Electrochemistry of proton-exchange-membrane electrolyte fuel cell (PEMFC) electrodes

The purpose of this research is mainly focused on four areas of the electrochemistry of PEMFCs: (1) developing a fast method to screen catalysts, (2) modifying carbon-supported Pt catalysts to improve their performance; (3) demonstrating a new method to accurately measure the catalyst active surface area by cyclic voltammetry; (3) reducing MeOH crossover in direct methanol fuel cells by modifying the electrolyte membrane. -- In order to achieve the above goals, a new electrochemical apparatus was designed to study the oxygen reduction reaction (ORR) on gas diffusion electrodes. Different kinds of electrode & membrane assemblies (MEA) were made at various platinum loadings and different catalysts (10 % and 20 % Pt on Vulcan XC-72R) using the hot bonding method. Several electrochemical techniques (cyclic voltammetry, transient and steady state polarization, and ac impedance) have been used to evaluate the key parameters that determine the oxygen reduction performance of the electrodes, such as active catalyst surface area, Tafel slope, and resistance. Polarization results indicate that a self-written program for collecting current provides useful data for both transient and steady state polarizations. The transient (10s) polarization can fully represent the electrode performance rapidly and reliably. AC impedance is a useful technique for measurement of the cell performance. -- Commercial carbon-supported Pt catalysts were modified by chemical oxidation (HNO₃, H₂SO₄, H₃PO₄, HCIO₄, NaCIO, H₂O₂, or (NH₄)₂S₂O₈) to enhance the catalytic activity for oxygen reduction reaction. Catalysts modified by either acidic or non-acidic oxidants can significantly improve ORR performance. The catalysts treated by acids, especially by HNO₃, increase the number of carbon surface functional groups such as - OH and - COOH allowing protons more easy access to the catalyst surface. The improved ionic conductivity on the catalyst surface leads to a higher performance based on the triple contact mechanism for the ORR in the catalyst layer. -- Furthermore, a technique using CO₂ electroreduction/reoxidation was evaluated to accurately measure the active surface area of carbon-supported catalysts as an alternative to the conventional use of hydrogen adsorption. The reduced CO₂ oxidation charge can be used to compare the active surface areas of different catalysts although charge should be used due to the complexity of the products of CO₂ reduction. This method retains the resolution advantage of the CO stripping method, and avoids the overestimation of the active surface area. Moreover, peak and onset potentials for reduced CO₂ oxidation are shown to be representative parameters for explaining catalyst tolerance to MeOH and CO, especially the oxidation onset potentials. -- Finally, membranes modified with conducting polymers by different polymerization methods such as Fe³⁺, (NH₄)₂S₂O₈, H₂O₂, and O₂ with UV irradiation were evaluated in terms of ionic conductivity and permeability to methanol. Electrochemical measurements showed that the modified membranes can significantly reduce methanol crossover. The inhibition of methanol crossover is realized by a lower methanol diffusion coefficient in the membrane as measured by chronoamperometry. The stability of modified membrane needs to be evaluated under practical operating conditions of direct methanol fuel cells

Analysis of the Characteristics of Pore Pressure Coefficient for Two Different Hydrate-Bearing Sediments under Triaxial Shear

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient A can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between A value responses and deformation behaviors remain elusive. In this study, effects of hydrate saturation and effective confining pressure on the characteristics of pore pressure coefficient A are explored systematically based on published triaxial undrained compression test data of hydrate-bearing sand and clay-silt sediments. Results show that there is a higher value of the coefficient A with increasing hydrate saturation at small strain stage during shearing. This effect becomes more obvious when the effective confining pressure increases for hydrate-bearing sand sediments rather than hydrate-bearing clayey-silt sediments. An increasing hydrate saturation leads to a reduction in A values at failure. Although A values at failure of sand sediments increase with increasing effective confining pressure, there are no same monotonic effects on clayey-silt specimens. A values of hydrate-bearing sand sediments firstly go beyond 1/3 and then become lower than 1/3 at failure even lower than 0, while that of hydrate-bearing clayey-silt sediments is always larger than 1/3 when the effective confining pressure is high (e.g., >1 MPa). However, when the effective confining pressure is small (e.g., 100 kPa), that behaves similar to hydrate-bearing sand sediments but always bigger than 0. How the A value changes with hydrate saturation and effective confining pressure is inherently controlled by the alternation of effective mean stress

Long chain diol index (LDI) as a potential measure to estimate annual mean sea surface temperature in the northern South China Sea

