bluuebunny/crossref_metadata_2025 huggingface dataset:
Stationary 2D FEM Model Framework for SOFC Stack Performance Prediction
Authors: Geisler Helge , Kromp Alexander , Joos Jochen , Weber André , Ivers-Tiffée Ellen
Published: February 2020

High performing anode-supported cells (ASCs) undergo a significant power reduction when contacted by a metallic interconnector (MIC) and operated in a stack layer [1,2]. The performance limiting factors can be identified by a stationary 2D FEM model, which calculates current/voltage (C/V) characteristics for specific SOFC stack layer setups. The required parameter set which describes (i) electrical/ionic conduction, (ii) electrochemical charge transfer and (iii) porous media gas transport is obtained via electrochemical impedance spectroscopy (EIS), 4-point DC conductivity measurements and 3D microstructures extracted from FIB-tomography [3]. The applicability of our stationary 2D FEM model was verified by comparing simulated and measured C/V characteristics over a broad range of operation conditions, and published in [3,4].

For state-of-the-art LSCF cathodes as thin as 45 µm, the simulation results demonstrate significant performance loss due to O2starvation underneath the MIC contact ribs and/or electrical cross conduction limitations in the cathode material underneath the gas channel area in each individual stack layer [3,4]. Following up these results, a systematic variation of the cathode parameters thickness, porosity, tortuosity, mean pore diameter, and electrical conductivity as well as of the MIC flowfield design has been carried out numerically. In this way, the sensitivity of each parameter on stack layer performance became transparent.

It will be shown in this work, that (i) choosing an optimized cathode/MIC design combination can increase current density at 0.7V by more than 25% in an SOFC stack assembly (see figure 1), that (ii) the magnitude of possible loss minimization requires the ability to precisely control the system parameters, and that (iii) an optimized MIC design avoids microstructural changes occurring in the cathode/CCL material because of its inherent chemical stability limitations.

[1] L. Blum, W. A. Meulenberg, H. Nabielek, R. Steinberger-Wilckens, Worldwide SOFC Technology Overview and Benchmark, International Journal of Applied Ceramic Technology
2, pp. 482-492 (2005).

[2] M. Kornely, A. Leonide, A. Weber, E. Ivers-Tiffée, Journal of Power Sources, Volume 196, Issue (17), pp. 7209-7216 (2011).

[3] H. Geisler, A. Kromp, A. Weber, E. Ivers-Tiffée, Journal of the Electrochemical Society, 161 (6) F778-F788 (2014).

[4] H. Geisler, A. Kromp, A. Weber, E. Ivers-Tiffée, Proceedings of the 11th European Solid Oxide Fuel Cell Forum, A0904 (2014).

Figure 1
Current Status of Rapid Evaluation of Durability of Six SOFC Stacks within Nedo Project
Authors: Yokokawa Harumi
Published: February 2020

Six solid oxide fuel cell stacks manufactured by Kyocera, MHPS, TOTO, NGK spark plug, NGK Insulator, Murata Manufacturing have been investigated to identify their performance degradation behavior, extract the major sources of degradation factors, clarify the degradation mechanism, and predict the time dependent contribution of such degradation to life time.  Stack performance was analyzed into several contributions, and an interesting correlation of overpotential values and ohmic loss has been obtained in addition to identification of respective degradation rates.  Respective degradation issues have been examined experimentally with SIMS, FIB-SEM, STEM and described as key simulation techniques to predict the long life up to 90,000 h.  Emphases have been placed on degradation of cathodes with Cr or S; to ensure the appropriateness for overcoming such poisoning effects, care is made to measure the impurity level after long term operation in various environments.  For simulation, focus was made to overcome some gaps between knowledge on stacks and on single (or buttons) cells appearing in several aspects.
CFY-Stack Technology: The Next Design
Authors: Bienert Christian , Brandner Marco , Skrabs Stefan , Venskutonis Andreas , Sigl Lorenz S. , Megel Stefan , Becker Wieland , Trofimenko Nikolai , Kusnezoff Mihails , Michaelis Alexander
Published: February 2020

The stack concept of electrolyte supported cells (ESC) is applicable for both, solid oxide fuel cells as well as solid oxide electrolysis cells. High power densities and efficiencies comparable to anode supported cells (ASC) can be achieved, however, for the stack design presented in this paper, in unison with high robustness. Incorporating an electrolyte made of fully scandia stabilized zirconia paired with chromium-based CFY (Cr5FeY) interconnects of matched CTE (coefficient of thermal expansion), a feasible stack concept was created with the focus on long-term stability in regard to high temperature corrosion and thermal cycling. Stack data of more than 20.000 hours of a single stack is presented in addition to industrial onward development of the stack design. Using the same material combinations and the same footprint as for the MK351-design, the newly developed MK352-design was simulated, drafted and produced and allows for easier stack setup and system integration. Minor changes in interconnect (IC) design enable lower pressure drops over the stack, which, paired with enhanced tolerance chains lead to enhanced stack performance, especially in regard to thermal cycling. Cross-checked, test results for the MK352-stack are in good agreement with the simulation.
Development of Intermediate Temperature (550 - 650oC) Metal Supported Solid Oxide Fuel Cells (SOFCs) Using Plasma Processes
Authors: Rupa Pantula K. P. , Goli V. R. , Balasubramanian Krishnamurty , Tomov R I , Kumar R Vasant , Glowacki B A , Krishnan Venkatesan Venkata
Published: February 2020

The primary problem in preparation of metal supported SOFCs, in almost all cases is reconciling the sintering characteristics of the metal support vis-a-vis the ceramic electrolyte/ anode functional layer, with which it is in contact. The sintering temperatures of dense ceramics are much higher than those of metal supports (Ferritic steels in this case), as also are differences in sintering atmosphere. 

