Porous Solid Oxide Fuel Cell Research Articles

Accessing Elastic Properties of Porous Solid Oxide Fuel Cell Electrodes Using 2D Image-Based Discrete Element Modeling and Deep Learning

Accessing Elastic Properties of Porous Solid Oxide Fuel Cell Electrodes Using 2D Image-Based Discrete Element Modeling and Deep Learning

Breakthrough in atmospheric plasma spraying of high-density composite electrolytes: Deposition behavior and performance of plasma-sprayed GDC-LSGM on porous metal-supported solid oxide fuel cells

The potential application of plasma spraying in the preparation of ceramic electrolyte for porous metal-supported solid oxide fuel cells (SOFCs) is highlighted by its ability to eliminate the need for a high-temperature sintering process. However, the challenge of achieving highly dense electrolytes through plasma spraying remains to be addressed. In this study, a novel electrolyte for porous metal-supported SOFCs (PMS-SOFCs) is developed. This involved the preparation of a highly dense structure of gadolinium-doped ceria (GDC)-lanthanum strontium gallium magnesium oxide (LSGM) composite coating using plasma spraying under atmospheric conditions. The composite electrolyte is prepared using atmospheric plasma spraying (APS). The addition of the low-melting-point LSGM phase enhanced the microstructural densification of the GDC-based composite coating and diminished electron loss in a reducing atmosphere, thereby improving the cell's open-circuit voltage. At 36 kW plasma arc power, the single cell with composite electrolyte exhibited a maximum power density of 371 mw/cm2 at 750 °C and achieved the highest open-circuit voltage (1.03 V) at 600 °C. Moreover, the open-circuit voltage remained stable over a 100-h test. These findings suggest that using APS to deposit a composite electrolyte with added low-melting-point secondary phases presents a promising approach for achieving relatively high OCV in PMS-SOFCs based on cerium oxide electrolytes.

Operando optical studies of sulfur contamination in syngas operation of solid oxide fuel cells

Near-infrared thermal imaging, voltammetry, and chronocoulometry have been used to examine electrochemical oxidation mechanisms in solid oxide fuel cells (SOFCs) operating with model syngas mixtures (CO + H2) at 800 °C with and without a sulfur-containing contaminant. Questions persist regarding the electrochemical oxidation of H2 vs. CO especially on the role of CO activation and CO mobility in porous SOFC cermet anodes. Chronocoulometry, electrochemical impedance spectroscopy, and operando optical methods are also used to characterize contaminant effects of SOFC operation at 800 °C in the presence of 50 ppmv sulfur to gain insight into its effect on syngas SOFC operation. The chronocoulometry results indicate that in the absence of sulfur contaminants, electrochemical oxidation of adsorbed CO does not contribute to current production, possibly because of rapid carbon formation at the anode surface. This discovery suggests that initially adsorbed H2 is the primary source of charge at early times in the electro-oxidation of syngas fuels. Furthermore, in the presence of sulfur contaminants, chronocoulometry data show that S competitively occupies ~30% of the electroactive sites at the triple phase boundary and contributes to the total initial charge likely via electrochemical oxidation of S to SO2.

Atomic Layer Deposition for Surface Area Characterization of Porous Solid Oxide Fuel Cell Electrodes and Beyond

