Experimental approach for the study of SOFC cathodes

The suitability of impedance measurements in Solid Oxide Fuel Cells (SOFCs) is an important concern, especially in case of measuring separately the behaviour of one of the electrode when an overvoltage is applied. In this case a thin electrolyte-supported cell with the RE (Reference Electrode) coplanar with the WE (Working Electrode) is experimentally convenient, but many authors highlighted that incorrect results can be obtained if an inappropriate geometric configuration is used. In this work LSM cathodes ((La0.8Sr0.2)MnO3-x) were investigated in a Yttria-stabilised Zirconia (YSZ) electrolyte-supported cell, using an electrolyte 3 mm thick. Two types of cells were prepared: the first (Cell1) according to the geometric requirements suggested in literature: little WE (diameter 3 mm) aligned to the CE (Counter Electrode) and with equal Rpol(polarisation resistance) and time constant; RE co-planar around the WE and placed at a distance greater than three-electrolyte thicknesses from the WE; the second one (Cell2) equal to Cell1 but with a bigger WE (diameter 8 mm). Impedance measurements were carried out both in two- and three- electrode configuration, at OCV (Open Circuit Voltage) and under applied overpotentials. A preliminary comparison between the results extracted from Cell2 at two- and three- electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV and small overpotentials. On the other side, when an overpotential over 0.2 V is applied, a comparison between Cell1 and Cell2 gives quite different results. The investigation here presented considers also an experimental approach useful for the comprehension of the main phenomena governing the kinetic of the process

Utilisation of methylcellulose as a shaping agent in the fabrication of Ba_{0.95}Ca_{0.05}Ce_{0.9}Y_{0.1}O_3 proton-conducting ceramic membranes via the gelcasting method

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Inductance correction in impedance study of solid oxide fuel cells

A procedure for evaluation and elimination of errors, caused by parasitic inductance and resistance in EIS studies of two solid oxide fuel cells (SOFC) materials: yttria stabilized zirconia (YSZ) electrolyte and lanthanum strontium manganite (LSM)/YSZ composite cathode is presented in this paper. It is shown that for these low impedance systems the parasitic inductance can affect not only the high frequencies but also the middle and low ones. The parasitic errors correction procedure increases significantly the reliability of the electrochemical impedance spectroscopy (EIS) results

Electrochemical performance of Ni-based anodes for solid oxide fuel cells

The catalytic activity of Ni-based anodic materials was investigated in complete solid oxide fuel cells (SOFCs) by electrochemical analysis. Button cells, consisting of supporting yttria-stabilized zirconia (YSZ) electrolyte layer, (La1-xSrx)y MnO3 (LSM) cathode and (cermet) Ni0.5Co0.5\u2013YSZ anode were employed. Powders for anodes were obtained by wet impregnation. This pro- cedure allowed easy production of composite electrodes with homogeneous distribution of phases and controlled microstructure. Two electrodes impedance spectroscopy was carried out at different temperatures and partial pres- sures of reacting gases in order to evaluate contribution of each component to overall cell losses. Current\u2013voltage characteristic curves were also collected. Feeding with CH4 was tested and compared to H2. No deterioration of cell performance due to carbon formation at anode was observed over a test period of 100 h

Experiments on metal-Glass-metal samples simulating the fuel inlet/outlet manifolds in SOFC stacks

The investigations performed on state-of-the-art SOFC stacks operated at various electrical load for several thousand hours have underlined the importance to better understand how the sealant materials evolve during the operation period. The opportunity to operate a stack and to have access to post-experiment samples is quite unique and opened the possibility to design and operate in a suitable rig samples replicating the metalglass-metal of a stack manifold. Samples prepared with the same materials and manufacturing method as for stacks have been aged at operating conditions of the fuel inlet and outlet for 500h under a polarization of 0.8V and a temperature of 700\ub0C in dual atmosphere (i.e. air, fuel). The evolution of the glass properties has been followed in by Electrochemical Impedance Spectroscopy (EIS) with measurements performed at Open Circuit Voltage (OCV) and under polarization. EIS measurements allowed to monitor the behaviour of the investigated system during the ageing process. The bulk resistance of the glass was measured and related to the evolution of the microstructural features investigated by post experiment characterization on the cross-sections. The combination of different fuel stream composition and temperature resulted in a quite stronger evolution of the glass at the outlet

Groundwater pollution: an energy consumption challenge for electrochemical advanced oxidation processes

