SOFC Operation Research Articles

Effects of water on the degradations in the Ni-YSZ anode of the direct ammonia solid oxide fuel cells

Direct ammonia solid oxide fuel cells (DA-SOFCs) are promising next-generation energy conversion devices. Despite their possibilities, the electrochemical performance of DA-SOFCs degrade severely over long-term operation. However, the degradation mechanisms are still unveiled. Here, we reveal that the degradations of DA-SOFCs are significantly initiated by the presence of the water, which is the by-product of the SOFC operation, inside the fuel electrode. We measured the stability of the DA-SOFCs by varying the fuels and water partial pressure in the fuel. Thorough surface characterizations verified that Ni becomes favorable for oxidation when ammonia is present in the water, resulting in a reduction of the catalyst reactivity for NH3 decomposition and electric conductivity. Furthermore, we conducted a density functional theory calculation and found that N* adsorbed onto Ni decomposed from NH3 facilitates the H2O decomposition and results in nickel oxidation.

A novel pathway for achieving efficient integration of SOFC/SOEC and addressing photovoltaic duck curve challenge

This study presents a novel method that directly integrates SOFC oxy-fuel combustion and SOEC co-electrolysis to tackle the challenges of high-energy consumption of SOFC/SOEC integration and photovoltaic duck curve. To explore the viability of this method, a biomass-solar complementary scenario is pioneered and its performance is assessed from thermodynamic and economic standpoints. The results show that the studied scenario achieves CHP and exergy efficiencies of 50.67% and 43.68% with the corresponding total product cost rate (Ċtot) of 45.44 $/h and the levelized cost of products (cp) of 14.61 $/GJ under the design conditions. The system annually produces 17485 tons of freshwater and 1432 tons of oxygen. Exergy analysis identifies the afterburner (AB-II) and supercritical CO2 recompression power cycle (SCRPC) as the primary contributors to exergy destruction with 381.74 kW and 343.62 kW, which alone account for over half of the total irreversibility. Increasing the isentropic efficiency of gas turbine (ηGT) and SOFC operating pressure (Pop) improves the overall thermodynamic performance, but enhancing SOFC current density and gasification temperature has adverse effects. Across varying Pop andηGT, there exist optimal Ċtot (44.83 $/h) and cp (14.47 $/GJ), which are obtained at ηGT of 80% and Pop of 600 kPa, as well as at ηGT of 87.14% and Pop of 800 kPa, respectively. This research can provide a reference for the development of cyclic energy storage technologies and the low-carbon transition in the energy fields.

Thermodynamic performance evaluation of a novel solar-assisted multi-generation system driven by ammonia-fueled SOFC with anode outlet gas recirculation

The cracking of ammonia to produce hydrogen and nitrogen at high temperatures, without carbon dioxide emissions, has great potential for future energy development. Hence, a novel solar-assisted multi-generation system driven by ammonia-fueled SOFC with anode outlet gas recirculation is proposed and the thermodynamic models are constructed. Under the design conditions, the exergy efficiency, and primary energy saving rate of the novel system in summer are 47.83%, and 51.36%, respectively, while in winter, they are 50.52%, and 52.01%, respectively. Meanwhile, the thermodynamic performances of the new system and two reference systems are compared, in which the reference system 1 isn't coupled with a solar collector, and SOFC adopts an anode outlet gas recirculation layout, the reference system 2 has neither coupled solar collector nor anode outlet gas recirculation arrangement for SOFC. The results demonstrate that the anode outlet gas recirculation layout and coupling solar collectors in SOFC help to reduce the fuel consumption of the system. Moreover, the subsystem's exergy flow diagrams under the design condition reveal the exergy destruction of the components. And the impacts of key factors including SOFC operating temperature, fuel utilization rate, direct normal irradiance, and hydrogen recovery factor are analyzed.

Development and performance evaluation of (Sm1-xNdx)0.2O2-δ electrolyte for low-temperature solid oxide fuel cell application

Major hurdle for SOFC operation at lower temperature is it’s ion migration through electrolyte crystal lattice reduces as more energy required and lowered number of ion vacancies. In order, researchers from all over the world trying to develop SOFC which operates at lower temperature which let to erupt the electrolyte performance. Currently available electrolyte YSZ (Yttria Stabilized Zirconia) shows high efficiency above 1100 °C. Fluorite structure doped/codoped ceria electrolyte shows better ionic conductivity below the 1000 °C. In current study ceria co doped with the Sm+3 and Nd+3 having ionic radius close to host ceria through oxalate co precipitation route. Crystallite size and structure is observed through X-ray diffraction for purity of doping with nano crystallite size transpose from 13.21 to 22.21 nm. Furthermore, Energy dispersive X-ray spectroscopy (EDXS) validates maximum 0.2 Codoping achieved through oxalate rought. Moreover grain size and grain boundary compaction enhances with sintering electrolyte pellet at 1300˚C for 2 h observed under FE-SEM. AC and DC conductive properties are studied with diverse codoping percentage and it is analyzed through Electrochemical Impedance Spectroscopy (EIS). Ce with 0.1 codoping Sm+3 and Nd+3 respectively enhances ion migration with minimum activation energy and relaxation time in lattice site.

