Development and evaluation of a 3-cell stack of metal-based solid oxide fuel cells fabricated via a sinter-joining method for auxiliary power unit applications

Recently, metal-based solid oxide fuel cells (SOFCs) receive much attention for potential application in auxiliary power units (APUs). In this study, a sinter-joining method with a silver bonding layer is proposed. This method enables the fabrication of metal-based SOFCs by joining metal plates and conventional ceramic cells using a silver bonding layer. This sinter-joining method has the advantage of full-sintering of the cathode at 1100 °C, which facilitates a lower area specific resistance (ASR) of the cathode. Furthermore, the entire manufacturing process is conducted under air atmosphere. A 5 × 5 cm² metal-based cell is successfully fabricated by the sinter-joining method, and a maximum power density of 433 mW cm^?2 and a low polarization resistance of 0.12 Ω cm² is obtained. Using the metal-based cells, a prototype 3-cell SOFC stack is developed considering mechanical robustness and diesel reformate fuel supply for future APU system applications. The stack exhibits a maximum power density of 100 mW cm^?2 and is tested for 120 h. After the test, a post-mortem analysis is conducted, and the causes of the low electrochemical performance and degradation issue are investigated. In the conclusion, the sinter-joining method is considered as one of the methods for metal-based SOFCs.     

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.     

Effect of bi-layer interconnector design on mass transfer performance in porous anode of solid oxide fuel cells

We propose a novel interconnector design, termed bi-layer interconnector, for solid oxide fuel cells (SOFCs). It can disturb the fuel gas and air on the planes normal to the SOFC three-phase-boundary (TPB) layer. In this paper, a two-dimensional half-cell model is developed to study the concentration overpotentials in the fuel side of the SOFC stack with conventional and novel bi-layer interconnectors. The numerical results show that the novel bi-layer interconnector can increase the velocity of the fuel gas in the porous anode. The results of mole fraction distribution illustrate that the novel bi-layer interconnector can effectively disturb the fuel flow. The average H₂ mole fraction in the porous anode of SOFC with bi-layer interconnector is about 4.7% higher than that of conventional SOFC. The average H₂ mole fraction at TPB interface is about 9.2% higher. The concentration overpotential of the novel SOFC design is lower than that of the conventional SOFC design by 5%. It can enhance the mass transfer in porous electrode and improve the performance of SOFC.     

Performance enhancement of planar solid oxide fuel cell using a novel interconnector design

This study presents a novel interconnector design, termed groove and rib-finned interconnector, to improve the performance of the planar solid oxide fuel cell (SOFC). We have conducted a detailed comparative study on the flow characteristics and electrical performance of conventional straight channel interconnectors and novel interconnectors through a three-dimensional model. Compared with the conventional straight channel interconnector, the result shows that the novel interconnector can provide higher fuel utilization, and the output power density at a low fuel flow is still higher than that of the conventional design at a high fuel flow. The novel interconnector increases the velocity and vorticity of the reactant gas, and promotes gas disturbance, and enhances the mass transport in the electrode. The novel interconnector eliminates the oxygen-free zone of the cathode under the rib and provides sufficient oxygen with uniform concentration distribution for the electrochemical reaction. Therefore, the novel interconnector significantly reduces the activation and concentration overpotentials and improves the electrical performance of the SOFC stack.     

A novel interconnector design of SOFC

In this study, a novel interconnector design is proposed, which is named as the X-type interconnector. The solid oxide fuel cell (SOFC) models are established for the conventional interconnector and the X-type interconnector. The results indicate that the design of the X-type interconnector is beneficial to the transport of gas in SOFC, which has a higher oxygen concentration under rib than that of the conventional interconnector. Furthermore, compared with the X-type interconnector, the potential difference in cathode is larger for the conventional interconnector, which indicates that the X-type interconnector reduces the current path and improves the performance of SOFC. For any porosity and conductivity of anode and cathode, the X-type interconnector is superior to the conventional interconnector. Moreover, when the cathode conductivity is smaller, the advantage of the X-type interconnector becomes more remarkable.     