Conventional surface area determination techniques are inadequate for calculating the surface areas of porous solids with absolute totals less than ~1 m2, but with nm-scale feature sizes, such as state-of-the-art, lab-scale solid oxide fuel cell electrodes. Meaningful contextualization of their electrochemical performance requires knowing system surface area, but the two predominant classes of techniques, tomography and gas adsorption theory, either lack the resolution or require prohibitive amounts of material. Presented here is a new method for the accurate determination of absolute surface areas on the order of 1-1000 cm2 with precision ±0.3 cm2 utilizing atomic layer deposition (ALD) of commonly available trimethylaluminum (TMA) and water vapor to conformally deposit alumina over internal features. The area of alumina can then be quantified using plasma spectroscopy and the known nm-scale thickness of the layer. As a model system, scaffolds of La0.8Sr0.2MnO3-δ/Ce0.9Gd0.1O2-δ (LSM/GDC) of 2.6 m2/g were measured under the new technique and compared against the B.E.T. method, with remarkably similar results obtained, but with ~1000 times less mass needed for the experiment. Under the modest ALD reactor soak times used (~10 s), and with conservative precursor pulse durations, the penetration depth of the coating was found to be ~50 µm, exceeding the requirement for the electrodes under study. To demonstrate the technique’s capabilities further, the LSM/GDC scaffolds were infiltrated with PrOx nanoparticles of ~50 nm, and the increase in surface area (+ ~25%) and enhanced electrochemical performance were measured. After annealing for 1000 hours at 700°C, both were again measured, and the surface area was found to have reverted to non-infiltrated levels, despite the infiltrated cells maintaining their higher performance. The technique, due to its ease versus other surface area determination methods and its pairing with an established analytical chemistry method, enables a useful combined chemical/morphological approach to the characterization of next-generation solid oxide cell electrodes, and may find other uses among nanostructured yet low absolute surface area systems in the broad field of heterogeneous catalysis.Figure 1. Measured specific surface areas for La0.8Sr0.2MnO3-δ/Ce0.9Gd0.1O2-δ (LSM/GDC) scaffolds and the ALD Al2O3-based technique, in lab-scale mg quantities, versus the B.E.T. method result for gram-scale LSM/GDC flakes. Shown is ALD-derived surface area quantified both via inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Blue) and gravimetrically (Green), with the ICP-OES offering comparable accuracy and precision to the B.E.T. (Purple). Error bars reflect repetition of the same sample (intra-cell) and among different samples (inter-cell). Figure 1

Numerical study of detecting crack initiation in a planar solid oxide fuel cell

AbstractConverting chemical energy into electricity is done by an electrochemical device known as a fuel cell. Thermal stress is caused by high operating temperature, between 700 and 1,000°C, of solid oxide fuel cell (SOFC). Thermal stress is the main cause of crack initiation and crack propagation. This phenomenon may cause gas leakage, structure instability and cease operation of the SOFC before its lifetime. The aim of this study is to present a method that predicts the initiation of cracks in an anisotropic porous planar SOFC. The coupled governing nonlinear differential equations are solved numerically as for heat transfer, fluid flow, mass transfer, mass continuity, and momentum. An in‐house computer code which is based on computational fluid dynamics, computational structural mechanics and extended finite element method is utilized and developed. This code, according to Darcy and Navier–Stokes thermofluid model, determines the temperature and stress distribution. The results show that the highest thermal stress occurs at the upper corners of cathode and at the lower corners of the anode. The maximum temperature occurs at the middle of the electrolyte cathode and electrolyte anode, while the maximum pressure occurs at the middle of the upper and lower section of the anode and cathode. In addition, the thickness of the cathode electrode at the left side is increased by 1.5%. Finally, the crack initiation occurs at the left side between the upper and lower corners of the cathode.

Development of solid oxide fuel cells using high-frequency plasma spraying technologies

The paper describes the topical issues and the latest achievements in the development of materials with high electrochemical characteristics for single cells and small assemblies of solid oxide fuel cells (SOFC). The authors propose planar SOFC technology which includes sequential deposition of all layers of a thin-film cell on a porous metallic substrate by plasma techniques. The method allows developing porous SOFC anode and cathode, which increases the active area of electrochemical reaction, reduces the spread of technological parameters (in particular, the spread in thickness and porosity of a solid electrolyte), improves SOFC performance and reduces the mass and size of SOFC assemblies. The paper also describes the physics of plasma spraying process in individual cells and SOFC structures. The proposed method is an alternative to traditional SOFC production methods; it eliminates the necessity to use agglomeration technology or other heat treatment. Moreover, it opens up the prospects for automated continuous production process. The appropriate SOFC production equipment is developed, for stationary and mobile use. Further studies will be focused on the task of reducing the weight of SOFC assemblies, which allows using them as an auxiliary power unit for on-board power supply, for example, for cars, airplanes and spaceships.

