bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi Length-Scale Quantification of Hierarchical Microstructure in Designed Microtubular SOFC Electrodes
Authors: Cooper Samuel J , Li Tao , Bradley Robert S , Li Kang , Brandon Nigel P. , Kilner John A.
Published: February 2020

Extrusion techniques can be used to produce microtubular SOFCs with a wide range of hierarchical microstructures. It is believed that by controlling the pore structure of the thicker supporting electrode, the transport properties can be optimised.

X-ray micro-tomography has been used to image the finger-like voids in a microtubular anode. These structures are on the order of 10 microns in diameter; however, smaller characteristic lengthscales can be imaged using FIB/SEM, which reveals the packing of the sintered particles, as well as the pore structure within the particles. Quantifying the tortuosity factor at each of these lengthscales offers insight into the effect of tailoring the pores.

The tortuosity factor of porous SOFC electrodes is used to account for the apparent reduction in species diffusion caused by the geometry of the network. It affects both the electrical transport in the solid phases as well as gas diffusion in the pores. The tortuosity factor is ideally calculated by solving the steady-state diffusion equation; however in systems spanning several orders of magnitude it is very computationally expensive. The figure shows the distribution of a scalar field in an interdigitated void structure, calculated using a finite element method.

This work combines the calculated tortuosity factors of each lengthscale to try and understand the significance of the hierarchical pore structure. The straight, radial pores of the microtubes imaged were found to augment the system significantly compared to an isotropic network of equivalent porosity. The importance of recasting the tortuosity factor as a function of distance from the electrolyte for hierarchical pore systems is also discussed.

Figure 1
Lanthanum Nickelate Infiltration into Porous Gd-Doped Ceria As Cathode for Solid Oxide Fuel Cells
Authors: Nicollet Clément , Flura Aurélien , Vibhu Vaibhav , Rougier Aline , Bassat Jean-Marc , Grenier Jean-Claude
Published: February 2020

To increase Solid Oxide Fuel Cells performances, the optimization of the oxygen electrode is mandatory as its activation overpotential is higher than the one of the hydrogen electrode. One way to increase the cathode electrocatalytic properties and to decrease its polarization resistance is to increase the Triple Phase Boundaries zone, where the oxygen is reduced. A promising solution to achieve this goal is to disperse an oxygen reduction catalyst on the surface of a porous ionic conductor. The lanthanum nickelate La2NiO4+δ (LNO) has remarkable surface exchange reaction rate and oxygen diffusion coefficient. However, when the electrode is shaped by classical screen printing technique, it displays higher polarization resistance (≈ 1 Ω.cm²) than the standard lanthanum cobaltite-based materials (≈ 0.1 Ω.cm² for La0.6Sr0.4Co0.2Fe0.8O3-δ).

In this study, cathodes were prepared by infiltration of lanthanum nickelate into a Gd-doped ceria (GDC) backbone sintered on 8YSZ electrolyte pellets. The influence of the preparation parameters of the composite electrodes on their electrochemical activity will be presented. The optimization of the preparation parameters led to a large decrease of the polarization resistance, down to 0.1 Ω·cm² at 600 °C. Using the Adler-Lane-Steele model, the impedance diagrams were fitted, allowing the determination of the surface exchange rate and ionic conductivity of the electrode: the obtained values well agree with those previously reported in the literature. Using this infiltration process, single cells were made starting from commercial half cells (Ni cermet/8YSZ), the cathode being the composite LNO/GDC. Voltammetry measurements showed power density higher than 1 W.cm-2 at 800°C.
Smart Fuel Cell Demonstration Project: A Challenge to Realize SOFC-Powered Campus
Authors: Sasaki Kazunari , Taniguchi Shunsuke , Shiratori Yusuke , Hayashi Akari , Oshima Toshihiro , Nishihara Masamichi , Tachikawa Yuya , Daio Takeshi , Kawabata Tsutomu , Fujita Miki , Zaitsu Ayumi
Published: February 2020

Based on various efforts, commercialization of fuel cell technologies has been just started in Japan. Energy Basic Plan in Japan has been revised where hydrogen is clearly stated as one of the important energy carriers. After this revision, roadmap for fuel cell and hydrogen technologies is established up to 2040. While more than 100,000 residential fuel cell units have been sold and the commercialization of fuel cell vehicles (FCV) has just been started, industrial fuel cells as distributed power systems will be commercialized probably from 2017. For such larger-scale stationary applications, SOFC is the promising technology.

 

In Kyushu University, “Smart Fuel Cell Demonstration Project” is going on where our University Campus will become a campus where fuel cell and hydrogen technologies are widely applied. While FCV, hydrogen fuelling station, and various residential fuel cells are in operation, installation of a few types of SOFC systems is going on, including a 250kW SOFC-MGT power generation system, 5kW SOFC cogeneration systems, and 1kW-class SOFC cogeneration units. These SOFC systems can cover ca. 5 % of the whole electricity demand of this university campus with a maximum electricity contract of 6000 kW. As shown in the Figure, the university campus will be a future energy society where SOFC is playing an important role as the efficient power generation system. The electric power generated from solar, wind, and SOFC will be displayed on the web site of our campus to improve the social acceptance of SOFC technologies. SOFC will act as a base power generator, while hydrogen refueling station will act as an energy storage station for the renewable electricity and FCV will be a zero-emission vehicle using CO2-free hydrogen. Concept for future energy society will be discussed where SOFC will take a major role, based on our demonstration experiences.

