arXiv subjects

Vijay A. Sethuraman

Publications and source records attributed to Vijay A. Sethuraman.

At least 19 recordsLinked to original sources

arXiv2020

Measurement of Volume Changes and Associated Stresses in Ge Electrodes Due to Na/Na$^+$ Redox Reactions

In situ electrochemical cells were assembled with an amorphous germanium (a-Ge) film as working electrode and sodium foil as reference and counter electrode. The stresses generated in a-Ge electrodes due to electrochemical reaction with sodium were measured in real-time during the galvanostatic cycling. A specially designed patterned a-Ge electrode was cycled against sodium and the corresponding volume changes were measured using an AFM; it was observed that sodiation/desodiation of a-Ge results in more than 300% volume change, consistent with literature. The potential and stress response showed that the a-Ge film undergoes irreversible changes during the first sodiation process, but the subsequent desodiation/sodiation cycles are reversible. The stress response of the film reached steady-state after the initial sodiation and is qualitatively similar to the response of Ge during lithiation, i.e., initial linear elastic response followed by extensive plastic deformation of the film to accommodate large volume changes. However, despite being bigger ion, sodiation of Ge generated lower stress levels compared to lithiation. Consequently, the mechanical dissipation losses associated with plastic deformation are lower during sodiation process than it is for lithiation.

cond-mat.mtrl-sci↗

arXiv2020

In-situ Synthesis of Bismuth/Reduced Graphene Oxide Nanocomposites as High Capacity Anode Materials for Mg-ion Battery

Herein, in-situ reduction of bismuth and graphene oxide was performed by a solvothermal method under a nitrogen atmosphere, and the resulting Bi on RGO nanocomposites were used as an anode material for Mg-ion batteries. The nanocomposite of 60% Bi:40% RGO is a beneficial anode material, delivering a discharge capacity as high as 413 and 372 mAh per g at the specific current of 39 mA per g in the 1st and 50th cycles, respectively. In addition, it shows high-rate capability with the discharge capacities of 381, 372, 354, 295, and 238 mAh per g at the specific currents of 53, 100, 200, 500, and 700 mA per g, respectively. The better electrochemical performance of the nanocomposite is due to improvement in the electronic conductivity and significant reduction of volume changes during electrochemical cycling. This study demonstrates the bismuth (Bi) on reduced graphene oxide (RGO) nanocomposite as a promising high-capacity anode for magnesium-ion batteries with longer life cycle and high-rate performance.

physics.app-ph↗

arXiv2020

The Influence of Elastic Strain on Catalytic Activity Towards the Hydrogen Evolution Reaction

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 towards 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 activity were accelerated by compression, while Cu 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.

physics.app-ph↗

arXiv2020

Mathematical Model of a Direct Methanol Fuel Cell

A one dimensional (1-D), isothermal model for a direct methanol fuel cell (DMFC) is presented. This model accounts for the kinetics of the multi-step methanol oxidation reaction at the anode. Diffusion and crossover of methanol are modeled and the mixed potential of the oxygen cathode due to methanol crossover is included. Kinetic and diffusional parameters are estimated by comparing the model to data from a 25 cm2 DMFC. This semi-analytical model can be solved rapidly so that it is suitable for inclusion in real-time system level DMFC simulations.

physics.chem-ph↗

arXiv2020

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Tin Coatings

Here, we study the role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass. 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 surface stress; this also allowed studying role of out-of-plane stress gradient in whisker growth. Both bulk stress and surface stress in 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 surface stresses readily established a negative out-of-plane stress gradient, required for spontaneous growth of whiskers. The importance of 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. It was consistently observed that whisker growth was more in the coatings under external tensile stress, however, with higher negative out-of-plane stress gradient. The results conclusively indicate the critical role of negative out-of-plane stress gradient on whisker growth, as compared to only the nature (i.e., sign and magnitude) of stress.

