Enhanced Sulfur Tolerance of Nickel-Based Anodes for Oxygen-Ion

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Enhanced sulfur tolerance of nickel-based anodes for oxygen-ion conducting solid-oxide fuel cells by incorporating a secondary water storing phase Feng Wang, Wei Wang, Jifa Qu, Yijun Zhong, Moses O. Tade, and Zongping Shao Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Sep 2014 Downloaded from http://pubs.acs.org on September 29, 2014

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Environmental Science & Technology

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Enhanced sulfur tolerance of nickel-based anodes for

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oxygen-ion conducting solid-oxide fuel cells by

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incorporating a secondary water storing phase

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Feng Wang1, Wei Wang2*, Jifa Qu1, Yijun Zhong1, Mose O. Tadé2, Zongping Shao1,2*

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State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry &

Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, China 2

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

* Corresponding Authors

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Telephone: +61-8-92665602. Email: [email protected] (W. Wang)

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Telephone: +86-25-83172242. Email: [email protected] (Z. Shao)

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1

ACS Paragon Plus Environment


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TABLE OF CONTENTS

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ABSTRACT

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In this work, Ni+BaZr0.4Ce0.4Y0.2O3-δ (Ni+BZCY) anode with high water storage

17

capability is used to increase the sulfur tolerance of nickel electrocatalysts for solid oxide fuel

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cells (SOFCs) with oxygen-ion conducting Sm0.2Ce0.8O1.9 (SDC) electrolyte. Attractive power

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outputs are still obtained for cell with Ni+BZCY anode operating on hydrogen fuels containing

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100-1000 ppm H2S, while for a similar cell with Ni+SDC anode it displays much reduced

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performance by introducing only 100 ppm H2S into hydrogen. Operating on a hydrogen fuel

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containing 100 ppm H2S at 600 oC and a fixed current density of 200 mA cm-2, a stable power

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output of 148 mW cm-2 is well maintained for a cell with Ni+BZCY anode within a test period of

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700 min, while it was decreased from an initial value of 137 mW cm-2 to only 81 mW cm-2 for a

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similar cell with Ni+SDC anode after a test period of only 150 min. After the stability test, loss of

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Ni percolating network and reaction between nickel and sulfur are appeared over Ni+SDC anode,

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but it is not observed for Ni+BZCY anode. It highly promises the use of water-storing BZCY as

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anode component in improving sulfur tolerance for SOFCs with oxygen-ion conducting SDC

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electrolyte.

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1. INTRODUCTION

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Fuel cells are well recognized as an outstanding clean power generation technology,

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characterized by high energy conversion efficiency, low emissions of greenhouse gas such as

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CO2 and environmental pollutants like NOx, and size flexibility. Among various types of fuel

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cells, solid oxide fuel cells (SOFCs) have received particular attention because of some additional

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important advantages, such as fuel flexibility, high-quality exhaust heat and a wide range of

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material selection for main cell components.1-3 Actually all combustible fuels, such as hydrogen,

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hydrocarbons, carbon, alcohols, natural gas and biogas, could be direct fuels of SOFCs.4-8

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For scientific in-lab research purpose, a fuel with simple component is often used for the

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investigation of fuel cell performance, such as pure hydrogen, methane and hydrocarbons.

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However, for practical applications, the fuel composition is much more complicated. As we know,

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there are no natural resources of hydrogen, while most hydrogen is produced by reforming of

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natural gas or coal. Since natural gas and coal contain a certain level of sulfur impurity, while the

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state-of-the-art Ni-based cermets anodes are very sensitive to sulfur poisoning,9-14 the poisoning

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of the anodes by sulfur in a practical fuel is thus a big concern. Actually, natural gas or biogas,

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the most promising fuel for SOFCs, contain a high concentration of H2S (> 50 ppm),15,

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well exceeds the upper limit (10 ppm) for stable operation of a SOFC with conventional Ni-based

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cermet anodes.17,

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introducing into fuel cell system. For example, hydrodesulfurization is a useful technique for

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reducing sulfur content in high sulfur-containing fuels; however, it is difficult to reduce the sulfur

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concentration to lower than 10 ppm.19, 20 Some sorbents such as activated carbon, zeolites are

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useful for reducing sulfur levels to a lower extent than hydrodesulfurization and have received

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increasing attention recently.21-24 However, there are several drawbacks associated with this

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which

Thereby, fuel processing is required to reduce the sulfur content before

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process, such as production of toxic carbonyl sulfide (COS) gas and difficulty to regenerate the

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activated carbon and zeolites.