A Study on Electrochemical Properties of Fuel-Electrode-Supported Reversible Solid Oxide Cells

Introduction Reversible solid oxide cells (r-SOCs) enable both power generation and steam electrolysis, and have attracted attention as a solid state electrochemical energy device for a decarbonized society (1). R-SOCs have the advantage of high efficiency due to high temperature operation, but there exist various technical issues remained in materials selection, start-up and shutdown, and long-term durability. Therefore, reducing operating temperature is desirable for practical applications (2,3). Electrode-supported cells, in which the electrode acts as a structural support, enable lower temperature operation by reducing the thickness of the electrolyte, which has certain electrical resistivity. Here in this study, the current-voltage characteristics and reversible cycle durability under r-SOC operating conditions are evaluated using fuel-electrode-supported cells, with the aim of developing r-SOCs capable of highly efficient fuel cell power generation and steam electrolysis. Experimental For the experiments, fuel-electrode-supported cells were fabricated using fuel electrode-supported half-cells (Japan Fine Ceramics, Japan), schematically shown in Fig. 1. The half-cell consists of scandia-stabilized zirconia (ScSZ: 10 mol%Sc2O3-1mol%CeO2-89 mol%ZrO2) or yttria-stabilized zirconia (YSZ: 8 mol%Y2O3-92 mol%ZrO2) electrolyte and a Ni-cermet fuel electrode, such as Ni-ScSZ or Ni-YSZ. (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was applied for the air electrode, and Gd0.1Ce0.9O2 (GDC) was inserted between the electrolyte and the air electrode to suppress interdiffusion and chemical reactions. Four types of cells were used with different combinations of the electrolyte component (YSZ or ScSZ) in the electrolyte layer or the supporting fuel electrode.In electrochemical tests, 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. R-SOC initial performance tests were conducted using an electrochemical analyzer (1255B, Solartron). Cell voltage and impedance were measured at 700-800°C at current densities ranging from -0.5 A cm-2 to +0.5 A cm-2. Positive current density means the value in SOFC mode, while negative current density means the value in SOEC mode. In r-SOC 1,000 cycle durability tests, cycles of switching between SOFC and SOEC operation were repeated 1,000 times by varying current density. The range of current density in the cycle tests was between -0.2 A cm-2 and +0.2 A cm-2. The cell impedance was measured before the cycling test and every 100 cycles. After each test, the cells were analyzed by using a focused-ion beam scanning electron microscopy (FIB-SEM) to observe and evaluate the electrode microstructure. Results and discussion Figure 2 (a) shows the r-SOC initial performance of each fuel-electrode-supported cell. The cell with the Ni-YSZ fuel electrode and the YSZ electrolyte showed better current voltage characteristics than other cells. Figure 2 (b) shows the r-SOC 1000-cycle durability of the cell with the Ni-YSZ fuel electrode and the YSZ electrolyte. The results showed a decrease in power generation and electrolysis performance with increasing the number of cycles in both SOFC and SOEC modes. Possible degradation mechanisms will be discussed. Acknowledgments This paper is based on results obtained from a project (Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration), JPNP20005, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Collaborative support by Prof. H. L. Tuller and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References (1) Venkataraman, M. Pérez-Fortes, L. Wang, Y. S. Hajimolana, C. Boigues-Muñoz, A. Agostini, S. J. Mcphail, F. Mar échal, J. V. Herle, and P. V. Aravind, J. Energy Storage, 24, 100782 (2019).(2) Subotić, S. Pofahl, V. Lawlor, N. H. Menzler, T. Thaller, and C. Hochenauer, Energy Proc., 158, 2329 (2019).(3) B. Mogensen, Current Opinion Electrochem., 21, 265 (2020). Figure 1

Influence of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

In the present study, the electrochemical performance and microstructure evolutions of Ni-M (M = Fe, Cu, Mn) bimetallic fuel electrodes were investigated under SOFC and SOEC operations. Ni-Fe, Ni-Mn, Ni-Cu and pure Ni patterned fuel electrodes were sputtered on YSZ pellets. During the SOFC tests, the electrochemical performance of all Ni-M bimetallic fuel electrodes were lower than pure Ni electrode, while the degradation rates of Ni-Fe and Ni-Mn electrodes were smaller than the others. The spreading of Ni film on YSZ surface was observed for all samples, and such Ni migration was suppressed by Fe and Mn addition, whereas it was enhanced by Cu addition. During the SOEC tests, the cell performance degraded with Cu and Fe addition, but improved with Mn addition. The adhesion between Ni film and YSZ substrate was enhanced by doping Fe and Mn, which correlated well with the inhibited degradation in both fuel cell and electrolysis operations. The Ni migration phenomenon is discussed by the strength of Ni-O bond, surface tension and melting point, which are influenced by the addition of transition metal elements.

