Enhancing Sulfur Tolerance of Ni-Based Cermet Anodes of Solid

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Enhancing sulfur tolerance of Ni-based cermet anodes of solid oxide fuel cells by ytterbium-doped barium cerate infiltration Meng Li, Bin Hua, Jing-li Luo, San Ping Jiang, Jian Pu, Bo Chi, and Jian Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00925 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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ACS Applied Materials & Interfaces

Enhancing sulfur tolerance of Ni-based cermet anodes of solid oxide fuel cells by ytterbium-doped barium cerate infiltration Meng Lia,b, Bin Huac, Jing-li Luoc, San Ping Jiangb,*, Jian Pua, Bo Chia, Jian Lia,* a

Center for Fuel Cell Innovation, State Key Laboratory for Coal Combustion, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China. b

Fuels and Energy Technology Institute & Department of Chemical Engineering,

Curtin University, Perth, WA 6102, Australia. c

Department of Chemical and Materials Engineering, University of Alberta, Edmonton,

Alberta T6G 2G6, Canada.

KEYWORDS

Ni-GDC anode; sulfur tolerance; ytterbium-doped barium cerate; impregnation; solid oxide fuel cells.

ABSTRACT

Conventional anode materials for solid oxide fuel cells (SOFCs) are Ni-based cermets, which are highly susceptible to deactivation by contaminants in hydrocarbon 1

ACS Paragon Plus Environment


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fuels. Hydrogen sulfide is one of the commonly existed contaminants in readily available natural gas and gasification product gasses of pyrolysis of biomasses. Development of sulfur tolerant anode materials is thus one of the critical challenges for commercial viability and practical application of SOFC technologies. Here we report a viable approach to substantially enhance the sulfur poisoning resistance of Ni-gadolinia doped ceria (Ni-GDC) anode through impregnation of proton conducting perovskite BaCe0.9Yb0.1O3-δ (BCYb). The impregnation of BCYb nanoparticles improves the electrochemical performance of Ni-GDC anode in both H2 and H2S containing fuels. Moreover, more importantly, the enhanced stability is observed in 500 ppm H2S/H2. The SEM and XPS analysis indicate that the infiltrated BCYb fine particles inhibit the adsorption of sulfur and facilitate sulfur removal from active sites, thus preventing the detrimental interaction between sulfur and Ni-GDC and the formation of cerium sulfide. The preliminary results of the cell with BCYb+Ni-GDC anode in methane fuel containing 5000 ppm H2S show the promising potential of the BCYb infiltration approach in the development of highly active and stable Ni-GDC based anodes fed with hydrocarbon fuels containing a high concentration of sulfur compounds.

2


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1. Introduction Fuel cells which can directly and efficiently convert chemical energy in fuels to electricity have attracted significant interests due to the increased needs for clean, sustainable and environmentally friendly energy sources. Among all types of fuel cells, solid oxide fuel cells (SOFCs) offer excellent prospects for the fuel flexibility, attributing to their high operation temperature which facilitates the cleavage of chemical bonds to release chemical energy.1,2 Various fuels, such as hydrogen, natural gas, gasified coal and even solid carbonaceous fuels, can be used as potential fuels for SOFCs. The utilization of readily available hydrocarbon fuels decreases operation cost of SOFC system by eliminating the need for the production, storage and transportation of high-grade purified hydrogen. However, a well-known problem associated with the direct utilization of hydrocarbon fuels in SOFCs is that most readily available hydrocarbon fuels like natural gas contain sulfur compounds to some extent.3,4 The sulfur compounds either exist as hydrogen sulfide (H2S) or would convert to H2S under the fuel oxidation conditions as the results of the presence of steam and the presence of H2S is extremely detrimental to the Ni-based cermet anodes even at the level of ppm concentrations.5-9 Reducing the operation temperature of conventional SOFCs to an intermediate temperature range of 500-750 °C can not only significantly extend the choice range of SOFC materials and prolong their lifetime but also reduce the manufacturing cost of SOFC systems. Matsuzaki and Yasuda10 investigated the temperature dependence of 3



