Effects of anode surface modification on the performance of low temperature SOFCs

An anode functional layer (AFL, ∼5 μm) for improving the cell performance was fabricated by the slurry spin coating method on the porous surface of an anode substrate. The effects of the AFL on the anode/electrolyte interfacial morphology and the Sm_0.2Ce_0.8O_1.9 (SDC) film deposition process were evaluated. And the electrochemical characteristics of the cells with and without the AFL were tested for comparison. With the AFL layer, the cell performance was greatly improved and the maximum power density was increased from 0.733 to 0.884 W cm⁻² at 600 °C and from 1.085 to 1.213 W cm⁻² at 650 °C. The systematical analysis indicated that the AFL could effectively reduce the anode polarization loss by increasing the three-phase boundary (TPB) length.     

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.     

Direct borohydride fuel cell using Ni-based composite anodes

In this study, nickel-based composite anode catalysts consisting of Ni with either Pd on carbon or Pt on carbon (the ratio of Ni:Pd or Ni:Pt being 25:1) were prepared for use in direct borohydride fuel cells (DBFCs). Cathode catalysts used were 1 mg cm⁻² Pt/C or Pd electrodeposited on activated carbon cloth. The oxidants were oxygen, oxygen in air, or acidified hydrogen peroxide. Alkaline solution of sodium borohydride was used as fuel in the cell. High power performance has been achieved by DBFC using non-precious metal, Ni-based composite anodes with relatively low anodic loading (e.g., 270 mW cm⁻² for NaBH₄/O₂ fuel cell at 60 °C, 665 mW cm⁻² for NaBH₄/H₂O₂ fuel cell at 60 °C). Effects of temperature, oxidant, and anode catalyst loading on the DBFC performance were investigated. The cell was operated for about 100 h and its performance stability was recorded.     

Performance of dimethoxymethane and trimethoxymethane in liquid-feed direct oxidation fuel cells

The present study involves the evaluation of dimethoxymethane (DMM) (formaldehyde dimethyl acetal, or methylal) and trimethoxymethane (TMM) (trimethyl orthoformate) in direct oxidation liquid-feed fuel cells as novel oxygenated fuels. We have demonstrated that sustained oxidation of TMM at high current densities can be achieved in half-cells and liquid-feed polymer electrolyte fuel cells 1, 2 and 3. In the present study, the performance of dimethoxymethane and trimethoxymethane was compared with that of methanol in 2″ × 2″ (25 cm² electrode area) and 4″ × 6″ (160 cm² electrode area) direct oxidation fuel cells. The impact of various parameters upon cell performance, such as cell temperature, anode fuel concentration, cathode fuel pressure and flow (O₂ and air), was investigated. Fuel crossover rates in operating fuel cells were also measured for methanol, DMM, and TMM and characterized in terms of concentration and temperature effects. Although DMM and more particularly TMM may present some logistical advantages over that of methanol, such as possessing a higher boiling point, higher flash point, and lower toxicity, the overall performance was observed to be inferior to that of methanol under typical fuel cell operating conditions.     

Performance improvement of anode-supported electrolytes for planar solid oxide fuel cells via a tape-casting/lamination/co-firing technique

Recently, solid oxide fuel cells (SOFCs) have attracted considerable attention because of their low emissions, high-energy conversion efficiency, and flexible usage of various fuels. One of the key problems of applying flat-type SOFCs to large-scale power generation is that unit cells of large area and with a high degree of flatness cannot be manufactured satisfactorily.In this study, the effects of tape-casting, laminating, and co-firing conditions on the flatness of anode-supported electrolyte unit cells have been investigated to improve the cell performance of unit cells. The cells are composed of a Ni-yttria-stabilized zirconia (YSZ) anode, a Ni-YSZ anode functional layer (AFL), a YSZ electrolyte, and a lanthanum strontium manganate (LSM)-YSZ cathode. The flatness of the anode-supported electrolyte is optimized by controlling the firing schedule, the lamination method, and the applied load during firing. A 5 cm × 5 cm (active area 4 cm × 4 cm) unit cell having a reasonable flatness of 55 μm/5 cm shows a higher power output of 11.4 W as compared with 7.7 W a unit cell with a flatness of 200 μm/5 cm, when operating at 800 °C with humidified hydrogen fuel.     

