Surface combinatorial studies of IR properties of nanostructured Ru film electrodes using CO as probe molecule

Influence of Carbonate Solvents on Solid Electrolyte Interphase Composition over Si Electrodes Monitored by In Situ and Ex Situ Spectroscopies

A solid electrolyte interphase (SEI) layer on Si-based anodes should have high mechanical properties to adapt the volume changes of Si with low thickness and good ionic conductivity. To better understand the influence of carbonate solvents on the SEI composition and mechanism of formation, systematic studies were performed using dimethyl carbonate (DMC) or propylene carbonate (PC) solvent and LiPF₆ as a salt. A 1 M LiPF₆/EC-DMC was used for comparison. The surface chemical composition of the Si electrode was analyzed at different potentials of lithiation/delithiation and after a few cycles. Ex situ X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry results demonstrate that a thinner and more stable SEI layer is formed in LiPF₆/DMC. The in situ Fourier transform infrared spectroscopy proves that the coordination between Li⁺ and DMC is weaker, and fewer DMC molecules take part in the formation of the SEI layer. The higher capacity retention during 60 cycles and less significant morphological modifications of the Si electrode in 1 M LiPF₆/DMC compared to other electrolytes were demonstrated, confirming a good and stable interfacial layer. The possible surface reactions are discussed, and the difference in the mechanisms of formation of SEI in these three various electrolytes is proposed.

Surface Segregation in Bimetallic Nanoparticles: A Critical Issue in Electrocatalyst Engineering

Bimetallic nanoparticles are a class of important electrocatalyst. They exhibit a synergistic effect that critically depends on the surface composition, which determines the surface properties and the adsorption/desorption behavior of the reactants and intermediates during catalysis. The surface composition can be varied, as nanoparticles are exposed to certain environments through surface segregation. Thermodynamically, this is caused by a difference in surface energy between the two metals. It may lead to the enrichment of one metal on the surface and the other in the core. The external conditions that influence the surface energy may lead to the variation of the thermodynamic steady state of the particle surface and, thus, offer a chance to vary the surface composition. In this review, the most recent and important progress in surface segregation of bimetallic nanoparticles and its impact in electrocatalysis are introduced. Typical segregation inducements and surface characterization techniques are discussed in detail. It is concluded that surface segregation is a critical issue when designing bimetallic catalysts. It is necessary to explore methods to control it and utilize it as a way towards producing robust, bimetallic electrocatalysts.

In-situ infrared spectroscopic studies of electrochemical energy conversion and storage

With their ability to convert chemical energy of fuels directly into electrical power or reversibly store electrical energy, systems such as fuel cells and lithium ion batteries are of great importance in managing energy use. In these electrochemical energy conversion and storage (EECS) systems, controlled electrochemical redox reactions generate or store the electrical energy, ideally under conditions that avoid or kinetically suppress side reactions. A comprehensive understanding of electrode reactions is critical for the exploration and optimization of electrode materials and is therefore the key issue for developing advanced EECS systems. Based on its fingerprint and surface selection rules, electrochemical in-situ FTIR spectroscopy (in-situ FTIRS) can provide real-time information about the chemical nature of adsorbates and solution species as well as intermediate/product species involved in the electrochemical reactions. These unique features make this technique well-suited for insitu studies of EECS. In this Account, we review the characterization of electrode materials and the investigation of interfacial reaction processes involved in EECS systems by using state-of-the-art in-situ FTIR reflection technologies, primarily with an external configuration. We introduce the application of in-situ FTIRS to EECS systems and describe relevant technologies including in-situ microscope FTIRS, in-situ time-resolved FTIRS, and the combinatorial FTIRS approach. We focus first on the in-situ steady-state and time-resolved FTIRS studies on the electrooxidation of small organic molecules. Next, we review the characterization of electrocatalysts through the IR properties of nanomaterials, such as abnormal IR effects (AIREs) and surface enhanced infrared absorption (SEIRA). Finally, we introduce the application of in-situ FTIRS to demonstrate the decomposition of electrolyte and (de)lithiation processes involved in lithium ion batteries. The body of work summarized here has substantially advanced the knowledge of electrode processes and represents the forefront in studies of EECS at the molecular level.

Interface for time-resolved electrochemical infrared microspectroscopy using synchrotron infrared radiation

A description of a coupled electrochemical and spectrometer interface using synchrotron infrared radiation is provided. The interface described allows for the precise and accurate timing needed for time-resolved IR spectroscopic studies of electrochemical systems. The overall interface uses a series of transistor-transistor logic trigger signals generated from the commercial FTIR spectrometer to regulate the recording of control, electrochemical, and IR signals with reproducible and adjustable timing. The instrument has been tested using a thin-layer electrochemical cell with synchrotron light focused through microscope optics. The time-resolved response of the benzoquinone/dihydroxybenzoquinone redox couple is illustrated as an example of the instrument's capability.

