Local Electrochemical Reactions Research Articles

Dynamically visualizing battery reactions by operando Kelvin probe force microscopy

Energy storage devices using electrochemical reactions have become an integral part of our daily lives, and further improvement of their performance is highly demanded. An important task for this purpose is to thoroughly understand the electrochemical processes governing their chemistry. Here we develop a method based on Kelvin probe force microscopy that enables dynamic visualization of changes in the internal potential distribution in an operating electrochemical device and use it to characterize an all-solid-state lithium ion battery. Observation of the cathode composite regions during a cyclic voltammetry operation reveals differences between the behavior of local electrochemical reactions in the charge and discharge processes. Based on careful inspection of the results, we show that the difference arises from a change in the state of an electronic conductive path network in the composite electrode. Our method provides new insights into the local electrochemical reactions during electrochemical operation of devices.

Scanning Electrochemical Cell Microscopy for Analysis of Solid Electrolyte Interface on Negative Electrodes in Lithium-Ion Batteries

Lithium-ion battery is one of the most popular secondary batteries for practical applications such as mobile phones and electrical cars because of their high energy density and good cyclability. For further improvement of their total performance with highly efficient cyclability, it is necessary to be in precise control of lithium-ion transport at the interface between electrolyte and electrodes. There is a major issue for efficient ion transport due to solid electrolyte interface (SEI) on negative composite electrodes such as graphite and/or silicon with some binders and electrical additives. The SEI is in general formed by decomposition of electrolyte. However, the SEI formation process or SEI itself is not still clear how the decomposition process proceeds due to the surface structure, binders or dispersion of active materials. Here, in this work we have used a scanning electrochemical microscopy (SECCM) with a single barrel nanopipette inside glove-box for analyzing local electrochemical reaction for SEI formation. A 50 nm radius nanopipette was filled with organic electrolyte (1.0 M LiPF6 in ethylene carbonate:diethyl carbonate = 1:1 in vol. %) with a Li reference electrode. When the pipette was approached to the sample surface, a meniscus was created. Then, through the meniscus as a nanoscale electrochemical simulator, cyclic voltammogram was measured to introduce the SEI in localized area. The location SEI formed and their created potential were mapped by scanning the pipette on the sample. Further, on the particular structure such as basal and edge of graphite, the decomposition process was also analyzed for investigation of material structures for SEI formation. Those information would be particularly important to understand their mechanism and to control SEI formation.

Developing Plasmonic Imaging Technique to Understand Battery Solid Electrolyte Interface Formation and Li Nucleation in-Situ

During the operation of lithium-ion battery (LIB), a solid electrolyte interface (SEI) layer forms on the anode electrode materials due to side reactions with the electrolyte solvent and salt. It is essential to the performance of LIBs and it decides the initial capacity loss, self-discharge characterizstics, cycle life and safety. Therefore to understand the SEI formation is extremely important for battery research. Currently, most of characterization methods of SEI are based on electron microscope, although powerful, it is difficulty to directly map the SEI formation in-situ. We have developed a novel plasmonic imaging technique to image the local SEI formation and measure the electrochemical reaction of the anode electrodes. Unlike conventional electrochemistry that measures the electrical current from entire electrode, the plasmonic-based electrochemical microscope (PECM) technique is based on sensitive imaging of surface plasmonic signal caused by electrochemical reaction, which is measured optically, thus allowing for fast and non-invasive imaging of electrode processes. PECM can image localized chemical reactions on the electrode surface ins-situ, this information will be extremely important for us to understand SEI formation and electrochemical reaction on the anode electrode. The electrochemical reaction will always be accompanied by a conversion of the chemical species between oxidized and reduced states, and plasmonic imaging is extremely sensitive to the species generated (or consumed) on electrode materials. Therefore, the PECM has great potential to characterize the local electrochemical reactions and energy storage process.

