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The effect of the preferential orientation of supporting bacterial cellulose (BC) nanofibrils on the conductivity of composite proton conducting electrolytes with poly(4-styrene sulfonic acid) (PSSA) is reported. Data obtained by impedance spectroscopy show that the in-plane conductivity at 40% relative humidity (RH) is more than half order of magnitude higher than that measured through-plane, indicating significant discontinuity of proton transport at the PSSA/BC interface. The difference becomes less than 20% in nearly saturated conditions (98% RH), demonstrating the key role of water in ensuring proton transport through those interfaces. The negative impact of the conductivity anisotropy in fuel cell performance is mitigated due to operation in wet conditions and fuel cell tests of PSSA/BC-based membrane electrode assemblies under humidified hydrogen/air gradients at room temperature yield 40 mW cm-2 at 125 mA cm-2, which is amongst the highest values reported for a biopolymer-based electrolyte. It also results from the presented investigation that conventional electrode preparation used for thermoplastic polymer electrolytes must be modified in order to ensure proper adhesion to BC-based MEAs and thus to lower polarization losses.

The synthesis and characterization of nanocrystals of the zeolitic imidazolate framework-8 (ZIF8) from zinc nitrate and 2-methylimidazole (Hmim) by a colloidal chemistry route is described in the search for novel protonic conductors. The ZIF8 powders have the sodalite cubic structure with a unit cell parameter slightly in excess of 1.7 nm, a crystallite size between 30 and 50 nm, and specific surface area between 800 and 2251 m(2)g(-1), associated to high levels of microporosity and some contribution of mesopores. The impedance spectra of ZIF8 powder compacts collected under 98% relative humidity (RH) depict at least 3 contributions, which could be ascribed to the bulk resistance at high frequency, and to external interfacial phenomena at intermediate and low frequencies. The conductivity data extracted from these data display strong humidity dependence, increasing by more than 4 orders of magnitude between 20 and 98% RH at 94 degrees C, ascribed to proton transport along adsorbed water molecules. This is also supported by the enhancement of 1 order of magnitude of the conductivity of samples with highest surface area, attaining a maximum of 4.6 x 10(-4) Scm(-1) at 94 degrees C and 98% RH. The value is low, but in agreement with the reported low water uptake capacity of these materials. Surprisingly, the activation energy for conduction is as high as 110 kJmol(-1), which may denote a particular influence of the structure on proton diffusion. While the transport properties are still short for any technological application, these first results demonstrate the potential of the ZIF8 structure to combine exceptional thermal and hydrolytic stability with the potential to easily introduce high levels of proton transport by suitable functionalization. (C) 2014 Elsevier Ltd. All rights reserved.

Ceria-based composite electrolytes (50 and 70 vol%) including one mixture of Li and Na carbonates (1:1 molar ratio) were prepared using either pure CeO2 (modest conductor) or Gd-doped ceria (excellent oxide-ion conductor) as ceramic matrix. The composite electrolytes and their phase constituents were tested under pure CO2, air, and H-2 diluted in N-2, at temperatures ranging from 300 to 580 degrees C, to study their electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural characterization. Low temperature (below melting) impedance shows in a clear manner the relevance of the matrix functionality on the performance of these composites, but also reveals additional microstructural and even unusual composite effects. These data were used to test equivalent circuit models derived from the composite constituent properties, and to assess their potential to design composites for target functionalities at higher temperature. (C) 2013 Elsevier Ltd. All rights reserved.

Composite electrodes based on several ceramic oxides were tested for their chemical compatibility and electrochemical performance in systems based on composite Gd-doped ceria + alkaline carbonate electrolytes. Reactivity tests performed at 700 degrees C (accelerated testing conditions), involving the ceramic electrode materials and one mixture of Na and Li carbonates, identified two promising perovskyte-type ceramics, LaCoO3 and La0.8Sr0.2Co0.2Fe0.8O3, as the most stable. Co-pressed and co-sintered cell assemblies (electrode/electrolyte/electrode) involving composite electrodes (perovskytes mixed with alkaline carbonates)were studied in detail by scanning electron microscopy and tested by impedance spectroscopy in air in the temperature range 300-600 degrees C. These tests showed the potential of these composite electrodes for intermediate temperature applications, clearly surpassing in performance standard Au electrodes, here used as reference. (c) 2012 Elsevier Ltd. All rights reserved.

