Cathode Microstructure Research Articles

Effect of the Activation Process on the Microstructure and Electrochemical Properties of N-Doped Carbon Cathodes in Li-O2 Batteries.

Lithium-oxygen (Li-O2) batteries have the potential to provide high energy densities; however, they suffer from low actual specific capacity and poor cycle performance. Hence, it is urgent to design a satisfactory oxygen electrode for a Li-O2 battery. In this study, carbonaceous materials, denominated CA, CB, and CC, from chitin were prepared by the three activators of H3PO4, KOH, and KHCO3 as oxygen electrode materials for Li-O2 batteries. The different carbon structural characteristics from the same precursor were regulated and controlled by different chemical reagents. Finally, the spherical particle cluster structure of CA has a high specific surface area, rich N doping, good connectivity, and uniform surface chemistry, so that CA acts as an oxygen electrode presenting excellent electron conductivity, providing sufficient, and stable electrochemical activity sites for oxygen reduction reaction and storing abundant discharge products. The electrochemical measurements indicate that at a current density of 0.02 mA/cm2, a CA-based battery delivers a high specific capacity of 16 600 mA h/g and a stable cycle performance of 210 cycles. This study proposes a functional carbonaceous material from chitin as a cathode oxygen electrode, which provides an economical and sustainable way for the improvement of oxygen electrodes and the application of Li-O2 batteries.

Modeling of solid oxide fuel cell (SOFC) electrodes from fabrication to operation: Microstructure optimization via artificial neural networks and multi-objective genetic algorithms

In this study, a modeling framework is proposed for the optimization of the solid oxide fuel cell (SOFC) electrode microstructures. This involves sequential simulations of the SOFCs from initial powder to final electrochemical performance with artificial intelligence-assisted multi-objective optimization. The effects of starting powder parameters such as particle size, particle size distribution (PSD) and pore former content on cathodic overpotential and degradation rate of SOFCs are studied. It is shown that fine particle size and/or low pore former content lead to low cathodic overpotential but high degradation rate in the investigated range of the parameters. Predictive models for the cathode overpotential and degradation rate are established by an artificial neural network using the simulation data. The Sobol global sensitivity study suggests that particle size and pore former content play important roles in determination of the cathode overpotential and degradation rate while the PSD effect is insignificant. A multi-objective genetic algorithm (MOGA) is used to minimize both the overpotential and degradation rate of the cathode. The Pareto front is obtained for the optimal design of cathode microstructures. Compared to the grid search method, the MOGA proves to be more robust and efficient for SOFC electrode microstructure optimization.

Production of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode with graded porosity for improving proton-conducting solid oxide fuel cells

Utilization of 500-nm-diameter polystyrene (PS) nanospheres as pore former in a screen printing process to tailor the porous structure of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode for improving the performance of protonic solid oxide fuel cells is reported. The effects of PS nanosphere amount on cathode microstructure and cell performance are investigated. It is found that PS nanospheres can undergo a self-organized distribution in the screen-printed LSCF cathode due to the large difference in density between PS and LSCF, resulting in a porosity gradient in the cathode structure. The fuel cell with a 15 wt% PS-tailored cathode exhibits a much higher power density compared to that without tailoring by PS. The enhanced cell performance can be ascribed to the graded porosity in the cathode structure, which significantly reduces the ohmic and polarization resistances. It seems that such a graded-porosity cathode structure not only facilitates the generation and migration of O−ad from catalytic sites to triple phase boundaries (TPBs) but also promotes transfer of protons from the electrolyte to the TPBs.

Influence of Water Vapor on Performance Degradation and Microstructural Change of (La,Sr)(Co,Fe)O3−δ Cathode

In this study, single cells employing a La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) cathode were operated with a supply of humidified oxygen to the cathode at 1000 °C for 100 h to investigate the influence of water vapor on the performance and microstructure of LSCF cathode. When the gaseous mixture of 20% H2O–80% O2 was supplied to the cathode, the performance of LSCF cathode continuously lowered during discharge at 300 mA cm–2 for 100 h. Then, the microstructures of surface and cross-section of LSCF cathode were observed by scanning electron microscopy. The surface morphology was drastically changed by discharge operation. The SrO layer was formed at the outermost surface of cathode, indicating that the strontium segregation was accelerated by water vapor. In response to this phenomenon, the formation amount of cobalt- and/or iron-based oxides enlarged inside the electrode. These microstructural and phase changes would be responsible for the performance deterioration of LSCF cathode.