Analysis of Influence of Impurities in Hydrogen on PEFC Performance, Based on Measurement of Current Distribution Using Electrochemical Impedance Distribution Analyzer

Polymer electrolyte fuel cells (PEFC) systems have attracted considerable attention in that they can be powered by the hydrogen that is produced as a by-product of various industrial processes, such as caustic soda production and petrochemical processes. Most of the by-product hydrogen is discarded as a waste stream or used for heat production. Therefore, such plants could realize significant energy savings by installing PEFC systems, allowing them to generate power with a high level of efficiency by using the by-product hydrogen to fuel PEFC systems. However, the by-product hydrogen produced by these plants typically contains impurities. Although most of these impurities can be removed, a small amount inevitably remains in the hydrogen and can cause the performance of a PEFC system to degrade. Therefore, the influence of these impurities must be clarified before installing a PEFC system. Given the above, we have been developing a means of quickly evaluating the influence of impurities on a PEFC. The measurement of the current distribution is an important analysis approach that provides information on the local electrochemical reaction conditions in each of the cells of a PEFC. These cells are composed of an electrolytic membrane, a catalyst layer, a diffusion layer, and a flow channel. We have devised a means of measuring the current distribution using current distribution measurement sensors (S++ Simulation Services, Germany) which are inserted between the gas flow channel and current collector of either electrode. This method of measuring the current distribution allows us to quickly analyze the influence of impurities on the PEFC performance, relative to conventional voltage measurements, and can clarify which areas in a cell have degraded because the voltage change is the average of the values for every area in a cell, whereas the current change appears as an absolute value in a local area. Electrochemical impedance spectroscopy (EIS) is a valuable electrochemical analysis tool that can be applied to many kinds of electrochemical devices, including fuel cells. EIS can provide much useful information on the electrolyte membrane, the catalyst layer, and the reactant transfer in a PEFC cell. Although the measurement of the electrochemical impedance distribution in a cell provides a useful means of analyzing the influence of impurities on the PEFC performance, to the best of our knowledge, no previous studies have attempted a high-resolution analysis of the distribution. Therefore, we have developed a new measurement system that enables us to measure both the electrochemical impedance distribution and current distribution. The measurement system is an improved current distribution measurement sensor, intended for application to a JARI-standard cell, with an active area of 25 cm2 and which has 81 segmented internal current shunt resistances for measuring DC current and 81 segmented external connectors for measuring AC current signals. In the present study, to simulate impurities in the hydrogen, we added NH3 (25 ppm) and analyzed its influence on the PEFC performance. We used the measurement system to measure the change in the electrochemical impedance distribution and current distribution over time. The operating temperature, humidification temperatures of the anode and cathode, and current density of the cell were set to 65 ºC, 65 ºC, and 0.3 A/cm2 respectively. The AC current for measuring the electrochemical impedance was 1 Ap-p and the frequency range was 100 mHz–100 kHz. The current distribution measurement (Fig. 1) revealed that the influence of the NH3 was greatest at the inlet to the cell. Furthermore, from the results of the electrochemical impedance distribution measurement, it was found that the current distribution and some aspects of the resistance distribution, as determined from the electrochemical impedance, were correlated. Meanwhile, the most significant factor influencing the effect of NH3 on the PEFC performance is the increase in the activation overvoltage at the inlet of the cell. In addition, it was also thought that the cathode reaction resistance of the cell increases, which is caused by the catalyst-specific surface area decreasing at the inlet of the cell. In conclusion, our new measurement method provides an effective means of determining the largest degraded area and the largest degradation factor affecting PEFC performance, as a result of the presence of impurities in hydrogen. Therefore, relative to conventional measurements, we can quickly evaluate the influence of impurities on the PEFC performance by focusing on and analyzing these areas and factors. In the future, we will apply our new measurement method to the evaluation of the influence of impurities on the PEFC performance by acquiring information on the influence of such impurities on PEFC performance. Figure 1

Mesoscale Complexations in Lithium Electrodeposition.

Mechanistic understanding of lithium electrodeposition and morphology evolution is critical for lithium metal anodes. In this study, we deduce that Li deposition morphology evolution is determined by the mesoscale complexations that underlie due to local electrochemical reaction, Li surface self-diffusion, and Li-ion transport in the electrolyte. Li-ion depletion at the reaction front for higher reaction rates primarily accounts for dendritic growth with needlelike or fractal morphology. Large Li self-diffusion barrier, on the other hand, may lead to the formation of porous Li film for lower reaction rates. Enhanced ion transport in the electrolyte contributes to homogeneous deposition, thereby avoiding nucleation for Li dendrite formation. This study also demonstrates that the substrate surface roughness strongly affects dendritic growth localization over the protrusive surface features. A nondimensional electrochemical Damkohler number is further proposed, which correlates surface diffusion rate and reaction rate and allows constructing a comprehensive phase map for lithium electrodeposition morphology evolution.

