High-temperature mechanical properties of a solid oxide fuel cell glass sealant in sintered forms

High-temperature mechanical properties of a silicate-based glass sealant (GC-9) for planar solid oxide fuel cell have been studied in sintered forms. Ring-on-ring biaxial flexural tests are carried out at room temperature to 800 °C for the sintered GC-9 glass. The results are also compared with those in cast bulk forms. From the force-displacement curves, the glass transition temperature (T_g) of the non-aged, sintered GC-9 glass is estimated to be between 700 °C and 750 °C, while that of the aged one is between 750 °C and 800 °C. Due to a crack healing effect of the residual glass at high temperature, the flexural strength of the sintered GC-9 glass at temperature of 650 °C to T_g point is greater than that at room temperature. At temperature above T_g, the flexural strength and stiffness are considerably reduced to a level lower than the room-temperature one. The sintered GC-9 glass with pores and crystalline phases has a flexural strength lower than the cast bulk one at temperature of 650 °C and below. Due to a greater extent of crystallization, the flexural strength and stiffness of the sintered GC-9 glass are greater than those of the cast bulk one at 700-800 °C.     

Effects of crystallization on the high-temperature mechanical properties of a glass sealant for solid oxide fuel cell

Effects of crystallization on the high-temperature mechanical properties of a newly developed silicate-based glass sealant (GC-9) are investigated for use in planar solid oxide fuel cell (pSOFC). The aged, crystallized GC-9 glass is produced by heat treatment of the original GC-9 glass at 900 °C for 3 h. Not only crystalline phases are formed but the residual glass is also changed in the aged GC-9 glass after the heat treatment. Mechanical properties of the aged GC-9 glass are determined by four-point bending technique at temperature from 25 °C to 750 °C. The glass transition temperature of the given glass is reduced but the softening temperature is increased by such a crystallization heat treatment. The aged GC-9 glass exhibits a greater flexural strength and Young's modulus than the non-aged one at temperature below 650 °C due to the existence of crystalline phases. At temperature of 700 °C and 750 °C, a greater extent of stress relaxation is found in the aged GC-9 glass such that its strength and stiffness are much lower than those of the non-aged one. The changes in the thermal and mechanical properties through the given aging treatment are favorable for application of the GC-9 glass sealant in pSOFC.     

High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells

Novel aromatic polyether type copolymers bearing side chain polar pyridine rings as well as combination of main and side chain pyridine units have been evaluated as potential polymer electrolytes for proton exchange membrane fuel cells (PEMFCs). The advanced chemical and physicochemical properties of these new polymers with their high oxidative stability, mechanical integrity and high glass transition temperatures (T_g's up to 270 °C) and decomposition temperatures (T_d's up to 480 °C) make them promising candidates for high and medium temperature proton exchange membranes in fuel cells. These copolymers exhibit adequate proton conductivities up to 0.08 S cm⁻¹ even at moderate phosphoric acid doping levels. An optimized terpolymer chemical structure has been developed, which has been effectively tested as high temperature phosphoric acid imbibed polymer electrolyte. MEA prepared out of the novel terpolymer chemical structure is approaching state of the art fuel cell operating performance (135 mW cm⁻² with electrical efficiency 45%) at high temperatures (150-180 °C) despite the low phosphoric acid content (<200 wt%) and the low platinum loading (ca. 0.7 mg cm⁻²). Durability tests were performed affording stable performance for more than 1000 h.     