Performance and Stability of Cathode Electrode Under Carbon Capture Operating Conditions

Introduction The high-temperature molten carbonate fuel cell (MCFC) is one of the most advanced clean power generating devices. To date >8 billion kWh of clean electricity has been produced commercially using this new technology. The high operating temperature of MCFC (550-650°C) dramatically improves the reaction kinetics and eliminates the need for a noble metal catalyst. The electrochemical reactions taking place during cell operation involve CO2 transfer from cathode to anode through electrolyte matrix in the form of carbonate ions (Figure 1). The transport of CO3 = is equivalent to that of O= and CO2 (CO3 = ⇔ O= + CO2). Therefore, MCFC stack technology can be utilized for efficient simultaneous power generation and CO2 separation/capture by integrating with conventional combustion-based coal and/or natural-gas power plants. The state of the art MCFC cathode is porous lithiated NiO. The cathode performance and stability are affected by several factors such as gas composition, temperature, electrode structure, and electrolyte chemistry. To ensure long-term performance and material stability (such as NiO cathode dissolution, polarization loss and CO2 capture efficiency), material selection, operating characteristics, and electrode design need to be carefully considered. FCE has operated numerous bench-scale single cells (100 W) and technology stacks (30 kW) under carbon capture operating conditions (4-5% CO2 in the cathode inlet as opposed to >15% in baseline MCFC systems) to understand parameters affecting performance, life, and to investigate design solutions for further enhancement. Figure 2 shows similar NiO cathode stability under standard as well as carbon capture conditions, projecting to useful life of >7-years under both operation conditions. More endurance carbon capture tests are being conducted for further verification. This paper will review the cathode material stability, microstructure, and durability under long-term carbon-capture operations. The effect of parameters such as electrolyte fill, gas composition and electrolyte chemistry, as well as approaches to enhance the CO2 capture efficiency and life, will be discussed. Figure 1

Modeling of solid oxide fuel cell (SOFC) electrodes from fabrication to operation: Correlations between microstructures and electrochemical performances

Abstract In this study, a numerical framework which jointly uses discrete element method, kinetic Monte Carlo method and lattice Boltzmann method is proposed to predict the electrochemical performance of solid oxide fuel cell cathode from raw powder. A total of 770 La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes with different microstructures are numerically synthesized using various particle size, size distribution, pore former content and sintering time. Best performance is found at a porosity of approximately 0.40. The cathode performance degrades tremendously at about a critical porosity of 0.10–0.25 because the pores become disconnected. The critical porosity is found to have a positive linear dependency on the mean pore size. Cathodes that have smaller mean pore size may still operate functionally at a low porosity of 0.10. A response surface model (RSM) is proposed to predict the overpotential from microstructural parameters. The RSM is also found to be valid for real cathode microstructures and can be served as an off-line assessment tool for the cathode performance.

High performance BaCe0.5Fe0.5-xBixO3-δ as cobalt-free cathode for proton-conducting solid oxide fuel cells

To explore the space for further optimization of BaCeO3-based single-phase cathode, series of perovskite materials BaCe0.5Fe0.5-xBixO3-δ (x = 0, 0.1, 0.2, 0.3) as cobalt-free cathodes for proton conducting solid oxide fuel cells are evaluated in this assignment. The influence of the firing temperature on cathode microstructures and the electrochemical performance of fuel cells is evaluated, which states clearly that the best sintering temperature for BaCe0.5Fe0.3Bi0.2O3-δ cathode is 900 °C. Moreover, a special attention is paid to the effect of the Bi doping amount on the phase structure and electrochemical performance. With the cathode BaCe0.5Fe0.3Bi0.2O3-δ, the single cell achieves the polarization resistance of 0.061 Ω cm2 and the maximum power density of 943 mW cm−2 at 700 °C. These encouraging electrochemical results demonstrate that BaCe0.5Fe0.3Bi0.2O3-δ is a promising cobalt-free cathode for proton-conducting solid oxide fuel cells.