Performance analysis and multi-objective optimization of a novel poly-generation system integrating SOFC/GT with SCO2/HDH/ERC

The SOFC-based poly-generation system is an effective and environmental-friendly technology. To further improve energy efficiency of poly-generation system, a novel poly-generation system is developed based on solid oxide fuel cell/gas turbine (SOFC/GT) as the prime mover and supercritical carbon dioxide (SCO2) Brayton cycle, humidification and dehumidification (HDH) desalination and ejector refrigeration cycle (ERC) as the waste heat recovery units to produce power, cooling and freshwater. In the proposed system, a novel configuration of combining the SCO2 cycle with HDH is applied to recover the waste heat of SOFC/GT hybrid system. A mathematical model is developed to explore the system performances from the energy, exergy and economic perspectives. The effects of key parameters such as fuel flowrate, fuel utilization factor, SOFC operating pressure and temperature, and desalination top temperature on system performances are investigated. Multi-objectives optimization is conducted to obtain the optimal design parameters of the poly-generation system. The results show that the system net power output, cooling capacity and freshwater production are 273.769 kW, 12.997 kW and 24.23 g/s with the total cost being 14.01 $/h under the given condition, respectively. The electrical and exergy efficiencies and total gained output ratio of the system are 68.35 %, 67.23 % and 85.25 %, respectively. The parametric analysis indicates that an appropriate increase in fuel flowrate, fuel utilization factor and SOFC operating pressure and temperature can improve the electrical and exergy efficiency. The optimization results demonstrate that the system exergy efficiency and total cost are 67.48 % and 13.94 $/h.

Green hydrogen production and utilization in a novel SOFC/GT-based zero-carbon cogeneration system: A thermodynamic evaluation

Owing to the rise in global energy demand and environmental issues caused by energy supply and distribution, the attempt to design and develop novel eco-friendly and efficient cogeneration energy systems is one of the current research focuses. In this regard, this study presents a novel zero-carbon cogeneration energy system based on a pressurized solid oxide fuel cell-gas turbine (SOFC/GT) with a carbon capture and storage (CCS) process. The designed system accomplishes the joint production of energy (electricity, cooling, and heating) and matter (hydrogen, oxygen, condensate water recovery, and carbon dioxide). The proposed system also includes an advanced alkaline electrolyzer (AAE) system, an organic Rankine cycle, a LiBr-H2O absorption refrigeration cycle, and an LNG regasification cycle. A combination of a solar concentrated photovoltaic/thermal (CPV/T) system and wind turbines feed the AAE system, resulting in green hydrogen production. According to the simulation results, the suggested system under the design conditions generates power, cooling, and domestic hot water with 315 kW, 137.5 kW, and 1.012 kg/s, respectively. Also, this system can capture and store carbon dioxide, recover condensate water, and supply natural gas to the pipeline with values of 74.88 kg/h, 137.9 kg/h, and 624.24 kg/h, respectively. The results from thermodynamic analysis indicate that the CPV/T system has the highest contribution to the exergy destruction rate, and the presented cogeneration system operates with energy and exergy efficiency of 58.09% and 38.58%, respectively. Furthermore, a comprehensive parametric study has been conducted on the developed system in which the impact of important parameters, such as SOFC operating temperature, current density, solar radiation, air velocity, etc., on the output parameters of the system has been examined.

Performance of SOFCs using model waste gases: A case study

Municipal solid waste landfills and steelmaking industry produce large amounts of high-calorific waste gases which can be utilized for energy generation. The possibility of SOFC operation using model landfill and coke-oven gases without external steam reforming of methane was experimentally demonstrated employing a SOFC short-stack comprising two electrolyte-supported membrane-electrode assemblies of planar geometry. The current-voltage dependencies, polarization resistances and composition of the effluent fuel oxidation products were tested as function of time during approximately 600 h. In all cases, complete conversion of methane occurs at the anodes even under open-circuit conditions. The maximum power density of 183 mW/cm2 was achieved for the humidified landfill gas at 850 °C and current density of 320 mA/cm2; the fuel utilization was 67% at current density of 222 mA/cm2. Neither microstructural changes nor carbon deposition were revealed by microscopic analyses after the SOFC tests. However, thermodynamic estimations showed that local anode coking may occur in a narrow zone near the fuel inlet where the fuel utilization only starts. This process correlating with local cooling detected in the vicinity of fuel inlet, may be responsible for degradation in the SOFC performance observed in the waste gas utilization regime.