As an alternative to using aids to lower the sintering temperature of ceramics (GDC and YSZ), thereby matching that of the substrate (which can lower electrical conductivity of the electrolyte), plasma spray deposition and plasma assisted co-sintering methods can be used effectively for multi-layered deposition, bonding and sintering. 

In this work, an entire cycle for fabricating a metal supported cell is discussed, using porous SS430L as anode substrate, NiO-GDC as anode functional layer, GDC as electrolyte and LSCF-GDC as cathodic layer.  Other techniques such as tape casting for metallic substrates and Inkjet printing have also been incorporated into the manufacturing operations to engineer a metal supported SOFC cell.  Key steps involve -

Tape Casting of porous SS430L substrate layers, followed by vacuum sintering (1100 C)

Inkjet printing/ Screen printing of anode (NiO + GDC) functional layers, followed by hot plasma surface treatment to induce sintering and bonding (‘Plasma Glazing’)

Plasma Spray (powder) deposition of GDC electrolyte layer leading to bonding and sintering

Ink-Jet printing of GDC colloidal suspension to give better finish to the ‘rough’ electrolyte surface, followed by ‘Plasma Glaze’

Brush printing/ Screen printing of cathode (LSCF + GDC) layers on the electrolyte surface followed by ‘Plasma Glazing’

The above mentioned process is part of an ongoing Indo-UK program wherein NFTDC, with its partner, University Of Cambridge, UK, has taken a multi-disciplinary approach to developing a metal supported SOFC stack.  This program includes - Materials synthesis and processing (GDC, LSCF), Stack design and Interconnect fabrication, Cell-Stack assembly, along with Electrochemical and microstructural characterization, and finally, process design of the overall system that can operate on commercial Natural Gas or Higher hydrocarbon fuels.
Thermal Integration of SOFC and Plate Heat Exchanger Desorber
Authors: Venkataraman Vikrant , Pacek Andrzej W , Steinberger-Wilckens Robert
Published: February 2020

SOFCs are a potential candidate for use as Auxiliary Power Units (APU’s) on board heavy duty trucks. Heat from the SOFC exhaust is generally used either for cabin heating or for recuperative heat exchange and even after that there is still a considerable amount of high quality heat available. The unique selling point of an SOFC is the availability of both electricity and high quality heat. Not using the heat from an SOFC stack is tantamount to using only half the available useful energy from the fuel.

The ongoing research work reported here focuses on the design & development of a compact Solid Oxide Fuel Cell –Vapour Absorption Refrigeration System (SOFC-VARS) unit for refrigerated truck applications. This report presents a numerical model and simulation for thermal integration of an SOFC stack with a plate heat exchanger desorber via a specially designed tube in tube heat exchanger with internal fins where thermal oil is heated to the required desorber temperature and then serves as the coupling fluid in the plate heat exchanger desorber.

The sizing of the heat exchanger and desorber has been carried out to fit a small refrigerated van, to cater to a 1 kW cooling load. The modelling focuses on the heat transfer aspects at the SOFC and heat exchanger end and on both heat & mass transfer aspects at the desorber end.

The results show that a plate heat exchanger desorber is able to produce the required quantity of refrigerant needed for a 1 kW cooling load. A thorough sensitivity analysis on the plate heat exchanger desorber has also been carried out to identify the parameters that affect desorption performance the most and the parameters that have least effect on the desorption performance.

The use of plate heat exchangers as desorbers not only gives a high heat transfer surface needed for desorption but also leads to considerable reduction in desorber volume when compared to conventional falling film desorbers. Based on the results obtained from modelling, appropriate design maps have been drawn which showcase the sizing of the plate heat exchanger desorber and the SOFC stack and also the mass flow rate of thermal oil needed.
A Lumped Dynamic Modelling Approach for Model-Based Control and Diagnosis of Solid Oxide Fuel Cell System with Anode Off-Gas Recycling
Authors: Marra Dario , Sorrentino Marco , Pohjoranta Antti , Pianese Cesare , Kiviaho Jari
Published: February 2020

The interest towards anode off-gas recycling (AOGR) in Solid Oxide Fuel Cell (SOFC) systems has recently increased, mainly due to the potential electrical efficiency improvement, as well as to the possibility of avoiding feeding external water for fuel reforming, when adopting such a configuration. However, advanced system control is necessary to ensure such relevant benefits are achieved. Proper control actions and system performance optimization, when AOGR is introduced, entail developing detailed thermodynamic system analysis, in such a way as to remove or at least limit any hazard associated to solid carbon formation, while suitably meeting thermal and electric requirements. To this end, an adequate SOFC stack model is necessary. This work focuses on the identification and validation of an SOFC stack model that accounts for AOGR inclusion. A lumped dynamic modelling approach is adopted to simulate temperature and gas composition at the stack anode outlet. The model is able to simulate the dynamic response of the stack during transients, as shown in Figure 1. Experimental data from a real SOFC system is used in model validation and the modeling approach adopted here ensures achieving a satisfactory compromise between estimation accuracy and computational burden. These factors support using the model as state estimator in model-based control and diagnosis algorithms.