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Electrode materials modification and operation conditions optimization are two most

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important ways for improving the sulfur tolerance of Ni-based anodes. For example, surface

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modification with some oxides such as CeO2 and doped CeO2 was found to be effective in

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improving sulfur tolerance of Ni+YSZ anode.25-27 In those studies, CeO2 or doped CeO2 particles

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were deposited over Ni+YSZ anode surface randomly through infiltration, which could have a

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negative effect on the electrode porosity. In addition, Ni-Sm doped ceria (SDC) often showed

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better sulfur tolerance than Ni+YSZ since CeO2 has been used as a high temperature desulfurizer

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agent.28 In another study, it was found the presence of 10 vol.% water in fuel gas effectively

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promoted the performance recoverability to a large extent of SOFCs operating on sulfur-

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containing fuel.29 Wang and Liu further predicted the regeneration of sulfur-poisoned nickel by

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O2 and H2O and found that sulfur adsorption on the nickel surface can only be cleaned with

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water.30 However, since water is not a fuel, to maximize the fuel efficiency, the water content in a

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fuel gas should be as low as possible.

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Recently, M.L. Liu from Georgia Institute of Technology reported the superior sulfur

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tolerance/coking resistance of proton-conductor based anodes, in which the proton-conducting

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phase acted as the main component of anode ceramic phase, used to decorate conventional

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Ni+YSZ anode surface or in-situ formed in the Ni+YSZ anode by the addition of BaCO3.31-33 As

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we know, for SOFCs with oxygen-ion conducting electrolyte, the fuel oxidation will occur at the

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anode side. If a hydrocarbon or hydrogen is applied as the fuel, water will be produced at the

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anode side under current polarization. This water can be used for the elimination of carbon/sulfur

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deposition over the anode. However, the water concentration varies from time to time in the 5



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anode chamber of a SOFC with oxygen-ion conducting electrolyte, depending on the operation

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conditions. A similar variation of oxygen concentration was also appeared in automobile exhaust

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treatment. In that case, a ceria is used to act as an oxygen storage-and-release component to

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stabilize the local oxygen partial pressure at the catalyst surface even when the air-to-fuel ratio in

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the engine exhaust fluctuates with time.34 Similarly, a water storage material may be beneficial

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for improving carbon/sulfur removal over the anode. M.L. Liu et al. demonstrated that nano BaO

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modified Ni+YSZ anode showed high sulfur tolerance, and they further pointed out that the high

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performance was closely related to the water adsorption capability of nanosized BaO.35 As

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compared to surface water adsorption of BaO nanoparticles, proton-conducting perovskite oxides

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have bulk water storage capability, in particular at lower temperatures.36-38 More recently,

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Sengodan et al. have used infiltration method to modify the conventional Ni/YSZ anode by

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BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) to improve the sulfur tolerance.32 However, due to limited

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amount of proton conductor in the Ni-based anode by infiltration, the sulfur tolerance was not

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satisfied at higher H2S contents in the fuel. Indeed, we have demonstrated recently that those

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proton conducting phases could be used as water storage material and the whole ceramic phase to

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significantly improve the coking resistance of nickel-based anodes for operating on ethanol

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fuel.39

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Similarly, a water-storing material may also be applied to facilitate the elimination of

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sulfur adsorption over the nickel surface, thus improving the sulfur tolerance of SOFC anode.

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Figure 1 shows the proposed mechanism for water-induced sulfur removal process on an anode

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with water storage capability based on proton conducting oxide. Firstly, the H2 in the fuel is

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oxidized by the O2- from the cathode on the triple phase boundary (TPB) with the generation of

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water; in parallel, sulfur absorbs on the surface of Ni to produce surface-adsorbed sulfur (SNi*).

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Secondary, the water incorporation or storage into the proton-conducting oxide with the

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formation of (OH)o species is happened. Then, the incorporated (OH)o reacts with SNi* to

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generate SO2 and H2. Finally, SO2 is removed from the Ni surface while H2 is oxidized by O2- to

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form H2O.

