Surface Chemistry Of Electrodes Research Articles

Charge Injection Engineering of Ambipolar Field-Effect Transistors for High-Performance Organic Complementary Circuits

Ambipolar π-conjugated polymers may provide inexpensive large-area manufacturing of complementary integrated circuits (CICs) without requiring micro-patterning of the individual p- and n-channel semiconductors. However, current-generation ambipolar semiconductor-based CICs suffer from higher static power consumption, low operation frequencies, and degraded noise margins compared to complementary logics based on unipolar p- and n-channel organic field-effect transistors (OFETs). Here, we demonstrate a simple methodology to control charge injection and transport in ambipolar OFETs via engineering of the electrical contacts. Solution-processed caesium (Cs) salts, as electron-injection and hole-blocking layers at the interface between semiconductors and charge injection electrodes, significantly decrease the gold (Au) work function (∼4.1 eV) compared to that of a pristine Au electrode (∼4.7 eV). By controlling the electrode surface chemistry, excellent p-channel (hole mobility ∼0.1-0.6 cm(2)/(Vs)) and n-channel (electron mobility ∼0.1-0.3 cm(2)/(Vs)) OFET characteristics with the same semiconductor are demonstrated. Most importantly, in these OFETs the counterpart charge carrier currents are highly suppressed for depletion mode operation (I(off) < 70 nA when I(on) > 0.1-0.2 mA). Thus, high-performance, truly complementary inverters (high gain >50 and high noise margin >75% of ideal value) and ring oscillators (oscillation frequency ∼12 kHz) based on a solution-processed ambipolar polymer are demonstrated.

Offset prediction for charge-balanced stimulus waveforms

Functional electrical stimulation with cuff electrodes involves the controlled injection of current into an electrically excitable tissue for sensory or motor rehabilitation. Some charge injected during stimulation is ‘lost’ at the electrode–electrolyte interface when the charge carrier is translated from an electron to an ion in the solution. The process of charge injection through chemical reactions can reduce electrode longevity and implant biocompatibility. Conventionally, the excess charge is minimized by complex hardware solutions, which are often not appropriate for robust long-term implantable solutions. Here, we present a method of waveform design that minimizes irrecoverable charge during continuous pulsing through the use of biphasic waveforms with unequally charged phases. We developed an equivalent electrical model of the electrode–electrolyte impedance based on the electrode's surface chemistry during psuedo-bipolar stimulation conditions. Simulations with the equivalent circuit determined the uncompensated charge to be a function of stimulus parameters. In vitro stimulation experiments in saline confirmed that we could preemptively compensate for the excess charge following biphasic stimulus waveforms. As a result, there was a 92% reduction in the pre-pulse potential after a pulse train with this new waveform design when compared to stimulation with conventional biphasic waveforms.

Effect of Flame-Retarding Additives on Surface Chemistry of Electrodes in Li-Ion Batteries

This study examined the properties of 1 wt.% vinylene carbonate, vinyl ethylene carbonate, and diphenyloctyl phosphate additive electrolytes as a promising way of beneficially improving the surface and cell resistance of Li-ion batteries. The additive electrolytes were dominant both in surface formation and internal resistance. In particular, electrochemical impedance spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy confirmed that diphenyloctyl phosphate is an excellent additive to the electrolyte in the Li-ion batteries due to the improved co-intercalation of the solvent molecules.

Effect of Flame-Retarding Additives on Surface Chemistry of Electrodes in Li-Ion Batteries

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Engineering nanostructured electrodes and fabrication of film electrodes for efficient lithium ion intercalation

Lithium ion batteries have been one of the major power supplies for small electronic devices since the last century. However, with the rapid advancement of electronics and the increasing demand for clean sustainable energy, newer lithium ion batteries with higher energy density, higher power density, and better cyclic stability are needed. In addition, newer generations of lithium ion batteries must meet the requirements of low and easy fabrication cost and be free of toxic materials. There have been many novel approaches to gain high energy storage capacities and charge/discharge rates without sacrificing the battery cyclic life. Nanostructured electrodes are seemingly the most promising candidate for future lithium ion batteries. Modification of the electrode surface chemistry and the control of appropriate crystallinity are also reported to improve the electrode intercalation capabilities. The study of appropriately designed nanostructures, interfaces and crystallinity has also promoted and is accompanied with the development of thin film electrodes without the addition of binders and conductive carbon that are typically used in the fabrication of traditional lithium ion battery electrodes, simplifying the electrode fabrication process and enhancing electrode storage density. In this perspective, we summarize and discuss the efforts of fabricating nanostructures, modifying surface chemistry and manipulating crystallinity to achieve enhanced lithium ion intercalation capacities, rate capabilities and cyclic stability, as well as the direct fabrication of binderless film electrodes with desirable nano- and microstructures.

