Anode Microstructure Research Articles

In Operando Study on Nickel Morphological Change at Three-Phase-Boundary in Solid Oxide Fuel Cell Using Patterned Nickel-Film Electrode

In this study, an in operando observation technique based on confocal laser scanning microscope is developed to demonstrate the in operando morphological change of nickel (Ni) at active three-phase-boundary (TPB) under anodic polarization in solid oxide fuel cell (SOFC) anode. The observation reveals that the complex morphological change of Ni is determined by the competition between the mechanisms of enhanced Ni wettability on yttria-stabilized zirconia (YSZ) surface and the breaking up of thin Ni-film caused by Ni coarsening. The result explains the rapid performance degradation of anode under high current density. It is also expected that the in operando observation of Ni morphological change can largely enrich our understanding of the SOFC anode microstructure change.

Ammonia as Promising Fuel for Solid Oxide Fuel Cells: Experimental Analysis and Performance Evaluation

In the course of this study the direct utilization of ammonia in different types of solid oxide fuel cells (SOFCs), such as anode- and electrolyte-supported SOFC, is investigated. Experiments in low fuel utilization, exhibited excellent performance of ammonia in SOFCs, although the power outputs of equivalent hydrogen/nitrogen fuels were not attained due to the incomplete endothermic ammonia decomposition. Next, the single cells were operated under high fuel utilization conditions and methane was added to the humidified ammonia stream, where they showed excellent ammonia- and methane conversions. The stability of the cells used was proven over a period of at least 48 hours with a variety of fuel mixtures. Post mortem scanning electron microscopy analysis of the anode micro-structures indicated nitriding effects of nickel, as microscopic pores and enlargements of the metallic parts occurred. Finally, a long-term test over 1,000 hours was carried out using a ten-layer stack consisting of electrolyte-supported cells.

Development of proton conducting fuel cells using nickel metal support

This research reports the fabrication of a metal supported fuel cell based on the well-known proton conducting electrolyte (PCE) BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) material. The impact of the addition of YSZ on the anode microstructure is also described. This solid oxide fuel cell (SOFC) consists of Ni:yttria stabilized zirconia (YSZ) support (with different weight ratios of Ni:YSZ), Ni-BZCYYb anode functional layer (AFL), BZCYYb electrolyte and LSCF-BZCYYb cathode. The results show that addition of only 10 wt% YSZ to the support improves the microstructure of the cell by preventing severe Ni grain growth inside the support, and providing more accessible triple phase boundaries (TPBs) at the contact surface of the support/AFL and better distribution of the fuel gas to the accessible TPBs. As a result of microstructural improvement, polarization resistance of the cells with 10 wt% YSZ decreases compared with the cell having pure Ni metal. The cell containing YSZ in the support shows 67 h of stable operation under potentiostatic SOFC condition without any degradation. The current metal supported tubular fuel cell shows promising electrochemical and microstructural features which has the possibility to replace the conventional (Ni-PCE) or even Ni-YSZ anode supports.

Na2SnO3 as a novel anode for high performance lithium storage and its electrochemical reaction mechanism

Herein, Na2SnO3 is employed as an anode for rechargeable lithium ion battery (LIB). We thoroughly investigated the electrochemical performance of Na2SnO3 in comparison with the most commonly used Sn based oxides, such as SnO2 and Li2SnO3. It is found that Na2SnO3 is greatly superior to SnO2 and Li2SnO3 in terms of capacity, cycling stability and rate capability. Impressively, Na2SnO3 presents favorable specific capacity of 480 mA h g−1 at current density of 200 mA g−1 after 100 cycles and still delivers a capacity of 439 mA h g−1 at extremely large current density of 1000 mA g−1, which are leading the performance in anodes for LIBs. Ex situ SEM analysis of anodes after different cycles revealed the surface microstructure of anodes plays a critical role in determining cycling stability. The SEM results show big cracks on the surface of electrode for SnO2 after less 15 cycles and for Li2SnO3 after more 100 cycles, resulting from their severe volume change during charging-discharging process. However, Na2SnO3 electrode exhibits uniform surface morphology after 100 cycles. It is concluded the “Na2O″ intrinsic matrix of Na2SnO3 combining with “Li2O″ formed from the conversion reaction can act as a mixture buffering matrix that contributes to keeping the electrochemically formed nanoscale Sn particles apart and preventing their agglomeration during Li−Sn alloy formation and decomposition, thus inhibiting the volume expansion and the capacity fading by maintaining the electrode integrity. In addition, the electrochemical reaction mechanism of Na2SnO3 with Li is investigated by ex situ XRD technique. The findings in this study provide a new valuable anode for high-performance LIBs and an insightful viewpoint of developing anode materials with high electrochemical performance by introducing the electrochemical inactive intrinsic matrix.

