Key parameters of active layers affecting proton exchange membrane (PEM) fuel cell performance

The 2ⁿ full factorial design was applied to identify the key parameters of the active layer affecting the performance of a proton exchange membrane (PEM) fuel cell. Three main selected parameters were considered: carbon-type (Vulcan XC 72R and Black Pearls 2000 conducting furnace blacks, Cabot Corporation Boston, MA), Pt loading (0.1 and 0.5 mg/cm²), and Nafion™ sulfonic acid fluoropolymer (Du Pont de Nemours, Wilmington, DE) ionomer content (10% and 60%) for variables A, B, and C, respectively. The results from full factorial analysis indicated that the key factors affecting the exchange current density or activation loss were Pt loading whereas the key factors controlling the resistance due to ohmic loss were Nafion content and carbon type. In addition, there are the interactions between these parameters controlling the thin-film active layer performance, especially the interaction of carbon type and Nafion content. From cyclic voltammograms and cell performance testing, a Nafion content of 30% in a catalyst layer consisting of 0.5 mg/cm² Pt on Vulcan XC 72R is optimal.     

Dynamic modeling of direct ethanol fuel cells upon electrical load change

A transient, two‐dimensional, and two‐phase model was developed for predicting cell potential decay behavior of direct ethanol fuel cells operated galvanostatically after applying a step change in cell current density. To predict dynamic changes in anode overpotential and product distributions, a multi‐step reaction mechanism based on literatures involving many adsorbed intermediates (CH₃CH₂OH, CH₃CHO, CH₃CO, OH) was incorporated into this model. The kinetic rates of reactions involving electron transfers are described by the Butler‐Volmer equation. The surface coverage balance was used to determine the fractional coverage for each adsorbed species. The model also accounts for ethanol crossover through the membrane to cause cathode mixed potential. From the simulation results, a gradual increase of anode overpotential was caused by the acetyl bottleneck effect, leading to a slow decay of cell potential with time. Based on one set of model parameters, the model could accurately predict the dynamic response of cell voltage behavior after the cell current densities were stepped up as well as product selectivity.     

Prevalence and antimicrobial susceptibilities of Vibrio, salmonella, and Aeromonas isolates from various uncooked seafoods in Thailand

Uncooked seafood samples were collected from open markets and supermarkets in Bangkok, Thailand, and were examined for the presence of Vibrio, Salmonella, and Aeromonas species from January to February 2008. From 120 samples, 272 bacterial isolates were identified through biochemical testing. Of all sea bass, shrimp, oyster, and blood cockle samples (30 of each) that were processed for culture, 114 (95%) samples had at least one detectable isolate of Vibrio, Salmonella, or Aeromonas, leaving only 6 (5%) samples free of them. All oyster sample (100%) had at least one pathogen, followed by sea bass (97%), blood cockles (97%), and shrimp (90%). Overall, 111 (92%) of all samples had detectable Vibrio spp., 32 (27%) had detectable Aeromonas spp., and 25 (21%) had detectable Salmonella enterica. There was no overall difference between positive samples collected from fresh markets versus supermarkets (relative risk, 0.97; 95% CI, 0.89 to 1.05). Resistance to ampicillin among isolated pathogens was relatively high (56%), while resistance to 12 other antibiotics, including azithromycin, ciprofloxacin, and trimethoprim-sulfamethoxazole, was relatively low (0, 0, and 3%, respectively). Study results indicate that uncooked seafood in Bangkok, Thailand, commonly harbors enteric pathogens and that consumption of uncooked seafood should be avoided to reduce foodborne illnesses.     

The role of an anode microporous layer in direct ethanol fuel cells at different ethanol concentrations

Ethanol crossover and ethanol electrooxidation kinetic effects on direct ethanol fuel cell (DEFC) performance were determined at different ethanol feed concentrations for cells fabricated with and without an anode microporous layer (MPL). Several characterization techniques were used, including cell performance curves, anode polarization, electrochemical impedance spectroscopy (EIS) and ethanol crossover by the voltammetric method. It was found that the optimum ethanol feed concentration depended on the anode structure design and the cell current density operation. A microporous layer could reduce ethanol crossover but induced high mass transfer resistance, resulting in a slow ethanol electrooxidation reaction rate. However, ethanol crossover was not the dominant factor affecting DEFC performance for the ethanol feed concentration range (0.5–5.0 M) studied. The MEA without an anode MPL exhibited better performance than the one with an MPL for the entire range of ethanol concentration.     

