Tunability of the M(II)M(III)/M(II)(2) and M(III)(2)/M(II)M(III) (M=Mn, Co) couples in bis-μ-O,O'-carboxylato-μ-OR bridged complexes

A comparison of the electrochemical properties of a series of dinuclear complexes [M(2)(L)(RCO(2))(2)](+) with M = Mn or Co, L = 2,6-bis(N,N-bis-(2-pyridylmethyl)-sulfonamido)-4-methylphenolato (bpsmp(-)) or 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato (bpbp(-)) and R = H, CH(3), CF(3) or 3,4-dimethoxybenzoate demonstrates: (i) The electron-withdrawing sulfonyl groups in the backbone of bpsmp(-) stabilize the [M(2)(bpsmp)(RCO(2))(2)](+) complexes in their M(II)(2) oxidation state compared to their [M(2)(bpbp)(RCO(2))(2)](+) analogues. Manganese complexes are stabilised by approximately 550 mV and cobalt complexes by 650 mV. (ii) The auxiliary bridging carboxylato ligands further attenuate the metal-based redox chemistry. Substitution of two acetato for two trifluoroacetato ligands shifts redox couples by 300-400 mV. Within the working potential window, reversible or quasi-reversible M(II)M(III)? M(II)(2) processes range from 0.31 to 1.41 V for the [Co(2)(L)(RCO(2))(2)](+/2+) complexes and from 0.54 to 1.41 V for the [Mn(2)(L)(RCO(2))(2)](+/2+) complexes versus Ag/AgCl for E(M(II)M(III)/M(II)(2)). The extreme limits are defined by the complexes [M(2)(bpbp)(CH(3)CO(2))(2)](+) and [M(2)(bpsmp)(CF(3)CO(2))(2)](+) for both metal ions. Thus, tuning the ligand field in these dinuclear complexes makes possible a range of around 0.9 V and 1.49 V for the one-electron E(M(II)M(III)/M(II)(2)) couple of the Mn and Co complexes, respectively. The second one-electron process, M(II)M(III)? M(III)(2) was also observed in some cases. The lowest potential recorded for the E°(M(III)(2)/M(II)M(III)) couple was 0.63 V for [Co(2)(bpbp)(CH(3)CO(2))(2)](2+) and the highest measurable potential was 2.23 V versus Ag/AgCl for [Co(2)(bpsmp)(CF(3)CO(2))(2)](2+).     

Oxygen reduction on lanthanum strontium manganites investigated by the use of cone shaped electrodes

The oxygen reduction kinetics has been investigated by impedance spectroscopy on two different cone shaped electrodes of La_0.85Sr_0.15MnO₃ in contact with either YSZ or CGO electrolyte pellets. The contact area has been varied by varying the mechanical load on the electrode on both polarized and unpolarized electrodes. The experiments showed the activation of the electrode observed after a cathodic polarization to be caused by the creation of an active zone on the electrolyte close to the triple phase boundary. As a result of this, the oxygen reduction on polarized electrodes probably takes place by several parallel routes making a kinetic investigation complicated. Impedance analysis of unpolarized electrodes at different oxygen partial pressures showed a charge transfer process at high frequencies involving oxygen defects and a mass transport limited reaction at lower frequencies. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Structural and dielectric properties of Er substituted sol-gel fabricated PbTiO3 thin films

Er substituted lead titanate thin films were processed via chemical solution deposition. Their structural properties were investigated by means of atomic force microscopy, X-ray diffractometry and micro-Raman spectroscopy. The Curie temperature, T_C, was obtained from dielectric measurements. The results show that the grain size, tetragonality factor, c/a, E(1TO) transverse optical phonon frequency and the ferroelectric-to-paraelectric phase transition are affected by Er doping. Particularly the lattice tetragonality and Curie temperature of Er substituted films are found to be substantially lower in comparison to undoped specimen. In contrast, the E(1TO) phonon frequency is only slightly reduced by Er doping. The Results obtained for pure and Er substituted specimens are discussed in terms of combined effects of grain size, stress development and doping on the structural characteristics. PACS 68.55.-a; 77.80.Bh; 78.30.Hv     

Microscopy studies on pronton exchange membrane fuel cell electrodes with different ionomer contents

Proton Exchange Membrane (PEM) fuel cell electrodes with different ionomer contents were studied with various microscopic techniques. The morphology and surface potential were examined by Atomic Force Microscopy (AFM) and Kelvin Probe Microscopy (KPM), respectively. The particulate nature of the electrode was well displayed in the topography and phase images. The particle and pore size (Z) distributions showed the most frequent values at 30–40 nm and 20–30 nm, respectively. The particle size corresponds to the size of the carbon support for the platinum catalyst. Catalyst agglomeration was observed in high ionomer content electrodes. The surface potential images showed distinct difference to the topography images. The overall grain size was seen to increase, the pore volume to decrease, the surface roughness to decrease, and the surface potential variation to increase with the increase of ionomer content in the catalyst layer. Transmission electron microscopy (TEM) was carried out on selective electrodes to provide additional information and confirmed with the AFM results. Cyclic voltammetry (CV) showed that the electrode containing 30 wt.% ionomer has maximum catalyst utilization.     

