Eda Yilmaz

Promoting formation of noncrystalline Li2O2 in the Li-O2 battery with RuO2 nanoparticles.

Low electrical efficiency for the lithium-oxygen (Li-O2) electrochemical reaction is one of the most significant challenges in current nonaqueous Li-O2 batteries. Here we present ruthenium oxide nanoparticles (RuO2 NPs) dispersed on multiwalled carbon nanotubes (CNTs) as a cathode, which dramatically increase the electrical efficiency up to 73%. We demonstrate that the RuO2 NPs contribute to the formation of poorly crystalline lithium peroxide (Li2O2) that is coated over the CNT with large contact area during oxygen reduction reaction (ORR). This unique Li2O2 structure can be smoothly decomposed at low potential upon oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the more typical Li2O2 structure with a large size, small CNT contact area, and insulating crystals.

Real-Time XRD Studies of Li-O2 Electrochemical Reaction in Nonaqueous Lithium-Oxygen Battery.

Understanding of electrochemical process in rechargeable Li-O2 battery has suffered from lack of proper analytical tool, especially related to the identification of chemical species and number of electrons involved in the discharge/recharge process. Here we present a simple and straightforward analytical method for simultaneously attaining chemical and quantified information of Li2O2 (discharge product) and byproducts using in situ XRD measurement. By real-time monitoring of solid-state Li2O2 peak area, the accurate efficiency of Li2O2 formation and the number of electrons can be evaluated during full discharge. Furthermore, by observation of sequential area change of Li2O2 peak during recharge, we found nonlinearity of Li2O2 decomposition rate for the first time in ether-based electrolyte.

One-Dimensional Peptide Nanostructure Templated Growth of Iron Phosphate Nanostructures for Lithium-Ion Battery Cathodes

Template-directed synthesis of nanomaterials can provide benefits such as small crystalline size, high surface area, large surface-to-volume ratio, and structural stability. These properties are important for shorter distance in ion/electron movement and better electrode surface/electrolyte contact for energy storage applications. Here nanostructured FePO4 cathode materials were synthesized by using peptide nanostructures as a template inspired by biomineralization process. The amorphous, high surface area FePO4 nanostructures were utilized as a cathode for lithium-ion batteries. Discharge capacity of 155 mAh/g was achieved at C/20 current rate. The superior properties of biotemplated and nanostructured amorphous FePO4 are shown compared to template-free crystalline FePO4.

Photoresponse of PbS nanoparticles-quaterthiophene films prepared by gaseous deposition as probed by XPS

Semiconducting lead sulfide (PbS) nanoparticles were cluster beam deposited into evaporated quaterthiophene (4T) organic films, which in some cases were additionally modified by simultaneous 50 eV acetylene ion bombardment. Surface chemistry of these nanocomposite films was first examined using standard x-ray photoelectron spectroscopy (XPS). XPS was also used to probe photoinduced shifts in peak binding energies upon illumination with a continuous wave green laser and the magnitudes of these peak shifts were interpreted as changes in relative photoconductivity. The four types of films examined all displayed photoconductivity: 4T only, 4T with acetylene ions, 4T with PbS nanoparticles, and 4T with both PbS nanoparticles and acetylene ions. Furthermore, the ion-modified films displayed higher photoconductivity, which was consistent with enhanced bonding within the 4T organic matrix and between 4T and PbS nanoparticles. PbS nanoparticles displayed higher photoconductivity than the 4T component, regardless of ion modification.

Probing the Charge Build‐Up and Dissipation on Thin PMMA Film Surfaces at the Molecular Level by XPS

