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.     

Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell

A novel approach of fabricating SOFC anode comprising graded compositions in constituent phases having layer wise microstructural variation is reported. Such anode encompasses conventional NiO–YSZ (40 vol% Ni) with higher porosity at the fuel inlet side and Ni–YSZ electroless cermet (28–32 vol% Ni) with less porosity toward the electrolyte. Microstructures and thicknesses of the bilayer anodes (BLA) are varied sequentially from 50 to 250 μm for better thermal compatibility and cell performance. Significant augmentation in performance (3.5 A cm⁻² at 800 °C, 0.7 V) is obtained with engineered trilayer anode (TLA) having conventional anode support in conjunction with layers of electroless cermet each of 50 μm having 28 and 32 vol% Ni. Engineered TLA accounts for substantial reduction both in cell polarization (ohmic ASR: 78 mΩ cm² versus 2835 mΩ cm²; cell impedance: 0.35 Ω cm² versus 0.9 Ω cm²) and degradation rate (76 μV h⁻¹ versus 219 μV h⁻¹) compared to cells fabricated with conventional cermet.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

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.     

Effect of anode structure on performance of cone-shaped solid oxide fuel cells fabricated by phase inversion

Anode-supported cone-shaped tubular solid oxide fuel cells (SOFCs) are successfully fabricated by a phase inversion method. During processing, the two opposite sides of each cone-shaped anode tube are in different conditions--one side is in contact with coagulant (the corresponding surface is named as “W-surface”), while the other is isolated from coagulate (I-surface). Single SOFCs are made with YSZ electrolyte membrane coated on either W-surface or I-surface. Compared to the cell with YSZ membrane on W-surface, the cell on I-surface exhibits better performance, giving a maximum power density of 350 mW cm⁻² at 800 °C, using wet hydrogen as fuel and ambient air as oxidant. AC impedance test results are consistent with the performance. The sectional and surface structures of the SOFCs were examined by SEM and the relationship between SOFC performance and anode structure is analyzed. Structure of anodes fabricated at different phase inversion temperature is also investigated.     

Use of a catalyst layer for anode-supported SOFCs running on ethanol fuel

Solid oxide fuel cells (SOFCs) operating directly on hydrocarbon fuels have attracted much attention in recent years. A two-layer structure anode running on ethanol was fabricated by tape casting and screen printing technology in this paper, the addition of a Cu–CeO₂ catalyst layer to the supported anode surface yielded much better performance in ethanol fuel. The effect that the synthesis conditions of the catalyst layer have on the performances of the composite anodes was investigated. Single cells with this anode were also fabricated, of which the maximum power density reached 566 mW cm⁻² at 800 °C operating on ethanol steam. Long-term performance of the anodes was presented by discharging as long as 80 h without carbon deposition.     

Nano Ni layered anode for enhanced MCFC performance at reduced operating temperature

Nanoparticles of Ni and Ni–Al₂O₃ were coated on a molten carbonate fuel cell (MCFC) anode by spray method to enlarge the electrochemical reaction sites at triple phase boundaries (TPBs). Both nano Ni coated anode and nano Ni–Al₂O₃ anode exhibited significant reduction of anode polarization, thanks to smaller charge transfer resistance. The maximum power density of nano Ni coated anode was 159 mW cm⁻² at current density of 300 mA cm⁻² operating at 600 °C. This is about 7% increase from the standard cell performance tested and compared in the study. Although low performance of nano coated Ni–Al₂O₃ cell is observed due to electrolyte consumption, the stability of cell performance during operation time is more favorable in MCFCs operation.     

Microstructure tailoring of YSZ/Ni-YSZ dual-layer hollow fibers for micro-tubular solid oxide fuel cell application

YSZ/NiO-YSZ dual-layer hollow fibers with a thin YSZ top layer integrated on a porous NiO-YSZ (60:40 in weight) support, have been developed by one step method via a co-spinning-sintering process. Hydrogen reduction was performed to form YSZ/Ni-YSZ micro tube as the half solid oxide fuel cells (SOFCs). The microstructure of the dual-layer hollow fibers was tailored by adding ethanol as non-solvent in the initial mixture dopes for NiO-YSZ anode spinning. LSM cathode containing 20 wt%-YSZ was deposited on the electrolyte surface by dip-coating method to fabricate micro-tubular SOFCs. Experimental results indicate that the dual-layer hollow fibers from the anode dopes containing 15–20 wt% of ethanol possess the desired microstructure with optimized properties, such as the bending strength of 180 MPa, the porosity of 38–35% and the conductivity of 3000 S cm⁻¹ at room temperature. The micro-tubular SOFCs fabricated from such hollow fibers show a maximum power density up to 485 mW cm⁻² at 850 °C with 20 mL min⁻¹ of H₂ as fuel and 30 mL min⁻¹ air as oxidant, respectively.     

