Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.     

PEM fuel cells as membrane reactors: kinetic analysis by impedance spectroscopy

Proton exchange membrane fuel cells (PEMFC) are considered as electrochemical reactors, performances of which are regarded in the context of the various effects influencing FC output, such as mass transports, kinetic of electrode reactions and charge transfer in polymer electrolyte membrane (PEM). An experimental approach, involving the employment of impedance spectroscopy (IS), which allows a deep insight into the nature of these effects, is discussed and its applications to the different aspects of PEMFC functioning are reported. As examples of the use of IS in PEMFC studies, the investigations of the membrane conductivity and in situ studies of the anode and the cathode processes during FC operation are presented.     

Iron porphyrin-based cathode catalysts for polymer electrolyte membrane fuel cells: Effect of NH3 and Ar mixtures as pyrolysis gases on catalytic activity and stability

Ten different catalysts were prepared by loading 66 wt% ClFeTMPP on N330, a furnace grade carbon black, and pyrolyzing this catalyst precursor for 10 min at 950 °C in a NH₃/Ar gas mixture with various NH₃ volume fractions (from 0% to 100%). The activity and stability of these catalysts were measured in a fuel cell and compared. The only stable catalyst, although the least active, among these was the one pyrolyzed in pure Ar. A notable leap in catalytic activity, but drop in stability, was observed for all catalysts pyrolyzed in gas mixtures containing NH₃, even with a mere volume fraction of 1.3% NH₃ in the pyrolysis gas mixture. Catalytic activities increased, while stability decreased with increasing volume fraction of NH₃. The physicochemical properties of these catalysts were correlated with their electrochemical behaviour observed in fuel cell tests. It was found that a volume fraction of only 1.3% NH₃ was enough to double the micropore surface area, the surface nitrogen and iron concentrations in the resulting catalyst. Since the active sites are believed to be of the Fe/N/C type, the sharp increase in catalytic activity with as little as 1.3% NH₃ is attributed to the concurrent increase in microporous surface area, N and Fe surface contents in these catalysts. The only property that apparently correlates with stability is the degree of graphitization of the catalyst, which was estimated either from either X-ray diffraction and Raman spectroscopy measurements. Lastly, it was found that the catalysts’ peroxide yield, resulting from the partial reduction of O₂, does not correlate with their degree of stability.     

Solid electrolyte membrane reactor for controlled partial oxidation of hydrocarbons: Model and experimental validation

A mathematical model of a solid electrolyte membrane reactor is presented which accounts for the prevailing physical phenomena of the electrochemical partial oxidation of n-butane to maleic anhydride. From an analysis of characteristic dimensionless numbers it was concluded that the reactor behaviour can be described by a one-dimensional pseudo-homogeneous approach with respect to the anodic gas channel and a one-plus-one-dimensional electrochemical model. Beside mass and charge transport processes, electrochemical charge transfer reactions as well as heterogeneously catalysed oxidation reactions are considered. As kinetic model a modified Mars–van Krevelen approach is suggested. Experimental results of oxygen pumping and butane oxidation experiments were used to determine kinetic parameters and to validate the model.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Inorganic membranes and membrane reactors

The inorganic membrane reactor is a combined unit operation of chemical reactions and membrane separations. By having a membrane reactor, the downstream separation load can be reduced. Also, the yields can be increased and conversion can be improved for equilibrium limited reactions. However, many of the industrial chemical reactions take place at high temperature that the conventional polymeric membranes cannot withstand. A great deal of research has been done recently to develop ion-conducting ceramic membranes. Many of these have been successfully employed to form membrane reactors for many industrially relevant chemical reactions, such as hydrogenation, dehydrogenation, oxidation, coupled reactions, and decomposition reactions. An overview is given for the area of inorganic membrane preparations and membrane reactors. Many examples of petrochemical interests are presented, including hydrocarbon conversions and fuel cell applications.     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.     

Thermodynamic Analysis of Methane-fueled Solid Oxide Fuel Cells Considering CO Electrochemical Oxidation

abstract Thermodynamic analyses in the literature have shown that solid oxide fuel cells (SOFCs) with proton conducting electrolyte (H-SOFC) exhibited higher performance than SOFC with oxygen ion condu...     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

A polymer electrolyte membrane for high temperature fuel cells to fit vehicle applications

Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H₃PO₄ were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35.19 MPa. After doping with the phosphoric acid, the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0.3 S cm⁻¹ at a relative humidity 8.4% and temperature of 180 °C with a 300% H₃PO₄ doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1.2 W cm⁻² at cell voltages >0.4 V and current densities of 3.0 A cm⁻². The PTFE/PBI/H₃PO₄ composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 °C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs.     

