Influence of toluene contamination at the hydrogen Pt/C anode in a proton exchange membrane fuel cell

For fuel cells run on hydrogen reformate, traces of hydrocarbon contaminants in the hydrogen gas may be a concern for the performance and lifetime of the fuel cell. This study focuses on the influence of low concentrations of toluene on the adsorption and deactivation chemistry in a proton exchange membrane (PEM) fuel cell. For this purpose cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques were employed. Results from adsorption and desorption (by oxidation or reduction) experiments performed in a humidified nitrogen or hydrogen flow in a fuel cell test cell with a mass spectrometer system connected to the outlet are presented. The influence of adsorption potential, temperature, and humidity are discussed. The results show that toluene adsorbs on the catalyst surface in a broad potential window, up to at least 0.85 V versus RHE at 80 °C. Adsorbed toluene oxidizes to CO₂ with peak potentials above 1.0 V for temperatures below 95 °C. Some desorption of toluene (or reduced products) may take place at potentials below 0 V. In a hydrogen flow, toluene contamination in per mille concentrations leads to a continuous growth of the charge transfer resistance, while a 10-fold dilution of the toluene concentration resulted in a low and constant charge transfer resistance even for longer exposures. This indicates that a competition between toluene and hydrogen may take place on the active platinum surface at the anode.     

Characterization of catalysts and membrane in DMFC lifetime testing

The lifetime and performance of a direct methanol fuel cell (DMFC) were investigated to understand the correlation between the structure of catalysts/membrane and cell performance versus time. The cell polarization and performance curves were obtained during the DMFC operation with the time. The catalysts and Nafion^® membrane of the membrane electrode assembly (MEA) from the lifetime test were comprehensively examined by XRD, HRTEM, FTIR and Raman spectroscopy techniques. The results revealed that there was significant performance degradation during the first 200 h operation; while the degradation was slowing down between 200 and 704 h operation. The degradation became worse after 1002 h operation. The increases of the catalyst particle size from both anode and cathode catalysts were observed after the DMFC lifetime test. The changes of microstructure, surface composition, the interfacial structure of the MEA, and the aging of Nafion^® under the DMFC lifetime tests were also observed.     

CO tolerance effects of tungsten-based PEMFC anodes

The performance of proton exchange membrane fuel cells (PEMFC) fed with CO-contaminated hydrogen was investigated for anodes with PtWO_x/C and phosphotungstic acid (PTA) impregnated Pt/C electrocatalysts. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with anodes containing 0.4 mg PtWO_x cm⁻² and also for those with 0.4 mg Pt cm⁻² impregnated with ca. 1 mg PTA cm⁻². A decay of the single cell performance with time is observed, and this was attributed to an increase of the membrane resistance due to the polymer degradation promoted by the crossover of the tungsten species throughout the membrane.     

Nanostructured palladium-silver coated nickel foam cathode for magnesium-hydrogen peroxide fuel cells

A novel multiscale Pd-Ag catalyzed porous cathode for the magnesium-hydrogen peroxide fuel cell was prepared by electrodeposition of Pd onto Ag coated nickel foam surface from an aqueous solution of palladium chloride. The structure, morphology and composition of the electrodeposited catalyst layer were characterized using SEM, EDS and XPS analysis. Magnesium-hydrogen peroxide fuel cell tests with the Pd-Ag deposited cathode were carried out and compared with the Ag-deposited electrode. The effects of temperature, H₂O₂ flow rate and H₂O₂ concentration on cell performance were investigated, and the electrode stability test was carried out. The Pd-Ag deposited electrode showed higher catalytic activity for the reduction of hydrogen peroxide than that of the Ag-deposited Ni foam cathode, and gave much improved fuel cell performance. The magnesium-hydrogen peroxide fuel cell with nanostructured Pd-Ag coated nickel foam cathode presented a maximum power density of 140 mW cm⁻², but the Mg-H₂O₂ fuel cell with Ag coated Ni foam cathode gave only 110 mW cm⁻² under the same operation condition.     

Performance of an anode-supported SOFC with anode functional layers

Before fabrication of dense yttria-stabilized zirconia films, several thin anode functional layers (AFL) were fabricated onto porous NiO/yttria-stabilized zirconia anode substrates using slurry spin coating. The effect of AFL thickness on gas impermeability and performance of a cell was investigated by studying the effect of AFL thickness on the open-circuit voltage, ohmic resistance, I-V characteristics and electrode overpotential of cells. The results of investigation indicated that as the AFL thickness increased, the gas impermeability of cells was generally improved and the ohmic resistance of cells was increased. The cell with a 5-μm-thick AFL exhibited an excellent cell performance, for example, a single cell with this AFL exhibited an output power of 2.63 W cm⁻² at 800 °C when hydrogen was used as fuel and an oxygen was used as oxidant.     