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sulfur poisoning effect on the Ni-yttria-stabilized zirconia (Ni-YSZ) anode and showed that the critical H2S concentration in H2 was 2, 0.5 and 0.05 ppm at 1000, 900 and 750 °C, respectively. This indicates that the critical concentration at which the effect of sulfur poisoning becomes significant decreases rapidly as the operation temperature decreases. Therefore, development of sulfur tolerant anodes becomes increasingly important and critical for the intermediate temperature SOFCs or IT-SOFCs. Ni-based cermet, the most common anode in SOFCs due to its high electrical conductivity, structural stability and compatibility with YSZ electrolyte, and abundance and its high performance, has been extensively studied.7,11-14 It has been well established that Ni-based anodes have very low tolerance and resistance towards sulfur and experience severe performance degradation in fuels containing only a few ppm of H2S, primarily due to sulfur adsorption and thus blocking the active sites on the Ni surface for the oxidation reaction of fuels.10,15,16 The low resistance of Ni towards sulfur poisoning is most likely related to the low energy barriers for the H2S dissociation on transition-metal surfaces, due to the weak H-S bonds and high exothermicity related to the strong metal-S bonds as the results of significant overlap between the p orbital of S and the d orbital of metal atoms, as shown by the density function theory calculations.17 Thus, many researchers keep their work focused on developing Ni-free metal oxides as potential sulfur resistant anodes. One class of them is the lanthanum-, chromium- and titanium-based single and double 4


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perovskites,18,19 such as La0.4Sr0.6TiO3−δ,20 La1-xSrxVO3−δ,21 La0.75Sr0.25Cr1-xMnxO3−δ,22 Ce0.9Sr0.1Cr0.5Fe0.5O3-δ,23 and Sr2Mg1-xMnxMoO6-δ24. These metal oxides display improved sulfur poisoning tolerance with higher critical pH2S/pH2 ratio than Ni-based anodes. However, the practical application of Ni-free metal oxide based materials as anodes of SOFCs is limited by their low electrical conductivity, poor catalytic activity towards hydrocarbon fuels oxidation reactions, limited thermal, chemical and/or physical compatibility with the electrolyte at high temperatures during fabrication.6,25 On the other hand, the sulfur tolerance and resistance of Ni-based cermets in sulfur containing fuels can be significantly enhanced. One of the viable approaches is to modify Ni-based anodes via incorporation or impregnation of ionic conductivity or catalytic active materials, such as CeO2,26 Nb2O5,27 Sc2O3,28 BaO,29 Sn and Sb30. Incorporation of Ba(Ce1-xZrx)O3-based materials in Ni-based cermet anodes has been shown to be a promising approach to achieve better sulfur tolerance.9,31,32 Most recently, we studied in detail the electrochemical performance of Ni-GDC anode modified with BaCe0.9Yb0.1O3-δ (BCYb) nanoparticles in methane.33 The results reveal that the infiltration of proton conducting perovskite BCYb can improve the electrocatalytic activity of Ni-GDC anode. Moreover, more importantly, the infiltrated BCYb fine particles inhibit carbon formation and deposition on Ni grains in methane fuel, resulting in remarkably enhanced stability. It is thus anticipated that the addition of BCYb to the Ni-GDC anode would also inhibit the sulfur deposition and poisoning, alleviating the sulfur poisoning issue of Ni-GDC anode. 5



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2. Experimental 2.1 Materials The preparation of BaCe0.9Yb0.1O3-δ aqueous solution (0.2 M) is as same as that described in ref. 33. The pH of the solution was adjusted to ~7 using NH3·H2O (25-28 wt. %, Sinopharm, Shanghai). To investigate chemical compatibility between BCYb and Ni-GDC anode, BCYb powder (derived from the solution) was mixed with NiO (Fisher Scientific) and Ce0.9Gd0.1O1.95 (GDC, Fuel Cell Materials) powder respectively at a weight ratio of 1:1. Then the BCYb-NiO and BCYb-GDC mixtures were both calcined in air at 1000 °C for 3 h. The stability of the infiltrated BCYb particles under reducing conditions was examined at 650 °C by passing H2 for 2 h. NdBa0.75Ca0.25Co2O5+δ (NBCaC)34 double perovskite was used as the cathode powder. To prepare NBCaC cathode powder, Nd(NO3)3·6H2O, Ba(NO3)2, Ca(NO3)2 and Co(NO3)3·6H2O (all from Sinopharm, Shanghai) were dissolved in distilled water with ethylene glycol and citric acid added as the chelate agent. The molar ratio of total metal ions, ethylene glycol and citric acid was 1:1.5:3. The solution was stirred and heated at 80 °C for 2 h, and then vaporized in an oven at 300 °C, followed by calcination in air at 900 °C for 5h to form layered perovskite NBCaC. 2.2 Cell fabrication The anode-supported full cells (ϕ12 mm×1 mm) were prepared by the die-pressing and co-sintering process. To prepare the anode support, NiO and GDC powder were mixed with corn starch (Fisher Scientific) as a pore former at a weight ratio of 6