Development of a four‐cell DMFC stack with air‐breathing cathode and dendrite flow field

A four‐cell direct methanol fuel cell (DMFC) stack with an air‐breathing cathode with an active area of 0.48 cm² for each cell is designed, fabricated and tested. A pure copper sheet 300 µm thick with innovative perforated flow plates (dendrite type) is fabricated and used for the cathode. For the anode, conventional serpentine flow channels made of pure copper sheets 250 µm thick are used. An extensive parametric study is conducted to determine the optimum working conditions for the fuel flow rate (anode), methanol solution concentration, channel‐to‐land ratio and stack temperature. Comparisons are made with conventional serpentine flow channels. In addition, CO₂ (water) bubbles in the anode (cathode) channels are visualized, and the results are presented and discussed. It is found that the maximum stack power of the four‐cell μDMFC stack is up to 40 mW/cm² with a limiting current density of 335 mA/cm² at a maximum volumetric and gravimetric power density of 11.16 mW/cm³ and 3.13 W/kg, respectively. Copyright © 2014 John Wiley & Sons, Ltd.     

Three-dimensional simulation of solid oxide fuel cell with metal foam as cathode flow distributor

In this study, the use of metal foam as a flow distributor at cathode is evaluated numerically by a comprehensive three-dimensional solid oxide fuel cell (SOFC) model. The results show that the adoption of metal foam improves the power density by 13.74% at current density of 5000 A m⁻² in comparison with conventional straight channel design. It is found that electronic overpotential, oxygen concentration and reaction rates distribute more uniformly without the restriction of ribs. The effects of cathode thickness on the two different flow distributors are compared. Compared with conventional straight channel, the metal foam is found to be more suitable as a distributor for anode supported SOFC with thin cathode gas diffusion layer. Moreover, when metal foam is applied to the fuel cell with a larger reaction area, a more uniform velocity distribution and a lower temperature distribution can be achieved. It is also found that an appropriate permeability coefficient should offer a reasonable pressure drop, which is beneficial for the fuel cell system performance improvement.     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

Setup and experimental validation of a 5 kW HT-PEFC stack

This paper presents a performance analysis of a 5 kW_el high temperature polymer electrolyte fuel cell (HT-PEFC) stack. Stack design and sizing is adapted to auxiliary power unit (APU) applications assuming the use of middle distillates. The parameter study comprises the variation of the fuel type (reformate, with pure hydrogen as a reference), the stoichiometry on the anode (1.3–2.7) and cathode (1.25–4.0) sides and the carbon monoxide (CO) concentration (0.9–3.5%) in the reformate. At 0.5 A cm^?2, a coolant inlet temperature of 160 °C, stoichiometric factors of 1.3 on the anode and 2 on the cathode side and reformate operation, two interconnected full stacks produced 5 kW of electric power. The focus of the present work is an examination of the robustness of the full stacks through an analysis of the 70 single cell voltages. By comparison to other published operational parameter studies, this paper makes a significant contribution to the application of the methodology not to single cells or short stacks, but to a stack with technical relevance in the high power class.     

Electrochemical performance assessment of low-temperature solid oxide fuel cell with YSZ-based and SDC-based electrolytes

In this work, solid oxide fuel cells (SOFCs) based on different electrolytes, i.e., the yttria-stabilized zirconia (YSZ) and the samaria-doped ceria (SDC), were investigated to study their performances at low-temperature operation. The predicted performance of both SOFCs was validated with the experimental results. The verified models were implemented to study the impact of operating conditions, i.e., cell temperature, pressure, thicknesses of cathode, anode, and electrolyte, on their performances. The decrease in the operating temperature from intermediate range (800–900 °C) to low range (550–650 °C) has a considerable effect on the performance of the YSZ-based SOFC as conventional type, which dropped from 0.67–1.40 W/cm² to 0.027–0.13 W/cm². Under the low operating temperature range, the performance of SDC-based SOFC was superior to that of the YSZ-based SOFC, due to the lower ohmic loss. Nevertheless, the SDC-based SOFC has higher concentration overpotentials than the YSZ-based SOFC. The concentration overpotentials of the SDC-based SOFC can be reduced by the thinner anode and cathode thicknesses. In addition, the SDC-based SOFC at low operating temperature with the pressurized operation could significantly improve its power density, about 20% at 2 bar, which was close to that of YSZ-based SOFC at intermediate temperature of 800 °C.     

Time dependent failure probability estimation of the solid oxide fuel cell by a creep-damage related Weibull distribution model

In present paper, a new model is proposed and embedded into the finite element software ABAQUS to estimate the time dependent failure probability of the solid oxide fuel cell stack. The results show that sealant is the potential failure region of the solid oxide fuel cell stack, while the failure probability of the anode, electrolyte and cathode are very small within the operation time of 50,000 h. The creep and damage distribution of the components reflect that the proposed model can reasonably predict the time dependent failure probability of the solid oxide fuel cell stack. Increasing either the characteristic strain, Weibull modulus or decreasing the operating temperature can decrease the failure probability of the SOFC stack. For the sealant, to ensure the high temperature integrity of the SOFC stack, the characteristic strain should be larger than 0.01 or Weibull modulus should be higher than 8.0 under the operating temperature of 600 °C.     