Highly conductive epoxy/graphite composites for bipolar plates in proton exchange membrane fuel cells

Carbon-filled epoxy composites are developed for potential application as bipolar plates in proton exchange membrane (PEM) fuel cells. These composites are prepared by solution intercalation mixing, followed by compression molding and curing. Electrical conductivity, thermal and mechanical properties, and hygrothermal characteristics are determined as function of carbon-filler content. Expanded graphite and carbon black are used as synergistic combination to obtain desired in-plane and through-plane conductivities. These composites show high glass transition temperatures (T_g ∼ 180 °C), high thermal degradation temperatures (T₂ ∼ 415 °C), in-plane conductivity of 200–500 S cm⁻¹ with 50 wt% carbon fillers, in addition to offering high values of flexural modulus, flexural strength, and impact strength, respectively 2 × 10⁴ MPa, 72 MPa, and 173 J m⁻¹. The presence of carbon fillers helps reduce water uptake from 4 to 5 wt% for unfilled epoxy resins to 1–2 wt%. In addition, morphology, electrical, mechanical, and thermal properties remain unchanged on exposure to boiling water and acid reflux. This data indicate that the composites developed in this work meet many attributes of bipolar plates for use in PEM fuel cells.     

Role of the glass transition temperature of Nafion 117 membrane in the preparation of the membrane electrode assembly in a direct methanol fuel cell (DMFC)

The glass transition temperature (T_g) of the Nafion 117 membrane was traced by DSC step by step during the preparation of the membrane electrode assembly (MEA). Wide-angle x-ray diffraction and frequency response analysis were used for the determination of the crystallinity and proton conductivity of the membrane. As-received Nafion 117 membrane showed two glass transition temperatures in the DSC thermogram. The first T_g, caused by the mobility of the main chain in the polymer matrix, was 125 °C; the second T_g, derived from the side chain due to the strong interaction between the sulfonic acid functional groups, was 195 °C. During the pretreatment of the membrane, the T_g of the Nafion 117 membrane drastically decreased because of the plasticizer effect of water. In the hot-pressing process, the T_g of the Nafion 117 membrane gradually increased due to the loss of water. When the Nafion 117 was completely dried, the T_g of the membrane finally reached 132 °C. Thermal heat treatment was then applied to the MEA to obtain high interfacial stability; however, the membrane developed a crystalline morphology that led to reduced water uptake and proton conductivity. Therefore, the thermal heat treatment of the MEA should be carefully controlled in the region of the glass transition temperature (120–140 °C) of the Nafion 117 membrane to ensure the high performance of the MEA.     

Hygrothermal effects on properties of highly conductive epoxy/graphite composites for applications as bipolar plates

The hygrothermal effects on mechanical, thermal, and electrical properties of highly conductive graphite-based epoxy composites were investigated. The highly conductive graphite-based epoxy composites were found to be suitable for applications as bipolar plates in proton exchange membrane (PEM) fuel cells. The hygrothermal aging experiments were designed to simulate the service conditions in PEM fuel cells. Specifically, the composite specimens were immersed in boiling water, aqueous sulphuric acid solution, and aqueous solution of hydrogen peroxide. The water uptake, changes in surface appearance and dimensions, glass transition behavior and thermal stability, and electrical and mechanical properties were evaluated. The water uptake at short time increased linearly with the square root of time as in linear Fickian diffusion. The presence of graphite significantly reduced both the rate and extent of water uptake. No discernible changes in specimen dimensions, surface appearance, and morphology of the composites were observed. The electrical conductivity and mechanical properties remained almost unchanged. The wet specimens showed slight reduction of glass transition temperature (T_g) due to plasticization of epoxy networks by absorbed water, while the re-dried specimens showed small increase of T_g. The composites maintained high electrical conductivity of about 300–500 S cm⁻¹ and good mechanical properties and showed thermal stability up to 350 °C.     

Effects of substrate temperature on the structural and electrical properties of Cu(In,Ga)Se2 thin films

Polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films were deposited onto soda-lime glass substrates using the three-stage process at the substrate temperature (T_sub) varying from 350 to 550 °C. The effect of T_sub on the structural and electrical properties of CIGS films has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect measurement. We found that the surface roughness, constituent phases, film morphologies, resistivity (ρ) and carrier concentration (N_P) of as-grown CIGS films indicated different change trends with increase in T_sub. The higher T_sub gives smooth surface, large grain size and single-phase CIGS films. The values of N_P and ρ have two demarcated regions at T_sub of 380 and 450 °C. At lower T_sub of 380 °C, larger N_P and lower ρ were dominated by the existence of secondary-phase Cu_xSe with lower resistivity. In the case of 450 °C, the obvious changes in N_P and ρ can be attributed to the sufficient Na incorporation diffused from the glass substrate. Finally, the correlation of cell parameters with T_sub was analyzed.     

