Anode Microstructure Research Articles

3D Microstructure Effects in Ni-YSZ Anodes: Influence of TPB Lengths on the Electrochemical Performance

3D microstructure-performance relationships in Ni-YSZ anodes for electrolyte-supported cells are investigated in terms of the correlation between the triple phase boundary (TPB) length and polarization resistance (Rpol). Three different Ni-YSZ anodes of varying microstructure are subjected to eight reduction-oxidation (redox) cycles at 950 °C. In general the TPB lengths correlate with anode performance. However, the quantitative results also show that there is no simplistic relationship between TPB and Rpol. The degradation mechanism strongly depends on the initial microstructure. Finer microstructures exhibit lower degradation rates of TPB and Rpol. In fine microstructures, TPB loss is found to be due to Ni coarsening, while in coarse microstructures reduction of active TPB results mainly from loss of YSZ percolation. The latter is attributed to weak bottlenecks associated with lower sintering activity of the coarse YSZ. The coarse anode suffers from complete loss of YSZ connectivity and associated drop of TPBactive by 93%. Surprisingly, this severe microstructure degradation did not lead to electrochemical failure. Mechanistic scenarios are discussed for different anode microstructures. These scenarios are based on a model for coupled charge transfer and transport, which allows using TPB and effective properties as input. The mechanistic scenarios describe the microstructure influence on current distributions, which explains the observed complex relationship between TPB lengths and anode performances. The observed loss of YSZ percolation in the coarse anode is not detrimental because the electrochemical activity is concentrated in a narrow active layer. The anode performance can be predicted reliably if the volume-averaged properties (TPBactive, effective ionic conductivity) are corrected for the so-called short-range effect, which is particularly important in cases with a narrow active layer.

Fabrication of NiO-Y:BaZrO3 Composite Anode for Thin Film-Protonic Ceramic Fuel Cells using Tape-Casting

Optimization of the fabrication process of NiO-yttrium doped barium zirconate (BZY) composite anode substrates using tapecasting for high performance thin-film protonic ceramic fuel cells (PCFCs) is investigated. The anode substrate is composed of a tens of microns-thick anode functional layer laminated over a porous anode substrate. The macro-pore structure of the anode support is induced by micron-scale polymethyl methacrylate (PMMA) pore formers. Thermal gravity analysis (TGA) and a dilatometer are used to determine the polymeric additive burn-out and sintering temperatures. Crystallinity and microstructure of the tape-cast NiO-BZY anode are analyzed after the sintering.

Catalytic effect of praseodymium oxide additive on the microstructure and electrical property of graphite anode

Based on the revealed interactions (dissociation energy) between several typical metals (transition and rare-earth elements) and carbon atoms through the DFT quantum mechanics method, Pr can be predicted to possibly be a new potential catalyst. Indeed, Praseodymium oxide (Pr6O11) was insightfully found to catalyze the graphitization more efficiently than the other selected additives. The effects of Pr6O11 catalyst on the microstructure of the graphite anode were investigated by XRD, Raman spectroscopy, SEM, and TEM. The results indicate that Pr6O11 addition remarkably improves the formation of crystallites, thereby largely enhancing the electrical property of graphite anode. The graphite anode with a low electrical resistivity of 5.0 μΩ m was successfully available by adding 3 wt. % Pr6O11 at 2800 °C. Finally, the catalytic graphitization mechanism of rare-earth element was proposed based on the experimental results.

Characterization of time-varying macroscopic electro-chemo-mechanical behavior of SOFC subjected to Ni-sintering in cermet microstructures

In order to perform stress analyses of a solid oxide fuel cell (SOFC) under operation, we propose a characterization method of its time-varying macroscopic electro-chemo-mechanical behavior of electrodes by considering the time-varying geometries of anode microstructures due to Ni-sintering. The phase-field method is employed to simulate the micro-scale morphology change with time, from which the time-variation of the amount of triple-phase boundaries is directly predicted. Then, to evaluate the time-variation of the macroscopic oxygen ionic and electronic conductivities and the inelastic properties of the anode electrode, numerical material tests based on the homogenization method are conducted for each state of sintered microstructures. In these homogenization analyses, we also have to consider the dependencies of the properties of constituent materials on the temperature and/or the oxygen potential that is supposed to change within an operation period. To predict the oxygen potential distribution in an overall SOFC structure under long-period operation, which determines reduction-induced expansive/contractive deformation of oxide materials, an unsteady problem of macroscopic oxygen ionic and electronic conductions is solved. Using the calculated stress-free strains and the homogenized mechanical properties, both of which depend on the operational environment, we carry out the macroscopic stress analysis of the SOFC.

