Nafion/sulfated β-cyclodextrin composite membranes for direct methanol fuel cells

Proton-conducting composite membranes based on H⁺-form sulfated β-cyclodextrin (sb-CD) in a Nafion matrix are prepared via the solution-casting method and their methanol permeabilities, proton conductivities, proton diffusion coefficients and cell performances are measured. The methanol permeabilities of the composite membranes increase very slightly with increases in their sb-CD content. As a result of adding sb-CD with its many sulfonic acid groups into the Nafion matrix, the proton conductivities of the composite membranes increase with increases in their sb-CD content. The methanol permeability and proton conductivity results are used to show that the best selectivity of the membranes is that of the NC5 membrane (‘NCx’ denotes a Nafion/sb-CD composite membrane containing x wt.% sb-CD). The proton diffusion coefficients are measured with ¹H pulsed field gradient nuclear magnetic resonance (PFG-NMR) and found to increase with increase in the sb-CD content in the order NC5 > NC3 > NC1 > NC0. Thus the presence of sb-CD in the Nafion membranes increases the proton diffusion coefficients as well as the proton conductivities, ionic cluster size, water uptakes and the ion-exchange capacities (IECs). A maximum power density of 58 mW cm⁻² is obtained for the NC5 membrane. The combination of these effects should lead to an improvement in the performance of direct methanol fuel cells prepared with Nafion/sb-CD composite membranes.     

Nafion/Pd–SiO2 nanofiber composite membranes for direct methanol fuel cell applications

One of the major challenges for direct methanol fuel cells is the problem of methanol crossover. With the aim of solving this problem without adverse effects on the membrane conductivity, Nafion/Palladium–silica nanofiber (N/Pd–SiO₂) composite membranes with various fiber loadings were prepared by a solution casting method. The silica-supported palladium nanofibers had diameters ranging from 100 nm to 200 nm and were synthesized by a facile electro-spinning method. The thermal properties, ionic exchange capacities, water uptake, proton conductivities, methanol permeabilities, chemical structures, and micro-structural morphologies were determined for the prepared membranes. It was found that the transport properties of the membranes were affected by the fiber loading. All of the composite membranes showed higher water uptake and ion exchange capacities compared to commercial Nafion 117 and proved to be thermally stable for use as proton exchange membranes. The composite membranes with optimum fiber content (3 wt%) showed an improved proton conductivity of 0.1292 S cm⁻¹ and a reduced methanol permeability of 8.36 × 10⁻⁷ cm² s⁻¹. In single cell tests, it was observed that, the maximum power density measured with composite membrane is higher than those of commercial Nafion 117.     

Electroplated Ni–Fe2O3 composite coating for solid oxide fuel cell interconnect application

Ni–Fe₂O₃ composite coating was applied onto ferritic stainless steel using the cost-effective method of electroplating for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. By comparison, the coated and bare steels were evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the coated steel was close to that of the bare steel after initially rapid mass gain. The mass gain of the coated steel was higher than that of the bare steel owing to the formation of double-layer oxide structure with an outer layer of (Ni,Fe)₃O₄/NiO atop an inner layer of Cr₂O₃. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr₂O₃ scale thermally grown on the bare steel.     

Nanostructured palladium–La0.75Sr0.25Cr0.5Mn0.5O3/Y2O3–ZrO2 composite anodes for direct methane and ethanol solid oxide fuel cells

A palladium-impregnated La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ/yttria-stabilized zirconia (LSCM/YSZ) composite anode is investigated for the direct utilization of methane and ethanol fuels in solid oxide fuel cells (SOFCs). Impregnation of Pd nanoparticles significantly enhances the electrocatalytic activity of LSCM/YSZ composite anodes for the methane and ethanol electrooxidation reaction. At 800 °C, the maximum power density is increased by two and eight times with methane and ethanol fuels, respectively, for a cell with the Pd-impregnated LSCM/YSZ composite anode, as compared with that using a pure LSCM/YSZ anode. No carbon deposition is observed during the reaction of methane and ethanol fuels on the Pd-impregnated LSCM/YSZ composite anode. The results show the promises of nanostructured Pd-impregnated LSCM/YSZ composites as effective anodes for direct methane and ethanol SOFCs.     

