Charge Transfer Step Research Articles

First-Principles Based Microkinetic Modeling of CO2 Reduction at the Ni/SDC Cathode of a Solid Oxide Electrolysis Cell

Understanding of the CO2 electroreduction mechanism at the three-phase boundary (TPB) is of great importance for the development of a solid oxide electrolysis cell (SOEC). In this study, the effect of oxygen vacancy locations on the CO2 reduction reaction (CO2RR) at the TPB of Ni(111)/samarium-doped ceria (SDC) surface was investigated using periodic density functional theory (DFT) + U calculations. It was found that interface oxygen vacancy can notably boost CO2 adsorption and reduction. Based on DFT results, a microkinetic analysis was conducted to determine the rate-controlling step under various solid oxide electrolysis cell operating voltages at 1000 K. Possible charge transfer steps, including one- or two-electron charge transfer, were considered and discussed. The analysis reveals that, on Ni/SDC with noninterface oxygen vacancy, the rate-controlling step will change from the oxygen spillover step to the CO desorption step with an increase in cathode overpotential. On Ni/SDC with interface oxygen v...

Heterobinuclear Light Absorber Coupled to Molecular Wire for Charge Transport across Ultrathin Silica Membrane for Artificial Photosynthesis.

Coupling of robust, all-inorganic heterobinuclear light absorbers to metal oxide catalysts for water oxidation across an ultrathin product-separating silica membrane requires charge transfer through organic molecular wires embedded in the silica. A synthetic approach for assembling the bimetallic units on the silica surface is introduced that is compatible with the presence of encapsulated organic molecules. Accurate selection and fine tuning of the concentration of embedded conducting wires are enabled by a two-step method consisting of surface attachment of a tripodal anchor, trimethoxysilyl aniline, followed by attachment of p-oligo(phenylene vinylene) through amide linkage. Each step of the assembly process was monitored and characterized by a combination of Fourier transform infrared, Fourier transform-Raman, and UV-vis spectroscopy techniques. Hole transfer was observed from transient CoIII, formed by TiIVOCoII → TiIIIOCoIII charge transfer excitation of the chromophore, to p-oligo(phenylene vinylene) molecule within the 8 ns width of the photolysis laser pulse by transient optical absorption spectroscopy of the wire radical cation. The rectifying property of the light absorber-wire assembly enabled by appropriate selection of redox potentials of metals and embedded wire obviates the need for a molecularly defined linkage between the components. Combined with the previously observed ultrafast hole injection from the embedded wires to Co oxide catalyst, the result implies visible-light-induced hole transfer from visible-light-excited binuclear light absorber to water oxidation catalyst across the silica separation membrane in a few nanoseconds or faster. Demonstration and understanding of this interfacial charge-transfer step is critical for developing nanoscale core-shell architectures for complete photosynthetic cycles.

Interface Engineering with Ionic Liquid Composite Materials for Efficient and Durable Electrocatalysis