Ionic conductivity and interfacial properties of nanochitin-incorporated polyethylene oxide–LiN(C 2F 5SO 2) 2 polymer electrolytes

Nanocomposite polymer electrolytes (NCPE) composed of poly(ethylene oxide) and nanochitin for different concentrations of LiN(C 2F 5SO 2) 2 (LiBETI) were prepared by a completely dry, solvent-free procedure using a hot press. The thermal stability of NCPE membranes was investigated by DSC and TG-DTA. The membranes were subjected to SEM, ionic conductivity and FTIR analysis. Li/NCPE/Li symmetric cells were assembled and the variation of interfacial resistance as a function of time was also measured. The surface chemistry of lithium electrodes in contact with NCPE revealed the formation of Li–O–C and LiN compounds. LiFePO 4/NCPE/Li cell was assembled and the cycling profile showed a well-defined and reproducible shape of the voltage curves thus indicating a good cycling behavior of the cell at 60 °C.

Chitin-Incorporated Poly(ethylene oxide)-Based Nanocomposite Electrolytes for Lithium Batteries

Nanocomposite polymer electrolytes (NCPE), with different proportions of poly(ethylene oxide)/LiClO(4)/chitin were prepared by a hot press method. Nanochitin, a biopolymer, poly(beta-(1-->4)-N acetyl-d-glucosamine) was incorporated as a filler in poly(ethylene oxide) (PEO). The ionic conductivity of the composite polymer electrolytes was enhanced by one order upon addition of nanochitin. The lithium transference number, t(Li)(+), was increased from 0.24 to 0.51 upon chitin addition. The membranes were subjected to scanning electron microscopy, thermogravimetric-differential thermal analysis, differential scanning calorimetry, ionic conductivity, and Fourier transform infrared (FTIR) spectroscopy analysis. The free volume V(f) was probed by positron annihilation lifetime spectroscopy studies at 30 degrees C. Li/NCPE/Li symmetric cells were assembled, and the thickness of the solid electrolyte interface as a function of time was analyzed. This paper also describes FTIR spectroscopic studies of the interface between lithium metal and NCPE, which suggests that the surface chemistry of lithium electrodes in contact with NCPE is dominated by compounds with C-N-Li and C-O-Li bonding.

On the formation of lithium oxides and carbonates on Li metal electrodes in comparison to LiCoO 2 surface phases investigated by photoelectron spectroscopy

The study delivers detailed information about different lithium phases contributing to the complex surface chemistry of active electrodes in lithium ion batteries. Photoemission spectroscopy has been used to investigate the oxidation of metallic lithium as well as the reaction with air. From XP and UP valence band spectra characteristic emissions have been separated in order to help identifying lithium containing surface phases on intercalation electrodes. Characteristic spectral shapes of Li 2CO 3, Li 2O 2, Li 2O and LiOH compounds are presented. The spectral features of a reacted lithium surface are compared to carbonate emissions on a LiCoO 2 thin film cathode.

Influence of cell adhesion and spreading on impedance characteristics of cell-based sensors

Impedance measurements of cell-based sensors are a primary characterization route for detection and analysis of cellular responses to chemical and biological agents in real time. The detection sensitivity and limitation depend on sensor impedance characteristics and thus on cell patterning techniques. This study introduces a cell patterning approach to bind cells on microarrays of gold electrodes and demonstrates that single-cell patterning can substantially improve impedance characteristics of cell-based sensors. Mouse fibroblast cells (NIH3T3) are immobilized on electrodes through a lysine–arginine–glycine–aspartic acid (KRGD) peptide-mediated natural cell adhesion process. Electrodes are made of three sizes and immobilized with either covalently bound or physically adsorbed KRGD (c-electrodes or p-electrodes). Cells attached to c-electrodes increase the measurable electrical signal strength by 48.4%, 24.2%, and 19.0% for three electrode sizes, respectively, as compared to cells attached to p-electrodes, demonstrating that both the electrode size and surface chemistry play a key role in cell adhesion and spreading and thus the impedance characteristics of cell-based sensors. Single cells patterned on c-electrodes with dimensions comparable to cell size exhibit well-spread cell morphology and substantially outperform cells patterned on electrodes of other configurations.