High performance Ni–Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells

A novel anode consisting of Ni and Sm₂O₃ with negligible oxygen-ion conductivity was developed for intermediate-temperature solid oxide fuel cells (SOFCs). Its triple phase boundary length is pretty small compared with the conventional Ni-samaria doped ceria (SDC) anode, of which SDC is one of the electrolytes having high oxygen-ion conductivity. Even so, single cells with Ni–Sm₂O₃ anodes generated peak power density of 542 mW cm⁻² at 600 °C, comparable to, if not higher than those with the Ni–SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni–Sm₂O₃ exhibited lower interfacial polarization resistance than Ni–SDC. The high electrochemical performance, which might be related to the high catalytic activity of Sm₂O₃ and the unique microstructures of the Ni–Sm₂O₃, suggests a viable alternative to the conventional anodes for SOFCs.     

Pr2NiO4–Ag composite cathode for low temperature solid oxide fuel cells with ceria-carbonate composite electrolyte

Pr₂NiO₄–Ag composite was synthesized and evaluated as cathode component for low temperature solid oxide fuel cells based on ceria-carbonate composite electrolyte. X-ray diffraction analysis reveals that the formation of a single phase K₂NiF₄–type structure occurs at 1000 °C and Pr₂NiO₄–Ag composite shows chemically compatible with the composite electrolyte. Symmetrical cells impedance measurements prove that Ag displays acceptable electrocatalytic activity toward oxygen reduction reaction at the temperature range of 500–600 °C. Single cells with Ag active component electrodes present better electrochemical performances than those of Ag-free cells. An improved maximum power density of 695 mW cm⁻² was achieved at 600 °C using Pr₂NiO₄–Ag composite cathode, with humidified hydrogen as fuel and air as the oxidant. Preliminary results suggest that Pr₂NiO₄–Ag composite could be adopted as an alternative cathode for low temperature solid oxide fuel cells.     

Performance of organic–inorganic hybrid anion-exchange membranes for alkaline direct methanol fuel cells

A series of organic–inorganic membranes were prepared through sol–gel reaction of quaternized poly(vinyl alcohol) (QAPVA) with different contents of tetraethoxysilanes (TEOS) for alkaline direct methanol fuel cells. These hybrid membranes are characterized by FTIR, X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDXA) and thermo gravimetric analysis (TGA). The ion exchange content (IEC), water content, methanol permeability and conductivity of the hybrid membranes were measured to evaluate their applicability in fuel cells. It was found that the addition of silica enhanced the thermal stability and reduced the methanol permeability of the hybrid membranes. The hybrid membrane M-5, for which the silica content was 5 wt%, showed the lowest methanol permeability and the highest ion conductivity among the three hybrid membranes. The ratio of conductivity to methanol permeability of the membrane M-5 indicated that it had a high potential for alkaline direct methanol fuel cell applications.     

Thermophysical properties and devitrification of SrO–La2O3–Al2O3–B2O3–SiO2-based glass sealant for solid oxide fuel/electrolyzer cells

Thermal behaviors and stability of glass/glass–ceramic-based sealant materials are critical issues for high temperature solid oxide fuel/electrolyzer cells. To understand the thermophysical properties and devitrification behavior of SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system, glasses were synthesized by quenching (25 − X)SrO–20La₂O₃–(7 + X)Al₂O₃–40B₂O₃–8SiO₂ oxides, where X was varied from 0.0 mol% to 10.0 mol% at 2.5 mol% interval. Thermal properties were characterized by dilatometry and differential scanning calorimetry (DSC). Microstructural studies were performed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). All the compositions have a glass transition temperature greater than 620 °C and a crystallization temperature greater than 826 °C. Also, all the glasses have a coefficient of thermal expansion (CTE) between 9.0 × 10⁻⁶ K⁻¹ and 14.5 × 10⁶ K⁻¹ after the first thermal cycle. La₂O₃ and B₂O₃ contribute to glass devitrification by forming crystalline LaBO₃. Al₂O₃ stabilizes the glasses by suppressing devitrification. Significant improvement in devitrification resistance is observed as X increases from 0.0 mol% to 10.0 mol%.     