Electrets are well-known and utilized materials that develop a permanent electrostatic potential or a dipole moment. However, the nature of this “electrification” at the atomic and molecular scale is poorly understood. A better understanding of the fundamental processes that lead to electret formation could allow more intelligent utilization of these materials. Kelvin-probe atomic force microscopy (KP–AFM) has been the most advanced tool for probing, mapping, and quantifying the development of charge at submicrometer length scales. However, as in most electrical-based measurements, it lacks chemical specificity. In contrast, spectroscopic techniques such as IR, Raman, or NMR spectroscopy have excellent chemical specificity, but are not sensitive to charge accumulation. In this respect, ESR and EPR techniques have been quite successful in analysis of trapped charges. However, the use of these techniques is also limited to only radicals and paramagnetic species. Unlike optical techniques, X-ray photoelectron spectroscopy (XPS) is a charged-particle-based technique and is also very sensitive to the presence of electrical potentials on the analyzed surfaces. Moreover, the photoelectron emission process itself leads to the creation of positive potentials in nonconductive samples as a result of uncompensated charges, and elaborate charge compensation methods have been developed using low-energy electrons or ions to eliminate sample charging. However, complete removal, that is, achieving the point of zero charge (PZC), is only an ideal. Besides, the measurement of the sign and the extent of the potentials developed can reveal significant information. Herein, we describe a contactless analysis technique to investigate the nature of the charging process of polymer surfaces at the molecular level, using XPS, whereby poly(methyl methacrylate) (PMMA) films are analyzed either in their pristine state or deliberately charged using a flood gun as an external electron source, and by applying external bias to control the extent of charging resulting from a combination of the photoemission process and the compensating electrons from the flood gun. Insulating materials such as polymers, salts, metal oxides, and nitrides have large band-gap values, and electrons are localized, leading to extremely low conductivities. In these materials, other electronic states, such as interface and impurity states, as well as defect sites, completely dominate their electrical properties. In addition, the electrical properties of these materials are influenced by external stresses, such as exposure to light, energetic particles, mechanical distortions, slicing, and ball milling, which is attributed to insertion of localized electrons or ions at interfaces, grain boundaries, cracks, or in bulk sites such as cavities. This charge insertion can even lead to chemical oxidation–reduction reactions. Contact electrification has recently been in focus. Two different mechanisms were proposed as its cause, one being electron transfer and the other ions or materials transfer, and sound experimental findings support both mechanisms. PMMA, with an average molecular weight of 120000 (Aldrich) was used to prepare films from 0.4% (w/w) solution in chlorobenzene by spin coating onto conducting Si wafers. XPS measurements were carried out using a Thermo Fischer K-Alpha spectrometer, which was modified for the introduction of external bias to the substrate in the form of directcurrent (d.c.) or square-wave potential pulses with varying frequencies (10 3 to 10 Hz), as described previously. The instrument also provides a facility to record a narrow region of the spectrum in the snapshot mode with less than 50 ms steps. Different modes of data gathering are used to probe the sign, the extent, and the dynamics of charging/discharging in both the C1s and O1s regions. As prolonged exposure to Xrays causes decomposition of the PMMA films, and because of the long-lasting nature of charging (several hundreds of seconds), extreme care was exercised to always probe a pristine region of the PMMA films for each and every measurement with an approximately 400 mm X-ray spot size. Note also that, although the nature of charging is consistent, the measured potentials exhibit strong fluctuations from one film to the other and also across each film. Therefore, our results should be considered as qualitative findings for a proof of the principle. Figure 1 displays the O1s and C1s regions of XP spectra that were recorded at three different charged states of a PMMA film, as judged by their positions referenced to the Si2p3/2 peak of the substrate, and using the tabulated peak positions given in Table 1. Accordingly, if the recorded binding energy (BE) positions are less than the reference values, the kinetic energy of the photoelectron is increased with respect to neutral (PZC) state, and hence the sample is negatively charged (and vice versa). As can be gathered from the figure, in addition to the overall shift of peaks, measurable [*] Dr. E. Yilmaz, Dr. H. Sezen, Prof. S. Suzer Department of Chemistry, Bilkent University Bilkent, TR-06800 Ankara (Turkey) E-mail: suzer@fen.bilkent.edu.tr

Synthesis of Mesoporous Lithium Titanate Thin Films and Monoliths as an Anode Material for High-Rate Lithium-Ion Batteries

Mesoporous Li4 Ti5 O12 (LTO) thin film is an important anode material for lithium-ion batteries (LIBs). Mesoporous films could be prepared by self-assembly processes. A molten-salt-assisted self-assembly (MASA) process is used to prepare mesoporous thin films of LTOs. Clear solutions of CTAB, P123, LiNO3 , HNO3 , and Ti(OC4 H9 )4 in ethanol form gel-like meso-ordered films upon either spin or spray coating. In the assembly process, the CTAB/P123 molar ratio of 14 is required to accommodate enough salt species in the mesophase, in which the LiI /P123 ratio can be varied between molar ratios of 28 and 72. Calcination of the meso-ordered films produces transparent mesoporous spinel LTO films that are abbreviated as Cxx-yyy-zzz or CAxx-yyy-zzz (C=calcined, CA=calcined-annealed, xx=LiI /P123 molar ratio, and yyy=calcination and zzz=annealing temperatures in Celsius) herein. All samples were characterized by using XRD, TEM, N2 -sorption, and Raman techniques and it was found that, at all compositions, the LTO spinel phase formed with or without an anatase phase as an impurity. Electrochemical characterization of the films shows excellent performance at different current rates. The CA40-350-450 sample performs best among all samples tested, yielding an average discharge capacity of (176±1) mA h g-1 at C/2 and (139±4) mA h g-1 at 50 C and keeping 92 % of its initial discharge capacity upon 50 cycles at C/2.