Syntheses and Characterization of Lithium Alkyl Mono- and Dicarbonates as Components of Surface Films in Li-Ion Batteries

A homologous series of lithium alkyl mono- and dicarbonate salts was synthesized as model reference compounds for the frequently proposed components constituting the electrolyte/electrode interface in Li-ion batteries. The physicochemical characterization of these reference compounds in the bulk state using thermal analyses and X-ray photoelectron, nuclear magnetic resonance, and Fourier transform infrared spectroscopies establishes a reliable database of comparison for the studies on the surface chemistry of electrodes harvested from Li-ion cells.

Studies of cycling behavior, ageing, and interfacial reactions of LiNi 0.5Mn 1.5O 4 and carbon electrodes for lithium-ion 5-V cells

The cycling and storage behavior of LiNi 0.5Mn 1.5O 4 and MCMB electrodes for 5-V Li-ion batteries was investigated at elevated temperatures using a variety of electrochemical (CV, EIS) and spectroscopic (XPS, micro-Raman) tools. It was established that LiNi 0.5Mn 1.5O 4 electrodes could be cycled highly reversibly, demonstrating sufficient capacity retention at 60 °C by a constant current/constant voltage mode in DMC–EC/1.5 M LiPF 6 solutions. By studying the influence of temperature on the impedance of LiNi 0.5Mn 1.5O 4 electrodes, we conclude that when the initial electrode's surface chemistry is developed at a high temperature (60 °C) it becomes nearly invariant, and hence, their impedance remains steady upon cycling and storage. Prolonged storage of these electrodes at 60 °C may result in local Mn and Ni dissolution and transformation of the active material to λ-MnO 2. We have found that the surface chemistry of aged LiNi 0.5Mn 1.5O 4 electrodes (free of carbon black and PVdF) involves the formation of LiF, C–F and P–F x species. Storage of MCMB electrodes in LiPF 6 containing solutions at open circuit conditions (before their first lithiation) leads to significant morphological changes and the formation of lithium fluoride on the electrode surface, as determined by the XRD studies. LiF is probably a product of a catalytic thermal decomposition of LiPF 6. These initial changes further influence the impedance and kinetics of the lithiated electrodes.

Enhanced signal generation for use in the analysis of synthetic pyrethroids using chemical ionization tandem quadrupole ion trap mass spectrometry

Synthetic pyrethroids fragment extensively under electron ionization (EI) conditions to give low mass ions, most of them with the same m/z ratios. This fragmentation is primarily due to the labile ester linkage found in these compounds. In this research we established the best gas chromatography (GC) conditions in the EI mode that served as a benchmark in the development of a chemical ionization (CI) protocol for ten selected synthetic pyrethroids. Based on proton affinity data, several reagent gases were evaluated in the positive CI ionization mode. Methanol was found to produce higher average ion counts relative to the other gases evaluated, which led to the development of an optimized method consisting of selective ejection chemical ionization (SECI) and MS/MS. Standard stainless steel ion trap electrodes produced significant degradation of chromatographic performance on late eluting compounds, which was attributed to electrode surface chemistry. A dramatic improvement in signal-to-noise (S/N) ratios was observed when the chromatographically inert Silcosteel coated electrodes were used. The resulting method, that has significant S/N ratio improvements resulting from a combination of septum programmable injections (SPI), optimized CI and inert Silcosteel-coated electrodes, was used to determine instrument detection limits.

An Interface-Free-Energy Approach to Semiconductor Electrode Chemistry : Examples of Si and Ge

At a solid electrode, the solid/liquid interface free energy affects the wetting behavior of the liquid on the solid surface. This property may be used as a tool for monitoring changes in the electrode surface chemistry. Here such investigations have been carried out using contact angle measurements. The variations of the advancing and receding contact angles during a potential scan have been measured by a gravimetric method for Si and Ge electrodes, whose surfaces can be turned into a reduced or an oxidized state. For p-Si in dilute fluoride electrolyte, the change from the reduced state to the oxidized state (electropolishing regime) is associated with a large decrease of the contact angles. For Ge in or electrolyte, the change of the contact angles between the reduced state and the oxidized state is much smaller than for silicon, and is dependent upon crystal orientation, the (111) orientation actually giving an increase of the contact angles when the surface becomes oxidized. This strikingly different behavior for Si and Ge can be related to differences in the chemistry of these two electrodes in the oxidized state, namely the presence of a hydrated oxide on Si, as opposed to a hydroxyl monolayer on Ge. The present results demonstrate that gravimetric measurements of the contact angles can be turned into a convenient and sensitive technique for monitoring changes in interface chemistry. © 2004 The Electrochemical Society. All rights reserved.