LaNbO4 toughened NiO–Y2O3 stabilized ZrO2 composite for the anode support of planar solid oxide fuel cells

NiO–Y₂O₃ stabilized ZrO₂ (YSZ) composite is the state-of-the-art material for the anode support of planar solid oxide fuel cells (SOFCs). To improve its fracture toughness (K_C), lanthanum orthoniobate LaNbO₄ is synthesized by the method of solid state reaction and added to the mixture of YSZ and NiO at the weight ratio of 47:53. The content of LaNbO₄ in the composite is in the range between 5 and 30 wt%. The microstructure of the composites is examined by scanning electron microscopy (SEM); and the chemical compatibility among the components is evaluated by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Vickers hardness test is performed for estimating the K_C of the composites. The results indicate that the K_C increases with the addition of LaNbO₄ in the composites; and the toughening effect is associated with the grain-refinement in the composite and the domain switch in the monoclinic LaNbO₄.     

Evaluation of Sr0.88Y0.08TiO3–CeO2 as composite anode for solid oxide fuel cells running on CH4 fuel

Sr_0.88Y_0.08TiO₃ (YST) was synthesized and the performance of a YST–CeO₂ composite as an alternative anode for the direct utilization of CH₄ in solid oxide fuel cells (SOFCs) was investigated. X-ray diffraction showed that YST had good chemical compatibility with CeO₂ and YSZ (8 mol% Y-doped ZrO₂). The shrinkage of the YST–CeO₂ composite on sintering was less than that of pure YSZ and CeO₂, and its thermal-expansion behavior was similar to that of YSZ. With YSZ as electrolyte, ScSZ (10 mol% Sc-doped ZrO₂)–LSM (La_0.8Sr_0.2MnO₃) as cathode, and YST–CeO₂ composite as anode, single cells were prepared and tested in both H₂ and CH₄. The maximum power density obtained at 900 °C was 161.7 mW cm⁻² in H₂ atmosphere and 141.3 mW cm⁻² in CH₄. The results demonstrated the potential of using YST–CeO₂ composite as the anode for SOFCs.     

SiO2–Al2O3–Y2O3–ZnO glass sealants for intermediate temperature solid oxide fuel cell applications

In this study, eleven alkaline earth metal and B₂O₃-free SiO₂–Al₂O₃–Y₂O₃–ZnO glass sealants were prepared. The thermal stability, adhesion, and sealing properties of the glass systems with respect to SUS430 stainless steel (SUS430) were assessed for use in intermediate temperature solid oxide fuel cells (ITSOFCs). The glass transition points fell in the range of 694 °C and 833 °C, while the glass softening points, varying from 725 °C to 885 °C, decreased linearly with the ZnO/SiO₂ ratio. Among the glasses evaluated, the YAS1-50, YAS4-50, YAS5-50, and YAS5-90 glasses coupled with SUS430 showed fine adhesion and no detectable inter-diffusion across the interface. The coefficient of thermal expansion (CTE) of the YAS5-50 glass escalated from 7.01 to 9.30 × 10⁻⁶/K with 50 wt% Bi_1.5Y_0.5O₃ (BYO) fillers. The leak rates of the composite seals comprising the YAS5-50 glass and 50 wt% BYO fillers were measured at 700 °C after joining at 850 °C. The measurement used helium with a pressure at 1 psi across the glass seals and a slight load of 1.0 psi to minimize compressive seal effect on the glass. It appeared that a low leak rate of 0.052 sccm/cm was obtained and stayed unchanged as the system was soaked at 700 °C for 500 h, indicating a fine thermal stability against the extended duration benefited from the limited crystallization of the YAS5-50 glass. This promising long-term stability qualified the glass for use as SOFC seal.     

(La,Sr)MnO3–(Y,Bi)2O3 composite cathodes for intermediate-temperature solid oxide fuel cells