Synergistic role of Biomolecules and Bio-chelating agents in the sustainable development of an efficient BaCe0.97M0.03O3-δ (M = Sm, Gd) perovskite electrolyte for IT-SOFC

Here, BaCe0.97M0.03O3-δ (M = Sm, Gd) perovskite electrolyte materials were synthesized via a semi-organic combustion route by employing the synergetic role of biomolecules and bio-chelating agent of lemon juice for IT-SOFC. The synthesized BaCe0.97M0.03O3-δ (M = Sm, Gd) electrolyte was compared to BaCe0.97M0.03O3-δ (M = Sm, Gd) synthesized with a typical chemical combustion route. The structural analysis confirmed the orthorhombic phase of all perovskite electrolytes while the microstructural analysis revealed dense microstructure for electrolytes synthesized with bio-chelating agents compared to those synthesized with chemical combustion route. The compositional analysis confirmed the presence of all elements and the thermal analysis established the stability within the SOFC operating temperature range of BaCe0.97M0.03O3-δ (M = Sm, Gd) synthesized with both routes. The electrochemical measurements exhibited a maximum power density of 0.47 W cm−2 for BaCe0.97M0.03O3-δ (M = Sm) synthesized with lemon juice as a chelating agent suggesting its potential use as an efficient electrolyte for IT-SOFCs. The outcomes show that the semi-organic route can be successfully used for the synthesis of perovskite structure materials with minimal negative impact on the environment compared to chemical routes.

Comparative Study of Thermodynamic Performance and System Characteristics of Solid Oxide Fuel Cell System with Three Different Fuels

The global demand for hydrogen is increasing as the global warming problem emerges. Hydrogen, which does not produce air pollutants, is attracting attention in various fields such as transportation and power generation. However, there are drawbacks in storing and transporting hydrogen in its own form. Hydrogen must be stored in the form of low-temperature and high-pressure liquefied hydrogen, which requires a lot of energy, and transportation costs are also high due to the low mass energy density of hydrogen. Therefore, there is a need to store and transport hydrogen in different forms, and a means to convert and use these different forms of hydrogen is needed.Solid oxide fuel cell (SOFC) is one of the energy conversion devices capable of using various types of hydrogen with high efficiency. Due to the high operating temperature of SOFC, fuel can receive adequate heat for reforming process from high temperature SOFC exhaust gas. Therefore, SOFC can use various types of fuels, such as methane and ammonia.Due to the fuel flexibility of SOFC, many SOFC system study using various fuels have been conducted. However, the SOFC operating conditions and system layout used in each study are all different. Also, most of studies lacked parasitic power consideration due to the pressure drop in system components. These limitations make it difficult to compare each SOFC system and find out effect of fuel on SOFC system.In this study, a quantitative comparison of SOFC using three different fuels, methane, ammonia, and hydrogen, was conducted. Thermodynamic models of a SOFC stack and balance of plant components developed from the previous study are used for the system simulations. Two system layouts, non-recirculation, and anode recirculation were chosen, and thermodynamic feasibility test was conducted based on T-Q diagram analysis at heat exchanger and reformer. With these system layouts, thermodynamic performance of the system and the design characteristics of the system components were compared. Boundary conditions, such as SOFC operating temperature, air and fuel utilization remains the same to compare each system result quantitatively. Also, most of design variables of system components are fixed to find out the effect of fuel on system components.Among three non-recirculation systems, ammonia fed non-recirculation system showed the highest electrical efficiency, while methane fed anode recirculation system showed the highest electrical efficiency among the anode recirculation systems. With the anode recirculation, SOFC power drop occurs due to the lower hydrogen concentration in the SOFC. However, fuel input drop due to anode recirculation is greater than the SOFC power drop, which positively affects system performance. But parasitic power is also increased due to recycle blower and higher mass flow rate of anode recirculation flow. In the case of ammonia fueled system, this increase in parasitic power leads to decrease in electrical efficiency.In this study, there are many constraints in operating conditions and BoPs designs to compare each system with different fuel. Therefore, operating conditions and BoPs designs are not optimized, so each system performance for each fuel may increase with appropriate system design. This optimization will be done in further study. Figure 1

Investigation of Surface Oxide Layer Structure to Improve Durability of Stainless Steels Under Humidified Hydrogen

