單氣室甲烷固態氧化物燃料電池陽極改質之研究

行政院國家科學委員會專題研究計畫 成果報告

單氣室甲烷固態氧化物燃料電池陽極改質之研究 研究成果報告(精簡版)

計 畫 類 別 ： 個別型

計 畫 編 號 ： NSC 98-2221-E-011-076-

執 行 期 間 ： 98 年 08 月 01 日至 99 年 10 月 31 日 執 行 單 位 ： 國立臺灣科技大學化學工程系

計 畫 主 持 人 ： 蕭敬業

處 理 方 式 ： 本計畫涉及專利或其他智慧財產權，2 年後可公開查詢

中 華 民 國 100 年 01 月 03 日

行政院國家科學委員會補助專題研究計畫成果報告

單氣室甲烷固態氧化物燃料電池陽極改質之研究

計畫類別： 個別型計畫

計畫編號： NSC98-2221-E-011-076

執行期間： 98 年 08 月 01 日 至 99 年 10 月 31 日 執行單位： 國立台灣科技大學 化工系

計畫主持人：蕭 敬 業

計畫參與人員：張 仁 禎、謝 喻 明、彭 定 宏 成果報告類型： 精簡報告

處理方式： 二年後可公開查詢

中 華 民 國 99 年 12 月 31 日

Impact of anode porosity for anode-supported solid oxide fuel cell operated under single chamber atmosphere and intermediate temperatures

Ching-Yeh Shiau, Jen-Chen Chang, Yu-Ming Hsieh, Din-Hong Peng NSC-98-2221-E-011-076

Abstract

In this work, the impact of anode porosity on a NiO-SDC (Ce0.8Sm0.2O1.9) anode-supported solid oxide fuel cell is investigated by addition of pore formers (graphite, starch, polymethyl methacrylate) which create a controlled porous architecture. The anode-supported NiO- SDC/SDC/SDC-SSC (Sm0.5Sr0.5CoO3) single cell is evaluated under single chamber operation conditions with a mixture of methane and oxygen. Various weight ratios of the pore former in the electrode slurry are studied and characterized. Multi-layer anode support is employed to create the porosity gradient for better understanding the functionality of the porous microstructure.

Introduction

The solid oxide fuel cell (SOFC) is a promising candidate for clean energy technologies. It possesses several unique advantages, including flexible fuels, cheaper materials for each component and higher overall efficiency (~80%) taking into account the combined utilization of the produced power and heat. It is particularly suitable in stationary applications, for example, distributed power plants.

To achieve higher power output, there are several concerns in terms of materials and geometry of each component in a single SOFC. Among these issues, the electrode microstructure is highly correlated with the reaction kinetics. The percolation of electronic, ionic and reactant/product pathways determines the effective three-phase-boundary length of the electrode, which directly links to its performance [1]. It is extremely important for SOFCs of electrode-supported configuration, where the microstructure strongly influences the utilization of the electrode support, more specifically, the ease of access of reactants to the electrode layer close to the electrolyte/electrode interface where has been claimed to be the active region of the electrode [2].

To fulfill the requirement mentioned above, a porosity gradient microstructure was recently proposed, which can be achieved with fine adjustment of the slurry composition (active materials, pore former and binder) along the depth from the electrode surface [3, 4]. Improvement of the power output was achieved by porosity gradient anode with [5] or without [6] composition gradient.

The effectiveness of the porosity gradient electrode was demonstrated with the electrode-supported SOFC operating in dual-chamber mode (hydrogen and oxygen fed to anode and cathode, respectively) [7-12].

For SOFCs fueled by hydrocarbons, the electrode catalytic ability is critical. For instance, methane, commonly used as the fuel, can minimize the gas phase reaction with oxygen and is particularly suitable for single chamber SOFC (SC-SOFC) [13-17]. However, the high stability of methane results in poor oxidation activity over the SOFC anode without addition of precious metal as the catalyst [13]. In our previous work, it was found that an anode- supported Ni-SDC/SDC/SSC cell, without the addition of precious metal to the anode, was able to produce an obvious increase in the power density over that of an electrolyte-supported one [13]. This increase is proposed to have higher number of active sites for partial oxidation of methane provided by anode support and less resistance with the thin SDC film [18].

In this work, an attempt is done for SC-SOFC with porosity gradient anode support for further improvement. Methane-air mixture was fed as the fuel. The porosity gradient anode was tuned by adjusting the pore formers of various weight ratios during preparation. It was found that the performance can be greatly improved with careful control of the porosity gradient of the depth along the electrode surface.