Figure 1 – Comparison between measured and simulated fuel gas temperature at anode outlet

Figure 1
Development of High Performance Anodes for Metal-Supported Fuel Cells
Authors: Rojek Veronika , Roehrens Daniel , Brandner Marco , Menzler Norbert H. , Guillon Olivier , Opitz Alexander Karl , Bram Martin
Published: February 2020

Metal-supported solid oxide fuel cells (MSCs) offer various potentials like cost reduction, improved mechanical robustness and abuse tolerance, which make them particularly attractive for use in auxiliary power units (APUs). The implementation of the metallic substrate requires specific adaption of SOFC processing technologies since sintering of functional layers at high temperatures under oxidizing atmosphere – as typically for anode-supported fuel cells – is restricted by the metallic substrate. Therefore, one of the current challenges of MSC technology is the development of an anode functional layer, which can be sintered under reducing atmospheres and improves the ability of the anode to withstand sulfur contaminations of the fuel gas. In the present work, a new anode concept is presented, which is based on a Ni/CGO cermet replacing the established Ni/YSZ cermet. In literature, the application of Gd-doped ceria (CGO) instead of YSZ is discussed to be advantageous regarding the sulfur tolerance of the anode. Due to its mixed ionic-electronic conductivity under reducing atmospheres the surface of CGO becomes electrochemically active. Thus, CGO still enables the electrochemical charge transfer required for the hydrogen oxidation if triple phase boundaries are blocked by sulfur compounds. The present study sheds light on the sintering behavior of CGO under reducing atmosphere with and without addition of Ni particles. The sintering study is supported by some preliminary results of characterizing the electrochemical performance of the Ni/CGO anodes by impedance spectroscopy. In addition, thin CGO layer is currently used as diffusion barrier layer between the anode and the metallic substrate. A systematic study of the stability of this barrier layer under anode sintering conditions is given.
SOFC Anode Based on YSZ Pillars
Authors: Nagato Keisuke , Shinagawa Shunta , Shikazono Naoki , Iwasaki Shiko , Nakao Masayuki
Published: February 2020

In anode of solid oxide fuel cells (SOFCs), yttria-stabilized zirconia (YSZ), Ni, pore are distributed, and oxygen ions, electrons, H2 and H2O gases are transported in each path. And the species chemically reacted each other at the boundaries (triple phase boundaries, TPBs). In the view point of tortuosity, aligned YSZ and Ni particles and TPB are promising structures for a low resistances of matter transporting and their chemical reaction. In our previous work [1], it was proposed and developed that Ni particles were aligned by magnetic field during the drying process after screen-printing Ni/YSZ paste. A second step of low-tortuosity path configuration is completely straight along the specie transporting direction. Because the nominal conductivity of oxygen ion in YSZ is the lowest among the specie (ion, electrons, gas) transportation conductivities, YSZ should be straight preferentially. In this study, we prototyped pillar pattern of YSZ, infiltrated Ni, and investigated the effect of pillar width, height, and surface area of pillar corresponding to TPB length on the generation performance. The YSZ pillars were fabricated by an excimer laser. The figure shows that the width were varied 5, 10, 20, 40 μm, and their trenches were fabricated deeper in wider pillar. The trenches of YSZ pattern were infiltrated by Ni particles. The finer pillar resulted in higher generation performance as also shown in the figure. Furthermore, the electrochemical phenomena was discussed by Lattice-Boltzmann Method simulation.

[1] K. Nagato, N. Shikazono, A. Weber, D. Klotz, M. Nakao, E. Ivers-Tiffée, ECS Trans. 57, 1307-1311 (2013).

Figure 1
Lifetime and Performance Prediction of SOFC Anodes Operated with Trace Amounts of Hydrogen Sulfide
Authors: Riegraf Matthias , Schiller Guenter , Costa Rémi , Friedrich K. Andreas , Latz Arnulf , Yurkiv Vitaliy
Published: February 2020

Chemical degradation of Solid Oxide Fuel Cell (SOFC) anodes can be caused by various types of impurities present in practical fuels, as e.g. sulfur, chlorine, phosphorus and siloxane. To allow for a deeper understanding of the processes leading to sulfur poisoning, this study presents a modeling work of SOFC operating on H2/H2O and CH4/H2/H2O gas mixtures with different hydrogen sulfide (H2S) concentrations. In order to interpret experimental measurements, an elementary kinetic model is developed comprising a detailed multi‐step reaction mechanism of sulfur formation and oxidation at Ni/YSZ anodes coupled with channel gas-flow, porous-media transport and elementary charge-transfer chemistry.