We study degradation mechanism of stainless steels under SOFC operating conditions to improve durability of SOFC stack. To reduce material cost, a low-cost stainless steel is preferred because metal interconnect occupies considerable amount in the stack. From commercially available stainless steels, four types of alloy composition were chosen as model materials, which are widely used in the other applications. The alloy test pieces were exposed to hydrogen with 10%-50% H2O at 600oC-700oC, and mass gain after the test was measured. The mass gain increased with time, indicating that steam oxidation proceeded. The oxidation rate was smaller for JIS SUS430J1L than the other three types of the alloy for all the test conditions. The proficient durability came mainly from the thin Cr-rich oxide layer covering the alloy uniformly with little defect or segregation. The additives in the alloy such as Nb also may have an effect on suppressing elemental diffusion through the oxide layer. Because morphology of the surface oxide layer initially formed on the surface should affect the process of steam oxidation during SOFC operation, we investigated influence of pre-heat treatment condition (700oC-900oC) of the alloy. The surface oxide layer formed by the pre-heat treatment at 900oC in air for 2 h had an effect of reducing steam oxidation rate. Microstructure of the surface oxide layer before and after the durability test was analyzed by STEM. From the difference in the microstructure of the surface oxide layer, predominant factor to improve durability will be discussed. This paper is based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Visualization of Micro Damage in SOFC Using Terahertz Waves

As SOFC commercialization progresses and mass production is realized, quality control during production will become extremely important. Currently, quality control is based on visual inspection and testing of single cells and stacks under operating conditions. This method, although reliable, is time-consuming, costly, and unsustainable.Therefore, the authors have been developing evaluation methods with the aim of developing a system that enables rapid, safe, nondestructive, automatic, and accurate quality assurance assessment. One of them is developing evaluation methods for internal microdefects using THz waves. Previously, the authors have been conducting nondestructive evaluation during SOFC operation using the acoustic emission (AE) method [1-5]; the AE method detects the sound (elastic waves) generated when cells, stacks, etc. are deformed or mechanically damaged, and is suitable for in-situ observation during operation. In this situation, we have also succeeded in classifying deformation and various damages of cells and stacks by machine learning of the obtained signals [6,7,8]. However, there are two main disadvantages of using SOFC as a quality assurance inspection method: First, it is passive. In other words, it cannot detect deformation or damage that has already occurred. The second is that it is dependent on the measurement environment, since AE signals are not physical quantities and therefore change depending on the measurement environment. In other words, the waveform shape, amplitude, and frequency components of the signals may change significantly by attaching a sensor to the waveguide. Therefore, signals that have been successfully classified may need to be readjusted or re-verified if the measurement conditions change.On the other hand, signals obtained from THz waves are often highly reproducible physical quantities. In addition, it is rapid and nondestructive because of the contactless and pulsed wave. However, THz waves have a long wavelength, which results in low resolution in multilayer thin-film structures. If it becomes possible to detect small defects in the layers and visualize the internal structure by utilizing machine learning, it will surely be a very effective evaluation method for SOFCs.In this paper, we report on the clustering and recent results of machine learning using fluctuations of THz reflected waves.AcknowledgementsThis research was conducted as part of the New Energy and Industrial Technology Development Organization (NEDO) (JPNP20003). In addition, it was supported by Grant-in-Aid for Scientific Research (22K18865) and (22H01353).[1] K. Sato et al et al., Journal of Testing and Evaluation 34, JTE12707 (2006).DOI: 10.1520/JTE12707[2] K. Sato et al., Journal of Power Sources 195, 5481-5486 (2010).DOI : 10.1016/j.jpowsour.2010.03.077[3] K. Sato et al.,ECS Trans., 7 (1) 455 (2007).DOI: 10.1149/1.2729123[4] K. Kumada et al.,ECS Trans., 91 (1), 825 (2019).DOI: 10.1149/09101.0825ecst[5] T. Tsuchikura et al.,ECS Trans., 91 (1), 579 (2019).DOI: 10.1149/09101.0579ecst[6] D Inaba et al., Transactions of the Japanese Society for Artificial Intelligence 27 (3), 121-132 (2012).DOI: 10.1527/tjsai.27.121[7] D. Inaba et al., Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 7301 LNAI(PART 2), 49–60(2012).DOI: 10.1007/978-3-642-30220-6_5[8] K. Fukui et al.,ECS Trans., 57(1), 571–580 (2013).DOI: 10.1149/05701.0571ecst

Development of High-Entropy Alloy (HEA) Anode for Carbon-Free and Electrochemically-Stable Operation of SOFC on Hydrocarbon Fuels

State of the art SOFCs, consisting of Ni base anode, experience excessive localized endothermic cooling and carbon deposition during internal reforming of hydrocarbon fuels. To mitigate the above issues, we developed high-entropy alloy (HEA) anode consisting of optimized mixture of Cu, Ni, Co, Fe and Mn transition metals. Alloys were synthesized using co-precipitation technique and experimentally evaluated for internal reforming and electrochemical activity under SOFC operating conditions. As-synthesized HEA catalysts were also characterized by techniques including N2 adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. Compared with standard Ni/YSZ and Ni/GDC anodes, HEA anode containing gadolinium-doped ceria (GDC) showed controlled reforming of methane. Raman spectroscopy and SEM microscopy confirmed the absence of carbon on the HEA/GDC anode.