Study of slurry spin coating technique parameters for the fabrication of anode-supported YSZ Films for SOFCs

A slurry spin coating method was developed to fabricate gas-tight anode-supported YSZ films for solid oxide fuel cells (SOFCs). Several technique parameters for slurry spin coating, such as the slurry viscosity, spinning speed, number of coating cycles, film thickness and their effects on YSZ electrolyte film were investigated. SEM results, open-circuit voltage (OCV) values and cell performance indicated that these parameters had crucial and obvious influences on YSZ film quality and fuel cell performance. Based on the optimized parameters, anode-supported YSZ films and several single fuel cells were successfully fabricated and tested. An OCV as high as 1.06 V was obtained at 800 °C and maximum power densities of 900, 1567, 2005 mW cm⁻² were achieved at 700, 750, 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Effect of nanostructured anode functional layer thickness on the solid-oxide fuel cell performance in the intermediate temperature

Effect of anode functional layer thickness on the performance of solid-oxide fuel cells (SOFCs) has been investigated in the intermediate temperatures of 600–650 °C. Three types of cells with different thickness (0, 4, 10 micron) of nanostructured anode functional layer (AFL) consisting of Ni-ScSZ (Scandia stabilized zirconia) are prepared. The SOFCs consist of Ni-3YSZ (3 mol% yttria stabilized zirconia) anode tube support with the AFL, ScSZ electrolyte, and LSCF (lanthanum strontium cobalt ferrite) and GDC (gadolinium doped ceria) mixture cathode. It is shown that the performance of the cell is improved as the thickness of the anode functional layer increases. Power densities of the cell with 10 micron thick AFL at 600 and 650 °C are shown to be 0.22 and 0.27 W/cm² at 0.75 V, respectively. According to impedance spectroscopy, improvement of both ohmic and polarization resistances has been observed by increasing the thickness of the AFL, suggesting that the AFL also acts as a better contact layer between the electrolyte and the anode support, and the effectiveness of the AFL by optimizing the thickness.     

Effect of carbon black concentration in carbon fiber paper on the performance of low-temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) made from carbon fiber paper containing carbon black in proton exchange membrane fuel cells (PEMFCs) in order to determine the relationship between carbon black content and fuel cell performance. The connection between fuel cell performance and the carbon black content of the carbon fiber paper is discussed, and the effects of carbon black on the carbon fiber paper's thickness, density, and surface resistivity are investigated. When a carbon fiber paper GDL contains 10 wt% phenolic resin and 2% carbon black, and reaction area was 25 cm² and operating temperature 40 °C, tests show that a carbon electrode fuel cell could achieve 1026.4 mA cm⁻² and maximum power of 612.8 mW cm⁻² under a 0.5 V load.     

Solvent processible, high-performance partially fluorinated copoly(arylene ether) alkaline ionomers for alkaline electrodes

A solvent processable, low water uptake, partially fluorinated copoly(arylene ether) functionalized with pendant quaternary ammonium groups (QAPAE) was synthesized and uses as the ionomer in alkaline electrodes on fuel cells. The quaternized polymers containing fluorinated biphenyl groups were synthesized via chloromethylation of copoly(arylene ether) followed by amination with trimethylamine. The resulting ionomers were very soluble in polar, aprotic solvents. Highly aminated ionomers had conductivities approaching 10 mS cm⁻¹ at room temperature. Compared to previous ionomers based on quaternized poly(arylene ether sulfone) (QAPSF) with similar ion exchange capacity (IEC), the water uptake of QAPAE was significantly less due to the hydrophobic octafluoro-biphenyl groups in the backbone. The performance of the fuel cell electrodes made with the QAPAE ionomers was evaluated as the cathode on a hybrid AEM/PEM fuel cell. The QAPAE alkaline ionomer electrode with IEC = 1.22 meq g⁻¹ had superior performance to the electrodes prepared with QAPSF, IEC = 1.21 meq g⁻¹ at 25 and 60 °C in a H₂/O₂ fuel cell. The peak power densities at 60 °C were 315 mW cm⁻² for QAPAE electrodes and 215 mW cm⁻² for QAPSF electrodes.     

Continuously gradient anode functional layer for BCZY based proton-conducting fuel cells

This study examined the effects of a continuously gradient anode functional layer (AFL) on the performance of the BaCe_0.5Zr_0.35Y_0.15O_3−δ (BCZY) based proton conducting fuel cell. The gradient AFL composed of NiO and BCZY was fabricated by the electrostatic slurry spray deposition (ESSD) technique with a rotation stage. For the comparison, the single cells without AFL and with homogeneous AFL were also prepared and the electrochemical properties were evaluated by full cell test and electrochemical impedance spectroscopy (EIS). Power density of the single cell with the gradient AFL exhibited 521 mW/cm² at 700 °C which was higher by 48% and 16% compared to single cells without the AFL and with the homogeneous AFL. In addition, impedance spectra showed that both ohmic and polarization resistances under open circuit voltage (OCV) at 700 °C decreased from 0.3218 and 0.3462 to 0.2168 and 0.2203 Ωcm². This performance enhancement is primarily attributed to modified microstructure of the AFL which has a continuously gradient interface to effectively reduce a mismatch between electrolyte and anode.     