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Figure 1. Proposed mechanism for water-induced sulfur removal process on the Ni-based anode

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with a water-storing phase.

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To support our above consideration, we fabricated similar cells with Ni+BZCY anode and

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SDC electrolyte, which were previously used for coking resistance test,39 and operated them on

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H2S-containing fuels. The effect of H2S contents (100, 200 and 1000 ppm) in the H2 fuel gas on

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the cell performance was systematically studied. For comparison, Ni+Sm0.2Ce0.8O1.9 (SDC) anode,

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which has negligible water storing capability, was conducted in the same way. In addition, the

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possible reasons for the different tolerance of these two anodes towards sulfur were proposed.

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This study will provide some useful guidelines for the development of sulfur-resistant anodes for

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SOFCs operating on the fuel with high H2S amount at lower temperatures.

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2. EXPERIMENTAL SECTION

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NiO+BZCY and NiO+SDC anodes with 60 wt.% NiO were synthesized by a solution

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combustion method based on glycine nitrite process.39 The powders from the direct combustion

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process were further calcined at 1000 oC for 5 h in static air to yield the primary anode powders

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for the dry-pressing process. Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF), Sm0.5Sr0.5CoO3- (SSC) cathodes

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and SDC electrolyte were prepared by an EDTA-citrate complexing process.35, 36 The fuel cells

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used in this study were in a 60 wt.% NiO + 40 wt.% BZCY or SDC cermet anode-supported thin-

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film SDC electrolyte configuration, and the bilayer cells were fabricated by a dual dry-

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pressing/sintering process.40 The BSCF or SSC cathode slurry was sprayed on the central surface

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of the electrolyte layer for the bi-layer cells and fired at 1000 oC in static air for 2 h to form the

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complete cells for later performance investigation. The thicknesses of the anode, cathode and the

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electrolyte are about 500, 15 and 20 µm, respectively. The active area of the cell is 0.48 cm-2. The

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dimensions of the three SOFC cells are about 13 mm in diameter after calcined at 1400 oC. The

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porosities of the SOFC cells are about 40 % after the hydrogen reduction process.

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The phase structures of the various samples were examined by an X-ray diffractometer

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(XRD, D8 Advance, Bruker, Germany) equipped with a Cu K radiation (λ=0.1541 nm). The

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cross-sectional morphologies of the fuel cells were examined by a scanning electron microscope

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(SEM, S3400) equipped with an EDX detector. The surface morphologies of the fuel cells were

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examined by a scanning electron microscope (SEM, S4800). The water storage in the anode

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materials were investigated by Fourier Transform Infrared spectroscopy (Thermo Nicolet iS10).

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For this investigation, the samples were first reduced at 750 oC for 1 h and then exposed to 20

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vol.% H2O/Ar at 150 oC for 1 h.

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The I-V polarization curves of the fuel cells were obtained using a Keithley 2420 source

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meter in the 4-probe mode. During the measurements, H2 or H2S-containing H2 fuels was fed into

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the anode chamber while ambient air was used as the cathode atmosphere. The flow rate of H2 or

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H2+H2S fuel gas was controlled at 80 mL min-1 [STP]. The cell resistance was determined using

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electrochemical impedance spectroscopy (EIS) with a Solartron 1260 frequency response

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analyzer in combination with a Solartron 1287 potentiostat. The frequencies used for the EIS

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measurements ranged from 0.1 to 1000 kHz for signal amplitude of 20 mV.

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3. RESULTS AND DISCUSSION

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Figure S1 shows XRD patterns of Ni+SDC and Ni+BZCY anode materials after the calcination at

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1400 oC for 5 h at a ramping rate of 5 oC min-1, before and after the hydrogen reduction process.

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SDC and BZCY crystalline phases were detected respectively for Ni+SDC and Ni+BZCY

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samples, before and after the hydrogen reduction. NiO was completely reduced to Ni in the

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reduction process for these two anodes. Figure S2a&b shows the FE-SEM images of the reduced

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Ni+SDC and Ni+BZCY anodes. It was found that the two phases in the anodes are well

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connected and no etched interfaces of Ni and BZCY/SDC were observed.