Hybrid inorganic–organic proton conducting membranes based on Nafion and 5 wt% of MxOy (M = Ti,Zr, Hf,Ta and W). Part II: Relaxation phenomena and conductivity mechanism

In this report, we are presenting studies of the effect of M_xO_y nanopowders on the thermal, mechanical and electrical properties of [Nafion/(M_xO_y)_n] membranes with M = Ti, Zr, Hf, Ta and W and n = 5 wt%. Five homogeneous membranes with thicknesses ranging from 170 to 350 μm were studied. The thermal transitions characterizing [Nafion/(M_xO_y)_n] materials were investigated by modulated differential scanning calorimetry (MDSC). The mechanical parameters and relaxation processes were studied on temperature by dynamical mechanical analyses (DMA). Broadband dielectric spectroscopy (BDS) was used to study the conductivity mechanism and dielectric relaxation events in bulk materials. DMA investigations showed two distinct relaxation events. The first relaxation phenomenon, detected at about 19 °C, was attributed to the mechanical β-relaxation mode of Nafion. The second event, revealed in the temperature range 100–135 °C, was assigned to the mechanical α-relaxation mode of Nafion. The electric response of membranes was studied by BDS measurements in the frequency and temperature range respectively of 40 Hz–10 MHz and 5–135 °C. Real and imaginary components of permittivity (?*(ω) = ?′ (ω) − i?″(ω)) and conductivity spectra (σ*(ω) = σ′(ω) + iσ″(ω)) were analyzed. Conductivity spectra allowed us to accurately determine the σ_dc of membranes at 100% RH as a function of T. Two relaxation peaks were detected in the ?*(ω) profiles. The low frequency relaxation event was attributed to the α-relaxation mode of fluorocarbon chains of Nafion. The high frequency relaxation peak corresponds to the β-relaxation of acid side groups. The results allowed us to conclude that M_xO_y influences: (a) the relaxations of both the hydrophobic and the hydrophilic domains of Nafion polymer host; (b) the thermal stability range of conductivity (SRC) and the σ_dc of membranes.     

Galliosilicate glasses for viscous sealants in solid oxide fuel cell stacks: Part I: Compositional design

Galliosilicate glasses were developed for sealing intermediate temperature planar solid oxide fuel cell (SOFC) stacks. Candidate sealing glasses were identified for use at operating temperatures of 750 °C and at 850 °C after assessing flow behavior, thermal expansion properties, and crystallization behavior. A series of non-alkali glasses was identified for use at 750 °C within a strontium boro-galliosilicate compositional region that exhibited glass transition temperatures (T_g) between 658 and 675 °C and coefficients of thermal expansion (CTE) between 8 and 9.4 × 10⁻⁶ K⁻¹. Glass frits and powders flowed below 850 °C and did not crystallize dramatically after 500 h at 750 °C. Several glasses containing 5 mol% mixed alkali were identified in a strontium boro-galliosilicate compositional region for use at 850 °C. The glasses exhibited T_g between 615 and 620 °C with CTE from 7.8 to 9.7 × 10⁻⁶ K⁻¹. Glass frits flowed well below 850 °C and retained remnant glass phase after partial crystallization at 850 °C. The galliosilicate glasses developed in this work enable viscous sealing of SOFC stacks.     

