Anode Microstructure Research Articles

Parametric stochastic 3D model for the microstructure of anodes in lithium-ion power cells

The microstructure of anodes in lithium-ion batteries has a strong influence on their electrochemical performance and degradation effects. Thus, optimizing the morphology with respect to functionality is a main goal in battery research. Doing so experimentally in the laboratory causes high costs with regard to time and resources. One way to overcome this problem is the usage of parametric 3D microstructure models, which allow the realization of virtual morphologies on the computer. The functionality of microstructures generated with such models can be investigated by means of numerical transport simulations. The results of this procedure, which is called virtual materials testing, can be used to design anodes with improved morphologies that lead to a better electrochemical performance. Recently, a particle-based stochastic microstructure model for anodes in lithium-ion energy cells has been proposed. In the present paper, an extension of this model to describe the morphology of anodes in power cells, whose structure strongly differs from energy cell anodes, is introduced. The extensions include techniques to model anisotropic morphologies with a low volume fraction of the particle phase and strongly irregular particle shapes. The model is fitted to 3D image data of a power cell anode and validated using morphological image characteristics. Furthermore, we show examples of modifications of our microstructure model that can be made for generating further virtual morphologies. Finally, we briefly explain how electrochemical characteristics can be estimated using thermodynamically consistent transport theory. To illustrate this, we compute the cell potential over time during lithiation for image data of real microstructures as well as corresponding microstructures simulated by our model.

Stochastic microstructure modeling and electrochemical simulation of lithium-ion cell anodes in 3D

Abstract Thermodynamically consistent transport theory is used to compare 3D images of real anode microstructures from lithium-ion batteries to virtual ones created by a parametric stochastic 3D microstructure model. Half-cell simulations in 3D with spatially resolved microstructures at different applied currents show that for low currents the deviations between various electrochemical quantities like current density or overpotential are negligibly small. For larger currents small differences become more pronounced. Qualitative and quantitative differences of these features are discussed with respect to the microstructure and it is shown that the real and virtual structures behave similar during electrochemical simulations. Extensions of the stochastic microstructure model, which overcome small differences in electrochemical behavior, are proposed.

Bimetallic Nickel/Ruthenium Catalysts Synthesized by Atomic Layer Deposition for Low-Temperature Direct Methanol Solid Oxide Fuel Cells.

Nickel and ruthenium bimetallic catalysts were heterogeneously synthesized via atomic layer deposition (ALD) for use as the anode of direct methanol solid oxide fuel cells (DMSOFCs) operating in a low-temperature range. The presence of highly dispersed ALD Ru islands over a porous Ni mesh was confirmed, and the Ni/ALD Ru anode microstructure was observed. Fuel cell tests were conducted using Ni-only and Ni/ALD Ru anodes with approximately 350 μm thick gadolinium-doped ceria electrolytes and platinum cathodes. The performance of fuel cells was assessed using pure methanol at operating temperatures of 300-400 °C. Micromorphological changes of the anode after cell operation were investigated, and the content of adsorbed carbon on the anode side of the operated samples was measured. The difference in the maximum power density between samples utilizing Ni/ALD Ru and Pt/ALD Ru, the latter being the best catalyst for direct methanol fuel cells, was observed to be less than 7% at 300 °C and 30% at 350 °C. The improved electrochemical activity of the Ni/ALD Ru anode compared to that of the Ni-only anode, along with the reduction of the number of catalytically active sites due to agglomeration of Ni and carbon formation on the Ni surface as compared to Pt, explains this decent performance.

LBM prediction of effective electric and species transport properties of lithium-ion battery graphite anode

Numerical models play a vital role in the developing and performance optimization of lithium-ion batteries. The key factor to the prediction accuracy of macro-scale models is the specification of effective transport properties. This study, based on the anisotropic microstructure of graphite anode reconstructed by an ellipsoid-based simulated annealing method (SAM), established a mesoscopic model of diffusion process to predict the effective electric and species transport properties of lithium-ion battery graphite anode via lattice-Boltzmann (LB) method. The effect of particle size on the transport properties of graphite anode was discussed in detail. In the electrode through-plane direction, if the ellipsoidal particles are thinner and flatter, both the effective electric and species transport properties decrease; in the other two directions, the effective electronic charge transport properties barely change with the change of particle size while the effective species transport properties increase along with the increase of the size of particles. In addition, to get a more accurate replica of the real graphite anode, we assumed the sizes of solid particles follow a normal distribution and reconstructed the microstructure of electrode. The LB calculation results reveal that the normal distribution of particle size increases the electronic charge conductivity in the electrode through-plane direction and decreases in the other two directions, compared to the electrode of constant-sized particles; the effective species diffusivities (or ionic charge conductivities) in electrode through-plane direction for different microstructures are closer.

