Triazole and triazole derivatives as proton transport facilitators in polymer electrolyte membrane fuel cells

Some basic aspects pertaining to the application of triazole and its derivatives as proton transport facilitators for membranes for high temperature fuel cell operations are investigated. Performance as proton transport facilitators is studied for compounds in their native solid state and as a dopant in a polymer membrane. Some key parameters which influence the proton transport in the system are the proton affinity, pKa or acidity, activation energy and the ease of formation of hydrogen bonding network. Theoretical calculations of the proton affinity of the compounds are presented. The effect of proton affinity of the compound on the activation energies for proton transport is investigated. Proton conductivity is measured for acid doped triazoles in both pellet form (powder triazole mixed with acid) and in composite forms wherein the acid group is contained in a polymer matrix. The effect of formation of a hydrogen bonding network by the triazoles and its impact on the proton conductivity are studied. Also, the effect of ion exchange capacity (IEC) of the host polymeric electrolytes and loading of triazoles in the composites were investigated.     

Auto-combustion synthesis and properties of Ce0.85Gd0.15O1.925 for intermediate temperature solid oxide fuel cells electrolyte

A typical composition of the system Ce_1 − xGd_xO_2 − δ with x = 0.15 (CGO15) has been synthesized by auto-combustion method. DTA/TGA of the precursor compound indicated the completion of reaction at about 270 °C. Greater than 95% of the theoretical density has been achieved by sintering at 1300 °C for 10 h. Single phase formation in as-burnt stage has been confirmed by its powder X-ray diffraction (XRD) pattern. The structural morphology was studied employing bright field transmission electron micrograph (BFTEM) and high resolution transmission electron micrograph (HRTEM). BFTEM image indicates that particles are highly agglomerated and appear to be dispersed in amorphous matrix. Also BFTEM image reveals that the average particle size is 26 ± 5 nm. The presence of amorphous phase in as-prepared ash was also confirmed by HRTEM and selected area diffraction (SAD). The scanning electron micrograph (SEM) of the thermally etched system shows grains having an average size of 400 nm. Impedance measurements have been made in the frequency range 1 Hz to 1.3 MHz between 200 and 500 °C and the total conductivity was measured. An enhanced conductivity value is observed which may make this system suitable for application as a solid electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs).     

Nafion/mordenite hybrid membrane for high-temperature operation of polymer electrolyte membrane fuel cell

Nafion/mordenite hybrid membranes for the operation of polymer electrolyte membrane fuel cells (PEMFCs) above 100 °C were prepared by mixing of H⁺-form mordenite powder and perfluorosulfonylfluoride copolymer resin. PEMFC operation above 100 °C reduces CO poisoning as well as passivation of the Pt anode electrocatalyst by other condensable species. The physico-chemical properties of hybrid membranes were investigated by tensile strength and proton conductivity measurements. As the mordenite content increases at the high temperature region, the proton conductivity of hybrid membranes increased due to the late dehydration rate of existent water in the mordenite. Also, from the results of current–voltage relationship for single cells under 130 °C of operation condition, the hybrid membrane cell with 10 wt.% mordenite showed better performance than that of the others over the entire current density range. This result indicated that the existent water in the hybrid membrane containing 10 wt.% mordenite was higher than that with the others, thereby maintaining its conductivity. The Nafion/mordenite hybrid membrane prepared by this present method is thought to be a satisfactory polymer electrolyte membrane for PEMFC operation above 100 °C.     

NH4PO3/SiO2 composite as electrolyte for intermediate temperature fuel cells

NH₄PO₃/SiO₂ composite based electrolyte with SiO₂ as supporting matrix was prepared. A thermogravimetric analysis was performed. Its electrochemical properties were investigated by an impedance spectroscopy within the temperature range of 100–300 °C under dry and humid atmospheres. The maximum conductivity is 6 mS cm^− 1 at 300 °C under dry N₂ and 0.1 S cm^− 1 at 200 °C under humid N₂.     

Electrical properties of triple-doped bismuth oxide electrolyte for solid oxide fuel cells

In this study, the quaternary solid solutions of (Bi₂O₃)_(0.8?x)(Tb₄O₇)_0.1(Ho₂O₃)_0.1(Dy₂O₃)_x (x = 0.05, 0.10, 0.15, 0.20) as an electrolyte were synthesized for solid oxide fuel cells by the technique of solid-state synthesis.

The products were characterized by X-ray powder diffraction, differential thermal analysis/thermal gravimetry and the four-point probe technique (4PPT). The total electrical conductivity is measured on the temperature and the doped concentration by 4PPT.

All samples have been obtained as the δ-phase. According to the measurements of the 4PPT, the electrical conductivities of the samples increase with the temperature but decrease with the amount of doping rate. The value of the highest conductivity (σ) is found as 1.02?×?10^?1 S cm^?1 for the system of (Bi₂O₃)_0.75(Tb₄O₇)_0.1(Ho₂O₃)_0.1(Dy₂O₃)_0.05 at 850 °C. The thermal gravimetry (TG) curve shows that there is no mass loss of sample during the measurement. The analyses of differential thermal reveal that there are neither endothermic peaks nor exothermic peaks during the heating and cooling cycles (ranging from 30 to 1000 °C).     