Long chain alkyl diols (LCDs) were studied to probe the ability of long chain diol index (LDI) as a sea surface temperature (SST) or temperature record at depth in the open northern South China Sea (SCS). The results showed that riverine LCDs, which may affect LDI-derived SSTs, was less significant due to the substantially low fractional abundance (generally < 20% and averaged 10%) of 1,15-C-32 diol in core sediments. The LDI-derived SSTs demonstrated a generally similar trend as SST records calculated from other proxies in nearby cores, suggesting that the LDI proxy can be a potential index to estimate ancient annual mean SST in the open northern SCS. The integration of novel data from the open sea with already published data in the coastal area would provide comprehensive information on the paleo-temperature evolution of the northern SCS

Alcohol compounds in Azolla imbricata and potential source implication for marine sediments

This study investigated the composition of long-chain alkyl diols, triols, sec-alcohols, hydroxyl acids, and other hydroxylated compounds in Azolla imbricata and compared the organic alcohol components of Azolla filiculoides, Azolla microphylla, and South China Sea (SCS) sediments in order to investigate the possible indication of Azolla being the biological source of diols and triols in SCS sediment. Large amounts of diols, monohydroxy acids, and sec-alcohols with internal hydroxy groups at omega 20 were detected in the three types of Azolla. Among these, 1,omega 20-diol and omega 20-hydroxy acid exhibited strong even-odd predominance distribution, whereas omega 20-sec-alcohol exhibited strong odd-even predominance distribution. In addition, small amounts of diols, triols, and dihydroxy acids with internal hydroxy groups at 9, 10 or omega 9, omega 10 were detected, among which the chain length of C-29 was predominate. Compounds having similar structures as those in Azolla reflected a similar biosynthetic pathway: omega 20-hydroxy acid exhibiting even-odd predominance distribution is decarboxylated to omega 20-sec-alcohol exhibiting odd-even predominance distribution and converted to 1, omega 20-diol with even-odd predominance distribution by acyl reduction; omega 9, omega 10-hydroxy acid is converted to 1,20,21(1, omega 9, omega 10)-triol by acyl reduction, and then converted to 9,10-diol by hydrogenation and dehydration. The alcohol components in A. imbricata were clearly not the biological source of 1,13/1,14/1,15-C-28,C- 30,C- 32 diols and 1,3,4-C27-29 triols in the SCS sediment. Certain marine diatoms might be the source of 1,14-C-28,C- 30 diol in inshore sediment, but the biological source of diols and triols in the SCS sediment requires further investigation

Undrained Shear Properties of Shallow Clayey-Silty Sediments in the Shenhu Area of South China Sea

Suction piles are used to ensure wellhead stability during natural gas hydrate production in the Shenhu area of the South China Sea (SCS). Undrained shear properties of clayey-silty sediments play a critical role in the stability analysis of suction piles. However, it has not been fully studied. This study conducts a series of undrained triaxial shear tests on shallow clayey-silty sediments in the Shenhu area of SCS, and stress–strain curves under different overconsolidation ratio (OCR) conditions are obtained. OCR effects on undrained shear properties of clayey-silty sediments are discussed, and a model to predict the pore pressure coefficient at failure is proposed. Results show that the isotropic compression index is 0.175, and the isotropic swelling index is 0.029. The undrained shear strength is proportional to the effective confining pressure, and the proportionality coefficient is 0.42 for normally consolidated specimens, while the undrained shear strength of OC specimens nonlinearly increases with OCRs increasing. The proposed model aptly predicts the pore pressure coefficient at the failure of clayey-silty sediments of SCS with different OCRs

Detection and indication of 1,3,4-C27-29 triol in the sediment of northern South China Sea

After the detection of the 1,20,21-C-29 long-chain triol in some sediments and freshwater pteridophytes, in this study, a new homologous long-chain triol, 1,3,4-C27-29, is detected for the first time in the Site4B core sediment in the northern South China Sea. The hydroxyl location and length of the carbon chain of this newly discovered triol differ from those of 1,20,21-C-29 triol. The test results of its molecular distribution and individual carbon isotope reveal that 1,3,4-C(29)triol has a good correlation with n-C26-30 even carbon-numbered long-chain fatty alcohols, with R (2) (n=68) values of 0.905, 0.929 and 0.903, respectively, and its carbon isotope composition, at-32.3aEuro degrees +/- 1.9aEuro degrees, is similar to that of n-C26-30, at-29.13aEuro degrees +/- 0.87aEuro degrees,-32.98aEuro degrees +/- 1.28aEuro degrees, and-32.98aEuro degrees +/- 1.28aEuro degrees. 1,3,4-C-29 triol from the Site4B core sediment and terrigenous long-chain fatty alcohol (n-C26-34) show highly consistent distribution trends in the entire section; thus, the former could serve as a proxy indicator of the terrigenous input. Considering that the 1,20,21-C-29 triol in previous research belongs to Azolla, which are fresh water pteridophytes, the 1,3,4-C27-29 triol identified in this study might have similar biogenetic derivation. Thus, determination of its biogenic area and growing environment could provide potential organic geochemical evidence supporting the terrigenous input and source in the northern South China Sea