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In our previous work, we have demonstrated that Ni+BZCY anode catalyst had a strong

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water storage capability, much higher than the conventional Ni-based catalysts such as Ni-

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Ce0.8Zr0.2O2 and Ni-Al2O3.39 In this study, the water storage capabilities of the Ni+BZCY and

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Ni+SDC anodes were also comparatively studied by FTIR with the corresponding spectra

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presented in Figure 2. The small peaks of the two anodes before water treatment come from the

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surface absorbed water in the preparation process for the samples. As can be seen, after the

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treatment in water-containing atmosphere, Ni+BZCY anode displayed a much bigger water

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desorption peak at around 3430 cm-1 in the FTIR spectra than the fresh reduced Ni+BZCY anode

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and also the water-treated Ni+SDC anode. In addition, the difference in intensity of the water

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desorption peaks for the Ni+SDC anodes before and after the water treatment not as large as

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Ni+BZCY, suggesting the poor water storage capability of the Ni+SDC anode. These results

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suggested that the current Ni+BZCY anode also had a much higher water storage capability than

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Ni+SDC anode. As we know water vapor can facilitate the sulfur oxidation reaction, thus

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improved sulfur tolerance is expected, which will be studied in later text.

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Figure 2. Fourier Transform Infrared spectroscopy of the reduced anodes before and after the

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water adsorption.

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Anode-supported thin-film SDC electrolyte-based fuel cells using BSCF as a cathode were

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fabricated and tested. The thickness of the electrolytes for different cells was fixed at around 20

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μm by precisely controlling the amount of SDC powder in the dry pressing process. Figure S3

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shows typical SEM images of the fuel cells with reduced Ni+BZCY and Ni+SDC anodes from

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the cross-sectional view. The SDC electrolyte was well densified without any penetrating

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pinholes. In this section, the various fuel cells were first tested using pure hydrogen as the fuel.

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Typical power outputs of these two fuel cells at various temperatures are shown in Table 1. As

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can be seen, the cell delivered peak power densities (PPDs) of 678, 1018, 1143, 1043 and 926

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mW cm-2 at 550, 600, 650, 700 and 750 oC, respectively, which are comparable to the literature

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results with similar cells.41, 42 The lower power outputs at 750 and 700 oC than that at 650 oC is

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due to the increased current leakage within the SDC electrolyte. The fuel cell with Ni+BZCY

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anode also demonstrated promising power outputs with PPDs of 499, 596, 729, 894 and 1032

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mW cm-2 at 550, 600, 650, 700 and 750 oC, respectively. On the other hand, as shown in Figure

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S4, high open circuit voltages (OCVs) were obtained by the fuel cell with Ni+BZCY anode at all

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temperatures and for instance, 1.01 V at 750 oC, much higher than 0.709 V for a cell with

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Ni+SDC anode. The improved OCVs of the cell with Ni+BZCY anode should be attributed to the

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beneficial reaction of BZCY and SDC to form an interfacial layer between the anode and

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electrolyte.43

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In addition to H2 fuel, we have also tested the fuel cells on H2 fuels containing different

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amounts of H2S (1000 ppm, 200 ppm and 100 ppm). Table 1 also lists the PPDs of the fuel cells

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with Ni+SDC and Ni+BZCY anodes at different temperatures operating on H2+1000 ppm H2S,

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H2+200 ppm H2S and H2+100 ppm H2S gas mixtures as the fuels. For the Ni+SDC anode, when

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operated on H2+1000 ppm H2S as the fuel, PPDs of 64, 189, 477, 738 and 846 mW cm-2 were

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reached at 550, 600, 650, 700 and 750 oC, respectively. A sharp decrease in cell power output

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was observed with the decrease of temperature when operating on H2+1000 ppm H2S fuel, and

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the PPD decreased to 738 mW cm-2 at 700 oC, which is only 70.7 % compared to the value of

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hydrogen fuel. On the other hand, for the same H2+1000 ppm H2S fuel, Ni+BZCY anode showed

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a better sulfur tolerance sine the PPD reached 98.1 % of the value of H2 fuel. However, at the

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temperatures lower than 700 oC, the power outputs on H2+1000 ppm H2S fuel with Ni+BZCY

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anode were still much lower than those of H2.