Experimental investigation into tungsten carbide thin films as solid oxide fuel cell anodes

Abstract

Development of plasma sprayed Ni/YSZ anodes for metal supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) offer a promising technique for producing electricity by clean energy conversion through an electrochemical reaction of fuel and air. Plasma spraying could be a potential manufacturing route for commercial SOFCs, as it provides a distinct advantage especially in case of metal supported cells (MSCs) by allowing rapid processing at relatively low processing temperatures preventing thus the degradation of the metallic substrate. The objective of this work was to develop nickel/yttria stabilised zirconia (Ni/YSZ) anodes with high porosity and homogeneous phase distribution by atmospheric plasma spraying for MSCs. Various feedstock material approaches were explored in this study, both with single injection as well as separate injection of different feedstock materials , and with and without the use of pore formers to create additional porosity. The advantages and issues with each material route were investigated and discussed. It was shown that agglomerated Ni/YSZ/polyester feedstock material resulted in the best distribution of Ni and YSZ in the anode microstructure with homogeneous porosity. Subsequently, the Ni/YSZ/polyester material route with different amounts and size distributions of polyester was chosen to develop anode symmetrical cells using a commercial zirconia sheet as support for electrochemical testing. The Ni/YSZ/polyester anode powder with 10wt.% standard size polyester exhibited the best electrochemical performance. The results show that plasma spraying of the agglomerated Ni/YSZ/polyester could be a promising route to achieve high performance and rapid production anodes without using the carcinogenic nickel oxide.

Origin of Electrochemical Capacitance of Ni/YSZ Anode and Its Application to Evaluation of Active Reaction Site

1. INTRODUCTION Electrochemical performance and stability of a Ni-YSZ anode depend on its microstructure. It is indispensable to understand how microstructure changes during operation to ensure the long-term durability of solid oxide fuel cells (SOFCs). It has been reported to observe microstructure change before and after the operation by FIB-SEM [1]. FIB-SEM observation gives us the detain information about anode microstructure, but can’t be used for real time monitoring. Thus, it is better to be combined with a real-time microstructure analysis. As for gadolinium-doped ceria (GDC), which is a non-stoichiometric oxide, Nakamura et al. [2] have reported that chemical capacitance could be used to estimate effective reaction length because chemical capacitance was related to oxygen non-stoichiometry. While YSZ is not nonstoichiometric oxide, we believe that an electrode capacitance can be used to analyze the microstructure change of Ni-YSZ anode during operation. The purpose of this study is to clarify the relationship between microstructure and electrode capacitance of Ni-YSZ anode and to evaluate microstructure degradation by the electrode capacitance. 2. EXPERIMENTAL Here we assumed relationship between electrode capacitance and double-phase boundary (DPB) between Ni and YSZ because capacitance. Then, electrode capacitance, c pwas represented by the following equation, cp =A (LNi-YSZ/2)2(1) where A is constant; L Ni-YSZ is the diameter of osculating circle between Ni and YSZ particles, where both particles is assumed to be a sphere shape. This relationship is verified by a model electrode experiment. After this experiment, the same relationship is confirmed with porous Ni-YSZ electrode by C Mcalculated by model electrode experiment. The microstructure information was obtained from ternarized cross-section images by FE-SEM. Firstly, to test the hypothesis, the model nickel electrode was used. Model nickel electrodes, which had diameters of 1000, 500, 100, 50, and 20 μm, were prepared on the substrate of 8YSZ by a pulsed laser deposition. The electrochemical measurement was conducted in an infrared-furnace in 2.3% H2O-H2atmosphere at 600 - 765 °C. A three-probe method was employed. Pt foil was used as a current collector of CE. As to the WE, a Pt-coated tungsten probe was applied to touch onto the electrode surface. Electrode resistance and electrode capacitance were obtained by AC impedance measurement. Secondly, the above method was applied to a practical porous Ni-YSZ cerment anode. The cermet anode was 50vol% Ni-YSZ prepared from 8YSZ and NiO commercial powder. AC impedance measurement was in frequency range of1 M-0.01 Hz at 800 °C. 3. RESULT AND DISCUSSION 3.1. Model electrode experiment Fig. 1 shows the result of capacitance shows clear dependence on the DPB area. Electrodde capacitance was proportional to the square of diameter. This result supported our hypothesis. Since verification of the assumption in porous electrode, area specific capacitance, C M, was calculated in this experiment. In this study, C M are 1.3-2.1 × 10-4 Fcm-2 at 750 °C. Bieberle et al.[3] reported C M of 2.6 – 3.9 × 10-4at 700 °C with the model strip electrode. The result agrees with the reported data. The measured electrode capacitance reflect a certain physical phenomenon. It was confirmed that the electrode capacitance correlated to DPB. 3.2. Porous electrode experiment The experimental value of the capacitance need be compared with DPB area to verify the same relationship for the practical electrode. The experimental value, volume specific capacitance, c p, and resistance, r p, were measured by the equivalent model, Gerischer model. The measured capacitance and estimated value by C Mmeasured by the model electrode were compared. Although absolute values of measured- and estimated electrode capacitances are different order of magnitude, these capacitances are proportional. Therefore, there was relationship between electrode capacitance and anode microstructure. REFERENCES [1] Z. Jiao ,N. Shikazono, J. Electrochem Soc., 161(2014) F577 - F582 [2] T. Nakamura, et al, J. Electrochem. Soc., 155 (2008) B1244-B1250 [3] A. Bieberle, L.J. Gaukler, J. Electrochem. Soc., 148 (2001) A646-A656 Fig. 1 Diameter (DPB area) dependence of capacitance in 2.3% H2O-H2at 765 °C Figure 1