Figure 1
Thermodynamic Performance of a High Efficient Power Plant Based on Faecal Biomass Gasification, Solid Oxide Fuel Cells and Micro Steam Turbine
Authors: Recalde Mayra , Woudstra Theo , Liu Ming , Aravind P.V.
Published: February 2020

Affordable sanitation around the world with the recovery of valuable energy and clean water from faecal biomass in an off-grid system is a challenge. An energy neutral sanitation  system  concept  based on plasma gasification and solid oxide fuel cell (SOFC) was presented by TU Delft {Mings et al.}. The system is further optimised and  novel small scale CHP toilet system concept  has been developed which can generate power very efficiently and produce clean water. Pre-dryed faecal biomass, is fed to the integrated system. The system combines a dryer, a plasma gasifier, and a gas cleaning unit with an ambient pressure stack and a micro steam turbine (MST). The gasifier uses the heat supplied from the combustion of SOFC depleted fuel. With this concept syngas with high heating value can be produced, which can be efficiently converted in a SOFC system. This is reflected by the net electrical efficiency of the integrated system configuration  of the order of 50%. Additional power is produced combining it with an MST and an efficiency increase of the order of 5% is achieved. Variation of the different operating conditions in the system reveals an optimum for the temperature, steam to biomass ratio and oxidant to fuel ratio in the gasifier. Furthermore, the SOFC operating temperature influences the system performance and self-sustainability. As a result, from a thermodynamic point of view it is demonstrated that a high efficiency power generation through faecal biomass can be realized.

Ming Liu, T. Woudstra, E.J.O. Promes, S.Y.G. Restrepo, P.V. Aravind. System development and self-sustainability analysis for upgrading human waste to power.
Fundamental Impact of Humidity on SOFC Cathode Degradation
Authors: Huang Yi-Lin , Pellegrinelli Christopher , Wachsman Eric D.
Published: February 2020

Exposure of solid oxide fuel cell (SOFC) cathodes to atmospheric air contaminants, such as humidity, impacts the oxygen reduction reaction (ORR) mechanism and can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and the cathode is essential to develop improved performance cathodes with enhanced durability. To study the effects of humidity on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). In these experiments, heavy water, D2O (with a mass/charge ratio of m/z=20), was used to avoid the overlapping of the H2O and the 18O2 cracking fraction, which both have a peak at m/z=18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that H2O and O2 share the same surface exchange sites. Therefore, the presence of H2O competes with O2 for the available surface sites. Our findings show that H2O prefers to exchange with LSM and LSCF at a lower temperature range, around 300-450°C. For LSM, O2 is more favorable than H2O to be adsorbed on LSM surface and the presence of O2 limits H2O exchange with the LSM surface. For LSCF, H2O has two different exchange mechanisms, each dominating in a different temperature region. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials.
An Elementary Kinetic Model for the Electrochemical Reduction of Oxygen on LSM/YSZ Composite Cathodes
Authors: Banerjee Aayan , Deutschmann Olaf
Published: February 2020

A detailed computational model of the oxygen reduction reaction in composite strontium-doped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) SOFC cathodes is presented.  The coupled interactions of elementary heterogeneous chemistry, elementary electrochemistry (at two separate electrochemical double layers) and transport through the porous electrode are studied. Species diffusion and convection through the porous cathode are evaluated using the Dusty Gas Model while a distributed charge transfer model is used to study the ionic and electronic transport through the cathode. Charge-transfer reactions are modeled using a spillover mechanism with no a priori assumption of rate limiting steps unlike the frequently published Butler-Volmer approach. The model is validated against overall area-specific resistance (ASR) values previously reported by Barbucci et al. [Electrochemica Acta,
47 (2002)] over a temperature range of 950-1200 K. A sensitivity analysis reveals the impact of various kinetic parameters on the ASR and subsequently leads to the identification of rate-limiting steps. Preliminary results indicate that the ASR is sensitive to the rate of dissociative adsorption of O2 on LSM and oxygen ion transfer from the YSZ surface to the YSZ bulk.
Surface Oxygen Exchange Properties of Sr Doped La2NiO4+δ as SOFC Cathode: Thin-Film Electrical Conductivity Relaxation Investigation
Authors: Guan Bo , Li Wenyuan , Zhang Xinxin , Liu Xingbo
Published: February 2020