Lattice Boltzmann modelling of the coupling between charge transport and electrochemical reactions in a solid oxide fuel cell with a patterned anode

A fundamental understanding of the electrochemical reactions and the associated transport processes in electrodes of solid oxide fuel cells (SOFCs) is critical to the development of new electrode materials. To date, however, our understanding of the electrode processes is still very limited due to the lack of well-designed experiments and carefully-validated predictive models. To facilitate studies in this area, we have developed a numerical model that has taken into consideration of the coupling between the transport of mobile charged species (e.g., ions and electrons) in conducting phases and the electrochemical reactions at the three phase boundaries of an SOFC anode. The validity of this model has been confirmed by the electrochemical performance of test cells with a patterned anode consisting of well-defined electronic and ionic conducting phases. The model is then applied to quantifying the factors that critically influence the performance of a patterned anode, resulting in three key dimensionless parameters governing the coupling of transport and reactions in the anode: the ratio of electronic-to-ionic conductivity (σel/σion), the dimensionless exchange current density (iex/i0), and the dimensionless electric potential (Fϕ0/RT). In particular, it is found that only iex/i0 and Fϕ0/RT play a significant role under typical SOFC operating conditions: anode performance increases with the increase in iex/i0 and Fϕ0/RT. Accordingly, we have constructed a phase map to demonstrate the combined effect of iex/i0 and Fϕ0/RT, which is helpful for rational design and operation of SOFC patterned electrodes of different materials and geometries. More importantly, our present model is also applicable to the study of actual porous SOFC electrodes with known 3D microstructures.

Sr Surface Segregation on La0.6Sr0.4Co0.2Fe0.8O3-δ Porous Solid Oxide Fuel Cell Cathodes

In recent years, mixed ionic-electronic conducting (MIEC) materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) have been studied and developed as solid oxide fuel cell (SOFC) cathodes, due to their high activity for the oxygen reduction reaction at temperatures below 800°C.1 However, a number of such MIEC materials that contain Sr, including LSCF, exhibit Sr surface segregation at elevated temperatures. In prior work done mainly on dense thin-film MIEC electrodes, Sr-rich surface phases (e.g., SrO) have been shown to hinder the oxygen surface exchange process.2-5 A recent study, where the surface Sr coverage was quantified by a chemical etching method combined with 3-dimensional (3D) tomography, demonstrated increased amount of surface segregated Sr, along with increased polarization resistance, on LSCF porous electrodes after ageing at 800°C for 800 h.6 In this work, we present a study of degradation mechanisms of porous LSCF electrodes under thermal ageing. LSCF symmetric electrode cells with Gd0.1Ce0.9O1.95 (GDC) electrolytes were maintained in ambient air at various temperatures ranging from 550°C to 850°C for different periods of time up to 1500 h, without current/polarization. Electrochemical impedance spectroscopy (EIS) measurements were taken once per day at 700°C, for cells aged at 700°C and 850°C. Increases in the polarization resistance with time by as much as 2 times were found for both ageing temperatures. The electrode morphological changes and Sr surface segregation were examined utilizing a combination of 3D tomography via focused ion beam-scanning electron microscopy (FIB-SEM) and chemical etching with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection. The 3D imaging showed that the LSCF electrodes did not coarsen measurably over 700 h at 700°C or 1500 h at 850°C. On the other hand, the ICP-OES analysis detected an increased amount of water-soluble Sr with increasing ageing temperature and time, as shown in Fig. 1. Application of Adler-Lane-Steele (ALS) model using the measured impedance and microstructural parameters suggested that the increased polarization resistance resulted from a substantial decrease in both the surface exchange kchem and oxygen diffusion coefficients D* . The LSCF electrodes aged below 650°C showed no significant increase in surface Sr content compared to the as-prepared sample, indicating there may be a kinetic limitation on Sr surface segregation at lower temperatures. 1. Gao, Z., Mogni, L. V., Miller, E. C., Railsback, J. G. & Barnett, S. A. A perspective on low-temperature solid oxide fuel cells. Energ Environ Sci 9, 1602-1644 (2016). 2. Pan, Z. H., Liu, Q. L., Zhang, L., Zhang, X. W. & Chan, S. H. Effect of Sr Surface Segregation of La0.6Sr0.4Co0.2Fe0.8O3-delta Electrode on Its Electrochemical Performance in SOC. J Electrochem Soc 162, F1316-F1323 (2015). 3. Zhao, L., Drennan, J., Kong, C., Amarasinghe, S. & Jiang, S. P. Insight into surface segregation and chromium deposition on La0.6Sr0.4Co0.2Fe0.8O3-delta cathodes of solid oxide fuel cells. J Mater Chem A 2, 11114-11123 (2014). 4. Liu, Y. H. et al. Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-delta cathodes under solid oxide fuel cells operation conditions. Int J Hydrogen Energ 39, 15868-15876 (2014). 5. Simner, S. P., Anderson, M. D., Engelhard, M. H. & Stevenson, J. W. Degradation mechanisms of La-Sr-Co-Fe-O3SOFC cathodes. Electrochem Solid St 9, A478-A481 (2006). 6. Wang, H. Q. et al. Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O3-delta Solid Oxide Fuel Cell Cathodes. J Electrochem Soc 163, F581-F585 (2016). Figure 1

