Kunal Karan

A combined QCM and XPS investigation of asphaltene adsorption on metal surfaces

To investigate asphaltene-metal interactions, a combined quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS) study of asphaltene adsorption on a gold surface was conducted. Adsorption experiments were conducted at 25 degrees C with solutions of asphaltenes in toluene at concentrations ranging from 50 to 1500 ppm. QCM measurements yielded information on the kinetics of adsorption and further assessment of the data allowed the estimation of equilibrium adsorption levels. XPS analysis of adsorbed and bulk asphaltene demonstrated the presence of carboxylic, thiophenic, sulfide, pyridinic and pyrrolic type functional groups. The intensity of the main carbon (C-H) peak was related to surface coverage of adsorbed asphaltene as a function of asphaltene concentration by a simple mathematical model. The mass adsorption data from the QCM experiments also allowed estimation of the surface coverage, which was compared to those from XPS analyses. Surface coverage estimates as a function of asphaltene concentration could be described by a Langmuir (type-I) isotherm. The free energy of asphaltene adsorption was estimated to be -26.8+/-0.1 and -27.3+/-0.1 kJ/mol from QCM and XPS data, respectively assuming asphaltene molar mass of 750 g/gmol. QCM and XPS data was also analyzed to estimate adsorbed layer thickness after accounting for surface coverage. The thickness of the adsorbed asphaltene estimated from both XPS and QCM data analyses ranged from 6-8 nm over the entire range of adsorption concentrations investigated.

An Experimental Investigation of Water Transport in PEMFCs The Role of Microporous Layers

This experimental study was undertaken to resolve the contrasting viewpoints on the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the net water transport in a polymer electrolyte membrane fuel cell (PEMFC). Experimental results on single cells with and without cathode MPL show no statistically significant change in the net drag coefficient that could be attributed to the presence of the MPL. In contrast to the two prevailing but contrasting viewpoints, our results indicate that the MPL on the cathode neither enhances back-diffusion nor increases water removal from the cathode catalyst layer to the PTL.

Characterization of La0.5Ba0.5CoO3 − δ as a SOFC Cathode Material

This study reports the physical properties and electrochemical behavior of La 0.5 Ba 0.5 CoO 3-δ (LBC), a perovskite material and a potential candidate for the solid oxide fuel cell (SOFC) cathode. LBC was synthesized by a solid-state route, and phase purity was checked by X-ray diffraction measurement. Thermogravimetric analysis of LBC was performed both in air and in argon atmosphere to observe its phase stability. Total conductivity, measured by a four-point dc polarization method, exhibited a pseudometallic-type behavior. Ionic conductivity was determined in air over a 650-775°C temperature range by a dc polarization technique using electron-blocking electrodes. The high total conductivity, combined with a sufficiently high ionic conductivity (1.2 × 10 -3 S/cm at 650°C), indicates that LBC possesses a certain degree of mixed ionic-electronic conducting nature. The oxygen reduction behavior of porous LBC electrodes on a gadolinium-doped ceria electrolyte was studied in a symmetrical cell configuration by impedance spectroscopy. The CO 2 tolerance and the high temperature stability of the cell were also investigated. Impedance spectra obtained at zero dc bias conditions consisted of two semicircular arcs, which indicate that the overall oxygen reduction comprises at least two distinct processes. The area specific resistance (ASR) for electrode polarization was determined over a wide range of temperatures (600-850°C). The ASR value at 600°C was determined to be 0.16 Ω cm 2 , which offers the promise of utilizing LBC as a cathode for low temperature SOFCs.

Understanding the Ionomer Structure and the Proton Conduction Mechanism in PEFC Catalyst Layer: Adsorbed Nafion on Model Substrate

To understand the structure/property of the ionomer in the CL, adsorbed Nafion thin films were prepared on model substrate, SiO2 terminated silicon wafer, by spontaneous adsorption of Nafion from its 0.1 to 5 wt% solution. Fitting of Variable Angle Spectroscopic Ellipsometry (VASE) data yielded thickness of adsorbed films ranging from 4 nm at low concentrations (0.1 wt% film) to 306 nm at high concentrations (5wt% film). Contact angle measurements indicated that the low (0.1 wt%) and high (3 wt%) films to posses hydrophobic surface but the intermediate concentration (0.5 wt% and 1.0 wt%) film surface to be hydrophilic. Conductivity measurements of adsorbed films over a range of RH showed that all films prepared from 1 wt% and lower concentrations solutions to possess similar conductivities at high RH. However, the 5wt% film showed much higher conductivity indicating a different proton conduction mechanism for low and high concentration films.

Electrochemical Activity and Catalyst Utilization of Low Pt and Thickness Controlled Membrane Electrode Assemblies

An improved catalyst deposition methodology based on a piezo-electric printing technique has been developed and used to fabricate catalyst coated membranes (CCM) with thin catalyst layers (1-5 μm) and ultra-low Pt loadings (0.02-0.12 mg Pt /cm 2 ). The performance of these CCMs was examined in proton exchange membrane fuel cells (PEMFCs). The catalyst utilization was observed to increase with decreasing catalyst layer thickness (decreasing Pt loading). The printed CCM with two layers containing an ultra-low Pt loading (0.02 mg Pt /cm 2 ) exhibited Pt utilizations of 100%. Neglecting the anode contributions, the mass activity at 850 mV for the printed CCM is nearly 76.5 mA/mg Pt which is 3.5 times higher than that for the CCM fabricated by conventional spraying method (22.5 mA/mg Pt ).

