Chaojie Song

Temperature Dependent Performance and In Situ AC Impedance of High-Temperature PEM Fuel Cells Using the Nafion-112 Membrane

In this paper, temperature dependent performance of a Nafion 112-based proton exchange membrane (PEM) fuel cell was investigated at different temperatures, 30 psig back pressure, and 100% relative humidity (RH). High cell performance of ca. 0.637 V at 1.0 A/cm 2 was obtained at 120°C. Cell voltage decreased when the temperature increased within the range of 80-20°C. An in situ ac impedance spectroscopy method under load was developed to diagnose the performance reduction. A semi-empirical treatment was initiated to obtain expressions for extinguishing the individual performance drops caused by reaction kinetics (charge-transfer resistance), membrane resistance, and mass-transfer limitation, respectively.

Discrepancies in the Measurement of Ionic Conductivity of PEMs Using Two- and Four-Probe AC Impedance Spectroscopy

Two- and four-probe cells were designed for comparative studies of ionic conductivity of proton conducting membranes (PEMs) using electrochemical impedance spectroscopy. Nafion 115 membrane was employed to examine the influence of cell configuration, probe geometry, and arrangement of probe. Whereas a single arc and linear responses, were observed in Nyquist plots using the 2-probe cell, multiple arcs were observed using the 4-probe cell. The linear response observed in the 2-probe configuration and the low frequency arc observed in the 4-probe configuration are due to transmission line behavior that results from a distributed interfacial capacitance coupled with the membranes ionic resistance at the Pt/PEM interfacial region. Increasing the distance between the probes and/or reducing the electrode contact area reduces or eliminates these low-frequency artifacts so that accurate data for ionic resistance can be obtained. Equivalent circuits for 2-probe and 4-probe cell geometries are constructed and used to extract conductivity data.

Temperature and pH Dependence of Oxygen Reduction Catalyzed by Iron Fluoroporphyrin Adsorbed on a Graphite Electrode

The graphite electrode surface adsorbed by 5,10,15,20-tetrakis(pentafluorophenyl)-21H, 23H-porphine iron (III), abbreviated as Fg I I I TPFPP, displays strong electrocatalytic activity toward O 2 reduction. The surface adsorption of Fe I I I TPFPP was investigated at different pH levels and temperatures by cyclic voltammetry. The kinetics of catalyzed O 2 reduction at different temperatures, as measured by cyclic voltammetry and a rotating disk electrode, was analyzed and the corresponding reaction mechanism was proposed. A four-electron/four-proton process was found to be the dominating pathway for catalyzed O 2 reduction. Furthermore, an increase in temperature from 20 to 70°C was shown to significantly enhance the reduction rate. The implication of such non-noble electrocatalysts for the cathode reaction in low-temperature proton exchange membrane fuel cells is discussed.

Ta and Nb co-doped TiO2 and its carbon-hybrid materials for supporting Pt–Pd alloy electrocatalysts for PEM fuel cell oxygen reduction reaction

To explore possible substitutes for carbon supports in PEM fuel cell catalysts, both Ta and Nb co-doped TiO2 (TaNbTiO2) and carbon–TaNbTiO2 (C–TaNbTiO2) hybrid support materials are successfully synthesized by a modified thermal hydrolysis method. These materials are employed as support candidates for Pt–Pd alloy catalysts for the oxygen reduction reaction. Among several supported catalysts, 20 wt% Pt0.62Pd0.38/C75wt%–(Ta0.01Nb0.03Ti0.96O2)25wt% is found to have an oxygen reduction reaction (ORR) mass activity of 260 mA mgPt−1 and an ORR loss of 30%, which are slightly better than those of the commercially available Pt/C baseline catalyst which has a mass activity of 110 mA mgPt−1 and an ORR loss of 40% probably because of a strong physicochemical interaction among the Pt–Pd alloy catalyst particle, carbon–TaNbTiO2 support and carbon, as well as the stability of oxide to protect carbon from corrosion by the distribution of oxide onto carbon, respectively.

Single PEMFC Design and Validation for High-Temperature MEA Testing and Diagnosis up to 300 ° C

A single proton exchange membrane fuel cell (PEMFC) and associated hardware were designed for high-temperature membrane electrode assembly (MEA) testing and diagnosis in the range of 160-300°C. The fuel cell heating strategy and its design considerations with respect to heat balance are also discussed in this paper. The fuel cell performance of PEMEAS H 3 PO 4 -doped polybenzimidazole (PBI)-based MEAs and the corresponding ac impedance diagnosis demonstrated that this single cell and its associated hardware designs are feasible for high-temperature MEA testing. The apparent exchange current densities for the cathodic oxygen reduction reaction as a function of temperature were obtained through ac impedance measurement. Rapid performance degradation was observed at extreme temperatures, such as 300°C, suggesting an accelerated fuel cell testing method. AC impedance results showed that the activity loss of the catalyst/catalyst layer was responsible for this degradation.