The possibility of multi-layer nanofabrication via atomic force microscope-based pulse electrochemical nanopatterning

Pulse electrochemical nanopatterning, a non-contact scanning probe lithography process using ultrashort voltage pulses, is based primarily on an electrochemical machining process using localized electrochemical oxidation between a sharp tool tip and the sample surface. In this study, nanoscale oxide patterns were formed on silicon Si (100) wafer surfaces via electrochemical surface nanopatterning, by supplying external pulsed currents through non-contact atomic force microscopy. Nanoscale oxide width and height were controlled by modulating the applied pulse duration. Additionally, protruding nanoscale oxides were removed completely by simple chemical etching, showing a depressed pattern on the sample substrate surface. Nanoscale two-dimensional oxides, prepared by a localized electrochemical reaction, can be defined easily by controlling physical and electrical variables, before proceeding further to a layer-by-layer nanofabrication process.

Nanosized Conducting Filaments Formed by Atomic-Scale Defects in Redox-Based Resistive Switching Memories

Redox-based resistive switching phenomena are found in many metal oxides and hold great promise for applications in next-generation memories and neuromorphic computing systems. Resistive switching involves the formation and disruption of electrically conducting filaments through ion migration accompanied by local electrochemical redox reactions. These structural changes are often explained by point defects, but so far clear experimental evidence of such defects is missing. Here, nanosized conducting filaments in Fe-doped SrTiO3 thin-film memories are visualized, for the first time, by scanning transmission electron microscopy and core-loss spectroscopy. Conducting filaments are identified by a high local concentration of trivalent titanium ions correlating to oxygen vacancies. Strontium vacancies and lattice distortions also exist in the filaments. Despite a high concentration of defects in the filaments, their general SrTiO3 perovskite structure is essentially preserved. First insights into the switching...

Plasmonic Imaging of Electrochemical Reactions of Single Nanoparticles

Electrochemical reactions are involved in many natural phenomena, and are responsible for various applications, including energy conversion and storage, material processing and protection, and chemical detection and analysis. An electrochemical reaction is accompanied by electron transfer between a chemical species and an electrode. For this reason, it has been studied by measuring current, charge, or related electrical quantities. This approach has led to the development of various electrochemical methods, which have played an essential role in the understanding and applications of electrochemistry. While powerful, most of the traditional methods lack spatial and temporal resolutions desired for studying heterogeneous electrochemical reactions on electrode surfaces and in nanoscale materials. To overcome the limitations, scanning probe microscopes have been invented to map local electrochemical reactions with nanometer resolution. Examples include the scanning electrochemical microscope and scanning electrochemical cell microscope, which directly image local electrochemical reaction current using a scanning electrode or pipet. The use of a scanning probe in these microscopes provides high spatial resolution, but at the expense of temporal resolution and throughput. This Account discusses an alternative approach to study electrochemical reactions. Instead of measuring electron transfer electrically, it detects the accompanying changes in the reactant and product concentrations on the electrode surface optically via surface plasmon resonance (SPR). SPR is highly surface sensitive, and it provides quantitative information on the surface concentrations of reactants and products vs time and electrode potential, from which local reaction kinetics can be analyzed and quantified. The plasmonic approach allows imaging of local electrochemical reactions with high temporal resolution and sensitivity, making it attractive for studying electrochemical reactions in biological systems and nanoscale materials with high throughput. The plasmonic approach has two imaging modes: electrochemical current imaging and interfacial impedance imaging. The former images local electrochemical current associated with electrochemical reactions (faradic current), and the latter maps local interfacial impedance, including nonfaradic contributions (e.g., double layer charging). The plasmonic imaging technique can perform voltammetry (cyclic or square wave) in an analogous manner to the traditional electrochemical methods. It can also be integrated with bright field, dark field, and fluorescence imaging capabilities in one optical setup to provide additional capabilities. To date the plasmonic imaging technique has found various applications, including mapping of heterogeneous surface reactions, analysis of trace substances, detection of catalytic reactions, and measurement of graphene quantum capacitance. The plasmonic and other emerging optical imaging techniques (e.g., dark field and fluorescence microscopy), together with the scanning probe-based electrochemical imaging and single nanoparticle analysis techniques, provide new capabilities for one to study single nanoparticle electrochemistry with unprecedented spatial and temporal resolutions. In this Account, we focus on imaging of electrochemical reactions at single nanoparticles.