Ceria co-doped with calcium (Ca) and strontium (Sr): a potential candidate as a solid electrolyte for intermediate temperature solid oxide fuel cells

Co-doped samples of Ce_0.95?x Ca_0.05Sr _x O_1.95?x , where (x?=?0.00, 0.01, 0.02, and 0.03), have been prepared by auto-combustion method and characterized to explore their use as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Crystal structure, microstructure, and ionic conductivity have been characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy, respectively. All the compositions have been found to be single phase. Results show that the samples co-doped with Ca and Sr exhibit higher ionic conductivity than the samples singly doped with Ca in the intermediate temperature range. Ce_0.93Ca_0.05Sr_0.02O_2?δ exhibits maximum conductivity among all the compositions. This may be a potential candidate as a solid electrolyte for IT-SOFCs.     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

Highly dispersed anodes for solid oxide fuel cells using NiO/YSZ/BZY triple-phase composite powders prepared by spray pyrolysis

NiO/Y₂O₃-stabilized ZrO₂ (YSZ)/Y-doped BaZrO₃ (BZY) triple-phase composite powders were prepared by spray pyrolysis and evaluated for Ni/YSZ/BZY cermet anodes, which are considered effective for dry CH₄ operation in solid oxide fuel cells. The structure of the particles in these powders was fine crystal fragments, and the individual material phases were clearly separated and highly dispersed within the particles. The Ni/YSZ/BZY cermet anodes fabricated with these composite powders maintained a fine electrode microstructure equivalent to that in a simple Ni/YSZ cermet anode manufactured using a composite powder prepared by spray pyrolysis. Furthermore, the addition of BZY improved the anode performance in humidified H₂ and dry CH₄ operation.     

Preparation and evaluation of ionomeric membranes based on sulfonated-poly (styrene_isobutylene_styrene) membranes for proton exchange membrane fuel cells (PEMFC)

Sulfonated polystyrene-block-poly-(ethylene-ran-butylene)-block-polystyrene membranes with different sulfonated levels have been prepared and evaluated as proton exchange membrane for polymer electrolyte membrane fuel cell. The polymer was sulfonated by chlorosulfonic acid. Homogeneous membranes were prepared by solvent casting method. Ion exchange capacity, degree of sulfonation, absorption, and solubility of the membranes were studied. The membranes were characterized by Fourier transform infrared, thermogravimetric analyzer, differential scanning calorimetry, and impedance spectroscopy.     

XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells

In electrodes of low temperature fuel cells like polymer electrolyte membrane fuel cells (PEFC) or alkaline fuel cells (AFC) the reactants and the water must be transported. For this purpose the pore system in the electrodes needs a hydrophilic character for the transport of the water and a hydrophobic character for the transport of the gases. The degree of the hydrophobicity determines whether the pore system will be flooded by the reaction water. In the case of PEFC, this is also determined by the degree of the required humidification of the reaction gases. In AFC hydrophobicity determines the extension of the three-phase reaction zone. Caused by the strong influence of hydrophobicity on the transport processes, the electrochemical performance and the optimized operation conditions are also affected by hydrophobicity.Typically polytetrafluoro-ethylene (PTFE) is used to make the electrodes hydrophobic, because PTFE has a high chemical stability. Hydrophobicity depends on the concentration of PTFE on the electrode surface. The PTFE concentration, which is related to the hydrophobic character, can be determined by XPS. The changes in the PTFE content and structure of the electrode of a PEFC was investigated by cyclic voltammetry and XPS and correlated with the performance of the cell in long-term operation. With both methods an initial significant increase in free and electrochemically active surface platinum area is observed. This activation is associated with a degradation of the PTFE in the electrode which is responsible for the hydrophobic properties of the electrode. With further operation the performance of the cell decreases because the water management becomes more critical. Generally, it is shown that XPS can be used for the investigation of the hydrophobicity of electrodes prepared by various manufacturing techniques as well as of changes in their hydrophobicity induced by the electrochemical operation.     

