Understanding the role of elastic strain in modifying catalytic reaction rates is crucial for catalyst design, but experimentally, this effect is often coupled with a ligand effect. To isolate the ...strain effect, we have investigated the influence of externally applied elastic strain on the catalytic activity of metal films in the hydrogen evolution reaction (HER). We show that elastic strain tunes the catalytic activity in a controlled and predictable way. Both theory and experiment show strain controls reactivity in a controlled manner consistent with the qualitative predictions of the HER volcano plot and the d‐band theory: Ni and Pt's activities were accelerated by compression, while Cu's activity was accelerated by tension. By isolating the elastic strain effect from the ligand effect, this study provides a greater insight into the role of elastic strain in controlling electrocatalytic activity.
Strain‐tuned catalysis: Elastic strain affects catalytic reaction rates, but is often coupled with a ligand effect. To isolate the strain effect, three thin metal films on elastic substrates were subjected to uniaxial loading and unloading, and investigated for their activity in the hydrogen evolution reaction.
We report
in situ measurements of stress evolution in a silicon thin-film electrode during electrochemical lithiation and delithiation by using the multi-beam optical sensor (MOS) technique. Upon ...lithiation, due to substrate constraint, the silicon electrode initially undergoes elastic deformation, resulting in rapid rise of compressive stress. The electrode begins to deform plastically at a compressive stress of
ca. −1.75
GPa; subsequent lithiation results in continued plastic strain, dissipating mechanical energy. Upon delithiation, the electrode first undergoes elastic straining in the opposite direction, leading to a tensile stress of
ca. 1
GPa; subsequently, it deforms plastically during the rest of delithiation. The plastic flow stress evolves continuously with lithium concentration. Thus, mechanical energy is dissipated in plastic deformation during both lithiation and delithiation, and it can be calculated from the stress measurements; we show that it is comparable to the polarization loss. Upon current interruption, both the film stress and the electrode potential relax with similar time constants, suggesting that stress contributes significantly to the chemical potential of lithiated silicon.
We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that ...cycling at potentials that correspond to low lithium concentrations in Li
x
C (0
≤
x
<
0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (
x
>
0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (
U
<
0.15
V
vs. Li/Li
+). Steep Li
+ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li
x
C
d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C–C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.
Cycling efficiency and rate capability of porous copper-coated, amorphous silicon thin-film negative electrodes are compared to equivalent silicon thin-film electrodes in lithium-ion batteries. The ...presence of a copper layer coated on the active material plays a beneficial role in increasing the cycling efficiency and the rate capability of silicon thin-film electrodes. Between 3C and C/8 discharge rates, the available cell energy decreased by 8% and 18% for 40
nm copper-coated silicon and equivalent silicon thin-film electrodes, respectively. Copper-coated silicon thin-film electrodes also show higher cycling efficiency, resulting in lower capacity fade, than equivalent silicon thin-film electrodes. We believe that copper appears to act as a glue that binds the electrode together and prevents the electronic isolation of silicon particles, thereby decreasing capacity loss. Rate capability decreases significantly at higher copper coating thicknesses as the silicon active material is not accessed, suggesting that the thickness and porosity of the copper coating need to be optimized for enhanced capacity retention and rate capability in this system.
A combination of experimental measurements and numerical simulations are used to characterize the mechanical and electrochemical response of thin film amorphous Si electrodes during cyclic ...lithiation. Parameters extracted from the experiment include the variation of elastic modulus and the flow stress as functions of Li concentration; the strain rate sensitivity; the diffusion coefficient for Li transport in the electrode; the free energy of mixing as a function of Li concentration in the electrode; the exchange current density for the Lithium insertion reaction; as well as reaction rates and diffusion coefficients characterizing the rate of formation of solid-electrolyte interphase layer at the electrode surface. Model predictions are compared with experimental measurements; and the implications for practical Si based electrodes are discussed.