Bio-Surfactant Assisted Infiltration of SOFC Electrodes

Wet nano-catalyst impregnation of porous Solid Oxide Fuel Cell (SOFC) electrodes is a recognized process to enhance electrochemical performance. This impregnation method typically includes multiple pipet deposition steps of aqueous salt solutions onto the SOFC electrodes in order to achieve a desired catalyst loading level. In the current work, it was found that this labor-intensive and time-consuming process could be simplified through the use of a bio-adhesive (or bio-derived surfactant). The surfactant-assisted impregnation protocol allowed for the homogenous incorporation of a nano-catalyst within the electrode in a single salt solution deposition step. The process results in a lower electrode overpotential with enhanced long-term stability. In the current study, bio-inspired catechol-based surfactants were used to act as a wetting and local chelating agent which adhered within the 3-D electrode architectures. The work initially assessed the use of polymerized dopamine (PDA) and/or nor-epinephrine (PNE) neurotransmitters as the bio-template layer for metal oxide or hydroxide deposition. The adhered bio-template on the pore walls was shown to provide higher nano-catalyst loading by avoiding segregation of the precipitate within the porous structure, which typically results in the migration and distribution of the solute near the surface during the drying process. Another advantage of using the protocol developed in this work was that both electrodes could be infiltrated simultaneously by utilizing a simple dip-coating method within a single thermal step. Additional studies in this work focussed on the deposition and aggregate (and/or film) formation kinetics of the dopamine and epinephrine surfactants in order to better understand the nano-catalyst deposition morphology. The work investigated the deposition of these layers on YSZ substrates, which would act as an ideal surface to characterize (over the 3-D electrode layers). This work then led to the investigation of other catechol molecules, such as caffeic acid, cinnamic acid, and gallic acid onto YSZ planar substrates. In order to complete this study, commercial YSZ single crystals were submersed into surfactant solutions for 1 to 24 hours. The surface was dried and gently inscribed by a fine needle tip to measure the surfactant thickness by Atomic Force Microscopy (AFM). The result showed that the polymerization method and surfactant type had a significant effect on the adhered layer thickness, which varied from 30 to 500 nm within 24 hours of immersion time. In addition, the effect of these surfactants on the chelation and precipitation of metal oxide nano-catalyst from precursor solutions were also studied The optimal conditions were resolved from the surfactant coating studies which were used to impregnate nano-ceria (CeO2) based nano-catalyst into the electrodes of anode-supported SOFC button cells. The final studies of this work investigated the effect of the critical nano-catalyst infiltrant concentration on the fuel cell performance. Commercial button cells were infiltrated with norepinephrine (PNE) and then impregnated with different molarities of cerium salt solution for both electrodes by dip-coating method. After deposition, the samples were only exposed to a singular drying/firing step. The process achieved loadings ranging between 0.4 to 8.3 mg of nano-ceria after the firing (calcination) step at 750oC. Infiltrated cells were operated at 750oC using humidified hydrogen fuel for ~300 hours. The SOFC performance was evaluated by electrochemical impedance spectroscopy (EIS) in the 48th, 150th and 300th hours. EIS analysis showed that the infiltrated cells did not show a significantly lower resistance than the baseline cell in the first 48th hour. However, in the 150th to 300th hour, the resistance of the infiltrated cells reduced in both the high-frequency and mid-frequency regimes (associated with the anode and cathode, respectively). Furthermore, the resistance of the low-frequency regime (associate with mass diffusion) decreased between the 48th to 300th hour. The resistance relaxation in this region was probably due to increased gas diffusion related to nano-ceria sintering (and opening of the microstructure porosity). In the study, a gradual decrease in the total electrode overpotential was observed up to the 5 mg nano-ceria loading level. The cell displayed a 35% reduction in polarization over the baseline cell after 300 hours of testing. The overpotential of 8.5 mg nano-ceria infiltrated cell displayed a similar electrode overpotential value as the baseline cell after 300 h of testing (i.e., with no nano-catalyst loading). The correlation between the cell performance (as a function of loading time) to the nano-catalyst distribution/morphology is currently being investigated. In addition, methods for controlling the grain boundary migration rate using various solid-solution or secondary-phase stabilization strategies are also being investigated in order to achieve long-term stability of the infiltrated cells.