(Invited) Enhancing High Temperature Electrode Performance with Secondary Phases

This work summarizes an exhaustive effort intended to identify the mechanisms responsible for improved electrode performance that results from adding small amounts of aluminum titanate (Al2TiO5, ALT) to Ni-YSZ cermet anodes used in solid oxide fuel cells (SOFCs). Specifically, high temperature Raman spectroscopy and X-ray diffraction combined with electrochemical impedance measurements and ex situchemical analysis show that during electrode processing, ALT reacts to form secondary phases including NiAl2O4and Zr5Ti7O24. These secondary phases confer unexpected and cooperative benefits to the electrode’s mechanical and electrocatalytic properties as well as its operational resilience. Electrodes in high-temperature energy conversion devices must fulfill several critical tasks in order for these devices to realize their full potential of efficient operation, fuel flexibility and scalable architecture. Specifically, electrodes must remain stable at temperatures as high as 900˚C and be tolerant of incident contaminants. Traditional SOFC anodes consist of ceramic-metallic (or cermet) composite materials that are comprised of an oxide ion conducting yttria-stabilized zirconia (YSZ) and nickel oxide that is reduced to elemental Ni prior to SOFC operation. Ni is an outstanding and inexpensive electrocatalyst but is susceptible to particle coarsening during extended operation and poisoning by contaminants including sulfur and chlorine. Furthermore, when operating with carbon containing fuels, Ni-YSZ anodes can suffer from carbon accumulation (or coking) that blocks electrochemically active sites and will eventually disintegrate the electrode through metal dusting. A final liability of Ni-based SOFC electrodes is their inability to withstand reduction-oxidation cycling. Repeated Ni oxidation followed by NiO reduction creates large mechanical stresses in the anode microstructure that lead to electrode delamination from the electrolyte and electrode cracking. Recent studies have reported that small amounts (3-5% by mass) of ALT added to the Ni-YSZ cermet during anode fabrication endow electrodes with improved performance, enhanced strength, carbon tolerance and resilience to redox cycling.1-4 Specifically, adding 5% ALT (by mass) to a Ni-YSZ cermet coupon improves its mechanical strength measured with three point bending by 40% and functional ALT-enhanced Ni-YSZ anodes show two to three fold improved resilience to both electrochemical and environmental redox cycling. Additional studies presented elsewhere at this meeting demonstrate that adding ALT to Ni-YSZ anodes also improves electrode carbon tolerance, suppressing carbon accumulation by up to 75% in operating SOFCs. While the benefits of enhancing Ni-YSZ cermet anodes with ALT are clear, the mechanisms responsible for observed enhancements are not. Results presented in this work suggest that alumina (Al2O3) formed when NiAl2O4is reduced serves to anchor Ni particles and preserve microstructure. Al2O3also appears to play a reforming role that limits carbon accumulation on ALT-enhanced anodes. Electrochemical measurements demonstrate that Zr5Ti7O24has both oxide and electron conducting capabilities, allowing this MIEC material to expand the anode’s three phase boundary. Prior studies have shown that Zr5Ti7O24’s electronic conduction results from electron hopping from Ti3+and Ti4+sites and suggest that this n-type conduction can occur when titanium ions occupy as few as 4% of the YSZ cation sites.5 References. M. Welander, et al.“Operando Studies of Redox Resilience in ALT Enhanced NiO-YSZ SOFC Anodes” J. Electrochem. Soc. 165(2018) F152-F157.Hunt, et al.“Degradation Rate Quantification of Solid Oxide Fuel Cell Performance with and without Al2TiO5Addition” Int. J. Hydrogen Energy 43(2018) 15531-15536.R. Driscoll, et al. “Enhancement of High Temperature Metallic Catalysts: Aluminum Titanate in the Nickel-Zirconia System” Applied Catalysis A - General 527(2016) 36-44.McCleary and R. A. Amendola “Effect of Aluminum Titanate (Al2TiO5) on the Mechanical Performance of Solid Oxide Fuel Cell Ni-YSZ Anodes” Fuel Cells 17(2017) 862-868.K. E. Swider and W. L. Worrell “Electronic Conduction Mechanism in Yttria Stabilized Zirconia-Titania under Reducing Atmospheres” J. Electrochem. Soc. 143(1996) 3706-3711.