Stochastic 3D modeling of the microstructure of lithium-ion battery anodes via Gaussian random fields on the sphere

The performance and durability of lithium-ion batteries are highly dependent on the microstructures of their components. Recently, methods have been developed that make possible the simulation of electrochemical processes on 3D representations of lithium-ion batteries. However, it is difficult to obtain realistic microstructures on which these simulations can be carried out. In this paper, we develop a stochastic model that is able to produce realistic microstructures of lithium-ion battery anodes, which can serve as input for the simulations. We introduce the use of Gaussian random fields on the sphere as models for the particles that form the anodes. Using this new approach, we are able to model realistic particle geometries. The stochastic model also uses a number of techniques from stochastic geometry and spatial statistics. We carry out validation of our model, in order to demonstrate that it realistically describes the key features of the anode’s microstructure.

Simulation and Prediction of 3-D Microstructural Evolution and Long Term Performance of Ni-YSZ Anode

The electrochemical performance of SOFCs degrades over the operation time due to a number of factors. The electrochemical activity of the anode is one of these changing parameters due to microstructure coarsening. The change in microstructure of a Ni cermet anode is mainly due to the mobile Ni phase at high temperatures and can be accelerated by the presence of water vapour. The reduced triple phase boundary (TPB) density caused by coarsening leads to degradation of electrochemical performance of the anode. In order to manufacture SOFCs with long- lasting high performance and predict their useful lifetime, it is necessary to understand and predict the microstructural evolution of the anode under different operation conditions. In this contribution, we will demonstrate that the evolution of a 3D real microstructure of a Ni-YSZ anode can be simulated and predicted using our newly developed cellular automaton approach. For this simulation, the YSZ is assumed to be immobile while the mobility of Ni is by the route of evaporation-condensation. In the model the surface energy of Ni and the YSZ/Ni interface energy are simulated based on a new concept of material structural imbalance. The 3D microstructures of anodes subjected to different aging conditions are reconstructed experimentally using focus ion beam (FIB) slice and view. It is shown that the simulation results fit the experimental results very well. Using these short time experimental observations as a calibration of the model, we demonstrate that it is feasible to predict the long time microstructure evolution of the Ni-YSZ anode and hence its long term performance and lifetime.