Influence of microstructure on the electrochemical behaviour of LSCF-SDCC

The correlation of cathode microstructure with the electrochemical behavior of LSCF-SDCC has been studied. In this study, LSCF-SDCC composite cathode powders were hamimah@uthm.edu.my prepared using ball milling and calcined at 700-900°C. Cathode pellets were fabricated using the calcined powders via uniaxial press and sintered at 600°C. A reduction of pellet porosity (33.1-29.8%) was observed as the calcination temperature increased, which was also in line with the cross-sectional view of the pellet. In electrochemical analysis, dominant resistance was observed in all samples which corresponded to low adsorption of oxygen. Meanwhile, cathode prepared with calcined powders of7 50°C exhibited the least area specific resistance that yielded single cell performance of 139m W/cm2 at 550°C.

Combining tomographic imaging and in silico computation for rapid effective PEMFC cathode transport characterization

An approach for determination of effective oxygen diffusivity within a polymer electrolyte membrane fuel cell cathode catalyst layer by combining tomography and geometric calculations is presented. Accurate determination of effective oxygen diffusivity is critical for modeling and characterization of cathode design architectures. In silico geometric characterization methods exploit the increasing availability, capability, and decreasing cost of tomographic imaging and high-performance computing. Geometric methods can be made simple and fast and could accelerate efforts to iteratively improve cathode design. The method described herein relied on mathematically exact and statistically significant definitions of geometric tortuosity and geometric constrictivity, in addition to porosity. To demonstrate generality and robustness, a correlation for obstruction factor underpinned by these parameters was statistically induced from hundreds of instantiations of stochastic cathode microstructures generated by a simulated annealing method and validated versus the obstruction factor of real cathode microstructures obtained separately by Star and Fuller in a previous publication and Ziegler et al. at the University of Freiburg. This work demonstrated the utility and viability of adopting a combined tomography/geometric-computational approach for determination of the effective diffusivity of reactant in the cathode catalyst layer and its underlying transport properties. This analysis was based on a conventional cathode microstructure with platinum catalyst, carbon black support, and perfluorosulfonic acid ionomer. However, the method could easily be extended to platinum group metal-free cathodes in which accurate characterization may be of even greater importance due to the thicker cathode layers required in these systems. This approach could be widely adopted and relied upon for characterization as the materials and processing conditions of cathode layers are iteratively tested and optimized.

Near-Surface Material Phases and Microstructure of Scandate Cathodes.

Scandate cathodes that were fabricated using the liquid-solid process and that exhibited excellent emission performance were characterized using complementary state-of-the-art electron microscopy techniques. Sub-micron BaAl2O4 particles were observed on the surfaces and edges of tungsten particles, as seen in cross-section samples extracted from the scandate cathode surface regions. Although several BaAl2O4 particles were observed to surround smaller Sc2O3 nanoparticles, no chemical mixing of the two oxides was detected, and in fact the distinct oxide phases were separately verified by chemical analysis and also by 3D elemental tomography. Nanobeam electron diffraction confirmed that the crystal structure throughout W grains is body-centered cubic, indicating that they are metallic W and did not experience noticeable changes, even near the grain surfaces, as a result of the numerous complex chemical reactions that occur during cathode impregnation and activation. 3D reconstruction further revealed that internal Sc/Sc2O3 particles tend to exhibit a degree of correlated arrangement within a given W particle, rather than being distributed uniformly throughout. Moreover, the formation of Sc/Sc2O3 particles within W grains may arise from W surface roughening that occurs during the liquid-solid synthesis process.

