Conversion of methane to syngas in a solid oxide fuel cell with Ni-SDC anode and LSGM electrolyte

A solid oxide fuel cell constructed from Ni-SDC anode and LSGM electrolyte was applied to the partial oxidation of methane to syngas (CO+H₂) at 700-800 °C with the merits of co-generation of electricity and controllable O₂ supply. It was found that the co-generated syngas at H₂/CO ratio of 1.4-2.0 varied with applied current densities, CH₄ flow rates and operating temperatures. The cell voltage at 100 mA cm⁻² and 800 °C was 0.90 V, i.e. about 90 mW cm⁻² power density could be obtained. The cell operating at 50 mA cm⁻² for 24 h almost showed no degradation of the cell performance. The observed carbon deposition seemed mainly taking place by CH₄ cracking reaction.     

Tape Casting of Anode Supports for Solid Oxide Fuel Cells at Forschungszentrum Jülich

This contribution describes the development of tape casting for solid oxide fuel cells (SOFCs) anode supports starting with the characterization of the powders and ending with manufacturing of cells for stack testing. After casting the support, full cells were prepared by screen printing and sintering of the functional layers. The results of single‐cell and stack tests of the novel SOFC will be discussed. The new cell showed excellent electrochemical performance in single‐cell tests with more than 1.5 A/cm² (800°C, 0.7 V). Furthermore, stack tests showed no significant difference from earlier standard cells when operated at 800°C with a current density of 0.5 A/cm².     

Effect of pre-calcined ceramic powders at different temperatures on Ni-YSZ anode-supported SOFC cell/stack by low pressure injection molding

Recently, powder injection molding (PIM) has been exploited in the field of solid oxide fuel cells (SOFCs), especially for fabricating anode supports. The current study employs low pressure injection molding (LPIM) to manufacture near net shape, porous, tubular NiO-yttria stabilized zirconia (YSZ) anode supports for anode-supported SOFCs. The study investigates the effects of pre-calcining temperature of the ceramic powder on the microstructure, porosity and electrochemical performance of the cells in detail. Archimedes tests reveal that the porosity of an unreduced NiO-YSZ anode with 900 °C pre-calcined powder reaches a high of 25.9%, approaching the optimal value of 26%. Meanwhile, the anode prepared under this condition possesses more porous and homogeneous microstructures. At 800 °C, with humidified hydrogen as fuel and ambient air as the oxidant, the single cell with 900 °C pre-calcined powder delivers a maximum power density of 671 mW cm⁻² while the cell with raw powder, 555 mW cm⁻², and the cell with 1000 °C pre-calcined powder, 648 mW cm⁻². A four-cell stack is assembled by connecting four single cells in series. The stack could provide a maximum output power of 4.6 W and an open circuit voltage of 3.2 V when fuelled with humidified hydrogen at 800 °C.     

Nano-sized Sm0.5Sr0.5CoO3−δ as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9

Nano-sized Sm_0.5Sr_0.5CoO_3−δ (SSC) was fabricated onto the inner face of porous BaCe_0.8Sm_0.2O_2.9 (BCS) backbone by ion impregnation technique to form a composite cathode for solid oxide fuel cells (SOFCs) with BCS, a proton conductor, as electrolyte. The electro-performance of the composite cathodes was investigated as function of fabricating conditions, and the lowest polarization resistance, about 0.21 Ω cm² at 600 °C, was achieved with BCS backbone sintered at 1100 °C, SSC layer fired at 800 °C, and SSC loading of 55 wt.%. Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the migration of proton and the dissociative adsorption and diffusion of oxygen, respectively. There was an additional arc peaking at 1 Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions. With electrolyte about 70 μm in thickness, the simulated anode, cathode and bulk resistances of cells were 0.021, 0.055 and 0.68 Ω cm² at 700 °C, relatively, and the maximum power density was 307 mW cm⁻² at 700 °C.     

