Gold, as the noblest metal, is an appropriate model electrode to study electrochemical double layers. This study examines the frequency dispersion of the double layer of polished gold electrodes in perchloric acid with impedance spectroscopy under amplitude, electrolyte concentration, and potential variation. The dynamic perturbation of the double layer equilibrium by impedance measurements shows a constant phase (CP) response (phase angle of approximately −75°), which is responsible for a frequency dispersion of the capacitance. The response is almost independent of the excitation amplitude, for which the CP behavior is ascribed to resistive−capacitive (RC) contributions of the electric field-driven ion separation instead of the previously reported diffusionlimited adsorption processes. The RC character of the double layer in combination with the electrolyte resistance is accompanied by a relaxation that can damp the ion movement and the related ion separation. At the relaxation frequency, the capacitance is found to be independent of the electrolyte concentration, which is attributed to a constant ratio of the contributions of damped ion movement and dielectric polarization of water molecules.
Several porous backbone layers of proton conducting BaZr 1-x-y Ce x Y y O 3-δ (BZCY) were prepared by spray-coating or painting on BaZr 0.7 Ce 0.2 Y 0.1 O 2.95 (BZCY72) button cells followed by high temperature firing in air. This step enabled to produce ceramic backbones with various grain and pore sizes. The backbones were further infiltrated with mixed conducting double perovskite Ba 0.5 Gd 0.8 La 0.7 Co 2 O 6-δ (BGLC587) suspensions and annealed in air. The resulting composite layers were tested as oxygen/ steam side electrodes for proton ceramic fuel cells and electrolysers (PCFCs-PCEs). The results confirm that a high performing electrode material such as BGLC587 with partial proton conductivity and high thermal expansion can be applied on a proton conducting electrolyte with low thermal expansion by use of backbone infiltration without losing electrochemical functionality. Indeed, the electrodes display an apparent polarization resistance of only 0.03 cm 2 at 700 • C in oxygen humidified with 2.7% H 2 O. We further extracted and parameterized the impedances associated with the charge and mass transfer reactions in a system where protons are the dominating charge carriers at intermediate to low temperatures and oxide ions dominate the overall transport at high temperature. The fitting revealed to what extent the charge and mass transfer reactions are short-circuited by electronic leak current across the sample at high temperatures and pO 2 's. The acquired activation energies and pre-exponential values were used to explain materials-specific and micro-structural differences between the different electrode architectures.
In this report, we describe fabrication and electrochemical‐performance testing of tubular, anode‐supported fuel cells based on the protonic ceramic BaCe0.2Zr0.7Y0.1O3–δ (BCZY27). These devices are comprised of a 20‐μm‐thick BCZY27 electrolyte spray‐coated and co‐fired onto an extruded, tubular 9.8‐mm‐diameter, 1.25‐mm‐thick 65 wt.% NiO/35 wt.% BCZY27 anode support. Reactive sintering with NiO forms the BCZY27 material from parent oxides. An La0.6Sr0.4 Co0.2Fe0.8O3–δ (LSCF) cathode is applied following co‐sintering. While anode supports can be extruded to 3‐m lengths, the active area of the cells tested here is 7.5 cm2. Performance is quantified through polarization measurements across a range of temperatures with hydrogen‐air reactants. Peak power ranges from 78 to 189 mW cm–2 over the 700–850 °C temperature range. Open‐circuit voltage decreases with increasing operating temperature due to the co‐diffusion of the multiple charge carriers present. The ionic transference number is determined over a range operating temperatures and anode‐gas compositions, and is found to range from 0.77 to 0.88. Finally, an external power supply is used to drive hydrogen across the BCZY27 membrane. At an applied current density of 1 A cm–2 and 700 °C operating temperature, hydrogen flux is measured at 7.5 smL min–1 cm–2 active area.
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