Degradation of SOFCs By Phosphorus Impuriteis: Impedance Spectroscopy and Microstructural Analysis

Introduction SOFC is considered as a promising power generation system due to high electric efficiency and fuel flexibility. But, coal gas, for example, contains various impurities. In this study, phosphorus poisoning effect to Ni-ScSZ cermet anodes is analyzed by measuring Electrochemical Impedance Spectroscopy (EIS). Spectroscopy data are analyzed by the distribution of relaxation times (DRT) method1to investigate details of electrode resistance. Furthermore, microstructural analysis was made by electron microscopy. Experimental Electrolyte-supported cells with ScSZ (10 mol% Sc2O3 - 1 mol% CeO2-89 mol% ZrO2) plate (20 mmφ×0.2 mmt) were used in this study. Mixture of 56 wt% NiO and 44 wt% ScSZ was used for the anode and was sintered at 1300 oC for 3 h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ with a weight ratio of 1:1 was used for the cathode. Electrode area was 8×8 mm2 and Pt mesh was used as the current collector. Figure 1 describes the experimental setup. P2O5 container is locating at the fuel upstream to mix phosphorus impurity which is evaporated as PH3. PH3 concentration was specified as 2ppm by a TG-DTA measurement. The fuel consisted of 67% H2 and 33% H2O. Cell temperature was 800oC. Table 1 shows EIS condition. EIS data analyzed by DRT enable identification of electrode processes. FESEM-EDX was applied to investigate changes in the anode microstructure and composition by the cell performance tests. Results and discussion Figure 2 shows EIS spectra measured after 1h, 50h, and 100h. Figure 3 shows the DRT result of the Fig. 1 data. DRT peak becomes gradually visible due to the increase in impedance. DRT peak around 10-1~10-2 sec may originate from the diffusion of H2 gas2. Figure 3 shows SEM-WDX mapping of nickel and phosphorus. Ni particle is condensed and Ni-P compounds are formed on the anode surface. Therefore, cell performance degradation by inhibiting gas diffusion and forming Ni-P compound in the phosphorus poisoning of Ni-based anodes. References (1) A. Leonide et al., Journal of The Electrochemical Society, 155(1), B36-B41 (2008). (2) T.H. Wan et al., Electrochimica Acta, 184, 483-499 (2015). Figure 1