In Operando Optical Studies of SOFCs Operating with Butanol

In operando optical methods coupled with electrochemical measurements were employed to explore the effects of dry and humidified butanol on the performance and durability of anode supported Ni-YSZ membrane electrode assemblies in functioning solid oxide fuel cells (SOFCs). n-Butanol serves as a useful model biofuel given its ready availability from agricultural feedstocks and its tendency to undergo complex reforming and pyrolysis in high temperature environments. (Combustion and Flame, 158 (2011) 16) Butanol’s complicated gas phase and surface chemistry, however, has hampered efforts to characterize and quantify electrochemical oxidation mechanisms as well as reactive pathways that lead to SOFC component degradation and failure. In order to assess directly the effects of butanol in SOFCs operating at 700˚C and 800˚C, near-infrared thermal imaging was used to measure temperature gradients across the anode while vibrational Raman spectroscopy quantified the tendency for carbon to accumulate in different regions of the anode. FTIR emission spectroscopy monitored gas phase composition above the anode and identified adsorbed surface intermediates. Butanol reforming led to modest cooling (DT = -4˚C) of the anode at OCV. Humidifying the fuel mitigated this effect slightly (DT ~ -2˚C). Polarizing the cell had very little effect on anode temperature changes that resulted from endothermic fuel surface reactions. (Phys. Chem. Chem. Phys. 16, (2014) 227) Within the high temperature environment of the SOFC, butanol underwent significant gas phase reforming with methane, ethylene, formaldehyde and carbon monoxide all being observed in the SOFC fuel exhaust. This heterogeneous mixture led to substantial carbon accumulation on the anode observed in operando, Subsequent studies, however, indicated that carbon distribution was very heterogeneous in the anode microstructure. Post mortem, ex situ experiments showed that heterogeneity in carbon accumulation extended both across and through the anode microstructure with the highest concentration of surface carbon closest to the fuel inlet. Such observations are consistent with recent reports describing carbon accumulation on Ni/ceria electrodes performing electrolysis with CO/CO2 mixtures. (Phys. Chem. Chem. Phys. 16 (2014) 13063.) Carbon that formed from butanol was moderately well ordered, with Raman spectra showing a large graphitic “G” response and a small, but measurable “D” feature corresponding to grain boundaries and/or site defects. (J. Phys. Chem. C 112 (2008) 5232) Polarizing the cell slowed, but did not prevent, carbon from forming under a neat butanol feed, but humidifying the butanol suppressed carbon formation completely. FTIR emission spectroscopy showed measurable increases in both CO(g) and CO2(ads) as a function of cell polarization. The latter species, CO2(ads), is a surface intermediate formed during electrochemical oxidation of adsorbed carbon. (J. Phys. Chem. Lett. 4 (2013) 1310). Humidifying butanol (either 1:1 or 1:3 butanol:steam) led to significantly diminished electrochemical performance and decreased cell longevity. These observations emphasize the need to consider carefully how incident fuel mixtures affect SOFC performance and durability. The figure shows (a) the near infrared thermal image acquired from the anode at 800˚C while eposed to plain butanol and (b) Raman spectra acquired from the anode at 800˚C following exposure to both plain and humidified butanol at OCV. Figure 1

Effect of Non-Uniform Electrode Microstructure in Gas Diffusion Impedance

Gas diffusion resistance represents a significant contribution to anode overpotential in anode-supported solid oxide fuel cells (SOFC). Gas diffusion limitations are often detected through the analysis of impedance spectra. Typically, equivalent circuits are adopted to fit the measured impedance data. However, equivalent circuits do not account for the non-uniform microstructure of SOFC anodes, which commonly consist of a fine functional layer and a thicker and coarser supporting layer. In this study, the impedance response of gas diffusion phenomena in anodes with non-uniform microstructure is simulated by solving the dusty-gas model in frequency domain. The microstructural parameters are evaluated from the tomographic reconstruction of the two layers, as published in the literature. Simulations indicate that the non-uniform electrode microstructure affects the gas diffusion dynamics, thus producing an additional feature in the Nyquist plot at high frequency. Such an additional feature may be erroneously attributed to other phenomena (e.g., charge transfer processes) when phenomenological equivalent circuits are superimposed to fit the data. The study shows that the gas diffusion fingerprint in impedance spectra can substantially differ from the expected finite-length Warburg response in electrodes with non-uniform microstructure. The paper highlights the potential misinterpretations that can arise when interpreting impedance spectra with phenomenological models, especially when the coupling between microstructural characteristics and electrode dynamics are not properly taken into account.

