Performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack fabricated by dip coating technique

A simple and feasible technique is developed successfully to fabricate the cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack. The cone-shaped tubular anode substrates and yttria-stabilized zirconia (YSZ) electrolyte films are fabricated by dip coating technique. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 35.9 μm is successfully obtained. The single cell, NiO–YSZ/YSZ/LSM–YSZ, provides a maximum power density of 1.08 and 1.35 W cm⁻² at 800 and 850 °C, respectively, using moist hydrogen (75 ml/min) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC was assembled and tested. The maximum total power at 800 °C was about 3.7 W.     

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.     

Micro-tubular solid oxide fuel cells with graded anodes fabricated with a phase inversion method

Micro-tubular proton-conducting solid oxide fuel cells (SOFCs) are developed with thin film BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) electrolytes supported on Ni-BZCYYb anodes. The substrates, NiO-BZCYYb hollow fibers, are prepared by an immersion induced phase inversion technique. The resulted fibers have a special asymmetrical structure consisting of a sponge-like layer and a finger-like porous layer, which is propitious to serving as the anode supports for micro-tubular SOFCs. The fibers are characterized in terms of porosity, mechanical strength, and electrical conductivity regarding their sintering temperatures. To make a single cell, a dense BZCYYb electrolyte membrane about 20 μm thick is deposited on the hollow fiber by a suspension-coating process and a porous Sm_0.5Sr_0.5CoO₃ (SSC)-BZCYYb cathode is subsequently fabricated by a slurry coating technique. The micro-tubular proton-conducting SOFC generates a peak power density of 254 mW cm⁻² at 650 °C when humidified hydrogen is used as the fuel and ambient air as the oxidant.     

Direct operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell stack with methane

Operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell (SIS-SOFC) stack directly on methane is studied. A cone-shaped solid oxide fuel cell stack is assembled by connecting 11 cone-shaped anode-supported single cells in series. The 11-cell-stack provides a maximum power output of about 8 W (421.4 mW cm⁻² calculated using active cathode area) at 800 °C and 6 W (310.8 mW cm⁻²) at 700 °C, when operated with humidified methane fuel. The maximum volumetric power density of the stack is 0.9 W cm⁻³ at 800 °C. Good stability is observed during 10 periods of thermal cycling test. SEM-EDX measurements are taken for analyzing the microstructures and the coking degrees.     

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.     

Fabrication and electrochemical performance of thin-film solid oxide fuel cells with large area nanostructured membranes

Thin-film solid oxide fuel cells (SOFCs) with large (5-mm square) membranes and ultra-thin La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) cathodes have been fabricated and their electrochemical performance was measured up to 500 °C. A grid of plated nickel on the cathode with 5–10 μm linewidth and 25–50 μm pitch successfully supported a roughly 200-nm-thick LSCF/yttria-stabilized zirconia/platinum membrane while covering less than 20% of the membrane area. This geometry yielded a maximum performance of 1 mW cm⁻² and 200 mV open-circuit voltage at 500 °C. Another approach toward realizing large area fuel cell junctions consists of depositing the membrane on a smooth substrate, covering it with a high-porosity material formed in situ, then removing the substrate. We have used a composite of silica aerogel and carbon fiber as the support, and show that this material can be created in flow channels etched into the underside of a silicon chip bonded to the top of the SOFC membrane. We anticipate these integrated fuel cell devices and structures to be of relevance to advancing low-temperature SOFCs for portable applications.     

Effect of the anode microstructure on the enhanced performance of solid oxide fuel cells

Anode microstructure has a vital effect on the performance of anode supported solid oxide fuel cells. High electrical conductivity, gas permeability and low polarization are the required features of anodes to achieve high power densities. The desired properties of the anodes were obtained by modifying their microstructural development using pyrolyzable pore former particles without introducing any functional layers and compositional modifications. The microstructures of fabricated anodes were characterized using scanning electron microscopy and mercury intrusion porosimetry techniques while their electrochemical properties were identified using impedance spectroscopy and voltammetric measurements. Detailed investigations demonstrated that the pore structure has a major impact on the electrical conductivity, polarization and gas permeability of the anodes. Through tailoring of conventional anode microstructures, a significantly high power density of 1.54 W/cm² was achieved at 800 °C using diluted hydrogen (10% H₂ in argon) as fuel.     

Transient modeling of anode-supported solid oxide fuel cells

An isothermal 2-D transient model is developed for an anode-supported solid oxide fuel cell. The model takes into account the transient effects of both charge migration and species transport in PEN assembly. Due to the lack of transient experimental data, the transient model, under steady state operating conditions, is validated using experimental results from open literature. Numerical results show that the cell can obtain very quick transient current response when subjected to a step voltage change, followed by a slow current transient period due to species diffusion effects within porous electrodes. It is also found that the transient response of the cell current is sensitive to oxygen concentration change at cathode/channel interface, whereas the current response is slow when step change of hydrogen concentration is applied at anode/channel interface. The cell transient performance can be improved by increasing porosity or decreasing tortuosity of electrodes.     