Polymer electrolyte membrane fuel cells (PEMFCs) are among the most promising candidates in carbon neutral renewable energy systems because of their superior efficiency, environmentally friendly principles, as well as a wide range of potential applications 1. The development of catalytic materials for electrochemical oxygen reduction has received significant attention as the sluggish kinetics of cathodic oxygen reduction reaction (ORR) is the major source of efficiency loss in current PEMFCs. To this end, significant achievements have been made in the past decade to improve both intrinsic and mass-based activity through the evolution of nanoscale morphology and the compositional profile 2-4. However, these adjustments on geometric and compositional profiles remain insufficient for the widespread integration of fuel cells into industry applications 5. It is necessary to look for new strategies, such as shifting focus away from the catalyst and move toward the optimization of electrocatalytic interfaces. Electrocatalyst development for reactions involving multiple charge transfer steps and more than one adsorbed intermediate is severely limited by the scaling relations that govern the free energy of adsorption of those intermediates. Adsorbed intermediate scaling relations create a thermodynamic minimum overpotential of 0.37 V due to the fixed difference in free energy of adsorption (ΔΔGad) for *OH and *OOH of ~3.2 eV which is independent of the identity of the catalyst 1,6. It is found that by decreasing the degree of intermediate solvation, ΔΔGad can approach 2.46 eV which is the value required for ORR to take place at zero overpotential 7. Here we present our progress in addressing intermediate scaling for the ORR through the manipulation of metal/electrolyte interface with chemically tailored ionic liquids (ILs). Figure 1 demonstrates the impact of IL on the ORR on Pt(111). A significant increase in the ORR current is observed at both low overpotential and the potential region at which an adsorbed monolayer of hydrogen is formed on Pt. At low overpotential we attribute the enhanced activity to the disruption of *OH-H2O hydrogen bonded structure due to the control of the interaction of water with the catalytic surface in the presence of the IL. We argue that it is this change in water interaction with the surface, potentially in combination with other properties of the IL including reactant solubility, that results in the enhanced ORR activity. We also studied the manner by which ILs impact the stability of 3-dimensional, nanoporous nanoparticle electrocatalysts, such as nanoporous PtNi nanoparticles (np-NiPt). In addition to the standard mechanisms of catalyst degradation including Pt dissolution/Ostwald ripening and coalescence/aggregation, nanoporous materials can lose electrochemically active surface area (ECSA) through surface smoothening driven coarsening 8. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategies to mitigate coarsening through incorporating ILs at the metal/electrolyte interface to directly mitigate electrochemically enhanced surface diffusion, which leads to ECSA loss. The hydrophobicity of these ILs is known to change the interaction of water with the surface, positively shifting the onset potential of Pt oxidation. The interfacial IL acts to limit the charge dependent formation of a surface metal/electrolyte anion complex which is responsible for the potential dependent enhancement in surface diffusion. The oxygen reduction reaction activity, ECSA of these complex composite nanoporous electrocatalysts, as shown in Figure 2, was tracked as a function of cycle number during accelerated durability testing (ADT) providing insight into the mechanism of catalyst degradation and the effect of ILs on catalyst stability. Nørskov, J. K. et al., Phys. Chem. B 108 (2004) 17886–17892.Stamenkovic, V. R. et al., Nature Material 6 (2007) 241–247.Chen, C. et al., Science 343 (2014) 1339-1343.Snyder, J. et al. Am. Chem. Soc. 134 (2012) 8633-8645.Gasterger, H. A. et al. Applied Catalysis B: Environmental 56 (2005) 9-35.Hansen, H. A., et al., Phys. Chem. C 118 (2014) 6706–6718.Viswanathan, V., et al., Catal., 27 (2014) 215-221.Li, Y. et al., ACS Catal., 7 (2017) 7995-8005. Figure 1

Elementary reaction modeling of reversible CO/CO2 electrochemical conversion on patterned nickel electrodes

CO/CO2 are the major gas reactant/product in the fuel electrode of reversible solid oxide cells (RSOC). This study proposes a two-charge-transfer-step mechanism to describe the reaction and transfer processes of CO-CO2 electrochemical conversion on a patterned Ni electrode of RSOC. An elementary reaction model is developed to couple two charge transfer reactions, C(Ni)+O2−(YSZ) ↔ CO(Ni)+(YSZ) +2e− and CO(Ni)+O2−(YSZ) ↔ CO2(Ni)+(YSZ)+2e−, with adsorption/desorption, surface chemical reactions and surface diffusion. This model well validates in both solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes by the experimental data from a patterned Ni electrode with 10 μm stripe width at different pCO (0–0.25 atm), pCO2 (0–0.35 atm) and operating temperature (600-700 °C). This model indicates SOEC mode is dominated by charge transfer step C(Ni)+O2−(YSZ)↔CO(Ni)+(YSZ) +2e−, while SOFC mode by CO(Ni)+ O2−(YSZ)↔CO2(Ni)+(YSZ)+2e− on the patterned Ni electrode. The sensitivity analysis shows charge transfer step is the major rate-determining step for RSOC, besides, surface diffusion of CO and CO2 as well as CO2 adsorption also plays a significant role in the electrochemical reaction of SOEC while surface diffusion of CO and CO2 desorption could be co-limiting in SOFC.

Aggregation of P3HT as a preferred pathway for its chemical doping with F4-TCNQ.

The chemical doping reaction of P3HT with F4-TCNQ was studied by optical absorption spectroscopy, atomic force microscopy, and Kelvin probe force microscopy. We demonstrate that P3HT aggregation preferentially occurs before the actual charge transfer step takes place, emphasizing the impact of morphology on the chemical doping reaction of conjugated polymers at the molecular level.