Experimental

Synthesis of raw materials. All the chemicals were purchased and used as received. The electrolyte and cathode materials were synthesized by the citrate-gel method. For the SDC powders, 0.04 mole of Ce(NO3)3 6H2O (Acros, 99.5%) and 0.01 mole of Sm(NO3)3 6H2O (Acros, 99.9%) were dissolved in 100 mL of de-ionized water. 0.1 mole of citric acid (Acros, 99.5%) was dissolved in 100 mL of de-ionized water and added dropwise to the metal precursor solution with stirring. The mixed solution was gelled, dried and finally heat-treated at 600 ^oC. A similar procedure was used for the synthesis of SSC powders but with heat treatment of the solid gel at 1000 ^oC for 5 hr.

Preparation of anode support with single/multi- functional layers. Solid powders including commercial NiO (Acros, 97%), the synthesized SDC powders and a pore former (graphite, starch, polymethyl methacrylate) were mixed in ethanol in a weight ratio of 1 : 1. The mixture was ball-milled with zirconia balls in a PE bottle for 24 hr. 0.6 g of the pulverized powder was poured in a mold 16 mm in diameter and uni-pressed under a pressure of 73.5 MPa. Single/ multi functional layers were prepared by repeating the above process with the required powder mixture on the pelletized disk. Later the disk was heated at 750 ^oC for 6 hr at ambient atmosphere with a ramping rate of 1 ^oC min^-1. The calcined disk was served as the anode support for the deposition of the SDC film.

Preparation of anode-supported SOFC cell. Electrophoretic deposition (EPD) was employed for the deposition of SDC film on the NiO-SDC support. Basically, the process follows our previous work [18]. Firstly the SDC suspension was prepared with the mixed solvent of acetone and ethanol in a volumetric ratio of 3 : 1. 0.6 g of iodine and 5 g of SDC powders were added to the mixed solvent.

The above solvent was ultrasonically treated for a few minutes to form the required SDC suspension.

For EPD of SDC film on the anode support, a stainless steel disk 15.5 mm in diameter and the graphite-bonded NiO-SDC support were served as the anode and cathode, respectively. The distance between the two electrodes was fixed at 10 mm. The two electrodes were immersed in the SDC suspension, and a voltage of 60 V was applied. The deposition process was performed for 2 min. The deposited SDC film was co-fired with a heating program of 5 ^oC min^-1 to 1000 ^oC and held for 1 hr, which was followed by 2 ^oC min^-1 to 1350 ^oC and held for 12 hr.

The SDC-SSC cathode was prepared by screen printing. The slurry was prepared with the synthesized SDC powders, SSC powders, ethyl cellulose and α-terpineol in a weight ratio of 180 : 20 : 9 : 250. The above mixture was ball-milled for 24 hr to obtain homogeneous slurry. The slurry was applied on the SDC film by screen printing, followed by heating at 1100 ^oC for 5 hr.

The thickness of the prepared anode-supported SOFC cell is about 700, 17 and 22 μm for NiO-SDC, SDC and SSC-SDC layers, respectively (by SEM). The thickness of the single and multi-layer anode support is the same, except for the variation in the porosity gradient.

Characterization. The phase confirmation of the raw materials was characterized by X-ray

diffraction (XRD) technique using an X-ray diffractometer (Rigaku D/Max-RC, Japan) operated under Cu Kα radiation at 40 kV and 100 mA. The particle size distribution of the SDC powders was determined with a laser light scattering system. The microstructures and thickness of the electrode and the electrolyte were characterized by scanning electron microscope (SEM, JEOL JSM-6500F).

For electrochemical characterization, Ag net and Ag wires were attached to the electrode surfaces for current collection.

The impedance of the SOFC cell was characterized using an electrochemical system combining a Solartron 1260 Frequency Response Analyzer and a Solartron 1286 Potentiostat. The applied frequency ranged from 10⁵ to 10^-1 Hz with the signal amplitude of 20 mV. Prior to characterization, the anode was reduced at 600 ^oC for 6 hr in a flowing mixture gas of H2 and Ar in a volumetric ratio of 1 : 9 and a flow rate of 100 mL min^-1. The reactant gas was composed of methane and air premixed in a ratio of 3 : 7. The total flow rate was fixed at 300 sccm.

Results and Discussion

First of all the impact of various kinds of pore formers (5 wt%) was evaluated during preparation of the Ni-SDC anode support. The corresponding microstructures are shown in Fig. 1.

Compared to the Ni-SDC anode support without pore former (Fig. 1(a)), slab-like pores several to 10 μm in length and ~ 1 μm in width were found in the one with graphite as the pore former (Fig.

1(b)). For electrode supports with starch (Fig. 1(c)) and PMMA (Fig. 1(d)), much larger irregular pores are found. When examining the discharging curves of the Ni-SDC/SDC/SSC-SDC cells made by different pore formers (Fig 2(a)), only the one with graphite as the pore former exhibited better performance than that without pore former. The peak power density for each single cell was 269, 230, 212 and 255 mW cm^-2 for the cell with 5 wt% graphite, 5 wt% starch, 5 wt% PMMA and none as the pore former, respectively. Considering the open circuit potential (OCP) of the single cell, only a slight change with pore former was observed. Similar OCP (~0.84 V) was found for the cells without pore former and with 5 wt% graphite. The OCP for the rest was slightly lower than 0.84 V.