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45:33:22, using ball milling in isopropanol. The mixed powder was die-pressed into a disk and pre-sintered at 1050 °C in air for 2 h. The electrolyte layer on the anode support substrate was prepared by spin coating of a GDC slurry. Then the anode and the electrolyte were co-sintered at 1450 °C in air for 5 h to densify the GDC electrolyte layer. BCYb+Ni-GDC anode was prepared by infiltrating the BCYb solution into the NiO-GDC anode backbone and calcined in air at 1000 °C for 3 h. The loading of infiltrated BCYb was 5 wt. %. NBCaC-GDC composite cathode was prepared by mixing 60 wt. % NBCaC and 40 wt. % GDC in organic binder (ethyl-cellulose and terpineol) and applied to Ni-GDC anode supported GDC electrolyte cells by slurry painting, followed by sintering in air at 950 °C for 4 h. The area of the NBCaC-GDC cathode was 0.32 cm2. Two full cells with Ni-GDC and BCYb+Ni-GDC anodes are denoted as NG cell and B+NG cell respectively. 2.3 Characterization For electrochemical performance measurement, Pt paste (Sino-Platinum Metals Co., Ltd) was painted on the anode and cathode of the as-prepared cells and sintered in air at 800 °C for 2 h. Then a glass sealant (Ceramabond®, Aremco Product, Inc.) was used to seal the cells onto an Al2O3 tube. Pure H2 was led into the cell holder to reduce the anodes in situ at 650 °C for 2 h prior to electrochemical measurements. Pt wires and mesh (Sino-Platinum Metals Co., Ltd) were used as measuring probes and current

collector.

An

electrochemical

interface

(Solartron

1287)

and

an

impedance/gain phase analyzer (Solartron 1255) were adopted for the measurements. 7



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The

initial

electrochemical

impedance

spectra

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(EIS)

and

current

density-voltage-power density (I-V-P) curves were obtained in H2 at temperatures from 550 to 650 °C. In the Nyquist plot, the intercepts of the cell impedances with the real axis at high and low frequencies represent the ohmic resistance of the cell including electrode and electrolyte (RΩ) and the total resistance of the cell (RT), respectively. The tolerance to sulfur poisoning was investigated in 500 ppm H2S/H2 at 650 °C under a constant current of 640 mA cm-2. To further demonstrate the tolerance towards sulfur, the initial performance of the cells was also studied in CH4 containing 5000 ppm H2S. The flow rate of fuel gas was kept constant at 100 ml min-1. The phase formation of BCYb powder and NBCaC powder as well as the stability of BCYb under reducing conditions and chemical compatibility between BCYb and Ni-GDC anode were identified using X-ray diffraction (XRD, X’Pert Pro, PAN Analytical B.V.). The microstructures of the anodes before and after the test were examined by scanning electron microscopy (SEM, JEOL 6301F). 3. Results and discussion 3.1 Phase identification and microstructure characterization The XRD pattern of BCYb powder calcined at 1000 °C in air for 3 h is shown in Fig. 1a. The pattern exhibits only diffraction peaks in agreement with the standard spectrum of BaCeO3, indicating the formation of pure perovskite BCYb phase. The XRD pattern of BCYb powder after treatment in H2 for 2 h is identical to that of BCYb calcined in air, indicating that BCYb is stable under reducing conditions. The 8


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XRD pattern of as-prepared NBCaC powder (Fig. 1a) shows the peaks associated with double perovskite structure, according to the JCPDS card 00-053-0130 of NdBaCo2O5.92. This indicates that the as-synthesized NBCaC powder is a pure double perovskite phase. As shown in Fig. 1b, the XRD spectra show no splitting of main reflections or extra peaks for both BCYb-NiO and BCYb-GDC powders, suggesting that there is no chemical between BCYb, NiO and GDC at 1000 °C. Fig. 2a shows the backscattering electron image of the cross-section of a NiO-GDC anode supported cell. The NiO-GDC anode support layer is porous with interconnected large and small pores. The thickness of GDC electrolyte is ~8 µm and is sufficiently dense with no visible connecting pores. The NBCaC-GDC cathode is 25~30 µm thick and is well adhered to the electrolyte. NiO-GDC anode is characterized by uniformly distributed NiO (1.7-2.6 µm) and GDC (0.7-1.9 µm) particles (Fig. 2b). NiO grains are characterized by the formation of distinct facets while GDC grains show smooth surface. There are few isolated white nanoparticles along the grain boundaries, which may be related to some impurities of the raw materials. Regarding BCYb infiltrated NiO-GDC anode, BCYb+NiO-GDC, deposition of a large number of fine BCYb nanoparticles on the surface of Ni and GDC grains is observed. However, it appears that there is a preferred BCYb deposition on the surface of nickel particles (Fig. 2c). The size of BCYb is in the range of 20-60 nm. The results show that NiO and GDC particles are well connected and covered by a thin BCYb nanoparticle layer. 9