The Effects of Cell Design on Li Plating in Li-Ion Cells: A Real Insight into Cell Design Optimization for Minimizing Li Plating

Li-ion batteries are extensively used in personal electronic devices and electric vehicles (EVs) because of high energy density and high voltage. Although the Li-ion battery technology is satisfying for regular applications, several challenges such as cost, safety, longevity, power density, and energy density remain major hurdles to be overcome for mass production of next generation EVs. Among these challenges, battery lifetime has been recently paid special attention because of detrimental effects that fast charging has on EVs battery lifetime. The main objective of the present work is to study the effects of design parameters on Li-plating on the negative electrode. Li plating is understood to occur during fast charging especially at lower temperatures [1]. Li plating as well as other degradation mechanisms is the main cause of capacity fade and cell degradation [1, 2]. Therefore, understanding the Li-plating phenomena becomes indispensable particularly during aggressive operations of Li-ion cells (fast charging). This knowledge highly proves necessary for cell design and optimization. According to our modeling results Li plating can happen approximately within a distance equal to 35% of the negative electrode thickness, close to the separator. Therefore, the negative electrode design is critical for minimizing Li plating. Design parameters such as type and size of the negative electrode active particles, the negative electrode thickness, along with structural properties, including porosity and tortuosity [1, 3, 4] are primarily factors that influence Li-plating rate during fast charge that should be taken into account for cell design, development, and optimization. Particularly, a hierarchical anode microstructure and its effects on the amount of Li plating under different Crates and temperatures is studied in this work. A hierarchical anode microstructure can be established by moving some portion of the conductive additive and binder towards the negative electrode current collector that results in higher porosity close to the separator and lower porosity close to the current collector. The goal is to design a cell that exhibits little to no Li plating, while being charged (0% SOC to 80% SOC) in less than 45 minutes which is the target for the current EVs charging algorithm [5]. In this work, we deploy a mathematical Multiphysics model, developed in COMSOL Multiphysics 5.4, a finite element based commercial software, to provide in-depth knowledge of the relationship between Li plating amount and the negative electrode design and structural parameters for cell design and optimization purposes. Li plating is considered as an unwanted electrochemical reaction with an equilibrium potential of zero that is coupled with a so-called Pseudo 2D (P2D) Newman-type model [6]. When the Li plating starts (e.g. at high SOC during charge) there is a competition between Li-plating and intercalation currents evaluated by the model and can be changed with different model parameter sets. The fidelity of the model was validated by predicting the experimental data under various cell operations. [1] M.M. Forouzan, B.A. Mazzeo, D.R. Wheeler, J. Electrochem. Soc., 165 (2018) A2127-A2144. [2] A. Barré, B. Deguilhem, S. Grolleau, M. Gérard, F. Suard, D. Riu, Journal of Power Sources, 241 (2013) 680-689. [3] J.E. Vogel, M.M. Forouzan, E.E. Hardy, S.T. Crawford, D.R. Wheeler, B.A. Mazzeo, Electrochimica Acta, (2018). [4] M.M. Forouzan, M. Wray, L. Robertson, D.R. Wheeler, J. Electrochem. Soc., 164 (2017) A3117-A3130. [5] L. Donaldson, Materials Today, 21 (2018) 105-106. [6] T.F. Fuller, M. Doyle, J. Newman, J. Electrochem. Soc., 141 (1994) 1-10.