Highly efficient and durable anode-supported SOFC stack with internal manifold structure

We have developed a solid oxide fuel cell (SOFC) stack with an internal manifold structure. The stack, which is composed of 25 anode-supported 100-mm-diameter SOFCs, provided an electrical conversion efficiency of 56% (based on the lower heating value of methane, which was used as a fuel) and an output of 350 W when the fuel utilization, current density, and operating temperature were 75%, 0.3 A cm⁻², and 1073 K, respectively. The electrical efficiency and the output were maintained for 1100 h. The cell voltage fluctuation was ±2% for 25 cells. The relationship between average cell voltage and current density in the 25-cell stack was as almost the same as that in the 1- and 10-cell stacks, which suggests that our stack provides almost the same cell performance regardless the number of the cells.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

Fabrication and characterization of micro PEM fuel cells using pyrolyzed carbon current collector plates

A novel fabrication technique for micro proton exchange membrane fuel cells (μPEMFCs) based on carbon-MEMS (C-MEMS) was optimized to yield higher performance cells. Polymer manufacturing is relatively easy compared to directly patterning graphite as is typically done to make fuel cell bipolar plates. In a C-MEMS approach, fuel cell bipolar plates are fabricated by first patterning polymer Cirlex^® sheets. By subsequently pyrolyzing the machined polymer sheets at high temperature in an inert atmosphere, carbon bipolar plates with intricate groove structures to distribute the reactants are obtained. Using an improved assembly technique such as polishing the carbonized plates to minimize the contact resistance between gas diffusion layers (GDL) and bipolar plates, better pyrolysis temperature control and a better end plate design, a μPEMFC with a 0.64 cm² active surface was fabricated using the newly developed bipolar plates. At 1 atm and 25 °C a maximum power density of ∼76 mW cm⁻² was obtained, and at 2 atm and 25 °C ∼85 mW cm⁻² was achieved. These data are comparable with data reported in the literature for μPEMFCs and are a dramatic improvement over earlier results reported for the same C-MEMS based fuel cell. Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry were carried out to characterize steady-state and transient characteristics of the novel C-MEMS fuel cell.     

Fabrication and testing of coplanar single-chamber micro solid oxide fuel cells with geometrically complex electrodes

Coplanar single-chamber micro solid oxide fuel cells (SC-μSOFCs) with curvilinear microelectrode configurations of arbitrarily complex two-dimensional geometry were fabricated by a direct-write microfabrication technique using conventional fuel cell materials. The electrochemical performance of two SC-μSOFCs with different electrode shapes, but comparable electrode and inter-electrode dimensions, was characterized in a methane–air mixture at 700 °C. Both cells exhibited stable open circuit voltage and peak power density of 0.9 V and 2.3 mW cm⁻², respectively, indicating that electrode shape did not have a significant influence on the performance of these fuel cells.     

Effect of carbon fiber cloth with different structure on the performance of low temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different structure in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the structure of the carbon fiber cloth and fuel cell performance.The paper discusses the relationship between fuel cell performance and structure of the carbon fiber cloth, and also examines the effect of the carbon fiber cloth’s thickness, air permeability, surface resistivity, XRD and elemental analysis. Carbon fiber cloth is carbonized at rates of 190, 220, 250, 280, and 310 °C min⁻¹ respectively, and the resulting carbon fiber cloth is tested in cells. When the test piece area is 25 cm², the test temperature 40 °C, the gasket thickness 0.36 mm, and the carbonization rate 280 °C min⁻¹, a fuel cell using the carbon fiber cloth achieves a current density of 1968 mA cm⁻² and a maximum power density of 633 mW cm⁻² at 0.3 V.     

Effect of carbon fiber paper made from carbon felt with different yard weights on the performance of low temperature proton exchange membrane fuel cells

This study employed fuel cell gas diffusion layers (GDLs) consisting of carbon fiber paper made from carbon fiber felt with different yard weights in proton exchange membrane fuel cells (PEMFCs), and investigated the relationship between the yard weight of the carbon fiber paper and the fuel cell performance and thickness of the gasket. In this paper we discuss the relationship between carbon fiber felt with different yard weights and fuel cell performance and also explore the effect of carbon fiber paper thickness, air permeability, surface resistivity, and structural study. We focused on the material used for the gas diffusion layer in this study. Carbon fiber paper made in-house in this study contained 10 wt% (all percentages are by weight unless otherwise noted) phenolic resin. When the tested area was 25 cm², the test temperature was 40 °C, the gasket thickness was 0.06 mm, and the yard weight 70 g m⁻², fuel cell current density was 1968 mA cm⁻² at a load 0.3 V. When the gasket thickness was 0.36 mm and yard weight was 190 g m⁻², fuel current density was 1710 mA cm⁻² at a load of 0.3 V.     