Preparation of mesoporous carbon templated by silica particles for use as a catalyst support in polymer electrolyte membrane fuel cells

Mesoporous carbons were prepared using commercial silica particles and a formaldehyde–resorcinol resin as a template and carbon precursor, respectively. By changing the molar ratio of template to carbon precursor, mesoporous carbons with different mesoporosities (MC-X, X represents the molar ratio of template to carbon precursor) were produced. The resulting MCs had a high-surface area and large pore volume. In particular, the highest mesoporosity was observed for MC-3. Pt catalysts-supported on MC-X were prepared using formaldehyde as a reducing agent for use as a cathode catalyst in a polymer electrolyte fuel cell (PEMFC). The size of Pt crystallite was dependent on the properties of corresponding carbon support. As a whole, a carbon support with a high-surface area and high-mesoporosity served the best in terms of a high-dispersion of Pt nanoparticles. In a unit cell test of the PEMFC, a Pt catalyst with a high-mesoporosity and fine dispersion of metal showed an enhanced performance. The findings indicate that the surface area combined with the mesoporosity had a positive influence on the metal dispersion and the distribution of ionomer, leading to the enhanced cell performance.     

Effect of the geometry and pattern of the flow channel on the performance of polymer electrolyte membrane fuel cell

This research focuses on the effect of the geometry and patterns of the gas flow channel on the PEM fuel cell performance. Simulation was conducted and the results were verified by experiments. Three-dimensional, single phase, compressible and isothermal models of 5 cm² electrodes, anode and cathode, were developed and studied by utilizing a commercial Computational Fluid Dynamics (CFD) software, FLUENT 4.5. Two types of gas flow channel were investigated: conventional and interdigitated. The results showed that the flow channel pattern does not have a significant effect on the anode cell performance, whereas it has a strong effect/influence on the cathode cell performance. The interdigitated design provides a higher limiting current density and cell performance than the conventional design on the cathode side. Moreover, the cell performance does not depend on the inlet and outlet channel widths. On the contrary, for the interdigitated design, it was influenced by the shoulder width. Finally, experiments were conducted to validate the simulation results.     

Conversion of Methane Facilitated by Solid Oxide Electrolysis Cells

A novel Fe-Al/SiO₂ catalyst for direct non-oxidative conversion of methane to ethylene, benzene, and naphthalene was synthesized. An Fe-Al/SiO₂ catalyst integrated solid oxide electrolysis cell (SOEC) membrane reactor design is further developed. It could be successfully demonstrated that the SOEC enabled the in situ removal of H₂, which helps to shift the chemical equilibrium of the methane conversion reaction to enhance the conversion efficiency.     

Oxidative coupling of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ tubular membrane reactors

A dense membrane tube made of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was prepared by plastic extrusion from BSCF oxide synthesized by the complexing EDTA-citrate method. The membrane tube was used in a catalytic membrane reactor for oxidative coupling of methane (OCM) to C₂ without an additional catalyst. At high methane concentration (93%), about 62% C₂ selectivity was obtained, which is higher than that achieved in a conventional reactor using the BSCF as a catalyst. The dependence of the OCM reaction on temperature and methane concentration indicates that the C₂ selectivity in the BSCF membrane reactor is limited by high ion recombination rates. If an active OCM catalyst (La-Sr/CaO) was packed in the membrane tube, C₂ selectivity and CH₄ conversion increased compared to the blank run. The highest C₂ yield in the BSCF membrane reactor in presence of the La-Sr/CaO catalyst was about 15%, similar to that in a packed-bed reactor with the same catalyst under the same conditions. However, the ratio of C₂H₄/C₂H₆ in the membrane reactor was much higher than that in the packed-bed reactor, which is an advantage of the membrane reactor.     

A new family of Cu-doped lanthanum silicate apatites as electrolyte materials for SOFCs: Synthesis,structural and electrical properties

La_9.67Si_6-xCu_xO_26.5-x (LSC, x = 0, 0.1, 0.3 and 0.5) are synthesized by a citric-nitrate method. Substitution Si with Cu promotes the densification process of silicate apatite. Unit cell parameters and volume increase linearly with Cu content. The Rietveld refinement reveals a much more distorted (Si,Cu)O₄ tetrahedra in the oxygen stoichiometric La_9.67Si_5.5Cu_0.5O₂₆ sample. The structural observation from high temperature XRD implies a second-order phase transition in La_9.67Si_5.5Cu_0.5O₂₆. Cu-doping decreases the activation energy of oxygen ion conduction and increases the conductivity of LSC materials in the temperature range of 550–800 °C. La_9.67Si_5.5Cu_0.5O₂₆ shows the conductivity values of 29.3 and 12.3 mS cm⁻¹ at 800 °C and 650 °C, respectively. The oxygen ion transference number of La_9.67Si_5.5Cu_0.5O₂₆ is higher than 0.99. These attractive properties make the La_9.67Si_5.5Cu_0.5O₂₆ a promising oxygen ion conducting electrolyte for applications of solid oxide fuel cells, oxygen sensors, oxygen separation membranes, etc.     