Validation of an externally oil-cooled 1 kWel HT-PEMFC stack operating at various experimental conditions

The performance of 1 kW_el 48-cell HT-PEMFC at various experimental conditions is presented, particularly at several CO concentrations (up to 1.0%). Polarization curves measured at various anode (1.0–2.5) and cathode (1.6–4.0) stoichiometries; stack operating temperatures (120–160 °C) and gas pressures (up to 0.5 bar_g) are reported and analysed. The minimum gas stoichiometries of 1.25 and 2.0 were determined for the anode and cathode, respectively. The highest stack power density of 225 mW cm⁻² was measured at 160 °C and 0.4 A cm⁻². Operation at CO concentrations up to 1% was achieved, although a loss of performance of about 4% was observed for low CO concentrations. The operating temperature enhanced fuel cell performance and tolerance to CO, even when supplied with higher CO concentration in the anode feed gas.     

Upgrading the performance of La2Mo2O9-based solid oxide fuel cell under single chamber conditions

Various anode-supported solid oxide fuel cells (SOFC), based on 10 mol% Dy-doped La₂Mo₂O₉ (LDM) electrolyte, are prepared analytically and operated under single chamber conditions to explore the connections between electrode and power performance. The cathode of tested SOFCs is compositionally graded with three composites of samarium strontium cobaltite and Gd-doped ceria (GDC) to relax the thermal stress, because of sizable thermal expansion differences above 400 °C. We focus the research attention on varying the anode pore structure and composition to promote the power performance in methane/air mixture at 700 °C. For the one-layer support of GDC+NiO+LDM anode, addition of 10 wt% graphite minimizes its mass transport resistance through creating 8–5 μm long and ∼1 μm wide slit-shaped pores. The graphite pore former raises the peak power value by 80 mW cm⁻². Adopting a more porous and active outer layer, the double-layer support further enhances the cell power. The peak power was first raised by 48 mW cm⁻², using an outer layer that was prepared with 63 wt% NiO. Dosing 3% Pd on this outer layer uplifts another 59 mW cm⁻². In this study, with an improved anode, the peak power value reaches 437 mW cm⁻².     

Iron incorporated Ni–ZrO2 catalysts for electric power generation from methane

On the purpose to perform as functional layer of SOFCs operating on methane fuel, NiFe–ZrO₂ alloy catalysts have been synthesized and investigated for methane partial oxidation reactions. Ni₄Fe₁–ZrO₂ shows catalytic activity comparable to that of Ni–ZrO₂ and superior to other Fe-containing catalysts. In addition, O₂-TPO analysis indicates iron is also prone to coke formation; as a result, most of NiFe–ZrO₂ catalysts do not show improved coking resistance than Ni–ZrO₂. Anyway, Ni₄Fe₁–ZrO₂ (Ni:Fe = 4:1 by weight) prepared by glycine-nitrate process shows somewhat less carbon deposition than the others. However, Raman spectroscopy demonstrates that the addition of Fe does reduce the graphitization degree of the deposited carbon, suggesting the easier elimination of carbon once it is deposited over the catalyst. Ni₄Fe₁–ZrO₂ has an excellent long-term stability for partial oxidation of methane reaction at 850 °C. A solid oxide fuel cell with conventional nickel cermet anode and Ni₄Fe₁–ZrO₂ functional layer is operated on CH₄–O₂ gas mixture to yield a peak power density of 1038 mW cm⁻² at 850 °C, which is comparable to that of hydrogen fuel. In summary, the Ni₄Fe₁–ZrO₂ catalyst is potential catalyst as functional layer for solid-oxide fuel cells operating on methane fuel.     

A new nickel–ceria composite for direct-methane solid oxide fuel cells

Various Ni–La_xCe_1−xO_y composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La_0.1Ce_0.9O_y showed the highest catalytic performance and also the best coking resistance. The Ni–La_xCe_1−xO_y catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La_0.1Ce_0.9O_y anode presented the best catalytic activity and coking resistance in the various Ni–La_xCe_1−xO_y catalysts with different ceria contents. In addition, the Ni–La_0.1Ce_0.9O_y also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La_0.1Ce_0.9O_y anode and a catalyst yielded a peak power density of 850 mW cm⁻² at 650 °C while operating on a CH₄–H₂O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La_0.1Ce_0.9O_y anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO₂ composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.     