Ultrafast bridge planarization in donor-\u03c0-acceptor copolymers drives intramolecular charge transfer

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

Charge migration and charge transfer in molecular systems.

The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60 years after the seminal work of R. A. Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.

Electrocatalytic Interface Engineering with Ionic Liquids

Electrocatalyst development for reactions involving multiple charge transfer steps and more than one adsorbed intermediate is severely limited by the scaling relations that govern the free energy of adsorption of those intermediates. These scaling relations impart a minimum overpotential (thermodynamic), limiting activity independent of the identity of the catalyst. Consider the associative oxygen reduction reaction (ORR) mechanism which proceeds through *OOH, *O and *OH adsorbed intermediates1 , 2 , 3 , 4. Generally, the ORR is found to be rate limited by the conversion of *OH to H2O and it is concluded that weakening the strength of adsorption of *OH with the catalyst surface will lead to improved activity5 , 3 , 4 , 6. A shift in *OH binding strength, typically accomplished through manipulation of the electronic and geometric structure of the catalyst5 , 6, however, will also cause a shift in the energy of adsorption of other intermediates, including *OOH. This effect, termed intermediate scaling, rather than resulting in a continued increase in catalytic activity with decreasing *OH binding strength, will at some point shift the limiting reaction to the activation of O2, Sabatier/volcano relationship. Adsorbed intermediate scaling relations create a thermodynamic minimum overpotential of 0.37 V due to the fixed difference in free energy of adsorption (ΔΔGad) for *OH and *OOH of ~3.2 eV which is independent of the identity of the catalyst2 , 3 , 7 , 8. The ideal ORR catalyst, near zero overpotential, is found to have a difference in binding energy between *OH and *OOH (ΔΔGad) of 2.46 eV9. The origin of the effect of water on the progression of the ORR, as well as other elementary electrochemical reactions, is considered to be two-fold: (1) through solvation and hydrogen bonding, H2O stabilizes adsorbed reaction intermediates on the surface to differing degrees. It is found to stabilize *OH by about 0.5 eV and *OOH by 0.25 eV, contributing to the fixed ΔΔGad value9; (2) a mixed *OH-H2O hydrogen bonded structure is found to readily form on metal surfaces where charge redistribution due to the hydrogen bonding increases the binding strength and surface coverage of the *OH10 , 11 , 12 , 13. If strategies can be developed to reduce the binding energy difference between *OH and *OOH and prevent the formation of an ordered water stabilized *OH spectator structure, than the activity and selectivity of multi-intermediate reactions could be potentially maximized. Here we will present our progress in addressing intermediate scaling for the ORR through the manipulation of metal/electrolyte interface with chemically tailored ionic liquids (IL). Figure 1 demonstrates the impact of the formation of a composite electrocatalyst architecture through incorporation of a protic IL into a carbon supported PtNi alloy catalyst. The cyclic voltammogram, Fig. 1(top), shows a significant decrease in OH/O coverage at higher potentials which can be attributed to a decrease in adsorption free energy for those intermediates/spectators due to the manipulation of the interaction of water with the catalytic surface in the presence of the IL. We argue that it is this change in water interaction with the surface, potentially in combination with other properties of the IL including reactant solubility, that results in the enhanced ORR activity observed in Figure 1(bottom). Through the testing of well-defined IL/single crystal composite electrodes, we will present a systematic study of the mechanism by which ILs affect the kinetics of the ORR. The insight developed can be applied to other multi-intermediate reactions where the control of water interaction with the catalytic surface and adsorbed intermediate solvation is a viable strategy to approach the breaking of scaling relations. Figure 1: (top) CV of PtNi/C with (red) and without (blue) [MTBD][beti] IL in Ar purged 0.1 M HClO4. (bottom) ORR polarization curve for PtNi/C with (red) and without (blue) [MTBD][beti] IL in O2 saturated HClO4. 1. Gómez-Marín, A. M., et al., Catal. Sci. Technol. 4,1685 (2014). 2. Hansen, H. A., et al., J. Phys. Chem. C 118,6706–6718 (2014). 3. Nørskov, J. K. et al., J. Phys. Chem. B 108,17886–17892 (2004). 4. Qi, L., et al., J. Catal. 295,59–69 (2012). 5. Stamenkovic, V. R. et al., Science 315,493–497 (2007). 6. Stephens, I. E. L., et al., Energy Environ. Sci. 5,6744 (2012). 7. Viswanathan, V., et al., ACS Catal. 2,1654–1660 (2012). 8. Koper, M. T. M., Chem. Sci. 4,2710 (2013). 9. Viswanathan, V., et al., Top. Catal. 57,215–221 (2014). 10. Casalongue, H. S. et al., Nat Commun 4,2817 (2013). 11. Ogasawara, H., et al., Top. Catal. 59,439 (2016) 12. Pettersson, L. G. M. et al., J. Phys. Chem. C 114,10240–10248 (2010). 13. Schiros, T., et al., J. Phys. Chem. C 111, 15003–15012 (2007). Figure 1