Synthesis and properties of a barium aluminosilicate solid oxide fuel cell glass–ceramic sealant

A series of barium aluminosilicate glasses modified with CaO and B₂O₃ were prepared and evaluated with respect to their suitability in sealing planar solid oxide fuel cells (SOFCs). At a target operating temperature of 750 °C, the long-term coefficient of thermal expansion (CTE) of one particular composition (35 mol% BaO, 15 mol% CaO, 10 mol% B₂O₃, 5 mol% Al₂O₃, and bal. SiO₂) was found to be particularly stable, due to devitrification to a mixture of glass and ceramic phases. This sealant composition exhibits minimal chemical interaction with the yttria-stabilized zirconia electrolyte, yet forms a strong bond with this material. Interactions with metal components were found to be more extensive and depended on the composition of the metal oxide scale that formed during sealing. Generally alumina-scale formers exhibited a more compact reaction zone with the glass than chromia-scale forming alloys. Mechanical measurements conducted on the bulk glass–ceramic and on seals formed using these materials indicate that the sealant is anticipated to display adequate long-term strength for most conventional stationary SOFC applications.     

Low temperature behaviour of TiO2 rutile as negative electrode material for lithium-ion batteries

High surface nanosized rutile TiO₂ is prepared via a sol-gel method from an ethylene glycol-based titanium-precursor in the presence of a non-ionic surfactant, at pH 0. Its electrochemical behaviour has been investigated at low temperature using two different potential windows. Typically, the potential window of the rutile system is 1-3 V but the use of an enlarged potential window (0.1-3 V), leads to an excellent reversible capacity of 341 mAh g⁻¹ which is comparable to graphite anodes. The electrochemical performance was investigated by cyclic voltammetry and galvanostatic techniques at temperatures ranging from −40 to 20 °C. Nanosized TiO₂ exhibits excellent rate capability (341 mAh g⁻¹ at 20 °C, 197 mAh g⁻¹ at −10 °C, 138 mAh g⁻¹ at −20 °C, and 77 mAh g⁻¹ at −40 °C at a C/5 rate) and good cycling stability. The superior low-temperature electrochemical performance of nanosized rutile TiO₂ may make it a promising candidate as lithium-ion battery material.     

Solar cell junction temperature measurement of PV module

The present study develops a simple non-destructive method to measure the solar cell junction temperature of PV module. The PV module was put in the environmental chamber with precise temperature control to keep the solar PV module as well as the cell junction in thermal equilibrium with the chamber. The open-circuit voltage of PV module V_oc is then measured using a short pulse of solar irradiation provided by a solar simulator. Repeating the measurements at different environment temperature (40-80 °C) and solar irradiation S (200-1000 W/m²), the correlation between the open-circuit voltage V_oc , the junction temperature T_j , and solar irradiation S is derived.The fundamental correlation of the PV module is utilized for on-site monitoring of solar cell junction temperature using the measured V_oc and S at a short time instant with open circuit. The junction temperature T_j is then determined using the measured S and V_oc through the fundamental correlation. The outdoor test results show that the junction temperature measured using the present method, T_jo, is more accurate. The maximum error using the average surface temperature T_ave as the junction temperature is 4.8 °C underestimation; while the maximum error using the present method is 1.3 °C underestimation.     

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%.     

Effects of magnesium doping on electronic conductivity and electrochemical properties of LiFePO4 prepared via hydrothermal route