Sr Surface Segregation on La0.6Sr0.4Co0.2Fe0.8O3-δ Porous Solid Oxide Fuel Cell Cathodes

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) symmetric-electrode cells with Gd0.1Ce0.9O1.95 (GDC) electrolytes were aged in ambient air at temperatures ranging from 550 to 800 °C for times up to 1400 h, without current/polarization. Electrochemical impedance spectroscopy measurements, taken periodically during the tests at a testing temperature of 700 °C for cells aged ³ 700 °C, showed a continuous increase in polarization resistance. Focused ion beam-scanning electron microscopy (FIB-SEM) 3D tomography showed that the LSCF electrode did not coarsen measurably even at the highest ageing temperature, 800 °C, ruling out LSCF microstructural changes as the mechanism behind the resistance increase. On the other hand, Sr surface segregation, determined by chemical etching with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection, was found to increase with increasing ageing temperature > 650 °C. The effect of Sr surface segregation on the oxygen surface exchange and diffusion processes is quantified by comparing the measured electrochemical and microstructural data.

Numerical investigation of coupled mass transport and electrochemical reactions in porous SOFC anode microstructure

To investigate the coupled mass transport and electrochemical reactions in porous anode of solid oxide fuel cell (SOFC), a pore scale model employing the multi-component Lattice Boltzmann (LB) model for mass transport in the pore phase and a six-step reaction for electrochemical reactions at three phase boundaries is established. This model is superior in including the effects of heterogeneity of anode microstructure, making it qualified to quantitatively simulate local distributions of various physical fields without dependence on any statistical parameters. The characteristics of coupled interactions of mass transport and electrochemical reaction are analyzed visually by comparing H2 molar fraction distributions within the pore phase of the anode. It is demonstrated that, for anodes with different heterogeneous microstructures but with same porosity and tortuosity factor, the mass transport and electrochemical reaction process can be quite different, even under the same boundary conditions. Meanwhile, based on the same anode microstructure, species molar fraction varies similarly along the primary transport direction, even though the operating conditions are quite different. Besides, the influences of fuel composition, activation overpotential and operating temperature are also discussed at pore scale by the present model.

Thermomechanical analysis of porous solid oxide fuel cell by using peridynamics

Solid oxide fuel cell (SOFC) is widely used in hybrid marine propulsion systems due to its high power output, excellent emission control and wide fuel suitability. However, the operating temperature in SOFC will rise up to 800–1000 ℃ due to redox reaction among hydrogen and oxygen ions. This provides a suitable environment for ions transporting through ceramic materials. Under such operation temperatures, degradation may occur in the electrodes and electrolyte. As a result, unstable voltage, low capacity and cell failure may eventually occur. This study presents thermomechanical analysis of a porous SOFC cell plate which contains electrodes, electrolytes and pores. A microscale specimen in the shape of a plate is considered in order to maintain uniform temperature loading and increase the accuracy of estimation. A new computational technique, peridynamics, is utilized to calculate the deformations and stresses of the cell plate. Moreover, the crack formation and propagation are also obtained by using peridynamics. According to the numerical results, damage evolution depends on the electrolyte/electrode interface strength during the charging process. For weak interface strength case, damage emerges at the electrode/electrolyte interface. On the other hand, for stronger interface cases, damage emerges on pore boundaries especially with sharp corner.