Investigation of the Electronic and Local Structural Changes during Li Uptake and Release of Nanocrystalline NiFe2O4 By X-Ray Absorption Spectroscopy

As the most promising power sources for smart phones, laptops, electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium ion batteries have attracted much attention in past decades. In order to meet the ever-increasing demand for high energy density, high capacity and long service life, various transition metal oxides and compounds (TMOs) have been investigated because of their much higher theoretical capacity based on the conversion reactions [1-3]. During Li uptake, TMOs are firstly reduced to extremely small metallic nanoparticles dispersed in a Li2O matrix. And during subsequent charging, these metallic particles are oxidized. The reaction mechanism can be described using the following general equation [1]: TM x O y + 2yLi+ +2ye- → xTM + yLi2O (1) Among these TMOs, iron based compounds are particularly interesting candidates to investigate as they are less expensive and more environmental friendly than cobalt contained oxides. As a member of iron based compounds, NiFe2O4 comes into notice since it contains electrochemically active nickel and displayed a high theoretical capacity of 914 mAh g-1. However, these conversion reaction electrodes showed a fast capacity fading in few cycles because of the formation of an insulating polymeric layer around the particles and poor integrity caused by the drastic volume changes upon cycling [4-5]. Although the conversion reaction 1 of simple monoxides (CoO, CuO, FeO or NiO)[1] is thermodynamically favorable and has been widely investigated by ex situ/in situ X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Mössbauer spectroscopy during discharge and charge. Lots of detailed information for the reaction is still not well understood yet due to the intriguing nature of amorphous Li2O and ultrafine metal particles and limitations for the above techniques. A better understanding of the reactions occurring during Li uptake and release is crucially important to search for new high performance electrode materials. In this work, nanocrystalline NiFe2O4 particles were successfully synthesized and used as active electrode material for lithium ion battery. X-ray absorption spectroscopy (XAS) was applied to investigate the local structural changes and electrochemical reactions of NiFe2O4 in the cell during 1st discharge and charge process. As lithium inserted into the structure, Fe3+ ions are firstly reduced to Fe2+, but Ni2+ ions are unaffected which can be seen in fig.1. The matrix spinel structure collapses and transform to a rock salt monoxide phase with further lithiation. Along with the conversion reaction, the formation of SEI on the electrode consumed a large amount of lithium ions. The rock salt monoxides are reduced to metallic products at the end of discharging with a small particle size and highly disorder. The refined EXAFS results show that these metallic Fe/Ni nanoparticles are not well refined with a nearest number of neighbors of 6 and 8 for Fe and Ni respectively. All these ultrafine metal particles are reoxidized to Fe2O3and NiO phase instead of pristine spinel structure during the following charging. Reference [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496-499. [2] R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, J.C. Jumas, J. Olivier-Fourcade, Electrochemistry Communications 5 (2003) 16-21. [3] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Advanced Materials 22 (2010) E170-E192. [4] X. Guo, X. Lu, X. Fang, Y. Mao, Z. Wang, L. Chen, X. Xu, H. Yang, Y. Liu, Electrochemistry Communications 12 (2010) 847-850. [5] J.-M. Tarascon, S. Grugeon, M. Morcrette, S. Laruelle, P. Rozier, P. Poizot, Comptes Rendus Chimie 8 (2005) 9-15. Figure 1

Joule Heating Effect in the Electroforming Process of Pt/WO 3-X /Pt Nanoionics-Based Memristive Devices