Numerical simulation of La0.6Sr0.4Co0.2Fe0.8O3- Gd0.1Ce0.9O1.95 composite cathodes with micro pillars

In the current study, electrochemical performances of La0.6Sr0.4Co0.2Fe0.8O3-Gd0.1Ce0.9O1.95 (LSCF-GDC) composite cathode microstructures are numerically simulated in order to clarify the effects of GDC pillar which is introduced to enhance the effective ionic conductivity of the solid oxide fuel cell (SOFC) cathode. The numerical simulation is carried out based on the three-dimensional microstructure reconstructed with focused ion beam scanning electron microscopy (FIB-SEM). In order to further investigate the characteristics of the GDC pillars, we examine the electrochemical effects of GDC pillars inside the pure LSCF and LSCF-GDC composite cathode microstructures with different particle sizes. According to the simulation results, the GDC pillars effectively improve the performance of the pure LSCF cathode, and the improvements by the GDC pillars are more pronounced in the microstructures with small particle sizes.

Influence of Water Vapor on Performance Degradation and Microstructural Change of (La,Sr)(Co,Fe)O3−δ Cathode

In this study, single cells employing a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode were operated with a supply of humidified oxygen to the cathode at 1000°C for 100 h to investigate the influence of water vapor on the performance and microstructure of LSCF cathode. When a gaseous mixture of 20% H2O−80% O2 was supplied to the cathode, the performance of LSCF cathode continuously lowered during a discharge at 300 mA cm−2 for 100 h. Then, the microstructures of surface and cross-section of LSCF cathode were observed by scanning electron microscopy. The surface morphology was drastically changed by the discharge operation. A SrO layer was formed at the outermost surface of cathode, indicating that the strontium segregation was accelerated by water vapor. In response to this phenomenon, the formation amount of cobalt- and/or iron-based oxides enlarged inside the electrode. These microstructural and phase changes would be responsible for the performance deterioration of LSCF cathode.

Ultrahigh length-to-diameter ratio nickel phosphide nanowires as pH-wide electrocatalyst for efficient hydrogen evolution

The hydrogen evolution reaction (HER) represents one promising solution for energy and environmental sustainability. Nickel phosphide with various morphology has been recently investigated as promising earth abundant electrocatalysts for HER. Compared with three or two dimensional morphology, one dimensional (1D) nickel phosphide could provide larger specific surface area and hence improves the HER electrocatalytic performance. However, most reported 1D nickel phosphides show a small length-to-diameter ratio (less than 50). Herein, ultrahigh length-to-diameter ratio Ni2P nanowires synthesized by one-step phosphorization-sulfuration treatment are reported for the first time. The as-prepared nanowires have extremely high length to diameter ratio which ranges from 500 to 1000. The formation mechanism of such morphology is illustrated by a systematic study of the morphology, microstructure and elementary distribution of the nanowire array cathode. The ultralong nanowire morphology clearly helps improve the HER performances, since the self-supported Ni2P nanowire array cathode exhibits outstanding HER electrocatalytic activity and long-term durability in all of the acidic, alkaline, and neutral conditions.

Thermal behavior and microstructures of cathodes for liquid electrolyte-based lithium batteries

Lithium-ion batteries are widely used as a power source for portable equipment. However, the use of highly flammable organic solvents in the liquid electrolyte component in these batteries presents a serious safety concern. In this study, the thermal stability of battery cathodes comprising LiNi1/3Mn1/3Co1/3O2 (NMC) and LiPF6-based electrolyte solutions have been investigated using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) methods. Ex situ TEM measurements revealed that significant structural change occurred in the charged NMC composite after heating at a temperature above the exothermal peaks. It was found that LiF nanocrystallites precipitated in LiPF6 and that a number of nanoscale stacking faults developed in the Rbar{3}m layered structure of NMC. The results suggested that the decomposition reaction of LiPF6 and the structural change of NMC were directly associated with the exothermic reaction in the liquid electrolyte-based NMC electrode composite.

