Biocatalysts Used to Enable Hydrogen Fuel Cell with No Proton Exchange Membrane
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Harlingen at the University of Illinois at Urbana-Champaign have confirmed a complex order parameter in ruthenate superconductors. “We have pretty unambiguous evidence for ‘p-wave’ symmetry with a complex order parameter that breaks timereversal symmetry in this ruthenate superconductor,” said Van Harlingen, a Willett Professor and head of the Department of Physics at Illinois. Until now, this complex, odd symmetry state had been predicted by theoreticians, but not fully confirmed. Van Harlingen, Y. Maeno of Kyoto University, and their colleagues reported their latest findings in the November 24, 2006, issue of Science (p. 1267). The order parameter of a superconductor characterizes the nature of the pairing interaction that forms Cooper pairs. It controls many of the superconductor’s properties and provides a crucial clue to the microscopic mechanism responsible for the superconductivity. Conventional superconductors that form Cooper pairs through phonon interactions have an “s-wave” symmetry with an isotropic order parameter. Unconventional superconductors, however, have anisotropy in either or both the phase and magnitude of the order parameter. Ten years ago, Van Harlingen’s group pioneered the Josephson interferometer technique that showed the high-temperature superconducting cuprates had “d-wave” symmetry. They are now applying the technique to a wide range of superconducting materials suspected of having unconventional symmetry. “Our technique can directly measure phase differences in the superconducting order parameter,” said Van Harlingen, who is also a member of the Frederick Seitz Materials Research Laboratory and a professor in the university’s Center for Advanced Study. “This allows us to make an unambiguous determination of the pairing symmetry in unconventional superconductors,” he said. To use their interferometer technique, the researchers begin by constructing a corner Josephson junction that straddles different faces of a single crystal of the ruthenate superconductor. They then measure the magnetic-field modulation of the supercurrent that reveals the phase shift between different tunneling directions. If all areas of a Josephson junction have the same order parameter phase, the critical current—measured as a function of applied magnetic field—will create a Fraunhofer diffraction pattern, analogous to a single-slit optical diffraction pattern. However, phase differences in the order parameter on adja92
O2. Highly selective catalysts would make a PEM unnecessary and power could even be generated from very low levels of H2 in air. Recently, F.A. Armstrong, B. Friedrich, and co-researchers at the Inorganic Chemistry Laboratory, University of Oxford, United Kingdom, and the Institute of Biology/Microbiology, Humboldt University of Berlin, Germany, used catalysts found in nature to fabricate without a PEM a fuel cell that generates electricity from just 3% H2 in air, which is below the combustion limit. As reported recently in Chemical Communications (p. 5033; DOI: 10.1039/ b614272a), Armstrong, Fri
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