Cube-type micro SOFC stacks using sub-millimeter tubular SOFCs

Fabrication and characterization of tubular SOFCs under sub-millimeter (0.8 mm), bundles and stacks for low temperature operation were shown. The materials used in this study were Gd doped CeO₂ (GDC) for electrolyte, NiO–GDC for anode and (La, Sr)(Co, Fe)O₃ (LSCF)–GDC for cathode, respectively, and LSCF for supports of the tubular cells for bundle fabrication. After applying a sealing layer and current collector for each bundle of five micro tubular SOFCs, each bundle was stacked vertically, to build a four-storey cube-type stack with volume of about 0.8 cm³. The performance of the stack was shown to be 3.6 V OCV and 2 W maximum output power under 500 °C operating temperature. Preliminary quick start-up test was also conducted at the condition of 3 min start-up time from 150 to 400 °C for 5 times, and the results showed no degradation of the performance during the test.     

Performance of an anode-supported tubular solid oxide fuel cells stack with two single cells connected by a co-sintered ceramic interconnector

Two anode-supported tubular solid oxide fuel cells (SOFCs) have been connected by a co-sintered ceramic interconnector to form a stack. This novel bilayered ceramic interconnector consists of La-doped SrTiO₃ (La_0.4Sr_0.6TiO₃) and Sr-doped lanthanum manganite (La_0.8Sr_0.2MnO₃), which is fabricated by co-sintering with green anode at 1380 °C for 3 h. La_0.4Sr_0.6TiO₃ (LST) acts as a barrier avoiding the outward diffusion of H₂ to the cathode; while La_0.8Sr_0.2MnO₃ (LSM) prevents O₂ from diffusing inward to the anode. The compatibility of LST and LSM, as well as their microstructure which co-sintered with anode are both studied. The resistances between anode and LST/LSM interconnector at different temperatures are determined by AC impedance spectra. The results have showed that the bilayered LST/LSM is adequate for SOFC interconnector application. The active area is 2 cm² for interconnector and 16 cm² for the total cathode of the stack. When operating at 900 °C, 850 °C, 800 °C with H₂ as fuel and O₂ as oxidant, the maximum power density of the stack are 353 mW cm⁻², 285 mW cm⁻² and 237.5 mW cm⁻², respectively, i.e., approximately 80% power output efficiency can be achieved compared with the total of the two single cells.     

Development of a 700 W anode-supported micro-tubular SOFC stack for APU applications

A 700 W anode-supported micro-tubular solid-oxide fuel cell (SOFC) stack for use as an auxiliary power unit (APU) for an automobile is fabricated and characterized in this study. For this purpose, a single cell was initially designed via optimization of the current collecting method, the brazing method and the length of the tubular cell. Following this, a high-power single cell was fabricated that showed a cell performance of at 0.7 V and using H₂ (fuel utilization=45%) and air as fuel and oxidant gas, respectively. Additionally, a fuel manifold was designed by adopting a simulation method to supply fuel gas uniformly into a single unit cell. Finally, a 700 W anode-supported micro-tubular SOFC stack was constructed by stacking bundles of the single cells in a series of electrical connections using H₂ (fuel utilization=49%) and air as fuel and oxidant gas, respectively. The SOFC stack showed a high power density of ; moreover, due to the good thermo-mechanical properties of the micro-tubular SOFC stack, the start-up time could be reduced by 2 h, which corresponds to 6/min.     

Fabrication and operation of a 1 kW class anode-supported flat tubular SOFC stack

A 1 kW class anode-supported flat tubular SOFC stack for intermediate temperature (700–800 °C) operation was fabricated and operated in this study. For this purpose, we fabricated anode-supported flat tubular cells by optimization of the current collecting method and the induction brazing process. After that, we designed a compact fuel and air manifold by adopting a simulation technique to uniformly supply fuel and air gas into the stack and a unique seal and insulation method to make a more compact stack. To assemble the stack, the prepared anode-supported flat tubular cells with an effective electrode area of 90 cm² were connected in series to 30 bundles, in which one unit bundle consists of two flat tubular cells connected in parallel. The performance of the stack in 3% humidified H₂ and air at 750 °C showed a maximum electrical power of 921 W (fuel utilization ratio = 25.2%).     

