Direct ammonia solid oxide fuel cell based on thin proton-conducting electrolyte

Thin proton-conducting electrolyte with composition BaCe_0.8Gd_0.2O_3−δ (BCGO) was prepared over substrates composed of Ce_0.8Gd_0.2O_1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm⁻² at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.     

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.     

Short review on cobalt-free cathodes for solid oxide fuel cells

Cobalt-containing cathodes are known for their ability to operate under high-temperature applications in solid oxide fuel cells (SOFCs). Reducing the operation temperature into intermediate temperature-to-low temperature (IT-LT) zones may lead to a mismatch in the thermal expansion coefficient between the cathodes and the developed IT-LTSOFC electrolyte materials. Hence, cathode materials are adjusted to resolve this issue. Studies on IT-LTSOFC propose cobalt-free cathodes as an alternative way to produce high electrochemical performance cells for operation within the IT-LT range. Novel cobalt-free cathode powders are developed using perovskite structured materials, such as strontium ferrite oxide, as the main components together with dopants. This paper reviews various studies on cobalt-free cathode development, including the most important parameter in determining cathode performance, namely, the polarization resistance of SOFC cathodes.     

Improved performance of ammonia-fueled solid oxide fuel cell with SSZ thin film electrolyte and Ni-SSZ anode functional layer

Ammonia offers several advantages over hydrogen as an alternative fuel. However, using ammonia as a hydrogen source for fuel cells has not been received enough attention. In present paper, Scandia-stabilized Zirconia (SSZ) thin film electrolyte and Ni-SSZ anode functional layer were developed by tape casting in order to obtain high power output performance in ammonia, the results of a SOFC running on ammonia were described and its performance was compared with that when running on hydrogen. In order to improve the performance of the cell at higher temperatures, the anode was modified by iron through infiltration. A direct comparison of the performance of the modified cell running on either hydrogen or ammonia showed that the cell in ammonia generated slightly higher power densities at 700 and 750 °C. The performance in ammonia, using the anode catalyst, was comparable to that in hydrogen indicating ammonia could be treated as a promising alternative fuel by selecting an appropriate catalyst.     

A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

A direct carbon fuel cell based on a conventional anode-supported tubular solid oxide fuel cell, which consisted of a NiO-YSZ anode support tube, a NiO-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode, has been successfully achieved. It used the carbon black as fuel and oxygen as the oxidant, and a preliminary examination of the DCFC has been carried out. The cell generated an acceptable performance with the maximum power densities of 104, 75, and 47 mW cm⁻² at 850, 800, and 750 °C, respectively. These results demonstrate the feasibility for carbon directly converting to electricity in tubular solid oxide fuel cells.     

Development of the spinel powder reduction technique for solid oxide fuel cell interconnect coating

Mn_0.9Y_0.1Co₂O₄ spinel coatings are developed for solid oxide fuel cell (SOFC) alloy interconnects by a novel powder reduction technique. Material properties, electrical performance and long-term stability of the coatings are explored. The coating is about 9 μm in thickness and adheres well to the alloy substrate without any cracking or delamination. The area specific resistance (ASR) remains almost unchanged and is less than 3 mΩ cm² even though the coated alloy undergoes oxidation at 800 °C for 1017 h and ten thermal cycles from 800 °C to room temperature. The coated alloy presents excellent electrical performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.     

Metallic interconnects for solid oxide fuel cell: A review on protective coating and deposition techniques

With the reduction of solid oxide fuel cells (SOFCs) operating temperature to the range of 600 °C–800 °C, metallic alloy with high oxidation resistance are used to replace traditional ceramic interconnects. Metallic interconnects is advantageous over ceramic interconnects; in terms of manufacturability, cost, mechanical strength, and electrical conductivity. To date, promising candidates for metallic interconnects are all Cr-containing alloys, which are susceptible to volatile Cr migration that causes cell degradation. As such, protective coatings have been developed to effectively inhibit Cr migration; as well as maintain excellent electrical conductivity and good oxidation resistance. This article reviews the progress and technical challenges in developing metallic interconnects; different types of protective coatings and deposition techniques for metallic interconnects for intermediate-temperature SOFC applications.     

