Sulfur Poisoning of SOFCs: Voltage Oscillation and Ni Oxidation

Yoshizumi, Takuma; Taniguchi, Shunsuke; Shiratori, Yusuke; Sasaki, Kazunari

doi:10.1149/2.032211jes

Cited by 38 publications

(40 citation statements)

References 28 publications

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“…We identify the key parameters associated with these bifurcations, and the regions in parameter space where self-sustained oscillatory dynamics occur. Although a broad range of experimental data on SOFC oscillatory dynamics is, at present, unavailable, the predictions established in this paper appear to be broadly in line with, for example, results given in [3][4][5], which include bifurcations, multiple steady states and oscillatory phenomena. This is encouraging in underpinning the model we have developed here as capturing the fundamentals of oscillatory dynamics in SOFCs.…”

Section: Introductionsupporting

confidence: 84%

A fundamental model exhibiting nonlinear oscillatory dynamics in solid oxide fuel cells

2014

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In this paper, we address the phenomenon of temporal, self-sustained oscillations which have been observed under quite general conditions in solid oxide fuel cells. Our objective is to uncover the fundamental mechanisms giving rise to the observed oscillations. To this end, we develop a model based on the fundamental chemical kinetics and transfer processes which take place within the fuel cell. This leads to a three-dimensional dynamical system, which, under typical operating conditions, is rationally reducible to a planar dynamical system. The structural dynamics of the planar dynamical system are studied in detail. Self-sustained oscillations are shown to arise through Hopf bifurcations in this planar dynamical system, and the key parameter ranges for the occurrence of such oscillations are identified.

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Section: Introductionsupporting

confidence: 84%

A fundamental model exhibiting nonlinear oscillatory dynamics in solid oxide fuel cells

2014

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“…10 The fact that we do not observe irreversible performance loss in the YSZ-no-S test, but we do observe this in the YSZ-Hi test, points to that this is not the controlling mechanism causing irreversible loss. Based on the results reported here we propose that it is the combination of the continued feed of H 2 S and the high overpotential (i.e.…”

Section: Post-mortem Analysis Of the Anode Layers-mentioning

confidence: 59%

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

Hauch

Hagen

Hjelm

et al. 2014

J. Electrochem. Soc.

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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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“…It should be noted that the potential of the RE could be shifted relative to the potential in the middle of the electrolyte plates in between the electrodes, which can affect the values obtained for the ohmic loss. 16,17 Since the anode potential was measured between the anode and the reference electrode at the cathode side, it has a higher value than the cell voltage by the potential difference between the cathode and the reference electrode. Pt mesh (80 mesh acting) was attached to each electrode surface as a current collector.…”

Section: Methodsmentioning

confidence: 99%

SOFC Durability against Standby and Shutdown Cycling

Hanasaki

Uryu

Daio

et al. 2014

J. Electrochem. Soc.

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To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H 2 S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail. Solid oxide fuel cells (SOFCs) have several advantages including high efficiency, fuel flexibility, and utilization of non-noble metal, Pt-free catalysts, due to their relatively high operation temperature. Commercialization of SOFC systems for residential electric power applications began in Japan in 2011. Such systems are frequently stopped and restarted in normal operation, e.g. when power is not required, or in an emergency. Such start-stop operation results in thermal cycling, and is often associated with an interruption in fuel supply. Although it is well known that thermal and redox cycling under startstop operation deteriorates SOFC electrochemical performance, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] there are only a limited number of studies that systematically focus on this technologically relevant issue.Continuous SOFC stack operation with constant power output generally results in a gradual degradation in performance during long-term operation (SOFCs should run for up to a decade). However, SOFCs can suffer to a greater degree from changes in operation conditions. Due to thermal expansion mismatch between the different components, the cells can suffer from mechanical degradation mechanisms, such as delamination and crack formation with simple thermal cycling. Changes in atmosphere can result in more serious degradation. The influences of thermal cycling and current density cycling on cell degradation have been previously investigated.1-4 The change in volume causes Ni agglomeration. [5][6][7] Redox cycles are typically associated with oxidation of Ni particles at the anode. [8][9][10][11][12] Furthermore, redox cycling results in the formation of Ni hydroxides at a certain vapor pressure in oxidizing atmosphere, with a high water vapor concentration. 13,14 In real residential SOFC power units, cells and stacks are not subjected to these different conditions independently; the changes occur much more dynamically. Therefore, cycle durability studies should be performed using reali...

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Sulfur Poisoning of SOFCs: Voltage Oscillation and Ni Oxidation

Cited by 38 publications

References 28 publications

A fundamental model exhibiting nonlinear oscillatory dynamics in solid oxide fuel cells

A fundamental model exhibiting nonlinear oscillatory dynamics in solid oxide fuel cells

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

SOFC Durability against Standby and Shutdown Cycling

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