Cathode contact optimization and performance evaluation of intermediate temperature-operating solid oxide fuel cell stacks based on anode-supported planar cells with LaNi0.6Fe0.4O3 cathode

This paper reports the development of intermediate temperature-operating solid oxide fuel cell stacks using anode-supported planar cells with LaNi_0.6Fe_0.4O₃ (LNF)cathode. We developed metallic separators with radial gas flow channels and an anode seal structure. To achieve good power-generating characteristics, we propose two cathode contact methods. According to a performance evaluation at 800 °C, power density of 0.5 W cm⁻² is obtained at the current density of 1.0 A cm⁻² when operating with a sufficient fuel amount, and power conversion efficiency of over 50% LHV is obtained at the current density of more than 0.2 A cm⁻² when operating at a high fuel utilization rate.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

3D modeling of anode-supported planar SOFC with internal reforming of methane

Deposition of carbon on conventional anode catalysts and formation of large temperature gradients along the cell are the main barriers for implementing internal reforming in solid oxide fuel cell (SOFC) systems. Mathematical modeling is an essential tool to evaluate the effectiveness of the strategies to overcome these problems. In the present work, a three-dimensional model for a planar internal reforming SOFC is developed. A co-flow system with no pre-reforming, methane fuel utilization of 75%, voltage of 0.7 V and current density of 0.65 A cm⁻² was used as the base case. The distributions of both temperature and gas composition through the gas channels and PEN (positive electrode/electrolyte/negative electrode) structure were studied using the developed model. The results identified the most susceptible areas for carbon formation and thermal stress according to the methane to steam ratio and temperature gradients, respectively. The effects of changing the inlet gas composition through recycling were also investigated. Recycling of the anode exhaust gas, at an optimum level of 60% for the conditions studied, has the potential to significantly decrease the temperature gradients and reduce the carbon formation at the anode, while maintaining a high current density.     

Testing of a cathode fabricated by painting with a brush pen for anode-supported tubular solid oxide fuel cells

We have studied the properties of a cathode fabricated by painting with a brush pen for use with anode-supported tubular solid oxide fuel cells (SOFCs). The porous cathode connects well with the electrolyte. A preliminary examination of a single tubular cell, consisting of a Ni-YSZ anode support tube, a Ni-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode fabricated by painting with a brush pen, has been carried out, and an improved performance is obtained. The ohmic resistance of the cathode side clearly decreases, falling to a value only 37% of that of the comparable cathode made by dip-coating at 850 °C. The single cell with the painted cathode generates a maximum power density of 405 mW cm⁻² at 850 °C, when operating with humidified hydrogen.     

A corrugated mesoscale structure on electrode-electrolyte interface for enhancing cell performance in anode-supported SOFC

For enhancing the power density of a solid oxide fuel cell, mesoscale-structure control of electrode-electrolyte interfaces in an anode-supported cell is proposed. We define ‘mesoscale’ as a size range of the order of 10-100 μm which is larger than the ‘microscale’ of electrode particles but smaller than the ‘macroscale’ of cell geometries. Mesoscale-structure control enlarges the electrode-electrolyte interface, and this enlargement extends an active electrochemical reaction zone where a charge-transfer reaction occurs actively near the interface. A corrugated mesoscale electrolyte was adopted which enlarged the interface structures of both anode and cathode sides. We performed a 2-D numerical simulation, and discussed the effects of such structure not only on the overall performance but also on the detailed distributions of electric potentials, gas concentrations and local electrochemical reaction rate. As a result, it was observed that the corrugated mesoscale structure reduced both activation overpotential and ohmic loss by ion transport, and hence enhanced the power generation performance. When the interface area enlargement factor was 1.73, an enhancement of a power density having a maximum value of 59% was achieved with the mesoscale-corrugated cell rather than with the flat cell.     