physics.app-ph↗

arXiv2020

Effect of Di-phenyl Siloxane on the Catalytic Activity of Pt on Carbon

The effect of silicone on the catalytic activity of Pt for oxygen reduction and hydrogen adsorption was studied using di-phenyl siloxane as a source compound at a rotating disc electrode (RDE). Di-phenyl siloxane did not affect the catalytic activity of Pt when it was injected into the electrolyte. However, it blocked the oxygen reduction reaction when it was premixed with the catalyst. Proton transport was not blocked in either case. We postulate that di-phenyl siloxane induces hydrophobicity and causes local water starvation thereby blocking oxygen transport. Hence, the slow leaching of silicone seals in a fuel cell could cause silicon accumulation in the electrode, which will irreversibly degrade fuel cell performance by blocking oxygen transport to the catalyst sites.

physics.app-ph↗

arXiv2020

Hydrogen Peroxide Formation Rates in a PEMFC Anode and Cathode: Effect of Humidity and Temperature

Hydrogen peroxide (H2O2) formation rates in a proton exchange membrane (PEM) fuel cell anode and cathode were estimated as a function of humidity and temperature by studying the oxygen reduction reaction (ORR) on a rotating ring disc electrode (RRDE). Fuel cell conditions were replicated by depositing a film of Pt/Vulcan XC-72 catalyst onto the disk and by varying the temperature, dissolved O2 concentration and the acidity levels in hydrochloric acid (HClO4). The HClO4 acidity was correlated to ionomer water activity and hence fuel cell humidity. The H2O2 formation rates showed a linear dependence on oxygen concentration and square dependence on water activity. The H2O2 selectivity in ORR was independent of oxygen concentration but increased with decrease in water activity (i.e., decreased humidity). Potential dependent activation energy for the H2O2 formation reaction was estimated from data obtained at different temperatures.

physics.app-ph↗

arXiv2020

Capacity Fade due to Side-reactions in Silicon Anodes in Lithium-ion Batteries

It is shown that continuously occurring electrolyte-reduction reaction on freshly-exposed electrode surfaces during lithiation/delithiation cycles causes the lowering of cycling efficiency, and hence, capacity fade in well-cycled silicon anodes in lithium-ion batteries. Using galvanostatic lithiation/delithiation data from multiple cycles on a Li/Si half-cell, a methodology to separate the charge due to the main reaction (lithiation/delithiation of Si) from the side-reaction (electrolyte-reduction) is presented. The rate of this parasitic side reaction is estimated on well-cycled amorphous silicon thin-film electrodes at ambient temperature for the following three commonly-used lithium-ion electrolyte formulations: mixtures of ethylene carbonate and diethylene carbonate (EC:DEC) with and without a fluoroethylene carbonate (FEC) additive, and propylene carbonate (PC), all containing 1.2 M lithium hexafluorophosphate. Among the three formulations, the electrolyte containing EC:DEC with the FEC additive exhibits the lowest coulombic losses due to side-reactions, followed by PC, and EC:DEC without the FEC additive (i.e., EC:DEC + FEC > PC > EC:DEC). The importance of estimating side-reaction rates on a well-cycled electrode is discussed in the context of self-discharge, capacity fade, development of battery management system algorithms and precise mathematical modeling of lithium-ion batteries.

cond-mat.mtrl-sci↗

arXiv2015

On the Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt

The effect of elastic strain on catalytic activity of platinum (Pt) towards oxygen reduction reaction (ORR) is investigated through de-alloyed Pt-Cu thin films; stress evolution in the de-alloyed layer and the mass of the Cu removed are measured in real-time during electrochemical de-alloying of (111)-textured thin-film PtCu (1:1, atom %) electrodes. In situ stress measurements are made using the cantilever-deflection method and nano-gravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon de-alloying via successive voltammetric sweeps between -0.05 and 1.15 V vs. standard hydrogen electrode, compressive stress develops in the de-alloyed Pt layer at the surface of thin-film PtCu electrodes. The de-alloyed films also exhibit enhanced catalytic activity towards ORR compared to polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of de-alloyed Cu is approximately 210 +/- 46 ng/cm2, which corresponds to a de-alloyed layer thickness of 1.2 +/- 0.3 monolayers or 0.16 +/- 0.04 nm. The average biaxial stress in the de-alloyed layer is estimated to be 4.95 +/- 1.3 GPa, which corresponds to an elastic strain of 1.47 +/- 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt (111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes.