Investigation of 30-cell solid oxide electrolyzer stack modules for hydrogen production

The 30-cell nickel-yttria stabilized zirconia (Ni-YSZ) hydrogen electrode-supported planar solid oxide electrolyzer (SOE) stack modules were manufactured and tested at 800 °C in steam electrolysis mode for hydrogen production. The electrolysis efficiency of the stack modules was higher than 100% at a total steam and hydrogen flow of 2.1 sccm cm⁻², a H₂O/H₂ ratio of 80/20, and a current density of <0.2 A cm⁻². The electrolysis efficiency, current efficiency, and actual hydrogen production rate of the stack modules increased with increasing H₂O/H₂ ratio at a constant current density. However, the electrolysis and current efficiencies decreased steadily at high current densities. During hydrogen production, the stack modules were operated at 800 °C and a constant current density of 0.15 A cm⁻² for up to 1100 h. A steam conversion rate of 62% and current efficiency of 87.4% were obtained; the actual hydrogen production rate reached as high as 103.6 N L h⁻¹. Post-mortem analysis showed that delamination of the LSM–YSZ oxygen electrode mainly occurred in the steam and air inlet area of the 10×10 cm² cells.     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm^–2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm^–2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm² and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.     

Performances of micro-tubular solid oxide cell with novel asymmetric porous hydrogen electrode

Asymmetric-porous hollow-fiber has been fabricated by a phase-inversion process and employed as the hydrogen electrode for micro-tubular solid oxide cell (MT-SOC). The microstructure and electrochemical properties of MT-SOC were investigated in detail. The asymmetric-porous hydrogen electrode possesses unique two layer finger-like porous micro-structure with a thin functional layer and a thick fuel delivery layer. When the MT-SOC was operated in fuel cell mode, maximum power densities of 0.54, 0.71 and 1.25 W/cm² were obtained at 800, 850 and 900 °C, respectively. On the other hand, when the MT-SOC was operated in electrolysis mode at 900 °C with an applied voltage of 1.3 V, current densities of 0.68 A/cm² and 2.57 A/cm² were obtained at 30 vol.% and 80 vol.% absolute humidity (AH), respectively. These results indicate that novel-microstructured MT-SOC can be effectively fabricated towards high performance fuel cell and electrolysis cell.     

Fabrication and characterization of composite electrolyte for intermediate-temperature SOFC

In this study, a ceria-based composite electrolyte was investigated for intermediate-temperature solid oxide fuel cells (SOFCs) based on SDC-25 wt.% K₂CO₃. Sodium carbonate co-precipitation process by which SDC powder was adopted and sound cubic fluorite structure was formed after SDC powders were sintered at 750 °C for 3 h. The crystallite size of the particle was 21 nm in diameter as calculated from data obtained through X-ray diffraction. The conductivity of the composite electrolyte proposed in this study was much higher than that of pure SDC at the comparable temperature of 550-700 °C. The transition of the ionic conductivity occurred at 650 °C. Based on this type of composite electrolyte, single cell with the electrolyte thickness of 0.3 mm were fabricated using dry pressing, with nickel oxide adopted as anode and SSC as cathode. The single cell was then tested at 550-700 °C on home-made equipment in this study, using hydrogen/air. The maximum power density and open circuit voltage (OCV) achieved 600 mW cm⁻² and 1.05 V at 700 °C, respectively.     

Performance of an anode-supported tubular solid oxide fuel cell (SOFC) under pressurized conditions

An anode-supported tubular solid oxide fuel cell (SOFC) with a 15-μm thick YSZ electrolyte and an active area of 100 cm² was successfully fabricated by co-firing process, and the cell performance was measured under both atmospheric and pressurized conditions. The experimental results showed that the cell performance was significantly improved under the pressurized condition. When the pressure was increased from 1 to 6 atm, the maximum power density increased from 135.0 to 159.0 mW cm⁻² at 650 °C, and from 266.7 to 306.0 mW cm⁻² at 800 °C. The maximum power density at 800 °C and 4 atm was decreased from 334.8 to 273.9 mW cm⁻² when increasing the fuel utilization from 10% to 90%. Under the test condition of 70% fuel utilization, 800 °C and 4 atm, the cell could run stably at 0.7 V and 350 mA cm⁻² for 50 h, almost without any performance loss.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