An abiotically catalyzed glucose fuel cell for powering medical implants: Reconstructed manufacturing protocol and analysis of performance

Although the first abiotically catalyzed glucose fuel cells have already been developed as sustainable power supply for medical implants in the 1970s, no detailed information concerning the fabrication of these devices has been published so far. Here we present a comprehensive manufacturing protocol for such a fuel cell, together with a detailed analysis of long-term performance in neutral buffer containing physiological amounts of glucose and oxygen. In air saturated solution a power density of (3.3 ± 0.2) μW cm⁻² is displayed after 10 days of operation that gradually decreases to a value of (1.0 ± 0.05) μW cm⁻² in the course of 224 days. A novelty of this work is the characterization of fuel cell performance with individually resolved electrode potentials. Using this technique, we can show that the major part of performance degradation originates from a positive shift of the anode potential, indicating that a more poisoning-resistant glucose oxidation catalyst would improve the degradation behavior of the fuel cell. As further factors influencing performance an incomplete reactant separation and a mass transfer governed cathode reaction under the relatively low oxygen partial pressures of body tissue have been identified. Consequently we propose an oxygen depleting electrode interlayer and the application of more effective oxygen reduction catalysts as promising strategies to further improve the fuel cell performance under physiological concentrations of glucose and oxygen.     

Enhanced performance of a single-chamber solid oxide fuel cell with an SDC-impregnated cathode

The electrochemical performance of anode-supported single-chamber solid oxide fuel cells (SC-SOFCs) with and without SDC-impregnated cathodes was compared in a diluted methane–oxygen mixture. These cells were made of conventional materials including yttrium-stabilized zirconia (YSZ) thin film, a Ni + YSZ anode and a La_0.7Sr_0.3MnO₃ (LSM) cathode. Our results showed that the cell performance was greatly enhanced with the SDC-impregnated LSM cathode. At a furnace temperature of 750 °C, the maximum power density was as high as 404 mW cm⁻² for a CH₄ to O₂ ratio of 2:1, which was 4.0 times higher than the cell with a pure LSM cathode (100 mW cm⁻²). The overall polarization resistance of the impregnated cell was 1.6 Ω cm², which was much smaller than that of the non-impregnated one (4.2 Ω cm²). The impregnation introduced SDC nanoparticles greatly extended the electrochemical active zone and hence greatly improved the cell performance.     

An onboard hydrogen generation method based on hydrides and water recovery for micro-fuel cells

Micro-proton exchange membrane fuel cells are considered to be the next generation power sources for micro-scale power applications, but onboard hydrogen storage and generation with high energy density at the small scale is still a technical barrier. This paper introduces a hydrogen generation method based on an onboard hydride fuel and a byproduct water recovery mechanism for micro-hydrogen PEM fuel cells. The water recovery is carried out by water diffusion from the more humid cathode side to the less humid anode side through the proton exchange membrane. The micro-fuel cells based on this water recovery method were constructed and tested. The results demonstrate that the relative humidity has a significant affect on the fuel cell performance as well as the opening area on the cover layer, the type of hydrides, and the thickness of the Nafion membrane also can affect the fuel cell performance. A 10 mm³ prototype water recovery micro-fuel cell has been built and tested, and the device has produced a maximum power density of 104 W L⁻¹ and a maximum energy density of 313 W h L⁻¹.     

Effect of hydrophobic gas diffusion layers on the performance of the polymer exchange membrane fuel cell

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different concentrations of hydrophobic agents in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the hydrophobic agent content of the carbon fiber cloth and fuel cell performance.The paper examines the effect of hydrophobic agent content on GDL thickness, contact angle, air permeability, and surface and through-plane resistivity. Carbon fiber cloth is impregnated with hydrophobic agent concentrations of 0, 3, 5, 10, 30, and 50 wt%, and the resulting GDLs are subjected to performance tests. When the test piece area is 25 cm², the test temperature 80 °C, the gasket thickness 0.36 mm, and the hydrophobic agent content 5 wt%, a fuel cell using the GDL has a current density of 1430 mA cm⁻² at 0.3 V.