Sulfur Poisoning of SOFC Anodes: Effect of Overpotential on Long-Term Degradation

Abstract

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...

Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability

“…SEM images obtained at 8-10 kV using the secondary electron detector and low-voltage SEM images were obtained at 0.9 kV using the in-lens detector. Lowvoltage in-lens SEM imaging allows percolating and non-percolating Ni to be distinguished in the Ni/YSZ cermet microstructure, as described by Thydén et al [32] and later illustrated several times for long-term tested SOC [7,[33][34][35]. Image analysis by line-intercept-method [36] allowed for determination of the area fraction and particle size distributions of Ni and pores, using the in-house developed Matlab based software, ManSeg.…”

Current Status of Rapid Evaluation of Durability of Six SOFC Stacks within Nedo Project

“…For example, nickel retreat reported by Hauch et al (3,10) can be regarded as a phenomena which took place within the NiO stable region. A similar phenomenon is observed in the NGK SPARK PLUG anode by Kyoto University although the oxygen potential determined from CRIEPI's analyses shows that oxygen potential of this anode-electrolyte interface is far from the NiO-Ni borderline (see Fig.2), suggesting that additional mechanism contributes to this phenomenon.…”

Effect of Au and/or Mo Doping on the Development of Carbon and Sulfur Tolerant Anodes for SOFCs—A Short Review

“…The vast literature refers and focusses on systems, which are fed with H 2 S in H 2 -containing fuels. They studied the deactivation mechanisms and materials application Zha et al, 2007;Li T. S. et al, 2009;Li et al, 2010;Hagen, 2009, 2010;Xu et al, 2010;Li and Wang, 2011a,b), summarized in several studies and reviews (Sasaki et al, 2006(Sasaki et al, , 2011Trembly et al, 2006;Gong et al, 2007;Goodenough and Huang, 2007;Sun and Stimming, 2007;Rasmussen and Hagen, 2009;Yang et al, 2010;Cheng et al, 2011;Hagen et al, 2011;Kromp et al, 2012;Hagen, 2013;Wang et al, 2013;Weber et al, 2013;Hauch et al, 2014). The effect of a sulfur-containing fuel is examined on the performance and durability of SOFCs through the variation of several factors, such as the: (i) operating temperature, (ii) applied current/voltage load, (iii) time on stream, (iv) H 2 S concentration and to a lesser extent the concentration of H 2 and H 2 O, and (v) the anodes materials.…”