Tubular solid oxide fuel cells fabricated by a novel freeze casting method

Freeze casting is an established method for fabricating porous ceramic structures with controlled porosity and pore geometries. Herein, we developed a novel freeze casting and freeze drying process to fabricate tubular anode supports for solid oxide fuel cells (SOFCs). Freeze casting was performed by injecting aqueous anode slurry to a dual-purpose freeze casting and freeze drying mold wrapped with peripheral coils for flowing a coolant. With the use of an ice barrier layer, proper control of the experimental setup, and adjustments in the drying temperature profile, complete drying of the individual anode tubes was achieved in 4 hours. The freeze-cast anode tubes contained radially aligned columnar pore channels, thus significantly enhancing the gaseous diffusion. SOFC single cells with conventional Ni/yttria-stabilized zirconia/strontium-doped lanthanum manganite materials were prepared by dip coating the thin functional layers onto the anode support. Single cell tests showed that the concentration polarization was low owing to the highly porous anode support with directional pores. With H₂/N₂ (1:1) fuel, maximum power densities of 0.47, 0.36, and 0.27 W/cm² were recorded at 800°C, 750°C, and 700°C, respectively. Our results demonstrate the feasibility of using freeze casting to obtain tubular SOFCs with desired microstructures and fast turn-around times.     

Novel CO2-tolerant Co-based double perovskite cathode for intermediate temperature solid oxide fuel cells

For the commercial application of solid oxide fuel cells (SOFCs), CO₂-tolerant cathode materials with high electrochemical activity are required. Here, we discuss the performance of double perovskite Pr_0.2Sr_1.8CoTiO_6?δ (P02STC) as a potential cathode material for SOFCs. P02STC has a cubic structure and keeps lattice structure stable in the CO₂ atmosphere. The average thermal expansion coefficient is 17.8 × 10^–6 K^–1 at 30–900 °C in air. The P02STC cathode exhibits good electrochemical performance with a low polarization resistance of 0.080 Ωcm² at 700 °C. The P02STC cathode shows good structure stability, electrochemical performance stability, and excellent tolerance to CO₂ poisoning for the symmetrical cells based on the 350 h stability test in air and the 150 h stability test in O₂ containing 5%CO₂ at 700 °C. The electrolyte-supported single cell with a P02STC cathode shows a maximum power density of 677 mW cm^? 2 at 800 °C. The single cell operates stably for 250 h at a constant current of 0.3 A/cm² without obvious degrading performance. According to all of the experimental results, the P02STC sample might be a promising candidate cathode for SOFCs.     

Synthesis and characterization of 10Sc1CeSZ powders prepared by a solid–liquid method for electrolyte-supported solid oxide fuel cells

10Sc1CeSZ is one of the most important electrolyte materials used for solid oxide fuel cells (SOFCs). A novel solid–liquid method (SLM) was adopted for the preparation of 10Sc1CeSZ nanopowder. High-purity, single-phase, homogeneous 10Sc1CeSZ powder was successfully prepared using this method. The resulting powders and ceramic pellets were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). A cubic structure was obtained when the value of the specific surface area (SSA) of the starting material ZrO₂ was greater than 60 m² g⁻¹. A conductivity of 0.14 S cm⁻¹ at 800 °C was achieved for the sintered pellets. The performance of the electrolyte-supported cell (ESC) NiO+GDC/10Sc1CeSZ /10Sc1CeSZ +LSM reached 0.66 W cm⁻² at 0.75 V and 850 °C.     

Properties of CuMn1.5Ni0.5O4 spinel as high-performance cathode for solid oxide fuel cells

Spinel oxide cathode has made great progress in solid oxide fuel cells (SOFCs) because of its special characteristics different from perovskite. In this study, a spinel-structured SOFC cathode, CuMn_1.5Ni_0.5O₄ (CMN), is proposed. Rietveld refinement shows that CMN takes the cubic structure of the space group of P4₃32. CMN shows a high conductivity of about 70.0–91.2 S cm⁻¹ at 600–800 ºC in the air and exhibits good catalytic activity for oxygen. A symmetric cell with CMN-GDC composite cathode demonstrates a low Rp of 0.047 Ω cm² at 800 ºC. The charge transfer of oxygen is the rate-limiting process at lower temperatures. The performance test results of the button cell with CMN-GDC composite cathode are excellent, with high power densities of 1342.4 mW cm⁻² at 800 ºC. After a110h long-term test, the cell runs stably, and no microstructure damage is observed.     