Micro Tubular Solid Oxide Reversible Cell Using LaGaO3 Electrolyte Film Prepared on Channel Size Controlled NiO-YSZ Substrate

Reversible operation of SOFC is now important subjects from energy storage of renewable electric power from solar or wind power. At present, planer type cell design is widely used for this purpose, however, because of tight gas sealing, tubular type cell design is more suitable. In this study, micro tubular solid oxide cell using La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) thin electrolyte film was prepared by dip-coating and co-sintering process.NiO-YSZ porous substrate with 10 mm diameter and 30 mm length was used for preparation of the cell. LaGaO3 based oxide film was deposited with dip coating method. Two kinds of NiO and YSZ particles with different particle size (0.91 and 0.65 mm for NiO and 0.62 and 0.25 mm for YSZ) were mixed and extruded for tubular substrate. 4 types of NiO-YSZ anode porous substrate were Average porosity of each 4 tubular substrate was ca. 70% after reduction. However, from SEM observation, channel size in NiO-YSZ substrate was changed and NiO-YSZ using larger NiO and YSZ particles shows larger channel diameter.Power generation property of 4 tubular cell using different NiO and YSZ particle size for substrate. It was found that the cell deposited on NiO-YSZ using larger particles shows lager power density, in particular, the cell using both NiO and YSZ larger particles shows the largest power density (0.37 W/cm2 at 873 K), which is reasonably large power density. On the other hand, in case of electrolysis, channel size is also strongly influenced on the electrolysis current. As similar manner with fuel cell operation, the LSGM cell prepared on larger particles size of NiO and YSZ shows larger steam electrolysis current. Impedance analysis suggests that the effects of particle size of NiO and YSZ was explained by small concentration overpotential, and so larger channel size is effective for improving the initial performance of the SORC performance. Long term stability of the steam electrolysis was performed and it was found that the LSGM tubular cell prepared on NiO-YSZ with larger particles shows the reasonable stable electrolysis performance in long term stability (-4%/1000h average degradation rate). The main reason for degradation was assigned to the increased IR loss however overpotential was hardly changed over 600 h and so it seems most likely that the reoxidation of NiO or aggregation of NiO is the main reason for degradation of the cell.This study reveals that the control of channel size in NiO-YSZ porous substrate is highly important for initial performance and long-term stability of SORC.

Influence of Microstructure on the Sulfur Tolerance of Ceria-Based Anodes in Low Temperature SOFC

Hydrocarbon fuels such as natural gas, propane or biogas commonly contain small amounts of impurities like sulfur which result in a strong degradation of the cell performance in the intensively studied Ni/YSZ cermet anode [1-3]. The sulfur poisons the nickel catalyst and hinders the electrooxidation of hydrogen [2-3].At common SOFC operation temperatures of 700 to 900 °C the polarization resistance of a Ni/CGO fuel electrode is less influenced by sulfur [3-5]. With a modification of the surface by infiltration of Ni and / or ceria nanoparticles the impact can be even reduced [5-6]. The trend to a much lower operating temperature of SOFCs [7-8] even below 600 °C raises the question to what extend the sulfur tolerance of ceria-based fuel electrodes is maintained.In this study we analyze the impact of H2S on the polarization resistance of ceria-based fuel electrodes at operation temperatures around 600 °C. Ni/ceria anodes, differing in their microstructure, are investigated by electrochemical impedance spectroscopy (EIS). The distribution of relaxation times (DRT) analysis is applied to deconvolute the electrochemical processes followed by a complex nonlinear least square approximation to quantify the loss processes and the impact of sulfur amounts of 0.1-3 ppm. Compared to previous results [5] the polarization resistance increases more significantly at 600 °C than at 750 °C. The influence of the microstructure will be discussed in detail.