Stability of Nickel-Infiltrated Anodes in Intermediate Temperature SOFCs

Solid oxide fuel cells (SOFCs) with a thin La1-xSrxGa0.8Mg0.2O3- δ (LSGM) electrolyte plus Ni-infiltrated LSGM anode functional layer and Sr0.8La0.2TiO3 (SLT) anode support have shown high power density (> 1 W cm-2) at temperatures ≤ 650°C and good stability in short-term testing (< 100 h). However, the long term stability of these cells is still largely unstudied. In this work, ex situ studies on the evolution of aqueous infiltrated Ni(NO3)2 to NiO and finally to Ni metal nanoparticles on LSGM pellets determined that the size and number of Ni particles can be controlled by dehydrating the samples before calcination, which could be used to advantageously control the microstructure of infiltrated anodes. SOFCs with Ni-infiltrated anodes were tested in humidified hydrogen and air at open circuit voltage (OCV) to determine stability for durations up to 600 h. Scanning electron microscopy indicated that Ni nanoparticles coarsened over time, which negatively affected performance. The polarization resistance of the anode doubled in magnitude over the course of the test, and ohmic polarization was found to increase with time, which could result from coarsening Ni closing off active areas in the anode functional layer. OCV remained within ~0.05 V of the theoretical OCV by the end of the test. In all cases, the majority of the polarization resistance resulted from degradation of the La0.6Sr0.4Fe0.8Co0.2O3-δ-Ce0.8Gd0.2O1.95 (LSCF-GDC) cathode. To better understand the effect of Ni coarsening on cell performance, a model was developed to predict the changes in triple phase boundary (TPB) length during coarsening and its effect on cell performance. Additional testing is required to determine how these preliminary results will extend to operating lifetimes and how to use the ex situ test results to control microstructures in processed cells.

Numerical Assessment of SOFC Anode Polarization with Microstructure Evolution

A phase field method was introduced to simulate the microstructural evolution of nickel-yttria stabilized zirconia composite anode of solid oxide fuel cell in high temperature sintering based on three-dimensional microstructures reconstructed by focused ion beam-scanning-electron-microscopy technique. Nickel phase morphology and crystal structure evolutions and the change of nickel network percolation in operation were simulated simultaneously. In order to quantitatively study the correlation between anode microstructure change and the anode electrochemical performance degradation, the evolution of three-phase boundary density and nickel specific surface area were calculated. Corresponding degradation experiments have been conducted to compare with the simulation results. The experimental results showed good agreement to the simulation results. Figure 1

The Influence of Reduction Conditions on a Ni-YSZ SOFC Anode Microstructure and Evolution

Polarization resistance on SOFC electrodes is principally due to resistances associated to the process taking place in the electrocatalytically active region. This region is known as triple phase boundary (TPB) in common, non-MIEC (Mixed Ionic -Electronic Conductors) based electrodes. For this reason the electrode microstructural configuration, as determining for the extent of TPB accessible for reaction, is decisive regarding the electrode polarization resistance. It is also critical in order to develop a high performing device to minimize ohmic resistances through it. A possible strategy towards this goal is to maximize the conducting phase connectivity. Although it is generally assumed that the cathode accounts for the major contribution to the global polarization resistance in the Ni-YSZ/YSZ/LSM-YSZ system, the anode contribution is susceptible to be optimized to some extent by maximizing the TPB length and phase connectivity present in said electrode. A general practice when manufacturing Ni-YSZ porous electrodes is to start from nickel oxide powders, and have them reduced after the last processing step and before the first operation of the electrode. This nickel oxide to metallic nickel reduction features a volumetric shrinkage close to 40%, forming a new porous network through the volume previously filled by the nickel oxide phase. As the ion conducting phase, in this case YSZ, generally shows negligible mobility at common reduction temperatures, it will act as a backbone during nickel reduction preventing the whole electrode to shrink. The final metallic nickel microstructure, will depend on the initial nickel oxide phase microstructure, but also on the reduction conditions. This final structure is on one hand determined by the thermodynamically most stable configuration, mainly defined by the minimization of the surface energy, but on the other hand it has been observed that kinetics also play a major role on this final configuration. It has been observed that nickel phases tend to coarsen at high temperatures during long term operations [1]. This is due to the fact that Ni atoms present a significant amount of diffusive mobility through the nickel phase in the most common operation conditions, and as the thermodynamic constraint of the minimization of surface energy will always be present, some degree of evolution is expected during long term operation. On this work several samples of Ni:YSZ:Pore anode cermet (proportions 1:1:2 in volume approx.) was characterized after reduction at different temperatures and gas compositions. Two initial nickel oxide particle sizes where also tested. An indirect measurement of the Ni phase connectivity can be made from electrical conductivity measurements [2]. Selected samples were observed using a field emission scanning electron microscope (FESEM). On the range of reducing conditions explored, a trend towards better performance was observed on samples reduced at higher rates and with finer initial oxide microstructures. The most extreme conditions tested lead to the formation of a singular sponge-like nickel structure. REFERENCES [1] Journal of The Electrochemical Society, 160 (11) F1293-F1304 (2013) [2] Solid State Ionics 189 (2011) 82–90