A critical look into effects of electrode pore morphology in solid oxide fuel cells

Knudsen diffusion, an important form of gas transport in sub‐micro/nanoscale porous electrodes of solid oxide fuel cells (SOFCs), is evaluated typically based on the assumption of isotropic cross‐sections of electrode pores. As a consequence, errors are induced in the evaluation of gas transport and polarization loss of SOFCs with irregular, anisotropic pore morphology. Here, a numerical model is derived to investigate the impact of pore morphology on Knudsen diffusivity and effective total diffusivity in porous SOFC electrodes. Based on the model, the correlation between pore morphology and important parameters of SOFCs, including limiting current density (LCD) and concentration polarization (CP), is evaluated. As the aspect ratio of pore cross‐section increases, the gas diffusivity in SOFC electrodes decreases, and then nontrivial variations in LCD and CP are induced. This work facilitates the accurate evaluation of gas transport in SOFCs as well as the rational design of electrode microstructure of SOFCs. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2312–2317, 2017

Performance and Durability of Thin Film Thermocouple Array on a Porous Electrode.

Management of solid oxide fuel cell (SOFC) thermal gradients is vital to limit thermal expansion mismatch and thermal stress. However, owing to harsh operation conditions of SOFCs and limited available space in stack configuration, the number of techniques available to obtain temperature distribution from the cell surface is limited. The authors previously developed and studied a thermocouple array pattern to detect surface temperature distribution on an SOFC in open circuit conditions. In this study, the performance in terms of mechanical durability and oxidation state of the thin film thermoelements of the thermocouple array on the porous SOFC cathode is investigated. A thin-film multi-junction thermocouple array was sputter deposited using a magnetron sputter coater. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterisation techniques were carried out to understand characteristics of the thin film before and after temperature (20 °C–800 °C) measurement. Temperature readings from the sensor agreed well with the closely placed commercial thermocouple during heating segments. However, a sensor failure occurred at around 350 °C during the cooling segment. The SEM and XPS tests revealed cracks on the thin film thermoelements and oxidation to the film thickness direction.

Cell integrated multi-junction thermocouple array for solid oxide fuel cell temperature sensing: N+1 architecture

Understanding the cell temperature distribution of solid oxide fuel cell (SOFC) stacks during normal operation has multifaceted advantages in performance and degradation studies. Present efforts on measuring temperature from operating SOFCs measure only the gas channel temperature and do not reveal the cell level temperature distribution, which is more important for understanding a cell's performance and its temperature-related degradation. The authors propose a cell-integrated, multi-junction thermocouple array for in-situ cell surface temperature monitoring of an operational SOFC. The proposed thermocouple array requires far fewer numbers of thermoelements than that required by sets of thermocouples for the same number of temperature sensing points. Hence, the proposed array causes lower disturbance to cell performance than thermocouples. The thermoelement array was sputter deposited on the cathode of a commercial SOFC using alumel (Ni:Al:Mn:Si – 95:2:2:1 by wt.) and chromel (Ni:Cr – 90:10 by wt.). The thermocouple array was tested in a furnace over the entire operating temperature range of a typical SOFC. The individual sensing points of the array were shown to measure temperature independently from each other with equivalent accuracy to a thermocouple. Thus, the concept of multi-junction thermocouples is experimentally validated and its stability on a porous SOFC cathode is confirmed.

Bio-inspired surfactant assisted nano-catalyst impregnation of Solid-Oxide Fuel Cell (SOFC) electrodes

A bio-inspired surfactant was utilized to assist in the efficient impregnation of a nano-CeO2 catalyst throughout both porous Solid Oxide Fuel Cells (SOFC's) electrodes simultaneously. The process included the initial modification of electrode pore walls with a polydopamine film. The cell was then submersed into a cerium salt solution. The amount of nano-CeO2 deposited per impregnation step increased by 3.5 times by utilizing this two-step protocol in comparison to a conventional drip impregnation method. The impregnated cells exhibited a 20% higher power density than a baseline cell without the nano-catalyst at 750°C (using humid H2 fuel).

