funded by the DOE Office of Basic Energy Sciences and Los Alamos National Laboratory under DOE Contract DE-AC52-06NA25396. The work was funded in part by Sandia’s Laboratory-Directed Research and Development program.

Ab Initio Study Of Surface Segregation Effects And Li- O2 Cell Oxygen Reduction Reaction Activity On Pd3M (M=Fe, Co, Ni, Cu) Alloys

To achieve practical application as a replacement for Li-ion cells, current Li- O2 cells must overcome problems of (i) poor cycle life, (ii) high charge and discharge overpotentials resulting in low coloumbic efficiency and low power capability [1]. The poor cycle life reported in many articles can be traced to the oxidative instability of the electrolyte [2] in conjunction with electrode passivation due to irreversible product deposits and difficulty in decomposing Li2O2. The high overpotentials are also a consequence of the stability of the peroxide. ORR on Noble metals [1], metal oxide [3], metal alloys [4], and carbon nanotubes[5] has been reported to occur with reduced overpotential and longer cycle life. Also studies of the cathode surface have shown that the Li2O2 nucleates in various 3-D morphologies [5] and even after multiple cycles a significant portion of the surface is bare [5]. Thus the pristine surface retention is expected to have a significant role in the ORR kinetics. Previous DFT studies on fuel cell ORR have predicted that the Pd skin alloys have better catalytic activity compared to pure Pd. This has been also found to agree very well with experiment. The present study is examining these alloys as potential catalysts for the ORR. Periodic models of the close-packed (111) surfaces of these alloys of (2x2x4) L12 –type unit cell configuration were constructed and systematic permutation of the first two atomic layers of FCC (111) Pt3M and Pd3Mskin alloys slabs was carried out to model surface segregation phenomena and to find the most stable geometric surface configuration. This was repeated with 0.25ML of adsorbed O, O2, LiO, LiO2, Li2O2, and Li2O to understand the effects of adsorbate induced surface activity on the most stable surface configurations. The Oxygen Binding energy (BEO) an important parameter for ORR catalytic activity on transition metal alloys, was used to probe the ORR activity of Pd3M/C (M = Fe,Ni,Cu) as shown in figure 1.The reaction free energies were extrapolated to the rate constants of the rate determining step using molecular kinetics theory. The reaction free energy on these surfaces was analyzed to identify surfaces and sites on those surfaces that (i) favor LiO2 desorption (favorable to 1e- transfer reaction) (ii) favor LiO2 adsorption (favorable to 2e- transfer reaction)(iii) show least energy barrier for O2 dissociation (favorable to 4e- transfer reaction). After screening the various surfaces for the most likely reaction route, the activity of the various surfaces for each of the routes was calculated. The insight gained from this exercise can be used to guide the choice of catalyst for test in Li- O2 cells. Acknowledgement: Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.