Ultra-fast fabrication of tape-cast anode supports for solid oxide fuel cells via resonant acoustic mixing technology

Abstract Herein, for the first time, we demonstrate ultra-fast fabrication of a tape casted NiO-yttria stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFCs) using resonant acoustic mixing (RAM) technology. Due to its characteristics of non-contact and high-intensity acoustic field-assisted mixing, NiO-YSZ tape-cast slurry is prepared via a RAM process within ∼0.5 h, > 140 times faster than use of a conventional ball-milling (BM) process (∼72 h). During the RAM process, liquid binders more effectively penetrate into soft agglomerated ceramic powders and covered larger surface area than the case of BM process. The optimization of RAM procedure requires more subdivided mixing sequence and higher content of binders and plasticizers compared to that of the BM. Despite drastically reduced mixing time, the microstructures of RAM Ni-YSZ anode, quantified via a 3D reconstruction, are statistically identical to that of BM Ni-YSZ. The SOFC employing RAM Ni-YSZ anode support achieves 0.55 W/cm2 at 750 °C in peak power density and exhibits high durability for 300 h without noticeable degradation. Thus, our results demonstrate that the RAM process is a highly effective and ultra-fast mixing technology to produce high performance SOFC components.

Influence of Initial Powder Morphology on Polarization Characteristics of Nickel/Gadolinium-Doped-Ceria Solid Oxide Fuel Cells Electrode

Nickel-gadolinium doped ceria (Ni-GDC) composite anodes gain a lot of attention as they have potential to be a beneficial solution for the intermediate temperature solid oxide fuel cells. In this paper, the influence of the initial microstructure of Ni-GDC anode on the polarization characteristics and its deterioration in 100h operation are investigated. The initial microstructures of the investigated anodes are determined by the different sizes and aspect ratios of the GDC powders, but all anodes having the same initial compositions of NiO:GDC = 60:40 vol.%. It was found that the anode with the smallest GDC particle had the best initial performance, but also underwent the most severe degradation. The microstructures of the fabricated anodes were investigated by focused ion beam - scanning electron microscopy (FIB-SEM), and microstructural parameters were quantified to relate the electrochemical performance. From the experimentation, contribution of the Ni-GDC microstructure to the anode polarization characteristics is discussed. The morphological evolution after the operation is also in discussion, especially those of the GDC phase. It was concluded that the mobility of GDC could be an influential factor determining the change in polarization resistance.

Magnetic Alignment of Anode Microstructure in Solid Oxide Fuel Cell

Polarization resistance of solid oxide fuel cell electrodes consists of resistances due to mass transfer and chemical reaction. Therefore, higher ion conductivity, electron conductivity, and gas diffusivity and larger triple-phase boundary (TPB) density lead to lower polarization resistance. To increase effective conductivities and diffusivity, tortuosity factors in each mass-transfer path should be reduced. In this study, magnetic alignment method is developed to align the mass-transfer paths in the electrode microstructure. Since Ni as an electron-conducting material in anode has high magnetic permeability, the particles are polarized and aligned by a magnetic field. Therefore, the tortuosity factors can be reduced. In addition, this Ni alignment is expected to reduce the tortuosity factors of yttria-stabilized zirconia (YSZ) and pore phases as well. However, there is a risk for a decrease in TPB density. The effect of magnetic field on resultant microstructure and polarization resistance of Ni-YSZ anode are investigated by impedance measurement of anode symmetrical cells and by reconstruction of three-dimensional microstructure. The tortuosity factors of Ni and YSZ decreased and TPB density decreased by the magnetic field. As a result, the polarization resistance of anode was kept nearly unchanged. The effect of reducing Ni volume fraction is also investigated.

Preparation and Testing of Metal/Ce0.80Gd0.20O1.90 (Metal: Ni, Pd, Pt, Rh, Ru) Co-Impregnated La0.20Sr0.25Ca0.45TiO3 Anode Microstructures for Solid Oxide Fuel Cells