Transient analysis of hydrogen sulfide contamination on the performance of a PEM fuel cell

A transient kinetic model for the anode catalyst layer of a proton exchange membrane (PEM) fuel cell is proposed to describe the performance loss introduced by the hydrogen sulfide contaminant. Reaction rates are considered as functions of cell current density and contamination level and are estimated based on the available experimental data. It is found that at a constant cell current density the surface coverage of Pt\ue2\u20ac\u201cS increases faster with time and the anode overpotential rises sharply when increasing the contamination level from 1 to 10 ppm, leading to a faster and more severe cell performance degradation. At a constant contamination level, the surface coverage of Pt\ue2\u20ac\u201cS also increases faster with time when increasing the cell current density from 0.1 to 1.0 A cm\ue2\u2c6\u20192, resulting in faster and more severe cell performance degradation. Simulation shows that the contaminant surface coverage at steady state is governed by current density. With the same contaminant concentration, to maintain a higher current density output, a significant decrease of steady-state contaminant coverage is required, while at a lower current density, steady-state contaminants coverage increases significantly.NRC publication: Ye

Organic molecular evidence of seafloor hydrocarbon seepage in sedimentary intervals down a core in the northern South China Sea

The hydrocarbons in sedimentary organic matter (OM) at Site 4B in the Pearl River Mouth Basin (PRMB), northern South China Sea (SCS) were analyzed. The odd/even predominance (OEP) of long chain n-alkanes (> n-C-24) with CPI (carbon preference index) values from 1.62 to 3.80 and n-C-29/n-C-31 being the two most abundant in most of the samples are strongly indicative of a terrestrial higher plant source, and the even/odd predominance (EOP) distribution of mid-chain n-alkanes (n-C16-22) suggests a marine bacterial input, probably chemical autotrophic bacteria. However, the biomarker distributions at the 65-70 cm, 80-85 cm and 85-90 cm intervals show a similar to those of crude oils, which were characterized by a dominance of an unresolved complex mixture (UCM) in the n-C20+ region, low CPI values ( 9) of total tricyclic terpanes/alpha beta-C-30 hopane. These features are typical characteristics of mature OM that has experienced catagenesis and metagenesis, suggesting that outside hydrocarbons migrated into these three intervals. In terms of petroleum systems developed in the PRMB, hydrocarbon seepage from deep source rocks via a diaper structure and fault system is proposed to be responsible for the occurrence of this mature OM

Long chain 1,14-diols as potential indicators for upper water stratification in the open South China Sea

Long chain 1,14-diols have been widely studied in typical upwelling areas, but less investigated in open seas. In this study the 1,14-diols in a sediment core, covering a history between 42 ka and 12 ka, were studied to investigate their potential to indicate upper water stratification in the open northern South China Sea (SCS). Downcore records of 1,14-diol fractional abundances, diol index 1 (DI-1) and diol index 2 (DI-2) changed in parallel, with a prominent increase at -14.5 ka, likely corresponding with the Bolling warming event. This pattern is different from what has been observed in typical upwelling regimes, showing an insignificant relationship between DI-1 and DI-2. Moreover, these records showed a generally inverse variation pattern with records of primary productivity, further suggesting that they are not likely applicable as upwelling indicators in the study area. However, the 1,14-diol records changed positively with sea surface temperature (SST), as well as the difference between SST and subsurface-temperature, hence suggesting their potential to indicate upper water column stratification in the open SCS. This occurrence might be associated with the productivity of 1,14-diol precursor (i.e., Proboscia diatom), which is sensitive to the variation of mixed layer depth in oligotrophic open oceans and lacks of competitiveness relative to the more heavily silicified diatoms under normal Si replete upwelling conditions. Our result also suggested that the East Asian monsoon may have played an important role to modulate riverine terrigenous organic matter input to the study area