A thermodynamic and kinetic data set of sulfur formation and oxidation is derived based upon various literature sources including a coverage-dependent description of the enthalpy of surface-adsorbed sulfur. Firstly, the developed model is validated against literature-based sulfur chemisorption isobars, and subsequently against electrochemical button-cell experiments displaying a significant influence of operation temperature and applied potential on cell performance and degradation.

It is shown that sulfur surface coverage increases with increasing current density indicating a low sulfur oxidation rate. In order to gain for an advanced fundamental understanding of sulfur poisoning, sensitivity analyses towards total anode resistance and sulfur coverage for different operating conditions will be presented.

Furthermore, the identified elementary sulfur poisoning reactions are used to extend an existing reaction mechanism for methane steam reforming which is then validated based upon a variety of electrochemical experiments. It is shown that atomically adsorbed sulfur significantly influences heterogeneous reforming chemistry, causing a substantial decrease in OCV. Under polarization, at constant current densities the cell voltage decreases in a non-linear way. After the removal of hydrogen sulfide from the feed gas the cell shows a faster recovery than in H2/H2O mixtures. In addition, numerical impedance simulations over a wide range of operating conditions were performed, which allows a physically-based assignment of observed gas concentration, heterogeneous chemistry and electrochemical processes.
Accomplishments, Status, and Road Map for the US Department of Energy's Fossil Energy SOFC Program
Authors: White Briggs M. , Lundberg Wayne L. , Pierre Joseph F.
Published: February 2020

Development of electric power generation technology that efficiently and economically utilizes coal and natural gas while meeting environmental requirements is of crucial importance to the United States.  The U.S. Department of Energy (DOE) Office of Fossil Energy (FE), through the National Energy Technology Laboratory (NETL), is leading the research and development of advanced Solid Oxide Fuel Cells (SOFC) as a key enabling technology.  This work is being done in partnership with private industry, academia, and national laboratories. 

Program participants are making significant progress toward commercialization.  Milestones include: the development and validation of multiple generations of engineered seal concepts, cells with low degradation and high power density, development of integrated stack and system modeling tools, and validation of low-cost stainless steel interconnect solutions.  With advances in cost reduction, the program is now focused on research to improve durability and reliability, especially in an integrated realistic system context.  Integrated system tests at larger scales (10-100 kWe) are yielding respectable degradation rates in the 1 to 1.5%/1,000 hrs under cost-effective operating conditions.  

In this presentation, these highlighted accomplishments will be reviewed along with the status of the program and a road map for the future.
Towards the Next-Generation of Solid Oxide Fuel Cell Systems
Authors: Singh Vaibhav , Wagner Patrick Hubert , Wuillemin Zacharie , Diethelm Stefan , Schiffmann Jürg , Van herle Jan
Published: February 2020

Use of intermediate temperature solid oxide fuel cells for power generation is attractive, due to the highest achievable electrical efficiencies in the low power generation range. Nonetheless, there still exists a potential to improve the industry benchmark SOFC systems, which use natural gas or biogas as fuel and consider steam methane reforming, with external steam supply, for syngas production and usage in the stack.

Anode off-gas recirculation using a blower is the add-on to our next-generation SOFC system. Since the recirculating feed contains steam produced in the stack, no external steam supply is needed for reforming. This eliminates the use of expensive water de-ionisation sets. Further, it allows for high overall fuel utilization at low diffusion losses. System performance is evaluated through multi-objective optimization criteria, i.e. maximization of electrical efficiency and cogeneration efficiency. Evolutionary algorithms compare the different design alternatives, i.e. co-flow or counter-flow stack operation with hot or cold recirculation. The system flowsheet which includes models for BOP components and an inhouse experimentally validated SOFC stack model, is solved using the commercial software Belsim VALI.

The design variables identified for this system are 1. oxygen to carbon ratio before the external reformer 2. external to internal methane reforming fraction 3. reducing fuel species molar fraction at anode outlet 4. air-fuel equivalent ratio in the burner and 5. blower inlet temperature. Successive generations of population are obtained by reproduction and mutation of the existing population. Following the ‘survival of the fittest’ rule, the iteration is stopped when a non-dominated solution set represented by a Pareto-optimal front is obtained. The results obtained (eg. figure 1) suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable.

Figure 1
Performance Characteristics of 100 W Carbon Fuel Cell Stack Based on SOFC Technology
Authors: Mushtaq Usman , Kim Sun-Kyung , Song Rak-Hyun , Lim Tak-Hyoung , Lee Jong-Won , Lee Seung-Bok , Park Seok-Joo
Published: February 2020