Yttria-stabilized bismuth oxides (YSB) are cooperated to (La,Sr)MnO₃ (LSM) to form composite cathodes for intermediate-temperature solid oxide fuel cells. The composite electrodes are fabricated with screen-printing technique and characterized using electrochemical impedance spectroscopy. The interfacial polarization resistances (R_p) of the LSM–YSB electrodes on yttria-stabilized zirconia (YSZ), samaria-doped ceria (SDC), and YSB electrolytes are analyzed regarding the electrode composition and operating temperature. R_p decreases with the increase of YSB content up to 80 wt.% in the LSM–YSB composite. When YSZ is used as the electrolyte, the lowest R_p is 0.14 Ω cm² at 700 °C, which is only 1.8% of that for a pure LSM electrode, 5.6% of that reported for LSM–YSZ composites, and 13.2% of that for reported LSM–GDC (gadolinia-doped ceria) electrodes, demonstrating that YSB is very effective to enhance the performance of LSM-based cathodes. The electrode performance is also affected by the electrolyte substrate. LSM electrodes without any YSB exhibit obviously different performance on YSZ, SDC and YSB electrolytes. However, when YSB is cooperated, R_p on different electrolytes tends to become equivalent, especially for electrodes with high YSB content. Further analysis shows that their electrochemical performance is contributed dominantly from the electrode bulk whereas the contribution from the electrode/electrolyte interface is negligible, suggesting weak electrolyte effect on the performance of LSM–YSB composite electrodes.     

New RuO2 and carbon–RuO2 composite diffusion layer for use in direct methanol fuel cells

A RuO₂ diffusion layer is examined for use in direct methanol fuel cells (DMFC) by comparison with acetylene black and Vulcan XC-72R. In the test with a DMFC unit cell, the RuO₂ diffusion layer is superior to the other two materials. The difference in performance is interpreted in terms of structural and electrical properties which are evaluated by porosity, scanning electron microscopy and resistance measurements. The RuO₂ diffusion layer displays different behaviors at the anode and cathode sides. These characteristics can be attributed to a reduced loss of catalyst in the active catalyst layer, which leads to increased methanol diffusion at the anode and prevention of water flooding in the cathode. The effect of the RuO₂ diffusion layer on cell performance becomes more pronounced at lower temperatures and during operation in the presence of air. Finally, a carbon–RuO₂ composite is evaluated as a diffusion layer material for a DMFC.     

A novel doped CeO2–LaFeO3 composite oxide as both anode and cathode for solid oxide fuel cells

A novel composite oxide Ce(Mn,Fe)O₂-La(Sr)Fe(Mn)O₃ (CFM-LSFM) was synthesized and evaluated as both anode and cathode materials for solid oxide fuel cells. The cell with CFM-LSFM electrodes was fabricated by tape-casting and screen printing technique. The power-generating performance of this cell was comparable to that of the cell with Ni-SSZ anode and LSM-SSZ cathode. During the 120 h long-term test in hydrogen at 800 °C, the performance increased by 8.6% from 256 to 278 mW cm⁻². This was attributed to the decrease of polarization resistance and ohmic resistance during the test. The XRD results showed the presence of Fe, MnO and some unknown second phases after heat-treating the electrode materials in H₂ which may be beneficial to the anode electrochemical process. The gradual decrease of polarization resistance as increasing the current density possibly resulted from the increasing content of water in the anode.     

B2O3-free SiO2–Al2O3–SrO–La2O3–ZnO–TiO2 glass sealants for intermediate temperature solid oxide fuel cell applications

In this study, the chemical and thermal stabilities of eleven B₂O₃-free SiO₂–Al₂O₃–SrO–La₂O₃–ZnO–TiO₂ glasses were investigated, and the adhesion and sealing properties of the glasses with respect to Gd_0.2Ce_0.8O_2−δ (GDC) electrolyte and SUS 430 stainless steel (SUS430) were evaluated for use in intermediate temperature solid oxide fuel cells (ITSOFCs). It was found that the major crystallites formed in the glasses were initiated during the joining process at 950 °C, and only slight changes were observed in the intensity of crystallite peaks for the glasses subsequently soaked at 700 °C for 200 h. Experiments on the glass sandwiched with GDC electrolyte and SUS430 indicated that the SALSTi11 and SALSZT10 glasses provided good adhesion along the interfaces after heat treatment. According to the results of leakage test, the seals with the SALSTi11 and SALSZT10 glasses at 700 °C for a duration of 500 h showed good thermal stability with low leak rates of 0.007 and 0.003 sccm/cm at 0.5 psi, respectively. This property indicates a highly promising long-term thermal stability qualifying the sealing materials for ITSOFC applications.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