Insights into the simultaneous chronopotentiometric stripping measurement of indium(III), thallium(I) and zinc(II) in acidic medium at the in situ prepared antimony film carbon paste electrode

The simultaneous measurement of microgram per liter concentration levels of indium(III), thallium(I) and zinc(II) at the antimony film carbon paste electrode (SbF-CPE) is demonstrated. The antimony film was deposited in situ on a carbon paste substrate electrode and employed in chronopotentiometric stripping mode in deoxygenated solutions of 0.01 M hydrochloric acid (pH 2). The chronopotentiometric stripping performance of the SbF-CPE was studied and compared with constant current chronopotentiometric stripping and anodic stripping voltammetric operation. In comparison with its bismuth and mercury counterparts, the SbF-CPE exhibited advantageous electroanalytical performance; namely, at the bismuth film electrode, the measurement of zinc(II) was practically impossible due to hydrogen evolution, whereas the mercury film electrode exhibited a poorly developed signal for thallium(I). The SbF-CPE revealed favorable calculated LoDs (3σ) of 1.4 μg L⁻¹ for thallium(I) and 2.4 μg L⁻¹ for indium(III) along with good linear response in the examined concentration range from 10 to 100 μg L⁻¹ with correlations coefficients (R²) of 0.992 for thallium(I) and 0.994 for indium(III) associated with a 120 s deposition time. The chronopotentiometric stripping performance of the SbF-CPE was characterized also by satisfactory reproducibility of 1.62% for indium(III), 3.96% for thallium(I) and 2.11% for zinc(II) (c = 40 μg L⁻¹, n = 11).     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt_xM/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM_y/C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.     

The influence of the pore size and its distribution in a carbon paper electrode on the performance of a PEM Fuel cell

Porous conducting carbon paper with its unique combination of properties acts as the backing material of electrode in a fuel cell. It not only assists in the flow of electrons and reactant gases but also acts as an effective support for the electrolyte and the catalyst layer. The electrically conducting porous carbon paper was prepared by adopting a modified process of preparation which involves molding together several carbon fiber preforms in the form of laminates rather than a single preform. The method was found to influence the characteristics of the paper significantly and resulted in improved performance of the unit fuel cell employing the laminated paper as electrode. The I-V performance of the fuel cell using carbon paper formed by molding single ply showed a peak power density of 573 mW/cm² as compared to that of 722 mW/cm² for three ply laminates, a value very close to that of achieved by using Toray (Japan) carbon paper (782 mW/cm²) under identical operating conditions.     

Performances of micro-tubular solid oxide cell with novel asymmetric porous hydrogen electrode

Asymmetric-porous hollow-fiber has been fabricated by a phase-inversion process and employed as the hydrogen electrode for micro-tubular solid oxide cell (MT-SOC). The microstructure and electrochemical properties of MT-SOC were investigated in detail. The asymmetric-porous hydrogen electrode possesses unique two layer finger-like porous micro-structure with a thin functional layer and a thick fuel delivery layer. When the MT-SOC was operated in fuel cell mode, maximum power densities of 0.54, 0.71 and 1.25 W/cm² were obtained at 800, 850 and 900 °C, respectively. On the other hand, when the MT-SOC was operated in electrolysis mode at 900 °C with an applied voltage of 1.3 V, current densities of 0.68 A/cm² and 2.57 A/cm² were obtained at 30 vol.% and 80 vol.% absolute humidity (AH), respectively. These results indicate that novel-microstructured MT-SOC can be effectively fabricated towards high performance fuel cell and electrolysis cell.     

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.     

Transient carbon monoxide poisoning of a polymer electrolyte fuel cell operating on diluted hydrogen feed

The transient behavior of a 50 cm² PEM fuel cell fed on simulated reformate containing diluted hydrogen and trace quantities of carbon monoxide (CO) was experimentally investigated. It was found that the overall cell performance throughout the CO poisoning process can be described with a lumped model of hydrogen and CO adsorption, desorption, and electro-oxidation coupled with a current-voltage relationship for fuel cell performance. It was shown that while hydrogen dilution alone does not have an appreciable effect on cell polarization, in the presence of trace amounts of CO, hydrogen dilution amplifies the problem of CO poisoning. This is a result of the diluent reducing the partial pressure of reactants in the anode fed stream, thus retarding the already CO-impaired hydrogen adsorption onto the catalyst surface. In a diluted hydrogen stream, even low CO concentrations (i.e. 10 ppm), which are traditionally considered safe for PEM fuel cell operation, were found to be harmful to cell performance.     

