Anthropogenic CO2 emissions are contributing to global warming through the greenhouse effect. If left unabated, this will cause significant climate change. Decarbonisation of the energy supply will reduce CO2 emissions significantly. Fuel cell/electrolyser systems, such as solid oxide cells (SOCs) can play a part, both in energy storage and in carbon utilisation. SOCs consist of fuel and oxygen electrodes, and a solid electrolyte. Impedance spectroscopy (IS) can be used to separate its electrical response into individual processes for characterisation. The resulting immittance data can be represented in the following formalisms: impedance, Z*, admittance, Y*, permittivity, ε* and electric modulus, M*. In this work, immittance spectra were recorded for a variety of samples: commercial SOCs (NextCell, FuelCellMaterialsTM), YSZ (yttria stabilised zirconia) single crystals and YSZ ceramics sintered at temperatures ranging between 1275 and 1450 oC. YSZ is a typical electrolyte material used in SOCs.
SOC immittance spectra for commercial SOCs, measured at 850 oC, were influenced at high frequency due to a large contribution from jig inductance. Data were corrected for this unwanted contribution by using an inductor Ls in parallel with a resistor R0 in the equivalent circuit. Ls was estimated from Kramers-Kronig transform calculations because corrections from closed-circuit experiments were unreliable. The resulting spectra plotted in the form of Z* plots contained two broad arcs, each thought to represent multiple processes. However, despite analysis using Analysis of Differences in Impedance Spectra (ADIS) and Distribution of Relaxation Times (DRT), the number of processes present remained unclear and it was not possible to attribute the features to any specific process(es).
IS results from 8mol% YSZ single crystals, measured between 150 and 325 oC, exhibited a bulk response at high to mid frequency and electrode effects at mid to low frequencies, due to charge transfer and diffusion. In ε' spectroscopic plots, the bulk response exhibited two features: a high frequency plateau at ε' ~ 30, and a lower frequency plateau (visible mainly as a point of inflection in ε') at ~90-100. At lower frequencies, the ε' spectra were dominated by the electrode response.
Bulk YSZ data from the single crystals were fitted using a combination of resistor(s) (R), capacitor(s) (C) and a constant phase element (Q). Four circuits were compared to investigate which one gave i) the best fit in all four formalisms, and ii) the most reasonable physical trends in parameters with temperature. Circuits A, B and C have been used previously in literature to describe bulk YSZ, whereas circuit D is newly proposed in this work. Circuit A consisted of an R and a Q element in parallel. Circuit B contained an additional parallel capacitor, giving an RQC element, which results in a high frequency plateau in ε'. Circuit C combines an RQC element in parallel with a series Rd-Cd element, in which Rd and Cd represent local dipole interactions. Circuit D is also based on an RQC element, with an additional capacitance Cq in series with Q, which results in a low frequency plateau in ε', equal to the sum of the two capacitors. All equivalent circuits tested gave accurate values of bulk (ionic) conductivity (σ =1/R), which obeyed an Arrhenius law with an activation of ~1.03 eV, but did not all give accurate values of relative permittivity ε_r, expected to be temperature independent with a value of ~ 30. Circuit A gave a poor fitting in all formalisms except Z*. Circuits B and C fitted reasonably well in all formalisms but gave inaccurate fits of low frequency data in ε'. Circuit D gave both the best fit and the most reasonable physical parameter trends; in particular, ε_r was stable with temperature at ~30. Cq decreased moderately with increasing temperature, from 72 (a.u.) at 175 oC, to 57 at 325 oC. This additional capacitance indicates there is a distribution of characteristic frequencies for the bulk response, which decreases with increasing temperature. The origin of this is proposed to be associated with localised heterogeneity in the bulk, due to micro-clusters of oxygen vacancies. This response may be more general in functional oxides where local structure plays an important role in the electrical properties and not specific only to YSZ. Further investigation requires work on single crystals, not ceramics; the presence of grain boundaries obscures the low frequency ε' bulk plateau as described in the next paragraph.
In the immittance spectra of YSZ ceramics, measured between 175 and 325 oC, there was a bulk response at high frequency and a grain boundary response at mid frequency. Data were fitted using a parallel RQC element and a parallel RQ element connected together in series, representing the bulk and grain boundary responses, respectively. There was no need to include Cq in the bulk element as the low frequency plateau was swamped by the grain boundary response. The grain boundary response was most prominent for samples sintered at the lowest temperature investigated (1275 oC), which were also the most porous samples (66%). The highest density achieved was 96% for ceramics sintered at 1450 oC. In contrast to the electro-active grain boundaries, porosity was not represented as an individual response but instead affected the magnitude of the whole spectrum. The bulk conductivity increased with the relative density of the ceramics and, at 300oC, gave values of 3.1×10-6 and 8.2×10-6 S/cm for densities of 66 and 96%, respectively. The relative permittivity almost doubled, from 16 to 29, as the density increased by ~ 30% towards full theoretical density.
CORE (COnnecting REpositories)
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