The Influence of Sulfur Formation on Performance and Reforming Chemistry of SOFC Anodes Operating on Methane Containing Fuel

Abstract

This paper presents a detailed analysis of the influence of sulfur formation on performance and efficiency of Solid Oxide Fuel Cells (SOFC) operating on methane containing fuels. Our previously developed multi-step reaction mechanism of sulfur formation and oxidation is coupled with a complex heterogeneous mechanism of methane reforming, channel gas-flow, porous-media transport and elementary kinetic charge transfer and is used to describe sulfur-induced degradation and performance drops of Ni/YSZ anodes. Experimental literature data is used to validate the model and to interpret important aspects of cell performance degradation. Comparisons of the model predictions to the experiments illustrate that the developed model, without any modifications, reproduces the observed voltage decrease well and is able to capture the changes in fuel conversion and selectivity for different gas mixtures. It is shown that atomically adsorbed sulfur significantly influences heterogeneous reforming chemistry, causing substantial voltage degradation. At constant current densities, cell voltage decreases in a non-linear way with faster recovery than in H 2 /H 2 O mixtures.The enormous usage of fossil fuels over the last decades has led to a large release of green-house gases negatively impacting the climate change. Thus, there is an increased need to develop alternative energy conversion technologies with a reduced carbon-footprint. In this regard, the Solid Oxide Fuel Cell (SOFC) is a promising alternative technology for future energy supply. Due to a high operating temperature range (800 K-1100 K), SOFCs are capable of utilizing a variety of fuels, such as natural gas, biomass gasification products, or syngas. 1-3 However, most of these fuels contain compounds undesired for SOFC operation. One of such substances is hydrogen sulfide (H 2 S), which causes a drastic decrease of cell performance in a short period of time, particularly when a Ni/YSZ cermet is employed as anode material. Therefore, the major objective of this paper is to explore and interpret the influence of sulfur formation at the nickel surface of SOFC anodes on performance and in particular on methane reforming chemistry.Numerous studies have experimentally investigated the influence of sulfur poisoning on Ni/YSZ anodes operated on H 2 /H 2 O fuel gas mixtures identifying the electrochemical hydrogen oxidation process to be severely hindered under these conditions. 4-10 However, in recent years, a growing number of studies has been dedicated to the investigation of sulfur poisoning of Ni/YSZ SOFC anodes operating on hydrocarbon-containing fuels, such as methane, 11-13 reformates, 14 syn-15,16 and biogas. 1 Regardless of the employed fuel gas mixture, in all experimental studies a rapid power output drop was observed already for H 2 S concentrations in the ppm range accompanied by a large increase in total anode resistance. 11,[13][14][15]17 This drop is generally believed to be the result of the chemisorption of sulfur on the active surface sites resulting in a block...

Evaluation of the Effect of Sulfur on the Performance of Nickel/Gadolinium‐Doped Ceria Based Solid Oxide Fuel Cell Anodes

“…However,t he affected processi ss till at af requency 2-3 orders of magnitude lower than that of Ni/ YSZ, whichi sc aused by the double layer capacitance at the interface between Ni and YSZ. [2,7] Although the peak frequency of the affected process is significantly higherf or cell Bt han for cell A, the overall sulfur poisoning behaviorr emains similar both in regard to the magnitude of the performance drops and their evolution with H 2 Sconcentration and current density. Undert he assumption thatt he resistances of both anode processesa re approximately the same, the capacitance of the sulfur-affected process for the Ni/CGO40-based anode can be estimated to be twice that of Ni/CGO10.…”

“…[50] However, recent studies have suggested the converse behavior. [2,5,7] Thus, the observed mitigation effect might be relatedt oi mproveda node kinetics because of increased humidity levels in the anode compartment rather than to the actual sulfur oxidation to SO 2 and its subsequentr emoval from the anode surface. Furthermore, the increase of the ASR values for the OCV experiment are lower than at an appliedc urrent density of 0.25 Acm À2 ,w hich is inconsistent with the hypothesis of SO 2 formation.N onetheless, recentD FT calculations of sulfur oxidation on doped cerias urfaces showedt hat sulfur oxidation to SO 2 displays favorable kinetics as surface lattice oxygen could be used to oxidizethe ceria-adsorbed sulfur.…”

“…The sulfur poisoning of the commonly used Ni/yttria-stabilizedz irconia (YSZ) cermet anodes hasb een investigated widely,b oth experimentally and theoretically. [2][3][4][5][6][7] However,i nc omparison to Ni/ YSZ, fewer studies on the sulfur poisoning behavioro fN i/CGO anodese xist, although they show as ignificantly higher sulfur tolerance than Ni/YSZ. [8][9][10][11][12][13][14][15] Thus,t he underlying microscopic processes that define the performance characteristicsa nd the sulfur poisoning behavior of Ni/CGOe lectrodes are not yet fully understood.…”

Understanding the Impact of Sulfur Poisoning on the Methane-Reforming Activity of a Solid Oxide Fuel Cell Anode

“…Having identified the existence of sulfur adsorbates serving to mask active sites for hydrocarbon reforming, it may now be possible to propose a better solution for mitigating the problem. First, it appears that the free surface of Ni (Ni-gas two-phase boundary, 2PB) is directly related to the sulfur uptake property. , Thus, even with a finite Ni mass load, it is expected that smaller Ni particles, possibly nanoparticles, with a higher surface area can tolerate greater levels of sulfur contaminants. Moreover, as the particle diameter d decreases, the TPB length increases proportionally to d –2 ; hence, S* will have a greater chance to be removed as SO 2 with the aid of oxygen ion flux from the adjacent electrolyte phase, i.e., This further explains the excellent sulfur tolerance reported for anodes with exsolved Ni nanoparticles. ,− Given that Ni is likely to be dissolved into the YSZ phase at a high temperature and can later be exsolved, employing such nanoparticles may significantly enhance the tolerance to sulfur poisoning of Ni-based anodes.…”

Dry Reforming of Model Biogas on a Ni/SiO2 Catalyst: Overall Performance and Mechanisms of Sulfur Poisoning and Regeneration

“…The sulfur poisoning effect of nickel-based catalysts was first studied in the catalytic process of biomass/coal gasification , and steam reforming. ,− Recently, sulfur poisoning in the dry reforming process has also been investigated , because sulfur impurities are inevitably produced during the process of biogas production, and the presence of even low sulfur levels after desulfurization may lead to the deactivation of nickel catalysts. , It has been accepted for decades that catalysts can be deactivated by sulfur via the chemisorption of sulfur with active metal to form metal sulfides (eq ), which occupy the surface active sites, deactivating the catalyst. ,− However, the detailed sulfur poisoning performance and mechanism in a biogas dry reforming process with regard to the formation and elimination of Ni–S species still remains unclear. …”