Application of a General Gas Electrode Model to Ni-YSZ Symmetric Cells: Humidity and Current Collector Effects

Abstract

Electrolyte-supported symmetric Ni-YSZ cermet electrodes of ca. 23 µm were prepared by screenprinting and the impedance was measured as a function of humidity from 2% to 90% balanced in H 2 at a total flow rate of 50 sccm. The Ni felt current collector of 1 mm thickness exhibited a Gerischer-like gas concentration impedance in the low frequency range, which was similarly observed in the cermet-supported solid oxide cells, while the Pt paste collector exhibited only electrochemical polarization. The electrochemical polarization of both samples was modeled by a non-ideal diffusion-reaction transmission line model including CPEs with α = 0.5. In the case of the Pt paste collector, all the Bisquert parameters exhibited humidity dependence to the -1/2 power, supporting a non-faradaic chemical reaction mechanism at three phase boundaries. Consequently, the surface diffusivity and reaction rate increased linearly with humidity. Less pronounced humidity dependence and somewhat lower utilization length with an Ni felt collector can be attributed to the diffusion-limited gas flow through the collector.

Lattice parameters and site occupancy factors of magnetite–maghemite core–shell nanoparticles. A critical study

“…Fit 1 refers to the whole population of NPs, the dashed (AE1) lines highlighting the significance of the small q value changes. Considering also the q changes corresponding to the NP fraction >10 nm (not shown here for the sake of clarity), the extrapolated 'maghemitic' cells for the samples here investigated range between 8.3479 (6) and 8.3449 (2) Å , respectively, well in agreement with the reference value from Shin (1988) [8.3457 (-) Å ]; analogously, the extrapolated 'magnetitic' cell of 8.394 (1) Å closely matches that from Bosi et al (2009) [8.3967 (3) Å ]. Our results seem to indicate a clear linear trend of the lattice parameter versus the sample oxidation degree within the range defined by the two mentioned reference cell values.…”

“…Such an effect acts against the lattice contraction driven by size-dependent oxidation phenomena and -to the best of our knowledge -has never been detected before on this kind of NP. We believe that this might have caused the wide spread of the lattice parameters reported in the literature, where, for -Fe 2 O 3 , values range from 8.334 (2) Å (Petkov et al, 2009) to 8.3457 (-) Å (Shin, 1988), and the much narrower 8.394 (4)-8.400 (-) Å interval (indicates that s.u. values are not available; Gorski & Scherer, 2010;Gotic et al, 2009) is attributed to Fe 3 O 4 NPs.…”

“…We demonstrated that, despite the very close similarity of magnetite and cubic maghemite in terms of crystal structures and lattice parameters [a Magnetite = 8.3967 (3) Å (Bosi et al, 2009) and a Maghemite = 8.3457 Å (s.u. not available; Shin, 1988)] and the peak broadening due to the very small crystal domains, the size-dependent lattice parameter and stoichiometry of core-shell magnetite-maghemite NPs can be simultaneously determined from their synchrotron X-ray diffraction patterns and show a very good linear mutual correlation. Moreover, the number-and mass-based average crystal domain size and the size distributions were also estimated.…”

Electrode kinetics of porous Ni-3YSZ cermet operated in fuel cell and electrolysis modes for solid oxide cell application

“…Concerning the reaction mechanism in the Ni-YSZ cermet, several experimental works have characterized the electrode behavior using polarization curves (η-i curves) and Electrochemical Impedance Spectroscopy (EIS) [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Traditionally, these studies were carried out using simplified geometries, such as Ni patterned electrodes, to control the microstructure.…”

“…Besides, most of the works presented in the literature are focused on the reaction for hydrogen oxidation in SOFC mode, whereas there is a lack of investigations in electrolysis conditions. For example, very few studies have been specifically devoted to the electrode response in cathodic polarization [24,25,27] and even less have operated the electrode at high steam partial pressure (pH 2 /pH 2 O < 1) [27].…”