La0.20Sr0.25Ca0.45TiO3 (LSCTA-) is a novel mixed ionic and electronic conductor (MIEC) material which can act as a potential replacement Solid Oxide Fuel Cell (SOFC) anode ‘backbone’ microstructure, for the current state-of-the-art Ni-based cermet. By impregnating this ‘backbone’ with electrocatalytically active coatings of metal oxides and metallic particles, it is possible to create high performance SOFC anodes which offer improved redox stability and tolerance to non-optimal fuel gases. Here, we present short-term test data for SOFC containing LSCTA- anode ‘backbones’ impregnated with a variety of catalyst systems including: Ni/CGO, Pd/CGO, Pt/CGO, Rh/CGO and Ru/CGO. Electrolyte-supported SOFC containing Ni/CGO impregnated anodes showed large reductions in Area Specific Resistance (ASR), in comparison to previous generation research (0.55 Ω cm2 versus 1.2 Ω cm2, respectively). Exchange of the Ni component, for Pd and Rh, led to much lower ASR of 0.39 Ω cm2 and 0.41 Ω cm2 (in 97% H2:3% H2O, at 900°C and 0.8 V), respectively. Equivalent circuit fitting of AC impedance spectra revealed the absence of an anode charge transfer process for the Rh/CGO catalyst system above 875°C, in comparison to all other systems, identifying this system as a potential replacement for the Ni-based cermet.

Characterization and performance evaluation of ammonia as fuel for solid oxide fuel cells with Ni/YSZ anodes

Ammonia appears to be a promising fuel for solid oxide fuel cell systems: it is a carbon-free species, can be stored easily and offers an excellent energy density with a high hydrogen content. This work shows comprehensive investigations of the direct operation of ammonia on an industrial-sized solid oxide fuel cell with Ni/YSZ anode. In the course of this study, ammonia exhibited excellent performance as a fuel for solid oxide fuel cells, although test results equivalent to those of hydrogen/nitrogen fuel mixtures were not attained. Electrochemical impedance spectroscopy proved the reduced performance output of ammonia as fuel to result from its endothermic decomposition. This significantly increased the ohmic resistance, which is mainly influenced by the ammonia flow rate. Operation in counter-flow is more favorable than in co-flow, as lower ohmic and diffusion resistances were measured. Twenty-four-hour stability tests showed stable behavior at 800 °C and a voltage decrease of 2% at 700 °C. Investigations of the anode micro-structure suggest that nickel nitriding occurred, as microscopic pores, particle enlargements, and agglomerations were identified at the metallic parts.

Electrochemical properties and microstructure of Pb-Co anodes during electrolysis in H2SO4 solution

The Pb-Co anodes were prepared by hot press and the parameters of oxygen evolution dynamics of Pb-Co anodes were analyzed by Double-Slope Tafel, Cyclic Voltammograms and EIS carried out in 2 molL-1 H2SO4 solution. The results showed that with an increase Co content the OER over-potentials η of Pb-1%Co and Pb-2%Co anodes increase at 48 h of electrolysis but then decrease at 96 h of electrolysis when the passivation react. With an increase of Co content the corrosion current density icorr of Pb-Co anodes increase at 96 h of electrolysis, which revealed the formation process on the anodic layers of Pb-Co anodes at 96 h of electrolysis. The phase transformation and microstructure change were measured by XRD analysis and SEM observations. At 48 h of electrolysis, the anodic layers determined with orthorhombic crystals of α-PbO2. At 96 h of electrolysis, the anodic layers were mainly composed of tetragonal crystals of β-PbO2 phase, probably. It's obvious that the CoS2 phase appears on the anodic layer of Pb-3%Co anodes at 96 h of electrolysis in 2 molL-1 H2SO4 solution.

A high entropy oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O) with superior lithium storage performance

Here, a high entropy oxide (HEO), Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, was explored as an anode material for lithium ion batteries. The HEO anode provides a high initial discharge specific capacity of about 1585 mAh g−1 and exhibits superior cycling stability. A reversible capacity of 920 mAh g−1 was achieved at 100 mA g−1 after 300 cycles. Ex situ scanning electron microscopy (SEM) and selected-area electron diffraction (SAED) revealed that the surface morphology and microstructure of the HEO anodes were still stable even after long term cycling. The well-mixed cations together with an inactive material (MgO, formed after the initial discharge process) in the HEO anodes result in a remarkable cycling, rate performance and an applicable reversible capacity with an average voltage of ∼0.85 V. This work may provide a convenient method for obtaining component-controlled oxide nanostructured materials with high electrochemical performance.