High performance metal-supported solid oxide fuel cells fabricated by thermal spray

Metal-supported solid oxide fuel cells (SOFCs) have been fabricated and characterized in this work. The cells consist of porous NiO-SDC as anode, thin SDC as electrolyte, and SSCo as cathode on porous stainless steel substrate. The anode and electrolyte layers were consecutively deposited onto porous metal substrate by thermal spray, using standard industrial thermal spray equipment, operated in an open-air atmosphere. The cathode materials were applied to the as-sprayed half-cells by screen-printing and heat-treated at 800 °C for 2 h. The cell components and performance were examined by scanning electron microscopy (SEM), X-ray diffraction, leakage test, ac impedance and electrochemical polarization at temperatures between 500 and 700 °C. The half-inch button cells exhibit a maximum power density in excess of 0.50 W cm⁻² at 600 °C and 0.92 W cm⁻² at 700 °C operated with humidified hydrogen fuel, respectively. The half-inch button cell was run at 0.5 A cm⁻² at 603 °C for 100 h. The cell voltage decreased from 0.701 to 0.698 V, giving a cell degradation rate of 4.3% kh⁻¹. Impedance analysis indicated that the cell degradation included 4.5% contribution from ohmic loss and 1.4% contribution from electrode polarization. The 5 cm × 5 cm cells were also fabricated under the same conditions and showed a maximum power density of 0.26 W cm⁻² at 600 °C and 0.56 W cm⁻² at 700 °C with dry hydrogen as fuel, respectively. The impedance analysis showed that the ohmic resistance of the cells was the major polarization loss for all the cells, while both ohmic and electrode polarizations were significantly increased when the operating temperature decreased from 700 to 500 °C. This work demonstrated the feasibility for the fabrication of metal-supported SOFCs with relatively high performance using industrially available deposition techniques. Further optimization of the metal support, electrode materials and microstructure, and deposition process is ongoing.     

Entropy generation analysis in a monolithic-type solid oxide fuel cell (SOFC)

The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical reactions and electrochemistry. The fuel cell geometry is then modified to reduce the main sources of irreversibility and increase its efficiency.     

Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells

Tubular electrolyte-supporting solid oxide fuel cells directly operated on carbon fuel were fabricated and tested. Gadolinia doped ceria (GDC) mixed with silver was used as the anode to catalyze the electrochemical oxidation of CO while Fe-based catalyst was loaded on the carbon fuel to enhance the Boudouard reaction. The performance was significantly improved, with a maximum power density of 45 mW cm⁻², 10 times higher than that of the cell without any catalyst. Impedance measurements showed that the polarization resistance was decreased by tens of times through applying catalysts in the cell. An operation life of 10 h was observed at a constant current of 70 mA. The mechanism of the cell reaction was analyzed.     

A phase inversion/sintering process to fabricate nickel/yttria-stabilized zirconia hollow fibers as the anode support for micro-tubular solid oxide fuel cells

NiO/YSZ hollow fibers were fabricated via a combined phase inversion and sintering technique, where polyethersulfone (PESf) was employed as the polymeric binder, N-methyl-2-pyrrolidone (NMP) as the solvent and polyvinylpyrrolidone (PVP) as the additive, respectively. After reduction with hydrogen at 750 °C for 5 h, the porous Ni/YSZ hollow fibers with an asymmetric structure comprising of a microporous layer integrated with a finger-like porous layer were obtained, which can be served as the anode support of micro-tubular solid oxide fuel cells (SOFCs). As the sintering temperature was increased from 1200 to 1400 °C, the mechanical strength and the electrical conductivity of the Ni/YSZ hollow fibers increased from 35 to 178 MPa and from 30 to 772 S cm⁻¹, respectively but the porosity decreased from 64.2% to 37.0%. The optimum sintering temperature was found to be between 1350 and 1400 °C for Ni/YSZ hollow fibers applied as the anode support for micro-tubular SOFCs.     

Crack severity in relation to non-homogeneous Ni oxidation in anode-supported solid oxide fuel cells

The full oxidation of Ni-YSZ anode-supported cells at high temperatures (>700 °C) is shown here to lead to much more severe degradation (larger quantity and wider cracks in the electrolyte) than at lower temperatures. This correlates with the linear mass gain/time profile observed in TGA experiments at high temperatures, indicative of diffusion controlled Ni oxidation and thus the presence of O₂ (and Ni/NiO) concentration gradients into the depth of the anode layer. At low partial pressures of O₂, the severity of cracking also increases. SEM studies of partially oxidized anode layers confirmed that Ni oxidation is non-homogeneous when carried out at either high temperatures or low pO₂, in which case the outer regions of the anode (near the anode/air interface) become almost fully oxidized, while the inner regions (near the electrolyte) remain metallic. Under these conditions, the continued volume expansion associated with NiO formation can then only occur towards the electrolyte, increasing the compressive stress inside the anode as the Ni continues to be oxidized, leading to electrolyte cracking and warping (convex to the electrolyte). To prevent severe degradation to the cell, efforts should therefore be made to avoid gradients in NiO/Ni content during oxygen exposure of Ni-YSZ anode-supported cells at high temperatures.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