Harnessing of bioelectricity in microbial fuel cell (MFC) employing aerated cathode through anaerobic treatment of chemical wastewater using selectively enriched hydrogen producing mixed consortia

The possibility of bioelectricity generation from anaerobic chemical wastewater treatment was evaluated in a microbial fuel cell (MFC) [dual-chambered; mediator less anode; aerated cathode; plain graphite electrodes] employing selectively enriched hydrogen producing (acidogenic) mixed culture. Performance of MFC was evaluated at two organic/substrate loading rates (OLR) (1.165 Kg COD/m³-day and 1.404 Kg COD/m³-day) in terms of bioelectricity production and wastewater treatment at ambient pressure and temperature under acidophilic microenvironment (pH 5.5) using non-coated plain graphite electrodes (mediatorless anode; air cathode). Experimental data demonstrated the feasibility of in situ bioelectricity generation along with wastewater treatment. The performance of MFC with respect to power generation and wastewater treatment was found to depend on the applied OLR. Maximum voltage of 716 mV (2.84 mA; OLR −1.165 kg COD/m³-day) and 731 mV (2.97 mA; OLR-1.404 kg COD/m³-day) was observed at stable operating conditions. Substrate degradation rate (SDR) of 0.519 Kg COD/m³-day and 0.858 Kg COD/m³-day was observed at two OLRs studied. Maximum power yield (0.73 W/Kg COD_R and 0.49 W Kg/COD_R) and current density (339.87 mA/m² and 355.43 mA/m²) was observed at applied 50 Ω resistance. Fuel cell performance was evaluated employing polarization curve (100 Ω-30 KΩ), Coulombic efficiency (€_cb) and cell potentials along with sustainable power yield at stable phase of fuel cell operation. Designed MFC configuration, adopted operating conditions and used parent inoculum showed positive response.     

Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Three-dimensional non-isothermal modeling of carbon monoxide poisoning in high temperature proton exchange membrane fuel cells with phosphoric acid doped polybenzimidazole membranes

The performance of proton exchange membrane fuel cell (PEMFC) degrades when carbon monoxide (CO) is present in the supplied fuel, which is referred to as CO poisoning. Even though the high temperature PEMFC (HT-PEMFC) with a typical operating temperature range from 100 °C to 200 °C features higher CO tolerance than the conventional PEMFC operating at lower than 100 °C, the performance degradation of HT-PEMFC is still significant with high CO concentrations (e.g. ?0.5% CO by volume at 130 °C) in the supplied fuel. In this study, a CO poisoning model is developed for HT-PEMFCs with phosphoric acid doped polybenzimidazole (PBI) membranes. The present three-dimensional non-isothermal model compares well with published experimental data at various operating temperatures and CO concentrations in the supplied fuel. It is found that the CO adsorption/desorption processes follow Langmuir kinetics in HT-PEMFCs instead of the well-known Temkin kinetics in conventional PEMFCs. The results indicate that a HT-PEMFC can operate with sufficiently good performance at 130 °C or higher with hydrogen gas produced by methanol reforming with selective oxidation process, and at 160 °C or higher even without the selective oxidation process. At high current densities, it is also observed that severe performance degradation due to CO poisoning only occurs if the volume averaged hydrogen coverage is lower than 0.1 in the anode catalyst layer (CL).     

Hydrodesulfurization of jet fuel by pre-saturated one-liquid-flow technology for mobile fuel cell applications

To prevent the catalysts in fuel cell systems from poisoning by sulfur containing substances the fuel to be used must be desulfurized to a maximum of 10 ppm of sulfur. Thereby, damage to the catalysts in the fuel cell and the reformer can be avoided. Diesel fuel for road vehicles within the EU is already desulfurized at the refinery. However, jet fuel is permitted to have up to 3000 ppm of sulfur. Since the hydrodesulfurization process used in refineries is not suitable for mobile applications, the aim of the present work was to develop an alternative desulfurization process for jet fuel and to determine its technical feasibility.To this end, many processes were assessed with respect to their application in fuel cell based auxiliary power units (APUs). Among them, hydrodesulfurization with pre-saturation was selected for detailed investigations. Laboratory tests revealed that also syngas operation is possible without any performance loss in comparison to operation with hydrogen. Pure hydrogen is not available in a fuel cell system based on reforming of jet fuel. The effects of reaction temperature, operating pressure and liquid hourly space velocity (LHSV) were investigated. Different jet fuel qualities with up to 3000 ppm of sulfur were desulfurized to a level of 15-22 ppm.Finally, the technical applicability of hydrodesulfurization with pre-saturation was demonstrated in a pilot plant with an electrical power of 5 kW, going beyond the laboratory scale. In a 200-h experiment, a commercial jet fuel with 712 ppm of sulfur was desulfurized to a maximum sulfur content of 10 ppm. Besides this, H₂S separation by stripping with air turned out to be a suitable method for APU applications. The aim of developing a suitable process for the desulfurization of jet fuel in fuel cell APUs has thus been achieved.     