A particle-resolved 3D finite element model to study the effect of cathode microstructure on the behavior of lithium ion batteries

This manuscript presents the formulation and implementation of a particle-resolved 3D finite element (3DFE) model to study the effect of cathode microstructure on the electrochemical and mechanical responses of lithium-ion batteries (LIBs) during the discharge process. In this model, active particles (lithium iron phosphate) are explicitly resolved to allow the direct assignment of boundary conditions on particle surfaces. To incorporate the effect of binder phase in a numerically efficient manner, it is modeled as a homogenized matrix with a pore-filling electrolyte that allows the transport of both electrons and ions. A new virtual packing algorithm is implemented to reconstruct three cathode microstructures, one of which has an idealized microstructure with spherical particles of similar radii. This microstructural model is used for the verification of the 3DFE model through comparison with the pseudo two-dimensional (P2D) model. The other two cathodes, which are composed of two-size spherical and arbitrary-shaped particles, are used to study the impact of microstructure and discharge rate on the electro-chemo-mechanical behavior. The proposed model predicts the effect of electrode microstructure on active material utilization, as well as the magnitude and distribution of mechanical stresses developed in particles. We also study how the mechanical response of the cathode is affected by the binder stiffness.

How the colloid chemistry of precursor electrocatalyst dispersions is related to the polymer electrolyte membrane fuel cell performance

Polymer electrolyte membrane fuel cells (PEMFCs) operating at low temperature (60–80 °C, up to 110 °C) are mostly limited in their performance by the kinetics of the oxygen reduction reaction (ORR), leading to high loadings of platinum (Pt) in the cathode. Pt catalysts are without alternative in numerous industrial applications, and since Pt resources are limited, the associated high costs for low temperature fuel cells are hindering among other factors their commercialization. In order to increase the fraction of electrocatalytically available Pt towards ORR, this work is devoted to the factors responsible for the microstructure of the PEMFC cathodes. Typically, the active layers are coated by processes like spraying, doctor blading, printing etc. Therefore, the final structure actually is strongly dependent on the coating process and the physicochemical properties of the catalyst dispersions used. Selecting commercially available electrocatalysts from Johnson-Matthey and Tanaka as active material and ultrasonically assisted spraying as the coating method, systematic variations of the surface chemistry of the catalyst particles and their influence on catalyst layer morphology and therefore electrical and electrochemical properties of resulting membrane electrode assemblies (MEA) have been investigated. It could be shown, that the colloid–chemical properties of the catalyst dispersions have a profound influence not only on the microstructure of the MEAs but also on the performance under operating conditions.

Template-determined microstructure and electrochemical performances of Li-rich layered metal oxide cathode

This report unravels the dependence of the microstructure and electrochemical performances of Li-rich layered transition metal oxide (LLMO) on the molecular structure of polymer templates for the formation of oxalate precursor. A representative LLMO, Li1.2Mn0.54Ni0.13Co0.13, is synthesized by co-precipitation method, and three samples, LLMO-PVP, LLMO-PEG and LLMO-PVA, are obtained with polymer templates, polyvinyl pyrrolidone (PVP), polyethyleneglycol (PEG) and polyvinyl alcohol (PVA), respectively. The physical and chemical properties of the resulting products are analyzed with SEM, TEM, XRD, BET, ICP, and XPS, and their electrochemical performances as cathode of Li-ion battery are evaluated with EIS, GITT and charge/discharge tests. It is found that LLMO-PVP exhibits the best performances, followed by LLMO-PEG, while LLMO-PVA behaves poorest. This difference is ascribed to the various microstructures of the resulting products, which are determined by the molecular structure of polymer templates.

Analysis of the 3D microstructure of experimental cathode films for lithium-ion batteries under increasing compaction.

It is well known that the microstructure of electrodes in lithium-ion batteries has an immense impact on their overall performance. The manufacturing of the batteries includes the so-called calendering, where the electrodes are compressed with a certain pressure, which is called compaction load. This process step mainly determines the resulting morphology of the electrode and thus the properties of the battery. Therefore, eight cathodes with different compaction loads were manufactured and imaged by synchrotron tomography, which leads to 3D images containing detailed information about the inner structure of the cathode. This image data allows an extensive analysis of the 3D cathode microstructure with respect to increasing compaction. In order to quantify the microstructural changes we use several characteristics describing diverse properties of the morphology. Furthermore, the 3D image data can be used for the computation of characteristics which can not be determined by experiments. Therefore, 3D image data allows us to understand how the microstructure of cathodes is influenced by the compaction load. Finally, we are able to predict the distribution of a certain characteristic for arbitrary compaction loads. This information is valuable with regard to the development of improved lithium-ion batteries.