Performance simulation for an anode-supported SOFC using Star-CD code

Experimental activities and computational fluid dynamics (CFD) simulation are presented in this paper for investigating the performance of an anode-supported solid oxide fuel cell (SOFC). The goal of this work is to assess a commercial CFD code, Star-CD with es-sofc module, to simulate the current–voltage (I–V) characteristics with respect to the experimental data. Compiled with the geometry of cell test housing, a 3D numerical model and test conditions were established to analyze the anode-supported cell (ASC) performance including current density and temperature distributions, fuel concentration, and fuel utilization. After adjusting parameters in the electrochemical model, the simulation results showed good agreements with the experimental data. The results also revealed that the power density increased while the fuel utilization decreased as the fuel flow rate increased.     

Experimental characterization of an anode-supported tubular SOFC generator fueled with hydrogen, including a principal component analysis and a multi-linear regression

Solid oxide fuel cell (SOFC) power generators can now be commercialized as heat and power micro-cogenerator. Few well-documented field tests have been conducted to date on these units’ tubular cell architecture, however, and little has been done to derive general rules for a thorough understanding of these units’ operation. The present work focuses on characterizing the hydrogen-powered Acumentrics Gen521 (rated 2.5 kW) under various stable conditions. A test rig was installed at the Dipartimento di Energetica of the Università Politecnica delle Marche (Ancona, Italy) to ascertain the main characteristic curves of the Acumentrics Gen521. A multivariate data analysis was performed on the experimental data collected to establish the operating parameters most influential for the stack voltage (SV) and the DC stack output power generated in different working conditions. Some multi-linear response surfaces are suggested for predicting the SV and the DC power in different operating conditions.     

Low temperature anode-supported solid oxide fuel cells based on gadolinium doped ceria electrolytes

The utilization of anode-supported electrolytes is a useful strategy to increase the electrical properties of the solid oxide fuel cells, because it is possible to decrease considerably the thickness of the electrolytes. We have successfully prepared single-chamber fuel cells of gadolinium doped ceria electrolytes Ce_1−xGd_xO_2−y (CGO) supported on an anode formed by a cermet of NiO/CGO. Mixtures of precursor powders of NiO and gadolinium doped ceria with different particle sizes and compositions were analysed to obtain optimal bulk porous anodes to be used as anode-supported fuel cells. Doped ceria electrolytes were prepared by sol–gel related techniques. Then, ceria-based electrolytes were deposited by dip coating at different thicknesses (15–30 μm) using an ink prepared with nanometric powders of electrolytes dispersed in a liquid polymer. Cathodes of La_1−xSr_xCoO₃ (LSCO) were also prepared by sol–gel related techniques and were deposited on the electrolyte thick films. Finally, electrical properties were determined in a single-chamber reactor where propane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Flux rate values of the carrier gas and propane partial pressure were determinants for the optimization of the electrical properties of the fuel cells.     

Radiative and convective heat transport within tubular solid-oxide fuel-cell stacks

This paper develops a relatively simple model that is intended to rapidly evaluate design configuration and operating conditions for tubular anode-supported solid-oxide fuel-cell (SOFC) stacks. Heat is removed from the SOFC tubes by a combination of convection and radiation. Heat is convected to air that circulates outside the SOFC tubes and radiated to a surrounding cylindrical wall. The tubes are assumed to be arranged in hexagonal arrays in which the distance between tubes centers form equilateral triangles. The paper presents new configuration-factor formulas that are needed to represent arrays of staggered cylinders. The configuration factors are derived for long cylinders using the crossed-string method. These configuration factors have general utility beyond the application to fuel-cell systems. The model is applied to a particular cell and stack system and used to evaluate the effects of a range of design and operating conditions.     