Ca2+ Effects on a Pt/C Catalyst for the Oxygen Reduction Reaction

The proton exchange membrane fuel cell (PEMFC) is one of the most promising renewable energy sources due to benign exhaust emissions and high energy efficiency. Pt/C is currently the primary catalyst for the slow oxygen reduction reaction (ORR) kinetics at the cathode of a PEMFC. The ORR, O2 + 4H+ + 4e- → 2H2O, indicates that this process preferentially occurs at a triple phase boundary where reactant, electrolyte, and catalyst are in contact [1, 2]. The depletion of O2, H+ or e- would significantly affect reaction kinetics. The Nafion® perfluorosulfonic acid (PFSA) ionomer, composed of a carbon-fluorine backbone and side chains ending with sulfonic acid groups, is conventionally used to prepare catalyst layers by exploiting its binding, proton-conducting and O2 transport properties. Foreign cations replace ionomer protons and occupy sulfonic acid sites by ion exchange [3-6]. The replacement of H+ by foreign cations in the ionomer decreases not only the proton conductivity but also O2 permeability. Uddin et al. [6] model results showed that the presence of Na+ in the ionomer of the cathode catalyst layer, which decreases the O2 permeability, causes a decrease in dissolved O2 concentration. Durst et al. [7] found that the diffusion coefficient and the concentration of O2 in the H2SO4 electrolyte, and the kinematic viscosity of the electrolyte decrease in the presence of Co2+. Cationic impurities decrease the membrane water content in turn shrinking the ionomer and lowering the active catalyst surface area. Also, foreign cations promote the generation of H2O2, a side product of the ORR, which exerts an undetermined influence on the performance and durability of PEMFCs. Literature and our group studies indicated that foreign cations have severe effects on the performance and durability of the PEMFC. Among all cations, Ca2+ deserves special attention because it is one of the most abundant elements within the earth’s crust. Prior efforts have focused on the effects of Ca2+ on PEMFC operation [8-10] and membrane transport properties [11]. However, the effects of Ca2+ on the ORR kinetics have hardly been studied. Okada et al. [12] studied the effects of Ca2+ on the O2 reduction kinetics at a Nafion® film covered Pt rotating disk electrode. Their results showed that impurity ions, even in small amounts, significantly hinder the rate of the O2 reduction charge transfer step. However, the Nafion® film covered Pt electrode is different from the supported catalyst layer for a PEMFC. The O2 transport properties in a Ca2+ form ionomer are poorly understood. In addition, the H2O2 yield has not been measured. In this presentation, detailed analyses of the changes in the Pt/C electrochemical surface area, ORR diffusion-limited and kinetic currents, Tafel slope, H2O2 yield, O2 permeability and ionomer sulfonate site occupation will be presented and discussed. Acknowledgments Authors are grateful to the United States Department of Energy (award DE-EE0000467), the Office of Naval Research (award N00014-12-1-0496), and the Hawaiian Electric Company.

Multi-Physics Simulation of SOFC Button Cell with Multi-Step Charge Transfer Model in Composite LSM/YSZ Cathode

In the present study, a numerical simulation of a solid oxide fuel cell (SOFC) button cell with a multi-step oxygen reduction reaction (ORR) mechanism in a composite cathode is developed for better understanding of processes within the SOFC. The ORR mechanism involves both surface charge transfer through the triple phase boundary (3PB) and bulk charge transfer via a two phase boundary (2PB) pathway. The developed multi-physics simulation considers the charge conservation and species transportation through the porous media. Finally, the simulations of a full SOFC button cell were calibrated with measured cell performance (polarization curves and impedance behavior) at three different air/fuel utilization conditions. The surface absorption and dissociation in the proposed mechanism was identified as the rate-determining step. The cell performance is determined via combination of this rate-determining step, charge transfer steps in parallel pathways, and diffusion phenomena. The dependence of cell resistance on oxygen partial pressure was also quantified.