Roddlesden-Popper Phases, such as La2NiO4, have been proposed as promising intermediate temperature SOFC cathodes, due to their high surface oxygen exchange and bulk diffusivities. However, relatively low electrical conductivity of LNO phase is a concern for its application. It has been shown that Sr doping is an effective way to improve LNO’s electrical conductivity, but it is important to investigate the effect of such doping on other properties such as oxygen surface exchange coefficient. In this study, Sr doped lanthanum nickelates i.e., La2-xSrxNiO4+δ (0≤x≤0.4) thin and dense films on dense thick GDC (~1mm) substrate, were prepared by using spray-modified pressing method. The surface oxygen transport kinetics was investigated by electrical conductivity relaxation (ECR) technique. Since the thickness of thin films is ~15μm, much less than the characteristic length of LNO, the oxygen transport kinetics are controlled by the surface exchange steps and the oxygen ion concentration is uniform in the film at any time during the ECR process. Such situation enables a more accurate fitting results compared to the traditional fitting procedure based on thick pellets, because only k value is the unknown parameter. The fitted k for LNO is 4.02×10-5cm/s at 0.2atm 700oC, which decreases with lowered oxygen partial pressure. Sr doping harmed the surface exchange kinetics, k of Sr40 is about one order of magnitude smaller than undoped one. Interstitial oxygen and Ni oxidation state are suggested to be predominant roles in determining the surface kinetics. Both Sr doping and lowered oxygen partial pressure lead to the loss of interstitial oxygen and the oxidation of Ni2+, resulting the lowered surface exchange rate.
Electrochemical Analysis of Nano-Infiltrated Solid Oxide Fuel Cell Cathodes
Authors: Call Ann V , Barnett Scott A
Published: February 2020

Composite solid oxide fuel cell (SOFC) cathodes consisting of ionic conducting scaffolds infiltrated with mixed ionic and electronic conducting (MIEC) nano-particles have exhibited some of the lowest reported polarization resistances in the literature.  In this study, symmetric cell cathodes were prepared via wet infiltration of Sr0.5Sm0.5CoO3 (SSC) nano-particles via a nitrate process into porous Ce0.9Gd0.1O1.95 (GDC) scaffolds to be used as a model system to investigate performance under varying infiltration loadings.  Detailed analysis of the infiltrated cathodes was carried out using electrochemical impedance spectroscopy (EIS).  The results presented show that the infiltrated cathode microstructure has a direct impact on the initial performance of the cell.  Small initial particle sizes and high infiltration loadings (up to 30 vol% SSC) improved initial polarization resistance (RP).  A simple microstructure-based electrochemical model successfully explained these trends in RP. Further understanding of electrode performance was gleaned from fitting EIS data gathered under varying temperatures and oxygen partial pressures to equivalent circuit models.  Both RQ and Gerischer impedance elements provided good fits to the main response in the EIS data, which was associated with a combination of oxygen surface exchange and oxygen diffusion in the electrode. A gas diffusion response was also observed at relatively low pO2.
Solid Oxide Steam Electrolysis Anodes with Precisely Controlled Microstructure for Performance Analysis
Authors: Clancy Graeme C. , Peppley Brant A. , Pharoah Jon G. , Ryland Donald
Published: February 2020

Oxygen electrodes of Solid Oxide Electrolysis Cells (SOECs) have been manufactured using a spray deposition method with strictly controlled parameters to understand electrode transport, improve performance and extend electrolyser life time. The controlled parameters were the; particle size distribution, solid fraction of the ink, and spray deposition rate. The effect on physical properties; porosity, pore size, and the mass fraction of the individual ceramics in the solid phase as a function of distance from electrolyte, caused by controlling the above mentioned parameters was studied. The changes in performance of the cells with differing parameters were observed as; polarization responses, long term current-voltage behaviour, impedance spectroscopy and effective conductivities, measured with the van der Pauw method. The polarization responses gave valuable information to compare cells to those commercially available. The impedance spectroscopy of the cells allowed for the study of the changing physical properties effect on the electrochemical behaviour. Specifically the individual electrochemical processes; cell conductivity, oxygen gas diffusion, and oxygen evolution.
Stability of Bi2O3-Based Ionic Conductor and Its Application on Composite Cathode
Authors: Tsai Shu-Yi , Fung Kuan-Zong , Ni Chung-Ta , Chang Yu-Fan
Published: February 2020

In recent years, solid oxide fuel cells(SOFCs) has been considered as desired devices for generating electricity by electrochemical combination of a fuel with an oxidant. Oxygen ion conductors are usually used as the electolytes for SOFCs. Among several types of oxygen ion conductors such as CeO2, stabilized Bi2O3 and ZrO2, Bi2O3with stabilizers exhibit the highest oxygen ion conductivity. Although Bi2O3 tends to be reduced to Bi metal, Bi2O3 incorporated with proper dopants will show enhanced stability against hydrogen. The highest OCV for SOFC using Bi2O3-based electrolyte is around 0.5V which is still too low to be practical. On the other hand, Bi2O3-based system may make a good ionic component for a composite cathode.

In this study, we choose yttria-stabilized bismuth oxides (YSB) as ionic conductor and mixed with s
trontium-doped lanthanum manganite (LSM) for composite cathode. Size effect of LSM on the polarization resistance was investigated by using convertional solid-state reacted and nano-sized LSM.

The electrochemical performances of composite cathodes with different LSM:YSB ratio have been investigated at temperature ranging from 500℃to 650℃ using AC impedance spectroscopy. The polarization resistance measured from a symmetric cell consisting of nano-sized LSM-YSB electrodes on a YSB electrolyte is 50% lower than that of submicron-sized LSM-YSB cathodes  by SSR (0.21 ohmcm2) at 650℃.