Theoretical Modeling of the Electrochemical Lithiation Processes of Lithium-Sulfur Battery Cathodes Based on Sulfur/Graphene-Nanosheets Composites

The theoretical energy density of 2.5 kWh/kg of sulfur, plus its low-cost availability and environmentally friendliness, makes of lithium-sulfur (Li-S) batteries one of the most promising alternatives to fulfill the need of energy storage devices for high energy-demanding applications such as electric vehicles and smart grids. However, the successful commercialization of Li-S batteries is still challenged by the low utilization of sulfur, linked to its poor electronic/ionic conductivity and its continuous irreversible dissolution into the electrolyte in the form of long-chain lithium-polysulfides. So far, the mixing of sulfur with carbon has led to its better physical confinement and chemical sequestration in the cathode. However, the details of the sulfur-carbon interaction and its effects on the electrochemical lithiation processes during battery cycling are still largely unknown. Here we aimed at characterizing the structure of sulfur/graphene-nanosheets composites (SC composite) and understand its effects on the electrochemical lithiation processes during battery cycling. Reactive molecular dynamics (ReaxFF) calculations provided information on the cathode microstructure and the bulk reduction products upon lithiation. Ab initio molecular dynamics simulations (AIMD) along with density functional theory (DFT) optimizations, yield details of the effects of different electrolyte compositions on the SC composite capabilities for retention of sulfur and the polysulfide formation paths. We elucidated that a stronger sulfur-carbon interaction not only contributes to a better electronic conductivity but also leads to an alternative sulfur reduction path without the formation of long-chain polysulfides, which might help to explain the improved cycling stability observed in sulfurized polyacrylonitrile composites reported in the literature. In addition, we observed that different electrolyte compositions might be beneficial too by allowing an easier recovery of S-C bonds upon charging. We believe the results from this work can contribute to improving the synthesis methodologies of sulfur-carbon composites leading to a more rapid commercialization of lithium-sulfur batteries.

Flexibility and High-Rate Discharge Properties of Organic Radical Batteries with Gel-State Electrodes

Technology related to Internet of Things (IoT) and wearable devices has just begun to be actively developed. In that field, the demand for thinness and flexibility has increased in secondary batteries due to the structural compatibility of IoT or wearable devices such as vital sensors. We have been developing organic radical batteries (ORBs) with cathodes consisting of nitroxyl radical polymers such as poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl-4-yl methacrylate) (PTMA). Our previous work showed that ORBs with PTMA cathodes have high rate capabilities. PTMA is also known to be changed into a gel-like state due to the absorption of organic electrolytes. The PTMA gel in the electrodes is certain to be suitable for thin and flexible batteries because it is hardly damaged when it is repeatedly bent and unbent. However, the state of the PTMA cathodes in the battery has not been characterized, and the flexibility of ORBs with the PTMA cathodes has not been investigated so far. The objective of this work is to clarify the state of the cross-linked PTMA cathodes in the battery and flexibility of the ORBs. We also report the electrode preparation method of cross-linked PTMA cathodes to obtain practical ORBs with both high rate discharge properties and excellent cyclability. Cross-linked poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl-4-yl methacrylate) (PTMA) gel used in organic radical batteries (ORBs) is characterized by rheological measurements, and the micro structure of gel-state cathodes is observed using low vacuum SEM. The storage elastic modulus of the cross-linked PTMA gel is under 10 kPa which is in the range of soft gel. It suggests that the PTMA cathodes are fairy flexible in the battery. The ORBs with the cathodes made from cross-linked PTMA/vapor grown carbon fiber (VGCF) composite show excellent discharge rate properties (20C/1C capacity 91%). The structure of needle-like VGCF penetrating PTMA particles in the composite results in excellent rate capabilities. In the PTMA gel cathode, the cross-link structure of the PTMA has the effect of maintaining homogeneous dispersion of VGCF, which gives improvement in cyclability. The capacity and resistance of the ORBs are maintained in the initial state after repetitive bending 250 times. We demonstrate that the gel state cross-linked PTMA is a promising electrode material for practical flexible batteries.