Effect of B2O3–Bi2O3–PbO frit on the performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The effects of B₂O₃–Bi₂O₃–PbO (BBP) frit on the electrochemical performance, electrical conductivity, and thermal expansion of LaBaCo₂O_5+δ (LBCO) cathode were investigated. BBP frit was found to be effective in lowering the sintering temperature of LBCO cathode by about 200 °C and in improving its electrochemical performance within the intermediate-temperature range of 600–800 °C. LBCO with 5 wt.% BBP frit cathode based on Sm_0.2Ce_0.8O_1.9 electrolyte showed the best electrochemical performance, i.e., the lowest area-specific resistance (ASR) and cathodic overpotential. The ASR values were about 64.1%, 66.1%, and 74.5% lower than those of LBCO at 700, 750, and 800 °C, respectively. The cathodic overpotential decreased from 51.0 mV for LBCO to 8.2 mV at a current density of 0.2 A cm⁻² at 700 °C. The electrical conductivity of LBCO with 5 wt.% BBP frit was about 320–330 S cm⁻¹ at 600–800 °C in air.     

ZrO2–SiO2/Nafion composite membrane for polymer electrolyte membrane fuel cells operation at high temperature and low humidity

Recast Nafion^® composite membranes containing ZrO₂–SiO₂ binary oxides with different Zr/Si ratios are investigated for polymer electrolyte membrane fuel cells (PEMFCs) at temperatures above 100 °C. Fine particles of the ZrO₂–SiO₂ binary oxides, same as an inorganic filter, are synthesized from a sodium silicate and a carbonate complex of zirconium by a sol–gel technique. The composite membranes are prepared by blending a 10% (w/w) Nafion^®-water dispersion with the inorganic compound. All composite membranes show higher water uptake than unmodified membranes, and the proton conductivity increases with increasing zirconia content at 80 °C. By contrast, the proton conductivity decreases with zirconia content for the composite membranes containing binary oxides at 120 °C. The composite membranes are tested in a 9-cm² commercial single cell at both 80 °C and 120 °C in humidified H₂/air under different relative humidity (RH) conditions. Composite membrane containing the ZrO₂–SiO₂ binary oxide (Zr/Si = 0.5) give the best performance of 610 mW cm⁻¹ under conditions of 0.6 V, 120 °C, 50% RH and 2 atm.     

Anode performance of LST–xCeO2 for solid oxide fuel cells

La-doped SrTiO₃ (LST)–xCeO₂ (x = 0, 30, 40, 50) composites were evaluated as anode materials for solid oxide fuel cells in terms of chemical compatibility, electrical conductivity and fuel cell performance in H₂ and CH₄. Although the conductivity of LST–xCeO₂ composite slightly decreased from 4.6 to 3.9 S cm⁻¹ in H₂ at 900 °C as the content of CeO₂ increased, the fuel cell performance improved from 75.8 to 172.3 mW cm⁻² in H₂ and 54.5 to 139.6 mW cm⁻² in CH₄ at 900 °C. Electrochemical impedance spectra (EIS) indicated that the addition of CeO₂ into LST can significantly reduce the fuel cells polarization thus leading to a higher performance. The result demonstrated the potential ability of LST–xCeO₂ to be used as SOFCs anode.     

H2 and CH4 oxidation on Gd0.2Ce0.8O1.9 infiltrated SrMoO3–yttria-stabilized zirconia anode for solid oxide fuel cells

Strontium molybdate (SrMoO₃) as an electronic conductor was incorporated with yttria-stabilized zirconia (YSZ) to form an anode scaffold for solid oxide fuel cells. Gd_0.2Ce_0.8O_1.9 (GDC) nanoparticles were introduced by wet impregnation to complete the Ni-free GDC infiltrated SrMoO₃–YSZ anode fabrication. The effects of SrMoO₃ on the electrode conductivity and GDC infiltration on the catalytic activity were examined. A pronounced performance improvement was observed both on wet H₂ and CH₄ oxidation for the 56 wt.% GDC infiltrated SrMoO₃–YSZ. In particular, the polarization resistance decreased from 8 Ω cm² to 0.5 Ω cm² under wet H₂ (3% H₂O) at 800 °C with the introduction of GDC. Under wet CH₄ at 900 °C, a maximum power density of 160 mW cm⁻² was obtained and no carbon deposition was observed on the anode. It was found that the addition of H₂O in the anode caused an increase of electrode ohmic resistance and a decrease of open circuit voltage. Redox cycling stability was investigated and only a slight drop in cell performance was observed after 5 cycles. These results suggest that GDC infiltrated SrMoO₃–YSZ is a promising anode material for solid oxide fuel cells.