Influence of Zr and Be on microstructure and electrochemical behavior of AZ63 anode

In this study, the effect of Zr and Be on the microstructure and electrochemical properties of a common magnesium alloy anode, AZ63, used in cathodic protection, was investigated. The AZ63, as well as Zr (0.25, 0.50, 0.70, and 0.90 wt%) and Be (0.0001, 0.001, 0.01, and 0.1 wt%) bearing AZ63 anodes, were prepared. The microstructure and electrochemical characteristics of the cast anodes were characterized. The results showed that the addition of Zr refines the grain size of the AZ63 anode; and above 0.5 wt% Zr, forms Al2Zr and β‐Mg17Al12. In contrast, grain coarsening was observed with increasing Be content. Moreover, the addition of 0.1 wt% Be to the anode resulted in the spheroidization of the β‐phase. Also, it was shown that AZ63‐0.5 wt% Zr provides the lowest corrosion rate among the experimented anodes, and the higher the Be content, the lower the corrosion rate of the anode. Higher content of both Be and Zr alloying elements resulted in improved efficiency of the anode for 20% and 13%, respectively. The results of this study indicate higher impact of Be on the efficiency increment of the AZ63 anode.

Developing micro-scale heterogeneous numerical simulation of a solid oxide fuel cell anode

In this article a fully three dimensional, multiphase, meso-scale Solid Oxide Fuel Cell (SOFC) anode transport phenomena model is described. The Butler-Volmer model is combined with empirical relations for conductivity and diffusivity - notably the Fuller-Shetler-Giddings equation - for transport of gas reagents. Numerical simulation is used to obtain partial pressure and electric potential fields for each phase, accounting for activation and concentration overpotential as well as ohmic losses. Mesh generator and a solver have been developed to provide an in-house code for the computations. The findings are presented alongside a parametric study of a specific YSZ electrolyte-based SOFC anode microstructure. Despite high dependence of SOFC anode performance on the geometry of its anisotropic, three-phase microstructure, meso-scale numerical models simulating transport phenomena within these electrodes are not very common.

Lowering the sintering temperature of solid oxide fuel cell electrolytes by infiltration

A dense electrolyte with a relative density of over 95% is vital to prevent gas leakage and thus the achievement of high open circuit voltage in solid oxide fuel cells (SOFCs). The densification process of ceria based electrolyte requires high temperatures heat treatment (i.e. 1400–1500 °C). Thus, the minimum co-sintering temperatures of the anode-electrode bilayers are fixed at these values, resulting in coarse anode microstructures and consequently poor performance. The main purpose of this study is to densify gadolinia doped ceria (GDC), a common SOFC electrolyte, at temperatures lower than 1400 °C. By this aim, an approach involving the infiltration of polymeric precursors into porous electrolyte scaffolds, a method commonly used for composite SOFC electrodes, is proposed. By infiltrating polymeric precursors of GDC into porous GDC scaffolds, a reduction in the sintering temperature by at least 200 °C is achieved with no additives that might affect the electrical properties. Energy dispersive x-ray spectroscopy line scan analyses performed on porous GDC scaffolds infiltrated by a marker solution (polymeric FeOx precursor in this case) reveals a homogeneous infiltrated phase distribution, demonstrating the effectiveness of polymeric precursors.

Pre‐Oxidation‐Tuned Microstructures of Carbon Anodes Derived from Pitch for Enhancing Na Storage Performance

AbstractDisordered carbons have captured extensive interest as anode materials for Na‐ion batteries (NIBs) due to the abundant resources, competitive specific capacity, and low cost. Here, a facile strategy of pre‐oxidation is successfully adopted to tune the microstructure of carbon anode to facilitate sodium storage. Pitch is selected as the low‐cost and high carbon yield precursor. An easy pre‐oxidation treatment in air can enable pitch to realize an effective structural conversion from ordered to disordered at further carbonization processes. Compared with the carbonized pristine pitch, the carbonized pre‐oxidation pitch increases the carbon yield from 54 to 67%, the sodium storage capacity from 94.0 to 300.6 mAh g−1, and the initial Coulombic efficiency from 64.2 to 88.6%. Experiment results reveal that the introduction of oxygen based functional groups is the key to achieve the highly disordered structure, not only ensuring the cross‐linkage during low‐temperature pre‐oxidation process but also suppressing the carbon structure from melting and rearranging in the high‐temperature carbonization process. Most importantly, this facile pre‐oxidation strategy can also be extended to other carbon precursors to facilitate the low‐cost and high‐performance disordered carbon anodes for NIBs and beyond.