Study on the correlation between Solid Oxide Fuel Cell Ni-YSZ anode performance and reduction temperature

The influences of reduction temperature on the initial performance and shorttime durability of nickel-yttria-stabilized zirconia composite solid oxide fuel cell anode were investigated. Anode microstructures before and after operation were quantitatively analyzed by three-dimensional reconstruction based on focused ion beam-scanning electron microscopy technique. The anode reduced at 500 oC showed the worst initial performance and stability in operation and the anode reduced at 800 oC showed the smallest polarization resistance. The anode reduced at 1000 oC showed the most stable performance with polarization resistance enhanced with operation. It is found that higher reduction temperature leads to dense nickel and enhances nickel-yttria-stabilized-zirconia interfacial bonding, which can inhibit nickel sintering and improve the composite anode stability in long-time operation.

3D reconstruction size effect on the quantification of solid oxide fuel cell nickel–yttria-stabilized-zirconia anode microstructural information using scanning electron microscopy-focused ion beam technique

Self-made conventional nickel–yttria-stabilized zirconia composite anodes after reduction and 500h operation were analyzed by three-dimensional microstructure reconstruction based on focused ion beam-scanning electron microscopy technique. Interfacial area, three-phases-boundary density and phase volume fraction were measured based on the three-dimensional microstructure reconstruction to quantitatively study the statistical characterization of solid oxide fuel cell nickel–yttria-stabilized-zirconia anode microstructure before and after operation. It is found that for anode operated with long time, it is necessary to increase the corresponding three-dimensional reconstruction size to suppress the influence of microstructure variation caused by Ni agglomeration in order to obtain more accurate microstructural quantification information.

Fabrication of Tin-Plated Three-Dimensional Copper Nanostructure Using Electroless Plating and Its Anode Performance in Lithium-Ion Battery

Introduction Next-generation lithium-ion batteries with higher energy density are urgently needed for hybrid and electric vehicle applications, among others. Currently, graphite is typically used as the anode active material in lithium-ion batteries, but it is necessary to consider alternative, higher capacity materials. For example, tin has a larger theoretical capacity (991 mAh g-1) than that of graphite (372 mAh g-1). However, the volume of tin changes significantly during charging and discharging, causing it to slip down from the copper current collector, which deteriorates the cycling characteristics. Three-dimensional copper structures are effective at preventing such slippage.1) However, in general, the methods for fabricating such structures are very complicated. We have reported that a three-dimensional copper nanostructure (3DC-1) can be fabricated easily by one-step electrodeposition.2) In this study, a tin layer was formed on the 3DC-1 by electroless deposition, and its anode performance in a lithium-ion battery was evaluated. Experimental An acidic copper sulfate bath1) containing 0.85 M CuSO4∙5H2O + 0.55 M H2SO4 + 2.5×10-4 M polyacrylic acid (MW=5000) was used in the fabrication of the 3DC-1. Electrodeposition was conducted under galvanostatic conditions (1.2 A dm-2) at 25 °C without agitation. A bright Watts nickel plating bath (1 M NiSO4∙6H2O + 0.2 M NiCl2∙6H2O + 0.5 M H3BO3 + 0.01 M saccharin sodium dehydrate + 0.0025 M 1,4-butynediol) was prepared, and electrodeposition was conducted under galvanostatic conditions (3.0 A dm-2) at 25 °C without agitation. Tin was electrolessly plated onto the 3DC-1 at 80 °C for 15 s without agitation. An immersion plating bath containing 0.5 M K4P2O7 + 0.15 M Sn2P2O7+ 3 M thiourea was prepared. The pH was adjusted to 5 with hydrochloric acid. The microstructure of the tin anode was examined using field-emission scanning electron microscopy (FE-SEM). The phase structure of the deposits was analyzed by X-ray diffraction (XRD). A chemical composition analysis was carried out using X-ray fluorescence spectrometry (XRF) and energy-dispersive X-ray spectroscopy (EDX). Electrochemical studies of the tin anode were carried out with coin cells that were assembled in an Ar-filled glove box. Each coin cell consisted of a lithium foil as a counter electrode and a reference electrode. The plating film functioned as the working electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed in the range of 0.02–1.5 V (vs. Li/Li+) at a constant temperature of 25 °C. Results and Discussion Fig. 1 shows a surface SEM image of the tin anode, which reveals that the tin film was plated homogeneously onto the three-dimensional copper nanostructure. Fig. 2 shows a cross-sectional SEM image of the tin anode and corresponding EDX mapping results for copper, nickel, and tin. The tin was distributed homogeneously on the 3DC-1. Fig. 3 compares the cycling characteristics of tin anodes fabricated using 3DC-1 (a) and a flat copper foil (b). For the former, the discharge capacity was about 900 mAh g-1 after the first cycle. Even after 100 cycles, the discharge capacity remained over 700 mAh g-1. In contrast, for the latter, the discharge capacity was 650 mAh g-1 after the first cycle and 250 mAh g-1after 100 cycles. References 1) Shichao Zhang, Yalan Xing, Tao Jiang, Zhijia Du, Feng Li, Lei He, Wenbo Liu; Journal of Power Sources,196, 6915-6919 (2011) 2) S.Arai and T.Kitamura, ECS Electrochemistry Letters, 3 (5), D7-D9 (2014) Figure 1