Energy level engineering in ternary organic solar cells: Evaluating exciton dissociation at organic semiconductor interfaces

Electronic energy level engineering, with the aim to improve the power conversion efficiency in ternary organic solar cells, is a complex problem since multiple charge transfer steps and exciton dissociation driving forces must be considered. Here, we examine exciton dissociation in the ternary system poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester:2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (P3HT:PCBM:DIBSq). Even though the energy level diagram suggests that exciton dissociation at the P3HT:DIBSq interface should be efficient, electron paramagnetic resonance and external quantum efficiency measurements of planar devices show that this interface is not capable of generating separated charge carriers. Efficient exciton dissociation is still realised via energy transfer, which transports excitons from the P3HT:DIBSq interface to the DIBSq:PCBM interface, where separated charge carriers can be generated efficiently. This work demonstrates that energy level diagrams alone cannot be relied upon to predict the exciton dissociation and charge separation capability of an organic semiconductor interface and that energy transfer relaxes the energy level constraints for optimised multi-component organic solar cells.

Kinetics Tuning the Electrochemistry of Lithium Dendrites Formation in Lithium Batteries through Electrolytes.

Lithium batteries are one of the most advance energy storage devices in the world and have attracted extensive research interests. However, lithium dendrite growth was a safety issue which handicapped the application of pure lithium metal in the negative electrode. In this investigation, two solvents, propylene carbonate (PC) and 2-methyl-tetrahydrofuran (2MeTHF), and four Li+ salts, LiPF6, LiAsF6, LiBF4 and LiClO4 were investigated in terms of their effects on the kinetics of lithium dendrite formation in eight electrolyte solutions. The kinetic parameters of charge transfer step (exchange current density, j0, transfer coefficient, α) of Li+/Li redox system, the mass transfer parameters of Li+ (transfer number of Li+, tLi+, diffusion coefficient of Li+, DLi+), and the conductivity (κ) of each electrolyte were studied separately. The results demonstrate that the solvents play a critical role in the measured j0, tLi+, DLi+, and κ of the electrolyte, while the choice of Li+ salts only slightly affect the measured parameters. The understanding of the kinetics will gain insight into the mechanism of lithium dendrite formation and provide guidelines to the future application of lithium metal.

Employing electrochemical reduced graphene oxide as a co-catalyst for synergistically improving the photoelectrochemical performance of nanostructured hematite thin films

Abstract In this study, a series of electrochemical reduced graphene oxide (eRGO)-hematite nanocomposites were developed through a facile and environmental benign two-step electrodeposition method with high photoelectrochemical (PEC) performance. The resulting nanocomposites formed an intimate contact between the eRGO and hematite interface as supported by the field emission-scanning electron microscopy (FE-SEM) analysis. A remarkable 8-fold enhancement in the photocurrent density was observed on the eRGO-hematite-4 nanocomposite (using 2.0 mg/ml GO precursor) relative to the bare hematite under light irradiation. This improvement is ascribed to the finely controlled eRGO sheets that enhance the light absorption, increase PEC active surface area of hematite, improve efficient transfer of the photoinduced electrons from the conduction band of hematite to eRGO sheets and as a result leads to a minimised electron–hole pairs recombination rate. This was further evidenced with impedance characteristics, where the obtained surface charge resistance values of eRGO-hematite-4 nanocomposite are much lower than the bare hematite, revealing an efficient charge transfer step to impede the charge recombination. Lastly, a postulated mechanism for the PEC process associated with eRGO-hematite nanocomposite was presented.

Field-Controlled Charge Separation in a Conductive Matrix at the Single-Molecule Level: Toward Controlling Single-Molecule Fluorescence Intermittency.