Investigation of an Intermediate Temperature SOFC Operation in Series with a CPOx Reformer

Solid oxide fuel cells are known for their fuel flexibility which makes them a great candidate for decentralized power generation and off-grid applications. Depending on the target application, there have been different system configurations such as steam reforming vs. CPOx reforming, external reforming vs. internal reforming, anode recirculation vs. single pass. The CPOx reforming in series with SOFC with single pass configuration opens up the intermittent power source market, due to its unique benefit of low cost, short startup time and easy maintenance. In this scheme fuel cell electrochemical performance is directly related to the syngas composition, i.e. H2 and CO fractions in the fuel stream, as well as any unreformed methane.In this study a computational fluid dynamics (CFD) model has been developed to study the dynamic and steady state behavior of a commercial intermediate temperature SOFC stack operating in series with a CPOx reformer. The CFD model of the SOFC accounts for the charge, species, momentum and energy balances, whereas a reduced order model for the CPOx incorporates equilibrium thermodynamics. CPOx is an exothermic catalytic reaction with an overall balance of:CH4+1/2O2 → 2H2+CO (1)which is defined in stoichiometric ratios. In practice the air flow is regulated to control the reactor temperature and syngas yield. Hence, along with temperature and pressure the ratio of oxygen to carbon (O-C ratio) in the left hand side of Eq. 1 is a critical parameter for determining the overall performance and efficiency of the system. Moreover, the CPOx operation typically has different routes to achieve desired products which constitutes a system with various reaction equilibria [i]. However, apart from Eq.(1) the dominant reaction paths can be summarized as [ii]:Reformation CH4+CO2 ↔ 2H2+2CO (2)Water gas shift CO+H2O ↔ H2+CO2 (3)Boudouard CO2 + C ↔ 2CO (4)CH4 Combustion CH4+2O2 → 2H2O+CO2 (5)CO Combustion 2CO+O2 → 2CO2 (6)H2 Combustion 2H2+O2 → 2H2O (7)Of these, combustion reactions can be practically considered as irreversible. Overall system equilibrium is a result of these reactions and is a function of temperature, pressure and O/C ratio. Any changes in these parameters shift the reaction equilibrium, hence the conversion efficiency and syngas yield. As a result, SOFC performance will be directly dependent on these parameters.In this study different operating regimes of CPOX and SOFC have been investigated. The model is validated experimentally with polarization data and gas chromatography analysis. The model predictions show good agreement with the GC data for both the CPOX and SOFC outlet gas compositions.Performance of the SOFC running on syngas obtained as a result of different CPOx equilibria is compared and a fuel cell efficiency map as a function of temperature and O-C ratio has been constructed. Also, dynamic response of SOFC to the changes in CPOx operating parameters is estimated with the transient simulations. Moreover, selectivity of SOFC toward H2 and CO has been investigated by outlining the different energy conversion paths inside the fuel cell electrode. It is found that SOFC at intermediate temperatures has an overall tendency for CO conversion due to enhanced WGSR reaction inside the electrodes although the standalone CO oxidation overvoltage is higher than that of H2 oxidation. Finally, SOFC performance is analyzed for injecting various amounts of unreformed CH4 into the syngas stream.[i] Bjørn Christian Enger, Rune Lødeng, Anders Holmen, A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts, Applied Catalysis A: General 346 (2008) 1–27.[ii] Ruoshui Ma, Bang Xu, Xiao Zhang, Catalytic partial oxidation (CPOX) of natural gas and renewable hydrocarbons/oxygenated hydrocarbons—A review, Catalysis Today 338 (2019) 18–30. Figure 1

Development of Direct-Ammonia Solid Oxide Fuel Cells (DA-SOFCs) and the Effect of Incorporating Internal Ammonia Decomposition Catalysts

Direct-ammonia solid oxide fuel cells (DA-SOFCs) are among the most efficient power sources that use ammonia. The high operating temperature of SOFCs enables ammonia decomposition reaction to proceed inside the anode compartment of SOFCs. Coupling exothermal SOFC operation and endothermal ammonia decomposition reaction increases the efficiency of the device. DA-SOFCs function as well as hydrogen-fueled ones over 750 °C. Recently we have reported that DA-SOFCs suffer significant performance loss under 650 °C, especially the high-performance cells. We discuss a range of phenomena taking place inside the anode compartment of DA-SOFCs: interconnect, current collector, and anode support. Interconnect and current collector have the capability of decomposing significant amounts of ammonia. Nevertheless, incorporating a catalyst is important at low operating temperatures.