cond-mat.mtrl-sci↗

arXiv2015

Investigation of Initial Lithiation of Silicon (100) Using Solid-State 7Li NMR

Structural changes in amorphous LixSi during the initial lithiation of single crystal Si to amorphous LixSi were investigated as a function of Li flux and total charge using solid-state 7Li nuclear magnetic resonance (NMR) spectroscopy. Single crystal (100) Si wafers were used as a model system in this study, as the Li flux could be precisely controlled and uniformly distributed across the surface of the wafer. It was observed that peak shifts in solid-state 7Li NMR spectra varied as a function of the Li flux during the initial lithiation and stayed constant for samples of the same Li flux regardless of the total charge applied to the electrodes. We conclude from these results that the Li concentration in LixSi stays constant for a given flux regardless of the total coulombic charge applied to the electrode. The results from this study help better understand the kinetics, the reaction mechanisms, and the kinetic modeling of chemical reactions at the reaction front during the initial lithiation of Si (100).

cond-mat.mtrl-sci↗

arXiv2015

Stress Evolution in Lithium-ion Composite Electrodes during Electrochemical Cycling and Resulting Internal Pressures on the Cell Casing

Composite cathode coatings made of a high energy density layered oxide (Li1.2Ni0.15Mn0.55Co0.1O2, theoretical capacity ~377 mAh/g), polyvinylidene fluoride (PVdF) binder, and electron-conduction additives, were bonded to an elastic substrate. An electrochemical cell, built by pairing the cathode with a capacity-matched graphite anode, was electrochemically cycled and the real-time average stress evolution in the cathode coating was measured using a substrate-curvature technique. Features in the stress evolution profile showed correlations with phase changes in the oxide, thus yielding data complementary to in situ XRD studies on this material. The stress evolution showed a complex variation with lithium concentration suggesting that the volume changes associated with phase transformations in the oxide are not monotonically varying functions of lithium concentration. The peak tensile stress in the cathode during oxide delithiation was approximately 1.5 MPa and the peak compressive stress during oxide lithiation was about 6 MPa. Stress evolution in the anode coating was also measured separately using the same technique. The measured stresses are used to estimate the internal pressures that develop in a cylindrical lithium-ion cell with jelly-roll electrodes.

cond-mat.mtrl-sci↗

arXiv2015

Real-time Stress Measurements in Germanium Thin Film Electrodes during Electrochemical Lithiation/delithiation Cycling

An in situ study of stress evolution and mechanical behavior of germanium as a lithium-ion battery electrode material is presented. Thin films of germanium are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M LiPF6 in ethylene carbonate, diethyl carbonate, dimethyl carbonate solution (1:1:1, wt. %) as electrolyte. Real-time stress evolution in the germanium thin-film electrodes during electrochemical lithiation/delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. Upon lithiation a-Ge undergoes extensive plastic deformation, with a peak compressive stress reaching as high as -0.76 +/- 0.05 GPa (mean +/- standard deviation). The compressive stress decreases with lithium concentration reaching a value of approximately -0.3 GPa at the end of lithiation. Upon delithiation the stress quickly became tensile and follows a trend that mirrors the behavior on compressive side; the average peak tensile stress of the lithiated Ge samples was approximately 0.83 GPa. The peak tensile stress data along with the SEM analysis was used to estimate a lower bound fracture resistance of lithiated Ge, which is approximately 5.3 J/m^2. It was also observed that the lithiated Ge is rate sensitive, i.e., stress depends on how fast or slow the charging is carried out.

cond-mat.mtrl-sci↗

arXiv2013

Measurement and modeling of the mechanical and electrochemical response of amorphous Si thin film electrodes during cyclic lithiation

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.

cond-mat.mtrl-sci↗

arXiv2013

On plastic deformation and fracture in Si films during electrochemical lithiation/delithiation cycling