Carbon free composites Li_1−xMg_xFePO₄ (x = 0.00, 0.02) were synthesized from LiOH, H₃PO₄, FeSO₄ and MgSO₄ through hydrothermal route at 180 °C for 6h followed by being fired at 750 °C for 6 h. The samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), flame atomic absorption spectroscopy and electronic conductivity measurement. To investigate their electrochemical properties, the samples were mixed with glucose as carbon precursors, and fired at 750 °C for 6 h. The charge–discharge curves and cycle life test were carried out at 23 ± 2 °C. The Rietveid refinement results of lattice parameters of the samples indicate that the magnesium ion has been successfully doped into the M1 (Li) site of the phospho-olivine structure. With the same order of magnitude, there is no material difference in terms of the electronic conductivities between the doped and undoped composites. Conductivities of the doped and undoped samples are 10⁻¹⁰ S cm⁻¹ before being fired, 10⁻⁹ S cm⁻¹ after being fired at 750 °C, and 10⁻¹ S cm⁻¹ after coated with carbon, respectively. Both the doped and undoped composites coated with carbon exhibit comparable specific capacities of 146 mAh g⁻¹ vs. 144 mAh g⁻¹ at 0.2 C, 140 mAh g⁻¹ vs. 138 mAh g⁻¹ at 1 C, and 124 mAh g⁻¹ vs. 123 mAh g⁻¹ at 5 C, respectively. The capacity retention rates of both doped and undoped samples over 50 cycles at 5 C are close to 100% (vs. the first-cycle corresponding C-rate capacity). Magnesium doping has little effects on electronic conductivity and electrochemical properties of LiFePO₄ composites prepared via hydrothermal route.     

Lithium ion transport of solid electrolytes based on PEO/CF3SO3Li and aluminum carboxylate

A homogeneous, composite polymer electrolyte (PE) containing poly(ethylene oxide) (PEO), CF₃SO₃Li and 33 wt.% of aluminum carboxylate [RC(O)OAlEt₂]₂ with an oligooxyethylene group R = CH₂CH₂C(O)O(CH₂CH₂O)_nCH₃ (n = 7) (AlCarb7), characterized by low glass transition temperature T_g = −51.4 °C was prepared. The interaction of aluminum carboxylate with various lithium salts was characterized on the basis of ²⁷Al NMR spectroscopy in CDCl₃ solutions. The bulk conductivity of solid PE with AlCarb7 is of the order of 10⁻⁵ S cm⁻¹ at 60 °C and 10⁻⁴ S cm⁻¹ at 90 °C. Electrochemical tests of Li|PE|Li cells showed a decrease in the R_SEI with temperature, stabilizing at about 10 Ω cm⁻². The lithium ion transference numbers determined by ac–dc polarization experiments range from 0.7 to 0.9. ⁷Li, ¹⁹F and ¹H NMR spectra, the relaxation time and diffusion data were obtained. The calculated lithium transference number t₊ at 50 °C is equal to 0.995, which suggests practically complete immobilization of the triflate salt anions. In the range of high temperatures (130–180 °C) t₊ is equal 0.35–0.39. The dependence of t₊ on temperature should probably be connected with the partial dissociation of the aluminum carboxylate and lithium salt complex.     

High sintering activity Cu-Gd co-doped CeO2 electrolyte for solid oxide fuel cells

Nano-sized Ce_0.79Gd_0.2Cu_0.01O_2−δ electrolyte powder was synthesized by the polyvinyl alcohol assisted combustion method, and then characterized by crystalline structure, powder morphology, sintering micro-structure and electrical properties. The results demonstrate that the as-synthesized Ce_0.79Gd_0.2Cu_0.01O_2−δ was well crystalline with cubic fluorite structure, and exhibited a porous foamy morphology composed of gas cavities and fine crystals ranging from 30 to 50 nm. After sintering at 1100 °C, the as-prepared pellets exhibited a dense and moderate-grained micro-structure with 95.54% relative density, suggesting that the synthesized Ce_0.79Gd_0.2Cu_0.01O_2−δ powder had high sintering activity. The powders made by this method are expected to offer potential application in intermediate-to-low temperature solid-oxide fuel cells, due to its very low densification sintering temperature (1100 °C), as well as high conductivity of 0.026 S cm⁻¹ at 600 °C and good mechanical performance with three-point flexural strength value of 148.15 ± 2.42 MPa.     

