Search

A solid oxide short stack composed of 6 Ni-YSZ supported cells, YSZ electrolyte and GDC-LSC oxygen electrode has been tested for 10,700 hours in steam electrolysis. Initial degradation was followed by a global stabilization of the performance after lowering the current density, with a degradation rate below 0.5% kh(-1). Post-test analysis has been conducted on two repeating units (RUs) to highlight the most significant microstructure alterations. Nickel depletion was observed in the hydrogen electrode close to the interface with the electrolyte. Formation of small pores in the electrolyte was detected along the grain boundaries. A consequent detachment related to this phenomenon was observed in proximity of the GDC compatibility layer. At the oxygen electrode side, the formation of a approximate to 1 mu m dense mixed layer of GDC and YSZ was observed. Strontium from the LSC electrode migrated through GDC pores and reacted with YSZ, forming evident SrZrO3 inclusions. Distinct accumulation of silicon at the Ni/YSZ interface and chromium on the GDC barrier layer have been observed in both RUs. Despite this range of alterations observed, the stack degradation remained limited, testified from the fact that performance decay between 4,000 and 10,000 hours of operation was virtually nil.

The objective of the current work is to support the design of a pilot hydrogen and electricity producing plant that uses natural gas (or biomethane) as raw material, as a transition option towards a 100% renewable transportation system. The plant, with a solid oxide fuel cell (SOFC) as principal technology, is intended to be the main unit of an electric vehicle station. The refueling station has to work at different operation periods characterized by the hydrogen demand and the electricity needed for supply and self-consumption. The same set of heat exchangers has to satisfy the heating and cooling needs of the different operation periods. In order to optimize the operating variables of the pilot plant and to provide the best heat exchanger network, the applied methodology follows a systematic procedure for multi-objective, i.e. maximum efficiency and minimum number of heat exchanger matches, and multi-period optimization. The solving strategy combines process flow modeling in steady state, superstructure- based mathematical programming and the use of an evolutionary-based algorithm for optimization. The results show that the plant can reach a daily weighted efficiency exceeding 60%, up to 75% when considering heat utilization.

This work presents the poisoning effect of organic oxo-silicon compounds (siloxanes) which are generally found in sewage biogas. Lifetime and durability associated with solid oxide fuel cell (SOFC) technology is strongly related to the amount of contaminants that reach the stack. Several experiments on Ni anode-supported (AS) single cells are performed in order to clarify the mechanism of degradation and also the possibility of cell performance recovery. Three experiments focus on the degradation and recovery of AS Ni-YSZ fed with H-2, co-feeding 5 ppm D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as representative compound for the organic silicon species, at 800 degrees C. A fourth experiment focuses on the durability of the AS Ni-YSZ cell with variable concentrations of the impurity (0-5ppm), during steady state polarization (0.25Acm(-2)) for 250 h, using simulated biogas-reformate fuel H-2/CO/CO2/H2O: irreversible degradation was observed with the D4-impurity feed in the anode gas. Post-test scanning electron microscopy (SEM) results indicate formation of SiO2(s) deposits, which block pores and reduce the TPB length.

In this study, two short SOE stacks (6-cells) were characterized in steam-electrolysis mode. The first was based on Ni-YSZ supported cells with LSCF-based air electrodes. The second included LSC-based air electrodes specifically developed within the SOFC600 project. The stacks were fed with a 90% steam, 10% hydrogen mixture, and characterized between 600 and 700 degrees C. For the LSCF-based stack, an average cell voltage of 1.6V was reached at -1Acm(-2) and 700 degrees C, corresponding to 60% steam conversion. The LSC-based stack showed better performances, with 1.25V at -0.8Acm(-2) and 700 degrees C. Durability tests were performed around the thermoneutral voltage (1.35V) for resp. 2,400 (LSCF) and 1,500h (LSC). The average stack degradation rates were resp. +2.8% (kh)(-1) (LSCF) and +1.9% (kh)(-1) (LSC). Twelve load-cycles were also performed on the LSC-stack with no apparent degradation. In addition, co-electrolysis was performed between 750 and 850 degrees C by feeding the stack with a 60% H2O, 30% CO2, and 10% H-2 mixture. Ninety-five percent conversion was reached and the measured outlet syngas composition was close to that predicted by thermodynamics. Steam electrolysis tests were carried out in the same conditions for comparison. The stack performance in the co-electrolysis mode was slightly lower than in the electrolysis mode.

An alternative way to process data from polarization measurements for fuel cell model validation is proposed. The method is based on re- and subsampling of IV data, with which repetitive estimations are obtained for the model parameters. This way statistics such as standard deviations and correlations between the parameters may be experimentally derived. Histograms may also be produced, approximating the probability distributions that they follow. Two experimental case studies are discussed. In the first case, observations are made on the behavior of the parameter values for two mathematical models. As the number of data points (measurement points) employed in the estimation of the parameters increases, parameters with high variances converge to specific values. On the contrary, parameters with small variances diverge linearly. The parameters' histograms do not usually follow normal distributions rather they show a connection between the number of peaks in the graphs and correlations of the parameters. The second case study is an application on a fast degraded SOFC button cell, where the values and the histograms of the parameters are compared before and after degradation.