XPS Investigation of Surface Chemistry of Magnesium Electrodes in Contact with Organic Solutions of Organochloroaluminate Complex Salts

Metallic magnesium can be reversibly deposited from ethereal solutions of magnesium−aluminum complex salts of the general structure Mg(AlCl4-nRn)2, where R = alkyl. In contrast to the case of lithium surfaces, already thoroughly investigated, the surface state of magnesium electrodes in contact with organic solutions is yet unclear. In this paper we report on a systematic surface analysis of magnesium electrodes in contact with various organic solutions, using XPS. We find in both clean tetrahydrofurane (THF) and THF solutions of dibutylmagnesium or butylmagnesium chloride that the metal surface consists of magnesium oxide and hydroxide (probably developed during manipulation and sample transfer); however, it does not develop thick passivation layers. In THF solutions containing Mg(AlEtBuCl2)2, surface residuals of carbon, aluminum, and chlorine are detected yet are restricted to the outermost part of the surface, as physically adsorbed species. From their concentration one deduces that both the complex salt and the ether are not reduced at the magnesium surface but precipitate as an insoluble film. Metallic magnesium deposited from THF/Mg(AlEtBuCl2)2 solution on gold electrodes shows a very similar surface chemistry, providing an additional proof that, even in the most frail conditions available during electrochemical deposition, pure magnesium is deposited. The surface chemistry of magnesium in contact with propylene carbonate (PC) exhibits as well layered surface chemistry, most of it composed of magnesium oxide and hydroxide, but no evidence is found for reduction products of PC. It is concluded that the magnesium metal behaves like a surface film-free electrode in organo-haloaluminate/THF solutions. Our conclusions support several other studies on the properties of magnesium in such solutions.

An analysis of rechargeable lithium-ion batteries after prolonged cycling

We report herein on extensive post-mortem analysis of rechargeable Li-ion batteries after prolonged cycling at 25 and 40 °C. The 18650-type batteries produced by standard procedure at LG Chem. Inc. were comprised of graphitic carbon anodes, LiCoO 2 cathodes, and a commercial, Li-battery grade ethylene carbonate, ethyl–methyl carbonate (EC-EMC)/LiPF 6 solution. The batteries were dismantled under highly pure inert atmosphere after prolonged cycling by a specially designed system, followed by analysis of both electrodes by SEM, XRD, FTIR, and electrochemical methods (voltammetry, impedance spectroscopy). The residual solution was analyzed by gas chromatography–high-resolution mass spectrometry (GCMS), and by Karl Fischer titration (for trace H 2O content). It was found that the structure of both electrodes is basically retained during prolonged cycling. However, they developed surface films that considerably increase their impedance, thus badly affecting the battery performance after a few hundred charge–discharge cycles. It also appears that part of the cathode's active mass became electronically isolated from the current collector. Hence, the impedance of both electrodes increases considerably upon cycling. The residual solutions contained 10–15 times more water than the pristine solution, as well as products of trans-esterification of the alkyl carbonates, and some fluorinated and phosphorous-containing organic species. The surface chemistry of both electrodes seems to be dominated primarily by LiF formation. Capacity fading mechanisms of Li-ion batteries are discussed.

Reactivity of lithium battery electrode materials toward non-aqueous electrolytes: spontaneous reactions at the electrode–electrolyte interface investigated by FTIR

Spontaneous reactions occurring at the surface of LiNi 0.8Co 0.2O 2 and LiMn 2O 4-based electrodes during the storage in organic non-aqueous electrolytes have been investigated by diffuse reflectance FTIR technique. It is found that both materials spontaneously form different inorganic and organic compounds on their surface when in contact with electrolyte solutions. The nature of these self-acting reactions is moreover found to be similar to that of the processes occurring during electrochemical cycling of the electrodes. Reaction mechanisms and the final products depend on both electrode surface chemistry and the nature of electrolyte used. It appears that the spontaneous reactions are initiated by lithium deintercalation from the electrode active material. The influence of different factors, e.g. degree of lithiation of the active material, roughness of the electrode surface and temperature on the reaction rate is discussed.