An in situ study of deformation, fracture, and fatigue behavior of silicon as a lithium-ion battery electrode material is presented. Thin films (100-200 nm) of silicon are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M lithium hexafluorophosphate in ethylene carbonate, diethylene carbonate, dimethyl carbonate solution (1:1:1, wt.%) as electrolyte. Stress evolution in the Si thin-film electrodes during electrochemical lithiation and delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. The stress measurements have been corrected for contributions from residual stress arising from sputter-deposition. An indirect method for estimating the potential errors due to formation of the solid-electrolyte-interphase layer and surface charge on the stress measurements was presented. The films undergo extensive inelastic deformation during lithiation and delithiation. The peak compressive stress during lithiation was 1.48 GPa. The stress data along with the electron microscopy observations are used to estimate an upper bound fracture resistance of lithiated Si, which is approximately 9-11 J/m^2. Fracture initiation and crack density evolution as a function of cycle number is also reported.

cond-mat.mtrl-sci↗

arXiv2013

Parameter Estimates for a Polymer Electrolyte Membrane Fuel Cell Cathode

Five parameters of a model of a polymer electrolyte membrane fuel cell cathode (the porosity of the gas diffusion layer, the porosity of the catalyst layer, the exchange current density of the oxygen reduction reaction, the effective ionic conductivity of the electrolyte, and the ratio of the effective diffusion coefficient of oxygen in a flooded spherical agglomerate particle to the squared particle radius) were determined by the least square fitting of experimental polarization curves. The values of parameters obtained in this work indicate that ionic conduction and gas-phase transport are two processes significantly influencing the performance of PEMFC air cathodes. While ionic conduction influences cathode performance over a wide range of current densities, gas-phase transport influences cathode performance only at high current densities.

cond-mat.mtrl-sci↗

arXiv2013

Quantifying Oxidation Rates of Carbon Monoxide on a Pt/C Electrode

The electrochemical oxidation of carbon monoxide adsorbed (COad) on platinum-on-carbon electrodes was studied via a methodology in which pre-adsorbed CO was partially oxidized by applying potentiostatic pulses for certain durations. The residual COad was analyzed using stripping voltammetry that involved the deconvolution of COad oxidation peaks of voltammograms to quantify the weakly and strongly bound species of COad. The data obtained for various potentials and temperatures were fit to a model based on a nucleation and growth mechanism. The resulting fit produced potential- and temperature-dependent rate parameters that provided insight into the oxidation mechanism of the two COad species. Irrespective of the applied potential or temperature, the concentration of weakly bound COad species decreased exponentially with time. In contrast, the strongly bound COad species showed a gradual transition of mechanisms, from progressive nucleation at relatively low potentials to exponential decay at high potentials.

cond-mat.mtrl-sci↗

arXiv2013

Importance of Catalyst Stability vis-à-vis Hydrogen Peroxide Formation Rates in PEM Fuel Cell Electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H2O2) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts. The selectivity of these catalysts towards H2O2 formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H2O2 formation rates in a PEMFC anode and cathode. The effect of H2O2 formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H2O2 formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.

cond-mat.mtrl-sci↗

arXiv2013

Stress Evolution in Composite Silicon Electrodes during Lithiation/Delithiation

We report real-time average stress measurements on composite silicon electrodes made with two different binders [Carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF)] during electrochemical lithiation and delithiation. During galvanostatic lithiation at very slow rates, the stress in a CMC-based electrode becomes compressive and increases to 70 MPa, where it reaches a plateau and increases slowly thereafter with capacity. The PVDF-based electrode exhibits similar behavior, although with lower peak compressive stress of about 12 MPa. These initial experiments indicate that the stress evolution in a Si composite electrode depends strongly on the mechanical properties of the binder. Stress data obtained from a series of lithiation/delithiation cycles suggests plasticity induced irreversible shape changes in contacting Si particles, and as a result, the stress response of the system during any given lithiation/delithiation cycle depends on the cycling history of the electrode. While these results constitute the first in-situ stress measurements on composite Si electrodes during electrochemical cycling, the diagnostic technique described herein can be used to assess the mechanical response of a composite electrode made with other active material/binder combinations.

cond-mat.mtrl-sci↗