Groundwater represents one of the primary freshwater sources, but its pollution is rapidly increasing, and the development of tailored processes is mandatory. In this scenario, electrochemical advanced oxidation processes (EAOPs) represent a promising alternative to face this issue. Nevertheless, their application to groundwater is limited by low electrical conductivity, directly affecting cell voltage and energy consumption. Three different approaches to overcome this constrain are compared using a BDD anode for the oxidation of Mecoprop herbicide. The first is the addition of a supporting electrolyte as sodium sulfate (Na2SO4); despite its efficacy to reduce energy consumptions, some new problems are introduced in water source management and treatment plant for the electrolyte removal. An alternative approach is the reduction of gap-electrode from 5 mm up to some microns, equivalent to the thickness of a plastic mesh, used to avoid electrodes short-circuit. The third innovative method proposed here is the substitution of insulated mesh with an ionic conductor layer, a solid polymer electrolyte (SPE), like Nafion\uae117 sandwiched between the electrodes. This modification allows increasing electrochemical performance such as herbicide and COD removal, preserving energy consumptions. Indeed, the new cell setup shows a lower cell voltage (-31%) with a beneficial impact on energy consumption. An improvement in COD degradation is reached (from 70% with Na2SO4 addition over 90% with SPE). Moreover, SPE does not show any performance deterioration in the first 30 hours of utilisation. The results suggest that the usage of SPE can be a promising approach for the treatment of groundwater. In the second part of the talk, we will provide an energy consumption and removal efficiency analysis to compare the SPE-based system with the conventional groundwater treatment process. The aim is to highlight the strengths and weaknesses of the new proposed cell configuration and possible future groundwater treatments improvements

On the stabilization and extension of the distribution of relaxation times analysis

The Distribution of Relaxation Times (DRT) is a useful technique to provide an improved insight into the interpretation of Electrochemical Impedance Spectroscopy (EIS) data. Despite its capability to distinguish the characteristic frequencies of the processes involved in an electrochemical system, the DRT solution is an ill-posed problem that requires a regularization method. Tikhonov regularization is one of the most commonly adopted methods for DRT analysis. Nevertheless, the domain discretization, needed to solve the original continuous problem, introduces errors in DRT evaluation. This problem is quite relevant when the impedance is not sufficiently resolved at boundary frequency range (imaginary part of impedance does not approach 0), affecting the edges of the DRT spectrum and the internal part. Here, the zero-padding technique is applied to overcome this issue. The results show how this approach, frequently used in the signal theory but not in DRT, improves DRT evaluation quality with Tikhonov regularization. It consists of an extension of the experimental frequency domain, reducing the errors due to the frequency truncation. The extended algorithm, named ED-DRT, is compared with the conventional Tikhonov method on both simulated and real impedance data. The new approach increases the quality of DRT results, providing higher stability and accuracy in evaluating characteristic frequencies and global polarization resistance even with noisy EIS data

Characterisation of La0.6Sr0.4Co0.2Fe0.8O3-\u3b4 \u2013 Ba0.5Sr0.5Co0.8Fe0.2O3-\u3b4 composite as cathode for solid oxide fuel cells

A novel composite electrode, obtained by combining two high performing perovskite materials, such as La0.6Sr0.4Co0.2Fe0.8O3-\u3b4 and Ba0.5Sr0.5Co0.8Fe0.2O3-\u3b4, was investigated as promising cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The two perovskites possess high catalytic activity for the oxygen reduction (ORR), although some problems related to their chemical and structural stability have still to be overcome in view of improving durability of the cell performance. The purpose of this study was a stable and high-performing composite material, by mixing the powders through a very simple procedure. In principle, equilibrium at the LSCF-BSCF interface could be reached due to ions interdiffusion during the sintering treatment (1100 \ub0C), resulting in the chemical stabilization of the material. The composite-cathode/Ce0.8Sm0.2O2-\u3b4-electrolyte system was investigated by Electrochemical Impedance Spectroscopy (EIS) as a function of temperature, overpotential and time, exhibiting rather low activation energy for the oxygen reduction reaction ( 3c 96 kJ mol 121). XRD analysis was performed to detect any structural modification during thermal or operation stages and it was found that after the sintering the two starting perovskites were no longer present, while two new phases were identified: a Co-rich La0.5Ba0.5CoO3 cubic and a Sr-rich La0.4Sr0.6FeO3 rhombohedral phase. The polarization resistance at 700 \ub0C (0.07 \u3a9 cm2) appeared higher than for pure BSCF (0.036 \u3a9 cm2), but reasonable considering the improvement in durability achieved under the considered operating conditions (200 mA cm 122, 700 \ub0C): 5% degradation over 200 h for the composite, against 38% for pure BSCF over the same aging time and a 29% for pure LSCF over 72 h. The obtained results motivate further investigations to be performed on different compositions, in order to evaluate whether the LSCF to BSCF ratio has an influence on the structural stability during SOFC operating conditions