    The polarization of cathode is reduced because of the mixing of high surface area electric conducting materials and ionic conducting materials. The nanostructure of composite cathode is able to extend triple phase boundary(electronic conductor, ionic conductor,and oxygen gas) and provide numerous reaction sites for electrochemical reaction to occur.
Self Healing of Precoated AISI 441 for Solid Oxide Fuel Cell Interconnects
Authors: Berger Robert , Lundberg Mats W , Westlinder Jörgen , Holmberg Håkan
Published: February 2020

Sandvik Materials Technology develops nanocoatings for SOFC stainless steel interconnects. The self healing properties of cerium cobalt coated AISI441 (EN 1.4509) have been studied. A selectively coated sample was oxidized in air at 800°C for a total of 504 hours. A FIB trench, milled on the uncoated surface, was used as a reference point for the coated border. A second FIB trench, made after oxidation, revealed that the oxide formed on the uncoated part adjacent to the coated area contained both cobalt and cerium. Cerium was detected with EDS in the surface oxide of the uncoated area as far as 10 mm away from the coated edge, cobalt had diffused even further. These findings are encouraging since they suggests that cracks introduce in the coating during stamping and/or forming, which are usually in the 1-3 mm range, are likely to self heal (i.e. become enclosed by an upper Co, Mn, Cr-spinel)  upon oxidation.

Figure 1
Effects of Nitride Formation on Anode Catalytic Activity in Ammonia-Fueled SOFCs
Authors: Yokochi Ryuji , Hashinokuchi Michihiro , Doi Takayuki , Inaba Minoru
Published: February 2020

We have so far developed Ni-Fe and Ni-Mo anodes with high catalytic activity for ammonia(NH3)-fueled SOFCs [1], and have proposed that the performance of NH3 -fueled SOFCs was determined by a balance between NH3 adsorption (Fe, Mo) and nitrogen(N2) desorption (Ni) capability [2]. Furthermore, we also found that both Ni-Fe and Ni-Mo anodes react with NH3 to form metal alloy nitrides (FeNx, γ-Ni2Fe2N and Ni0.2Mo0.8N) and metal Ni under the operation condition, while pure Ni anode does not form nitrides. These results suggested that the nitrides enhance NH3 adsorption at the anode and led to the high anode activity for NH3 oxidation.

In the present study, we prepared three kinds of anodes, Ni-W, Ni-Ta and Ni-Nb and evaluated their performance as anode in NH3-fueled SOFCs to investigate the correlation between the nitride formation and the anode activity. These anodes were synthesized by impregnating W, Ta and Nb precursor solutions into a pre-sintered Ni-SDC anode cermet. The addition of W and Ta enhanced the anode activity for NH3 oxidation, but Nb did not.  XRD measurements of metal oxides (WO3, Ta2O5 and Nb2O5) after annealing in NH3 indicated that W and Ta in the anode reacted with NH3 to form nitrides, but Nb did not under the operation condition (i.e. at 973-1173 K in NH3). These results suggest that the easiness of the nitride formation is closely related to the high activity anode for NH3 oxidation.

Acknowledge

This work was supported by the Kyoto Environmental Nanotechnology Cluster from MEXT, Japan.

Reference

[1] W. Akimoto et al., Solid State Ionics, 256, 1–4 (2014).

[2] W. Akimoto et al., ECS Trans., 57 (1), 1639-1645 (2013).
The Impact of Toluene on the Performance of Anode-Supported Ni-YSZ SOFC Operated on Hydrogen and Biosyngas
Authors: Madi Hossein , Diethelm Stefan , Ludwig Christian , Van herle Jan
Published: February 2020

This paper presents an experimental study of the impact of toluene on the performance of anode-supported (AS) Ni-YSZ SOFC operating at ~800 °C, 0.25 A/cm2 fed with H2 and reformed biosyngas. Toluene was added to the fuel stream and its concentration increased stepwise from tens to thousands of ppm. Each poisoning test was followed by a recovery step (no toluene in the fuel stream) for the duration of 25 hours. The main goal of this work is to define a concentration threshold of toluene as a model tar compound in the fuel stream and later to identify the degradation mechanism caused by this tar compound. Polarization behavior, current-voltage and electrochemical impedance spectroscopy (EIS) were analyzed to evaluate the cell performance. A linear degradation is observed at concentrations above 1000 ppmv for the cell which was fed with pure H2 (cf Figure) The degradation was due to local C(s) deposition. In contrast, toluene even up to 3500 ppm did not cause a severe degradation when the cell was fed with reformed biosyngas.

Figure 1
The Influence of Sulfur Poisoning and YSZ Substrate Orientation on Electrochemical Properties of Nickel Pattern Anodes
Authors: Doppler Michael Christoph , Fleig Jürgen , Opitz Alexander Karl
Published: February 2020

In state of the art solid oxide fuel cells (SOFCs) the anode is usually a ceramic-metal composite consisting of a network of Ni and yttria-stabilized zirconia (YSZ) grains as well as pores allowing for gas transport. This approach, while yielding excellent performance, is rather impractical for kinetic studies of the electrochemical triple phase boundary kinetics of Ni/YSZ since ionic and gaseous transport may also influence the electrode polarization resistance. These interferences make extraction of electrochemical elementary steps as well as mechanistic studies of degradation phenomena (e.g. sulfur poisoning) highly challenging. To circumvent these issues sputter deposited thin film electrodes with well-defined structures can be utilized.