Low-temperature SOFCs using biomass-produced gases as fuels

The electromotive force (e.m.f) is calculated for solid oxide fuel cells (SOFCs) based on doped ceria electrolytes using biomass-produced gases (BPG, 14.7% CO, 14.2% CO₂, 15.3% H₂, 4.2% CH₄, and 51% N₂) as fuels and air as oxidant. It reveals that the BPG derived e.m.f. is very close to hydrogen when doped ceria is used as the electrolyte. A 35-m-thick samaria-doped ceria based single cell was tested between 450 and 650°C using BPG as fuel. Maximum power density of about 700 mW cm^–2 was achieved at 650 °C. The open-circuit voltage at 450 °C was 0.96 V, close to the calculated value. However, the cell power density using BPG as fuel was relatively lower than that using humidified hydrogen (3% H₂O), and close to that using humidified methane (3% H₂O). Impedance measurements indicate that the relatively lower power output may be attributed to the high anode--electrolyte interfacial polarization resistance when BPG is used as fuel.     

Na+ doping activates and stabilizes layered perovskite cathodes for high-performance fuel cells

A highly active mixed conductive cathode is required for solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) at reduced temperatures, which is one of the most important factors for their commercialization. Herein, we propose a Na⁺ doping strategy to activate and stabilize the triple-conducting (H⁺/O²⁻/e⁻) layered perovskite oxide of representative NdBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (NBSCF) for high-performance YSZ fuel cells. The results show that Na⁺ doping enhances the electrochemical properties of the NBSCF cathode, with polarization impedance decreasing from 0.105 to 0.080 Ω cm² at 750 °C and output power increasing from 946.05 to 1435.75 mW cm⁻² at 800 °C. Furthermore, high-temperature XRD (HT-XRD) and the oxygen temperature-programmed desorption (O₂-TPD) further confirm that Na⁺ doping can improve the structural stability of NBSCF. The single cell with a Na-doped NBSCF cathode showed no degradation of current density for more than 120 h at 700 °C and exhibited good stability. This work demonstrates the promise of Na⁺ doping for layered perovskite cathodes and an effective way to promote fuel cell performance.     

Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

A Novel Cell‐Array Design for Single Chamber SOFC Microstack

A novel design of single chamber solid oxide fuel cell (SC‐SOFC) microstack with cell‐array arrangement is fabricated and operated successfully in a methane–oxygen–nitrogen mixture. The small stack, consisting of five anode‐supported single cells connected in series, exhibits an open circuit voltage (OCV) of 4.74 V at the furnace temperature of 600 °C and a maximum power output of 420 mW (total active electrode area is 1.4 cm²) at the furnace temperature of 700 °C. A gas mixture of CH₄/O₂ = 1 leads to best performance and stability.     

Slip casting combined with colloidal spray coating in fabrication of tubular anode-supported solid oxide fuel cells

A simple and cost-effective slip casting technique was successfully developed to fabricate NiO–YSZ anode substrates for tubular anode-supported single SOFCs. An YSZ electrolyte film was coated on the anode substrates by colloidal spray coating technique. A single cell, NiO–YSZ/YSZ (20 μm)/LSM–YSZ, using the tubular anode supports with YSZ coating, was assembled and tested to demonstrate the feasibility of the techniques applied. Using humidified hydrogen (75 ml/min) as fuel and ambient air as oxidant, the maximum power densities of the cell were 760 mW/cm² and 907 mW/cm² at 800 °C and 850 °C, respectively. The observed OCV was closed to the theoretical value and the SEM results revealed that the microstructure of the anode fabricated by slip casting is porous while the electrolyte film coated by colloidal spray coating is dense.