Fabrication of an MWCNT-Reinforced Tin Anode for Use in Lithium Ion Batteries By Electrodeposition in Sulfuric Acid

Introduction Lithium ion batteries employing graphite as the anode active material exhibit higher energy densities than conventional rechargeable batteries, although increasing the capacity of these devices will require the use of alternative anode compounds. Since tin (Sn) has a higher theoretical capacity (991 mAh g-1) than graphite (372 mAh g-1), it has been considered for this role. However, Sn tends to fall off the current collector during charge-discharge cycles due to significant volume variations, degrading the battery performance. One approach to avoiding this problem is to anchor the Sn layer to an underlying copper (Cu) layer using fibrous materials. We have already reported the excellent charge-discharge characteristics of a new tin anode reinforced with multiwalled carbon nanotubes (MWCNTs), formed by a combination of Cu/MWCNT composite plating and electroless tin plating.In this study, we examined a new fabrication process for the MWCNT-reinforced tin anode structure based on tin electrodeposition using a sulfuric acid bath, and explored the correlation between the new negative electrode structure and its electrochemical characteristics. Experimental A Cu/MWCNT composite plating bath (0.85 M CuSO4 ●5H2O + 0.55 M H2SO4 + MWCNT + polyacrylic acid), and a tin plating bath (0.2 M SnSO4 + 0.5 M H2SO4 + additive) were prepared. Using these baths, a Cu/MWCNT film was applied to a pure copper plate and tin was subsequently electrodeposited on the Cu/MWCNT film, resulting in the MWCNT-reinforced tin anode structure. For comparison purposes, tin was also electrodeposited on a pure copper plate. Electroplating was carried out under galvanostatic conditions at 25 °C. The microstructure of the new tin anode was examined by field emission scanning electron microscopy (FE-SEM). The phase structure of the deposits was analyzed by X-ray diffraction (XRD). Electrochemical studies of the tin-based anodes were carried out using coin cells (2032) assembled in an Ar-filled glove box. Each coin cell consisted of a lithium foil counter and reference electrodes and a tin-based anode as the working electrode. The electrolyte was 1 M LiPF6in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed at a constant temperature of 25 °C. Results and Discussion Figure 1 shows a surface SEM image of the tin anode fabricated by the new process. MWCNTs are seen to be uniformly dispersed over the Sn layer. Figure 2 presents a cross-sectional SEM image of the tin anode. It can be seen that MWCNTs connect the Sn and Cu layers. No defects, such as voids or cracks, are observed between the two layers.The electrochemical characteristics of the tin anode will be discussed in detail during the presentation. References 1) S. Arai and R. Fukuoka, J. Appl. Electrochem., 46, 331 (2016). Figure 1

Fabrication and Electrochemical Evaluation of Tin Plated Three-Dimensional Copper Nanostructured Anode for Lithium Ion Battery