High performance intermediate temperature micro-tubular SOFCs with Ba0.9Co0.7Fe0.2Nb0.1O3−δ as cathode

In this study, anode supported intermediate temperature micro-tubular solid oxide fuel cells (MT-SOFCs) have been fabricated by combination of phase-inversion, dip-coating, co-sintering and printing method. The MT-SOFC consists of a ∼300 μm wall-thickness Ni–Sc₂O₃ stabilized ZrO₂ (ScSZ) anode tube, ∼10 μm ScSZ dense electrolyte layer, ∼10 μm Ce_0.9Gd_0.1O_2−δ (GDC) membrane buffer layer and ∼50 μm Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ (BCFN) cathode layer. SEM and electrochemical impedance spectroscopy (EIS) analysis suggested that the novel structured anode can remarkably diminish the porous anode geometrical tortuosity and improve the fuel gas diffusivity. High peak power densities of 0.34, 0.51 and 0.72 W cm⁻² have been achieved with humidified hydrogen as the fuel and ambient air as oxidant at 550, 600 and 650 °C, respectively. Further, the cell has demonstrated a very stable performance with no significant cell voltage degradation under a constant current of 0.6 A cm⁻² for over 213 h test at 650 °C.     

Electrochemical characteristics and performance of anode-supported SOFCs fabricated using carbon microspheres as a pore-former

This paper evaluates the influence of carbon microspheres (CMSs) as an electrode pore-former on the fabrication and electrochemical properties of the anode-supported solid oxide fuel cells (SOFCs). The anode supports are fabricated by dry-pressing of CMS and NiO/YSZ (nickel-oxide/yttria-stabilized zirconia) composite powder, and the YSZ electrolyte layer is prepared by the electrophoretic deposition technique. The ohmic and polarization resistances for NiO/YSZ–YSZ half cells at different testing temperatures (650–850 °C) are analyzed by electrochemical impedance spectroscopy (EIS). The polarization ASR (area specific resistance) for the fabricated half cells increases from 0.583 Ω cm² to 3.047 Ω cm² when the temperature decreases from 850 °C to 650 °C. The electrochemical performance of single cells is measured at different temperatures (700–850 °C) and the results indicate that the cells fabricated using CMS as the pore-former exhibit much higher electrochemical performance than those without using CMS. A maximum power density of 207.7 mW cm⁻², 431.2 mW cm⁻², and 571.6 mW cm⁻² is recorded at 850 °C for the cells fabricated by adding 0 wt. %, 2.5 wt. % and 5 wt. % of CMS, respectively. The maximum fuel utilization efficiency is also found to increase from 26.5% for the cell prepared without CMS to 47.0% and 59.6% for the cells prepared with 2.5 wt. % and 5 wt. % of CMS, respectively. The increase in the electrochemical performance by adding CMS as pore-former to anode-supports is attributed to higher porosity and pore size of the electrode.     

Enhanced performance of solid oxide fuel cell fabricated by a replica technique combined with infiltrating process

For the convenience of hermetic sealing, first time, a replica technique is successfully invented in this study to fabricate the dissymmetrical tri-layer structure of “porous La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) |dense LSGM| porous LSGM” skeleton by adopting a carbon layer. SEM analysis reveals that the bonding strength of interfacial contact between dense LSGM and porous LSGM can also be improved when using this new fabrication method. Metal Ni and layered perovskite oxide SmBa_0.5Sr_0.5Co₂O₅ (SBSCO) are then infiltrated into the dissymmetrical skeleton on each side to form the functional fuel cell. The OCV are close to the expected Nernst potentials which demonstrate that the cell fabricated in this study can be well sealed. The maximum power densities of functional fuel cell with configuration of “Ni–LSGM |LSGM| LSGM–SBSCO” are 0.12 W cm⁻², 0.38 W cm⁻², 1 W cm⁻² and 1.8 W cm⁻² at 400, 450, 500, 550 °C, respectively. Though long term stability testing shows a rapid performance degradation when discharged at 0.7 V for 80 h, by changing pure Ni to Ni–SDC mixed oxide, the performance of functional fuel cell with configuration of “Ni–SDC–LSGM |LSGM| LSGM–SBSCO” increases and the long term stability is largely improved.     