Studies of the interface between lithium electrodes and polymeric electrolyte systems using in situ FTIR spectroscopy

This paper describes studies of the interface between lithium electrodes and solid electrolyte systems using in situ FTIR spectroscopy in a single internal reflectance mode. In this method, the masking effect of the electrolyte matrix components is largely avoided. We studied gel electrolytes based on polyvinylidene difluoride-hexafluoropropylene with cyclic alkyl carbonates as plasticizers, suitable for ambient temperatures, and a solvent free polymer, derivatives of polyethylene oxide (PEO) with a branched structure: poly[ethyleneoxide-2-(2-methoxyethoxy)ethyl glycidyl ether] at elevated temperatures. We found that the surface chemistry of Li electrodes in contact with the gel matrixes is dominated by alkyl carbonate solvent reduction to ROCO 2Li surface species. In the case of the PEO-based polymer, the surface reactions of Li electrodes are dominated by salt and trace water reduction. The polymer itself seems to be stable with lithium even at 60°C.

Spectroelectrochemical studies of magnesium deposition by in situ FTIR spectroscopy

Magnesium can be reversibly deposited electrochemically from solutions of ethereal solvents, with Grignard reagents (RMgX) or complexes of Mg(AX 3− n R n+1 ) 2 stoichiometry as the electrolytes (A=Al, B; X=Cl, Br; R=alkyl or aryl groups). These processes are far from being simple reactions of the Mg/Mg ++ couple, since the above electrolytes in solutions have complicated structures in which the ether molecules play an important stabilization role. In addition, Mg deposition processes in all of the above solutions are accompanied by adsorption phenomena. The surface chemistry of magnesium electrodes was studied in situ by FTIR spectroscopy, using an internal reflectance mode. The electrolyte solutions studied included tetrahydrofuran (THF) solutions of the RMgX electrolytes (R=butyl, ethyl, methyl benzyl, and X=Cl, Br); Mg(AlCl 2BuEt) 2; Mg(AlCl 3Bu) 2 and Mg(BPh 2Bu 2); Bu, Et, Ph=butyl, ethyl and phenyl groups, respectively. It was clear from these studies that Mg electrodes do not develop stable passivation in these solutions (i.e. formation of surface films). The nature of the adsorbed species in the above systems is discussed, based on the spectral results.

Electrochemical and Electrical Behavior of (111)-Oriented Si Surfaces Alkoxylated through Oxidative Activation of Si−H Bonds

The reaction chemistry of H-terminated (111)-oriented Si surfaces has been explored in the presence of one-electron oxidants in alcohol solvents. Similar chemistry was observed for Si surfaces exposed to methanolic solutions of ferrocenium tetrafluoroborate or to methanolic solutions of I2, as well as for Si electrodes that were subjected to anodic electrochemical oxidation processes in methanol. Oxidation in alcohols activates the surficial Si−H bond toward nucleophilic attack, producing alkoxylated surfaces. This mechanism for Si−H activation is supported by infrared spectroscopy, temperature-programmed desorption spectroscopy, and by X-ray photoelectron spectroscopic studies of this system. These data explain a variety of observations regarding the surface chemistry and electrochemistry of Si electrodes and surfaces.

New insights into the interactions between electrode materials and electrolyte solutions for advanced nonaqueous batteries

In this paper we review some recent work with Li metal and Li–graphite anodes and LixMOy cathodes (M=transition metals such as Ni, Co, Mn). The emphasis was on the study of surface phenomena using in situ and ex situ FTIR spectroscopy, atomic force microscopy (in situ AFM), electrochemical quartz crystal microbalance (EQCM) and impedance spectroscopy (EIS). The performance of Li metal and Li–carbon anodes in secondary batteries depends on the nature of the surface films that cover them. The use of Li metal anodes requires the formation of highly uniform and elastic surface films. Thus, most of the commonly used liquid electrolyte solutions are not suitable for Li metal-based rechargeable batteries. In the case of Li–C-based batteries, the passivating films need not be elastic. Channeling the Li–C electrode surface chemistry towards the formation of Li2CO3 surface films provides adequate passivation for these electrodes. This can be achieved through the use of EC-based solutions of low EC concentration (cosolvents should be less reactive than EC). An interesting finding is that the behavior of many commonly used cathodes also depends on their surface chemistry, and that their overall Li insertion processes include the step of Li ion migration through surface films. Their origin is discussed herein, as well as possible oxidation processes of the relevant solutions.