An analytical model for solid oxide fuel cells with bi-layer electrolyte

A theoretical model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and analytical solutions of various important relationships, such as I–V relationship, distribution of oxygen partial pressure in the bi-layer electrolyte, leakage current density etc. are obtained. Based on the assumptions of constant ionic conductivity and reversible electrodes, the model takes into considerations of transports of both ions and electrons in the electrolyte. The modeling results are compared with both experimental data and results from other models in the literature and very good agreements are obtained.     

Sputtered MnCu metallic coating on ferritic stainless steel for solid oxide fuel cell interconnects application

MnCu (Mn:Cu = 1:1, atomic ratio) metallic coatings have been deposited by magnetron sputtering on bare and on 100 h pre-oxidized SUS 430 steel for planar solid oxide fuel cells interconnects application. After oxidation at 800 °C in air, the MnCu coating directly deposited on the bare steel has been thermally converted to (Mn,Cu)₃O₄ spinel with Fe, containing discrete CuO on the outer surface. Nevertheless, the converted (Mn,Cu)₃O₄/CuO layer from the MnCu coating deposited on the pre-oxidized steelis almost free of Fe. A double-layer oxide structure with a main (Mn,Cu)₃O₄ spinel layer atop a Cr-rich oxide layer has been developed on the bare and pre-oxidized steel samples with MnCu coatings after thermal exposure. The outer layer mainly consisted of (Mn,Cu)₃O₄ spinel has not only significantly suppressed Cr outward migration to the scale surface, but also effectively reduced the area specific resistance (ASR) of the scale. The sputtered MnCu metallic coating is a very promising candidate for steel interconnect coating material.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Fabrication of ceramic films for solid oxide fuel cells via slurry spin coating technique

Thin ceramic films of samaria-doped ceria (SDC) were deposited on green NiO–SDC substrate via a slurry spin coating technique followed by co-firing. The ceramic films as-prepared are homogenous and dense, without cracks and penetrating pinholes, as observed from cross-sectional SEM images. The thicknesses of the ceramic films for one coating run can be adjusted between 0.8 and 9 μm by altering the spin rate. The film thickness (h) is inversely proportional to the logarithm of the spin rate (Log(f)) in the range of 3000–10,000 rpm. The slurry spin coating procedure is largely a competition between the thinning and the drying. Half-cells with both 15 and 25 mm diameters were fabricated. In addition, YSZ electrolyte layer with a thickness of 15 μm was also deposited with a homogeneous and completely dense microstructure by two runs of slurry spin coating.     

Ni–Fe + SDC composite as anode material for intermediate temperature solid oxide fuel cell

Composite materials of Sm_0.2Ce_0.8O_1.9 (SDC) with various Ni–Fe alloys were synthesized and evaluated as the anode for intermediate temperature solid oxide fuel cell. The performance of single cells consisting of the Ni–Fe + SDC anode, SDC buffer layer, La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) electrolyte, and SrCo_0.8Fe_0.2O_3 − δ (SCF) cathode were measured in the temperature range of 600–800 °C with wet H₂ as fuel. It was found that the anodic overpotentials of the different Fe–Ni compositions at 800 °C were in the following order: Ni_0.8Fe_0.2 < Ni_0.75Fe_0.25 < Ni < Ni_0.7Fe_0.3 < Ni_0.9Fe_0.1 < Ni_0.95Fe_0.05 < Ni_0.33Fe_0.67. The single cell with the Ni_0.8Fe_0.2 + SDC anode exhibited a maximum power density of 1.43 W cm⁻² at 800 °C and 0.62 W cm⁻² at 700 °C. The polarization resistance of the Ni_0.8Fe_0.2 + SDC anode was as low as 0.105 Ω cm² at 800 °C under open circuit condition. A stable performance with essentially negligible increase in anode overpotential was observed during about 160 h operation of the cell with the Ni_0.8Fe_0.2 + SDC anode at 800 °C with a fixed current density of 1875 mA cm⁻². The possible mechanism responsible for the improved electrochemical properties of the composite anodes with the Ni_0.8Fe_0.2 and Ni₀₇₅Fe_0.25 alloys was discussed.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.     