A novel drug targeting system for acidic solid tumors has been developed based on ultra pH-sensitive polymer and cell penetrating TAT. The delivery system consisted of two components: 1) A polymeric ...micelle that has a hydrophobic core made of poly(
l-lactic acid) (PLLA) and a hydrophilic shell consisting of polyethylene glycol (PEG) conjugated to TAT (TAT micelle), 2) an ultra pH-sensitive diblock copolymer of poly(methacryloyl sulfadimethoxine) (PSD) and PEG (PSD-
b-PEG). The anionic PSD is complexed with cationic TAT of the micelles to achieve the final carrier, which could systemically shield the micelles and expose them at slightly acidic tumor pH. TAT micelles had particle sizes between 20 and 45 nm and their critical micelle concentrations were 3.5 mg/l to 5.5 mg/l. The TAT micelles, upon mixing with pH-sensitive PSD-
b-PEG, showed a slight increase in particle size between pH 8.0 and 6.8 (60–90 nm), indicating complexation. As the pH was decreased (pH 6.6 to 6.0) two populations were observed, one that of normal TAT micelles (45 nm) and the other of aggregated hydrophobic PSD-
b-PEG. Zeta potential measurements showed similar trend substantiating the shielding/deshielding process. Flow cytometry and confocal microscopy showed significantly higher uptake of TAT micelles at pH 6.6 compared to pH 7.4 indicating shielding at normal pH and deshielding at tumor pH. The confocal microscopy indicated that the TAT not only translocates into the cells but is also seen on the surface of the nucleus. These results strongly indicate that the above micelles would be able to target any hydrophobic drug near the nucleus.
The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer ...electrolyte membrane (PEM) fuel cell. At 70
°C, sequential hydrogen sulfide (H
2S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H
2S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H
2/O
2 polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H
2S on Pt anode is presented. H
2S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H
2S. As an implication, the local potential of a PEM fuel cell anode exposed to H
2S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.
Lithium Diffusion in Graphitic Carbon Persson, Kristin; Sethuraman, Vijay A; Hardwick, Laurence J ...
The journal of physical chemistry letters,
04/2010, Letnik:
1, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Graphitic carbon is currently considered the state-of-the-art material for the negative electrode in lithium ion cells, mainly due to its high reversibility and low operating potential. However, ...carbon anodes exhibit mediocre charge/discharge rate performance, which contributes to severe transport-induced surface structural damage upon prolonged cycling and limits the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized nonisotropic particles. To solve this problem for graphite, we use the Devanathan−Stachurski electrochemical methodology combined with ab initio computations to deconvolute and quantify the mechanism of lithium ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium ion diffusivity in the direction parallel to the graphene plane (∼10−7−10−6 cm2 s−1), as compared to sluggish lithium ion transport along grain boundaries (∼10−11 cm2 s−1), indicating the possibility of rational design of carbonaceous materials and composite electrodes with very high rate capability.
The role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass is examined. The bulk stress in Sn coatings was measured using a laser-optics-based curvature ...setup, whereas glancing angle x-ray diffraction was employed to quantify the stress near the surface; this also allowed studying the role of the out-of-plane stress gradient in whisker growth. Both bulk stress and near-surface stress in the Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and near-surface stresses readily reveals establishment of a negative out-of-plane stress gradient, which is required for the spontaneous growth of whiskers. The importance of the out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. Additional whisker growth occurred in the coatings subjected to external tensile stress; however, this was accompanied by a higher negative out-of-plane stress gradient. The results conclusively demonstrate the important role of the negative out-of-plane stress gradient on whisker growth, as compared to only sign and magnitude of stress.
Charge lost per unit surface area of a silicon electrode due to the formation of solid-electrolyte-interphase (SEI) layer during initial lithiation was quantified, and the species that constitute ...this layer were identified. Coin cells made with Si thin-film electrodes were subjected to a combination of galvanostatic and potentiostatic lithiation and delithiation cycles to accurately measure the capacity lost to SEI layer formation. While the planar geometry of amorphous thin films allows accurate calculation of surface area, creation of additional surface by cracking was prevented by minimizing the thickness of the Si film. The cycled electrodes were analyzed with X-ray photoelectron spectroscopy to characterize the composition of the SEI layer. The charge lost due to SEI formation measured from coin cell experiments was found to be in good agreement with the first-cycle capacity loss during the initial lithiation of a Si(100) crystal with planar geometry. The methodology presented in this work is expected to provide a useful practical tool for battery-material developers in estimating the expected capacity loss due to first cycle SEI layer formation and in choosing an appropriate particle size distribution that balances mechanical integrity and the first cycle capacity loss in large volume expansion electrodes for lithium-ion batteries.
► A method to quantify capacity loss per unit area due to initial SEI formation on Si. ► Identify species that constitute the SEI layer. ► Reported data can be used to predict first-cycle capacity loss on any geometry.
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