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Table 1. PPDs of fuel cells with Ni+BZCY and Ni+SDC anodes operating on different fuels at

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various temperatures. Anodes

Ni+BZCY

Ni+SDC

Fuels H2 1000 ppm H2S+H2 200 ppm H2S+H2 100 ppm H2S+H2 H2 1000 ppm H2S+H2 200 ppm H2S+H2 100 ppm H2S+H2

750 oC 1032 1010 1017 1019 926 846 894 904

PPDs at different temperatures (mW cm-2) 700 oC 650 oC 600 oC 550 oC 916 729 596 499 877 304 137 121 901 660 405 218 913 674 522 290 1043 1143 1018 678 738 477 189 64 905 572 293 103 1033 748 384 181

200 201

Taking PPDs of the cells with these two anodes operated on H2 as a criterion, the

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reduction ratios of the PPDs on the H2+200 ppm H2S fuel at different temperatures are shown in

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Figure 3. It was found that the reduction of operational temperature led to a much faster decrease

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in PPDs, suggesting much more serious sulfur poisoning at the reduced temperatures for these

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two anodes. This phenomenon was in good agreement with that reported in literatures since the

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sulfur desorption on the Ni surface became faster at higher temperature.9,

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Ni+BZCY anode displayed a much more moderate reduction ratio than Ni+SDC, for example,

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9.4 % compared with 50 % at 650 oC, suggesting much better sulfur tolerance of Ni+BZCY due

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to the higher water storage capability. With the further reduction of H2S concentration to 100

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ppm, it was found that the cell performance of the Ni+SDC anodes below 700 oC was still not

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satisfied although H2S contents was reduced from 1000 ppm to 100 ppm in the fuel. The above

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results suggested that Ni+SDC composites were not a practical anode for SOFCs operating on

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H2S-containing fuels at intermediate temperatures. For the fuel cell with Ni+BZCY anode, the

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power outputs on H2+100 ppm H2S fuel were comparable to those on hydrogen at the

10

In addition,

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temperatures from 600 to 750 oC, suggesting high sulfur tolerance of this anode for the fuel with

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100 ppm H2S. As compared with the results of Ni+SDC anode, it was found that Ni+BZCY

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anode displayed a much superior sulfur tolerance, which could be attributed to the enhanced

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water storage capability of Ni+BZCY.

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Figure 3. The reduction ratios of the PPDs for the cell with Ni+BZCY and Ni+SDC anodes

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operated on the H2+200 ppm H2S fuel compared with H2 fuel at different temperatures.

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More specifically, the I-V, I-P curves and EIS of the fuel cells with these two anodes

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operating on various fuels at 600 oC are presented in Figure 4. It is clear that the fuel cell with

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Ni+BZCY anode showed a slightly lower power output on H2+100ppm H2S or 200 ppm H2S

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compared with H2 fuel, while the addition of H2S even in the amount of 100 ppm could obviously

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decrease the electrochemical activity of Ni+SDC anode operating on H2 fuel. To obtain more

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information to interpret the cell performance with different anodes operating on various fuels, the

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EIS of the fuel cells was measured with the results shown in Figure 4c&d. In EIS, the high-

229

frequency offset on the real axis represents the electrolyte resistances, whereas the difference

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between the high and low frequency intercepts on the real axis is associated with electrode

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polarization resistances including the contributions of both anode and cathode. As shown in 13



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Figure 4c, the fuel cell with Ni+SDC anode operating on H2 fuel showed the lowest electrode

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polarization resistance and the fuel cell with H2+1000 ppm H2S fuel presented the largest value,

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which was in good agreement with the results in Figure 4a. For the fuel cell with Ni+BZCY

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anode, besides the H2+1000 ppm H2S fuel, the electrode polarization resistances were

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comparable for the other three fuels (Figure 4d). It was also found that there were some

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differences for the electrolyte resistances for the two fuel cells, especially for the cell with

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Ni+SDC anode. The difference in electrolyte resistance could be attributed to the reconstruction

239

and diffusion of Ni and the loss of percolating Ni network.44 The sharp increase in the electrode

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polarization and electrolyte resistances of Ni+SDC in H2S-containing fuels resulted in the

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obvious decrease in power outputs. In addition, the cell with Ni+BZCY anode displayed similar

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electrode polarization and electrolyte resistances for the 100 or 200 ppm H2S-containing fuels.