Electrochemical preparation and structural characterization of Co thin films and their anomalous IR properties

Nanometer scale cobalt thin films of different structures and thicknesses supported on glassy carbon were prepared by electrochemical deposition under cyclic voltammetric conditions (denoted nm-Co/GC(n)). The thickness of Co thin films was altered systematically by varying the number (n) of potential cycling within a defined potential range in electrodeposition. Electrochemical in situ scanning tunneling microscopy (STM) and ex situ scanning electron microscopy (SEM) were employed to characterize the surface structure of Co thin films. It has been illustrated that the Co thin films were uniformly composed of Co nanoparticles, whose structure and size varied with increasing n. The structure of nanoparticles inside the Co thin films underwent a transition from bearded nanoparticles to multiform nanoparticles and finally to hexagonal nanosheets, accompanying with an increase of average size. In situ FTIR reflection spectroscopic studies employing CO adsorption as probe reaction revealed that the Co thin films all exhibited anomalous IR properties; that is, along with their different nanostructures they presented abnormal IR effects, Fano-like IR effects, and surface-enhanced IR absorption effects. CO adsorbed on Co thin films dominated by bearded nanoparticles yielded abnormal IR absorption bands; that is, the direction of the bands is inverted completely, with enhanced intensity in comparison with those of CO adsorbed on a bulk Co electrode. The enhancement of abnormal IR absorption has reached a maximal value of 26.2 on the nm-Co/GC(2) electrode. Fano-like IR features, which describe the bipolar IR bands with their positive-going peak on the low wavenumbers side, were observed in cases of CO adsorbed on Co thin films composed mainly of multiform nanoparticles, typically on the nm-Co/GC(8) electrode. IR features were finally changed into surface-enhanced IR absorption as CO adsorbed on the nm-Co/GC(30) electrode, on which the Co thin film is dominated by Co hexagonal nanosheets.

Ubiquitous Strategy for Probing ATR Surface-Enhanced Infrared Absorption at Platinum Group Metal−Electrolyte Interfaces

A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers. Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently "pinhole-free" overlayers as evidenced by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed.

In Situ STM Studies of Electrochemical Growth of Nanostructured Ni Films and Their Anomalous IR Properties

We have extended the study of anomalous IR properties, which were initially discovered on nanostructured films of platinum group metals and alloys, to nanostructured films of nickel, a member of the iron group triad, and broadened the fundamental knowledge on this subject. Nanostructured thin films of nickel supported on glassy carbon [nm-Ni/GC(n)] were prepared by electrochemical deposition under cyclic voltammetric conditions, and the thickness of films was altered systematically by varying the number (n) of potential cycling within a defined potential range for electrodeposition. Electrochemical in situ scanning tunneling microscopy (STM) was employed to monitor the electrochemical growth of nanostructured Ni films. These in situ STM images illustrated that, along the increase of the film thickness, Ni films have undergone a transformation from layer structure to island structure and finally to lumpish arris structure. Investigations by in situ FTIR spectroscopy employing adsorbed CO as the probe revealed that these nanostructures of Ni films yield abnormal IR features, Fano-like IR features, and normal IR features, respectively. The IR bands of CO adsorbed on Ni thin films of a layer structure were inverted in their direction and enhanced in their intensity up to 15.5 times on an nm-Ni/GC(4) electrode. The Fano-like IR features, which are defined as a bipolar band with its negative-going peak on the low wavenumber side and its positive-going peak on the high wavenumber side, are observed for the first time on Ni thin films of an island nanostructure, i.e., at the nm-Ni/GC(16) electrode. IR features changed to normal absorption in CO adsorbed on the nm-Ni/GC(25) electrode, i.e., that with lumpish arris nanostructured Ni film of a larger thickness.

Anomalous IR properties of nanostructured films created by square wave potential on an array of Pt microelectrodes: an in situ microscope FTIRS study of CO adsorption

Thin films of different nanostructures on an array of nine Pt microelectrodes were prepared by applying a square wave potential treatment for different times (tau). It has been measured from the cyclic voltammetric studies that the relative surface roughness of the films was increased slightly and reached a maximal value of about 2.5. SEM studies demonstrated that with the increase of tau, the growth of island-shaped Pt crystallites on the films led to the formation of plumelike crystallites that can reach about 2-3.5 microm in length when tau exceeded 70 min. In situ microscope FTIR reflection spectroscopic studies illuminated that CO adsorbed on the array yielded different anomalous IR features. With the increase of tau, the direction of the CO L band (linearly bonded CO) was transformed from the negative-going direction (normal IR adsorption) to bipolar (Fano-like spectral line shape) and finally to the positive-going direction (abnormal IR adsorption). The intensity of the CO L band was enhanced significantly and a maximal enhancement factor of about 33 was measured when tau was 40 min; the center of the CO L band and the Stark tuning rate also showed regular changes. This study demonstrated that specific nanostructures of Pt thin films can be prepared through a square wave potential treatment for different times and revealed the intrinsic relationship between anomalous IR properties and surface nanostructures of the thin films.

Abnormal IR effects of Pt nanostructured surfaces upon CO chemisorption due to interaction and electron-hole damping

The abnormal IR effects (AIREs) characterized by a positive-going peak of platinum (Pt) nanostructured surface generated in a square-wave potential treatment upon CO molecule chemisorption was observed and analyzed with a consideration of the interparticle interaction and electron-hole damping between nanoislands and CO molecules. A theoretical simulation shows that the islanded nanostructured Pt surfaces, which gives rise to interparticle interaction, coupling with electron-hole mechanism, may contribute to the origins of positive-going peak (AIREs) observed by in situ Fourier transformation IR (FTIR) experiments.