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3.2 Cell performance in H2 and 500 ppm H2S/H2 Fig. 3a-b show the polarization and power output of NG and B+NG cells in 3% H2O/97% H2, measured at temperatures ranging from 550 to 650 °C. The open circuit voltages (OCVs) of the NG cell in 3% H2O/97% H2 fuel and air are 0.90, 0.85 and 0.82 V at 550, 600 and 650 °C, respectively. For the NG cell, the peak power density is 0.84, 1.23 and 1.41 W cm-2 at 550, 600 and 650 °C, respectively (Fig. 3a). It increases remarkably to 1.11, 1.49 and 1.75 W cm-2 at the same temperatures for the B+NG cell (Fig. 3b). Because the cathodes and GDC electrolyte thickness in the cells are almost identical, the significant performance enhancement is most likely as the result of differences in the anodes. This indicates that infiltration of BCYb significantly improves the electrochemical performance of Ni-GDC anode based cells. Fig. 3c-d show electrochemical impedance spectra of the NG and B+NG cells, measured under open circuit conditions at different temperature in 3% H2O/97% H2 fuel and air. Both ohmic resistance (RΩ) and total resistance (RT) of the cell decrease significantly with the increase in temperature due to the increased conductivity and catalytic activity. Compared to the NG cell, RT values remarkably decrease for the B+NG cell while RΩ values are also slightly lower. Since the electronic conduction in GDC is not negligible under the operation conditions, the interfacial resistances (Rp) of the cell may not be determined simply by the difference between the high and low frequency intercepts (R'p = RT -RΩ). The values of Rp can be obtained by taking into

10


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consideration of the contribution of the mixed ionic and electronic properties of the electrolyte,35,36 Rp = VOC EN

R'p

(1)

V R [1- Ω (1- OC )] RT

EN

where VOC is the measured open circuit voltage (OCV) and EN is the calculated Nernst potential. The Nernst potential under conditions of 3% H2O/97% H2 fuel and air as oxidant is 1.142, 1.134 and 1.126 V at 550, 600 and 650 °C, respectively, substantially higher than the measured VOC. The low OCV of the cell is due to the fact that GDC is a mixed electronic and ionic conductor in reducing environments, which reduces the ionic transfer number of the cell.37,38 As shown in Fig. 4, the VOC/EN decreases from 0.788 to 0.728, due to increased electronic conduction in GDC electrolyte at elevated temperatures. The values of R'p and Rp for two cells at different temperatures are listed in Table 1. For both cells, Rp is much larger than the corresponding R'p at all temperatures. The ratio of Rp to R'p becomes larger as the temperature is elevated, which is the consequence of increased electronic conductivity of the GDC electrolyte. For example, the ratio of Rp to R'p is 1.34 at 550 °C and increases to 1.64 at 650 °C. Fig. 5 shows the I-V-P curves and open circuit impedance spectra of the NG and B+NG cells in H2 and 500 ppm H2S/H2 fuels at 650 °C. Both cells suffer electrochemical performance degradation in 500 ppm H2S/H2 as compared with that in pure H2. The peak power density of the B+NG cell in 500 ppm H2S/H2 is 1.66 W cm-2, slightly lower than 1.75 W cm-2 in pure H2. On the other hand, for the NG cell, 11



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the peak power density is 1.41 W cm-2 in pure H2 and decreases remarkably to 0.69 W cm-2 in 500 ppm H2S/H2. The reduction in the power output of the B+NG cell in 500 ppm H2S/H2 as compared with that in pure H2 is 5%, substantially lower than 51% for the NG cell, tested under identical conditions. This evidently indicates that sulfur tolerance of the Ni-GDC anode is dramatically improved with the infiltration of 5 wt. % BCYb nanoparticles. The high performance of the B+NG cell is also supported by the impedance behavior of the cell (Fig. 5b and Table 1). For the NG cell, both RΩ and Rp increase significantly from 0.13 and 0.18 Ω cm2 in H2 to 0.26 and 0.50 Ω cm2 after exposure to 500 ppm H2S/H2. In terms of the B+NG cell, the increase in RΩ and Rp after exposure to 500 ppm H2S/H2 is much smaller. Assuming that the contribution of the cathodic reaction to the overall cell impedance would be more or less the same, the much smaller increase in both RΩ and Rp for the B+NG cell can be primarily attributed to the enhanced sulfur tolerance of the infiltrated BCYb nanoparticles. However, due to the overlapping of the cathodic and anodic reactions to the overall cell impedance,39 it would be difficult to ambiguously separate the anodic impedance from the overall cell impedance. 3.3 Stability and microstructure of cells in 500 ppm H2S/H2 Fig. 6 shows the durability of two cells measured in 500 ppm H2S/H2 at a constant current density of 640 mA cm-2 and 650 °C. In both cases, a swift initial drop occurs in the cell voltage when the sulfur is introduced to hydrogen. For the B+NG cell, the cell voltage is 0.738 V after stabilized in pure H2 and decreases to 0.704 V after 12