X-ray Raman spectroscopy of lithium-ion battery electrolyte solutions in a flow cell

The effects of varying LiPF6 salt concentration and the presence of lithium bis(oxalate)borate additive on the electronic structure of commonly used lithium-ion battery electrolyte solvents (ethylene carbonate-dimethyl carbonate and propylene carbonate) have been investigated. X-ray Raman scattering spectroscopy (a non-resonant inelastic X-ray scattering method) was utilized together with a closed-circle flow cell. Carbon and oxygen K-edges provide characteristic information on the electronic structure of the electrolyte solutions, which are sensitive to local chemistry. Higher Li+ ion concentration in the solvent manifests itself as a blue-shift of both the π* feature in the carbon edge and the carbonyl π* feature in the oxygen edge. While these oxygen K-edge results agree with previous soft X-ray absorption studies on LiBF4 salt concentration in propylene carbonate, carbon K-edge spectra reveal a shift in energy, which can be explained with differing ionic conductivities of the electrolyte solutions.

Catalytic Properties of Vanadium Diselenide: A Comprehensive Study on Its Electrocatalytic Performance in Alkaline, Neutral, and Acidic Media

Here, we report the synthesis of vanadium diselenide (VSe2) three-dimensional nanoparticles (NPs) and two-dimensional (2D) nanosheets (NSs) utilizing nanosecond pulsed laser ablation technique followed by liquid-phase exfoliation. Furthermore, a systematic study has been conducted on the effect of NP and NS morphologies of VSe2 in their catalytic activities toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline, neutral, and acidic conditions. Research on VSe2 clearly demonstrates that these morphologies do not have a significant difference for ORR and OER; however, a drastic effect of morphology was observed for HER. The ORR activity of both NSs and NPs involves ∼2.85 numbers of electrons with the Tafel slope of 120 mV/dec in alkaline and neutral pH. In alkaline solution, NPs are proved to be an efficient catalyst for OER with an onset potential 1.5 V; however, for HER, NSs have a better onset potential of −0.25 V. Moreover, the obtained NPs have also better catalytic activity with a 400 mV anodic shift in the onset potential compared to NSs. These results provide a reference point for the future application of VSe2 in energy storage and conversion devices and mass production of other 2D materials.

Nanohybrid structured RuO2/Mn2O3/CNF as a catalyst for Na–O2 batteries

A 3D RuO2/Mn2O3/carbon nanofiber (CNF) composite has been prepared in this study by a facile two step microwave synthesis, as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). RuO2 nanoparticles with the mean size of 1.57 nm are uniformly distributed on Mn2O3 nano-rods grown on electrospun CNFs. The electrocatalytic activity of the composites are investigated towards ORR/OER under alkaline condition. The ternary RuO2/Mn2O3/CNF composite showed superior ORR activity in terms of onset potential (0.95 V versus RHE) and Tafel slope (121 mV dec-1) compared to its RuO2/CNF and Mn2O3/CNF counterparts. In the case of OER, the RuO2/Mn2O3/CNF exhibited 0.34 V over-potential value measured at 10 mA cm-2 and 52 mV dec-1 Tafel slope which are lower than those of the other synthesized samples and as compared to state of the art RuO2 and IrO x type materials. RuO2/Mn2O3/CNF also exhibited higher specific capacity (9352 mAh [Formula: see text]) than CNF (1395 mAh [Formula: see text]), Mn2O3/CNF (3108 mAh [Formula: see text]) and RuO2/CNF (4859 mAh g carbon -1) as the cathode material in Na-O2 battery, which indicates the validity of the results in non-aqueous medium. Taking the benefit of RuO2 and Mn2O3 synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V versus Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.