In addition, the impedance for all the cells at OCP seems the same since it may be dominated by the smaller cathode (Fig. 2(b)). Therefore the difference of the electrochemical performance may result from the mass transport of methane, which is altered by the microstructure of the anode supports.

The impact of graphite content on the anode support was then considered. With the increase of the graphite content, more slab-like pores were shown in the prepared anode support (10 wt%, Fig.

1 (e)), indicating the space occupied by graphite was able to preserved during heating. Upon increasing the graphite content to 15 wt%, the mechanical strength of the prepared anode support became so weak that was unable to survive from the following process. Further, the corresponding electrochemical performance was evaluated at 600 ^oC (Fig. 3(a)). The peak power density for the single cell with 0, 5 and 10 wt% graphite was 255, 269 and 297 mW cm^-2, respectively, indicating better improvement with higher graphite content for anode support. This may have been due to the better mass transport of methane into the active site of the anode support, while the reaction kinetic is almost unchanged with porosity of anode support. It can be realized by the identical OCP values [19].

Multi-layer anode support was evaluated to further correlate the porosity gradient microstructure to their electrochemical behaviors. Two-layer anode supports composed of a first layer with 5wt%

graphite content fixed (in contact with electrolyte layer) and a second layer with graphite content

varied were firstly considered. The corresponding electrochemical performance at 600 ^oC strongly varied with the porosity gradient of the anode support (Fig. 3(b)). Clearly, the two-anode-layer SC-SOFC with a second layer of 10 wt% graphite content exhibited the pronounced performance.

However, only a slight improvement was found in the single-layer cell with 5 wt% graphite content for the anode support. This could be accounted for by the difficulty of the methane to enter into the first layer of 5 wt% graphite content where the electrode/electrolyte interface is the main contribution for electrochemical reaction. The results also reveal that the mass transport of methane in all two-layer cells was much easier than single-layer cell of 5 wt% graphite content. This is depicted by the change of the discharging curve in the high-current region and the symmetric shape of the current-power relationship. Further, the OCP of the SC-SOFC with a second layer graphite content higher than 10 wt% (15 and 20 wt%) was reduced, which probably resulted from the difficulty of electron transfer raised from highly porous second-layer microstructure. The SC-SOFC prepared by three-layer anode support showed similar electrochemical characteristics to that of the two-layer anode support (Fig. 3(c)), where the improvement is due only to the mass transport of methane, which enters into the second and third layer of anode support but was not able to enter into the first layer (electrode/electrolyte interface).

Conclusion

In this work, it was found that careful control of the anode support microstructure was possible by the use of graphite as the pore former, and the electrochemical performance of the SC-SOFC was able to be so refined. The anode support of 10 wt% graphite content showed the least mass transport polarization, and thus exhibited the most excellent performance. The porosity gradient anode supports were fabricated to understand their correlation to electrochemical performance. It can be concluded that the access of methane to the electrode/electrolyte interface is the key for high performance SC-SOFC.

Reference

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Fig. 1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0

Voltage(V) , V

Current(I) , A/cm²

power density(P) , W/cm2

5%-PMMA 5%-Starch 5%-Graphite 0%-Pore former

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.0 0.1 0.2 0.3 0.4

5% PMMA 5% starch 5% graphite no pore former

Z'' , Ω

Z' , Ω

Fig. 2

0.0 0.5 1.0 1.5

0.0 0.2 0.4 0.6 0.8 1.0

Current(I) , A/cm²

Voltage(V) , V power density(P) , W/cm2

10%-graphite 600°C 5% -graphite 600°C 0% -graphite 600°C

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0

Current(I) , A/cm²

Power density(P) , W/cm2

Voltage(V) , V

5 % Graphite First 5 % G- Second 10 % G First 5 % G- Second 15 % G First 5 % G- Second 20 % G

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0 0.2 0.4 0.6 0.8

1.0 5 % Graphite

First 5 % G- Second 10 % G First 5 % G- Second 10 % G - third 15%

Power density(P) , W/cm2

Voltage(V) , V

Current(I) , A/cm²

0.0 0.1 0.2 0.3 0.4 0.5

Fig. 3

國科會補助計畫衍生研發成果推廣資料表

日期:2011/01/03

國科會補助計畫

計畫名稱: 單氣室甲烷固態氧化物燃料電池陽極改質之研究 計畫主持人: 蕭敬業

計畫編號: 98-2221-E-011-076- 學門領域: 無機化工材料

無研發成果推廣資料

98 年度專題研究計畫研究成果彙整表

計畫主持人：蕭敬業 計畫編號：98-2221-E-011-076- 計畫名稱：單氣室甲烷固態氧化物燃料電池陽極改質之研究

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