Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Ni/YSZ and Ni–CeO2/YSZ anodes prepared by impregnation for solid oxide fuel cells

In this paper, Ni/YSZ and Ni–CeO₂/YSZ anodes for a solid oxide fuel cell (SOFC) were prepared by tape casting and vacuum impregnation. By this method, the Ni content in the anode could be reduced compared to the traditional tape casting method. It was found that adding CeO₂ into the Ni/YSZ anode by a Ni(NO₃)₂ and Ce(NO₃)₃ mixed impregnation could further enhance cell performance. This was investigated in H₂ at 1073 K. XRD patterns indicated that CeO₂ and Ni were separate phases, and the CeO₂ addition could enhance the Ni dispersion on the YSZ framework surface which was observed by SEM images. It was shown that adding CeO₂ into the Ni anodes could decrease the cell polarization resistance. The maximum power density for cells with 25 wt.% Ni, 5 wt.% CeO₂–25 wt.% Ni/YSZ, or 10 wt.% CeO₂–25 wt.% Ni/YSZ anode was 230 mW cm⁻², 420 mW cm⁻² and 530 mW cm⁻², respectively, in H₂ at 1073 K. The OCV for these cells was 1.05–1.09 V, indicating that a dense electrolyte film was obtained by co-firing porous YSZ layer and dense YSZ layer.     

Ni/YSZ anode - Effect of pre-treatments on cell degradation and microstructures

Anode supported (Ni/YSZ-YSZ-LSM/YSZ) solid oxide fuel cells were tested and the degradation over hundreds of hours was monitored and analyzed by impedance spectroscopy. Test conditions were chosen to focus on the Ni/YSZ anode degradation and all tests were operated at 750 °C, a current density of 0.75 A cm⁻². Oxygen was supplied to the cathode and the anode inlet gas mixture had a high p(H₂O)/p(H₂) ratio of 0.4/0.6. Commercially available gasses were applied. The effect of different types of pre-treatments on the Ni/YSZ electrode degradation during subsequent fuel cell testing was investigated. Pre-treatments included operating at OCV (4% and 40% H₂O in H₂) prior to fuel cell testing, cleaning of the inlet H₂ gas at 700 °C and processing the anode half cell via multilayer tape casting. Analyses of impedance spectra showed that the increase in the charge transfer reaction resistance in the Ni/YSZ (R_Ni,TPB) was decreased to ¼ or less for the pre-treated and fuel cell tested cells when compared with a non-pre-treated reference tested cell; all operated at the same fuel cell test conditions. Scanning electron microscopy and image analyses for the non-pre-treated reference tested cell and selected pre-treated cells showed significant differences in the area fractions of percolating nickel both in the active anode and support layer.     

Effect of PH3 poisoning on a Ni-YSZ anode-supported solid oxide fuel cell under various operating conditions

The Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can generate electrical power by using coal-derived syngas as the fuel. However, trace contamination of phosphine (PH₃) in the syngas can cause irreversible degradation in cell performance. A series of tests at 10 ppm PH₃ in the fuel gas was carried out under a variety of operating conditions, viz, with/without electrochemical reaction in syngas and with/without H₂O in H₂ fuel at 750 °C, 800 °C and 850 °C. The poisoning effects were evaluated by both electrochemical methods and chemical analyses. The post-mortem analyses of the SOFC anode were performed by means of XRD, SEM/EDS, and XPS. The results show that the degradation rate is larger at the higher cell working temperature using syngas with PH₃ in a 200 h test though PH₃ is more reactive with Ni in the anode at lower working temperature and produces a secondary nickel phosphide (Ni_xP_y) phase. The dominant compositions of Ni_xP_y on the cell anode are Ni₅P₂ with the presence of H₂O, and Ni₁₂P₅ without the presence of H₂O. The production of Ni_xP_y can be generated on the cell anode using syngas or dry H₂ fuel with 10 ppm PH₃ contaminant. Further, the appearance of Ni_xP_y phases is independent of the electrochemical reactions in the cell.     