Capillary zone electrophoresis of organic acids in beverages

Capillary zone electrophoresis (CZE) with photodiode array detection (PDA) was applied to simultaneously determine eleven organic acids in wine, beer, and fruit and vegetable juices after derivatisation with 2-nitrophenylhydrazine (2-NPH) in the presence of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl). The parameters affecting CZE separation included type and concentration of buffer, pH, organic additive and applied voltage. Optimum conditions at 25 °C were: 30 mmol l⁻¹ borate buffer, pH 10.0, containing 100 ml l⁻¹ acetonitrile, at 20 kV, sample injection at 0.5 psi for 5 s, with direct detection at 230 nm. Separation of eleven organic acids was achieved within 12 min. Linear calibration curves with good fit were obtained in the range 10.0-100.0 mg l⁻¹. Limits of detection ranged from 2.0 to 10.0 mg l⁻¹. Intra-day precision with RSD?4.0% for migration time and ?5.0% for peak area, and inter-day precision with RSD?6.0% and ?9.0% for migration time and peak area, were obtained. Recovery for all beverage samples was 97±15%.     

Investigation of membrane electrode assembly (MEA) hot-pressing parameters for proton exchange membrane fuel cell

The hot-pressing conditions for fabricating the membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC) was investigated by using a 2ⁿ full factorial design. Time, temperature and pressure were key parameters that were varied from 500 to 1500 psi, 1 to 5 min and 100 to 160 °C, respectively. The results from the full factorial analysis indicated that the order of significance of the main MEA fabricating effects was temperature, pressure, time–temperature interaction and pressure–time–temperature interaction. By examining the cell performance curves, the lower fabrication conditions of temperature and pressure were suitable for MEA preparation. The conductive layer between the membrane and the catalyst layer became thin at high pressure and high temperature, as seen from scanning electron microscopy (SEM) images. In the ranges of condition studied, the most suitable hot-pressing condition for MEA fabrication was at 100 °C, 1000 psi and 2 min. This condition provided the highest maximum power density from the MEA and the best contact at the interfaces between the gas diffusion layer, the active layer and the electrolyte membrane. The experimental results were verified by testing with a commercial MEA in the same operating condition and with the same equipment. The performance of the fabricated MEA was better than that of the commercial one.     

Effects of anode orientation and flow channel design on performance of refuelable zinc-air fuel cells

The effects of anode orientation (whether an anode is located above or under a cathode) and flow channel design (parallel or serpentine flow channel) on the performance of refuelable zinc-air fuel cells (RZAFC) continuously fed with KOH electrolyte were investigated. The performance test was conducted at different electrolyte flow rates of 2, 4, and 6 ml h^?1. A polarization test of the cell was conducted at the initial stage of operation, followed by a long-term current discharge test in potentiostatic mode. The spent zinc powders were characterized by a scanning electron microscope and X-ray diffraction. The experimental results revealed that the anode-bottom orientation in the cell performed much better than the anode-top orientation with 11.4 times higher zinc utilization. The performance reduction of the anode-top orientation cell was caused by the cathode overpotential, due to the flooding of the cathode by water crossover from the anode, which was induced by the gravity force. For the flow channel design effects, there was an optimum electrolyte flow rate, to yield a maximum current discharge capacity, of 4 ml h^?1 in this study. At this optimum flow rate, the total charge per gram of zinc delivered from the anode serpentine cell was 1.75 times higher than that from the anode-parallel one.     

Effect of pre-treatment approach of a carbon support on activity of PtSn/C electrocatalysts for direct ethanol fuel cells

Vulcan XC-72R carbon was pretreated using acid and thermal activation methods, and the carbons obtained were used as supports for a PtSn/C catalyst synthesized by a successive reduction process. Surface characteristics of the supports, including BET surface area, pH_PZC and functional group, were analyzed using physical N₂ adsorption, mass titration, acid–base titration, and Fourier transform infrared (FTIR) spectrometer technique, respectively. The prepared PtSn/C catalysts were characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDX), inductively coupled plasma–atomic emission spectrometry (ICP–AES), and transmission electron microscope (TEM) techniques, and then were examined for their behavior under ethanol oxidation as well as for their performance in a direct ethanol fuel cell (DEFC). The results showed that pretreatment by HNO₃ produced various oxygenated functional groups on the support surface and increased its acidic property. The strong acidity of the acid-treated support led to an unfavorable condition for the Pt reduction reaction and resulted in low Pt content but high Pt:Sn ratio in the PtSn/C catalyst. On the other hand, thermal activation increased the base functional groups on the carbon surface, which enhanced reduction of Pt precursor, and consequently, provided a small average metal particle size of 2.2 nm. The results from cyclic voltammetry, chronoamperometry and cell performance testing confirmed that the catalytic activity for ethanol oxidation and the performance in the direct ethanol fuel cell of the heat-treated carbon-supported PtSn catalyst was superior to the fresh PtSn/C catalyst and the acid-treated carbon-supported PtSn catalyst.