Development of metal supported solid oxide fuel cells based on powder metallurgical manufacturing route

Abstract

In the past few decades, stationary solid oxide fuel cell (SOFC) systems have been developed that can generate electricity and heat from the energy stored in hydrogen or hydrocarbons with total efficiencies up to 95%. While the mechanical cell support of stationary systems is commonly supplied by thick ceramic cell components (i.e. anode and electrolyte supported concepts), mobile systems demand a more robust design. This is ensured by a strong yet porous metallic substrate which serves as the mechanical backbone of thin film membrane electrode assemblies [metal supported cell (MSC) concept]. Porous PM Fe–Cr oxide dispersion strengthened alloys for use as MSC supports have recently been developed. These materials provide mechanical and chemical long term stability in typical SOFC atmospheres at operation temperatures up to 850°C. The substrates support a multilayer anode–electrolyte–cathode thin film assembly, constituting a high performance MSC repeat unit. These units are the building blocks for MSC stacks with superior properties for mobile applications.     

Porous Ni/8YSZ anode of SOFC fabricated by the plasma sprayed method

In the present study, porous electrode coating of Ni/8YSZ on the interconnector material was made by the plasma-spraying. By introducing the pore former into the composite powder, the porous structure of SOFC anode will be obtained. By using the plasma spraying technique for SOFC fabrication, we can avoid the thermal failure between the components of SOFC which made from the traditional sintering method at high temperature. In this study, two kinds of composite powders in the granulate form were prepared, one with the nano carbon as a pore former and the other without the carbon. The results showed that the porous structure of SOFC anode could be achieved by the plasma spraying technique. The porosity of the anode made from the composite powder with pore former was 40%. Without pore former the porosity in the anode coating after hydrogen reduction was almost 30%. These results suggest that this method exhibits the potential to manufacture the porous ceramic/metal composite anode of SOFC to achieve the larger triple phase boundary for fuel oxidation.     

Fabrication and characterization of metal-supported solid oxide fuel cells

Metal-supported solid oxide fuel cells (SOFCs) are an acceptable approach to solving the serious problems of SOFC technology, such as sealing and mechanical strength. In this work, commercial stainless-steel plates, STS430, are used as supporting bodies for a metal-supported SOFC in order to decrease the number of fabrication steps. The metal support for a single-cell has a diameter of 28 mm, a thickness of 1 mm, and a channel width of 0.4 mm. A thin ceramic layer, composed of yttria-stabilized zirconia (YSZ) and NiO/YSZ, is attached to the metal support by using a cermet adhesive. La_0.8Sr_0.2Co_0.4Mn_0.6O₃ perovskite oxide serves as the cathode material because of its low impedance on the YSZ electrolyte, according to half-cell tests. The maximum power density of the cell is 0.09 W cm⁻² at 800 °C. The effects of temperature, oxygen partial pressure, and current collection by pastes are investigated. The oxygen reduction reaction at the cathode dominates the overall cell performance, according to experimental and numerical analyses.     

Experimental study on dual recirculation of polymer electrolyte membrane fuel cell

Durability and start-up ability in sub-zero environment are two technical bottlenecks of vehicular polymer electrolyte membrane (PEM) fuel cell systems. With exhaust gas recirculation on the anode and cathode side, the cell voltage at low current density can be reduced, and the membrane can be humidified without external humidifier. They may be helpful to prolong the working lifetime and to promote the start-up ability. This paper presents an experimental study on a PEM fuel cell system with anodic and cathodic recirculation. The system is built up based on a 10 kW fuel cell stack, which consists of 50 cells and has an active area of 261 cm². A cathodic recirculation pump and a hydrogen recirculation pump are utilized on the cathode and anode side, respectively. Key parameters, e.g., stack current, stack voltage, cell voltage, air flow, relative humidity on the cathode side, oxygen concentration at the inlet and outlet of the cathode side, are measured. Results show that: 1) with a cathodic recirculation the system gets good self-humidification effect, which is similar to that with an external humidifier; 2) with a cathodic recirculation and a reduction of fresh air flux, the cell voltage can be obviously reduced; 3) with an anodic recirculation the cell voltage can also be reduced due to a reduction in the hydrogen partial pressure, the relative humidity on the cathode side is a little smaller than the case with only cathode recirculation. It indicates that, for our stack the cathodic recirculation is effective to clamp cell voltage at low current density, and a self-humidification system is possible with cathodic recirculation. Further study will focus on the dynamic model and control of the dual recirculation fuel cell system.     