Effect of Sm0.2Ce0.8O1.9 on the carbon coking in Ni-based anodes for solid oxide fuel cells running on methane fuel

To directly use hydrocarbon fuel without a reforming process, a new microstructure for Ni/Sm_0.2Ce_0.8O_2−δ (Ni/SDC) anodes, in which the Ni surface of the anode is covered with a porous Sm_0.2Ce_0.8O_2−δ thin film, was investigated as an alternative to conventional Ni/YSZ anodes. The porous SDC thin layer was coated on the pores of the anode using the sol–gel coating method. The cell performance was improved by 20%–25% with the Ni/SDC anode relative to the cell performance with the Ni/YSZ anode due to the high ionic conductivity of the Ni/SDC anode and the increase of electrochemical reaction sites. For the SDC-coated Ni/SDC anode operating with methane fuel, no significant degradation of the cell performance was observed after 180 h due to the surface modification with the SDC film on the Ni surface, which opposes the severe degradation of the cell performance that was observed for the Ni/YSZ anode, which results from carbon deposition by methane cracking. Carbon was hardly detected in the SDC-coated Ni/SDC anode due to the catalytic oxidation of the deposited carbon on the SDC film as well as the electrochemical oxidation of methane in the triple-phase-boundary.     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

Application of a Cu-CeO2/Ni-yttria-stabilized zirconia multi-layer anode for anode-supported Solid Oxide Fuel Cells operating on H2-CO syngas fuels

A Ni-yttria-stabilized zirconia (YSZ) anode and a Cu-CeO₂/Ni-YSZ multi-layer anode have been fabricated for use in anode-supported Solid Oxide Fuel Cells (SOFCs), and their performances and stabilities in H₂-CO syngas have been studied at 750 °C. A high CO content has been found to cause carbon deposition and crack formation in the Ni-YSZ anode after long-term operation, but the Cu-CeO₂ catalyst layer on the Ni-YSZ anode surface improves its stability in syngas with high CO content by facilitating the water gas shift reaction. The optimized single cell has run in syngas with a composition of 48.5%H₂-48.5%CO-3%H₂O for 460 h without obvious degradation of its performance, however, its performance decreases after 630 h due to carbon deposition in the anode functional layer and subsequent crack formation on the anode and electrolyte.     

Effects of coal syngas major compositions on Ni/YSZ anode-supported solid oxide fuel cells

This paper presents a systematical evaluation of the effects of CO₂, H₂O, CO, N₂ and CH₄ in the coal syngas on the properties of typical Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The results show that CO₂, H₂O, CO, N₂ and CH₄ have complicated effects on the cell performance and the electrochemical impedance spectra (EIS) analysis reveals the addition of these gases influences electrode processes such as the oxygen ion exchange from YSZ to anode TPBs, the charge transfer at the anode TPBs, gas diffusion and conversion at the anode. Two kinds of mixture gases with different compositions are thus constituted and used as fuel for aging test on two cells at 750 °C. No degradation or carbon deposition is observed for the cell fueled with 40% H₂-20% CO-20% H₂O-20% CO₂ for 360 h while the cell fueled with 50% H₂-30% CO-10% H₂O-10% CO₂ exhibits an abrupt degradation after 50 h due to the severe carbon deposition.     

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.     

Direct utilization of methanol on impregnated Ni/YSZ and Ni-Zr0.35Ce0.65O2/YSZ anodes for solid oxide fuel cells

Some of the limits on fuel cell development include the issues of hydrogen availability and storage. Methanol has many advantages as an alternative fuel for fuel cells but depending on the anode composition, the formation of carbon may be a problem. In this paper, the direct utilization of methanol in solid oxide fuel cells with impregnated Ni/YSZ and Ni-Zr_0.35Ce_0.65O_2−δ (ZDC)/YSZ anodes was investigated at 1073 K. Performance and stability of these anodes, as measured by steady-state polarization and electrochemical impedance spectroscopy, were improved by the presence of ZDC; although, the deposition of carbon, as detected by scanning electron microscopy and temperature-programmed oxidation analysis, was not entirely avoided. The impact of the carbon, however, was different depending on the anode. That is, carbon formation caused the delamination of impregnated Ni/YSZ anodes, while the structural integrity of Ni-ZDC/YSZ anodes was maintained and the cell performance was not negatively impacted. Increasing the fuel utilization decreased coking, as predicted by equilibrium calculations.     