High Fuel Utilization Operation of Solid Oxide Fuel Cells - a Modeling Study

High fuel utilization (Uf ) is desirable in fuel cells as it has a direct impact on efficiency – the higher the Uf the higher the fuel cell efficiency. However, the Ni phase in Ni based SOFC anodes is susceptible to oxidation to NiO when the fuel cell is operated at high fuel utilizations. This phase transition is problematic because a) NiO is inactive to the fuel oxidation reactions, and b) the larger specific volume of NiO results in damage to the anode microstructure if this Ni to NiO transition is reversed and repeated a few times. The second problem listed above is known as Ni redox cycling. Thus, there are conflicting objectives where the designer/operator of a SOFC system desires a high fuel utilization Uf while avoiding Ni oxidation. Ideally, one would like to identify a safe operating zone where the fuel utilization Uf can be maintained in a high enough range without risking Ni oxidation. In this presentation we will use simulation results from a detailed multiphysics model to outline some of the challenges of operating a SOFC at high fuel utilization and attempt to identify the above mentioned “safe operating zone”. The multiphysics model used in this work is for a planar anode supported geometry and includes a fully coupled description of the flow and mass transfer in the air/fuel channels and the electrodes, and the current/potential distribution in the electrodes. One of the mechanisms for Ni oxidation is through reaction (1). Ni + H2O ↔ NiO + H2 (1) The favourability of reaction (1) at a particular location in the anode is dictated by the thermodynamics of the reaction and the local fuel composition. As the model is able to predict the fuel side composition as a function of position, the output of the model can be used to predict whether there is a risk of Ni oxidation in the anode at any given set of operating conditions. We will present results from a parametric study over a wide range of different temperatures, cell voltage, as well as inlet fuel flow-rates and composition. An example set of results is given in Fig 1 where the inlet flow-rate is varied for a given cell voltage and temperature. The Ni oxidation reaction is thermodynamically favourable in the latter section of the anode where the partial pressure ratio PH2O/PH2 exceeds the threshold value given by the thermodynamics of the reaction and shown as a dot-dash line in the bottom plot. According to Fig 1, the threshold Uf for these conditions is ~99%. This is a much higher value than the typical 80-85% used in most SOFC designs. Another noteworthy point is that at this high fuel utlization, the last 1/3 of the cell is producing a very small fraction of the total current. Figure 1

Fabrication By Electrodeposition of a New Tin Anode Reinforced with Carbon Nanotubes for Lithium Ion Batteries

Introduction Lithium ion batteries using graphite as the anode active material have higher energy densities than conventional rechargeable batteries, but obtaining higher capacity will require use of an alternative anode material. Tin has a higher capacity than graphite (991 mAh g-1 vs. 372 mAh g-1) and was used in the current research as an active material. However, tin slips down from the current collector during charging and discharging due to a large volume change, and the cycling characteristics deteriorate. Anchoring the tin layer to the underlying copper layer with a fibrous material should improve adhesion. We previously demonstrated the excellent charge/discharge characteristics of a new tin anode comprising an electroless tin plating film deposited on a Cu/CNT (carbon nanotube) composite plating film.1) In this study, we created a new negative electrode structure incorporating CNTs by electrodeposition and explored the correlation between the structure and charge/discharge characteristics of the electrode. Experimental A Cu/CNT composite plating bath (0.85 M CuSO4・5H2O + 0.55 M H2SO4 + CNTs + polyacrylic acid) and a tin plating bath (1 M K4P2O7 + 0.25 M Sn2P2O7 + additive) were prepared. Cu/CNT composite plating film was plated on a pure copper plate, then tin was electrodeposited on the Cu/CNT composite plating film, generating a new CNT-reinforced tin anode structure. For comparison, tin was electrodeposited directly on a pure copper plate. Electroplating was carried out under galvanostatic conditions at 25°C. The Cu/CNT composite plating film on a pure copper plate and a platinum plate were used as the cathode and anode, respectively, for tin plating. The microstructure of the new tin anode was examined by field-emission scanning electron microscopy (FE-SEM). The phase structures of the deposits were analyzed by X-ray diffraction (XRD). Electrochemical studies of the tin-based anodes were carried out using coin cells assembled in an Ar-filled glove box. Each coin cell consisted of lithium foil as the counter and reference electrodes and the tin-based anode 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 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 new tin anode and demonstrates that tin particles had deposited on the CNTs. Fig. 2 shows a cross-sectional SEM image of the new tin anode; CNTs are clearly seen between the tin and copper layers, and no defects are observed. The charge/discharge characteristics of the new tin anode will be discussed at the meeting. References 1) R. Fukuoka, S. Arai, of the 44th Annual Meeting of Union of Chemistry-Related Societies in Chubu Area of Japan, 133(2013) Figure 1