Comparative study on the performance of tubular and button cells with YSZ membrane fabricated by a refined particle suspension coating technique

Both tubular and button solid oxide fuel cells (SOFCs) with configuration NiO–YSZ/YSZ/PNSM–YSZ were assembled and compared in their performance. A refined particle suspension coating technique was used for preparing thin dense YSZ electrolyte layer on the two types of anode supports, and the thickness of YSZ membrane was controlled by the time of tubular anode dipped into YSZ suspension and the suspension volume dropped onto the button anode, respectively. Current–voltage tests and AC impedance measurements were carried out to characterize the performance and ohmic resistances in the two cells. Compared with tubular cell, higher peak power density values of 933 mW cm⁻² at 850 °C was achieved, which is 2.2 times higher than the value of tubular cell. AC impedance indicated that lower performance of tubular cell was restricted by the ohmic loss at the operating temperatures.     

Carbon-resistant Ni-YSZ/Cu–CeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell

A NiO-YSZ/porous YSZ dual-layer hollow fiber with an asymmetric structure was fabricated by a co-spinning-sintering method. A dense YSZ electrolyte film was prepared on NiO-YSZ layer by dip-coating method and calcined at 1450 °C; subsequently a porous cathode was dip-coated on the dense YSZ electrolyte film using LSM-YSZ (in the weight ratio 4:1) ink to fabricate a micro tubular solid oxide fuel cell (MT-SOFC). Cu–CeO₂ catalyst was impregnated into the porous YSZ layer to form the second anode composition. The power output of the MT-SOFC with Ni-YSZ/Cu–CeO₂-YSZ graded anode was up to 242 mW cm⁻² operated at 850 °C using CH₄ as fuel and air as oxidant. Little carbon deposition was observed on the double anode using methane as the fuel after 60 h' stable operation.     

Synthesis and characterization of NiO/GDC–GDC dual nano-composite powders for high-performance methane fueled solid oxide fuel cells

GDC (gadolinium-doped ceria) is well known as a high oxygen ionic conductor and is a catalyst for the electrochemical reaction with methane fuel leading to the oxidation of deposited carbon that can clog the pores of the anode and break the microstructure of the anode. NiO/GDC–GDC dual nano-composite powders were synthesized by the Pechini process, which were used as an AFL (anode functional layer) or anode substrates along with a GDC electrolyte and LSCF–GDC cathode. The anodes, AFL, and electrolyte were fabricated by a tape-casting/lamination/co-firing. NiO–GDC anode and NiO/GDC–GDC anode-supported unit cells were evaluated in terms of their power density and durability. As a result, the NiO/GDC–GDC dual nano-composite demonstrated an improved power density from 0.4 W/cm² to 0.56 W/cm² with H₂ fuel/air and from 0.3 W/cm² to 0.56 W/cm² with CH₄ fuel/air at 650 °C. In addition, it could be operated for over 500 h without any degradation with CH₄ fuel.     

Electrochemical performance of a copper-impregnated Ni–Ce0.8Sm0.2O1.9 anode running on methane

Ni–Cu–Ce_0.8Sm_0.2O_1.9 anode-supported single cells were developed for the direct utilization of methane. An yttria-doped zirconia and Ce_0.8Sm_0.2O_1.9 bi-layer electrolyte and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ cathode layer were fabricated by slurry spin-coating. Cu was added to the anode by impregnation with a nitrate solution. The effects of Cu on the electrochemical performance of the anode were investigated in dry methane with respect to times of impregnation. Impregnation with Cu twice was determined to be optimal. Incorporating Cu into the anode improved electrochemical performance of the cells, reducing ohmic resistance and suppressing carbon deposition. At 700 °C, the single cell exhibited a maximum power density of 406 mW/cm² in dry methane. At a current density of 500 mA/cm², the cell maintained 98.6% of its initial voltage after operation for 900 min.     

Fuel-flexible operation of a solid oxide fuel cell with Sr0.8La0.2TiO3 support

Solid oxide fuel cells with Sr_0.8La_0.2TiO₃ anode-side supports, Ni- Sm-doped ceria adhesion layer, Ni- Y₂O₃-stabilized ZrO₂ (YSZ) anode active layer, YSZ electrolyte, and La_0.8Sr_0.2MnO₃(LSM)–YSZ cathode are described. These cells are stable in simulated natural gas at current densities as low as 0.2 A cm⁻². This represents much-improved stability against coking in natural gas, compared with conventional Ni–YSZ anode-supported SOFCs which rapidly coke, even at higher current densities. Cell operation in H₂ fuel with 50–100 ppm, H₂S results in an initial decrease in cell power density, but no long-term degradation occurs and full recovery to the initial performance level is observed after dry H₂ fuel flow is restored. Degradation is not observed during or after seven redox cycles between H₂ and air.