In recent years, nanoionics-based memristive devices with resistive switching behaviors have garnered significant attention as promising candidates for next-generation nonvolatile memory known as resistance random access memory (RRAM) due to their potential high scalability and low-power consumption. In addition, memristive devices are also promising for logic and neuromorphic computations. The memristive devices typically have a simple stacked structure of metal-insulator-metal (MIM), where the resistance can be reversed changed through local ion migration and electrochemical reaction after an initial electroforming process. Nanoionics–based memristive devices can be classified as cation migration cells and anion migration cells. When an electrochemically active electrode such as one made of Ag or Cu is used in a MIM device, resistive switching occurs through formation and annihilation of a metallic atom bridge resulting from the migration of highly mobile cations such as Ag+ and Cu+ ions. This type of nanoionics-based device is referred to as an atomic switch or electrochemical metallization systems.[1] Resistive switching behavior can also be observed for a MIM device without an electrochemically active electrode. In this case, the migration of anions, usually oxygen ions plays a crucial role. Local oxygen ion migration, which is better described as the migration of oxygen vacancy (V O ‥) and a resultant change in the electronic barrier at the metal electrode/TMO interface, is associated with the resistive switching behavior. In general, the cation migration cell and anion migration cell both need electroforming process to obtain resistive switching behaviors. For the cation migration cells, the formation of Ag or Cu bridges in the forming process has been directly observed by TEM or in-situ TEM.[2]In contrast, the mechanism of the electroforming process in the anion migration cell is still uncertain due to the lack of direct characterization. In this study, the electroforming process and resistive switching behaviors of the anion migration cell of Pt/WO3-x/Pt have been investigated as a function of temperature. As shown in Figure 1a, electroforming process with soft-breakdown was triggered as the sweep voltage was raised to about 6.0 V at 296 K and 396 K. But, at 110 K, no soft-breakdown was observed even by increasing the sweep voltage to 20 V, which indicates that the devices cannot be electroformed at ultralow temperature. In contrast, the device shows resistive switching behaviors at ultralow temperature after being electroformed at room temperature, see Figure 1b. In order to real-time observe the structure changes during electroforming process, we prepared a Pt/WO3-x/Pt devices that operated inside a TEM, see the inset of Figure 1g. As reported in Figure 1c to 1f, the initial amorphous WO3-x layer gradually crystallized by applying voltage weeping. And, crystallized nanoparticles of Magnéli phase were observed, as shown in Figure 1g. Actually, the current level of in situ measurements is much smaller than that in the conventional electroforming processes. This may be the reason why no conductive filament composed of the Magnéli phases was observed. The crystallization and formation of the conductive filament in WO3-x layer have been observed by cutting the electroformed device in our previous work.[3] Based on above results, it is proposed that the Joule heating effect plays a key role in the electroforming process. Under ultralow temperature, the current level of the device is too low to generate enough heating effect to introduce the filament formation. Moreover, the heat dissipates much quickly at ultralow temperature. Therefore electroforming process could not be triggered at ultralow temperature. In contrast, the resistive switching behavior obtained after electroforming process mainly occurs in the nanogap region between the electrode and the conductive filaments, where the V O ‥ migration under external electric field determines the resistive switching process. So, the resistive switching behavior can be observed at low temperature. Based on these results and discussions, it is reasonable to conclude that Joule heating effect domains the electroforming process, instead of the resistive switching processes. Terabe, K.; Hasegawa, T.; Nakayama, T.; Aono, M., Nature 2005, 433, 47-50. Yang, Y.; Gao, P.; Gaba, S.; Chang, T.; Pan, X.; Lu, W., Nat.Commun. 2012, 3, 732. Tan, Z. H.; Yang, R.; Terabe, K.; Yin, X. B.; Zhang, X. D.; Guo, X., Adv. Mater. 2016, 28, 377-384. Figure 1

Electrochemomechanics with flexoelectricity and modelling of electrochemical strain microscopy in mixed ionic-electronic conductors