A symmetrical solid oxide fuel cell prepared by dry-pressing and impregnating methods

In this study, a simple and cost-effective dry-pressing method has been used to fabricate a symmetrical solid oxide fuel cell (SOFC) where the dense yttria-stabilized zirconia (YSZ) electrolyte film is sandwiched between two symmetrical porous YSZ layers in which La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) based anode and cathode are incorporated using wet impregnation techniques. The maximum power densities (P_max) of a single cell with 32 wt.% LSCM impregnated YSZ anode and cathode reach 333 and 265 mW cm⁻² at 900 °C in dry H₂ and CH₄, respectively. The cell performance is further improved with additional impregnation of a small amount of Sm-doped CeO₂ (SDC) or Ni. When 6 wt.% Ni as catalyst is added to both the anode and cathode, P_max values of 559 and 547 mW cm⁻² can be achieved, which are better than with SDC. The effect of Ni on the cathode performance is also investigated by impedance spectra analysis.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Effect of milling methods on performance of Ni–Y2O3-stabilized ZrO2 anode for solid oxide fuel cell

A Ni–YSZ (Y₂O₃-stabilized ZrO₂) composite is commonly used as a solid oxide fuel cell anode. The composite powders are usually synthesized by mixing NiO and YSZ powders. The particle size and distribution of the two phases generally determine the performance of the anode. Two different milling methods are used to prepare the composite anode powders, namely, high-energy milling and ball-milling that reduce the particle size. The particle size and the Ni distribution of the two composite powders are examined. The effects of milling on the performance are evaluated by using both an electrolyte-supported, symmetric Ni–YSZ/YSZ/Ni–YSZ cell and an anode-supported, asymmetric cell. The performance is examined at 800 °C by impedance analysis and current-voltage measurements.     

Preparation of Cu-Ni/YSZ solid oxide fuel cell anodes using microwave irradiation

A microwave irradiation process is used to deposit Cu nanoparticles on the Ni/YSZ anode of an electrolyte-supported solid oxide fuel cell (SOFC). The reaction time in the microwave is only 15 s for the deposition of 6 wt% Cu (with respect to Ni) from a solution of Cu(NO₃)₂·3H₂O and ethylene glycol (HOCH₂CH₂OH). The morphology of the deposited Cu particles is spherical and the average size of the particles is less than 100 nm. The electrochemical performance of the microwave Cu-coated Ni/YSZ anodes is tested in dry H₂ and dry CH₄ at 1073 K, and the anodes are characterized with scanning electron microscopy, electrochemical impedance spectroscopy, and temperature-programmed oxidation. The results indicate that preparation of the anodes by the microwave technique produces similar performance trend as those reported for Cu-Ni/YSZ/CeO₂ anodes prepared by impregnation. Specifically, less carbon is formed on the Cu-Ni/YSZ than on conventional Ni/YSZ anodes when exposed to dry methane and the carbon that does form is more reactive.     

Applications of nano-composite materials for improving the performance of anode-supported electrolytes of SOFCs

In order to improve the performance of the anode-supported electrolyte of solid oxide fuel cells (SOFCs), the anode electrode is modified by inserting an anode functional layer of nano-composite powders between a Ni–YSZ electrode and YSZ electrolyte. The NiO–YSZ nano-composite powders are fabricated by coating nano-sized Ni and YSZ particles on the YSZ core particle by the Pechini process. The reduction of the polarization resistance of a single cell that is applied to the anode functional layer is attributed to the increasing reaction of three-phase boundaries (TPBs) within the layer and the micro-structured uniformity in the electrode. Two methods were used, namely tape-casting/dip-coating and tape-casting/co-firing, for studying the performance. It can be concluded that the cell with an anode functional layer thickness (15–20 μm) and a microstructure of NiO–YSZ nano-composite materials which was fabricated by the tape-casting/dip-coating method improved the output power (to 1.3 W cm⁻²) at 800 °C using hydrogen as fuel and air as an oxidant.     

The effect of phosphine in syngas on Ni-YSZ anode-supported solid oxide fuel cells

Ni-YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance, not only in hydrogen fuel, but also in a clean blended synthetic coal syngas mixture (30% H₂, 26% H₂O, 23% CO, and 21% CO₂). However, trace impurities, such as phosphine (PH₃), in coal-derived syngas can cause degradation in cell performance [J.P. Trembly, R.S. Gemmen, D.J. Bayless, J. Power Sources 163 (2007) 986-996]. A commercial solid oxide fuel cell was exposed to a syngas with 10 ppm PH₃ under a constant current load at 800 °C and its performance was evaluated periodically using electrochemical methods. The central part of the anode was exposed directly to the syngas without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the impurity PH₃ caused a significant loss of the Ni-YSZ anode electrochemical performance and an irreversible Ni-YSZ structural modification. Ni₅P₂ was confirmed to be produced on the cell surface as the dominant nickel phosphorus phase.     