Decompostion of pharmaceuticals (sulfamethazine and sulfathiazole) using oxygen-based membrane biofilm reactor

The subject of this research was the decomposition of pharmaceuticals (sulfamethazine and sulfathiazole) using an oxygen-based membrane biofilm reactor. The influent concentrations in pharmaceuticals feed-medium were (in ppb): sulfamethazine (40) and sulfathiazole (85). The oxygen-based membrane biofilm reactor system consisted of two membrane modules connected to a recirculation loop. The main membrane module contained a bundle of 32 hydrophobic hollow-fiber membranes inside a polyvinyl-chloride pipe shell, and the other module contained a single fiber used to take biofilm samples. Pure O₂ was supplied to the inside of the hollow fibers through the manifold at the base, and the O₂ pressure for both reactors was 13 kPa. (1 kPa = 0.0099 atm = 0.145 psi). HRT was 3 h. The decomposition ratio of pharmaceuticals (sulfamethazine and sulfathiazole) using oxygen-based membrane biofilm reactor was (%): sulfamethazine (77 ± 2), and sulfathiazole (87 ± 2).     

Electrochemical Characteristics and Performance of Platinum Nanoparticles Supported by Vulcan/Polyaniline for Oxygen Reduction in PEMFC

We report a Pt/Vulcan carbon–polyaniline (VC–PANI) catalyst for the oxygen reduction reaction (ORR). This electrocatalyst was prepared from Pt nanoparticles supported by a VC–PANI composite substrate. Electrochemical performance was measured using potentiostat/galvanostats technique and a proton exchange membrane fuel cell (PEMFC) test station. The electrochemical properties of the electrodes were characterized using linear sweep voltammetry, AC impedance spectroscopy and chronoamperometry. Electrochemical characterization by hydrogen adsorption/desorption cyclic voltammetry and CO stripping voltammetry indicates that the electrochemical active surface areas of the Pt/VC–PANI are comparable to the commercial catalyst. The performance of the Pt/VC–PANI and Pt/C(E‐TEK) + PANI electrocatalysts were found to be 1.82 and 1.33 times higher than of the Pt/C(E‐TEK) electrode. The surface morphologies of the electrodes were characterized by using scanning electron microscopy (SEM). PANI has a fibrous structure and the improved performance was attributed to the PANI effect and synergistic effects between the carbon Vulcan and the PANI fiber. These results indicate that Pt/VC–PANI is a promising catalyst for the ORR in PEMFCs using an H₂/O₂ feed.     

Vanadium doped ceramic matrix Li6.7La3Zr1.7V0.3O12 enabled PEO-based solid composite electrolyte with high lithium ion transference number and cycling stability

Solid polymer electrolytes (SPEs) have attracted much attention because of their potential in improving energy density and safety. Vanadium doped ceramic matrix Li_6.7La₃Zr_1.7V_0.3O₁₂ (LLZVO) was synthesized by high-temperature annealing, and formed a composite electrolyte with polyethylene oxide (PEO). Compared with pure PEO electrolyte membrane, the composite electrolyte membrane exhibited better ionic conductivity (30 °C: 3.2 × 10^?5 S cm^?1; 80 °C: 3.6 × 10^?3 S cm^?1). The combination of LLZVO was beneficial to improve the lithium ion transference number (t_Li⁺) of SPE, which was as high as 0.81. The Li/SPE/LiFePO₄ battery shows good cycling ability, with a specific capacity of 142 mAh g^?1 after a stable cycle of 150 cycles. Meanwhile, the symmetrical lithium battery with composite electrolyte can work continuously for 1200 h without short circuit at the current density of 0.1 mA cm^?2 at 50 °C, and the capacity is 0.176 mAh. Vanadium doped ceramic matrix LLZVO as an active ionic conductor, improved the overall performance of solid electrolyte.     

Corrosion resistance and sintering effect of carbon supports in polymer electrolyte membrane fuel cells

The corrosion resistance of carbon black, carbon nanofiber and carbon nanocage used as catalyst supports in fuel cells was investigated by monitoring CO₂ emission using on-line mass spectrometry when 1.4 V was applied for 30 min. The changes associated with the carbon corrosion were assessed through electrochemical methods. In general, graphitized carbon supports were more corrosion-resistant than amorphous carbon black. However, the degree of graphitization did not directly correlate with higher resistance to corrosion. Hydrophobicity was critical in enhancing resistance to corrosion. When sintering of Pt particles was considered, carbon nanocages were more resistant than nanofibers. The present findings thus indicate that the carbon nanocage is an appropriate catalyst support in fuel cell systems.