In this study, pattern Ni anodes were investigated in H2/H2O atmosphere by means of electrochemical impedance spectroscopy (EIS). A sample pretreatment method was established to obtain stable values of the polarization resistance (RP) and interface capacitance (C) – i.e. avoiding degradation with time – to allow comparison of different microelectrodes. The effect of different YSZ substrate orientations on Rpand C is statistically evaluated and the correlation between the two properties is discussed.

Based on this knowledge the influence of H2S on Rp and C was investigated. The poisoning influence of sulfur on RP as well as the regeneration behavior was found to be qualitatively consistent with cermet electrodes described in the literature. Interestingly, also the electrode capacitance was affected by H2S: it decreased under sulfur poisoning conditions and regenerated partially after removal. Combined with dependencies of Rp and C on temperature, bias voltage and geometry these findings allow valuable insights into the electrochemical properties of Ni electrodes and their degradation.
Carbon Deposition and Deformation of Ni-YSZ Cermet in Hydrocarbon-Containing Gases
Authors: Ohmura Nobuaki , Nakamura Takashi , Matsuoka Koji , Kudo Takao , Amezawa Koji
Published: February 2020

For the full-scale commercialization of SOFC systems, the durability and reliability of the cells and stacks should be improved. Carbon deposition and following deformation of the anode and is one of issues to be solved. For SOFC systems, hydrogen-rich gas, which is obtained through the steam reforming of hydrocarbons, is supplied to the anode. In practical SOFC systems, the steam/carbon molar ratio (S/C) is kept high enough to prevent carbon deposition thermodynamically. However, at the start-up/shut-down processes or failures in the reforming system, etc.,hydrocarbon fuel with low S/C may be introduced into the anode. In Ni-YSZ cermet, which is the most common anode material in SOFCs, Ni is highly active for the decomposition of the hydrocarbon gases and carbon formation. If the low-S/C hydrocarbon fuel was supplied to Ni-YSZ cermet, carbon would deposit on Ni. Such carbon deposition causes not only deterioration of conductivity and electrochemical activity but also significant volume expansion of Ni-YSZ cermet.

From the backgrounds mentioned above, in this study, we quantitatively evaluated the carbon deposition and expansion of Ni-YSZ cermet during exposure to 10%CH4-N2, 1%CH4-N2, 1%C2H6-N2 or 1%C3H8-N2with S/C = 0.1 or 1.0 in the temperature range between 973 and 1173 K. The dilatometry and thermogravimetry measurements as well as posterior analyses by SEM/EDX and XRD were performed for blocks of Ni-YSZ cermet.

When S/C was 0.1, significant carbon deposition and expansion were observed during the exposure to hydrocarbon gases. The expansion reached 1% within 5 hours for 1%CH4-N2 and 1 hour for the others. According to SEM/EDX analyses, the amount of deposited carbon was found inside of the specimen after the exposure, although its distribution differed among hydrocarbon gas species. Carbon segregated in the near surface of the specimens for 1%C2H6-N2 and 1%C3H8-N2, while distributed in the whole parts of the specimens for 1% and 10%CH4-N2. The temperature dependency of the expansion was also quite deferent among hydrocarbon gas species. The expansion ratio was smallest at 973 K for 10%CH4-N2 and 1%CH4-N2, while smallest at 1173 K for 1%CH2H6-N2 and 1%C3H8-N2. In the presentation, the expansion mechanism will be discussed based on the obtained results.
Impregnation of Nanoparticle Scaffolds for Syngas-Fed Solid Oxide Fuel Cell Anodes
Authors: Boldrin Paul , Ruiz-Trejo Enrique , Tighe Christopher , Chang Kee-Chul , Darr Jawwad , Brandon Nigel P.
Published: February 2020

We will present a strategy for fabrication of solid oxide fuel cell anodes with improved porosity and lower sintering temperatures by impregnation of nanoparticle-containing porous scaffolds of ceria-gadolinia (CGO). The CGO scaffolds are fabricated using a screen-printed ink containing nanoparticles and commercial particles of CGO and polymeric pore formers. The porosity and structure of these scaffolds is characterised by scanning electron microscopy and in situ ultra-small angle X-ray scattering, showing that incorporation of nanoparticles can increase the porosity by allowing a reduction in sintering temperature from 1300 to 1000 °C. The scaffolds are then impregnated with nickel nitrates to produce symmetrical cell and button cells. The electrochemical characterisation of the symmetrical cells shows that the cells sintered at 1000 °C possess similar electrode polarisation compared to those sintered at 1300 °C of around 10 Ωcm2 at 500 °C under 10% hydrogen despite being sintered at 300 °C lower temperature. The low frequency response in the impedance spectra usually assigned to diffusion is smaller in the low temperature calcined cells while the high frequency response is larger. Button cell testing showed that reducing the sintering temperature produced cells which perform better at 700 °C (and below) in hydrogen, and performed better at all temperatures using syngas, with power densities of up to 0.15 W cm-2 at 800 °C, while a further improvement in performance could be obtained by impregnating with a mixture of nickel and copper nitrate. This approach has the potential to allow the use of a wider range of nanomaterials, giving a finer control over microstructure.