Carbon Fuel Cells (CFCs) run on carbon-rich fuels such as coal and biomass. Owing to their high theoretical efficiencies, they have been studied as new stationary power generation. This work presents the fabrication and operation of a 100W SOFC-based CFC stack, consisting of 5 bundles with 6 tubular cells per bundle. 8 mol%Y2O3-stabilized ZrO2 (8YSZ) electrolyte and layers of La0.8Sr0.2MnO3/La0.6Sr0.4Fe0.8Co0.2O3 cathode were coated on Ni-8YSZ anode support, with an active electrode area of 60cm2 per cell. Six cells were embedded vertically on a fuel gas manifold and interconnected electrically in parallel within a bundle. Five bundles were sealed and stacked in series electrically. The stack was connected to carbon fuel chambers containing mixture of activated carbon and potassium carbonate. The carbon fuel mixture was agitated with carbon dioxide to promote reverse Boudouard reaction and thus providing carbon monoxide as stack fuel. The stack and the fuel chambers were operated simultaneously in a heating furnace. The CFC stack showed 111.5W at 850 oC and its average voltage degradation rate is 0.55 V/kh for 200hours. The CFC stack degradation during long-term operation was analyzed and discussed as related to microstructural change, poisoning by impurities and electrochemical cell polarization losses.
Anomalous Ohmic Loss in Solid Oxide Fuel Cells with Doped Ceria Interlayers
Authors: Zhou Xiao-Dong , Lu Z , Templeton J , Stevenson J W
Published: February 2020

The ohmic resistance of an anode-supported SOFC is usually composed of contributions from the following parts: anode, electrolyte, cathode, and contact between the cell components. All of the ohmic resistances of cell components should be as small as possible to achieve an efficient SOFC operation. It is often believed that most of the ohmic resistance attributes to the electrolyte since the ionic conductivity of the electrolyte is usually several orders of magnitude lower than the electrical conductivities of the electrodes.

In an SOFC single cell with YSZ as the electrolyte, however, the contribution of the electrolyte to the total ohmic resistance of the anode-supported SOFC has not yet been fully understood despite of the numerous studies in this area. Several groups reported that the YSZ films in real SOFC environment showed lower ionic conductivities than that of the bulk YSZ material. This is especially true when the doped ceria is screen printed on YSZ-based single cell, in which the magnitude of the ohmic losses in the cell was observed more than three times greater than that for YSZ.  The aim of this paper is to address the origin of the exaggerated ohmic resistance.
Evaluation of La2NiO4 as Anode Material for Solid Oxide Electrolyser Cells
Authors: Egger Andreas , Schrödl Nina , Sitte Werner
Published: February 2020

Rising capacities in renewable energy production pose major challenges resulting from the decoupling between supply and demand, which requires long-range distribution and storage of electrical power. Electrolysis of water is considered a promising method for converting, storing and distributing surplus electrical energy in chemical form. Stimulated by the large-scale installation of wind parks, interest in high temperature steam electrolysis as a highly efficient means for hydrogen production is reviving. This technology appears to be especially favourable if external heat sources from power plants or industrial facilities can be utilised to assist in sustaining operating temperatures above 600°C.

High temperature electrolysis is based on solid oxide electrolyser cells (SOEC) which adopt design concepts and materials developed for solid oxide fuel cells (SOFC). In this work the promising SOFC cathode material La2NiO4+δ is investigated with respect to its suitability as anode for SOEC application. Important material properties characterising electrical conductivity and oxygen transport kinetics are determined on densely sintered samples of La2NiO4+δ. In addition, its performance as air electrode is investigated on asymmetric cells under anodic polarisation at current densities of up to -410 mAcm-2 by means of impedance analysis and I-V measurements. Electrochemical characterisation is conducted at 800°C and 0.2 bar oxygen partial pressure and the long-term stability is studied over several thousand hours. Special emphasis is put on the effect of Cr-poisoning on the oxygen exchange kinetics and SOEC anode performance in dry and humid atmospheres. Results from post-test examination of porous electrode structures as well as dense samples of La2NiO4+δ by microstructural analysis with SEM-EDX and AFM are presented.
Electrochemical Impedance Spectroscopy Data from Solid Oxide Cells Undergoing Co-Electrolysis: The Influence of Rig Inductance
Authors: Thompson Alison R , Call Ann V , Cumming Denis J , Sinclair Derek C , Elder Rachael H
Published: February 2020

High temperature solid oxide cells (SOCs) can be used for co-electrolysis of carbon dioxide and steam to produce carbon monoxide and hydrogen: key reactants for the production of synthetic fuels. During co-electrolysis, two electrochemical reactions occur at the fuel electrode, in addition to the reverse water gas shift reaction (rWGSR). It is currently not well understood which processes relating to these reactions are rate-limiting. Here we propose a method that enables rate-limiting processes to be identified using electrochemical impedance spectroscopy (EIS). SOCs were operated at 850oC under varied fuel concentrations and current densities. Inductance effects were accounted for and the resulting EIS data was de-convoluted using the Distribution of Relaxation Times (DRT) method, giving the characteristic frequencies of rate-limiting processes occurring during cell operation. The resistances and capacitances associated with these processes were quantified using the impedance and electric modulus formalisms, respectively. DRT results in combination with the analysis of differences in spectra (ADIS) associated with the EIS data, plotted in both impedance and electric modulus formalisms, were used to justify the number of processes associated with the cell polarization resistance and to develop a meaningful equivalent circuit to model cell performance.
Effect of Ceria Content of CeScSZ Powder on the Phase Stability and Electrical Conductivity of SOFC Electrolyte
Authors: Choi Seung-Woo , Kim Kyeong-Joon , Kim Min-Young , Lim Jinsub , Yang Seung-Hoon , Kim Yu Sin , Kim Ho-Sung
Published: February 2020

Solid oxide fuel cell (SOFC) is attracting more attention in recent years for its lower pollution emission and high energy convert efficiency. The SOFC is conventionally operated at high temperatures between 900 and 1000 °C, because of the low ionic conductivity of YSZ below 800 °C. ScSZ-based electrolyte is one of the candidate materials for IT-SOFCs because of their higher ionic conductivities compared to any other zirconia electrolytes. In general, it is reported that 1Ce10ScSZ composition is available as ScSZ-based electrolyte of SOFC [1].