Linear discharge model,power losses and overall efficiency of the solid oxide fuel cell with thin film samarium doped ceria electrolyte. Part I: Linear discharge model

In this work, a solid oxide fuel cell with 60 μm samarium doped ceria film as the electrolyte is fabricated with a co-pressing technique. The performance of the cell is measured at 600, 650 and 700 °C. The corresponding maximum power outputs are 236, 331 and 401 mW cm^?2, respectively. The measured current–voltage (I–V) curves are straight lines. A linear discharge model is derived based on the Gorelov and Liu modified electromotive force (EMF) equations. The model fits the measured I–V curves with the maximum errors less than 1.5%. The overall activation overpotential of the cell is therefore postulated to be proportional to the polarization current.     

A promising NiCo2O4 protective coating for metallic interconnects of solid oxide fuel cells

The NiCo₂O₄ spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol-gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800 °C for 200 h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr₂O₃ and the formation of conductive spinel phases in the oxide scale.     

Comparison of solid oxide fuel cell anode coatings prepared from different feedstock powders by atmospheric plasma spray method

Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300 nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock.     

Theoretical analysis of the characteristics of the solid oxide fuel cells with a bi-layer electrolyte

From the analytical model derived earlier [17], analytical expressions for the relative thickness ratio r_s of a bi-layer electrolyte and the maximum power density of a fuel cell are developed. Using these expressions, together with the other relationships from the analytical model, the characteristics of solid oxide fuel cells (SOFCs) with a bi-layer electrolyte are analyzed and theoretical analysis of the effect of the configuration of a bi-layer electrolyte on the SOFC performance is performed. The results show that the effectiveness of the bi-layer electrolyte depends strongly on its configuration. In the analyses, the variations of open circuit voltage and the maximum power density with the thickness ratio at different total electrolyte thicknesses and different operating temperatures are obtained. Furthermore, by taking into considerations of the oxygen partial pressure at the interface between the two layers of electrolytes, an analytical expression for the critical relative thickness ratio, above which, the electrolyte is stable, is obtained.     

Basic properties of a liquid tin anode solid oxide fuel cell

An unconventional high temperature fuel cell system, the liquid tin anode solid oxide fuel cell (LTA-SOFC), is discussed. A thermodynamic analysis of a solid oxide fuel cell with a liquid metal anode is developed. Pertinent thermochemical and thermophysical properties of liquid tin in particular are detailed. An experimental setup for analysis of LTA-SOFC anode kinetics is described, and data for a planar cell under hydrogen indicated an effective oxygen diffusion coefficient of 5.3 × 10⁻⁵ cm² s⁻¹ at 800 °C and 8.9 × 10⁻⁵ cm² s⁻¹ at 900 °C. This value is similar to previously reported literature values for liquid tin. The oxygen conductivity through the tin, calculated from measured diffusion coefficients and theoretical oxygen solubility limits, is found to be on the same order of that of yttria-stabilized zirconia (YSZ), a traditional SOFC electrolyte material. As such, the ohmic loss due to oxygen transport through the tin layer must be considered in practical system cell design since the tin layer will usually be at least as thick as the electrolyte.     

CuFe2O4/CuO coating for solid oxide fuel cell steel interconnects

CuFe_0.8 (Fe:Cu = 0.8:1, atomic ratio) alloy layer is fabricated on both bare and pre-oxidized SUS 430 steels by direct current magnetron sputtering, followed by exposing at 800 °C in air to obtain a protective coating for solid oxide fuel cell (SOFC) steel interconnects. The CuFe_0.8 alloy layer is thermally converted to CuFe₂O₄/CuO coating, which effectively suppresses the out-migration of Cr. Pre-oxidation treatment not only initially accelerates the formation of CuFe₂O₄/CuO coating but also further inhibits the Cr and Fe outward diffusion. Suppressing outward diffusion of Cr could improve electrical property of oxide scale and decrease the risk of cathode Cr-poisoning. Blocking out-diffusion of Fe is beneficial to stabilize the CuO layer. After 2520 h oxidation, the scale ASR at 800 °C is 66.9 mΩ cm² for coated bare steel, 43.4 mΩ cm² for the coated pre-oxidized steel.