Figure 1
Electrochemical Performance and Stability of Manganese Doped La0.1Ca0.9TiO3 Anode of SOFC
Authors: Chung Yong Sik , Yoon Heechul , Chung Jong Shik , Sammes Nigel Mark
Published: February 2020

Manganese doped Lanthanum calcium titanate (La0.1Ca0.9Ti1-xMnxO3, LCTM, x= 0 – 0.8) perovskites were synthesized using a standard solid state reaction method. The crystallinity and microstructures were analyzed using X-ray diffraction and SEM. Furthermore, the electrical conductivity was measured under different PO2 atmosphere via a 4-probe method. LCTM was then screen printed onto a YSZ support, as the anode, to analyze the catalytic activity for both hydrogen oxidation and methane reforming reaction. On increasing manganese doping, the lattice parameter of the perovskite phase decreased due to smaller ionic radii of manganese compared to titanium. In addition, both electrical conductivity and power density were improved with increasing manganese doping up to a certain composition. Multi-oxidation states (likely to be oxidized) of manganese increased the concentration of oxygen vacancy and relative Ti4+ concentration compared to Ti3+, thus the electrochemical performance was improved. However, the electrical conductivity decreased with further increasing manganese doping due to decrease in the absolute concentration of titanium. In this work we study the effect of manganese doping on LCT, and analyzed the electrochemical and physical behavior of the material under anodic atmosphere.
Modeling of Solid Oxide Fuel Cell with Anisotropic Conductivity
Authors: Xu Min , Andersson Martin , Li Tingshuai , Sundén Bengt , Chen Fanglin (Frank)
Published: February 2020

Solid oxide fuel cells (SOFCs) are promising as an energy producing device, which at this stage of its development will require numerical modeling at different time- and length scales. A 3D model is developed based on the FEM, using COMSOL, of a single SOFC operating at an intermediate temperature range. Ion, electron, heat, gas-phase species and momentum, transport equations are implemented and coupled to the kinetics of the electrochemical reactions. The effects from direction dependent material properties (e.g., ion, electron and pore tortuosities as well as permeability are investigated. It is concluded that the consideration of direction dependent material properties is an important step for an improved (realistic) description of the nano/microscale porous structure.

Keywords: SOFC, Modeling, FEM, Electrochemical reactions, Porous media, material properties
Atmospheric Plasma-Sprayed Metal-Supported Solid Oxide Fuel Cells with Varying Cathode Microstructures
Authors: Harris Jeffrey , Kuhn Joel , Kesler Olivera
Published: February 2020

Metal-supported solid oxide fuel cells (SOFCs) use less expensive materials than traditional anode-supported or electrolyte-supported SOFCs.  The active layers of a metal-supported SOFC can be manufactured by plasma spraying without the need for high-temperature sintering.  Plasma spraying is a rapid-fabrication process that is scalable for large cell areas and for mass production.

Porous ferritic stainless steel (Sanergy HT, Sandvik, Sweden) supports were manufactured from powders by pressing and sintering pellets. The active cell layers consisted of a composite nickel - yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, and a La0.6­Sr0.4Co0.2Fe0.8O3-δ (LSCF) – Ce0.8Sm0.2O1.9 (SDC) cathode.

Plasma spraying produces unique microstructures, as the cell layers are built up by the rapid solidification of molten (or semi-molten) splats. These microstructures were tailored by adjusting the plasma spray process parameters and the ingredients in the feedstock.  Dense electrolytes were made using a high-power plasma, a very high torch pass speed (to minimize the plasma heat impulse to the substrate), and a feedstock consisting of YSZ suspended in a mixture of water, ethanol, and ethylene glycol. Cathodes with four different microstructures were produced by varying the plasma spray parameters and mixing carbon or starch-based pore forming agents into the ceramic feedstock powders.  Resultant microstructures are shown in Figure 1.

Electrochemical performances of  metal-supported button cells with varying cathode microstructures were measured.  At 750°C, the cells had open circuit potentials of up to 1.057 V and peak power densities as high as 562 mW/cm².  Electrochemical impedance spectra were measured at an operating point of 0.7 V, varying the cathode gas with different air-nitrogen mixtures.  With 100% air as the cathode gas, the lowest cell polarization resistance was 0.29 Ωcm² at 750°C. Equivalent circuit modeling was performed to analyse how the cathode microstructure affected the cell performance.