Oxygen Sorption and Desorption Properties of Selected Lanthanum Manganites and Lanthanum Ferrite Manganites

Temperature‐programmed desorption (TPD) with a carrier gas was used to study the oxygen sorption and desorption properties of oxidation catalysts and solid‐oxide fuel cell (SOFC) cathode materials (La_0.85Sr_0.15)_0.95MnO_3+δ (LSM) and La_0.60Sr_0.40Fe_0.80Mn_0.20O_3‐δ (LSFM). The powders were characterized by X‐ray diffractometry, atomic force microscopy (AFM), and BET surface adsorption. Sorbed oxygen could be distinguished from oxygen originating from stoichiometry changes. The results indicated that there is one main site for oxygen sorption/desorption. The amount of sorbed oxygen was monitored over time at different temperatures. Furthermore, through data analysis it was shown that the desorption peak associated with oxygen sorption is described well by second‐order desorption kinetics. This indicates that oxygen molecules dissociate upon adsorption and that the rate‐determining step for the desorption reaction is a recombination of monatomic oxygen. Typical problems with re‐adsorption in this kind of TPD setup were revealed to be insignificant by using simulations. Finally, different key parameters of sorption and desorption were determined, such as desorption activation energies, density of sorption sites, and adsorption and desorption reaction order.     

The Identification of the Sodium–Potassium Pump (Nobel Lecture)

Deemed too provocative , the word “pump” was omitted in the title of the 1957 publication “The Influence of Some Cations on an Adenosine Triphosphatase from Peripheral Nerves”, in which J. C. Skou showed that an ATPase in the membrane of crab nerves had such characteristics from the point of view of substrate ATP and effect of Na and K that it was reasonable to suggest that it was involved in the active transport of Na across the cell membrane.     

Electrochemical reduction of NO and O2 on oxide based electrodes

Cyclic voltammetry has been used to investigate the electrocatalytic properties of oxide based electrodes for the reduction of NO and oxygen. Perovskites of compositions La_xSr_1-xMnO_3+δ, (LSMx), with 0.05 ≤ x ≤ 0.5 and La_0.6Sr_0.4Fe_0.8Mn_0.2O_3-δ, (LSFM), the spinels Co₃O₄ and CuCr₂O₄ and the mixed oxide V₂O₅ have been selected and investigated in the temperature range 300 – 500 °C. The materials show good catalytic properties against both NO oxidation and NO reduction. The catalytic activity as well as the selectivity is dependent both on the redox behavior of the compound and on the oxygen defect concentration. Paper presented at 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Crosslinking and alkyl substitution in nano-structured grafted fluoropolymer for use as proton-exchange membranes in fuel cells

In order to develop cheaper and better fuel-cell electrolyte membranes than the polyperfluorosulfonic acids, an effort has been made to improve the fuel-cell relevant properties of sulfonated styrene/divinylbenzene-grafted poly(ethylene-alt-tetrafluoroethylene) membranes. Thus the influence of the crosslinking agent divinylbenzene has been investigated and its amount optimized. Substitution of styrene by methylstyrene and t-butylstyrene has been performed with the purpose of improving the chemical stability of the membranes. Grafting with a fraction of divinylbenzene in the order of 1–2 vol-% of the total monomers has been found to be the best compromise between high grafting yield, good chemical stability, and high proton conductivity of the final membrane. The use of methylstyrene and t-butylstyrene as grafting monomers instead of styrene results in substantially increased chemical stability, with reasonable proton conductivity still being possible to obtain.     

Die Identifizierung der Natrium-Kalium-Pumpe (Nobel-Vortrag)

Zu provokativ erschien J. C. Skou der Begriff „Pumpe”︁ für den Titel seiner Publikation „The Influence of Some Cations on an Adenosine Triphosphatase from Peripheral Nerves”︁, in der er 1957 beschrieb, daß eine ATPase in der Membran von Krabbennerven das Substrat ATP und den gerichteten Transport von Na- und K-Ionen in charakteristischer Weise beeinflußt. Diese Charakteristika ließen keinen anderen Schluß zu, als daß dieses Enzym, das heute ganz selbstverständlich als „Na⁺-K⁺-Pumpe” bezeichnet wird, am aktiven Transport von Na⁺ durch die Zellmembran beteiligt ist.     

Determination of The Oxygen Chemical Diffusion Coefficient in Perovskites by a Thermogravimetric Method

A thermogravimetric method has been used for the determination of the oxygen chemical diffusion coefficients in La_1–xSr_xMnO_3+δ; x=0; 0.05; 0.10; 0.15 (LSM). A temperature range of 700–1000°C was studied. The chemical diffusion coefficient varies between 1.6⋅10^–13 and 1.8⋅10^–10cm²s^–1 for the samples in the temperature range studied. The activation energy for oxygen chemical diffusion was determined to be 190–280 kJ mol^–1 for the LSM samples. The magnitude of the chemical diffusion coefficients of the LSM samples does not depend on the strontium site occupation factor. This revised version was published online in July 2006 with corrections to the Cover Date.