Stress and Electrochemical Analyses of LiNixMnyCo1-x-YO2(NMC)/Graphite Lithium-Ion Battery with Real 3D Microstructures

LiNixMnyCo1-x-yO2 (NMC) is one of promising cathode materials in lithium-ion batteries (LIBs) for power tools and electric vehicles due to their higher specific capacity and energy density. However, it is observed that repeated cycling leads to micro-scale fracture along the grain boundaries in the secondary particles of NMC, and it could eventually cause low cycling stability and irreversible capacity retention. The aim of this study is to investigate the correlation of mechanical stresses and electrochemical battery degradation. Specifically, mechanical stress and strain caused by lithium intercalation and diffusion in both cathode and anode were calculated and crack initiation in particles was predicted. Furthermore, a modified Butler-Volmer equation, which accounts for the influence of mechanical stress on electrochemical reactions, is considered. In this study, we adopted real 3D microstructures of both anode (graphite) and cathode (NMC) based on over 100 images from a synchrotron radiation X-ray tomographic microscopy (SRXTM). The 3D finite element model (FEM) was developed by COMOSL Multiphysics 5.2a and followed by coupled electrochemical and mechanical analyses of a whole cell, including cathode, anode, and electrolyte. As the first step, the electrochemical model based on the microstructure was compared with results from the previous study with a simple geometry (e.g. spherical particles). Specifically, the real 3D microstructure model, concentration, polarization, over potential, and von-Mises stress distribution across both electrodes (e.g. graphite and NMC) were computed under different C-rates. We observed that effects of phase transformation of NMC on mechanical stress was significant during discharging processes. Moreover, the model revealed that at particle connections where exhibit complicated geometries and the bulged regimes of smaller curvature showed higher stress and were vulnerable to particle cracking. Finally, it is confirmed that mechanical stress plays important role in electrochemical performance. This computational model can make a solid foundation to understand the relationship between mechanical stress and electrochemical performance in NMC/graphite battery chemistry. It might be helpful to understand key factors in the deterioration of Lithium-ion batteries with higher C-rates and longer cycle life.

In operando optical study of active three phase boundary of nickel-yttria stabilized zirconia solid-oxide fuel cell anode under polarization

Degradation of solid oxide fuel cell (SOFC) anode always takes place during long-term high temperature operation because of the decrease in three phase boundary (TPB) density and loss of nickel (Ni) network percolation due to Ni coarsening. In the present study, for the first time, an in operando observation technology using optical laser microscope is developed to demonstrate the real-time local morphological change of Ni at active TPB under polarization conditions. The local morphological change, which is determined by the competition between the mechanisms of enhanced Ni wettability on yttria-stabilized zirconia (YSZ) surface and thin Ni-film break up caused by Ni coarsening are observed. In operando observation reveals the reason of rapid performance degradation of SOFC anode under sever operation conditions. It is expected that the in operando observation of Ni morphological change can largely enrich our understanding of the SOFC anode microstructure change, which may further supply precious opportunities to investigate sever degradation mechanisms.

Improving sulfur tolerance of Ni-YSZ anodes of solid oxide fuel cells by optimization of microstructure and operating conditions

In this study, we propose an improvement in the sulfur tolerance of nickel-yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells (SOFCs) by simultaneously employing optimized operating conditions and microstructural modifications. An electrolyte supported SOFC is operated at 20 ppm H2S impurity at 750 °C for 20 h degradation and 10 h recovery test. The current cycles with a higher amplitude and small pulse time during the constant current operation are beneficial for the mitigation of sulfur poisoning. The effect of humidity on the sulfur degradation of Ni-YSZ anode is also studied. The synergetic effect of microstructure modification and current cycling conditions improves the sulfur tolerance of Ni-YSZ anode. It has been found that, when an anode with a modified microstructure by infiltrated CeO2 and Yb2O3 nanoparticles is operated on 20 ppm H2S poisoned gas at 10% relative humidity and the optimum pulsed current cycling conditions, about 7 times less degradation of the SOFC performance is observed. This study shows that at lower H2S concentration, a stable operation of a SOFC with minimum degradation can be achieved with the combination of optimization of operating conditions and modification of the anode microstructure.