Figure 1
The Effects of Cell Parameters on SOFC Cell Unit Performance
Authors: Moon Gi Jong , Ahn Myung Mo , Choi Young Jae , Kang In Seok
Published: February 2020

The effects of cell parameters on SOFC cell unit performance have been investigated experimentally and numerically. The experiment was conducted using the anode-supported SOFC cell unit (100cm2) and the CFD simulation model was also performed adapting same geometry, material properties, and cell parameters of SOFC cell unit. 3D SOFC cell unit simulation model including electrochemistry, heat and mass transfer, and fluid dynamics are solved by COMSOL Multiphysics. In the experiment, the voltage was measured under fixed current density condition, so current fixed method was used in numerical simulation. Because the simulation results show good agreement with experimental data, the effects of cell parameters on SOFC cell unit performance could be investigated by changing porosity, thickness of electrode and electrolytes using our simulation model. The cell performance was also analyzed by calculating each overpotentials (activation loss, concentration loss, and ohmic loss). With this model, we could find the optimal cell design parameters and we expect that the current findings can provide a guideline for the design of SOFC cell unit.
Reactive Force-Field Development for Metal/Ceramic SOFC Anode Modeling
Authors: Saha Leton Chandra , Iskandarov Albert , Nakao Kazuhide , Ishimoto Takayoshi , Umeno Yoshitaka , Koyama Michihisa
Published: February 2020

A solid oxide fuel cell (SOFC) system anode material is usually composed of a mixture of Ni and other oxide ceramics, such as doped-CeO2, and Sc stabilized ZrO2, due to their high activity and structural stability. Among them the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet is widely used today as an anode material of SOFC due to its good electrocatalytic property against low price.

For SOFC performance, it is essential to improve the stability of the Ni/YSZ cermet anode. Combination of Ni and YSZ has been studied experimentally, but the interaction is not well understood in the boundary between Ni and YSZ. However, there is no evidence about the atomic-scale Ni/YSZ interface dynamics and chemical changes in the structure during the operation of SOFC. Unfortunately, most classical force fields are incapable of describing chemical reactions. To overcome this difficulty, reactive force field (ReaxFF) potential was developed by van Duin and co-workers. Recently, Ni/YSZ/H2 interactions are reported by ReaxFF. But in this interaction Ni/YSZ interface stability is not taken into account. As a first step we developed our own ReaxFF potential to investigate Ni/ZrO2 interface and for determining stable Ni/YSZ interface for modeling of the triple-phase-boundary in SOFCs. Big difference is found between our developed and reported ReaxFF and that is work of separation. Using our ReaxFF potential, work of separation is in excellent agreement with literature. It is found that O around Zr4+cation is reduced in case of bulk and interface regions. Although interface region exhibited less reduction compared to bulk region. More details and Ni/YSZ interface stability through O vacancy may present at conference.
Thermal Gradients in Solid Oxide Fuel Cell Anodes: X-Ray Diffraction, Thermal Imaging and Model Prediction
Authors: Robinson James B. , Engebretsen Erik , Brown Leon D , Clague Ralph , Eastwood David , Reinhard Christina , Lee Peter , Brett Dan J. , Shearing Paul R.
Published: February 2020

Solid Oxide fuel cells (SOFC) are a widely discussed future power option due to their high energy efficiency and fuel flexibility in addition to low maintenance requirements.  However, the long term mechanical stability of the stack can be compromised due to simple operation as a result thermally mismatched materials (1).  The commonly used Ni / YSZ anode – electrolyte combination provides a high degree of electrochemical performance; however, the presence of the Ni catalyst which aids the performance of the cell results in a significant mismatch in the coefficient of thermal expansion (CTE) between the anode and electrolyte of cells (2).  Issues arising from mismatched CTE can be further exacerbated by the presence of thermal gradients within SOFCs.  Thermal gradients which can arise from simple operation, internal reforming and inhomogeneous current distribution amongst other factors (3-5) have been noted to significantly increase the prospect of cell failure (6). 

In this study the effect of thermal gradients on SOFC anodes is investigated using both experimental and computational methods.  Experiments conducted at Diamond Light Source using combined synchrotron X-ray diffraction and infrared thermal imaging (7)  are detailed highlighting the presence of non-uniform thermally derived stresses at operationally relevant temperatures within the Ni phase of the anode of cells.  Computationally derived thermal gradients have been generated using finite element method modelling in order to study the effect of cell operation utilising a three dimensional electrochemical model.  The results obtained from the model have been coupled with the experimental results in order to investigate the effect of various parameters upon the cell.  Operational conditions including cell polarization and fuel flow conditions and configurations are examined to highlight the importance of cell optimisation in minimising stresses within the cell anode to prolong cell lifetime whilst maximising performance.      

References

1.         S. Majumdar, T. Claar and B. Flandermeyer, J Am Ceram Soc, 69, 628 (1986).

2.         W. Z. Zhu and S. C. Deevi, Materials Science and Engineering: A, 362, 228 (2003).

3.         H. Apfel, M. Rzepka, H. Tu and U. Stimming, Journal of Power Sources, 154, 370 (2006).

4.         H. Severson and M. Assadi, Journal of Fuel Cell Science and Technology, 10, 061001 (2013).

5.         R. Clague, A. J. Marquis and N. P. Brandon, Journal of Power Sources, 210, 224 (2012).

6.         A. Nakajo, Z. Wuillemin, J. Van herle and D. Favrat, Journal of Power Sources, 193, 203 (2009).

7.         J. B. Robinson, L. D. Brown, R. Jervis, O. O. Taiwo, J. Millichamp, T. J. Mason, T. P. Neville, D. S. Eastwood, C. Reinhard, P. D. Lee, D. J. L. Brett and P. R. Shearing, Journal of Synchrotron Radiation, 21 (2014).
Kinetics of Internal Methane Reforming on the Anodes of Low Temperature Ceres Power Steel Cell SOFCs
Authors: Leah Robert Timothy , Lankin Mike , Bone Adam , Mukerjee Subhasish , Selby Mark
Published: February 2020

Ceres Power has developed a unique low-temperature metal supported SOFC design (the ‘Steel Cell’) based predominantly around the use of ceria. This unique design architecture offers greatly enhanced robustness to real-world operating conditions at a lower cost than conventional SOFC designs, whilst retaining the advantages of fuel flexibility, high efficiency and low degradation.