Novel polymer electrolytes based on triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO2

A novel polymer electrolyte based on triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO₂ inclusions has been designed. The electrolyte shows favorable features for ion migration such as low glass transition temperature and high concentration of amorphous phase. Combined with the effect of active SiO₂, its ionic conductivity is about 8.0 × 10⁻⁵ S cm⁻¹ at 30 °C, which exceeds that for the PEO-based systems. As applying them to cells with LiFePO₄-type cathodes, a capacity of about 147.0 mAh g⁻¹ is obtained at 60 °C, which is retained by more than 90% after 40 charge/discharge cycles. Moreover, about 100 mAh g⁻¹ could still be delivered as temperature decreases to 30 °C.     

Performance of LaBaCo2O5+δ-Ag with B2O3-Bi2O3-PbO frit composite cathodes for intermediate-temperature solid oxide fuel cells

The composite cathodes LaBaCo₂O_5+δ-x wt.% Ag (LBCO-xAg, x = 20, 30, 40, 50) were prepared by mechanical mixing method for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The experiment results indicated that the addition of a small amount of B₂O₃-Bi₂O₃-PbO (BBP) frit to LBCO-xAg can effectively improve the adhesion and strength of cathode membrane without damaging its porous structure. The BBP frit was proved effective for lowering the sintering temperature of LBCO-xAg to 900 °C. According to the electrochemical impedance spectroscopy and cathodic polarization analysis, the LBCO-30Ag exhibited the best performance and the optimal BBP frit content was 2.5 wt.%. For LBCO-30Ag with 2.5 wt.% BBP frit, the area-specific resistance based on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte decreased by about 57.6% at 700 °C, 60.5% at 750 °C and 75.9% at 800 °C compared to LBCO, and its cathodic overpotential was 10.7 mV at a current density of 0.2 A cm⁻² at 700 °C, while the corresponding value for LBCO was 51.0 mV. The addition of Ag and BBP frit to LBCO had no significant effect on the thermal expansion.     

Interlayer-free nanostructured La0.58Sr0.4Co0.2 Fe0.8O3−δ cathode on scandium stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells

LSCF powders with a specific surface area of 25.2 m² g⁻¹ and an average particle size of 89 nm are synthesized by the polymerizable complex method. The use of nanocrystalline LSCF powders allows the fabrication of an interlayer-free nanoporous cathode on top of an ScSZ electrolyte at a low temperature at which non-electrocatalytic secondary phases cannot form. The electrochemical performance of the interlayer-free cathode depends largely on the sintering temperature. A cathode sintered at below 750 °C lacks sufficient mechanical adhesion to the electrolyte, while the electrode surfaces are locally densified when sintered at above 800 °C. Impedance spectroscopy combined with microstructural evidence reveals that the optimum sintering temperature for LSCF is 750 °C. This avoids excess densification and grain growth, and results in the lowest polarization resistance (0.048 Ω cm² at 750 °C).     

Influence of the growth parameters of p-CdTe thin films on the performance of Au-Cu/p-CdTe/n-CdO type solar cells

R.F. sputter deposition of Sb doped CdTe thin films was carried out with targets containing different amounts of antimony (C_T: 0, 2.5, 10 and 20 at.%). The substrates were kept at different temperatures (T_s) of 200, 275, 350 and 450 °C. Three different argon pressure values: 2.5, 5 and 15 mTorr were used. The lowest dark resistivity (ρ) at room temperature (RT) was 9.0 × 10⁵ Ω cm, which is one of the lowest values reported in the literature for Sb doped CdTe. Highly transparent (∼90%) and conductive (ρ = 3.7 × 10⁻⁴ Ω cm) F doped CdO (n-type) thin films, prepared at room temperature by the sol-gel method, were employed as window and top-contact. The configuration of the fabricated solar cell was (Au-Cu)/p-CdTe/n-CdO/glass. Open-circuit voltage (V_oc) and short-circuit current density (J_sc) at room temperature have the highest values for high T_s, low P_g and C_T = 10 at.%. Despite the fact that V_oc and J_sc are lower than those reported in the literature, we think this work is useful as a basis for the search of more competitive CdTe/CdO based PV devices.