Sulfur impurities in carbon containing fuels for solid oxide fuel cells (SOFC), e.g. natural gas and biogas, typically lead to significant losses in performance due to the sulfur sensitivity of Ni/yttria-stabilized-zirconia (YSZ) anodes for SOFC. Full cells having Ni/YSZ anodes have been characterized during long-term galvanostatic operation in internal reforming gas mixture (CH 4 /H 2 O/H 2 :30/60/10), with 2 ppm H 2 S exposure to the anode for 500 hours at 850 • C, at different current densities. This work focus on the long-term effect of H 2 S exposure over a few hundreds of hours; and describes and correlates the observed evolution of anode performance, over hundreds of hours, with sulfur exposure at low cell overpotential (low current density) and at high overpotential (high current density) with and without H 2 S exposure. For tests at low overpotential with H 2 S exposure only a reversible loss in performance was observed and post-mortem SEM analysis showed an intact Ni/YSZ anode microstructure. For tests at high cell overpotential the H 2 S exposure caused both a reversible loss in performance and an irreversible long-term degradation. Post-mortem SEM analysis of the Ni/YSZ anode from this tests showed increased porosity and lack of percolating Ni in the few microns of the anode closest to the anode/electrolyte interface. Fuel flexibility is an attractive characteristic of solid oxide fuel cells (SOFC). They can for instance be fuelled with carbon containing fuels such as natural gas or fuels derived from biomass. A well-known drawback for the use of carbon containing fuels is the fact that they contain varying amounts of sulfur, which poisons Ni-based SOFC anodes.1-8 Therefore, it is important to investigate the SOFC tolerance toward sulfur poisoning and how it is affected by operating parameters.There are many studies concerning the initial, and mainly reversible, loss in performance occurring at SOFC anodes upon sulfur poisoning as a function of different operating conditions. 4,7-9 Fewer results are reported on the long-term, and often irreversible, loss of performance due to sulfur poisoning, 5,8,10 even though this will inevitably be of high importance for SOFC as an emerging energy conversion technology. In order to establish the acceptable limits for sulfur impurities in carbon containing fuels in order to avoid longterm, irreversible degradation, 5,11 it is necessary to understand how the sulfur impurity tolerance correlate to operating parameters such as gas mixtures, temperature and current density, and indirectly to the cell overpotential.It has been suggested that Ni-zirconia based SOFC anodes containing Sc are more tolerant toward H 2 S poisoning than Ni/YSZ based anodes 5,7 and in this work both short and long term tests were performed to investigate this hypothesis. Tests were mainly carried out at the same set of test conditions (e.g. temperature, current density, fuel) in order to compare Sc-Y co-doped (ScYSZ) and Y doped zirconia (YSZ) Ni cermet SOFC anodes. There are several studies s...

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Modified strontium titanates have received much attention recently for their potential as anode material in solid oxide fuel cells (SOFC). Their inherent redox stability and superior tolerance to sulphur poisoning and coking as compared to Ni based cermet anodes could improve durability of SOFC systems dramatically.Various substitution strategies can be deployed to optimise materials properties in these strontium titanates, such as electronic conductivity, electrocatalytic activity, chemical stability and sinterability, and thus mechanical strength. Substitution strategies not only cover choice and amount of substituent, but also perovskite defect chemistry, distinguishing between A-site deficiency (A 1Àx BO 3 ) and cationstoichiometry (ABO 3+d ). Literature suggests distinct differences in the materials properties between the latter two compositional approaches. After discussing the defect chemistry of modified strontium titanates, this paper reviews three different A-site deficient donor (La, Y, Nb) substituted strontium titanates for their electrical behaviour and fuel cell performance. Promising performances in both electrolyte as well as anode supported cell designs have been obtained, when using hydrogen as fuel.Performances are retained after numerous redox cycles. Long term stability in sulphur and carbon containing fuels still needs to be explored in greater detail.

Investigations of Ni/ScYSZ cermets were performed by electrochemical impedance spectroscopy (EIS) using different symmetric designs: electrolyte supported (ESC) and anode supported (ASC) cells. The obtained spectra were analyzed using distribution of relaxation times (DRT), and complex non-linear least squares fitting (CNLS). Depending on the cell design, one or two low frequency gas transport related processes have been identified, and fitted with generalized finite Warburg (GFW) elements. One was related to gas diffusion in a stagnant layer above the anode (ESC+ASC), and the other to gas diffusion in the anode support layer (ASC). A higher frequency process has also been identified, and correlated to the charge transfer (CT) combined with ionic conduction in the ceramic matrix. This has been fitted using a transmission line model (TML), which correlates the exhibited responses with the expected physical processes. Values of the ionic conductivity of the backbone structure, as well as the line specific resistance (LSR) for the charge transfer process have been determined. The impact of cell design on performance was found to be non-negligible. The cells with anode supports showed significantly lower resistance values associated with the TML electrode related processes.