Optimal Design of Ultralow-Platinum PEMFC Anode Electrodes

A computational, two-dimensional agglomerate anode electrode model is presented. The model provides insight on the mass and charge transport in the anode and implements the recent proposed dual-pathway kinetics for the hydrogen oxidation reaction. Results from this model highlight the potential for platinum reduction on the anode. In order to systematically assess the possible reductions on platinum loading, an optimization problem is formulated to minimize platinum loading while maintaining performance of typical state-of-the-art electrodes. The results reveal that platinum loading can be reduced by more than one order of magnitude, from 0.4 to less than 0.018 mg/cm 2 , by changing the gas diffusion layer (GDL) and catalyst layer (CL) composition. Furthermore, if the CL thickness and the GDL and CL compositions are optimized simultaneously, the amount of platinum can be further reduced by an extra order of magnitude by depositing a catalyst layer of 1.25 μm with a platinum loading of 0.0026 mg/cm 2 .

Impact of Nonuniform Potential in SOFC Composite Cathodes on the Determination of Electrochemical Kinetic Parameters A Numerical Analysis

Engineering of porous solid oxide fuel cell (SOFC) composite cathodes comprising a mixture of electrocatalyst and ionic conductor requires the knowledge of electrochemical kinetic parameters such as the reaction order and the charge-transfer coefficients. Conventional dc techniques are commonly employed in electrochemical measurements for composite cathodes. The results are often analyzed in terms of the low-or high-field approximations of the Butler-Volmer equation, which is based on the assumption that no potential or overpotential gradients exist within the electrode. In this study, numerical simulation of lanthanum strontium manganate-yttria-stabilized zirconia composite cathode was performed with an assumed oxygen reduction reaction mechanism and corresponding electrochemical kinetic parameters. The simulation results indicated that for composite cathodes significant gradients in the overpotential exist. The apparent reaction order and apparent charge-transfer coefficients, derived on the basis of nominal overpotential and net current, the two quantities that are typically accessible experimentally, were significantly different than the actual kinetic parameters used in the simulation. The extent of the variation between the actual and apparent electrochemical kinetics parameters was found to be dependent on the thickness and microstructure of the composite cathode. Simulation of conventional cathodes, which had no potential gradients within the bulk of the cathode, showed no errors.

Utilization of Biogas Generated from Ontario Wastewater Treatment Plants in Solid Oxide Fuel Cell Systems: A Process Modeling Study

This study reports an assessment of recoverable energy from biogas generated in anaerobic digestion units of wastewater treatment plants. Based on a survey of wastewater treatment plants (WWTPs) in the Canadian province of Ontario, the total and recoverable amount of WWTP-biogas energy was estimated. Compositional and flowrate data from three WWTPs – Ravenview in Kingston (73,000 m3/day), Humber in Toronto (337,000 m3/day) and Ashbridges Bay in Toronto (677,000 m3/day) – obtained from the survey were used as inputs for process flow simulation of a biogas-fuelled solid oxide fuel cell (SOFC) system. The system included H2S removal, reformation of the biogas and electrical and heat generation in a solid oxide fuel cell. Using a conservative basis of higher heating value (HHV), overall efficiencies of 55%, 58% and 60% for the three plants were calculated. If such biogas-SOFC systems were to be used at all the sites identified in the survey of Ontario WWTPs, a total of 1.3 GWh of electrical energy would be produced per day.

Effect of Relative Humidity on Electrochemical Active Area and Impedance Response of PEM Fuel Cell

The influence of humidity of supplied gases on on electrochemically active surface area and charge transfer resistance in cathode process of PEM fuel cell was studied. Impedance spectra for cells operated with various gas stream combinations - H2/Air, H2/O2, H2/H2 and H2/N2 - was analyzed to determine the physical/chemical origin of the spectra features. Cathode charge transfer resistance increased with a decrease in the humidity of supplied gases. As well, a reduction in the electrochemically active surface area with a decrease in relative humidity was observed.

Estimation of Chemical and Transport Processes in Porous, Stoichiometric LSM Cathodes Using Steady-State Polarization and Impedance Modeling

Electrochemical data collected for the porous La 1―x Sr x MnO 3±δ (LSM) cathode-yttria-stabilized zirconia electrolyte system were analyzed using a mathematical model capable of simulating both the steady-state and impedance responses. The model considered the distributed nature of the porous electrode and a parallel pathway for oxygen transport including gas transport and surface diffusion. Data were collected in low oxygen partial pressures ranging between 10- 4 and 10- 3 atm, where LSM was considered to be stoichiometric with respect to oxygen. Thermodynamically consistent kinetic and transport parameters were regressed from the steady-state polarization data. Three different parameter sets fit the general trend of the experimental data. The surface diffusion and adsorption/desorption parameters were cross-correlated, which underlines the difficulty in finding a unique set of fitting parameters. The parameter set with four adjustable parameters resulted in a high surface diffusion coefficient but slower adsorption/desorption kinetics compared to the other two parameter sets for which a lower surface diffusion coefficient was fixed. The predicted overall impedance at zero-dc bias conditions for three parameter sets had varying degrees of agreement with the data. Low and high frequency arcs attributed to adsorption and an oxygen transport process were observed in the faradaic impedance response.