 In this paper, CeScSZ powders with ceria-doped content were synthesized by co-precipitation process. The crystal lattice parameter and XRD peak pattern of powders were investigated with the increase of calcination temperature and the change of ceria-doped content at a certain calcination temperature. Ionic conductivities of the CeScSZ powders were also studied at the range of 500 ~ 900 °C. In here, the XRD peaks of cubic structure were found only on the ceria-doped powders more than 1 mole% even at the calcination temperature of 1500 °C. Meanwhile, the behaviour of ionic conductivities showed little different characteristics from the XRD pattern, namely, 1 mole% ceria-doped 10ScSZ powder showed the highest conductivity. Therefore, electrolyte film was fabricated by using the synthesized 1Ce10ScSZ powder and single cell of SOFC with anode-supported electrolyte was fabricated by  tape casting and co-sintering process. As a result, the SOFC cell showed power density of 1.2 W/cm2 at 2 A/cm2, 800 °C with low ohmic resistance of under 0.1 W·cm2.   

References

[1] Ju Hee Kang, Young Mi Kim, Ho-Sung Kim, Moo Sung Lee, Jae Hyuk Jang, Jin-Hun Jo “Fabrication characteristics of SOFC single cell using nanocrystalline 1Ce10ScSZ electrolyte powder prepared by co-precipitation process,” J. Full Cell and Technology, 2012, Vol 9, pp. 011015.
Degradation of Electrical Conductivity in Lanthanum Silicate Oxyapatite
Authors: Ishiyama Tomohiro , Maruyama Hiraku , Kishimoto Haruo , Bagarinao Katherine Develos- , Horita Teruhisa , Yamaji Katsuhiko , Yamazaki Atsushi
Published: February 2020

Lanthanum silicate oxyapatite (La9.33+xSi6O26+1.5x) is known to exhibit higher oxygen ionic conductivity than yttria-stabilized zirconia at temperatures below 700oC, making this material attractive for its potential use in intermediate-temperature fuel cells operated at 500~700 oC. Mineshige et. al. reported that the lanthanum content in oxyapatite phase is strongly related to its electrical conductivity, with the highest conductivity obtained when x=0.67 (La10Si6O27) [1]. It was further reported that La2SiO5 existed in addition to the oxyapatite phase even in the as-sintered sample. Another important aspect that can influence electrical conductivity is the phase stability of lanthanum apatite, which remains unclear. In this study, electrical conductivity measurements of lanthanum silicate apatite were conducted for over 100h and its phase stability was evaluated.

 Lanthanum silicate apatite with nominal composition of La10Si6O27 was made by conventional solid state reaction. The pellet was sintered at 1700oC for 10h after calcining at 1400oC for 10h. Electrical conductivities were determined by impedance spectroscopy measurements conducted at 600, 800 and 1000oC for 106~201h in dry air. The phase stability was evaluated by annealing at 1500oC for 20h.

 The degradation of electrical conductivity was observed at all temperatures. In particular, the conductivity decreased significantly from 5.57x10-2 Scm-1 to 3.57x10-2 Scm-1 after 201h at 1000oC. Crystallites having small facets with the size of several hundred nanometers were observed on the oxyapatite phase after conductivity measurement. On the other hand, significantly larger particles were found segregated on the surface of the sample annealed at 1500oC; these particles have a composition determined to be La2SiO5 by EDS analysis. This result suggests that the same phase segregation could have occurred at the temperature range of 600 ~ 1000oC but at a diminished degree. Moreover, the degradation of electrical conductivity could be correlated to the observed phase segregation. In this presentation, we will discuss the causes of the degradation of electrical conductivity in consideration of the phase equilibrium of lanthanum silicate.

Ref. 1: A. Mineshige, Y. Ohnishi, R. Sakamoto, Y. Daiko, M. Kobune, T. Yazawa, H. Yoshioka, T. Nakao, T. Fukutsuka, Y. Uchimoto, Solid State Ionics 192 (2011) 195.
Doped Apatites for Novel Solid Oxide Fuel Cell Applications
Authors: Ow Jia Wei Nicholas Earvin , Musso Federico , Bertuzzo Marcus , Alfredsson Maria , Corà Furio
Published: February 2020