Development of the anode pore structure and its effects on the performance of solid oxide fuel cells

Microstructural features, especially pore structure, has a substantial effect on the properties of the anode layer determining the electrochemical performance of the solid oxide fuel cells (SOFCs). Distinct anode pore structures were obtained by removal of various pyrolyzable pore formers (e.g. flake graphite, spheroidal graphite, spherical polymethyl methacrylate, random shaped sucrose, and spherical polystyrene particles). Determined processing parameters for the constituent layers allowed fabrication of Ni-YSZ anodes and complete multilayer fuel cells without macro defects (i.e. cracks, blisters and warpage). A systematic comparison was performed on the anode microstructures, as the fabricated fuel cells consisted of identical component layers (i.e. electrolyte, cathode and current collectors) supported by an anode layer with various pore structures. Voltammetric measurements and analyses of the corresponding impedance spectra on the developed fuel cells along with the investigations on the resultant microstructures using scanning electron microscopy and mercury intrusion porosimetry techniques led to the identification of the relationships between the anode pore structure and the electrochemical performance of the fuel cells. It was revealed that the anode pore structure has critical effects on the properties of the formed anode layers such as electrical conductivity, gas permeability and electrochemical polarization. The novel findings on the anode pore structure allowed increasing the power density of the fuel cells with identical components from 0.45 W/cm² to power densities over 1.75 W/cm² at 800 °C using diluted hydrogen (10% H₂ in Ar) as fuel.     

Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells

The microstructure of anode has a significant influence on the whole electrochemical performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-sintering techniques. As the anode porosity mainly came from the pore former (graphite) in the dip-coating process, different contents of graphite were added into the anode slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm⁻² at 850 °C, with carbon black (located 5% Fe) as the fuel and air as the oxidant.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

High performance of direct methane-fuelled solid oxide fuel cell with samarium modified nickel-based anode

Natural gas is one of the most attractive fuels for solid oxide fuel cell (SOFC), while the anode activity for methane fuel has a great influence on the performance and stability of SOFC. Samarium is a good catalyst promoter for methane reforming. In this work, samarium is used to modify nickel catalyst, which results in small nickel oxide particles. The SmNi-YSZ (yttria-stabilized zirconia) anode has smaller particles and better interfacial contact between nickel and YSZ compared with conventional Ni-YSZ anode. The fine structure of SmNi-YSZ anode results in high activity for electrochemical oxidation of hydrogen and low polarization resistance of the cell. The performance of SmNi-YSZ anode cell with humidified methane as fuel is greatly improved, which is similar to that with hydrogen as fuel. The maximum power densities of SmNi-YSZ anode cell are 1.56 W cm⁻² for humidified hydrogen fuel and 1.54 W cm⁻² for humidified methane fuel at 800 °C. The maximum power density is increased by 221% when samarium is used to modify Ni-YSZ anode for humidified methane fuel at 650 °C. High cell performance results in good stability of SmNi-YSZ anode cell and the cell runs stably for more than 600 min for humidified methane fuel.     

High performance praseodymium nickelate oxide cathode for low temperature solid oxide fuel cell

The praseodymium nickelate oxide Pr₂NiO_4+δ, a mixed conducting oxide with the K₂NiF₄-type structure, was evaluated as cathode for low temperature solid oxide fuel cells (T = 873 K). The electrochemical performance of the cathode has been improved by optimization of the microstructure of the porous cathode combined with the use of a ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr₂NiO_4+δ-powders with a median particle size of 0.4 μm, and sintering the screen printed layer at a sintering temperature of about 1353 K for 1 h. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions for the Pr₂NiO_4+δ cathode and tested. The best results were obtained when using a dense Ce_0.8Gd_0.2O_1.9 (20CGO) barrier layer. While a complete optimization of the cell preparation has not yet been achieved, the electrochemical performances of anode supported cells with Pr₂NiO_4+δ are higher than those with the well known state-of-the-art La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ (LSFC) material.     

Material and method selection for efficient solid oxide fuel cell anode: Recent advancements and reviews

The energy crisis has reached to an alarming situation due to increase in population. To overcome the shortfall of energy, solid oxide fuel cell (SOFC) being cheap, clean, and efficient renewable energy source is getting attention for electricity generation. Out of the three main components as anode, electrolyte, and cathode; anode/fuel electrode is an important component of SOFC because it allows the flow of electrons via external circuit to cathode generating the electric current and hence requires high electrical conductivity. In this review, anode materials synthesized until now are reviewed and by careful analysis categorized on the basis of operating temperature, conductivity, electrode polarization resistance, and structure. This comparison and categorization will provide selection criteria for state‐of‐the‐art and highly efficient anode materials for SOFC. In addition, the synthesis methods have been reviewed on the basis of their pros and cons, which will further facilitate the researchers to select the best synthesis method so as to get optimized properties of materials.