Accelerated Lifetime Testing for Proton Exchange Membrane Fuel Cells Using Extremely High Temperature and Unusually High Load

In this paper, two testing protocols were developed in order to accelerate the lifetime testing of proton exchange membrane (PEM) fuel cells. The first protocol was to operate the fuel cell at extremely high temperatures, such as 300 °C, and the second was to operate the fuel cell at unusually high current densities, such as 2.0 A/cm2 . A PEM fuel cell assembled with a PBI membrane-based MEA was designed and constructed to validate the first testing protocol. After several hours of high temperature operation, the degraded MEA and catalyst layers were analyzed using SEM, XRD, and TEM. A fuel cell assembled with a Nafion 211 membrane-based MEA was employed to validate the second protocol. The results obtained at high temperature and at high load demonstrated that operating a PEM fuel cell under certain extremely high-stress conditions could be used as methods for accelerated lifetime testing.

STEM HAADF Tomography of Molybdenum Disulfide with Mesoporous Structure

A highly ordered mesoporous molybdenum disulfide has been developed for catalysis in heavy oil refining. The morphology, structure, and composition of the material have been systematically characterized with advanced electron microscopy techniques. Scanning transmission electron microscopy with high‐angle annular dark field tomography has been used to investigate the porous structure to give spatial information on the nanometre scale, and offer a direct view of individual porous particles in three‐dimensions. The pore‐size distribution, connectivity of the pores, and the mesoporous surface area have also been analyzed and offer useful information towards catalyst design.

Electrochemical behavior of ruthenium-hexacyanoferrate modified glassy carbon electrode and catalytic activity towards ethanol electrooxidation

3- ion and three ruthenium forms (Ru(II), Ru(III) and Ru(IV)), characteristic of ruthenium oxide compounds. The modified electrode displayed excellent electrocatalytic activity towards ethanol oxidation in the potential region where electrochemical processes Ru(III)-O-Ru(IV) and Ru(IV)-O-Ru(VI) occur. Impedance spectroscopy data indicated that the charge transfer resistance decreased with the increase of the applied potential and ethanol concentration, indicating the use of the RuHCF modified electrode as an ethanol sensor. Under optimized conditions, the sensor responded linearly and rapidly to ethanol concentration between 0.03 and 0.4 mol L -1 with a limit of detection of 0.76 mmol L -1 , suggesting an adequate sensitivity in ethanol analyses.

HDS of dibenzothiophenes and hydrogenation of tetralin over a SiO2 supported Ni-Mo-S catalyst

AbstractA one-step synthesized Ni-Mo-S catalyst supported on SiO2 was prepared and used for hydrodesulphurization (HDS) of dibenzothiophene (DBT), and 4,6-dimethyl-dibenzothiophene (4,6-DMDBT), and for hydrogenation of tetralin. The catalyst showed relatively high HDS activity with complete conversion of DBT and 4,6-DMDBT at temperature of 280 °C and a constant pressure of 435 psi. The HDS conversions of DBTand 4,6-DMDBT increased with increasing temperature and pressure, and decreasing liquid hourly space velocity (LHSV). The HDS of DBT proceeded mostly through the direct desulphurization (DDS) pathway whereas that of 4,6-DMDBT occurred mainly through the hydrogenationdesulphurization (HYD) pathway. Although the catalyst showed up to 24% hydrogenation/dehydrogenation conversion of tetralin, it had low conversion and selectivity for ring opening and contraction due to the competitive adsorption of DBTand 4,6-DMDBT and insufficient acidic sites on the catalyst surface.

Catalyst Contamination in PEM Fuel Cells

The effects of impurities on fuel cells, often referred to as fuel cell contamination, is one of the most important issues in fuel cell operation and applications. Contamination is closely associated with proton exchange membrane fuel cell (PEMFC) durability and stability, both of which are important factors in the development and commercialization of PEMFC technology. Studies have identified that the membrane electrode assembly (MEA), the heart of the PEMFC, is the fuel cell component most affected by contamination. Impurities in the air and fuel streams damage the MEA by affecting both the anode and cathode catalyst layers (CLs), the gas diffusion layers (GDLs), as well as the proton exchange membrane (PEM), causing MEA performance degradation or even fuel cell failure. In general, PEMFC contamination effects can be categorized into three major types: (1) kinetic losses caused by the poisoning of both anode and cathode catalyst sites or a decrease in the catalyst activity; (2) ohmic losses due to an increase in the resistance of membrane and ionomer, caused by alteration of the proton transportation path; and (3) mass transfer losses due to changes in structure and in the ratio between the hydrophobicity and hydrophilicity of CLs, GDLs, and the PEM. Among those effects, the most significant is the kinetic effect of the anode and cathode electrocatalysts. This chapter presents PEMFC contamination with a focus on the anode and cathode catalyst layers. Catalyst contamination mechanisms, experimental results, modeling, as well as mitigation strategies are also covered in detail. For further information, such as contamination effects on other parts of PEMFCs, the reader is referred to a recent review paper [1].