Porous SOFC Electrode Infiltration Methods By Bio-Adhesive Templating

Nano-catalyst infiltration within porous Solid Oxide Fuel Cells (SOFC’s) electrodes is a well-known process to increase electrochemical performance. Conventional dripping methods usually require multiple infiltration steps in order to achieve optimum catalyst loading. In this study, an alternative process was developed to efficiently infiltrate nano-catalyst into the NiO/YSZ anode of an anode-supported commercial SOFC. Mussel inspired catechol-based adhesives were used as a surfactant layer within the 3-D electrode architecture to better control the infiltration kinetics and dispersion of the nano-catalyst. The work assessed the use of adhesive molecules, polymerized dopamine (PDA) and/or nor-epinephrine (PNE), as the bio-template layer for either metal oxide or hydroxide deposition. The bio-template adhesiveness on the pore walls was shown to provide higher nano-catalyst loading by avoiding segregation of the precipitate within the porous structure, which usually results in the migration and preferential deposition of the solute at the surface (during the drying process). The first objective was to better understand the templating kinetics of PDA and PNE with altering processing variables (eg. pH, solid loading, immersion time, etc.), in order to control nano-catalyst deposition amount. The second objective was to control the catalyst grain growth rate by manipulating the level of nano-catalyst percolation and by altering the composition of the catalyst to hinder diffusion and grain boundary mobility. The process started by first immersing the cell into the polymerized template solution up to 24 h and then immersed directly into the catalyst precursor solution, with the same protocol. Cerium oxide was used as a demonstration catalyst composition for this work, but later work will focus on other catalyst compositions. A dip-coating process for the entire cell was used for this work in order to evaluate the possibility of infiltrating both the anode and cathode at the same time. The final step included a singular firing to 750oC to form CeO2 catalyst. In that case, only one thermal firing step was needed to achieve high solids loading levels, which is unlike conventional dripping methods where multiple thermal steps are required. A correlation between ~300 hour cell performance and nano-catalyst characterization was developed by I-V-P testing and impedance spectroscopy at various duration during the constant current loading. In one heat cycle, dopamine assisted dip-coated cells reached 1.1 mg to 1.6 mg deposition of nano-ceria incorporation (for the combined anode and cathode). The most promising of the protocols included a) ex-situ polymerization, (where the biotemplate solution was polymerized in a separate beaker before infiltration) and b) in-situ polymerization (where the biotemplate solution was polymerized when the cell was in the bio-template medium). These methods showed an initial increase of the maximum power density at 750°C using humidifed H2 fuel by ~20.7 and 17.9%, respectively. However, the grain growth of the nano-catalyst within the in-situ polymerized fuel cell caused a decrease in performance from 11.5% to 5.6 % in 300 h. Also, PNE assisted dip-coating protocol resulted in the deposition of 1.8 mg of nano-ceria by the same process. The cell showed ~13.6% higher increase in the maximum power density at ~24 h, but the cell degraded and the maximum power density decreased to ~2.1% lower than the baseline cell performance. Future work is focusing upon methods to control the nano-catalyst growth at the operating temperature. Methods of controlling the grain boundary migration rate using various solid-solution or secondary phase stabilization strategies are currently being investigated in order to achieve long-term stability of the infiltrated cells. Acknowledgements: As part of the National Energy Technology Laboratory’s Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000. This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Three Dimensional Microstructural Characterization of Cathode Degradation in SOFCs Using Focused Ion Beam and SEM