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Figure 4. I-V, I-P curves and EIS for the fuel cells with Ni+SDC (a, c) and Ni+BZCY (b, d)

246

anode operating on different fuels at 600 °C.

247

To determine the effect of water storage capability of the anodes on the operational

248

stability, two similar fuel cells were first polarized under a constant current density of 200 mA

249

cm-2 for 20 h at 600 oC by operating on H2 to obtain a stable performance, and then the stability

250

tests were conducted by operating on 100 ppm H2S-containing H2 fuel with Ni+BZCY and

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Ni+SDC anodes. To avoid the possible CO2 poisoning and phase transition of BSCF cathode,

252

which may mask the effect of H2S poisoning on the degradation of cell performance, SSC was

253

used as cathode material for the stability test instead. In the stability test, a mesh-like

254

morphological structure of silver paste was drawn with a stick directly onto the SSC cathode

255

surface to create the current collector, and then fired at 180 °C for 1 h.45 Figure 5 showed the

256

time dependence of the voltage under different current densities and temperatures. For the cell

257

with Ni+SDC anode, the voltage was not stable and the cell was failed after a continuous

258

operation for 150 minutes under a current density of 200 mA cm-2 at 600 oC. In contrast, the cell

259

operation was stable for 700 minutes under different current densities and temperatures when

260

Ni+BZCY anode was applied. This improvement in the operational stability is clearly due to the

261

improved water storage capability and then enhanced sulfur tolerance of Ni+BZCY anode.

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Figure 5. Time-dependent voltages of the fuel cells with Ni+SDC and Ni+BZCY anodes

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operating on H2+100 ppm H2S as the fuel under different current densities and temperatures.

265

The element distributions over the anode in the fuel cells after the stability test were

266

investigated by SEM-EDX at selected regions and the typical SEM images are shown in Figure

267

S5. First of all, the sulfur content of the active layer of the anode (about 10-20 µm in depth near

268

the anode-electrolyte interface) was studied with results shown in Figure S5. For the comparison

269

purpose, the sulfur amount of the SDC electrolyte was also studied. It was found that the sulfur

270

contents of the active layer of the Ni+BZCY anode and the SDC electrolyte are comparable.

271

However, the sulfur amount of the active layer of the Ni+SDC anode was more than 10 times

272

higher than that of the similar position of the Ni+BZCY anode. As shown in Figure S6, the XRD

273

results of these two anodes after the operational stability test suggested that no obvious difference

274

was observed for the Ni+BZCY anode while there were some obvious diffraction peaks assigned

275

to NiSx phases in the XRD pattern of the Ni+SDC after the stability test. It suggested that more

276

severe sulfur poisoning of Ni+SDC anode was appeared, which could be accountable for the

277

poorer operational stability. Furthermore, the anode surface of used fuel cells was also conducted 16


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by SEM as shown in Figure 6. Comparison of micrographs for the anodes operated in pure H2

279

(Figure S2) and after operation on H2+H2S fuels in Figure 6 shows microstructural differences

280

expressed as etched interfaces of Ni and SDC originated from the reaction of Ni with S after the

281

operational stability test, which is in good agreement with the literature.46 On the other hand, for

282

the Ni+BZCY anode as shown in Figure 6, no obvious differences were observed as compared

283

with the SEM image in Figure S2. The EDX results of the surfaces of these two anodes were also

284

shown in Figure S7. It was found that the sulfur content on the Ni+SDC anode surface was 12

285

times higher than that of the Ni+BZCY anode, further confirming the excellent sulfur tolerance

286

of the Ni+BZCY anode.

287 288

Figure 6. SEM photos of the Ni+BZCY (a) and Ni+SDC (b) anodes after the operational stability

289

test from the surface view.

