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The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.

During early summer, the density of microplanktonic ciliates in the euphotic zone on Georges Bank (Northwest Atlantic) ranged from 600 to 13 000 cells 1 ' ; in the slope waters to the southeast of the bank, densities ranged from 1900 to 2800 cells 1 ' . Myrionecta rubra, a photosynthetic autotroph ciliate with a reduced algal endosymbiont, numerically comprised an average of 30 % of the microplanktonic ciliate fauna at stations in < 100m depth, but 3 % or less of the ciliate fauna at the deeper stations. Oligotrichous ciliates with chloroplasts are estimated to have contributed 34 O/ O of the ciliate fauna and were abundant at both shallow, unstratified stations on the bank and at deeper, stratified stations on the slope of the continental shelf and in the Gulf of Maine. Overall, about 50 O/O of the ciliates in the euphotic zone contained chlorophyll. At the shallow water stations, 2 species, M. rubra and Laboea strobila (a mixotrophic oligotrich) accounted for over 50 % of the biomass of ciliates with chlorophyll. At an irradiance of 100 pE m % l , M. rubra and L. strobila had photosynthetic rates of 85 and 465pg C fixed c e l l l h l , respectively. During summer, when phytoplankton biomass is low, autotrophic and mixotrophic ciliates may make an important contribution to photosynthesis in the larger size fractions and be an important source of food for larger organisms that rely on high quality, 2 15 to 20 pm food particles.

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The nature of platinum dissolution and precipitation in a polymer electrolyte membrane of a membrane electrode assembly ͑MEA͒ for a proton-exchange membrane fuel cell ͑PEMFC͒ was studied using a potential holding experiment at 1.0 V vs a reversible hydrogen electrode and high-resolution transmission electron microscopy. The electrochemically active surface area decreased depending on the holding time, and platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. However, platinum dissolution and deposition out of the catalyst layer were greatly reduced when a platinum black electrode was used. In the experiment using a double-layered catalyst layer, platinum redeposited not on the carbon black surface but rather on the platinum black surface.As promising candidate power sources for electrically powered vehicles, small-scale stationary power generators, and portable electronic devices, proton-exchange membrane fuel cells ͑PEMFCs͒, which electrochemically convert the chemical energy of a fuel directly into electrical energy with high conversion efficiency, have received considerable attention. 1-5 For the commercial success of PEMFC power sources, reliability and life are the most important considerations. However, there is as yet limited information available about failure modes for PEMFCs, and the causes and mechanisms of degradation are not fully understood. 6 A better understanding of the phenomena in a membrane electrode assembly ͑MEA͒ is needed.Carbon-black-supported platinum is used as an electrocatalyst for both electrode reactions in PEMFCs. The kinetics of cathode deterioration by the degradation of cathode platinum is a particularly important problem. Platinum sintering in the catalyst layer during PEMFC operation has been reported to be a factor in the degradation of a cathode. 7-10 Two mechanisms have been proposed to explain the surface area of carbon-supported platinum by a sintering phenomena in aqueous electrolyte: 11,12 dissolution/reprecipitation 13-15 and the migration ͑surface diffusion͒ of platinum particles. 16,17 Platinum dissolution has been studied mainly in sulfuric and phosphoric acid solutions. [18][19][20][21][22][23] The dissolution of platinum is considered to play an important role in the decrease in the surface area of platinum in the case of PEMFCs. 22-24 Some dissolved platinum ion species such as Pt 2+ redeposit on other platinum particles, resulting in platinum particle growth ͑electro-chemical Ostwald ripening͒, 12,25 and other dissolved platinum ion species diffuse out of the electrode and into the electrolyte membrane.In this study, we carried out potential holding experiments and analyzed the conditions in a MEA to investigate the phenomena in which platinum dissolves and diffuses into a polymer electrolyte membrane from the cathode catalyst layer. We applied highresolution transmission electron microscopy ͑TEM͒, which is the most effective technique for investigating the distribution of nanosize platinum particles in MEA, because e...

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SignificanceSingle-celled microorganisms are important in ecosystems, and their behaviors impact the Earth’s environments. To survive in harsh environments, these organisms frequently act as though exercising discretion. How do they achieve such intelligent behaviors? In this work, we focused on the accumulation of ciliates on solid/fluid interfaces, where they can obtain sufficient nutrients and a stable environment. This phenomenon is not described in the standard hydrodynamics of microswimmers. Our experiment and simulation revealed that simple principles, the anisotropic shape of the cell and the mechanosensing nature of cilia, induce the accumulation of ciliates on solid/fluid interfaces. The contribution of our work is that a simple response of the cellular apparatus and fluid dynamics explain the apparently clever behavior of ciliates.