The limited durability of solid oxide fuel cells (SOFCs) in practical applications has impeded the commercial adoption of these devices. A primary component of this degradation occurs within the cathode upon long-term exposure to various contaminants, including H2O, CO2, and Cr vapor. These contaminants cause significant microstructural and compositional changes within the cathode that adversely affect activation, polarization mechanisms, and ionic and electronic conductivities. Previous works (Gostovic et al. and Smith et al.) have demonstrated that a number of quantifiable microstructural characteristics can be directly related to SOFC performance, the most important of these being triple phase boundary length (LTPB) and pore surface area. These parameters have not been examined during cell degradation, and further analysis under these conditions will provide insight into specific cell degradation mechanisms, informing future fabrication and operation criteria.In this work, we present a three-dimensional quantification of porous SOFC cathode materials that have been aged in both clean and contaminated atmospheres. Symmetric cathode cells were produced by standard screen printing methods using an yttria-stabilized zirconia (Y2O3-ZrO2, YSZ) electrolyte, and a composite cathode. The composite cathode consisted of 50% lanthanum strontium manganite (La1−xSrxMnO3, LSM) and 50% YSZ by weight. We report here the quantification of and differences between three YSZ/LSM-YSZ symmetric cells: unaged, aged for 480 hours in a clean air environment, and aged for 480 hours under H2O atmosphere. This quantification was performed using a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) to serially mill 30nm slices of the cell, taking an image after each slice. In this way, a series of 2D images was acquired, aligned, and reconstructed into a fully quantifiable 3D volume rendering of the cell near the cathode/electrolyte interface, as shown in Figure 1.After developing a 3D model for each cell, the volumes were quantified by a number of parameters, including LTPB, particle/pore size and distribution, surface area coverage, porosity, phase volume fraction, and tortuosity. Additional information relating to phase connectivity was calculated by generating a “skeleton” network for each phase, and was compared for each cell. The microstructure was then related to cell performance data acquired from the same samples. Furthermore, the detailed microstructure, compositional changes, and the formation of impurity phases due to the contaminants was characterized by transmission electron microscopy (TEM), utilizing both energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS).1. Gostovic, D., Vito, N. J., O’Hara, K. A., Jones, K. S., & Wachsman, E. D. J. Am. Cer. Soc., 94(2), 620–627, (2011).2. Smith, J. R., Chen, A., Gostovic, D., Hickey, D., Kundinger, D. P., Duncan, K. L., & Wachsman, E. D. (2009). Solid State Ionics, 180(1), 90–98. Figure 1: Serial 2D slices of YSZ electrolyte (left) and composite LSM-YSZ cathode (right) acquired in FIB/SEM (a) are reconstructed into a representative 3D volume surface model (b) for further quantification. Data is from unaged symmetric cathode button cell.

Impedance of SOFC electrodes: A review and a comprehensive case study on the impedance of LSM:YSZ cathodes

It was shown through a comprehensive impedance spectroscopy study that the impedance of the classic composite LSM:YSZ (lanthanum strontium manganite and yttria stabilized zirconia) solid oxide fuel cell (SOFC) cathode can be described well with porous electrode theory. Furthermore, it was illustrated through a literature review on SOFC electrodes that porous electrode theory not only describes the classic LSM:YSZ SOFC cathode well, but SOFC electrodes in general. The extensive impedance spectroscopy study of LSM:YSZ cathodes consisted of measurements on cathodes with three different sintering temperatures and hence different microstructures and varying degrees of LSM/YSZ solid state interactions. LSM based composite cathodes, where YSZ was replaced with CGO was also studied in order to acquire further knowledge on the chemical compatibility between LSM and YSZ. All impedance measurements were acquired in the very broad temperature range of 200–900°C for complete elucidation of the impedance. All impedance spectra were analyzed in terms of porous electrode theory. Physical materials parameters were extracted from the analysis, which were in excellent accordance with literature values. Valuable insight about the dissolution of Mn in the cathode composite material YSZ during preparation was furthermore provided along with valuable engineering characteristics such as the electrochemical utilization thickness. From the combined impedance study and literature review, it is clear, that porous electrode theory is the most suitable framework for any type of porous composite SOFC electrode evaluation.

Relationship Between Particle and Plasma Properties and Coating Characteristics of Samaria-Doped Ceria Prepared by Atmospheric Plasma Spraying for Use in Solid Oxide Fuel Cells

Samaria-doped ceria (SDC) has become a promising material for the fabrication of high-performance, intermediate-temperature solid oxide fuel cells (SOFCs). In this study, the in-flight characteristics, such as particle velocity and surface temperature, of spray-dried SDC agglomerates were measured and correlated to the resulting microstructures of SDC coatings fabricated using atmospheric plasma spraying, a manufacturing technique with the capability of producing full cells in minutes. Plasmas containing argon, nitrogen and hydrogen led to particle surface temperatures higher than those in plasmas containing only argon and nitrogen. A threshold temperature for the successful deposition of SDC on porous stainless steel substrates was calculated to be 2570 °C. Coating porosity was found to be linked to average particle temperature, suggesting that plasma conditions leading to lower particle temperatures may be most suitable for fabricating porous SOFC electrode layers.