Recently, a new scanning probe microscopy approach, referred to as electrochemical strain microscopy (ESM), for probing local ionic flows and electrochemical reactions in solids based on the bias-strain coupling was proposed by Morozovska et al. Then, a series of theoretical papers for analyzing the image formation and spectroscopic mechanism of ESM were published within the framework of Fermi-Dirac statistics, the Vegard law, the direct flexoelectric coupling effect, the electrostriction effect, and so on. However, most of the models in these papers are limited to the partial coupling or particular process, and numerically solved by using decoupling approximation. In this paper, to model the ESM measurement with the coupling electrical-chemical-mechanical process, the chemical Gibbs function variational principle for the thermal electrical chemical mechanical fully coupling problem is proposed. The fully coupling governing equations are derived from the variational principle. When the tip concentrates the electric field within a small volume of the material, the inhomogeneous electric field is induced. So, both direct and inverse flexoelectric effects should be taken into account. Here, the bulk defect electrochemical reactions are also taken into account, which are usually omitted in the existing works. This theory can be used to deal with coupling problems in solids, including conductors, semiconductors, and piezoelectric and non-piezoelectric dielectrics. As an application of this work, a developed initial-boundary value problem is solved numerically in a mixed ion-electronic conductor. Numerical results show that it is meaningful and necessary to consider the bulk defect chemical reaction. Besides, the chemical reaction and the flexoelectric effect have an interactive influence on each other. This work can provide theoretical basis for the ESM as well as investigating the bulk chemical reaction process in solids.

Appling plasmonics based electrochemical microscopy to thin-layer electrochemistry

Plasmonics based electrochemical microscopy (PECM) has been introduced to image local electrochemical reactions optically. The algorithm used to convert the plasmonic signal to current, however, is not applicable to thin-layer electrochemistry, which is important for microfluidic detection and biosensing applications. Here we introduce a PECM algorithm for both semi-infinite and thin-layer electrochemical geometries. The new algorithm expresses plasmonic signal as local concentration of redox species on the electrode surface, which, together with the diffusion and Nernst equations, determines the current. We validate the algorithm experimentally by fabricating a thin-layer electrochemical cell, consisting of two-parallel electrodes separated with various distances.

A FEM model for investigation of micro-galvanic corrosion of Al alloys and effects of deposition of corrosion products

A finite element method model has been elaborated aiming at a deeper insight into the influence of microstructure on micro-galvanic corrosion of Al alloys. The model considers a dynamic corroding surface and takes into account kinetic data from local electrochemical reactions, transport of O2 and ionic species (e.g., Al3+, H+, Cl−), homogeneous reactions in the electrolyte, deposition of reaction products and its influence on both anodic and cathodic reactions. As a first step, an Al matrix with a micrometer-sized cathodic intermetallic particle exposed in 0.1M NaCl has been considered. The simulation predicts the dynamic changes of the corroding surface, and the flow and distribution of ionic species and of O2 in space and time. The calculated pH of the electrolyte inside and nearby the occluded corroding volume suggests the formation of insoluble Al(OH)3 on both the cathodic and anodic areas. This results in blocking effects of anodic and cathodic reactions and in eventual termination of the micro-galvanic corrosion. The predicted deposition of corrosion product is in good agreement with in-situ atomic force microscopy measurements.

The EChemPen: A Guiding Hand To Learn Electrochemical Surface Modifications

The Electrochemical Pen (EChemPen) was developed as an attractive tool for learning electrochemistry. The fabrication, principle, and operation of the EChemPen are simple and can be easily performed by students in practical classes. It is based on a regular fountain pen principle, where the electrolytic solution is dispensed at a tip to locally modify a conductive surface by triggering a localized electrochemical reaction. Three simple model reactions were chosen to demonstrate the versatility of the EChemPen for teaching various electrochemical processes. We describe first the reversible writing/erasing of metal letters, then the electrodeposition of a black conducting polymer “ink”, and finally the colorful writings that can be generated by titanium anodization and that can be controlled by the applied potential. These entertaining and didactic experiments are adapted for teaching undergraduate students that start to study electrochemistry by means of surface modification reactions.

Feasibility of in-plane GDL structuration: Impact on current density distribution in large-area Proton Exchange Membrane Fuel Cells

In the present work, the in-plane structuration of the electrode Gas Diffusion Layer (GDL) is proposed as an interesting approach to reduce the operating heterogeneities occurring within the cell along the gas flow fields, especially in dry operating conditions. This is especially important for transportation applications where the fuel cell is often operated in these conditions. Structured and homogeneous large-area (220 cm2) membrane electrode assemblies (MEAs) are tested in single-cell and 6-cell stack configurations.An S++® electronic card inserted in the stack is used to measure the current distribution within the cell, which is an indicator of the local electrochemical reaction conditions. The results of the current distribution measurements together with the cell performances obtained for the structured and the homogenous MEAs indicate that the electrode in-plane structuration is effective in reducing the functioning heterogeneities within the cell under specific operating conditions.