The fluorescence intermittency or “blinking” of single molecules of ATTO647N (ATTO) in the conductive matrix polyvinylcarbazole (PVK) is described in the presence of an external applied electric field. It is shown that due to the energy distribution of the highest occupied molecular orbital (HOMO) level of PVK, which is energetically close to the HOMO of ATTO, sporadic electron transfer occurs. As a result, the on/off dynamics of blinking can be influenced by the electric field. This field will, depending on the respective position and orientation of the dye/polymer system with respect to those of the electrodes, either enhance or suppress electron transfer from PVK to ATTO as well as the back electron transfer from reduced ATTO to PVK. After the charge-transfer step, the applied field will pull the hole in PVK away from the dye, increasing the overall time the dye resides in a dark state.

General Theory for Pulse Voltammetric Techniques at Rough Electrodes: Multistep Reversible Charge Transfer Mechanism

Theoretical model is developed for the voltammetric response of multistep reversible charge transfer mechanism at rough electrode. This theory is applicable for arbitrary as well as random roughness profiles. An explicit mathematical expression relating the staircase (SCV), cyclic staircase (CSCV), differential pulse (DPV) and square wave (SWV) voltammetric response of finite fractal electrode to the statistical morphological characteristics is obtained. The fractal characteristics, viz. fractal dimension (DH), lower length scale of fractality (ℓ) and topothesy length (ℓτ) are found to dominantly control the various voltammetric techniques responses of a multistep charge transfer mechanism. The heights of the voltammetric peaks corresponding to each charge transfer step is found to increase with increasing roughness. In CSCV, the anomalous intermediate scan rate window of Randles-Sevc˘ik plots shifts with the number of charge transfer step. The peak current for the charge transfer step occurring at lower potential has an early onset of the influence of morphology, i.e., the anomalous regime window shifts at lower scan rates compared to those occurring at higher potential values. The differential techniques show roughness induced current enhancement for all the peaks without affecting the position of the peak potential. Thus, roughness helps to overcome the major drawback of low Faradaic current in these techniques and improves their application for chemical sensing purposes.

A Simple, One-Dimensional Electrochemical Model for the Lithium-Sulfur Battery

The use of rechargeable batteries in electric and hybrid vehicles would not only reduce the carbon emission significantly but also decrease the dependence on foreign oil. In addition, the use of rechargeable batteries in grid storage would be beneficial for the storage of renewable energy sources such as solar and wind energy which are normally intermittent. Therefore, developing high energy density and low cost rechargeable batteries will play a critical role in overcoming many of the energy-related challenges. Lithium-sulfur (Li-S) batteries have gained significant attention in recent years due to their very high theoretical specific energy [1-6]. However for the commercial use of the Li-S battery, its capacity retention and cycle life must be improved [1-6]. Electrochemical modeling coupled with electrochemical characterization is essential to have a better understanding of the reaction and degradation mechanisms in these batteries in order to improve the battery performance and life. In this study, a simple one-dimensional electrochemical model for a Li-S cell is developed based on a previous model by Eroglu et al [1]. The Li-S cell in the model is composed of the lithium-metal anode, a porous separator and the porous sulfur-carbon cathode filled with the electrolyte. The concentration gradients in the cell as well as the polysulfide shuttle mechanism are neglected in the model. The model predicts the current-voltage relationship for each charge transfer step for an isothermal discharge of the Li-S cell. In the electrochemical model, anode kinetics is defined with the Butler-Volmer kinetics. The Ohmic losses from the transport of Li+ through the porous separator are also considered in the model using Ohm’s Law. The cathode is treated with the one-dimensional concentration-independent porous electrode model by Newman and Tobias [7]. The cathode kinetics is defined with a single reaction step, simplifying the multiple-step redox reactions for the sulfur cathode. By simplifying the sulfur reaction in the cathode into a single reaction, the cathode kinetics in the electrochemical model can be defined by a single kinetic parameter, exchange current density for the cathode (i0,pe). The electrochemical model of the Li-S cell is supported with the Electrochemical Impedance Spectroscopy (EIS) data from the literature [2-6]. The exchange current density for the cathode kinetics is estimated as a function of depth of discharge (Figure 1) by comparing the total cell resistance measured by the EIS data in the literature with the area-specific impedance (ASI) predicted by the model. It is seen in Figure 1 that the exchange current density for the cathode kinetics in a Li-S cell increases with depth of discharge (DOD) until 60% where it reaches a maximum. Further increase at DOD results in lower i0,pe. This could be explained by the increasing electronic conductivity of the sulfur cathode at the beginning of the discharge due to the consumption of insulating sulfur. However at higher DODs, the accumulation of the insoluble Li2S at the cathode surface becomes significant, decreasing the charge transfer rate. Figure 1. Exchange current density for the cathode kinetics predicted by the electrochemical model as a function of depth of discharge.