Solid oxide fuel cell cermet anodes with proton-conducting ceramic phases, Ni-SrZr0.95Y0.05O2.975 (Ni-SZY), Ni-CaZr0.95Y0.05O2.975 (Ni-CZY) and Ni-SrCe0.475Zr0.475Y0.05O2.975 (Ni-SCZY), have been analysed by electrochemical impedance spectroscopy. The anodes were sintered on opposing faces of yttria-stabilised zirconia (YSZ) electrolyte and the polarisation behaviour studied in the temperature range 600–900 °C in various regimes of H2 and H2O partial pressures. The ceramic component of the Ni-CZY and Ni-SCZY cermets form an insulating phase at the interface with YSZ. Impedance spectra are composed of two dominant rate-limiting contributions attributable to electrode processes with relaxation frequencies ca. 103 and 1 Hz at 800 °C. Both high- and low-frequency responses are sensitive to H2O partial pressure and temperature, with activation energies in the range 1.02–1.25 and 1.19–1.35 eV, respectively. Factors influencing the origin of the rate-limiting processes are discussed, including transport limitations (oxide-ion and electronic) in the solid phases and microstructure. Proton conductivity may assist in accelerating the kinetics of the anodic reaction by widening the effective reaction area in electrodes optimised in terms of Ni content, oxide-ion conductivity and microstructure.

Using the galvanic cell: Pt | Au , CO 2 , O 2 | a Na ‴ Na 2 CO 3 ( Au ) | a Na ″ NASICON | a Na ′ FeO + NiO ( borate glass ) | FeNi (48)| Au | Pt the p-electronic conduction parameter a ⊕ of NASICON was characterized as a function of the sodium activity a Na ″ and the temperature. For that purpose, the isothermal voltage response to successive variations of a Na ″ by changing the composition of the CO 2 –O 2 gas mixture was evaluated by a non-linear regression procedure. Within the temperature range under investigation (300–620 °C) it holds that: log a ⊕ =11.076±0.1− (18278.0±1100) K T (T≤500 ° C ) log a ⊕ =−3.519±1.7− (7904±1400) K T (500 ° C ≤T≤620 ° C ) Thus, in a CO 2 sensor with NASICON as solid electrolyte the sodium activity prevailing at the measuring electrode is outside the ionic domain of the electrolyte. This is in apparent contradiction to most of the literature reports on the behavior of such gas sensors, but confirms previous findings on the extent of the electronic conductivity in Na–beta-alumina under comparable conditions.

Non-negligible electrode polarisation in faradaic efficiency and emf electrochemical cells used for transport number determination results in apparent ion transference numbers which are lower than the true values. However, appropriate modifications of these techniques combined with use of electrodes having a high polarisation resistance, enable a precise determination of even minor electronic contributions to the total conductivity of solid electrolytes. Experimental modification of the faradaic efficiency method, taking into account the electrode polarisation resistance, is proposed and verified using Ce0.80Gd0.18Pr0.02O2−δ ceramics. Substitution of 2% gadolinium cations in the lattice of Ce0.80Gd0.20O2−δ solid electrolyte with praseodymium was found to increase the p-type electronic conductivity by 2.5–4 times, while the activation energy for the electron–hole transport decreases from 145 to 125 kJ mol−1. The oxygen ion transference numbers of Ce0.80Gd0.18Pr0.02O2−δ in air vary in the range 0.996–0.970 at 873–1223 K, decreasing with increasing temperature. No significant effect of co-doping with praseodymium on the ionic conductivity, crystal lattice and thermal expansion of ceria solid electrolyte was found.

The present work reports estimates for the hole conductivity of lanthanum gallate-based samples (La 0.9 Sr 0.1 Ga 1− x Fe x O 3−δ , with x =0 and x =0.2), in the range 700–1000°C. These estimates were based on electrochemical permeability measurements, and were compared to previous estimates obtained from ion-blocking measurements. The p-type conductivity of these materials was confirmed to be strongly influenced by the content of Fe. The activation energy for p-type conductivity was found to be in the range 0.9–1 eV, lower than usually observed for alternative electrolytes based in zirconia or ceria. The increase in p-type conductivity with Fe content can be explained by a relatively easy formation of Fe 4+ ions under oxidizing conditions, a basic requirement for the establishment of a small polaron-type conduction mechanism.