A widely quoted benefit of SOFCs is the ability to carry out a significant percentage of the reforming of hydrocarbon fuels to syngas within the anode of the cells, simplifying the design of the fuel processing system by comparison with other fuel cell types. In addition performing the endothermic reforming reaction within the stack significantly reduces the requirement for air cooling, which improves the efficiency of a typical SOFC system by reducing the parasitic power demand from the cathode air blower.

It has historically been viewed that a disadvantage of low temperature SOFC operation is that significant levels of internal methane reforming are no longer possible due to poor kinetics and thermodynamically limited methane conversion. The Ceres steel cell technology has been developed to perform relatively high levels (40-60%) of internal methane reforming. In this paper a method for approximately estimating the kinetics of methane reforming on Steel Cell anodes will be described, coupling experimental measurements to a mathematical model.
The Effect of Electrode Morphology on Solid Oxide Fuel Cell Performance
Authors: Pharoah Jon G. , Handl Lisa , Schmidt Volker
Published: February 2020

The morphology of the active electrode in a solid oxide fuel cell has a strong impact on both the transport properties and the active triple phase boundary length and distribution. The performance of the electrode emerges from the interaction of these two parameters and is impacted strongly by the size and shape of the starting powders used to make the ionically and electronically conducting phases of the electrode as well as by the manufacturing method used and the sintering process post manufacture.  This paper presents a numerical study over a range of electrode morphologies chosen to represent different manufacturing processes and presents comparisons of geometric characteristics, transport properties and electrode performance.  

 

Three different morphologies are generated from three different starting powders, spheres representing non aggregated powders, agglomerates of spheres and high aspect ratio cylinders modelling splats formed in plasma sprays.  Of particular interest in this study is the effect of anisotropy of the base particles on the resulting electrode.

 

For each base particle shape multiple stochastic realizations of densely packed particle systems are generated using a collective rearrangement algorithm.  The structures are then analysed geometrically with methods from spatial statistics using our own Java-based software, and meshed, solved and analysed with respect to transport and performance using a custom modified version of the open source computational fluid dynamics package, openFOAM.  Detailed comparisons are presented in terms of tortuosity of paths through each phase of the material, constrictivity and chord lengths in different directions, directional transport properties in each of the three phases, distribution of the triple phase boundary lines and electrode performance
Effective Conductivity of Solid Oxide Fuel Cell Electrodes
Authors: Zheng Keqing , Ni Meng
Published: February 2020

Effective conductivity of SOFC electrodes is an important parameter for predicting the ohmic loss in SOFC.  This work investigates the effective conductivity of SOFC electrodes via numerical simulation.  The microstructures of porous electrodes are reconstructed numerically by packing spherical particles in a computational domain, followed by dilation process to simulate the sintering procedure.  The important tortuosity is computed and compared with literature data for model validation.  The effects of various parameters on the effective conductivity of the electrodes are investigated, including the particle size, material composition, and porosity.  The results are useful for understanding the microstructure properties of SOFC composite electrodes and for subsequent design optimization of SOFC electrodes.
Chromium-Related Degradation of Thin-Film Electrolyte Solid Oxide Fuel Cell Stacks
Authors: Menzler Norbert H. , Sebold Doris , Fang Qingping
Published: February 2020

Solid oxide fuel cell short stacks for dezentralized stationary power delivery have been operated with special thin-film electrolyte anode-supported cells. After operation times of up to 3,000h the cells were post-test characterized. The cells have besides very thin electrolyte films (≤ 3µm) either LSCF or LSC cathodes (LSCF: La-Sr-Co-Fe oxide, LSC: La-Sr-Co oxide). Typical operation parameters were temperatures between 600 and 750°C, a current density of 500mA/cm², hydrogen with 20% water vapor and a fuel utilization of approx. 40%. Typical chromium evaporation protection layers like wet powder sprayed manganese oxide and cathode contact layers (a perovskite based on La-Mn-Co-Cu oxide) were used.

Microstructural post-test characterization reveals in the case of using a 1µm 8YSZ electrolyte prepared via sol-gel route and on top a sputtered diffusion barrier layer composed of GDC, the existence of SrCrO4 either on top of the LSCF cathode as expected or, unexpectedly the formation of the spinel phase at the borderline cathode / barrier (electrolyte) layer. In case of using a 3µm thin screen printed 8YSZ electrolyte and an LSC cathode, the chromite phase has not been detected at the cathode / barrier border.

SEM and TEM post-test analysis of the cells show in case of the sol-gel electrolyte and sputtered GDC also the formation of micro-pores within the GDC layer. Theoretical calculations reveal that the chosen current density may lead to either preferred oxygen reduction or preferred chromium reduction depending on the real local current density within the cathode / barrier (electrolyte) interface.