Introduction Lithium-ion batteries (LIBs) are among the most important energy storage and power conversion devices, and are widely used in portable electronics such as mobile phones and laptop computers. However, LIBs cannot satisfy the ever-growing needs of high-power applications due to the low energy density of commercial graphite. Tin and lithium form reversible alloys, and for the fully lithiated composition (Li4.4Sn), the specific capacity is 994 mA h g-1, which is almost three times higher than the theoretical value for a conventional graphite anode (372 mA h g-1). However, there are huge variations in the volume occupied by the atoms within the alloy structure because of aggregation of Sn and Li during the electrochemical alloying-dealloying process. These variations result in considerable mechanical stress, which leads to rapid capacity fade (short cycle life) due to pulverization of the material. To address this issue, several strategies have been proposed for improving the cyclability of tin materials by decreasing the particle size and using porous thin films of tin. We have developed a simple method for fabricating a three-dimensional (3D) copper nanostructure using electrodeposition. In this study, the microstructure of a tin-coated anode was analyzed and the charge/discharge characteristics of the anode were evaluated following direct electrodeposition of a pure tin layer on the 3D copper nanostructure. Experimental An acidic copper sulfate bath (0.85 M CuSO4×5H2O + 0.55 M H2SO4 + 3.0´10-4 M polyacrylic acid M.W=5000)1) was used to fabricate the 3D copper nanostructure. Electrodeposition was conducted under galvanostatic conditions (1 A dm-2) at 25°C without agitation. A tin alloy plating bath containing 1 M K4P2O7 + 0.25 M Sn2P2O7+ 0.002 M polyethylene glycol + 0.005 M HCHO was prepared. Electroplating was carried out under galvanostatic conditions at 25°C without agitation. The morphology and structure of the obtained samples were examined by field emission-scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectrometry (EDX). Electrochemical measurements were performed using coin-type cells assembled in an argon-filled glove box. LiPF6 (1 M) in ethylene carbonate and diethyl carbonate (1:1 vol%) was used as the electrolyte solution. Cycling tests were performed between 0.02 and 1.5 V (vs. Li/Li+) at a constant temperature of 25°C. Results and Discussion This new tin anode exhibited both a high reversible capacity and improved cyclability. The reversible capacity was 521.1 mA h g-1 after 100 cycles and 469.3 mA h g-1after 300 cycles at 0.5 C. Figure 1(a) shows a surface SEM image of the new tin anode, demonstrating the homogeneously plated tin film on the 3D copper nanostructure.Figure 1(b) shows an SEM image after 100 cycles, where a small amount of pulverization is evident. Figure 1(c) shows an SEM image of the anode after 300 cycles. Further pulverization is seen to have taken place. The results of a detailed electrochemical evaluation of this anode will be presented at the meeting. Reference 1) S.Arai and T.Kitamura, ECS Electrochem. Lett., 3 (5), D7-D9 (2014). Figure 1

Electrochemical Characteristics of Nickel and Ceria Infiltrated Nano-Structured La0.2Sr0.8TiO3 Anodes for Solid Oxide Fuel Cells By Infiltration

A La-doped SrTiO3 (LST) is an alternative anode material of solid oxide fuel cells (SOFC). The LST has a high electronic conductivity and chemically stable under a reduced atmosphere. In addition, it has excellent carbon deposition resistance and high tolerance of sulfur in hydrocarbon fuels. However, the LST suffers from a low catalytic activity toward the oxygen reduction reaction. Thus, it needs to improve the catalyst activity of the LST anode. In this study, nickel and gadolia-doped ceria (GDC) infiltrated nano-structured LST anodes were proposed in order to improve the catalytic activity of the LST-based oxide anode. Nano-sized Ni and GDC were homogeneously dispersed in a LST backbone by infiltration. Ni/GDC infiltrated LST showed higher catalytic activity than Ni and GDC infiltrated ones, which means that the synergistic effect can be expected in the Ni/GDC infiltrated LST. It seems that the polarization resistance (Rp) depended on the infiltrated Ni and GDC content and their dispersion state or microstructure. We analyzed the correlation between the microstructure of the infiltrated anode and electrochemical characteristics.