The effect of coal syngas containing AsH3 on the performance of SOFCs: Investigations into the effect of operational temperature,current density and AsH3 concentration

The performance of solid oxide fuel cells (SOFCs) using simulated coal-derived syngas, with and without arsine (AsH₃), was studied. Anode-supported SOFCs were tested galvanostatically at 0.25 and 0.5 A cm⁻² at 750 and 800 °C with simulated coal syngas containing 0.1, 1, and 2 ppm AsH₃. The tests with simulated coal syngas containing 1 ppm AsH₃ show little degradation over 100 h of operation. The tests with simulated coal syngas containing 2 ppm AsH₃ show some signs of degradation, however no secondary arsenide phases were found. Extended trial testing with 0.1 ppm AsH₃ showed degradation as well as the formation of a secondary nickel arsenide phase in the anode of the SOFC.     

SOFC anodes for direct oxidation of hydrogen and methane fuels containing H2S

Single cell solid oxide fuel cells using Ni-YSZ and Co-YSZ anodes were tested in H₂, CH₄, H₂S/H₂ and H₂S/CH₄ fuel mixtures. Their performance was found to quickly degrade in dry CH₄ due to carbon deposition and lifting of the anode from the electrolyte. In contrast, hydrogen or methane containing H₂S showed an increase in exchange current densities when compared to H₂,M/YSZ/LSM,air systems (M = Ni or Co) despite having less optimal anode microstructure. Conversion from metal to metal-sulfide in the presence of H₂S produced large, dense metal-sulfide particles surrounded by YSZ, thus decreasing the triple-phase boundary. Furthermore, CoS-based anodes showed phase segregation and densification toward the electrolyte. Despite this, long-term testing at η_a = 0.5 V of the H₂S/CH₄,CoS-YSZ/YSZ/LSM,air system showed no signs of degradation of the anode over a 6-day period. Only after removal of H₂S from the H₂S/CH₄ stream did the CoS-anode reduce back to Co, signifying H₂S is required to maintain the metal-sulfide active anode.     

Fabrication and characterization of Cu–Ni–YSZ SOFC anodes for direct use of methane via Cu-electroplating

The Cu–Ni–YSZ cermet anodes for direct use of methane in solid oxide fuel cells have been fabricated by electroplating Cu into a porous Ni–YSZ cermet anode. The uniform distribution of Cu in the Ni–YSZ anode was obtained by electroplating in an aqueous solution mixture of CuSO₄·5H₂O and H₂SO₄ for 30 min with 0.1 A of applied current. When the Cu–Ni–YSZ anode was exposed to methane at 700 °C, the amount of carbon deposited on the anode decreased as the amount of Cu in the Cu–Ni solid solution increased. The power density (0.24 W/cm²) of a single cell with a Cu–Ni–YSZ anode was slightly lower in methane at 700 °C than the power density (0.28 W/cm²) of a single cell with a Ni–YSZ anode. However, the performance of the Ni–YSZ anode-supported single cell degraded steeply over 21 h because of carbon deposition, whereas the Cu–Ni–YSZ anode-supported single cell showed enhanced durability up to 200 h.     

Electrochemical performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials

Yttrium-substituted SrTiO₃ has been considered as anode material of solid oxide fuel cells (SOFCs) substituting of the state-of-the-art Ni cermet anodes. Sr_0.895Y_0.07TiO_3−δ (SYT) shows good electrical conductivity, compatible thermal expansion with yttria-stabilized ZrO₂ (YSZ) electrolyte and reliable stability during reduction and oxidation (redox) cycles. Single cells based on SYT anode substrates were fabricated in the dimension of 50 mm × 50 mm. The cell performances were over 1.0 A cm⁻² at 0.7 V and 800 °C, which already reached the practical application level. Although Ti diffusion from SYT substrates to YSZ electrolytes was observed, it did not show apparent disadvantage to the cell performance. The cells survived 200 redox cycles without obvious OCV decrease and macroscopic damage, but performance decreased due to the electronic properties of the SYT material. The influence of water partial pressure on cell performance and coking tolerance of the cells are also discussed in this study.