Analysis of Interfacial Processes at the SOFC Electrodes by I n-Situ Raman Spectroscopy

Kinetics of redox reactions and the variations of real charge-carrier concentrations across the SOFC electrode / electrolyte interfaces have critical importance for the fuel cell performance. The microscopic mechanisms of electrochemical processes in the vicinity of triple-phase boundary (TPB) can be assessed employing impedance spectroscopy, current-voltage measurements and various pulse techniques. Except for the micro-electrode approaches, however, these methods are cumulative. Moreover, unambiguous interpretation of the electrochemical measurement results requires, as a rule, to introduce complimentary experimental methods and/or to use simplified models. The present work is centered on the developments of a new combined technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions. The Raman spectroscopy was already successfully used to investigate both cathodic [1,2] and anodic [3] processes in SOFC, but the resultant information was mainly related to outer boundaries of the model electrochemical cells, primarily electrode surface, due to low penetration depth of the excitation radiation. The most important zones of the electrode systems, where the electrochemical reactions occur and ionic charge carriers are generated, cannot be achieved viewing the surface and edge areas. In the present work, this problem was solved by employing optically transparent, single-crystal membranes made of 10 mol.% Sc2O3 and 1% mol.% Y2O3 stabilized zirconia (10Sc1YSZ). In order to provide simultaneous Raman and electrochemical measurements of the cell placed under an oxygen chemical potential gradient, such as air/H2, a special controlled-atmosphere chamber was elaborated and tested. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the TPB zone, passing the beam through single-crystal solid electrolyte onto the interface, as a function of temperature, atmosphere, current density and/or overpotential. The results of case studies focused on redox kinetics of Ni-containing cermet anodes, are presented. This work was supported by grant 14-29-04031 of Russian Foundation for Basic Research. References S. Loridant, L. Abello, E. Siebert, G. Lucazeau, Solid State Ion. 78 (1995) 249-258K. Blinn, H. Abernathy, M.Liu, Advances in Solid Oxide Fuel Cells V. (2010) 63-73W. Bessler, M.Vogler, H. Stoermer, D. Gerthsen, A. Utz, A. Weber, E. Ivers-Tiffee, Phys. Chem. Chem. Phys., 12 (2010) 13888-13903

Effective Diffusivity of Electrolyte in Porous Structure Using a Three-Dimensional Random Pore Network Model

In Li-ion cell, electrochemical discharge and intercalation/deintercalation occur at length scales that differ by an order of magnitude1. Analysis of these processes has been conventionally done using the classical pseudo 2D-model2 that is based on the porous-electrode theory, which solves the volume-averaged species conservation in the electrolyte. However, the species conservation in solid active material is solved by using the intrinsic diffusion coefficient since the model uses a microstructure that is composed of a single spherical particle placed at each grid location in the electrode. Hence, this model is unable to account for the percolation network in real particle clusters. Therefore, to make volume averaging process applicable at each grid location in the electrode, it is necessary to estimate the effective diffusivity in both particle and electrolyte phases. In this work, a model has been used to predict the effective diffusivity in the porous structure. It is assumed that the micro-porous structure of an electrode in Li-ion battery can be approximate by 3-D random pore network model as shown in Fig. 1. In this model, a pore is assumed to be circular bond segment which connects any two nodes. To make the model more realistic, length and diameter of each pore segment are chosen with the circular diameter of each bond segment distributed using a Gaussian distribution. The length of the bond segment, which is the distance between the nodes placed randomly is also prescribed using a second Gaussian. The lengths and diameters of each pore segment are iteratively adjusted such that the total pore volume fraction generated is equal to the porosity. When a pore geometry of required porosity is attained, fixed concentration boundary conditions are prescribed at two faces (with zero-flux imposed on all other faces) and the steady-state Fick’s diffusion equation is solved in the electrolyte phase. By applying mass balance equation at each node of the lattice, the effective mass diffusivity can be calculated. The sensitivity of number of nodes (from 3 ᵡ 3 ᵡ 3 nodes to 50 ᵡ 50 ᵡ 50 nodes or more if necessary) over effective diffusivity for a range of electrode porosities will be studied to determine the optimum number of nodes over which the transport parameter becomes independent of the node number. At optimum node network, the calculated effective diffusivity will be compared with the existing theoretical models based on idealizations and simplifying assumptions such as Bruggeman, Weissberg and Maxwell correlation. So far, no electrochemical reaction or transport is assumed at solid-liquid interface to evaluate the effective diffusivity in electrolyte phase. Apart from effective diffusivity and conductivity in electrode, the interfacial electrochemical reaction rate is another homogenized quantity based on idealized spherical particle in pseudo 2D model. Now to estimation effective diffusivity in solid phase and electrochemical reaction flux, the corresponding microscopic transport equations, for ion concentration and electric potential in solid and liquid phase in steady state is to be solve. To solve the above transport equations, specifying the state variables value at opposite end of the cube as boundary condition (Dirichlet boundary condition). Finally, the solution of transport equations is used to calculate the localized electrochemical reaction flux along the interface based on Butler-Volmer equation. To evaluate the normalized overall reaction current density, the flux integrate over the entire interface and is divided by total interfacial area. All the transport equations are solved in steady state, because diffusion and conduction occur over a much smaller distance in cubic geometry (placed at each node) than in electrode.