Hodrogen Oxidation Reaction Mechanisms in Mixed Ionic and Electronic Conducting Oxide Anode for Solid Oxide Fuel Cells

Ceramics anode offers certain advantages in terms of impurity tolerance and other properties, as compared to conventional Ni/YSZ anode for solid oxide fuel cells (SOFC). However, ceramic anodes typically have much lower electrochemical performance, and there is very few studies on the hydrogen oxidation reaction (HOR) mechanisms and kinetics in ceramic anodes. In this research, using Doped YCrO3 as baseline material, we systematically investigated the HOR mechanisms in mixed ionic and electronic conducting (MIEC) ceramic anodes, with the focus on the effect of electronic conductivity, electro-catalytic activity, triple-phase-boundary (3PB), and electrolyte, on the HOR kinetics. It is found that both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5 eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3 ~ 1/2 for Ni doped yttrium chromites. A model of H2oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen. The influence of electrolyte to electrode performance was investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain.

Investigation of the Lithiation Mechanism of Fe3O4‐Based Composite Anode: the Effect of the Carbon Matrix

AbstractIn this work, the Fe3O4/CNTs composite and Fe3O4/C composite were synthesized by a facile hydrothermal method without any reducing agents and pyrolysis of glucose, respectively, and the process of the first lithiation of commercial Fe3O4, Fe3O4/CNTs and Fe3O4/C electrode were systematically investigated by EIS as function of electrode potential. It is found that at intermediate degrees of intercalation, the characteristic Nyquist plots of Fe3O4 electrode are composed of three parts, namely, the small semicircle in high‐frequency region can be attributed to the migration of lithium‐ion through SEI film (the resistance of SEI film coupled with SEI film capacitance) as well as contact problems; the semicircle in the medium‐frequency region is attributed to the electronic properties of the material and the large arc in low‐frequency region is due to charge transfer step. Moreover, the changes of kinetic parameters for lithiation process of Fe3O4/CNTs composite as a function of electrode potential in the first discharge cycle was discussed in detail, and the effect of carbon matrix type on the electrochemical performance and lithiation mechanism of Fe3O4 electrode is given.

Modeling of corrosion of steel tubing in CO2 storage

AbstractIn this work, a mechanistic model was developed to predict the corrosion rate of steel tubing under carbon storage conditions. The model integrates a number of sub‐models that quantify contributions of interrelated steps to the corrosion process. The water chemistry sub‐model determines the solution pH and the concentrations of species. The electrochemical corrosion sub‐model assesses both charge‐transfer and mass‐transfer steps and their effects on corrosion. The scale formation and its role in corrosion are considered. The finite difference method is used to solve numerical equations, and a MetLab program was written for computation of the corrosion rate. The modeling results are validated by both laboratory testing and literature data. The parametric effects, including temperature, CO2 partial pressure, solution salinity, pH, time, etc., on corrosion are predicted. The limitations of the model are also discussed. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

The inherent coupling of charge transfer and mass transport processes: the curious electrochemical reversibility

As a complement to a previous contribution from us, the mass transport mechanisms of the electroactive species to and from the electrode in an uncomplicated electrode mechanism are considered. The electrode process as a whole is discussed, with emphasis to its reversibility degree, as results from the relevant responses in controlled potential techniques, such as chronoamperometry and current sampling voltammetry, linear sweep and cyclic voltammetry, and in rotating disk voltammetry. The electrode process as a whole, composed by charge transfer and mass transport steps that concur to condition the current flowing, is analysed on the basis of the relative rates of the two steps, as well of the time window within which the process is observed. The so-called ‘boundary value problem’ for uncomplicated charge transfers with different reversibility degrees is outlined. Supplementary Material is available, in which the simulated concentration profiles for reduced and oxidised species reacting at an electrode, at which a triangular potential waveform is applied, are linked to the corresponding current densities.