Micro-tubular solid oxide fuel cells and stacks

The properties and performance of micro-tubular solid oxide fuel cells are compared and the differentiating factors discussed. The best recorded power density for a single cell in the literature to date is 1.1 W cm⁻², with anode microstructure and current collection technique emerging as two key factors influencing electrical performance. The use of hydrocarbon fuels instead of pure hydrogen and methods for reducing the resultant carbon deposition are briefly discussed. Performance on thermal and reduction-oxidation (RedOx) cycling is also a critical issue for cell durability. Combining these individual cells into stacks is necessary to obtain useful power outputs. As such, issues of fluid and heat transfer within such stacks become critical, and computational modelling can therefore be a useful design tool. Experimentally tested stacks and stack models are discussed and the findings summarised. New results for a simple stack manufactured at the University of Birmingham are also given.     

RedOx study of anode-supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet (ceramic-metal composite) anode of nickel with yttria stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this technology is the tolerance of the anode towards reduction and oxidation (“RedOx”) cycles.In this study, two techniques are used to quantify the anode expansion after a RedOx cycle of the nickel at different temperatures. The first method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. In the second method, the anode porosity is measured using scanning electron microscopy (SEM) image quantification. The same measurement techniques are used to quantify anode expansion after consecutive RedOx cycles at constant temperature.The quantification technique is then applied to cells tested in real stack conditions. The cell corners can undergo several RedOx cycles depending on stack design and fuel utilization. The study of such zones allows estimating the number of cycles that the anode experienced locally.     

Redox-induced performance degradation of anode-supported tubular solid oxide fuel cells

Redox behavior of a Ni-Y₂O₃-stabilized ZrO₂ (YSZ) composite anode support and the performance degradation of an anode-supported tubular solid oxide fuel cell (SOFC) were studied under complete oxidation and reduction conditions (degrees of oxidation and reduction = 100%). Materials characterization studies showed that the exposure time in oxidizing and reducing atmospheres played a critical role in the degradation of the porous structures and the physical properties of the anode support. In particular, the redox cycling with an 8 h exposure time resulted in the cracking of YSZ network, leading to significant decay of the mechanical strength. The polarization experiments on the redox-cycled anode-supported tubular cell showed serious performance degradation as a result of the decreases of open-circuit potential and power density. The ac-impedance measurements combined with microstructural observations indicated that the performance degradation resulted mainly from (i) the degradation of anode support, (ii) microcracks across the whole cell, and (iii) interface delamination.     

CFD model for tubular SOFC stack fed directly by biomass

The energy transition can also be promoted by the sustainable use of biomass. Residual biomass in the Mediterranean areas can be exploited to a greater extent through highly efficient fuel cell systems. The Direct Biomass-SOFC project is based on a direct coupling between biomass power supply and SOFC tubular cells. This research project stems from the need to cover the electricity demand, avoiding the use of non-renewable sources. It will be investigated the unused or little-used biomass sources that can be exploited from the Mediterranean area.To this purpose, analyses were conducted to model a SOFC tubular cell stack by investigating the optimal configuration. The basic objective is to design a SOFC tubular cell stack, fed by syngas to produce at least 200 W. Two configurations were chosen: a square and a circular arrangement. Another objective of the study is to choose the best temperature control system. It have been selected a pressurised water system and an air system. The results show that the best performance is guaranteed by a square arrangement with an air temperature control system. The circular configuration provides less power than the square configuration, being limited by the multiple series connection to the lowest current value. The maximum electrical power produced with the square configuration is 225 W.     

Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.     

Thermochemical model and experimental validation of a tubular SOFC cell comprised in a 1 kWel stack designed for μCHP applications

Being aware of the needs for clean highly efficient micro combined heat and power (μCHP) systems for single and multifamily households, the Italian Ministry of Industry launched in 2009 the EFESO Project aiming to develop and operate four SOFC prototypes. An imperative part of the project foresaw computational modeling to optimize operating conditions of the power modules and pinpoint potential drawbacks in its design. This article deals with a 3-dimensional thermochemical model of a single SOFC tubular geometry cell comprised in a 1kW_el stack operating under similar conditions to the characterized power module. An analysis is presented on the effects of current density distribution, temperature distribution in the cell and pressure drop in the air and fuel channels, being these the most critical variables when operating the SOFC-powered μCHP system. This model will serve as a platform to generate a model of the whole stack which will be further validated by means of experimental activities.