Ex-Situ 3D Nano-Ptychography of Ni-YSZ SOFC Anode during Redox Cycling

Solid oxide cells (SOC) are used for efficient and cost-effective conversion of a wide variety of fuels (SOFC) for power generation or for energy storage via electrolysis (SOEC). However, SOC lifetime under harsh operating conditions is limited by electrode microstructure durability. In particular, the state of the art anode electrodes are composed of a porous cermet of nickel and yttria stabilized zirconia (YSZ). Electrochemical reactions occur where the ion and electron conducting phases meet the pores at the triple phase boundaries (TPB). Thus, the electrochemical activity of the electrode is strongly dependent on the density of TPBs and the quality of the connecting pathways through each phase1. Despite nickel remaining stable under reducing environments, nickel oxidation can occur in the electrode due to air leakage in the fuel system or by mechanical failure of the electrolyte. During operation, redox cycles can impact the lifetime of the electrode thus tracking their effect on the microstructure in 3D provides valuable information. 3D characterization of a Ni-YSZ electrode was first achieved in 20062 using focused ion beam scanning electron microscope (FIB-SEM) nanotomography and it has been routinely performed for the last decade followed by the development of near edge X-ray nano-tomography3 and most recently holotomography4. Near edge X-ray nano-tomography and FIB-SEM has been used to study the effects of oxidation on Ni-YSZ electrodes3 X-ray Ptychography is a scanning based technique in which a set of several local far-field scattering patterns is obtained by scanning the sample with a coherent micrometer-size X-ray beam. When used in tomography mode, ptychography provides a quantitative measurement of the local electron density of materials in 3D. Over the years, ptychography has demonstrated high sensitivity to mass density changes and the highest resolution among the X-ray imaging techniques5. As a proof of concept, we report the first results of ptycho-nanotomography applied to an “ex-situ” redox experiment performed at the cSAXS beamline at the Swiss Light Source (PSI). In the experiment the same Ni-YSZ anode microstructure is first analyzed in its pristine state, then oxidized and finally reduced again at 850°C in air and in 4%H2-96%N2 respectively. A specimen containing both Ni-YSZ electrode and YSZ electrolyte was created using a high precision mechanical polishing procedure followed by FIB ion milling. The “ex-situ” treatments were carried out in a small custom made tube furnace between each ptychography scan at the beamline. Datasets typically 20x20x12 µm in size were quantified to have a 3D spatial resolution of 55 nm using the Fourier shell correlation method5. The high mass density sensitivity of ptychography, resulted in high phase contrast between Ni, YSZ, NiO and pore, enabled us to effectively resolve all the phases present in the electrode (figure 1a,b,c). Therefore, an accurate segmentation procedure could be applied minimizing the associated geometrical errors of calculated microstructural parameters. Here we describe the major microstructural changes which can be noticed in the microstructure showing the evolution of the nickel particle size distribution, connectivity and tortuosity in this recent experiment. Moreover, the morphological changes in the YSZ phase are also discussed with emphasis on the observed micro-cracks (figure 1d) and their effect on the overall connectivity.

Controlled SOFC Anode Porosity Using Aligned Sacrificial Fibers

Solid oxide fuel cell anode supports serve a multipurpose role. This layer must be both the mechanical backbone of the cell and an active electrode. While high porosity is desired to maximize reaction sites, it negatively impacts strength. Therefore, efforts should be made to design anode microstructure for high strength while maintaining electrochemical performance. Currently, solid oxide fuel cell anodes are fabricated such that there is little control over the final microstructure. A new technique for creating porous anode support layers in solid oxide fuel cells was developed. Using sacrificial, high aspect ratio pore formers, anode support layers with non-random porosity networks were created utilizing dielectrophoresis. Variation in applied voltage and frequency during dielectrophoretic treatment allowed control over the resulting structure. Microstructure analysis and electrochemical testing of novel anode support layers was performed and compared to results of standard SOFC designs.

Efficient three-phase reconstruction of heterogeneous material from 2D cross-sections via phase-recovery algorithm.