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introducing 500 ppm H2S/H2. After polarization in 500 ppm H2S/H2 fuel for 18 h, the cell voltage is 0.702 V, showing rather stable performance in 500 ppm H2S/H2 fuel. Most interestingly, after changing the fuel to pure H2, the cell voltage improves and reaches 0.736 V, almost the same as 0.738 V before the introduction of 500 ppm H2S/H2. This indicates that the performance degradation of the B+NG cell in 500 ppm H2S/H2 can be completely recovered. For the NG cell, the cell voltage also decreases from 0.63 V in H2 to 0.62 V in 500 ppm H2S/H2. However, very different from the B+NG cell, the voltage of the NG cell is not stable in 500 ppm H2S/H2 and decreases with the polarization, reaching 0.59 V after polarization for 6 h. Upon removal of H2S from the fuel, the cell voltage remains at 0.58 V in H2, still lower than 0.63 V before the introduction of 500 ppm H2S in H2 fuel. This indicates that performance degradation of the NG cell in 500 ppm H2S/H2 is not reversible. The irreversibility of the performance deterioration of the Ni-GDC anode indicates the permanent structural damage of the Ni-GDC anode when exposed to 500 ppm H2S/H2. Fig. 7 shows the SEM micrographs of the anodes after the stability test in 500 ppm H2S/H2. For the Ni-GDC anode, a significant microstructural change, especially for the Ni grains, is observed (Fig. 7a). The surface of the Ni grains becomes very rough and porous, and there is the clear formation of a number of holes in the Ni grains. This is in contrast to the dense and crystalline faceted Ni particles before the test in H2S-containing H2. GDC phase also changed to particles with sharp edges. This indicates the severe interaction between H2S and Ni-GDC. Zhang et al also observed 13



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the significant morphology change of both Ni and GDC grains after the Ni-GDC anode was exposed to H2S-H2 fuel at 800 °C.40 As for the BCYb+Ni-GDC anode, there is no hole formation or roughening on Ni surface and no visible changes to the GDC grains. Both Ni and GDC particles remain more or less the same as that before the test (Fig. 7b). The infiltrated BCYb nanoparticle layer on the surface of Ni and GDC particles is still visible, and there is no significant agglomeration of the BCYb nanoparticles after the stability test for 24 h. The result indicates that the presence of BCYb nanoparticles substantially reduces the interaction between sulfur and Ni-GDC probably by inhibiting the adsorption of sulfur species on the Ni surface, thus inhibiting the detrimental effect of sulfur on morphology and microstructure of Ni-GDC anode. The XPS was used to characterize the chemical state and surface properties of the Ni-GDC and BCYb+Ni-GDC anodes after the stability test in 500 ppm H2S/H2. Fig. 8 shows the XPS spectra of S2p, Ce3d and O1s core level obtained on the surface of the anodes. XPS spectra were fitted with a Shirley-type background subtraction method after normalized with a C1s peak. The background-corrected XPS spectra were fitted by 80% Gaussian and 20% Lorentz functions for different chemical states of the elements. The S2p spectrum obtained from the Ni-GDC anode can be fitted using two separate doublets (Fig. 8a). One of the doublets has two overlapping peaks centered at 159.8 and 161.0 eV, which correspond to the S2p3/2 and S2p1/2 spin-orbit states,

14


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respectively. These peak positions are consistent with those reported for S2-,41-43 indicating the formation of a surface cerium sulfide, Ce2O2S: CeO2ሺsሻ +xH2(g) →CeO2-xሺsሻ +xH2 O(g)

(2)

2CeO2-x(s) +H2 S(g) +൫1-2x൯H2ሺgሻ ↔Ce2 O2 S(s) +2൫1-x൯H2 Oሺgሻ (x

Enhancing Sulfur Tolerance of Ni-Based Cermet Anodes of Solid

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