Fabrication of a New Tin Anode for a Lithium-Ion Battery Using a Three-Dimensional Copper Nanostructure

Introduction Lithium-ion batteries (LIBs) are one of the most important energy storage and conversion devices and are widely used in portable electronic devices such as mobile phones and laptops. 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 such as Li4.4Sn with maximum composition and have a specific capacity of 994 mA h g-1, 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 during the electrochemical alloying-dealloying process. These volume 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 to improve 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 the 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 PEG +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). A cross-section polisher was used to generate cross-sections of the samples. Electrochemical measurements were obtained using coin-type cells assembled in an argon-filled glove box. LiPF6 (1 M) in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%) was used as the electrolyte solution. Cycling tests were performed between 0.02-1.5V (vs. Li/Li+) at a constant temperature of 25°C. Results and Discussion Figure 1 (a) shows a surface SEM image of the new tin anode and demonstrates the homogeneous plating of 3D copper nanostructures on the tin film.Figure 1 (b) shows a cross-sectional SEM image of the new tin anode, revealing that the inside of the 3D copper nanostructure is also plated uniformly. This new tin anode exhibits both high reversible capacity and improved cyclability. Charge/discharge characteristics of this anode will be presented in detail at the meeting. References 1) S.Arai and T.Kitamura, ECS Electrochemistry Letters. 2014, 3 (5), D7-D9. Figure 1

(Invited) Microstructural Modeling of Heterogeneous Nucleation and Growth of Lithium Electrodeposits

Through numerical modeling and simulation, we explore the charge/discharge conditions that lead to the formation of lithium dendrites in rechargeable lithium-ion batteries. Firstly, by using high resolution X-ray tomography, a three-dimensional representation of a lithiated graphite anode microstructure from a commercial battery was used as input to simulate the local electrochemical behavior that will favor heterogeneous lithium nucleation. The analysis demonstrates that spatial variation in porosity, particle roughness and morphology significantly alter the local intercalation kinetics, state of charge and mechanical stability, and dendrite formation. Secondly, by using the phase field approach, we demonstrate the conditions that will enable an overpotential-controlled critical lithium electrodeposit to become energetically favorable and identify the regimes of dendrite nucleation, growth, and suppresion behavior. Figure 1

Effect of Pass Deformation on the Microstructure and Properties of Aluminum Alloy Anodes for Fuel Cell Application

he effect of pass deformation on the microstructure of Al-Mg-Sn-Bi-Ga-In alloy anode was investigated by using SEM, TEM, and EBSD. The results show that when controlling the rolling deformation at 40%, the segregation phases on the Al alloy matrix was the best distributed for the reason of dynamic recrystallization procedure. The electrochemical and anti-corrosion results show that uniform distribution of segregation phase will improve the related properties of Al alloy anode. After optimizing the rolling procedure, the Al alloy anode has more negative electrode potential of about -1.585V(vs.Hg/HgO) and lower hydrogen evolution rate of 0.092mL/ (min·cm2)