Synthesis and characterization of La0.9Sr0.1Ga0.8Mg0.2O3-δ electrolyte for intermediate temperature solid oxide fuel cells (ITSOFC)

Experimental investigations on new materials for application as electrolyte in electrolyte supported planar Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC) operating below 800 °C is in progress at our laboratory. Sr and Mg doped Lanthanum gallate (LSGM) powder was prepared by glycine — nitrate combustion method. The prepared LSGM powder is relatively finer than that prepared through other techniques such as solid-state reaction. The measurements comprising XRD, particle size, density, TGA/DTA were made. Thin sections of circular pellets were fabricated and annealed at different temperatures ranging between 1000 and 1300 °C. The sintering behaviour of LSGM was investigated to obtain information on the densification factor, relative percentage shrinkage/expansion in volume, while annealing and the resulting apparent porosity values. Bismuth oxide is found to be an effective sintering aid in general. So the effect of bismuth oxide addition on LSGM was investigated through sintering studies, XRD, TGA/DTA, SEM and conductivity measurements. The results obtained on LSGM with and without bismuth oxide addition are discussed with respect to the requirement of an electrolyte for ITSOFC applications. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

A stable NH4PO3-glass proton conductor for intermediate temperature fuel cells

A stable and conductive composite material based on NH₄PO₃ and amorphous oxides (SiO₂-P₂O₅) has been prepared by a wet-chemical route following with firing in ammonia. The composite showed high proton conductivity in both ambient air and humidified 5% H₂/Ar. A total conductivity of 6.0-19 mS/cm in the temperature range of 150-250 °C has been achieved. The conductivity (about 19 mS/cm) is stable in humidified 5% H₂/Ar during ageing at 175 °C for over 100 h. This material is a potential electrolyte for intermediate temperature fuel cells and other electrochemical devices.     

Stable and high conductivity ceria/bismuth oxide bilayer electrolytes for lower temperature solid oxide fuel cells

A highly conductive bismuth oxide/ceria bilayer electrolyte was developed to reduce solid oxide fuel cell (SOFC) operating temperatures. Bilayer electrolytes were fabricated by depositing a layer of Er_0.2Bi_0.8O_1.5 (ESB) of varying thickness via pulsed laser deposition and dip-coating on a Sm_0.2Ce_0.8O_1.9 (SDC) substrate. The open-circuit potential (OCP) and ionic transference number (t _i) of ESB/SDC electrolytes were tested in a fuel cell arrangement as a function of relative thickness, temperature, and with H₂/H₂O and CO/CO₂ on the anode side and air on the cathode side. These EMF measurements showed a significant increase in OCP and t _i with the bilayer structure, as compared to the cells with a single SDC electrolyte layer. Furthermore, improvement in the OCP and t _i of bilayer SOFCs was observed with increasing relative thickness of the ESB layers. Hence, the bilayer structure overcomes the limited thermodynamic stability of bismuth oxides and prevents electronic conductivity of ceria-based oxides in reducing atmosphere.     

Copper coated nickel foam as current collector for H2S-containing syngas solid oxide fuel cells

Copper coated nickel foam with Cu/Ni ratio of 2.5 was fabricated using electrochemical plating and characterized using XRD, SEM, EDS and Van der Pauw four point conductivity measurements. Copper coated nickel foam exhibited good stability of electronic conductivity and mechanical strength during more than 150 h exposure to 500 ppm H₂S-containing syngas at 750 °C. In contrast, uncoated nickel foam readily underwent severe carbon deposition and brittle fracture resulting in dramatic increase of electronic resistance under the same testing conditions. Copper coated nickel foam is a promising current collector for H₂S-containing syngas solid oxide fuel cells (SOFCs).     

Niobium based tetragonal tungsten bronzes as potential anodes for solid oxide fuel cells: synthesis and electrical characterisation

In this paper we report studies on a range of niobate based tungsten bronzes, with a view to analysing their potential as anode materials in SOFCs. Six systems were studied, (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃, Sr_0.6−xLa_xTi_0.2+xNb_0.8−xO₃, (Sr_0.4−xBa_x)Na_0.2NbO₃, (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃, Ba_0.5−xA_xNbO₃ (A=Ca, Sr), and Ba_0.3NbO_2.8, and the electrical conductivities were examined over a range of oxygen partial pressures (10⁻²⁰–1 bar). All the systems showed good conductivity in low oxygen partial pressures, with values as high as 8 S cm⁻¹ at 930°C (P(O₂)=10⁻²⁰ bar). As the oxygen partial pressure was raised the conductivity dropped showing in most cases an approximate [P(O₂)]^−1/4 dependence and good re-oxidation kinetics. Of all the samples studied the (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃ and (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃ systems appear most promising for potential use as anode materials in SOFCs.     

Li<Subscript>1−<Emphasis Type="BoldItalic">x</Emphasis></Subscript>Sm<Subscript>1+<Emphasis Type="BoldItalic">x</Emphasis></Subscript>SiO<Subscript>4</Subscript> as solid electrolyte for high temperature solid-state lithium batteries

Lithium lanthanoid silicates are projected as promising solid electrolytes for solid-state high-temperature lithium batteries. Synthesis of Li_1−x Sm_1+x SiO₄ (x = 0.2 to 0.6) was carried using sol–gel method, and these compounds were characterized by thermogravimetry differential thermal analysis, X-ray diffraction, Fourier transform infrared, and SEM. Impedance measurements were carried out at different temperatures, and conductivity at different temperatures was calculated. The effect of an increase of samarium content on the conductivity of the solid electrolyte was studied in this paper. It was found that less samarium content exhibits good conductivity at higher temperatures.