Analysis of Degradation Phenomena of SOC Stacks Operated in Reversible SOFC / SOEC Cycling Mode

The reversible SOFC/SOEC operation of solid oxide cell (SOC) stacks promise high overall electricity-to-electricity round-trip efficiencies and low storage costs. Although in recent years the degradation rates of SOFC and SOEC stacks in single mode long-term operation have been steadily decreased, the understanding of degradation mechanisms during reversible SOFC/SOEC operation remains an important and challenging issue. Therefore, the Korean-German project “Solid Oxide Reversible Fuel Cell / Electrolysis Stack” (SORFES) focuses on the development of the core component technology for a 1 kW reversible SOC stack in order to enhance the hydrogen productivity and its utilization. The primary goals are the improvement of the performance and the durability of SOC stacks during reversible SOFC/SOEC operation and the quantification and the qualification of the relevant degradation effects.The paper presents and compares the performance and degradation results of two SOC stacks which were operated mainly in galvanostatic steady-state SOFC mode and in reversible SOFC/SOEC cycling mode. The stacks with ASC cells of Elcogen (Estonia) were fabricated by the industrial project partner E&KOA (Daejeon, Korea). The reversible cycles consist of day/night switches between SOEC and SOFC, thus covering intermittent renewable electricity supply (e.g. of photovoltaics). The stacks were electrochemically characterized by jV-curves and electrochemical impedance spectroscopy (EIS).The first SOC stack with 10 cells was operated during 500 h in SOFC at constant current density followed by 500 h of operation under reversible SOFC/SOEC cycling conditions. The initial performance and homogeneity along the repeat units (RUs) of the stack in SOFC and SOEC at the beginning of operation are presented. In order to better understand the stack degradation, the results between reversible SOFC/SOEC cycling and galvanostatic steady-state SOFC operation are compared. The degradation, especially of the OCV, the power density and the area specific resistance (ASR) of the different RUs are analyzed and discussed. Moreover, the progression of the individual resistances, specifically of the ohmic-, the electrode polarization- and the gas concentration resistances of the RUs are evaluated and presented. The influence of temperature gradients and thermo-mechanical stresses during reversible exothermic (SOFC) and endothermic (SOEC) cycling are outlined and discussed.The results of the first stack test were used to improve the stack components and setup, e.g. the contacting and sealing of the cells in the stack and the protective coating on the interconnects. Moreover, the operating conditions during reversible SOFC/SOEC cycling were optimized. The second improved stack with 6 RUs was operated for 2800 h in galvanostatic steady-state SOFC mode and reversible SOFC/SOEC cycling mode with low degradation rate.The results of the present paper help to understand and improve the long-term stability of SOC stacks during reversible SOFC/SOEC cycling, thus promoting the SOC technology for renewable energy storage applications.

Direct Ammonia SOFC-Gas Turbine Hybrid Systems Modeling for Heavy-Duty Transportation Sector

Ammonia currently stands as being the second most produced chemical in the world [1]. While it is prominently used in the manufacturing of fertilizers, increase in production capacity through renewable sources such as green hydrogen and its potential as a carbon-neutral energy carrier make it a promising candidate to replace fossil fuels for power generation in the future [2]. Moreover, when compared to liquid hydrogen, ammonia can be stored at more practical temperatures (-253⁰C vs -33⁰C at 1 bar) and has a higher volumetric energy density (8.5 MJ/L vs 12.7 MJ/L) [3]. Though the toxicity of ammonia as a fuel is well known, there are well established safety and handling procedures for storage and transport, given its ubiquitous existence as an essential commodity required for agriculture.From a systems perspective, residual chemical exergy available in the SOFC anode gas exhaust can be utilized through a gas turbine to enable increased efficiency [4]. Direct ammonia SOFCs (DASOFCs) can eliminate the need for external cracking to hydrogen, thereby reducing system complexity. In addition, heat loss and thermal management in the stack periphery may be effectively managed via in-situ endothermic decomposition of ammonia [5]. Integration in a hybrid system is also beneficial to SOFCs due to the associated heat of compression for pressurized operation. The different physicochemical timescales of electrochemical fuel cells and mechanical turbomachinery are advantageous to dynamic operation which is important because of fluctuating power loads in a mobile application.In this work, the potential of an ammonia-based hybrid SOFC/GT system concept is assessed. A multiscale stack simulation using constitutive relations for heat, mass and charge transfer based on high power density SOFCs is integrated with turbomachinery and other auxiliary system components within the gPROMS process simulation framework. State variables for each point can then be defined to estimate the operating boundaries of the system.This presentation discusses the thermodynamics of SOFC operation and system concepts to utilize ammonia in a hybrid system for a kW-scale transportation application. This involves evaluation of system indicators such as thermodynamic efficiency, fuel utilization and specific power which can help determine optimal performance. The efficiency and specific power are dictated by the operating temperature of the fuel cell, preheating requirements and the relative utilization of fuel between the SOFC and gas turbine. The proposed system concept is shown to achieve LHV efficiencies of up to 60% (LHV) within reasonable operating limits. The presentation includes an examination of the prospects of ammonia as a power source for the mobility sector in comparison to contemporary and future renewable fuels, and concludes with practical challenges for deployment.