290

Hauch et al. have demonstrated a possible sulfur poisoning mechanism for nickel catalyst

291

that the sulfur poisoning could result in the loss of percolating Ni network.44 For the fuel cells

292

operating on H2, the Ni particles in the anodes are well distributed and a well percolating network

293

exists. In this study, EDX was used to study the Ni contributions in these two anodes before and

294

after sulfur poisoning, as shown in Table 2. The Ni distributions in the Ni+BZCY anodes after

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stability tests were uniform and Ni contents were comparable to those of the fresh reduced anode

296

and for instance, the Ni amount in the used anode was 61.3±5.6 wt.% compared with the value of

297

63.6±5.4 wt.% for the fresh one. In addition, the Ni amount in the used Ni+SDC anode was

298

28.8±15.9 wt.% compared with the value of 59.8±3.4 wt.% for the fresh one. In the Ni+SDC

299

anode, a percolating Ni network was lacking in the active layer of anode, resulting a rapid

300

decrease in cell performance. In summary, the degradation of the fuel cells with Ni+SDC anode

301

should be attributed to the loss of percolating Ni network and the harmful reaction of nickel and

302

sulfur. However, for the fuel cell with Ni+BZCY anode, the high water storage capability of the

303

anode could reduce the reaction between nickel and sulfur as well as the diffusion of Ni from the

304

active layer, and then, an improved operational stability was obtained.

305 306

Table 2. EDX results of the Ni contents for the two fuel cells before and after the operational

307

stability test. Anode Ni+SDC Ni+BZCY

Conditions

Ni contents in the different regions (%) Region 1

Region 2

Region 3

Region 4

Before stability test

59.2

56.4

62.3

57.8

After stability test

44.7

21.4

12.9

20.9

Before stability test

64.1

69.0

58.2

65.3

After stability test

63.7

63.4

66.9

55.6

308 309

In conclusion, a Ni+BZCY composite with high water storage capability was synthesized

310

and demonstrated as a promising alternative anode material for SOFCs operating on H2S-

311

containing fuels. The effect of H2S content and water storage capability on the sulfur poisoning

312

behavior of the anodes for SOFCs was systematically studied. The Ni+BZCY anode was superior

18


Page 19 of 36


313

to the state-of-the-art Ni+SDC anode in sulfur tolerance, especially for the lower temperatures

314

and higher H2S amount. A significant improvement in sulfur tolerance of the Ni+BZCY anode

315

was observed at low temperatures with the reduction of H2S amount in the fuel; however, there

316

was no increase in sulfur tolerance for Ni+SDC anode even with 100 ppm H2S in the fuel. An

317

obvious degradation was observed for the Ni+SDC anode while the power output of the fuel cell

318

with Ni+BZCY anode was well maintained. It was found that the degradation of the Ni+SDC

319

anode could be assigned to the loss of Ni percolating network and the harmful reaction between

320

nickel and sulfur of Ni and SDC ceramic phase. The high water storage capability of Ni+BZCY

321

anode could significantly reduce the above phenomenon in the stability tests. In sum, the above

322

results clearly suggest the potential applications of the Ni+BZCY composite with high water

323

storage capability for the anode materials of SOFCs operating on various sulfur-containing fuels.

324 325

ASSOCIATED CONTENT

326

Supporting Information

327

(1) XRD patterns, SEM photos, EDX profiles of Ni+BZCY and Ni+SDC anodes under different

328

test conditions, (2) I-V, I-P curves for the cells with Ni+SDC and Ni+BZCY anodes, This

329

material is available free of charge via the Internet at http://pubs.acs.org.

330

NOTES

331

The authors declare no competing financial interest.

332 333

19



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334

ACKNOWLEDGMENT

335

This work was supported by the program of “National Science Foundation for Distinguished

336

Young Scholars of China” under contract No. 51025209 and the Doctoral Fund of Ministry of

337

Education of China (20113221110002).

338

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TOC 180x100mm (150 x 150 DPI)



Proposed mechanism for water-induced sulfur removal process on the Ni-based anode with a water-storing phase. 199x149mm (150 x 150 DPI)


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Page 29 of 36


Fourier Transform Infrared spectroscopy of the reduced anodes before and after the water adsorption. 594x419mm (150 x 150 DPI)



The reduction ratios of the PPDs for the cell with Ni+BZCY and Ni+SDC anodes operated on the H2+200 ppm H2S fuel compared with H2 fuel at different temperatures. 594x419mm (150 x 150 DPI)


Page 30 of 36

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558x431mm (150 x 150 DPI)



558x431mm (150 x 150 DPI)


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577x404mm (150 x 150 DPI)



577x404mm (150 x 150 DPI)


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594x419mm (150 x 150 DPI)



254x95mm (150 x 150 DPI)


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Enhanced Sulfur Tolerance of Nickel-Based Anodes for Oxygen-Ion

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