Pt behavior in the Pt-C-S ± Fe-H2O system at 200–400°C and P tot = 1 kbar: Experimental results

The effect of sulfur on platinum adsorption on carbonaceous matter (CM) was experimentally studied at 200–400°C and Ptot = 1 kbar. The IR spectra of the experimental products indicate that sulfur accelerates HC condensation and aromatization, but the effect of sulfur on platinum concentrations in the organic fractions is within the analytical uncertainties. SEM images show the development of a multilayer porous carbonaceous film on the walls of the ampoules and platinum in physical contact with carbonaceous matter. The composition of the film varies, depending on its thickness (3–25 μm), within the following limits: 61.06–100 wt % C, 0–33.7 wt % Pt, 0–5.17 wt % O, and 0–0.74 wt % S. The film contains tiny Pt crystals, whose morphology varies with increasing duration of the experiments from nanometer- and micrometer-sized spheroids to subequant, tabular, and wire-like. Depending on their size, the composition of the crystals varies as follows: 23.30–52.45 wt % Pt, 49.57–73.52 wt % C, and 0–4.20 wt % O. According to our SEM data, the kerogen also contains tiny crystalline segregations of carbon aceous platinum whose morphology and composition are analogous to those on the film. The presence of carbon in the tiny platinum crystals deposited from solution can be explained by the background effect of the kerogen of the film and/or by their crystallization from organo-platinum complexes. In our kinetic experiments, local electrochemical reactions produced aggregates of nanometer-sized (60–250 nm) spheroids around larger micrometer-sized (up to 10 μm) spheroids, whose aggregation resulted in larger crystals and their further transformation. The polymorphism, hierarchical aggregation, and compositional variability of the platinum segregations are likely typical of car- bon-bearing systems because of their crystallization from metastable organo-platinum complexes.

Analysis of Interfacial Processes at the SOFC Electrodes by I n-Situ Raman Spectroscopy

The present work is centered on the developments of a new technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions, combined with electrochemical measurements. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the triple-phase boundary zone by passing the beam through transparent single-crystal solid electrolyte onto the interface, varying current density, temperature and atmosphere over the working electrode. The results of case studies, focused on reduction of Ni-containing cermet anodes, are presented.

Oxygen vacancy diffusion across cathode/electrolyte interface in solid oxide fuel cells: An electrochemical phase-field model

An electrochemical phase-field model is developed to study electronic and ionic transport across the cathode/electrolyte interface in solid oxide fuel cells. The influences of local current density and interfacial electrochemical reactions on the transport behaviors are incorporated. This model reproduces two electrochemical features. Nernst equation is satisfied through the thermodynamic equilibriums of the electron and oxygen vacancy. The distributions of charged species around the interface induce charge double layer. Moreover, we verify the nonlinear current/overpotential relationship. This model facilitates the exploration of problems in solid oxide fuel cells, which are associated with transport of species and electrochemical reactions at high operating temperature.