Digital reconstruction of a complex heterogeneous media from the limited statistical information, mostly provided by different imaging techniques, is the key to the successful computational analysis of this important class of materials. In this study, a novel approach is presented for three-dimensional (3D) reconstruction of a three-phase microstructure from its statistical information provided by two-dimensional (2D) cross-sections. In this three-step method, first two-point correlation functions (TPCFs) are extracted from the cross-section(s) using a spectral method suitable for the three-phase media. In the next step, 3D TPCFs are approximated for all vectors in a representative volume element (RVE). Finally, the 3D microstructure is realized from the full-set TPCFs obtained in the previous step, using a modified phase-recovery algorithm. The method is generally applicable to any complex three-phase media, here illustrated for an SOFC anode microstructure. The capabilities and shortcomings of the method are then investigated by performing a qualitative comparison between example cross-sections obtained computationally and their experimental equivalents. Finally, it is shown that the method almost conserves key microstructural properties of the media including tortuosity, percolation and three-phase boundary length (TPBL).

Cotton-Fibers Used as Pore Former for the Anode with High Porosity and Long Cylindrical Pores of Solid Oxide Fuel Cells

The anode microstructure is crucial for the performance of solid oxide fuel cell(SOFC). The porous anode with high porosity and long cylindrical pores was prepared by using cotton-fiber as pore-former. With Ni-yttria stabilized zirconia (YSZ) as anode, La0.85Sr0.15MnO3-YSZ as cathode, YSZ electrolyte-supported cell was fabricated by slurry coating method. For a comparison, the unit cell with the conventional flour as anode pore former was prepared. All the as-prepared cells were measured in hydrogen at 650 to 800 °C. The microstructure of tested cells was observed by scanning electron microscopy (SEM). The SEM shows that the long cylindrical pores produced by cotton-fibers were beneficial to provide continuous gas pathways for rapid diffusion of fuel and reaction products in the anode. Compared the flour of 373.74 mW·cm-2at 800 °C, the maximum density of unit-cell prepared by cotton-fibers was higher than 900 mW·cm-2, which was attribute to an enlarger active reaction sites created by cotton-fibers for hydrogen oxidation.

Modeling of gas transport with electrochemical reaction in nickel-yttria-stabilized zirconia anode during thermal cycling by Lattice Boltzmann method

Abstract This work reports an investigation of the impact of microstructure on the performance of solid oxide fuel cells (SOFC) composed of nickel yttria-stabilized zirconia (Ni YSZ). X-ray nano computed tomography (nano-CT) was used to obtain three-dimensional (3D) models of Ni-YSZ composite anode samples subjected to different thermal cycles. Key parameters, such as triple phase boundary (TPB) density, were calculated using 3D reconstructions. The electrochemical reaction occurring at active-TPB was modeled by the Lattice Boltzmann Method for simulation of multi-component mass transfer in porous anodes. The effect of different electrode geometries on the mass transfer and the electrochemical reaction in anodes was studied by TPB distributions measured by nano CT for samples subjected to different thermal cycles. The concentration polarization and the activation polarization were estimated respectively. The results demonstrate that a combined approach involving nano-CT experiments in conjunction with simulations of gas transport and electrochemical reactions using the Lattice Boltzmann method can be used to better understand the relationship between electrode microstructure and performance of nickel yttria-stabilized zirconia anodes.

Preparation and characterization of Ni-YSZ composite electrode for solid oxide fuel cells by different co-precipitation routes

In this study, two synthetic routes were used to prepare NiO-YSZ nanocomposite particles. The first one is an improved co-precipitation route, in which NiO-YSZ nanocomposite particles were synthesized by adding a nickel/yttrium nitrate solution to an organic base solution containing an anionic zirconium carbonate complex. The second one is the conventional co-precipitation method. The homogeneity and sinteractivity of the synthesized NiO/YSZ composite particles, as well as the subsequent microstructure and electrochemical performance of the resultant Ni-YSZ anode for the two synthetic routes were evaluated and compared in detail. The results show that the particles synthesized by the conventional co-precipitation route involved aggregated composites with a non-uniform distribution of NiO and YSZ phases. The particles showed poor sinteractivity, a coarse and inhomogeneous anode microstructure, and a moderate area-specific resistance (ASR) of 0.35 Ω cm2 at 800 °C under open circuit voltage (OCV). In contrast, homogeneous nanocomposite particles were successfully synthesized by the improved co-precipitation route. The particles showed better sinteractivity, a fine and uniform porous microstructure with a grain size in the range of 200–400 nm even at the sintering temperature of 1300 °C, and a low ASR of 0.16 Ω cm2 at 800 °C under OCV.