The aim of this project is to combine X-ray absorption fine structure (EXAFS) and atomistic simulations to determine local atomic environments in novel oxide ion conductors, with applications in solid oxide fuel cells (SOFC). The purpose is to gain insight into the local coordination of Ga and Ge in doped melilites (e.g. La0.46Sr1.54Ga2.95M0.05O7.27-d. where M=Zn2+, Al3+ and Ge4+). The latter compounds are believed to facilitate oxygen conduction by rearranging its local structure in the presence of oxygen excess. By understanding the oxygen conduction mechanism we can design materials with improved functionality.
Improvement of Protonic Ceramic Fuel Cells with Thin Film BCZY Electrolyte
Authors: Okuyama Yuji , Ebihara Noriaki , Okuyama Kenji , Mizutani Yasunobu
Published: February 2020

Fuel cell performance of protonic ceramic fuel cells (PCFCs) using thin film BaCe0.6Zr0.2Y0.2O3-δ (BCZY622) electrolyte have been studied. First reference cell with La0.8Sr0.2MnO3-δcathode and Ni anode showed poor performance and short-term degradation. Overpotential of anode and Ohmic loss was major limiting factor, and solid solution of Ni into BCZY622 electrolyte has been observed. The electrical conductivity and the proton concentration of BCZY622 and BCZY622 doped with 5wt% NiO were measured by impedance analysis and thermogravimetric analysis. The electrical conductivity and the proton concentration of BCZY622 were decreased by introducing NiO.

To avoid short-term degradation, cell construction, manufacturing process, and firing temperatures were improved. As a result, carefully designed PCFC with thin film BCZY622 showed good power density of 200-300mW/cm2 in wide temperature range at 873 - 1073K , and performed stable generation.
Three-dimensional Modeling of Internal Reforming SOFC with a Focus on the Interconnect Size Effect
Authors: Sun Qiong , Ni Meng
Published: February 2020

A comprehensive 3D mathematical model is developed for planar solid oxide fuel cells (SOFCs) running on hydrocarbon fuels.  The model fully considers the coupled gas transport, ionic conduction, electronic conduction, electrochemical reactions, internal reforming reaction, water gas shift reaction, and heat transfer.  The model is validated by comparing the simulation results with literature data.  Parametric simulations are conducted to investigate how the size of interconnect rib affect the various physical and chemical processes in the SOFC.  It’s found that the interconnect rib greatly influences the gas diffusion in the porous electrode thus influences the electrochemical/chemical reactions.  Optimal rib size is observed for internal reforming SOFCs.  The results are useful for design of SOFC stack with hydrocarbon fuels.
Electrochemical Behavior of Phosphine Induced Anode Performance Degradation in a Planar SOFC: A Numerical Study
Authors: Sezer Hayri , Celik Ismail Bektas , Yang Tao
Published: February 2020

A one-dimensional in-house computational model was developed to predict the performance degradation of the SOFCs anode exposure of fuel contaminants formed in typical coal syngas. This model is extended to a three-dimensional model to predict the phosphine induced performance degradation in relatively large planar cells operating on hydrogen. The model parameters are calibrated using button cell experiments conducted under accelerated tests conditions. These parameters are then used to perform simulations to predict fuel contaminant performance degradation of planar cells. The results from degradation simulations show that the contaminant coverage alter the initial current distribution, hence the fuel and oxygen distribution inside the anode significantly. The electrochemical characteristic of the degraded cell is analyzed by performing impedance and polarization simulations at cell operating current. The polarization and impedance simulations are implemented by dividing the cell into three regions along the fuel flow. Good agreement is achieved between polarization and impedance analysis of the cell at cell current load. The results show the degradation rates and the impedance behavior of planar cells are very different than those observed in button cells.
The Fate of Phosphine in SOFC Fuels
Authors: Finklea Harry O. , Zhang Wei , Jony Mahfuzur , Demirgok Berk
Published: February 2020

Phosphine (PH3) is present in part-per-million levels of coal syngas, a potential fuel for SOFCs.  Previous studies have shown that the SOFC loses power and ultimately fails during prolonged exposure to phosphine in wet and dry fuels.  The presence of nickel phosphide phases on the anode suggests that phosphine survives the operating conditions of the SOFC fuel.  In this study, a mass spectrometer was used to analyze fuel gases after passing the gases through a heated tube or after passing the fuel gases across the anode of an operating SOFC.  Phosphine appears to react with residual oxygen at intermediate temperatures of 400-600ºC in dry (P(H2O) < 1 torr) hydrogen, but does not react above 600ºC and consequently reaches the anode.  At higher water pressures (5-20 torr), phosphine reacts at temperatures above 400ºC.  Phosphine is detected in the exhaust gas of a SOFC using dry hydrogen while at open circuit, and disappears from the exhaust gas during current flow.  The inverse correlation between water content of the exhaust gas and the phosphine concentration suggests that phosphine is reacting with electrochemically generated water.  No new compounds containing phosphorus were detected in the m/z range of 45-100, eliminating HPO, HPO2 and HPO3 as possible reaction products.  There is evidence for the presence of nonvolatile products of phosphine in the supporting tubes after prolonged exposure to phosphine.  These products can generate measurable amounts of phosphine at 800ºC in dry hydrogen initially free of phosphine.
Development of Micro Power Generator Using LPG-Fueled Microtubular Solid Oxide Fuel Cells
Authors: Sumi Hirofumi , Yamaguchi Toshiaki , Suzuki Toshio , Shimada Hiroyuki , Hamamoto Koichi , Fujishiro Yoshinobu
Published: February 2020