The fading behavior of direct methanol fuel cells under a start-run-stop operation

Stability tests of direct methanol fuel cells (DMFCs) were conducted under two different operational modes, start-run-stop (SRS) and long-running (LR) modes, to investigate the difference in performance decay of the cells. Frequency response analysis (FRA), cyclic voltammetry (CV), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) were used to identify the causes of cell degradation. The cell performance test results showed that the fading behavior of the cell under the SRS operation was greater than that under the LR operation. The maximum power density was reduced approximately 20% and 32% of the initial value after operating under the LR and the SRS mode, respectively. This result was corresponded with the anode catalyst agglomeration data obtained from both XRD and TEM analysis. The increase of PtRu particle size under the SRS operation was higher than that under the LR operation. The FRA spectra showed that the anode reaction resistance increased from the initial value of 0.26 Ω cm² to 0.30 Ω cm² after life-testing under SRS mode for 45 h. A right-shift of the methanol oxidation peak and a 5.0% reduction of electrocatalyst surface area observed from the cyclic voltammograms also supported this finding. Finally the decay of cell performance was due to the Ru crossover, as verified by EDX results.     

The effect of nitrogen oxides in air on the performance of proton exchange membrane fuel cell

The effects of NO_x on the performance of proton exchange membrane (PEM) fuel cell were investigated through the introduction of a mixture containing NO and NO₂, in a ratio of 9:1, into the cathode stream of a single PEM fuel cell. The NO_x concentrations used in the experiments were 1480 ppm, 140 ppm and 10 ppm, which cover a range of three orders. The experimental results obtained from the tests of durability, polarization, reversibility and electrochemical impedance spectroscopy (EIS) showed a detrimental effect of NO_x on the cell performance. The electrochemical measurements results suggested that the impacts of NO_x are mainly resulted from the superposition of the oxygen reduction reaction (ORR), NO and HNO₂ oxidation reactions, and the increased cathodic impedance. Complete recovery of the cell performance was reached after operating the cell with clean air and then purging with N₂ for hours.     

Proton conducting membrane containing room temperature ionic liquid

A new proton conducting membrane containing room temperature ionic liquid: 2,3-dimethyl-1-octylimidazolium trifluoromethanesulfonylimide (DMOImTFSI) and polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) has been developed in the present work. The addition of bis(trifluoromethanesulphonyl)imide (HN(CF₃SO₂)₂) to this membrane results in an increase in conductivity by one order of magnitude at 25 °C. The membrane shows a conductivity of 2.74 × 10⁻³ S/cm at 130 °C along with good mechanical stability. The membrane was tested in a commercial fuel cell test station at 100 °C with dry hydrogen and oxygen gas reactants using Pt/C electrodes. The membrane containing the ionic liquid has been found to be electroactive for hydrogen oxidation and oxygen reduction at the platinum electrode and can be developed for use in proton exchange membrane fuel cell (PEMFC) under non-humid conditions at elevated temperatures.     

Testing the performance and compatibility of degummed soybean heating oil blends for use in residential furnaces

Degummed soybean heating oil (SHO) is a renewable energy resource, which can reduce dependence on foreign oil and create a new market for the soybean industry. This study demonstrated that SHO 20 (20% degummed soybean oil and 80% No. 2 fuel oil) is suitable for application in residential furnaces without modification. The tests conducted were: fuel properties, seal compatibility, long-term storage, and laboratory and field combustion. The physical property tests showed that the kinematic viscosity (0.0346 cm²/s) and the pour point of SHO 20 (−30 °C) were within the ASTM requirement for No. 2 fuel oil; and the net heating value of SHO 20 (43.9 MJ/kg) was only 1-3% lower than the No. 2 fuel oil value (45.6 MJ/kg). Compatibility tests performed on the rubber seals and gasket materials (Nitrile and Viton) found in typical heating fuel pump systems indicated that the tensile strength and hardness values were not significantly affected by SHO blends when compared with No. 2 fuel oil. A long-term storage test revealed that there was no significant change in heat content and no visible stratification of SHO 20 blend during three months of storage. The pump pressure and the type of nozzle used affected the concentration of NO_x, SO₂, and CO in the flue gas. As was expected, increasing the SHO fraction in the blend also reduced the SO₂ emission. The combustion of SHO 20 resulted in a higher flue gas temperature which increased the NO_x emission than with No. 2 fuel oil.