Effect of electrical current on the oxidation behavior of electroless nickel-plated ferritic stainless steel in solid oxide fuel cell operating conditions

Planar solid oxide fuel cell (SOFC) systems often employ metallic interconnects, which separate and connect individual cells in electrical series to create a stack. Coated and uncoated ferritic stainless steels (FSSs), are reported among the most promising materials currently being investigated for interconnect applications. In this study, FSS AISI 441 samples coated with electroless nickel (～25 μm) were subjected to intermediate temperature IT-SOFC operating conditions at 700 °C for 500 h with and without the application of electrical current (0.5 Acm^?2). The application of the electric current promotes Fe migration on both the cathode and the anode side. This phenomenon results in the formation of a ～4 μm thick Fe₂O₃ on the anode side responsible for increased ASR values. Comparative analyses of the current and no current exposures and resultant surface oxide layers, along with suspected mechanisms and implications are presented and discussed.     

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm² at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm² current density and 700 °C.     

Porous nickel-iron alloys as anode support for intermediate temperature solid oxide fuel cells: II. Cell performance and stability

Porous nickel–iron alloy supported solid oxide fuel cells (SOFCs) are fabricated through cost-effective ceramic process including tape casting, screen printing and co-sintering. The cell performance is characterized with humidified hydrogen as the fuel and flowing air as the oxidant. Effects of iron content on the cell performance and stability under redox and thermal cycle are investigated from the point of view of structural stability. Single cells supported by nickel and nickel–iron alloy (50 wt % iron) present relatively high discharge performance, and the maximum power density measured at 800 °C is 1.52 and 1.30 W cm^?2 respectively. Nickel supported SOFC shows better thermal stability between 200 and 750 °C due to its dimensional stable substrate under thermal cycles. Posttest analysis shows that a dense iron oxide layer formed on the surface of the nickel-iron alloy during the early stage of oxidation, which prevents the further oxidation of the substrate as well as the functional anode layer, and thus, making nickel-iron supported SOFC exhibits better redox stability at 750 °C. Adding 0.5 wt % magnesium oxide into the nickel-iron alloy (50 wt% iron) can inhibit the metal sintering and reduce the linear shrinkage, making the single cell exhibit promising thermal stability.     

Coating layer and influence of transition metal for ferritic stainless steel interconnector solid oxide fuel cell: A review

Progressive efforts on lowering working temperature of Solid Oxide Fuel Cell (SOFC) to approximately 600 °C enable application of stainless steel as interconnector instead of expensive ceramic. Prolonged exposure of stainless steel to SOFC operating conditions can lead to chromium poisoning owing to migration of chromium (IV) species to cathode and significantly reduces electrical conductivity of cells. Ferritic stainless steel is potential candidate as interconnector because it has appropriate chromium content and is less expensive compared other stainless steels. Protective coating layer on interconnector is essential in minimizing chromium poisoning phenomenon in aspect of area specific resistance (ASR), thermal expansion coefficient (TEC) and coating uniformity. Addition of transition metal to coating layer is an enhanced method to improve coating behavior. Nickel, copper, manganese and silver are promising metals that can be used as coating layer to inhibit chromium (IV) species from diffusing outward and improve electrical conductivity and excellent oxidation resistance. This paper reviews oxidation behavior of coating layer of interconnector of SOFC, sintering effect, protective coating technique and influence of transition metal in the coating layer.     

Development of cube-type SOFC stacks using anode-supported tubular cells

Tubular SOFCs have shown many desirable characteristics such as high thermal stability during rapid heat cycling and large electrode area per unit volume, which can accelerate to realize SOFC systems applicable to portable devices and auxiliary power units for automobile. So far, we have developed anode-supported tubular SOFCs with 0.8–2 mm diameter using Gd-doped CeO₂ (GDC) electrolyte, NiO-GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)-GDC cathode. In this study, a newly developed cube-type SOFC stack which consists of three SOFC bundles was designed and examined. The bundle consists of three 2 mm diameter tubular SOFCs and a rectangular shaped cathode support where these tubular cells are arranged in parallel. The performance of the stack whose volume is less than 1 cm³ was shown to be 2.8 V OCV and over 1 W at 1.6 V under 500 °C. Cathode loss factor due to current collection from cathode matrix was also estimated using a proposed model.