Solid oxide fuel cells (SOFCs) can, in principle, directly use hydrocarbon fuels such as methane and liquefied petroleum gas (LPG).  It was previously found that nickel-gadolinia doped ceria (Ni-GDC) anode has high durability under LPG utilization in SOFCs.  However, the performance under direct LPG utilization was low compared to hydrogen fuel, because anode polarization resistance increased by internal reforming.  Carbon was slightly deposited on the Ni-GDC anode after direct LPG utilization for 100 h at 650 oC.  In this work, a catalytic partial oxidation reformer was developed for LPG utilization, and high performance was obtained using reformate gas as well as hydrogen without carbon deposition on the Ni-GDC anode for 100 h at 650 oC.  Furthermore, microtubular SOFC stacks were developed using mass production technologies, and a proto-type 200 W micro power generator was demonstrated via NEDO project entitled "Technology Development for Promoting SOFC Commercialization".
Assessment of Pressure Effects on High Temperature Steam Electrolysis Based on Solid Oxide Technology
Authors: Bernadet Lucile , Gousseau Georges , Chatroux André , Laurencin Jérôme , Mauvy Fabrice , Reytier Magali
Published: February 2020

To manage the grid with renewable electrical sources and ensure their storage, hydrogen chains have to be considered. However, hydrogen must be pressurized before being used or stored. The first levels of compression are less energetic for liquids than for the hydrogen gas, therefore the electrolysis of steam directly under pressure has to be studied. On one hand, with the solid oxide technology, the Open Circuit Voltage (OCV) presents the drawback of increasing with pressure. On the other hand, the pressure might improve the reactive species supply to the electrochemical sites and reduce the related losses. Thus, what are the electrolysis performances under pressure? And is there an energetic gain at the system level to produce pressurized hydrogen?

In this context, experimental and modelling approaches have been adopted in order to understand the basic underlying mechanisms of pressurized electrolysis operation. In this frame, SOEC performance tests were realized under pressure in the range of 1 to 10 bars on two types of single commercial cells. They were based on conventional materials and differentiated by their microstructure. Concerning the set-up, the test platform requirements were achieved by developing an innovative mechanical design associated to a specific steam management. Moreover, a previously proposed electrochemical model of solid oxide electrolysis cell was adapted to take into account the pressure effects. The model allows computing all the most important operation parameters such as local current densities, local molar fraction, overpotentials and also the cell microstructural parameters [1].

According to the complete I-V curves from 0 to -1.3 A/cm2 at 800°C with high steam conversion, it has been observed that OCVs respect the Nernst relation under pressure while the limit current appears at higher current values. This work shows that the hydrogen production can be increased thanks to a 10 bars operation at the autothermal voltage.  Moreover, these trends have been analyzed with the model. The impact of pressure on the electrochemical performances is much more important in the range of 1 to 5 bars than above. In our experimental conditions, the diffusion improvement in the porous cathode alone can explain this behavior. Finally, the impact of each microstructural parameter on the cell performances with pressure is discussed.

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement 621173.

[1] J. Laurencin, D. Kane, G. Delette, J. Deseure, F. Lefebvre-Joud, Modelling of solid oxide steam electrolyser : impact of the operating conditions on hydrogen production, 2011, Journal of Power Sources, 196, 2080-2093.

Figure 1
Soft Sensor Design for Estimation of SOFC Stack Temperatures and Oxygen-to-Carbon Ratio
Authors: Dolenc Bostjan , Vrecko Darko , Juricic Dani , Pohjoranta Antti , Kiviaho Jari , Pianese Cesare
Published: February 2020

The lifespan of a solid oxide fuel cell (SOFC) stack depends on several factors, such as the internal stack temperature and temperature gradients as well as the fuel gas oxygen-to-carbon (O/C) ratio within the stack. An excessively high stack operating temperature generally accelerates degradation processes while large temperature gradients over the stack increase the thermal stress within the stack which may lead to the delamination of repeating unit components. A too low O/C ratio inflicts carbon deposition which quickly leads to stack breakage. Therefore, monitoring these variables is of vital importance. Although measuring temperatures within the stack and measuring the fuel gas composition at the stack inlet is feasible, this raises the overall equipment cost. In this paper a data-driven design of soft sensors for stack minimum and maximum temperatures as well as the O/C ratio at the anode and pre-reformer inlet is presented. For temperature estimation both dynamic and static models are derived and their performance is compared. The dynamic model is identified using subspace identification, which results in a causal state-space model. The non-causal static model assumes that a combination of process variables at the stack inlet and outlet describe its internal condition. The estimation of gas composition at the inlets, which is required for O/C ratio estimation, is based solely on static relationships. The soft sensors are designed in such a way that adding extra inputs yields no further increase in estimate accuracy. The empirical data required for modelling were obtained from a complete SOFC power generation unit. The results show that estimates of all the relevant variables can be accomplished by simple linear regression models.

Keywords: SOFC, soft sensor, O/C estimation, temperature estimation

Figure 1