In Operando Optical Studies of SOFCs Operating with Butanol

In operando optical methods coupled with electrochemical measurements were employed to explore the effects of dry and humidified butanol on the performance and durability of anode supported Ni-YSZ membrane electrode assemblies in functioning solid oxide fuel cells (SOFCs). In order to assess directly the effects of butanol in SOFCs operating at 700˚C and 800˚C, near-infrared thermal imaging was used to measure temperature gradients across the anode while vibrational Raman spectroscopy quantified the tendency for carbon to accumulate in different regions of the anode. FTIR emission spectroscopy monitored gas phase composition above the anode and identified adsorbed surface intermediates. Butanol reforming led to modest cooling (DT = -4˚C) of the anode at OCV. Humidifying the fuel mitigated this effect slightly (DT ~ -2˚C). Polarizing the cell had very little effect on anode temperature changes that resulted from endothermic fuel surface reactions. Within the high temperature environment of the SOFC, butanol underwent significant gas phase reforming with methane, ethylene, formaldehyde and carbon monoxide all being observed in the SOFC fuel exhaust. This heterogeneous mixture led to substantial carbon accumulation on the anode observed in operando, Subsequent studies, however, indicated that carbon distribution was very heterogeneous in the anode microstructure. Post mortem, ex situ experiments showed that heterogeneity in carbon accumulation extended both across and through the anode microstructure with the highest concentration of surface carbon closest to the fuel inlet. Polarizing the cell slowed, but did not prevent, carbon from forming under a neat butanol feed, but humidifying the butanol suppressed carbon formation completely.

Impedance Studies of the Reduction Process in NiO-YSZ SOFC Anodes

The aim of this study is the development of a methodology for deeper insight into the Ni-YSZ anode behavior by in situ impedance monitoring of the Ni network initial formation and changes during the redox process. The testing is performed on single bare anodes. This approach bypasses complications arising from the impedance data analysis of cells. For more complete description of the changes in the anode microstructure, gas permeability measurements are also introduced. The obtained results show that impedance can register the changes in the sample’s conductivity during anode reduction, as well as during redox cycling performed in mild oxidation conditions. The oxidation rate is much slower than the reduction one. The combination of impedance with gas permeability and microstructural measurements gives new opportunities for anode optimization in respect to the ohmic and the concentration polarization.

Performance Factors and Sulfur Tolerance of Metal Supported Solid Oxide Fuel Cells with Nanostructured Ni:GDC Infiltrated Anodes

Two metal supported solid oxide fuel cells (active area 16 cm2) with nanostructured Ni:GDC infiltrated anodes, but different anode and support microstructures were studied in respect to sulfur tolerance at the aimed operating temperature of 650ºC. The studied MS-SOFCs are based on ferretic stainless steel (FeCr) and showed excellent performance characteristics at 650ºC with area specific resistances (ASR) of 0.35 Ωcm2 and 0.7 Ωcm2 respectively. The sulfur tolerance testing was performed by addition/removal of 2, 5, and 10 ppm H2S in hydrogen based fuel under galvanostatic operation at a current load of 0.25Acm-2. The results were compared with literature on the sulfur tolerance of the conventional SOFC Ni/YSZ cermet anode. The comparison in terms of absolute cell resistance increase and relative anode polarization resistance increase indicate, that the nanostructured Ni:GDC MS-SOFC based anode is significantly more sulfur tolerant than the conventional Ni/YSZ cermet anode.

Numerical Assessment of SOFC Anode Polarization with Microstructure Evolution

In order to quantitatively investigate the correlation between the solid oxide fuel cell anode microstructure evolution and the resulting electrochemical performance, the evolution of three-phase boundary and nickel specific surface area were calculated. A three dimensional numerical simulation of the anode overpotential is conducted using a Lattice Boltzmann Method (LBM), which solves the species transport equations coupled with the electrochemical reaction at three-phases-boundary. Phase field method is introduced to simulate the microstructural evolution of nickel-yttria stabilized zirconia composite anode. The increase of anode overpotential during long time operation can be simulated based on the LBM and the phase field modeling.